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page iNational Nanofabrication Users Network Research Experience for Undergraduates Program

Cornell Nanofabrication Facility ..................................................... 1Binding Organic Molecules to Inorganic Substrates ........................................ 3Diffractive Cell Detection Using A Microcontact Printed Antibody Grating....... 7The Morphological Evolution of Silicon Films Deposited by Angled

Supersonic Molecular Beams of Silicon Containing Hydrides ................... 9Manufacture & Measurement of Sub-Micron Pillars of (La,Sr)MnO3 ............. 10Fabrication of a Mechanical Microchannel System in Silicon ........................ 12Fabrication of Bilayer Organic Light Emitting Diodes .................................... 15An Attempt to Eliminate Atomic Steps from GaAs(001) Surfaces ................. 18The Study of Next-Generation Photoresists .................................................. 20Fabrication of Modulated Diffraction Gratings ............................................... 24

Howard University School of Engineering .................................. 27AFM GURU: Software for the Atomic Force Microscope ............................. 29Miniaturized Dialysis Machine for Internal Use ............................................. 30Heteroepitaxial Growth And Characterization Of Beta Silicon Carbide

Films Using Chemical Vapor Deposition (CVD). ...................................... 31

The Pennsylvania State University............................................... 33Creating Chemical Libraries in Silicon ........................................................... 35Gas Sensing Capabilities of Ultra-Thin Metal Oxide Semiconductor

Capacitors ................................................................................................ 36Design & Fabrication of a Microelectro-Mechanical Accelerometer

using Thin-Film PZT as the Sensing Medium .......................................... 37Electron Beam Lithography Damage Done to Silicon ................................... 39

Table of Contents

Introduction................................................................................. iiiBlast from the Past ..................................................................... ivNNUN Director & Addresses...................................................... vi1997 NNUN REU Photo Legend................................................ vii

1997 NNUN REU Research Accomplishments:(Alphabetical by Site and Intern’s Last Name)

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page iiNational Nanofabrication Users Network Research Experience for Undergraduates Program

Stanford Nanofabrication Facility ................................................. 41Characterization of Spatial Variation of Thin Film Properties after

Deposition ................................................................................................ 43Computer Modeling of High Density Plasma Oxide Etching ......................... 44Measuring Charging Damage Due to Plasma Nonuniformity ........................ 45Processing Techniques in Fabricating Spin-Dependent Tunnelling

Devices .................................................................................................... 46The Optimization and Fabrication of Fabry-Perot Vertical Cavity Phase

Flip Modulators ........................................................................................ 47Microfabricated Check Valves for Intraocular Pressure Stabilizer for

Glaucoma Therapy................................................................................... 48Fabrication of a Single Electron Transistor Using an Atomic Force

Microscope C-V Profiling of InAs Quantum Dots in a GaAs Substrate .... 49Variances in Hillock Formation on Passivated Aluminum Due to

Grain Growth ............................................................................................ 50AFM Tip-induced Oxide Formation on Silicion .............................................. 51Dopant Redistribution in Polysilicon During TiSi2 Formation ........................ 52Use of the AFM and NIH Image for Rapid Statistical Characterization

of Grain Size in Polycrystalline Silicon Films ........................................... 53Scanning Capacitance Microscopy for Two-Dimensional Dopant

Profiling in CMOS Devices ....................................................................... 54

University of California at Santa Barbara .................................... 55Optical Characterization of InGaN Quantum Wells ....................................... 57Preparation of CdS Nanocrystals by Reaction Compartmentalization .......... 580-D Polaritons ................................................................................................ 59Quantum Dot Resonant Tunneling Diodes .................................................... 60Native Oxides as Insulators for Gallium Arsenide ......................................... 61Photoelectrochemical Etching of GaN ........................................................... 62Fabrication of GaAs Terahertz Fabry-Perot Resonators ............................... 63Testing Millimeter Wave Detectors in Low Background Conditions ............... 64Sub-Micron Magnetism.................................................................................. 65

Index of Interns and Mentors ........................................................ 67

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page iiiNational Nanofabrication Users Network Research Experience for Undergraduates Program

1997 marked the first year in which the NNUN combined their efforts andresearch sites to hold a joint REU Program. The National Science Foundationagreed that this pulling together of abilities and resources was a fine ideawhose time had come. The NNUN’s five-year contract was approved.

In June, forty-one undergraduates began their participation in the program,taking full advantage of the unique opportunities at each of their assignedsites. The culmination of those 41 research projects was a combined NNUNREU Convocation, held at Cornell University, in August. Undergraduates andfaculty alike were impressed by the scope of the investigations, and as onegraduate s tudent who sa t in on the presenta t ions sa id , “ I even learnedsomething new about the possiblities in my own field!”

While NSF provided the primary funding for the 1997 Research Experiencefor Undergraduates Program, the research could not have taken place withoutthe generous financial support of each of the five sites, and these companies:

Advanced Micro Devices LAM ResearchAnalog Devices Lucas NovasensorApplied Materials MotorolaCorning, Inc. National SemiconductorDuPont, Inc. Philips ResearchEricsson Rockwell International CorporationHewlett Packard Siemens AG, Corporate R&DHitachi Texas InstrumentsIBM VLSI Technology, Inc.Intel Xerox, Inc.

NationalNanofabricationUsers NetworkNNNNNNNNUUUUNNNN

This publication was formatted and laid out by Melanie-Claire Mallison, Corporate & Public Relations, CNF

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National Nanofabrication Users Network Page iv Research Experience for Undergraduates Program

Below is the result of my first effort to contact pastREU interns and find out what they are up to. Given thenatural state of this particular time in the students’ lives, Ihad a hard time finding people. However, I think this is acommendable first try, and I hope as we go along, the nextgenerations of REU’s will stay in touch, both with us andwith their fellow summer researchers.

REU Intern:Brendan Hill, Summer 1996Electrical Engineering, Villanova University, PADr. L. Rathbun, CNF, Knight LabBrendan Hill <[email protected]>Hi! This is my “Blast From the Past” biography.

Right now, I’m a senior at Villanova Universitymajoring in electrical engineering and minoring in computerscience. I am secretary of Villanova’s chapter of theInstitute of Electrical and Electronics Engineers (IEEE),and I am also secretary of Villanova’s solar car racing team,Team SolarCAT. In addition, I have membership in EtaKappa Nu (electrical and computer engineering honorsociety) and Tau Beta Pi (engineering honor society). Aftercollege, I hope to work for a year or two and then pursue amaster ’s degree. I’d like to work in either thetelecommunications field or in intelligent control systemsengineering.

Looking forward to reading about fellow REU’s!Brendan Hill

REU Intern:Jurgen Holleck, Summer 1993Physics, Oberlin College, OHProf. Noel C. Mac Donald, Electr Engr, CNFProject Title: Silicon Micromechanical AccelerometerHome Address:15 Rolling Lane, Wayland, MA 01778(508)358-5022email:[email protected]

Hi! I wanted to let you know what I’m up to since myexperience at Cornell in the REU program was probably thebest summer and best undergrad experience I’ve had. Iworked on micromechanics with Prof. MacDonald andfound the work fascinating and the people I worked withand the other REU students great. Anyway, I finished up atOberlin with a physics major, and chemistry minor in 1994and almost went to Cornell for the graduate program inapplied physics, but changed my mind when I realized that Iwanted to go to medical school.

I worked for a year at EIC laboratories in Norwood,MA while I applied to medical schools. At EIC, I worked ina materials science group using thin film depositiontechnology to develop cuff electrodes which could wraparound nerves with the ultimate goal being to stimulatenerves, e.g. in the case of paralysis. My experience at theNNF (now CNF) proved useful during my work at EIC. I

Blast from the Past

am currently in my 3rd year of medical school at theUniversity of Massachusetts Medical school and amenjoying my clinical rotations. I am very curious as to whatthe other students who did the REU program in my year aredoing and would be very interested at hearing about them.Thanks, Jurgen

REU Intern:Tina Jayaweera, Summer 1993Physics, Oberlin College, OHDr. Michael J. Rooks, CNFProject Title: Electronics for Circuit Development for

Electron Beam Lithographyemail address: [email protected]

Four years after my REU at the CNF (then NNF), Iseem to have found myself back at Cornell. I am currentlya second-year PhD student in Mechanical Engineering. Iwork with Prof. E. Fisher in the field of Combustion. Ourcurrent research is with fire-suppressing agents, in hopes tofind a viable alternative to the current, ozone-destroyinghalons. We have had success with phosphorous-containingcompounds and are in the process of doing a thoroughinvestigation of one such compound, dimethyl methyl-phosphonate. Between my research and course work, I’mdefinitely managing to keep busy!

REU Intern:Aaron Lamb, Summer 1995Electrical Engineering, Harvey Mudd College, CAProf. Al Phillips, Electr Engr, CUProject Title: Properties of Shallow p-n junctionsHome Address:3899 Nobel Dr. #1301San Diego, CA 92122(619)453-4542email: [email protected]

Dear Melanie,Here’s what I’ve been up to since 1995: Since

graduating in 1996, I’ve been working for Torrey ScienceCorporation, a quickly growing high-tech communicationcompany in San Diego. Our primary product is a satellitecommunicator for the first Low-Earth Orbit (LEO) satellitenetwork, called ORBCOMM. I am working on a projectcalled MUBLCOM, the next generation of the networkallowing both voice and data communication. I have takenpart in all aspects of the project, from helping to define theinitial system, to the design of the digital signal processingunit in the satellite, and finally to the system integration andRF testing of the prototype. We are all working toward alaunch date in 1998. Away from work, I’ve been trying tokeep up my tennis and volleyball games, as well as learn afew new pieces on the piano.

I look forward to getting a copy of the “Blast-from-the-Past”. Thanks, Aaron

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National Nanofabrication Users Network Page v Research Experience for Undergraduates Program

Aaron LambTorrey Science CorporationMember Technical StaffTorrey Science Corporation3550 General Atomics Court, Bldg. 14P.O. Box 910348San Diego, CA 92121Tel (619) 552-1052Fax (619) 552-1056E-mail: [email protected]

REU Intern:Raymond G Montemayor, Summer 1995Electrical Engineering, Harvey Mudd College, CAProf. Lester Eastman, EE, CUProject Title: High Power Pseudomorphic Modulation

Doped Field Effect Transistors (SMODFET)Permanent:19513 Hinsdale Ave.Torrance, CA 90503School:1836 Glendon Ave. #2Los Angeles, CA [email protected]

After my summer at the NNF [it was called theNational Nanofabrication Facility back then] in 1995, Ifinished my senior year at Harvey Mudd College andreceived a B.S. degree in Engineering in May of 1996. Ispent four months that summer interning at SverdrupTechnology, a contractor for the NASA Ames ResearchCenter. I then began a two-year M.S. program in theElectrical Engineering Department at UCLA, specializing inIntegrated Circuits and Systems (RF ICs in particular). Mythesis topic is "A 900-MHz Self-Calibrating Image-RejectReceiver." After completing the first year of my graduatework, I spent this past summer interning at RockwellSemiconductor Systems designing RF blocks for a newwireless transceiver. I am currently completing the secondyear of my program and will be graduating in June '98. Inaddition to school, I am the president of Pilipinos InEngineering (PIE), a relatively new organization in theSchool of Engineering and Applied Science. Please sendmy copy to my school address. Thanks a lot! I'm lookingforward to reading about my fellow REUs. Raymond

REU Intern:Samir Succar, Summer 1996Physics, Oberlin College, OHProf. H. Craighead, AEP, CUProject Title: Silicon Etching Using Ultra-Thin ResistsHome Address:260 N. 3rd St., Apt JSan Jose, CA 95112hm: 408 292-7674wk: 408 501-7108

Blast from the Past,continued

fx: 408 437-9031http://www.cs.oberlin.edu/~ssuccaremail: Samir Succar <[email protected]>

I just completed my undergraduate degree at OberlinCollege with a major in Physics this past May. I’m nowinvolved in R&D for a new line of e-beam based diagnosticequipment for Schlumberger Technologies, a company inSilicon Valley specializing in Automatic Test Equipment(among other things). I’m planning to return to school for agraduate degree in either EE or Applied Physics within thenext couple of years.

Hope this helps - good luck! Samir

REU Intern:John P. Wus, III, Summer 1991EE, Villanova Univ, PAProf Harold G. Craighead, AEP, CUProject Title: The Fabrication of Apertures, Scribe Lines,

and Front-to-Back Alignment Marks on SiliconHome Address1084 Oakwood Dr.Warminster, PA 18974Email: [email protected] (John Wus)

Hello, I am John Wus. Since being a CNF REU, Igraduated Summa Cum Laude from Villanova University inMay 1992 with the B.S. degree in Electrical Engineering.Upon graduation, I received the Dean’s Award for AcademicExcellence and the Dean’s Award for Meritorious Service.While I was an undergraduate, I completed design projectsdealing with the development of a housing unit in order toperform experiments on the Space Shuttle and thedevelopment of a voice recognition system for commercialuses. In addition to my engineering courses at Villanova, Iobtained a Business Minor which included courses inAccounting, Marketing, Finance, and Economics. I am amember of the Phi Kappa Phi Honor Society, Tau Beta PiHonor Society, and Eta Kappa Nu Honor Society.

In 1992, I was awarded the Gotshall Fellowship toattend Lehigh University. From September 1992 to June1997, I attended Lehigh University under the guidance ofProf. Weiping Li. In January 1995, I received the M.S.degree in Electrical Engineering from Lehigh University. Ihave authored and co-authored numerous papers in the areaof image and video compression, as well as two patentapplications. My research group has submitted severalproposals to the development committee of the MPEG-4video standard. I received my Ph.D. degree in ElectricalEngineering from Lehigh University in June 1997. Myresearch interests include signal processing, image andvideo processing, and VLSI. Some of my personal interestsare tutoring, bartending, ballroom dancing, and songwriting. I wish all the new REU’s the best of luck. Enjoyyour research and your summer at Cornell University. Iknow I did!

John Wus

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page viNational Nanofabrication Users Network Research Experience for Undergraduates Program

The National Nanofabrication Users Network (NNUN)URL: http://www.nnun.org/

is comprised of these five sites:

Cornell Nanofabrication FacilityDr. Joseph Ballantyne, DirectorCornell University, Knight LaboratoryIthaca, New York 14853-5403Voice: (607) 255-2329Fax: (607) 255-8601URL: http://www.cnf.cornell.edu/

Materials Science Center for ExcellenceDr. Michael Spencer, DirectorHoward University School of Engineering2300 Sixth St, NWWashington, D.C. 20059Voice: (202) 806-6618Fax: (202) 806-5367URL: http://www.msrce.howard.edu/~nanonet/nnun.htm

Electronic Materials & Processing Research LaboratoryDr. Robert Davis, DirectorMR Institute BldgThe Pennsylvania State UniversityUniversity Park, PA 16802Voice: (814) 865-0870Fax: (814) 865-6581URL: http://www.emprl.psu.edu/

Stanford Nanofabrication FacilityDr. James Plummer, DirectorStanford UniversityCIS BuildingStanford, CA 94305-4070Voice: (415) 725-6266Fax: (415) 725-6278URL: http://www-snf.stanford.edu/

University of California at Santa Barbara(UCSB) - Nanofabrication FacilityDr. Evelyn Hu, DirectorDepartment of Electrical & Computer EngineeringSanta Barbara, CA 93106-9560Voice: (805) 893-4130Fax: (805) 893-8170URL: http://www.ece.ucsb.edu/nnun/index.html

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page viiNational Nanofabrication Users Network Research Experience for Undergraduates Program

(Photo taken by Karlis Musa, CNF Staff, at the 1997 NNUN REU Convocation, held in August at Cornell University)

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(#) REU Intern Home Institution Mentor NNUN Site1. Adam Pyonin Washington Univ Dr. Nabeel Ibrahim SNF2. Rhasheda Nixon Tuskegee Univ Michael Rogosky PSU3. Brittany Mitchell Clarkson Univ Dr. Mike Deal SNF4. John Kim Yale Univ Prof. James Harris SNF5. George Paloczi Univ of California at SB Dr. Kathryn Wilder SNF6. John Fuhrman Univ of California at SB Dr. Steve DenBaars UCSB7. Mark Zwald Cornell Univ Prof. Harold Craighead CNF8. Matthew Owings Harvey Mudd College Prof. Emmanuel Giannelis CNF9. Arthur Beyder SUNY at Buffalo Prof. Nicholas Winograd PSU10. Kevin Ness Univ of California at SB Dr. Univmesh Mishra UCSB11. Nicole Jantzi Stanford Univ Dr. Mark Zdeblick SNF12. Paul Molnar Cornell Univ Dr. Jorge M. Garcia UCSB13. Andrej Sierakowski Stanford Univ Prof. James Plummer SNF14. Keith Hebert Texas A & M Univ Dr. Stephen Fonash PSU15. Dedra Demaree Univ of MD, Baltimore Cty Prof. Lester Eastman CNF16. Joshua Stamp Hanover College, IN Dr. David Awschalom UCSB17. Brett Sackett Univ of California at Davis Dr. Mark Su UCSB18. Benjamin Cawood James Madison Univ Prof. Michael Spencer HU19. Farouk Hadeed Yale Univ Prof. Shan Wang SNF20. Frances Uku Howard University Observing21. Ijeoma Akunyili Howard Univ Dr. Gary Harris HU22. Brandon Fleming Case Western Reserve Univ Dr. Peter Griffin SNF23. Craig Maloney Univ of Toledo Dr. Atac Imamoglu UCSB24. Daniel Culkin Fordham Univ Prof. James Engstrom CNF25. Brendan Moran The Johns Hopkins Univniv Prof. Jack Blakely CNF26. Bennet Tarng Stanford Univ Prof. Peter Krusius CNF27. Sandra Sattler Columbia Univ Dr. Mark Sherwin UCSB28. Jennifer Mei-An Popham Alabama A & M Univ Dr. Evelyn Hu UCSB29. Xinyi Zhou Cornell Univ Prof. Christopher Ober CNF30. Veronica Collier Alabama A & M Univ Dr. Kimberly Jones HU31. Maria Huang California Inst of Technology Dr. James Harris, Jr. SNF32. Mark Finlan William and Mary Dr. James McVittie SNF33. John Cross Santa Clara Univ Dr. James McVittie SNF

1997 NNUN REU Photo Legend

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Cornell

Nanofabrication

Facility

1997 REU Participants

From Left to Right:

REU Participant Home Institution Principal Investigator

Mary Heseltine Univ of Rochester Prof. Noel MacDonald

Dr. Mary D. S. Frame

Brendan Moran The Johns Hopkins Univ Prof. Jack Blakely

Dedra Demaree U of MD, Baltimore County Prof. Lester Eastman

Laurance Hiller SUNY at Buffalo Prof. Seok-Beom Yoon

Leeann Chandler James Madison Univ Prof. Noel MacDonald

Matthew Owings Harvey Mudd College Prof. Emmanuel Giannelis

Bennet Tarng Stanford Univ Prof. J. Peter Krusius

Mark Zwald Cornell Univ Prof. Harold Craighead

Yelena Chigirinskaya Cornell Univ Prof. Carl Batt

Prof. Harold Craighead

Not Pictured:

Daniel Culkin Fordham Univ Prof. James Engstrom

Franklin Grigsby Harvey Mudd College Prof. Yuri Suzuki

Xinyi Zhou Cornell Univ Prof. Christopher Ober

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National Nanofabrication Users Network Page 3 Research Experience for Undergraduates Program

Cornell Nanofabrication Facility Participant

Abstract:The abil i ty to analyze organic substances on a

molecular level would be a useful tool in both themedical and scientific fields. MEMS offers a way toachieve analysis on this scale, but typical processes arenot easily applied to organic materials. A method ofapplying organic molecules to inorganic substrates andanalyzing the reaction results is the goal of this project.Si, Ta, Au, and SiO

2 substrates were coated with poly-

heptafluorobutyl methacrylate and then hydrolyzed underan O

2/H

2O plasma courtesy of the Cordis Corporation.

This created bound carboxylic acids to which aphotolabile protecting group could be attached. The lightsensitive protecting group examined is t-butyl (2-bromo-2-nitrophenyl) acetate, which was derived from 2-nitrophenylacetic acid. The protecting group was reactedwith the substrate under a gentle benzene reflux to form aphotolyzable coating, which then can be patterned withphotolithography. The exposed pattern would becomethe substrate for the organic molecules to be individuallyattached. Since the molecules are adhered to a solidsurface, conventional chemical analysis cannot be done,so we are attempting Auger, IR, and XPS surfaceanalysis.

Introduction:The capability of working with organic substances on a

molecular level is a new realm of scientific analysis.Processes such as pharmaceutical screening and DNAanalysis can be done on a smaller scale, making themfaster and cleaner, using less material. An example oftaking laboratory work to a smaller scale is thepolymerase chain reaction that can now be done in a handheld instrument. On an even smaller scale, the ability tomanipulate and test actual molecules could eliminatesome of the obscurity of working with larger quantities,because i t is difficult to obtain a large sample ofmolecularly pure material . Probing an organicmonolayer with a single molecule attached tonanofabricated inorganic tips would offer possibilities inexamining forces between molecules, but the toolsneeded to work on the submicron scale are made ofinorganics, such as si l icon and sil icon dioxide.Therefore, an efficient method of binding differentorganic molecules to these substrates must be examined.

The method here involved a photolabile protecting

group that , when removed, leaves a substrate for avariety of other molecules. The protecting group chosenwas t-butyl (2-bromo-2-nitrophenyl) acetate, which canbe removed with ultraviolet light. The protecting groupwas first tested on a soluble polymer to show that itcould be added and removed in solution before reacting itwith the solid substrate. The protecting group was thenbound to silicon wafers that had been coated with an inertpolymer.

This project explored photo-deprotection methods forthe solid substrate and examined surface analysistechniques to monitor each step of the process. By usingphotolithography to remove the protecting group in adesired pattern, several steps could result in an array ofdifferent molecules patterned on the substrate where theprotecting groups were removed.

Coating the Wafers:In order to examine the affect of different substrates on

wafer chemistry, twelve, three inch wafers were coatedwith four different thin layered films; these were Si, Ta,Au, and SiO

2. The wafers were then coated with an inert

polymer, poly-heptafluorobutyl methacrylate, under a lowenergy plasma courtesy of the Cordis Corporation.Cordis then hydrolyzed under an H

2O/O

2 plasma to create

exposed carboxylic acids on the wafer surface, whichcould except other organic molecules. Due to the timeconstraints of the project and the fact that Si and SiO

2 are

popular materials for submicron tools, Si and SiO2 were

the only substrates examined further.

“Binding Organic Molecules to Inorganic Substrates”REU Intern:

Leeann Chandler James Madison University

Principal Investigators:Prof. Noel MacDonald, John Chong Electrical Engineering, CU

Contact: [email protected]

Associates: Bruce Ganem, Amber Rouch Organic Chemistry, CU

Acknowledgements: CNF, NSF, Cordis Corp, Ganem Group, COC, CUEE, MacDonald Group

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Making the Protecting Group:The t-butyl (2-bromo-2-nitrophenyl) acetate protecting

group was made from 2-nitrophenylacetic acid with a twostep synthesis reaction.

The first step used 5.2 g of 2-nitrophenylacetic acid,which was reacted with 116 ml of t-butyl acetate andapproximately 0.5 ml HC

lO

4 in a 500 ml RB flask,

protected with a drying tube [1]. This resulted in aslightly exothermic reaction and produced a small reflux.After 24 hours, 200 ml ether was added with ice coldsodium bicarbonate to produce vigorous bubbling. Agentle sodium bicarbonate extraction (3 x 200 ml) wasperformed to remove all acids. The ether layer waswashed once with base dried (MgSO

4) and rotovapped to

afford product. TLC (5:1 Hexanes:Ethyl Acetate) with aKMnO

4 dip showed only one spot and a proton NMR

showed the expected peaks for t-butyl (2-nitrophenyl)acetate. The product was a yellow oil (5.732 g, 84%).

The second reaction involved 1.18 g of the aboveproduct, 1.06 g n-bromosuccinimide (NBS), and 54 mlCCl

4 (alumina fi l tered) in a tared 100 ml RB flask

equipped with a one-inch stirbar. A reflux condensor wasadded and the system was evacuated and fi l led withArgon six t imes. The reaction ran 15 hours underconstant Ar and a sunlamp. This l ight init iatedbromination was faster than the published method, whichinvolved a peroxide init iat ion. The reaction wasmonitored with TLC (5:1 Hex:EA, UV light) and visually(NBS sinks and succinimide floats). The reaction liquidturned yellow with dark clumps of floating solid. Thesolid was filtered off with Watman paper and the liquidwas extracted (4X) with dH

2O in a 250 ml separatory

funnel. The water layers were back extracted with CCl4,

and the organic layers were combined and dried withMgSO

4. The solvent was rotovapped away to yield a

yellow oil (1.2509 g crude yield, 80%).The crude product produced many spots with TLC (5:1

Hex:EA, UV light), so a 60 mm flash column with 5.25inches of wet packed silica in 10:1 Hex:EA was run. 25ml fractions were collected as the column ran in 10:1Hex:EA, and they were examined with TLC (5:1 Hex:EA,UV light). There was a relatively clean separation ofspots, so the desired fractions were combined,rotovapped, and analyzed. NMR and IR spectra of the

yellow product (0.837 g, 53.3%) indicated that the t-butyl(2-bromo-2-nitrophenyl) acetate had been purified.

Using the Protecting Group in Solution:In order to examine the behavior of the protecting

group before attempting to react it with the solid wafersurface, it was first reacted with a soluble substrate,decanoic acid. The first step involved the synthesis of t-butyl (2-decanoic-2-nitrophenyl) acetate. This reactionrequired 0.209 g t-butyl (2-bromo-2-nitrophenyl) acetate,0.112 g decanoic acid, 0.128 g 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU), and 10 ml distilled benzene in a 25ml RB flask [3]. A reflux condenser was added, an Arline attached, and the apparatus was placed in an oil bathfor reflux. When the reaction warmed, the solutionturned from deep purple to reddish-brown and a brownsolid appeared. The reaction was allowed to runovernight (20 hrs), but the TLC (5:1 Hex:EA, UV light)showed little change after four hours.

When the reaction was stopped, it was first allowed tocool; then, the condenser walls were rinsed with freshbenzene and the reaction mix was rinsed into a 125 mlseparatory funnel. 20 ml ethanoic acid and 20 ml dH

2O

were used to extract the brown/gold organic layer fromthe yellow aqueous layer. The organic layer wasextracted twice and the aqueous layer was back extractedwith ethanoic acid. The organic layer was dried withNaSO

4 and the solvent was rotovapped away. The crude

product was a brown oil (0.1838 g, 69.3%).The crude product was run on a 22 mm flash column

with 6 inches of wet packed silica in 10:1 Hex:EA. 20ml fractions were collected as the column ran in 10:1Hex:EA, and they were examined with TLC (5:1 Hex:EA,UV light). The desired fractions were combined androtovapped to produce a yellow oil (0.138 g, 52%)determined with NMR to be t-butyl (2-decanoic-2-nitrophenyl) acetate.

In order to examine how t-butyl (2-decanoic-2-nitrophenyl) acetate could be deprotected with UV light,i t was photoreacted in a Rayonet reactor. The firstattempt involved bulbs emitting 350 nm light, but i tappeared that only starting material was left after 24hours in the reactor [2]. The second reaction was runwith 46.8 mg of t-butyl (2-decanoic-2-nitrophenyl)acetate, 7 ml dist i l led benzene, and l ight with awavelength of 254 nm. The solution was in a quartz testtube suspended in the center of the reactor. The reactionwas only allowed to run for 3 hours because there did notappear to be changes in the TLC (5:1 Hex:EA, UV lightand KMnO

4) results.

When the reaction was stopped, the tube and i tscontents were rinsed into a 60 ml separatory funnel andextracted three times with saturated sodium bicarbonate.The aqueous layer was back extracted with benzene andthe combined organic layers were dried with sodiumsulfate in a glass frit. The concentrated product was runon a 12 mm flash column wet packed with six inches ofsilica in 10:1 Hex:EA, and 5 ml fractions were collected.

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The combined aqueous layers were made acidic usingconcentrated HCl (to pH 1) and extracted using 20 mlether in a 60 ml separatory funnel. The ether layer fromthis extraction and the desired column fractions fromabove were both rotovapped to produce yellow oilyproducts. They were both examined by a proton NMRand it appeared that the organic layer yielded startingmaterial, but the acidified aqueous layer yielded a smallamount of decanoic acid (3 mg).

Reacting the Protecting Group with the Wafer:Silicon and silicon dioxide wafer chips that had been

coated with the inert polymer and hydrolyzed to exposesurface carboxylic acids were reacted with the nitrophenylprotecting group in a similar manner as the decanoic acidreaction. Chips of wafer (80mm x 80mm) from other stepsin the coating process (plain and those before hydrolysis)were reacted with the experimental chips as a control forbonding not related to the exposed acids.

The chips (numbers scribed in the back) were placed ina 25 ml RB flask with a tiny stir bar, 51 mg t-butyl (2-bromo-2-nitrophenyl) acetate, 30 ml DBU, and 10 mldistilled benzene. A condenser was placed on top of theflask and placed under Ar, and the aparatus was placed inan oil bath to warm the solution to reflux. The mixturewas stirred to maintain a homogenous temperature andchemical mix, but the wafers moved around little. As thesolution warmed, the dark blue starting color turned toreddish orange and a dark brown solid formed on thewalls of the flask. The reaction was allowed to runovernight, and the chips were then rinsed with benzeneinto a steel strainer. The strainer was dipped in sodiumbicarbonate, then water, and then ethyl acetate/water.The chips began to st ick together, so they wereindividually rinsed in EA and placed face-up in a vacuumchamber for complete drying.

Surface Analysis:It is crucial to know what molecular changes had been

made to the wafer surface, so two wafer surface analysismethods were tested to see which was the best method ofexamining the wafers between each step of theprocessing. The two methods examined were ScanningAuger microscopy, which bombards the wafer surfacewith electrons, and x-ray photoelectron spectroscopy(XPS), which shoots photons at the wafer surface.

Figure 3 is a diagram of what happens to the surfaceelectrons in both methods and how the energy of thoseelectrons can be used to determine the element fromwhich they came.

Auger analysis measures the energy of an electronejected from the L

1 orbital of surface atoms. This

electron is released due to a series of events initiated byfree electrons beamed at the surface. The incomingelectron displaces an electron in the K orbital, whichcreates an energy gap. This causes an electron from theL

1 orbital to move into its place. Since there is less

binding energy needed for the K orbital, the extra enegryis transferred to a neighboring L

1 orbital electron, which

ejects that electron out of the surface. The energy of theejected electron is equal to E

K-2E

L. An Auger spectrum

has peaks for each element whose electrons are releasedfrom the substrate surface. This method provides highspatial resolution, but is quite damaging to the wafersurface.

XPS, which is also called Electron Spectroscopy forChemical Analysis (ESCA), also measures the energy ofelectrons ejected from the substrate surface, but thoseelectrons get their energy from photons. In this method,the surface is bombarded with photons, and the lightenergy is transferred to electrons in the K orbital causingthem to be ejected from the surface (E=hv-E

K). This

method provides a better chemical description of thewafer surface and gives details about the binding energybetween the atoms. ESCA is also much less damaging tothe substrate surface than Auger. An ESCA spectrumgives peaks for each atom on the surface by plotting theelectrons released for different possible eV levels, andthe distance between peaks gives an indication of thebinding energy for each element.

Discussion:In order to examine how organic materials can be

bound to inorganic substrates for chemical and biologicalanalysis on a molecular level, Si and SiO

2 wafers were

put through a series of reaction and analysis steps. First,the wafers were coated with an inert polymer, and thatpolymer was hydrolyzed to expose carboxylic acids(Cordis). Next, a photolabile protecting group had to befound that could be bound and removed from the wafersurface in a controllable manner. The molecule of choicewas t-butyl (2-bromo-2-nitrophenyl) acetate, which wasmade through a two-step synthesis. The protecting groupwas then tested by reacting it with soluble decanoic acidand removing i t with UV light. Since working withmolecules in solution is less difficult than working withthose attached to the wafer surface, binding theprotecting group to decanoic acid provided the chance toexamine some of the reaction conditions needed forreacting and removing the protecting group. Thephotodeprotection produced only a small amount ofdecanoic acid, but perhaps a longer reaction time wouldprovide better separation. Because some decanoic acidwas retrieved, it appears that 254 nm light is sufficient toremove the protecting group. Then, chips from the firststep described above were reacted with the nitrophenyl

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protecting group to fill all of the exposed carboxylicacids with a photolabile organic group.

The final part of the project was perhaps the mostcrucial, because litt le good can be found from waferchemistry when there is no way to tell what moleculeshave been put on the wafer surface. Silicon wafer chipsfrom several steps of the process were put through Augerand ESCA surface analysis to determine which methodgave the best results. Since ESCA was less destructiveand gave better chemical distinction, it appeared to be thebetter of the two methods. Figure 4, below, shows theresults of the combined ESCA spectra for three waferchips. The three samples represent three different stagesin the wafer processing. One spectrum was from theplain silicon wafer that was not reacted with any othermolecules. The second spectrum is from a wafer that wastreated with the inert polymer, and i t shows a largefluorine peak because i t is coated with poly-heptafluorobutyl methacrylate. The other sample was achip covered with hydrolyzed polymer that was reactedwith the nitrophenyl protecting group. I t shows thelargest peak for carbon and nitrogen because there aremore of those elements in the protecting group than inthe polymer. It is also important to note that there aredistinct differences in the three spectra, which is anindication that the reactions on the wafer surface workedbecause the chips have a different chemical surfacecoating.

Though more analysis and testing of the processesdescribed above could be done, it appears that the initialgoals of this project were accomplished. A protectinggroup was found that can be bound and removed with UVlight. That group was also bound to an inorganicsubstrate, and ESCA surface analysis proved to be anacceptable method of determining the reaction results.

References:[1] Flitsch, W., and P. Russkamp. Leibigs Ann Chem.

1985, 1398-1412.[2] Gravel, D., et al. Can J Chem. 1983, 61, 400-410.[3] Wieboldt, et al. Proc Natl Acad Sci USA. 1993, 92,

8753-8756.

Figure 4

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“Diffractive Cell Detection Using A MicrocontactPrinted Antibody Grating”

REU Intern:Yelena Chigirinskaya Cornell University

Principal Investigators:Prof. Carl Batt Food Science, CU

Prof. Harold Craighead A & E Physics, CU


Abstract:An optical immunoassay-based detector has been

fabricated by stamping an antibody grating pattern onto aclean silicon substrate using a 40 µm period elastomerstamp, produced by photoli thographic process. Theresult ing si l icon surface has 10µm wide ridges ofantibody and 30µm spaces between them. The antibodiesretain their ability to bind antigen specifically whilebound to the silicon surface. Thus, E. coli 0157:H7 cellswere captured on the appropriate antibody-stamped lines,and the intensity of the first order diffraction beam beingthe result of cell attachment was measured. From theprevious study it was determined that the diffractionintensity increases as the number of bacteria cellsincrease per unit of area.

Figure 1 Escherichia coli 0157:H7 cells bound to the antibodygrating pattern on clean silicon surface. Size of the pattern is

10µm/30µm

Introduction:Figure 2, below, shows how the first order diffraction

intensity was measured. The process of detection isbased on the fact that diffraction signal is significantwhen target cells bind to antibody grating, but isnegligible when only antibody grating is “shot” with thelaser beam. This optical cell detector significantlyreduces complexity of an immunoassay. In particular, itis a fast process that does not require any surface coatingfor the silicon substrate.

Our goal in this investigation was to increase theefficiency of the detector by decreasing the overall sizeof the grating pattern to 20µm period–5µm ridges and15µm spaces. We expected the diffraction intensity toincrease when the grating pattern was reduce. Smallgrating results in a larger angle between different orderdiffraction beams, which could facil i tate measuringintensity.

The graphs in figure 3 (next page) show the differencein first order diffraction intensity measured with bacteriacells at tached to the antibody grating and measuredwithout bacteria cells, but with antibody grating only. Itcan be seen from the graph that on average smallergrating produced lower first order diffraction intensity.Smaller intensity of the signal for smaller size gratingimplies that the smaller size grating lowers the efficiencyof this detector. However, the graph also shows anextremely wide range of data, which make analysis of thedata difficult and somewhat unreliable. Small number ofdata points as well as their wide range make this datahighly inconclusive.

Figure 2

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Methodology:In order to collect more data as well as make data

more consistent, we worked on making the proceduremore reproducible. First of all, we eliminated somepossible inconsistencies in the process. Condensationtest was performed on a si l icon chip stamped withantibodies to check whether antibodies in fact do “stick”to the si l icon surface. Figure 4 shows a picture ofcondensation droplets on an antibody stamped siliconsurface shows linearity. Thus, the condensation testimplies that the problem does not l ie in antibodiesbinding to the silicon surface.

Figure 4

A slide agglutination test was performed to confirm theidentity of the bacteria culture used. The test gave apositive result that the bacteria cells being used wereindeed E.coli 0157:H7. Other possible sources ofinconsistencies could be the stamping ink used, theprocess of applying the antibody solution onto theelastomer stamp, or perhaps the process of transferringantibodies onto a silicon chip.

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0 1 2 3

20µm period 40µm period

Figure 3

We were using PBS of pH 7.4 as the stamping ink. Wetried to change the buffer solution and test the followingbuffers: carbonate pH 9.6, borate pH 8.6, and glycine pH3.0. Although carbonate buffer did appear to make theprocess more consistent, i t st i l l did not make theprocedure highly reproducible. This fact implied thatthere are more than one factor that have to becoordinated for the process always to work. We havealso tr ied making antibody stamping more uniformthroughout the chip by adding Tween to the dilution ofantibodies. However, no effect on uniformity of thestamped antibody grating on the chip was observed.

Plasma treatment will make the highly hydrophobicelastomer stamps more hydrophilic. Thus, putt ingelastomer stamps into plasma before wetting them withthe antibody solution could possibly make the stampingmore uniform and the procedure more reproducible.

Conclusion:In this investigation we have used the smaller size

antibody grating pattern (20µm period) in an attempt tooptimize the signal of the detector. We have comparedthe first order diffraction beam intensity in the twograting sizes. Although the larger size grating patternappears to be giving higher diffraction intensity, theprocedure has to become more reproducible and the datahas to become more consistent before any definiteconclusion about the effect of the smaller size gratingpattern on the signal intensity can be made.

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“The Morphological Evolution of Silicon Films Deposited by Angled

Supersonic Molecular Beams of Silicon Containing Hydrides”

REU Intern:Daniel Culkin Fordham University

Principal Investigator:Prof. James Engstrom Chemical Engineering, CU


The focus of my research was supporting the researchefforts of the Engstrom research group in their studies ofsemiconductor thin fi lm deposit ion using supersonicmolecular beams.

This was done in part through ex-situ characterizationof material deposited employing beams of tunabletranslational energy (0.2-3.0 eV). One inherit advantageto this approach is the capability to define a beam angleof incidence with respect to the substrate. To exploit theuniqueness of our approach, and in hopes of providing apath towards self assembly, a study was init iatedconcerning the deposition of epitaxial Si on Si(100) atvarious reactor conditions, featuring varied angle ofincidence. Both beam and substrate conditions wereexamined and included the following: substratetemperature, beam kinetic energy, beam incident angle,and thin film thickness.

It has been shown in simulation by a number ofresearchers that deposition at oblique angles of incidence(> 70°) leads to rough films of a columnar nature viashadow instabili ty. This effect has also been shownexperimentally with metals in which the obliqueincidence condition leads to anisotropic magnetic andelectrical properties. In our study, we examined filmmorphology and gained topographical information of theroughness and fi lm periodicity using atomic forcemicroscopy (AFM).

Of equal importance is the film cross-section, whichwas probed using scanning electron microscopy (SEM).Results indicate that films deposited at a near normalangle of incidence were relatively smooth and exhibitedno distinct columnar structure, whereas films deposited atoblique angles resulted in columns that were inclinedtowards the incident molecular beam flux. Columninclination angle did not follow the wellknown tangentrule in this study, however, the onset of columninclination did agree with the onset of film roughness asmeasured by AFM.

Other standard techniques were completed to furthercharacterize these films and included sheet resistance andfilm thickness measurements. My research duties alsoincluded wafer patterning and optical f i lmcharacterization for a related fi lm deposit ion study.Miscellaneous projects included electronic circuit designand fabrication to support a recent lab upgrade.

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“Manufacture & Measurement of Sub-Micron Pillarsof (La,Sr)MnO

3”

Abstract:Recently the observation of large magnetoresistance

effects at room temperature in epitaxial manganite thinfilms has renewed interest in the doped manganiteperovskite materials RE

1-xAE

xMnO

3 [for trivalent rare-

earth ions (RE) and divalent alkaline-earth ions (AE)] forpotential magnetic random access memories and read-heads for disk drives. These so-called “colossal”magnetoresistance materials have a large decrease inresistance at the Curie temperature. Thin films of thematerial exhibit planar preferred magnetic spinorientation due to geometrical or demagnetization effects.However the direction of preferred orientation shouldchange as the aspect ratio of the materials is changed. Inorder to study this effect , thin fi lms of themagnetoresist ive material were scored 0.3µm pillarsusing electron-beam lithography. The preferred axis isdependent on crystalline properties, shape, and strain. Atall pillar shape takes the preferred axis out of the film’splane. Subsequent measurements of magnetization showpromise for revealing the effects of strain on thematerial. The methods of manufacture and measurementof sub-micron pillars detailed here are a novel processuseful for studying the highly promising magneto-resistive properties of thin films such as (La,Sr)MnO

3.

Background:These materials have received an enormous amount of

scrutiny for two reasons: (i) to understand the origin ofthe anomalously large magnetoresistance effect and (ii)to determine i ts potential as the next generationmagnetoresistive read head material. Incorporation ofCMR materials into magnetic storage applications hasbeen encouraged by the recent success of thedevelopment of giant magnetoresistive (GMR) materialsof metallic multilayers in magnetoresistive read heads. Inthe past few years, CMR thin film growth techniqueshave opened the way for the investigation of structuraldistortions using thin film epitaxy. [see for example vonHelmolt 1993, Jin 1994, Chung 1995, Horwitz 1996,Modak 1996]

Several groups have carried out studies on the filmthickness and substrate dependence to determine theclamping role of the substrate in thin film samples. [Jin1995, Kwon 1996] We have recently reported that themagnetic anisotropy in doped manganite f i lms are

dominated by strain effects between the film and thesubstrate [Suzuki 1997]. We have carried out systematicstudies of the dependence of the magnetic anisotropy onfilm thickness and substrate. The extreme sensitivity ofthe magnetic anisotropy to “external” strain, in the caseof fi lms imposed by the latt ice mismatch with thesubstrate, is indicative of the important role of structuraldistortions in explaining the anomalously large magneto-resistance found in CMR thin films.

In this study we explore a new method for studying themagnetic properties of structurally sensitive thin filmmaterials such as the CMR materials in the form ofmagnetic nanostructures.

Experimental Procedure And Discussion:The first step was to determine the feasibility of the

fabrication of nanoscale magnets of these dopedmanganite thin fi lms on SrTiO

3 substrates. Over the

course of the summer, a procedure for fabricatingsubmicron pil lars was developed. This procedure isincluded in the appendix.

Before the samples were patterned, they werecharacterized structurally and magnetically. X-raydiffraction was used to determine the crystalline qualityof the samples. The film peaks exhibited full width halfmaximum of Dw=0.11°, thus suggesting the excellentcrystallinity of these materials. Magnetization measure-ments of these films were also performed with the fieldperpendicular to the plane of the fi lm. In this f ieldconfiguration, i t is energetically unfavorable for themoments to point normal to the plane of the fi lm.Therefore, as in figure 1, it takes fields of up to 5000 Oeto pull these moments out of the plane of the film.

REU Intern:Franklin Grigsby Harvey Mudd College

Principal Investigator:Prof. Yuri Suzuki Material Science, CU


Figure 1

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However if we pattern the thin film into tall pillarswith the longest dimension parallel to the directionnormal to the substrate surface, the moments will want topoint parallel to this normal direction. One can imaginethat we can minimize these shape effects by fabricatingstructures with an aspect ratio close to one. Then onecould possibly study the effects of the crystal structureand strain on this so-called magnetic anisotropy. Verybriefly, magnetic anisotropy is the tendency for spins topoint in certain directions due to energy minimizationarguments.

We then structurally characterized these array sampleusing atomic force microscopy and found periodicstructures of the doped manganite materials.

Figure 2

Magnetic characterization in the same field con-figuration as above revealed that the moments wereeasier to pull out of the plane ( i.e., required a smallerfield to saturate). Figure 3 shows how approximately1900 Oe is required to pull the moments out of the arrayof magnets as opposed to about 4300 Oe required to pullthe moments out of the plane of the thin film.

Figure 3

Summary:We successfully developed a procedure to fabricate

arrays of magnetoresistive materials. The methods ofmanufacture and measurement of sub-micron pil larsdetailed here are a novel process useful for studying themagnetic anisotropy of highly promising magneto-resistive thin films such as (La,Sr)MnO

3.

References:B. Chung, E.L. Brosha, F.H. Garzon, I.D. Raistrick, R.J.

Houlton and M.E. Hawley, J. Mater. Res. 10 2518(1995).

J.S. Horwitz, P.C. Dorsey, N.C. Koon, M. Rubinstein,J.M. Byers, D.J. Gillespie, M.S. Osofsky, V.G.Harris, K.S. Grabowski, D.L. Kreis, E.P. Donovan,R.E. Treece and D.B. Chrisey, Mat, Res. Soc. Symp.Proc. 401 525 (1996).

S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R.Ramesh and L.H. Chen Science 264 413 (1994).

S. Jin, T.H. Tiefel, M. McCormack, H.M. O’Bryan, L.H.Chen, R. Ramesh, D. Schurig, Appl. Phys. Lett. 67557 (1995).

C. Kwon, K.C. Kim, M.C. Robson, S.E. Lofland, S.M.Bhagat, T. Venkatesan, R. Ramesh, R.D. Gomez, toappear in J. Magn. Mag. Mater.

A. Modak and M. Krishnan, Mat. Res. Soc. Symp. Proc.401 443 (1996).

Y.Suzuki, H.Y. Hwang, S-W. Cheong, R.B. van Dover,Appl. Phys. Lett. 71 140 (1997).

Fabrication Procedure For Submicron Pillars:

1. Spin undiluted SAL-601 negative photoresist onto thesample:Spin Speed: 5000rpmSpin Time: 60 secThickness: 600nm

2. Bake resist in the oven:Temperature:90°CTime: 30 min.

3. Expose the resist on the Leica/Cambridge EBMF 10.5:Dose Matrix Job: FG9706DM.JOBReal Job: FG9706.JOBField Size: 1.6384nmPixel Size: 0.05µmBeam Current: 1.5nA/VRU 0/Aperture 2-400µm

4. Post bake on a hotplate:Temperature:115°CTime: 60 sec

5. Develop resist in MDC Developer:Time: 4 minRinse: in DI water and blow dry

6. Argon ion mill:Time: 6 minutesUse mung grease to hold sample in place.

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“Fabrication of a Mechanical Microchannel System

in Silicon”

REU Intern:Mary Heseltine University of Rochester

Principal Investigators:Prof. Noel MacDonald Electrical Engineering, CU

Mary Frame, Ph.D. Anesthesiology, Univ of Rochester


Abstract:A glass microchannel system has been developed at the

University of Rochester Medical Center, which mimicsarteriolar microcirculation conditions. However, there isa need to integrate pressure sensors along the walls ofthese microchannels for flow analysis. The channels,originally fabricated in glass, were constructed in siliconusing photoli thography and reactive ion etchingtechniques. A backside etch was then used to integrateoptical and fluorescent visibility within the channel. Theuse of silicon vs. glass will allow for greater versatilityin channel fabrication and in sensor integration.Endothelial cells, which normally line all blood vessels,were plated on the channels, using glass channels as thecontrol. It was found that the cells on the silicon plateshave the same cytoskeletal arrangement andmorphological properties as on the glass controls. Thinsil icon membranes were constructed by etching arectangular well along side the microchannel wall. Thecommon “wall” of the microchannel and the well formedthe prototype membrane. The next phase of this projectwill be to determine whether the membrane wall willdeflect as a change in pressure occurs within the enclosedchannel. The deflection will be monitored throughoptical, electromechanical, or capacitative devices. Thusboth the pressure and flow will be determined throughouta mechanical microchannel system, which is a scalereconstruction model of microcirculation.

Introduction:A well-defined and controllable microchannel system

has been designed to better understand the rheologicconditions of microcirculation. It has been used to studyflow and i ts effects on endothelial cell responses tomechanical forces associated with flow and pressure.Endothelial cells line all blood vessels and are exposed toboth the changes in pressure and wall shear stress. Onedrawback is that the microchannel system wasconstructed in glass, making it extremely difficult tointegrate any type of sensing device. Silicon, a moreversati le material than glass, was used to facil i tateintegrating membranes within the channel. The primarygoal of this project is to design and fabricate a flexiblemembrane that will withstand and detect the mechanicalforces of the model system. Existing sensors, such aspiezoresistors, capacitors, optical systems, and tunnelingwould then be integrated into the system to measure the

deflection of the membrane walls during flow experi-mentation. This system will test biomedical applicationsfor responses to the mechanical forces of transmuralpressure, and the effect transmural pressure has on, forexample, white blood cell migration out of thevasculature.

Materials and Methods:

Fabrication of the Microchannel:Two types of microchannels, semi-rectangular and

semi-circular, were etched in si l icon using photo-lithography and reactive ion etching techniques as shownin Figure 1. For this process, a 1.5 µm layer ofphotoresist (Shipley 1813) was deposited on si l iconwafers. A microchannel pattern, consisting of a T-shaped bifurcation was exposed onto the photoresistusing a 5x g-line Stepper. Two types of reactive ionetching were used to etch the channels (PlasmaThermSRL-770 Reactive Ion Etcher). An anisotropic verticaletch was used to construct semi-rectangular channels.Semi-circular channels were formed in a two stepprocess, in which the anisotropic vertical etch wasfollowed by an isotropic etch. The photoresist was thenstripped using the Branson/IPC P2000 Barrel Etcher.The feed and two branches were 5 mm in length and 20µm in depth as illustrated below.

1 - 1.5 µm

Silicon20 µm

Photoresist

Figure 1:Fabrication of Microchannel in Silicon

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Silicon vs. Glass for Biomedical Application:Endothelial cells were grown on both silicon and glass

control plates. Eighteen hours after plating, the cells onthe silicon plates were found to have normal cytoskeletalarrangement and morphological properties. However, thecells on the silicon plates were not nearly as confluent ason the glass controls. In order to improve cell adhesion,a layer of oxide was deposited on the microchannelsusing a Plasma Enhanced Chemical Vapor Deposition(PEVCD). The cells on oxide coated si l icon wereindistinguishable from those on glass controls inmorphology and cytoskeletal arrangement.

Fabrication of Backside View Ports:Most biomedical experimentation is observed using

optical microscopy, ranging from 400 to 800nm.Therefore, the microchannel system in silicon had to beadapted to fit this. Photolithography and reactive ionetching techniques were used to fabricate backsideviewing ports as i l lustrated in Figure 2, below.

Following the fabrication of the channel, a layer of oxidewas deposited on the front side of the wafer using aPlasma Enhanced Chemical Vapor Deposition (PEVCD),to protect the channel during backside etching. A thicklayer of photoresist, AZP 4903 was deposited on thebackside of the wafer. Using a reactive ion etch, thebackside of the wafer was completely etched up to thelayer of oxide. The photoresist was then stripped fromthe backside of the wafer using the Branson/IPC P2000Barrel Etcher. These ports were placed along the axiallength of the channel, and at the T-shaped bifurcation.The outline of the channel was optically visible usingtrans-illumination as illustrated in Figure 3, in the nextcolumn. Fluorescent microspheres, as fluid flow markerswere visible using standard fluorescent microscopy.During experi-mentation, both the channels and the viewports were able to withstand the applied forces of fluid,the assembly of the model system, sterilization and othermechanical factors. Therefore, this made it possible tooptically view the microchannels in a working system.

Oxi de 3 µm Chann el

Photore sis t

View Port477 µm Deep

Fabrication of Membranes:The same photoli thography techniques as in the

fabrication of a microchannel were used to constructmembranes within the channel walls as shown in Figure4, below. A straight channel with thin wall membranes

Figure 2:Fabrication of Backside View Ports

Silicon Dioxide Channel

Figure 3:Channel Trans-Illuminated from Backside

Channel WellSilicon Dioxide

Membrane

Figure 4:Fabrication of Microchannel Membrane

was patterned into the photoresist and were formed usingan anisotropic etch (PlasmaTherm SRL-770 Reactive IonEtcher). The layer of resist was stripped and a MOS-quality cleaning was applied to the wafer. A wet oxideprocedure was then used to grow a 3100 nm layer ofsilicon dioxide on the wafer. The layer of silicon dioxidethat formed during the process was stripped using a HFdip, leaving membranes as small as 200 nm intact. Thethin membranes consisted of rectangular wells, 20 µm indepth, 10 - 100 µm in length and 0.5 - 1.5 µm wide asillustrated in Figures 5 and 6, next page.

Discussion:Our goal is to better understand how the physical

forces, such as pressure and flow within arteriolarmicrocirculation, affect endothelial cell function. Amicrochannel system that is well defined and controllablehas been developed to study flow and its effects on cellresponses to quantitatively defined fluid velocity profiles

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(i.e., shear rate). This project has determined that siliconmicrochannels can be used as a model system withoutcompromising cell function. Endothelial cells grown onthe channel walls of either oxide coated silicon plates orglass plates have the same cytoskeletal andmorphological properties and grow and divide incomparable rates to those on glass channels. The primaryfinding of this project was that a silicon microchannelsystem, consist ing of a front and backside etch canwithstand the mechanical forces applied during use.

The next phase, currently in progress, will be tocritically evaluate membranes of various compositions,such as gortex, polyimide, acrylic enamel, andpolysilicon. We will select a material that will deflectappropriately as fluid is withdrawn from themicrochannel using a perfusion system. The deflectionwill be monitored and measured through the use of one ofthe following four existing devices: piezoresistors,capacitor, optical measurement, or tunneling. By placing

Figure 5:Microchannel with Membrane Walls

Figure 6:Cross-Sectional View of Membrane Walls

a pressure sensor in the system, both pressure and flowwill be closely monitored, and will be related to cellresponses. Further, with these modifications, this systemcould be used in other biomedical applications, such asexamining the physical forces associated with cancer cellmetastasis, red blood cell egression from bone marrow, orleukocyte (white blood cell) responses in the circulation.

Acknowledgements:

I would sincerely like to thank the following:

Cornell Nanofabrication Facility (The REU Program);

The National Science Foundation;

The Whitaker Foundation;

Professor Noel MacDonald, Electrical Engineering, Cornell;

Mary D. S. Frame, Ph.D., University of Rochester;

Hercules P. Neves, Ph.D., Electrical Engineering, Cornell;

John Chong, Electrical Engineering, Cornell University.

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Introduction:Organic light emitting diodes are devices that produce

light by the electrical stimulation of organic materials.The active chemicals that make up these devices includethe dye chemical which produces the light, and otherchemicals which aid in the conduction of electricitythrough the dye between the anode and cathodes of thedevices. In the bilayer devices that have been created,the bottom layer, which is deposited in contact with theanodes of the devices, contains chemicals which aid inhole transport. The chemicals that were used in thiscapacity were poly(9-vinylcarbozole), (PVK) and Bis(3-methylphenyl) diphenylbenzidine, (TPD). The secondlayer of the devices contains the dye chemical as well asthe electron transporting chemical. All devices weremade using the dye Coumarin-6 (C-6) which producesgreen light with a wavelength of 500 nm. Two differentchemicals were tested as the electron transport chemical.These were 2-(4Biphenyl)-5-(4-tertbutyl-phenyl)-1,3,4-oxadiazole (PBD) and (Alq3). The cathodes for thedevices are placed above this second layer.

Researching Device Components:The process of creating bilayer OLED’s involved the

optimization of several variables. In single layerdevices, all of the active ingredients are dissolved in asingle solvent and deposited onto the substrate by spincoating. In order to get two layer devices, i t wasnecessary to dissolve the active ingredients for each layerinto different solvents which were mutuallyincompatible. Many solvents were tested for mutualincompatibility, including Dimethyl Sulfoxide, Hexanes,Methanol and THF. With the results of these tests, it wasdecided that THF would be used as the solvent for thehole layer chemicals and Methanol would be used as thesolvent for the dye layer chemicals.

The principle hole layer chemical is the PVK, but it isthought that the addition of TPD to this layer increasesthe efficiency of the hole transport . All hole layersolutions were made with 200 mg of PVK and varyingamounts of TPD. Additionally, each hole layer solutionwas made in both 5 and 1 Oml volumes of THF. A filmwas made with each solution at several levels of filteringas well. It was found from these tests that, in general,the 10ml solutions were best and that amounts of TPD inexcess of 1 Omg ruined the quality of the film. Filteringthe hole layer solutions also seemed to be detrimental.

“Fabrication of Bilayer Organic Light EmittingDiodes”

All of the dye layer solutions were made with lmg ofC-6 and were all made in 10ml solutions of Methanol, butboth PBD and the Alq3 were tested in the same way. Theoptimum amount of PBD seemed to be 100mg. The Alq3solutions did not leave fi lms, but blotches. Theseblotches were proportional in size to the amount of Alq3powder that was put into the solution. After filtering justonce, the disposition was almost nonexistent. This ledthe investigator to believe that the Alq3 did not disolve inthe solvent but merely formed a suspension. In devicesusing Alq3, no filtering was done on the dye layer, sothat some material would be deposited, even if i t isirregular in character.

Device Substrates:The substrates were all si l icon wafers. The only

preparation required was the application of Acetone andEthanol, by spin coating. The Acetone cleans the siliconby removing oils and the ethanol improves the adhesionof the chemical layers to the silicon. For the actualdevices, glass substrates were used. This glass wascoated with a layer of SnO, which conducts electricityand has a sheet resistance of 10 n/o. The SnO layer hasto be patterned using CNF’s photolithography equipment.Several chemicals are applied to the substrate by spincoating. This is done at 3000 rpm for 30 seconds. Theglass is cleaned again, with acetone and isopropylalcohol, and then a layer of a chemical called HMDS isapplied to the substrate. This improves the adhesion ofthe photoresist to the substrate. The photoresist used isAZ1813. After the photoresist is applied, the substratesare baked on a 90 degree hot plate for 2 minutes. Thisdries the photoresist and eliminates stickiness. Next thephotoresist is patterned using a contact aligner whichemits mercury light, and a mask which keeps the lightfrom reacting with the photoresist in some places andallowing it to react in others. The mask used for thisprocess was made by Dr. Yoon and produces a total often devices on every 1 in 2 substrate with a central stripof SnO which acts as the anode for all ten.

Once the mercury exposure is finished, the SnO isdeveloped in MF-3 12 developer. After this developingprocess, there is still photoresist on top of the SnO whichis to remain on the glass and the SnO which will beremoved from the glass is exposed. The next step is toremove the unwanted, exposed SnO from the substrate.

REU Intern:Laurance Hiller SUNY at Buffalo

Principal Investigators:Prof Dieter Ast, Seok-Beom Yoon Materials Science Engr, CU


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This is done by placing the substrates in a bath ofhydrochloric acid and zinc powder. The zinc is presentas a catalyst in the etching process which is very violent,spitting and bubbling and producing a lot of heat. Theetching process takes only 2 minutes.

Acetone is now applied to the substrates, removing theremaining photoresist and exposing the patterned SnOsubstrates. The last step in the preparation of thesubstrates is three 10 minute sonic baths in deionizedwater to remove all the loose particles. While thisprocess for substrate preparation can work very well, itwas found statistically, that more often than not, thepatterning had a large amounts of imperfection, includinggaps and irregulari t ies in the central SnO strip andperimeter rectangles. It is believed that these gaps andirregularities changed the current carrying abilities of theSnO and for this reason caused a great number of deviceson each substrate to be dim, short out quickly, have ahigher turn on voltage, or not turn on at all.

To test the belief that the contact aligning method ofSnO pattering was largely at fault for the failure ofdevices, a new method of patterning was employed.After the SnO glass was cut into 1 inch squares, thedesired pattern was painted onto the substrates by handusing a clear, commercial nail polish. Because of theadmitted awkwardness of this process, the devices madein this manner have only eight devices per substrateinstead of ten. In this process, the nail polish is takingon the role of the photoresist, covering up the SnO whichis to remain on the glass. After the nail polish dries, allof the other steps in developing the substrates are sameas for the ones made with the photolithographic method.

In a comparison test between the two methods ofsubstrate manufacture, twelve complete devices weremade. Six of these were made on photolithographicsubstrates and six were made on manually patternedsubstrates, al l with identical chemical layers.Statistically, the manually made devices were over 3times as likely to work as the photolithographic ones.

Building Devices:After the substrates are patterned, the process of

adding the chemical layers begins. Before this can bedone, Aluminum tape is placed over all of the SnOperimeter rectangles and the bottom most portion of thecentral SnO strip. The chemical layers are applied byspin coating at 2000 rpm for 30 seconds with anacceleration of 0.4 sec2. The spinner used for thispurpose belongs to Prof. Gianelis’s group. Thesubstrates went back into the oven for another hour andwere then allowed to cool, under vacuum, overnight.The last process in building the devices is the addition ofthe cathodes. A mask is required for depositing thecathodes. The substrates patterned by photolithographicmethods use a mask that Dr. Yoon made. This maskwould not work on the manually patterned substratesbecause they have two less devices per substrate andtherefore, a different geometry. A mask had to be madefor the deposition of cathodes on these devices.

Testing the Devices:The final tests were done on three types of devices.

The first of these has a hole transport layer consisting ofPVK and TPD and a dye/electron transport layerconsisting of C-6 and PBD. The second device has thesame dye/electron transport layer as the first type, butcontains no TPD in its hole transport layer. The thirdtype of device had the same hole transport layer as thesecond device but has C-6 and Alq3 in the dye/electrontransport layer.

The first test was to find an approximate value of theturn on voltage of the devices and get a qualitative feelfor their brightness. This was done by stimulating thedevices with a direct current power source in a darkenedenvironment. The type one device was the brightest,followed by type two and then by type three. (This isactually the origin of their hierarchical “type” numbers.)The turn on voltage of the diodes made with PBD as theelectron transport layer (types 1&2) averaged around 7Volts. The type three device, with Alq3 as the electrontransport material had a turn on voltage of 3 volts. Testswere done to determine the thickness of each of thelayers of the devices. This was done at the CornellNanofabrication Facility using an alpha-step machine.The data retrieved from the stepper tests appears below.

Tests for the Photoluminescence of the devices weretaken in Phillips Hall with Prof Eastman’s group. Thesurface of the film was excited by an ultra-violet laserand the emitted light was scanned by a photodetectorover the entire band of visible light. In the test of eachdevice, there was a peak at 500 nanometers, which is thegreen dye, C6. However, there is another peak around400 nanometers, which is blue light. It is believed thatthe PVK is producing this light. In the type two device

Figure 1

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(with PBD), the green light intensity is twice as high asthe blue light, and the emitted light in fact appears green.The type three device (with Alq3) shows a blue intensityalso, but has a significantly smaller green peak. Thisdevice appears more blue than green to the eye. It makessense that both devices have the same blue peak, becausetheir hole layers are identical in make-up. The sameamount of green dye is present in the dye layer of bothdevices, so one becomes confused as to the discrepancyin green peaks. While one device has PBD and the otherhas Alq3 as the electron transport chemical, they bothhave the same mass of electron transport chemical; 100mg. The difference lies in the density of the materials.One hundred milligrams of Alq3 takes up about twice asmuch volume as the same mass of PBD. Therefore, thedye chemical in the type three (ALq3) devices is lessconcentrated, producing weaker light intensity.

Figure 2

Figure 3

Another test that was done was the PIV test, whichstands for Photocurrent, Electrical current and Voltage.This was done with Prof Valentine’s group using theHP4155A parameter analyzer. The PIV curve for the typeone device is shown in figure 4.

Figure 4

The last test that was done was to find the activelifetime of the devices. This was done by stimulating thedevices with D.C. voltage in a darkened environment andtiming with a watch how long the devices stayed on afterinitially lighting. The type three devices (Alq3) tendedto last only 2-3 minutes and were very dim. Thesedevices would spark when they went out.

The type two devices (PBD, no TPD) lasted, onaverage 6 minutes, but dimmed a considerable amountover that time, but did not spark when they went out.The type one devices (PBD, TPD) were the longest livingdevices. On one occasion they lasted for 55 minutes andon another occasion, 81 minutes. They didn’t go out bythemselves (from sparking or burning) but were turnedoff by the experimenter after they got too dim.

Conclusion:The bilayer devices described here are the first of their

kind to be made with these ingredients and spin coatingtechniques. Based on the results I have, it seems that thebest devices are the type ones, containing 200 mg of PVKand 10 mg of TPD in the hole transport layer and 100 mgof PBD and lmg C-6 in the dye/electron transport layer.They have the best diode characteristics, most intenseillumination and the longest active lifetime. Electro-lumenescence tests weren’t done due to a lack of time.

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“An Attempt to Eliminate Atomic Steps fromGaAs(001) Surfaces”

REU Intern:Brendan Moran Mat Sci, The Johns Hopkins University

Principal Investigator:Prof. Jack Blakely Materials Science & Eng, CU


Abstract:As a continuation of previous work which attempted

to create atomic step-free regions on regularly patternedGaAs, this report discusses novel approaches taken tosuccessfully fabricate such arrays. “Mesa” patterns ofvarious sizes were fabricated on the substrate to create aclean, confined surface on which, by vapor phaseepitaxy, to deposit additional GaAs. This deposition wasintended to enable the atomic steps to flow across thesurface. Though we believed that the migrating stepswould terminate at mesa edges, and thus create anatomically flat surface, wide scale flattening did notoccur, due to surface contamination among other factors.

Introduction:As the feature sizes for fabricated devices in the

microelectronic industry decreases into the nanometerrange, the presence of atomic steps (thus surfaceroughness) occurring on the substrate surface will figuregreatly into device performance. In such applications asHEMT’s and FET’s, it has been shown that these atomicsteps hinder the mobili ty of charge carriers andcontribute to field breakdown respectively whenfabricated to such small dimensions [1]. Though it hasbeen demonstrated that high temperature annealing hasbeen able to reduce surface roughness, it does not ensurea completely atomically flat surface on which to beginbuilding devices [2]. Therefore, a process which canensure regular arrays of entirely step free regions wouldbe highly desirable to industry.

The scheme to achieve these step free regions stemsfrom a study and research in surface kinetics by theBlakely group. It was determined that when annealed,steps began to evaporate and migrate. Step free regionson Silicon as large as 10 microns were fabricated [1].Due to material issues, the method that was employedfor GaAs, though based on similar principles, issomewhat complementary to the previous method. Inshort , trenches in regular patterns were etched intoGaAs, thus providing diffusional sinks for stepmigration and termination during subsequent OMVPEgrowth. This method seems desirable due to i tsdeposition temperature, which is half of the meltingtemperature. This will el iminate some problems ofdopant diffusion associated with high temperatures.

Experimental Procedure:A semi-insulating half wafer (3") of GaAs with a 0.1

degree miscut was obtained from Dr. Richard Shealy’sgroup in the Department of Electrical Engineering atCornell University. It was scribed and cleaved by handinto a few approximately 15 mm by 20 mm sections, andwas ready for processing. A thin (100 nm) SiO

2 layer

was deposited by CVD initially to protect the samplesurfaces from any aggressive chemicals used inprocessing. A two dimensional grid pattern was thuswritten in positive e-beam resist, and was transferred tothe sample using a chemically assisted ion beam (CAIBE)system. This etching resulted in a trenches where thepattern had been writ ten, with a depth of about 3microns. To further alleviate problems that can occurduring solution chemistry, the e-beam resist was removedsolely in an oxygen plasma. The final step was toremove the SiO

2 layer, which was accomplished by a

buffered oxide etch (30:1). The samples were thenverified to be visibly free of contamination by bothoptical and scanning electron microscopy, and were thusready for OMVPE growth. A zoomed SEM imageexemplary of these mesas can be seen in figure 1.

Figure 1

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Prior to growth, the samples were degreased inalcohol and subjected to an acidic bath etch to removethe native oxide (Ga

2O

3) from the sample surface. GaAs

was deposited on the sample at a temperature of 650°C,and the sample surfaces were then characterized using anatomic force microscope, in tapping mode.

Results and Conclusions:Mesas of various sizes were examined with AFM to

observe step flow and possible flattening of the surface.Though from the obtained images step flow was obvious,regions of atomically flat material were typically lessthan a square micron. Easily observable on the aftergrowth GaAs samples are frequent irregular surfacemorphologies, possibly an affect of surfacecontamination. These sites acted as nucleation sites fornew growth and thus impeded directional surface stepflow. The origin of this contamination is currentlyunknown, yet it is not entirely untraceable. Figure 2represents a typical flat region where further step flowwas impeded.

Figure 2

Though wide scale flat tening did not occur aftergrowth, several insights relating to the processing,handling, and growth characteristics were gained. Theprocess for fabrication was in some cases refined andmodified, thus leading to visibly cleaner surfaces forgrowth. Further work, however, must be done concerningthe growth process to determine what conditions yieldsufficiently rapid step flow without unwanted islandnucleation, given a clean surface.

References:[1] S. Tanaka, C. C. Umbach, J. M. Blakely, R. M.

Tromp, and M. Mankos Appl. Phys. Lett., vol. 69, p.1235, 1996.

[2] A. V. Latyshev, A. L. Aseev, A. B. Krasilnikov, andS. L. Stepin, Surface Science, vol. 213, p 157, 1989.

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Abstract:The distribution property of the Photo-Acid Generators

(PAGs) in the photoresist film is studied closely becauseof i ts important effect on the sensit ivity of thephotoresist. It is essential to find a compatible PAG-polymer mixture to optimize a photoresist system. Someof the polymers we studied are known to have good etchresistance and transparency at 193 nm, while others areused as comparisons for better understanding of the PAGdistribution differences between the homo-polymer, andblock, random copolymers. Several polymers, such aspoly(IBMA-b-tBMA), are found to have evendistributions throughout the photoresist film with most ofthe PAGs studied, which is essential for a practicalphotoresist system. For some polymers, phase separationoccurs between the photoresist and the PAG, with thePAG usually segregating towards the bottom. Thisdecreases the photoresist’s sensitivity dramatically. Also,we set up a Supercritical Fluid Extraction (SFE) systemthat will allow us to use supercritical carbon dioxide as adeveloper solvent for certain photoresist. It can possiblyreplace the expensive and environmentally unsafe organicsolvents that’s currently used.

Introduction:Integrated circuits are becoming more and more

important in technological development. Most integratedcircuits are produced now with the I-l ine (365 nm)exposure technology. Some of the latest generationintegrated circuits are being printed with deep UV (248nm) exposure. The decrease in the wavelength allows abig increase in the maximum achievable resolution. Withthe increasing demand for even more compact integratedcircuits, a higher resolution is necessary. 193 nmlithography system can provide that high resolutionneeded to manufacture the next generation integratedcircuits. [1]

Many factors have to be considered to implement the193 nm lithography system. It is most important for thephotoresist system to have very high sensit ivity toexposure at this wavelength. The polymers also mustadhere to the si l icon wafers well in order for i t tofunction properly. Perhaps the most difficult feat is tofind a material that is transparent at the exposurewavelength and stable to reactive etching conditions.This was at one time thought to be mutually exclusive,

since materials which are etch-resistant were believed tohave high absorption around 193 nm. Materials forphotoresists must be selected carefully to balance bothtransparency at the exposure wavelength and etchresistance.

The photoresist polymers themselves are not light-sensitive. A photo-acid generator (PAG) that’s reactiveto 193 nm exposure is added to the system. In a positivephotoresist system, the PAG breaks down to produce a H+

ion at a certain dosage (mJ/cm2) of exposure to react withthe polymer and change the polymer from insoluble tosoluble in the developer. An example of PAG andphotoresist [Poly(tBMA) in this case] reactions are:

Eight different PAGs from Shipley Co. andPolyscience, Inc. are studied. Not all PAGs mix wellwith the photoresists. Sometimes they phase separateand the PAG segregates out. This decreases thesensit ivity of the photoresist significantly. Thedistribution of the PAG within the photoresist polymerhas a major influence towards the photoresist’ssensit ivity, and i t is very important for a practicalphotoresist system to have a compatible PAG-photoresistmixture. Since the all PAGs molecule contain a self-label, such as a heavy element like antimony (Sb), iodine(I), or sulfur (S), the PAG distribution can be studiedeasily with Rutherford Backscattering Spectroscopy(RBS).

Rutherford Backscattering Spectroscopy (RBS) is usedto determine the distribution of PAG in the polymerphotoresist layer. The RBS accelerates a stream ofHelium nucleus ions (4He++) of energy E

o in orders of

MeV at a certain angle q towards the sample. The 4He++

ions hit the atoms in the sample, and some get reflected.The reflected beam is detected by an energy sensitivedetector which records of the energy of the reflectedbeams. The energy of the reflected beam depends on the

“The Study of Next-Generation Photoresists”

REU Intern:Xinyi Zhou MSE, Cornell University

Principal Investigators:Prof Christopher Ober, Narayan Sundararjan Mat Sci & Eng, Cornell University


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target’s molecular weight, and it can be estimated by theequation:

E

M M M

M ME=

+ −+

×[ cos sin ]

[ ]

0 1

2

0

2 2 2

0 1

2 0

θ θ

Where E0 is the energy of the incident beam. M

0 and M

1

are the molecular mass of the incident and targetparticles, respectively. q is the angle between the sampleand the incident beam. The depth concentration profilecan be derived from RBS since the reflected beams fromatoms deep inside the sample will have lower energy thanthe reflected beam from atoms on the surface of thesample. See below for a schematic of the RBS. [3]

Figure 1, RBS Schematic

All elements in the polymer layer are light elementssuch as carbon, hydrogen, or oxygen, except the heavyelements in the PAG (Sb, I , or S). Therefore i t isrelatively easy to measure the distribution of PAG merelyby measuring the distributions of Sb, I , or S in thephotoresist layer.

Several polymers are chosen to set up a systematic testwith the 8 different PAGs. They are homopolymers PHS,PTHPMA, and PtBMA, copolymers P(tBMA-iBMA) andP(SiMA-tBMA). A study of the different types ofpolymers with the different PAGs should help usunderstand the compatibil i ty of the photoresist-PAGsystems.

Another improvement in photoresist technology worthinvestigating is the use of supercritical carbon dioxide asa possible developer. Tradit ionally, l iquid organicsolvents are used as developers for photoresists.Supercritical fluid (SCF) has the potential to become anexcellent alternative to the conventional l iquiddevelopers. Liquid organic solvents used for photoresistdeveloping can rarely be recycled and are expensive todispose. They are volati le and can cause greatenvironmental harm if not disposed of correctly.Supercrit ical CO

2 can be easily recycled merely by

decreasing the pressure back to gaseous state andallowing the dissolved solutes to precipitate out. It canalso be released to the environment without any harm.

SCF has many advantages against conventional solvents.Supercrit ical Fluid’s lack of l iquid-gas interfaceeliminates resist pattern collapses due to surface tensionsof liquid developers. The high diffusivity of the SCF canalso potentially increase processing rates. [2]

Experimental1. Preparing Solutions and coating the wafers16% or 20% (by weight) polymer solutions are

prepared using the solvent PGMEA (propylene glycolmethyl ether acetate). The solutions are filtered through0.2 micron Teflon syringe fi l ters. I t is extremelyimportant to fi l ter the solutions since any impurityparticles can cause the formation of “comets” on a wafer,which leads to uneven thickness of the photoresist layer.5% or 2% (by weight) of PAGs are then added to thesolutions. The solutions are added onto 1.5-inchdiameter silicon wafers by droppers, and is spun around2000 rpm (the exact rpm depends on the sample) for 60seconds. For most of the polymer samples, adhesion ontothe silicon wafer does not seem to be a big problem afterthe wafers are primed. After the wafer is spun, half of itis cleaved off to be baked at 150oC for 60 seconds (PostApply Bake). This is to drive off all excess solvents thatdid not evaporate during the spinning. The other half ofthe wafer is not baked. The two halves are used to studythe effect of the Post Apply Bake on the distribution ofPAGs.

2. Thickness measurementsIn order to find out the distribution of PAG in the

polymer. It is necessary to first find the thickness of thepolymer layer on the wafers. This is done by using theSloan Dektak II profilometer at the Advanced PackagingFacility. A light scratch is made onto the resist-coatedwafer, then the depth of the scratch is measured by theprofilometer. The profilometer measures the depth of thescratch by dragging a very thin needle across the it. Itoutputs a graph like figure 2 below.

Figure 2, Profilometer Output

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From the output, one can see that this sample has thethickness of roughly 5300 Å. The profilometer outputsfor all of the samples look roughly like this and thethickness can be determined from the graphs.

3. Rutherford Backscattering SpectroscopyAfter measuring the thickness of photoresist samples

on a spun wafer, the Rutherford BackscatteringSpectroscopy (RBS) is performed on the wafer. The RBSproduces an energy profile of all reflected beams (see theIntroduction part for background on RBS). This data isrecorded down onto a computer disk, and by using acomputer program called RUMP, the data is analyzed.The computer program simulates the graph output of theRBS, and together with the thickness measurement, theprogram calculates the distribution of different elementsin the photoresist polymer layer. Since only PAGmolecules contain heavy elements such as Sb or I, thedistribution of heavy elements represents the PAGdistribution.

For certain polymers, the distribution of PAG in thephotoresist layer is uniform. Shown in figure 3 is theRBS spectra of Poly(MMA-b-tBMA), a block co-polymerof methyl methacrylate and tertiary-butyl methacrylate.It shows an even distribution of the element antimony(Sb), the label element in the PAG.

Figure 3, RBS Spectra of Poly(MMA-tBMA)

For some polymer-PAG mixtures, segregation occursbetween the PAG and the polymer. Shown in figure 4 isthe RBS spectra of Poly(SiMA-tBMA). From thespectra, one can see the segregation of the PAG towardsthe bottom of the photoresist layer. There’s almost noPAG in the top third of the resist film. Most of the PAGssink down to the bottom of the photoresist, leaving aresist rich layer on top. This decreases the sensitivity ofthe photoresist significantly. From the computersimulation using the RUMP software, the blockcopolymer shown on the bottom half of f igure 4segregates less than the random copolymer.

Figure 4, RBS Spectra of Poly(SiMA-tBMA)

4. Exposure, Development and Further AnalysisThe wafer then is exposed to a certain dose of 193 nm

light (in units of mJ/cm2). Then it is baked again (Post-Exposure Bake) at 150oC for 60 seconds. The exposedwafer is developed in a developer and further analyzedfor the photoresist’s imaging characterist ics such assensitivity, contrast, and etc.

Setting Up the SFE Extraction FacilitySupercritical CO

2 is known to have good solubility on

certain fluoro-polymers and silicon-containing polymers.A practical photoresist system can be developed forprocessing in SC CO

2. The first step is to set up a

Supercritical Fluid Extraction (SFE) facility for CO2.

The developing conditions for the fluoro-polymerphotoresist we have in mind is about 100 oC and 5000pounds per square inch (psi) pressure of supercriticalCO

2. We set up a SFE extraction facility that’s based on

a high pressure SFE vessel inside an oven. A highpressure syringe pump that’s capable of pressurizing upto 10,000 psi is used to pressurized the vessel . Apressure relieve valve linking to the SFE vessel is set upon the side of the oven for extraction. The wafer sitsinside the SFE vessel and is heated by the oven andpressurized by the pump. See figure 5, on the next page,for a rough schematic of the SFE extraction system.

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Figure 5, Schematic of SFE CO2 Extraction

Facility

Conclusion and Future Research:We need to finish the systematic study of the PAGs and

the photoresist polymers to investigate the compatibility,segregation, and distribution properties of the 8 differentPAGs. After we fully understand the interaction betweenthe PAGs and photoresist polymers, i t will be mucheasier to find a compatible and practical PAG-polymersystem than can be used as a 193nm photoresist.

After the setup for the SFE CO2 facility is complete,

we can start to test different ideas for photoresists usingSC CO

2 as a developer. It will allow us to compare the

developing rate and resolution between traditional liquiddevelopers and Supercrit ical CO

2 developer, and

hopefully enable us to find a superior Supercritical CO2

developer photoresist.

References:1. Uzodinma Okoroanyanwu, Tsutomu Shimokawa,

Jeffrey Byers, etc. New Single Layer Posit ivePhotoresist for 193 nm Photolithography.

2. Paula Gallagher-Wetmore, Gregory M. Wallraff ,Robert D. Allen. Supercritical Fluid Processing: ANew Dry Technique for Photoresist Developing.

3. Larry F. Thompson, C. G. Willson, M.J. Bowden,Editors. Introduction to Microlithography, 2nd Ed.American Chemical Society, Washington D.C., 1994.

4. Elliot , David J. Microlithography : processtechnology for IC fabrication. McGraw-Hill, NewYork, 1986.

5. Elsa Reichmanis, editor, Scott A. MacDonald, editor,Takao Iwayanagi, editor. Polymers inmicroli thography : materials and processes .American Chemical Society, Washington D.C., 1986.

6. R.R. Kunz, S.C. Palmateer, A.R. Forte, R.D. Allen,G.M. Wallraff , R.A. DiPietro, and D.C. Hofer.Limits to Etch Resistance for 193-nm Single LayerResists.

7. H. Ito. Chemical Amplification Resists: History andDevelopment within IBM. IBM J. Res. Development.Vol. 41, NO. _ January/March, 1997.

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“Fabrication of Modulated Diffraction Gratings”

REU Intern:Mark Zwald AEP, Cornell University

Principal Investigators:Prof Harold Craighead, Dustin Carr Applied Eng & Physics, Cornell University


Abstract:We have successfully fabricated a new type of optical

modulator using silicon micromachining technology. Thedevices are fabricated from nitride/oxide/si l iconsubstrates using photolithography, reactive ion etching,and wet etching processes. Two MEMS processingtechniques were implemented: critical point drying withcarbon dioxide to avoid stiction problems with suspendedstructures and the utilization of photoresist to create asidewall passivation layer during a buffered oxide etch.The completed structure is an array of suspended nitridebeams whose pitch can be modulated with an appliedvoltage signal. Measurements of 80 micron long beamshave yielded a resonance frequency of 550 kHz with 75%modulation of the zero-order reflected intensity using adriving voltage amplitude of 2.7 volts. Similar deviceshave previously been fabricated [1].

Summary:Using only optical lithography techniques, we have

fabricated a nano-electromechanical device to be used asa high-speed optical modulator. The completed deviceconsists of a grating composed of suspended sil iconnitride (Si

3N

4) beams 80 µm long, 1.0 µm wide, and 0.25

µm thick, where every other beam is attached to the sameconductor. The design of the device was done in such away as to test the abil i ty of photoresist to be usedeffectively as a sidewall etch passivator during a bufferedHF oxide etch.

The fabrication process begins with the growth of 500nm of oxide (SiO

2) by wet thermal TCA oxidation and

followed by the deposition of 250 nm of LPCVD nitride[2]. Following this, ~800 nm of OCG 897-7i positivetone photoresist is spun on to be exposed in the GCA-6300 10x i-line stepper. After resist development, thegrating structures have been defined, so the sample thengoes through an anisotropic reactive ion etch in thePlasmaTherm PT72. A CF

4 plasma is used to etch

through the 250 nm of nitride and a CHF3 plasma is used

to etch through the 500 nm of oxide. At this point, thebeams have been defined but now need to be undercut ofthe supporting oxide underneath. To do this, ~1.5 µm ofOCG 897-12i (same resist as 897-7i but higher viscosity)is spun on and patterned in the 10x stepper. Afterdevelopment what remains is a window in the resistwhere the nitr ide beams are to be undercut of the

supporting oxide. The oxide which is within the resistpassivation layer will be protected from the oxide etch tofollow. Once the resist sidewall passivation layer hasbeen established, the sample is placed in a buffered 6:1(HF/NH

4F) oxide etch to undercut the nitride beams. The

oxide underneath the nitride beams within the passivationlayer is slightly etched creating a conical shape in theoxide. The protected oxide that remains acts as supportfeatures for the suspended nitride beams; at this point thegrating has been defined.

To ensure a functional device, the sample must gothrough a crit ical point drying step. To do this, weemploy a technique which has previously been used toimprove yield in conventional MEMS processing [3].The sample must go through a three stage dilutionprocess from the buffered oxide etch solution into water,and then another three stage dilution from water toethanol. The sample is then placed in a sealed chamberwhich dilutes the ethanol into liquid CO

2 at 10°C. The

liquid CO2 is heated; at 31°C and ~78 barr, the density of

the liquid and gaseous CO2 is the same (critical point)

and the l iquid rapidly goes through a phasetransformation into a gas. This allows for the liquid todry without creating surface tension effects which canpull suspended structures down. Results show thatcritical point is necessary to ensure a functional device.Samples which underwent the normal air dryingprocedure suffered from stiction problems with the Sisubstrate, the nature of which is not fully understood andshould be an area for further study. The final fabricationsteps are to remove any remaining resist (most of whichwas removed by the ethanol) by using an O

2 plasma in

the Branson/IPC Barrel Etcher. 3 nm of Cr and 3 nm ofAu are then evaporated on the sample to form conductivesurfaces and increase the reflectivity of the grating. Thefinal step is to package and wire bond the sample forelectrical/optical testing.

The suspended nitr ide beams can be deflected byapplying a positive voltage. This creates a negativemirror charge underneath the beams creating an attractiveforce between the beams and the Si substrate. Thisapplied voltage changes the pitch of the grating sinceevery other beam is connected to the “hot” conductor,causing the diffraction pattern of the reflected light tochange. When the voltage is turned off, the spring force

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of the beam returns it to its initial state. Using a HeNelaser and an AC coupled amplified photodetector,measurements were made of the beam’s resonancefrequency and amplitude modulation of the reflectedlight. Measurements of the 80x1 µm grating yielded aresonance frequency of 550 kHz with 75% modulation ofthe zero-order reflected intensity using a driving voltageamplitude of 2.7 volts. The device responds to a voltagegreater than 0.8 V and is saturated at 3.9 V.

This device has applications in fiber-opticscommunications and thin film display technologies. Thelow switching voltage and fast response to voltagechanges make it ideal for creating digital displays. Witha low voltage signal (< 0.8 V), the zero-order intensity isbright and the first-order intensity is dim. With a highvoltage applied, more light is diffracted into the first-order maxima, lowering the intensity of the zero-ordermaxima. A pulse-width modulated signal can be used tocreate a grayscale by controlling the duty cycle of thedriving voltage.

Conclusion:In conclusion, we have successfully fabricated a spatial

light modulator using conventional planar and MEMSprocessing techniques. Photoresist has been determinedas an effective etch passivator of buffered HF and criticalpoint drying has been shown to be effective in avoidingstiction problems with suspended MEMS structures.Measurements have shown that the device has highswitching rates and low switching voltage making it wellsuited for thin film displays driven by digital electronics.All fabrication was carried out at the CornellNanofabrication Facility with funding form the NationalScience Foundation.

References:[1] O. Solgaard, F. S. A. Sandejas, and D. M. Bloom,

Optics Letters. 17, 688 (1992).[2] Thanks to Steve Turner for the MOS processing.[3] M. R. Houston, R. Maboudian, R. T. Howe,

Proceedings of the 8th International Conference onSolid State Sensors and Actuators, 1, 210 (1995).

Figure 1:Shown below are suspended silicon nitride beams that

were fabricated in the manner described. Thesesuspended beams can be deformed with an electrical

potential. When alternate beams are pulled down, theperiod of the grating is effectively doubled. Such a

system can be effectively used as an optical modulator.

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For those CNF REU Interns who did not turn in a written report,their Convocation Abstract is included below:

“Leakage Current Through Novel Materials”

REU Intern:Dedra Demaree U of MD Baltimore County

Principal Investigator:Prof. Lester Eastman Electrical Engineering, CU

Schottky contacts are used in many active devices (HEMTS, MESFETS, etc.) and are an important research topic. Leakage currentthrough these Schottky contacts on new materials creates a major limitation. An effort is being made to optimize Schottky barriers onnovel materials (AlGaN, InGaP, etc.) by minimizing leakage current for both forward and reverse bias conditions. The materials areprocessed to make diodes with Schottky contacts using the technology at the Cornell Nanofabrication Facility. These contacts aremeasured at varying temperatures to determine their current versus voltage characteristics. The theoretical leakage current is thendetermined using “C-band” (self-consistent solution of Schrodinger’s and Poisson’s equations) and physical equations for tunneling andthermionic emission through the Schottky barriers.

“Fabrication of Thin Film Capacitors”

REU Intern:Matthew Owings Harvey Mudd College

Principal Investigator:Prof. Emmanuel Giannelis Materials Science, CU

As computer chips continue to decrease in size, there grows a demand for smaller and better thin film capacitors for memory and signalconditioning applications. We are examining the use of sol-gel techniques to create tantalum oxide thin film capacitors with copperelectrodes. We seek to determine the inherent and process-dependent electrical properties of these films. Furthermore, as initialinvestigations show copper diffusing into the tantalum oxide dielectric, we hope to find methods of preventing this diffusion whileretaining the desirable electrical properties of the capacitor.

“Anisotropic Through-Wafer Etching Using the SRL 770”

REU Intern:Bennet Tarng Stanford University

Principal Investigator:Prof. J Peter Krusius Electrical Engineering, CU

Current wafer processing technology limits itself to creating devices on the front sides of wafers, leaving the remaining bulk silicon toserve as structural support. If conducting paths were provided through the substrate, it would become possible to fabricate devicesutilizing both sides of wafers. Issues involved in creating such channels, which must be of size on the same order as the devices (microns,in the case of CMOS), include increasing the speed of hole etching and controlling the profiles of the holes. High etch rates are achievedby using the PlasmaTherm SRL 770 etcher, which can etch up to 3 microns of silicon per minute (typical wafer thickness ~350 um). Inmy presentation, I will show the ability of the 770 not only to etch quickly, but also anisotropically, creating channels with the desiredhigh aspect ratios. I will present results on my efforts to balance the effects of channel widening, caused by inherent isotropy in alletching processes and channel narrowing, caused by the creation of passivating sidewalls specific to the 770’s etch process, in order tocreate vertical profiles suitable to serve as communication channels between structures on opposite sides of wafers.

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Howard University

School of Engineering


From Left to Right:


Frances Uku, observing Howard University

Ijeoma Akunyili Howard University Dr Gary Haris

Benjamin Cawood James Madison University Dr Michael Spencer

Veronica Collier Alabama A & M University Dr Kimberly Jones

Not Pictured:

Mesha Harris Alabama A & M University Dr Kimberly Jones

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Howard University Participant

This letter is to describe the work of Benjamin Cawoodin our REU (NSF) Sponsored Program. During thesummer of 1997, Benjamin Cawood was a part of ourREU Program in the Materials Science Research Centerof Excellence (MSRCE), Howard University. His projectfor the summer was to develop computer software for ourAtomic Force Microscope. The software that he createdanalyzed the images created by the AFM for irregularitiesin semiconductor software. In order to do this, hebecame acquainted with the properties of the imagescreated by the AFM and then developed an interfacebetween the AFM and the software he was developing.He then constructed the software such that a largenumber of previously loaded AFM images could beanalyzed without operator assistance. This processallowed a sample which contained thousands of images tobe processed by computers. At the end of the programBen gave a presentation to other REU students andfaculty at Cornell University.

REU Intern:Benjamin Cawood James Madison University

Principal Investigator:Dr. Michael Spencer MSRCE, Howard University


“AFM GURU: Software for the Atomic Force Microscope”

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Introduction:Our summer research project was to investigate the

treatment options available to patients suffering fromkidney failure or end-stage renal disease (ESRD) with anaim to improve the current options. Research has shownthat kidney failure or ESRD affl icts approximately50,000 Americans annually. There are many causes ofESRD, among which are kidney infections, diabetes, highblood pressure and birth defects. The kidney provides thevital function of detoxification of the body. Without thisfunction, wastes gather in the blood causing seriousconditions such as uremia, t issue swelling, lungcongestion, high blood pressure, and eventually death.Currently, the treatment options are limited to dialysis ortransplants neither of which are cures. Since there are noknown cures and none on the immediate horizon, theneed to improve the current treatment options isapparent. Today, we will present our representation ofthe condensed internal hemodialysis machine as apossible option for treatment.

I. Kidney Description:Pair of reddish-brown bean shaped organs about the

size of a fist, located in posterior part of the abdomenwith one on each side of the spine.

Functions:• Cleans and filters blood.• Helps to stimulate production of red blood cells.• Regulates blood pressure, and various substances in the

blood including minerals like sodium and potassium.• Controls acid-base balance.• Excretes metabolic and foreign substances.

II. Cleaning & Filtering Functions of the Kidney:• Nephron (smallest functional unit of the kidney)• Bowman’s Capsule (contains the primary filtering

device of the nephron)• Glomerulus (serves as a giant sieve for plasma)

III. Urinary System:• 2 Kidneys• 2 Ureters• 1 Bladder• 1 Urethra

“Miniaturized Dialysis Machine for Internal Use”

REU Interns:Veronica Collier Alabama A & M University

Mesha Harris Alabama A & M University

Principal Investigator:Dr. Kimberly Jones MSRCE, Howard University


IV. Kidney Failure /Treatment Options:Kidney failure causes the body to hold fluid, the blood

pressure to rise, and toxins to build up in the body.There are three treatment options:1. Hemodialysis - machine acts as an artificial kidney

by cleansing the blood.2. Peritoneal dialysis - internal membrane fi l ters

blood.3. Kidney Transplant - a new kidney from a cadaver or

a living person is placed into the body.

V. Our Project:We have attempted to duplicate the function of the

kidney as a filter by designing and building a filtrationsystem. Our artificial kidney is a thin wafer of siliconwhich acts as a semipermeable membrane and discrim-inates against particles based solely on their size.

VI. Summary/Future Work:• The objective of our research was to improve the

quality of life for people suffering from kidney failure.• We hope to eventually modify our system into a

miniature internal hemodialysis machine.• We also plan to conduct studies on the membrane’s

durability and contamination rate.

Acknowledgments:We wish express our sincere gratitude to the following

people for their time and support with the building andthe designing of this project.

NNUN — Dr.’s Michael Spencer, K Wongchotigul,Kimberly Jones

MSRCE — James Griffin, Anthony Gomez, WilliamRose, Crawford Taylor and other members of theMSRCE.

HUH — Members of the Renal Dialysis Center and theNephrology Department

NIH — Dr. Lawrence Agadoa of NIDDK department atNIH

GWU — Dr. A. AliVolunteer Research Assistant — Susan Vanderpool

This project was supported by the National ScienceFoundation (NSF) and sponsored by the NationalNanofabrication User’s Network (NNUN).

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For those Howard REU Interns who did not turn in a written report,their Convocation Abstract is included below:

“Heteroepitaxial Growth And Characterization Of Beta Silicon

Carbide Films Using Chemical Vapor Deposition (CVD).”

REU Intern:Ijeoma Akunyili Howard University

Principal Investigators:Dr. Gary Harris MSRCE, Howard University

I will, in the course of this presentation, discuss the growth of beta silicon carbide films on siliconsubstrates. Firstly, I will give a brief overview on silicon carbide, its distinguishing properties andtechnological importance. Next, I will discuss epitaxial growth of silicon carbide with particular attentionto heteroepitaxy which we worked on this summer. The various cleaning procedures we went through inpreparing the substrate will also be discussed. The focus of this presentation, however, will be on theactual growth of the crystals using a CVD reactor. I will also discuss the thickness, electrical and mobilitymeasurement processes we used for the characterization of the crystals.

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The Pennsylvania State

University


From Left to Right:


Keith Hebert Texas A & M University Prof Stephen Fonash

Rhasheda Nixon Tuskegee University Research Associate Michael Rogosky

Arthur Beyder SUNY at Buffalo Prof Nicholas Winograd

Not Pictured:

Brad Neiman The Pennsylvania State University Dr Robert Davis

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Pennsylvania State University Participant

“Creating Chemical Libraries in Silicon”

REU Intern:Arthur Beyder SUNY at Buffalo

Principal Investigator:Prof. Nicholas Winograd Chemistry, The Pennsylvania State University


Time-of-flight secondary ion mass spectrometry(TOF-SIMS) was used to create 2-D chemicalmaps of several organic dyes and biological

compounds.

Micro-vials of various dimensions werefabricated in Si <100>.

Wet anisotropic etch produced vials with well-defined structure and volume.

Analysis of the filled vials to find TOF-SIMSresolution limits.

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REU Intern:Keith Hebert Texas A & M University

Principal Investigator:Prof. Stephen Fonash ES&M, Penn State University


“Gas Sensing Capabilities of Ultra-Thin Metal OxideSemiconductor Capacitors”

Project Summary:Testing MOSCap sensitivity to different gas ambients

using Cyclic Current-Voltage method.Figure 1 shows the gate device structure and

components. Notice the wells on the edge of the Poly.This seems to be the most sensitive area.

What is happening? Theoretically, excess chargebuilds up in the wells and changes the equilibrium, thuschanging the I-V characteristic.

Figure 2 shows some data collected when the MOSCapwas coated with 5Å layer of Palladium which acts as acatalyst in Hydrogen disassociation. The forward sweepis shown on the left graph where the device goes fromaccumulation (negative voltage) to depletion (lowpositive voltage) to inversion (higher positive voltage).As the device sweeps from positive 4 volts to negative 4volts the inversion layer is dissipated creating current.

Preliminary results show sensitivity to Hydrogen (w/Pdcoating) and water vapor, but little sensitivity to Oxygen.

Figure 1. MOSCap Gate Device Structure

Figure 2. Pd Coated, 10ms td, forward andreverse sweep, Cyclic I-V Data

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“Design & Fabrication of a Microelectro-MechanicalAccelerometer using Thin-Film PZT as the Sensing Medium”

Introduction:My REU project was the design and fabrication of a

MicroElectro-Mechanical (MEM) accelerometer — usingthin-film PZT as the acceleration-sensing medium. Veryfew researchers have successfully fabricated a thin-filmpiezoelectric MEM accelerometer. When the acceler-ometer feels a force, i.e. acceleration, the PZT deformsand, as a result, produces a current. This current is thenmeasured and, therefore, the acceleration is calculated.The accelerometer will have dimensions on the order ofmicrons, be made of materials used in microelectronics,and fabricated using techniques similar to integratedcircuits processing. The possible applications for thisaccelerometer include aircraft navigation, condition-based maintenance, automobile chassis-control andairbags.

Device Structure:The geometry of the accelerometer consists of a

centrally-supported, circular surface, approximately 2.3microns in height and between 25 and 500 microns indiameter. To better visualize the structure, think of a veryflat mushroom.

The materials that will be used in fabricating thisaccelerometer are silicon dioxide (glass), silicon nitride,platinum and lead zirconate titanate (PZT). The layers ofthis device, in order of deposition, are summarized in thetable below:

Type Material Approx. Thickness [microns]

Sacrificial SiO2

0.2Structural Si

3N

40.3

Contacts (T & B) Pt 0.1Sensing Medium PZT 1.5

The mechanism that this accelerometer is based on isthe piezoelectric effect . The propagation of theacceleration to the resulting signal goes as follows:

1. The device feels an acceleration in the directionperpendicular to the free-standing surface.

2. The acceleration deflects the free-standing,centrally-supported, surface.

3. This deflection distorts the piezoelectric material(PZT) The significant material property of PZT is

that when it is distorted, it produces current.4. The current is picked up by the Top and Bottom

Contacts, and is measured.

The process in fabricating this device was Silicon-based Surface Micromachining techniques. Thesetechniques are similar to integrated circuit processing.The fabrication process steps include the following:

Design & creation of the photolithography Mask-set.Sacrificial layer:

Deposit by chemical vapor deposition (CVD)Pattern with chemical (HF) or plasma (RIE) etching

Structural layerDeposit by CVDPattern by plasma etching (RIE)

Bottom Contact layerDeposit by thermal evaporationPattern

Deposit PZT layerTop Contact layer

DepositPattern

Pattern PZT with chemical (HF) etchingWire bond from Top Contact to a Probe Pad

Applications:Depending on the sensitivity (through adjustments in

dimensions and/or material thicknesses) of these devices,possible applications include these areas: condition-basedmaintenance, inertial guidance/ navigation, automobilechassis control and air bags. When manufactured inmass-production, l ike integrated circuit chips, thisaccelerometer could replace current ‘macro-’ sensorswith its lower cost and reduction in size. Also, because ofthe device’s small size, this accelerometer could beapplied in areas where sensors used to be too large toplace.

Range [g] Application10-3 Inertial Guidance/ Navigation1-5 Automobile Chassis Control1-50 Condition-Based Maintenance (CBM)50-100 Automobile Airbag Trigger Systems

REU Intern:Brad Neiman PSU

Principal Investigator:Dr. Robert Davis EMPRL, The Pennsylvania State University


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Inertial Guidance/ Navigation:In particular this accelerometer, along with GPS, in

aircraft — could provide second-order corrections formore accurate navigation and possibly for automatictake-off and landing.

Automobile Chassis Control:This accelerometer could be used to measure

accelerations of various components of an automobilechassis, such as: suspension for improved shockabsorption and traction, the entire chassis for improvedbraking and handling, etc.

Condition-Based Maintenance:Because of their size, these accelerometers could be

placed throughout any mechanical system to measure andrecord the acceleration histories of numerous parts. Thesehistories would be used to help more accurately estimateaging, or damage, to parts, and predict when a part needsreplacing before it fails. The data extracted from thesesensors would reduce the number of costly and time-consuming maintenance cycles.

Automobile Airbag Trigger System:Currently, the most commercial application of MEMS

is the Analog Devices accelerometer in some automobileairbag triggering systems. That accelerometer is used tomeasure the deceleration of the automobile when it isdrastically slowing down. Once the measureddeceleration reaches some maximum-allowablemagnitude, it sends a signal to deploy the automobile’sairbag(s).

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Abstract:In this paper damage caused by electron beam

lithography is analyzed. Two silicon wafers underwentdirect write and were compared against one wafer thatunderwent optical l i thography. They were comparedagainst each other to label the damage caused by electronbeam systems. Deep level transient spectroscopymeasurements were taken for all three wafers and theresults were analyzed to characterize defects found.

Introduction:In its twenty-five plus years of existence, damage by

electron beam lithography has been an arising concern. Inorder to discuss the damage and defects of electron beamlithography, the process must f irst be understood.Electron beam systems use a beam of high energyelectrons ranging from a low of 10 kev to a high of 100kev to write small features on silicon wafers and photomasks coated with resist. In this process using electronsto write patterns on chips is liable to produce some typeof damage. The higher the energy the more damageexpected.

In this experiment, certain types of defects wereexpected to be found. Defects in the lattice structure suchas vacancies, dislocations or intersti t ial as well asimpurity defects which could be substi tutional orinterstitial were the main ones looked for.

Studying defects at different energy levels is importantin determining what energy of electrons causes the leastamount of damage to the si l icon wafers. In thisexperiment the two direct write wafers will undergoelectron beam li thography at three different energylevels. The damage will then be analyzed with a controlwafer that was only subjected to optical lithography.Optical lithography uses ultra violet radiation to form theimage on the wafer, which causes no damage. Electronbeam lithography damage is important to the growingsemiconductor industry in order to characterize thedamage at different energy levels.

Methods and Materials:Three four-inch si l icon wafers were used for the

experiment. Two of the wafers were coated with 496kPoly methylmethacralate (PMMA) and spun at 2000 rpmand 5000 rpm to produce two different thicknesses. Thethird wafer was coated with 1813 resist and spun at 4000

“Electron Beam Lithography Damage Done to Silicon”

REU Intern:Rhasheda Nixon Tuskegee University

Principal Investigator:Research Associate Michael Rogosky Materials Sci & Engineering, PSU


rpm. The first two wafers underwent direct write on anelectron beam lithography system. Three different energylevels, 20 kev, 50 kev, and 100 kev, were used to write inthree different areas on both of the wafers. The thirdwafer was to be used as the control wager forcomparison. This wafer was exposed to ultra violetradiation in optical l i thography. In this process thecontrol wafer was exposed to hard contact printing for .9seconds.

The control wafer was pre-baked at 100 degreesCelsius for 90 seconds and post baked at 100 degreesCelsius for 90 seconds. The two direct write wafers werepre-baked at 175 degrees Celsius for 50 minutes but werenot post baked.

The developer used for the control wafer was MF312.The direct write wafers were developed using a one toone ratio of Methyl Iso Butane Ketone to 2-PropanolAlcohol.

After electron beam and optical lithography, the threewafers were evaporated first with 200 Angstroms ofplatinum and then 600-800 Angstroms of aluminum. Thebackside of the wafer was evaporated with 300Angstroms of t i tanium and 600-900 Angstroms ofaluminum.

Deep level transient spectroscopy (DLTS), an electricalprocedure, is most commonly used to measure lowconcentrations of defects and impurit ies in si l iconwafers. In this process, DLTS, one sample is analyzed ata time. The sample is cooled to 10-14 degrees Kelvin andthe measurements of capacitance versus voltage andtemperature are then taken at he lowest roomtemperature.

We continue to work in this project.

Acknowledgments:I would like to thank all of the staff at EMPRL, and my

mentor, Michael Rogosky, for without his help andpatience I would have been totally lost. I would also liketo thank CURO and REU for giving me this opportunity.

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Bibliography:Benton, Janet L. “Characterization of Defects in

Semiconductors by Deep Level TransientSpectroscopy.” Journal of Crystal Growth 106(1990): 116-126.

Frank, L. “Quantification of the Electron Beam Damageof Thin Films.” Mikrochimica Acta 114/115 (1994):293303.

Harriott, Lloyd, and Alexander Liddle. “Electron-BeamLithography.” Physics World 10.4 (1997): 41-45.

Howitt, David G., and Douglas L. Medlin. “RadiationDamage and Electron Beam Lithography.” Scanning:International Journal of Scanning ElectronMicroscopy and Related Methods 13.Supplement I(1991): I-8.

Humphreys, C. “100 kev Electron Beam Damage ofMetals and Oxides.” Micron and Microscopia Acta22.1/2 (1991): 147-148.

Medlin, D. L. “Radiation Damage Processes AffectingElectron Beam Lithography of Inorganic Materials.”Scanning Electron Microscopy and Related Methods14.2 (1992): 86-90.

Paul, Douglas J. “Electron Beam Induced Damage ofSilicon Germanium.” Journal of Vacuum Science andTechnology 14.6 (1996): 3834-3837.

Svensson, B. G. “Point Defects in MeV Ion-ImplantedSilicon Studied by Deep Level TransientSpectroscopy.” Nuclear Instruments & Methods inPhysics Research 106.4 (1995): 183-189.

6E + 13

4E + 13

2E + 13

0

0

Objectives:

• To provide a better knowledge of damageassociated with high-energy electrons in electronbeam lithography.

• To be able to find and identify defects using deeplevel transient spectroscopy.

Results:

• Electron beam damage was found in the wafer.

• Capacitance difference.

• Differences between 50 keV and 100 keV.

• Current comparisons.

Future Work:

• More in-depth study of depletion regions.

• Deeper probing in the wafers.

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Stanford Nanofabrication

Facility


First Row, Left to Right:


John Kim Yale University Prof. James Harris

Nicole M. Jantzi Stanford University Dr. Mark Zdeblick

Maria Huang California Inst of Technology Dr. James Harris, Jr.

Rong Fan Stanford University Dr. James McVittie

Brandon Earl Fleming Case Western Reserve University Dr. Peter Griffin

Second Row, Left to Right:

Adam Pyonin Washington University Dr. Nabeel Ibrahim

George Paloczi Univ of California at Santa Barbara Dr. Kathryn Wilder

John Cross Santa Clara University Dr. James Plummer

Mark Finlan William and Mary Dr. James McVittie

Farouk Hadeed Yale University Prof. Shan Wang

Andrej Sierakowski Stanford University Prof. James Plummer

Not Pictured:

Brittany Mitchell Clarkson University Dr. Mike Deal

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Stanford Nanofabrication Facility Participant

1. SEM picture of the cross-section of anoverhang test structure after PECVD of SiO

2.

2. Three dimensional AFM picture of the cross-section of an overhang test structure (1.0 mm

gap, cantilevers removed) after PECVD ofSiO

2.

The spat ia l var ia t ion of phys ica l proper t ies of th in f i lms in ageometrically aggressive structure after deposition is unknown.

Film density variation was investigated after PECVD of SiO2 in overhang

test structures.

AFM and SEM were used to characterize the density of the depositedfilm after isotropic etching.

A correlation between film density and ion flux distribution has beenobserved and will be modeled for thin film processing simulation.

“Characterization of Spatial Variation of Thin FilmProperties after Deposition”

REU Intern:John Cross Santa Clara University

Principal Investigator:Dr James Plummer Stanford University


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REU Intern:Rong Fan Stanford University

Principal Investigator:Dr. James McVittie EE Department, Stanford University


“Computer Modeling of High Density Plasma Oxide Etching”

Projec t Objec t ive : Making tes t s t ruc tures for oxide e tching underdifferent reaction conditions.

Basic reactions observed in oxide etching include physical sputtering,ion assisted chemical etching done by high energy ions, and ion enhanceddeposition by low energy ions.

The model will be improved further by comparing experimental etchingprofile with simulation profiles.

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REU Intern:Mark Finlan William and Mary

Principal Investigators:Dr. James McVittie, Mayur Joshi Electrical Engineering, Stanford


“Measuring Charging Damage Due to Plasma Nonuniformity”

Proper funct ional i ty of th in oxide layers in MOS Devices andconsequently circuits is critical.

Local nonuniformity in plasma may cause charging damage to thin oxidelayers, and hence we need a method of measuring this damage.

We fabricated antenna structures to simulate, accentuate, and measurethis type of damage to thin oxides.

We were able to fabricate antenna structures with oxides as thin as 40 Åwhich s imula te oxide th icknesses tha t wi l l be used in fu ture ULSIdevices.

Depiction of Localized Plasma Charging onThin Oxide Layer

Schematic Profile of Antenna Structure Overhead SEM Picture of Individual AntennaStructure

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REU Intern:Farouk Hadeed Yale University

Principal Investigator:Prof. Shan Wang Electrical Engr & MSE, Stanford University


“Processing Techniques in Fabricating Spin-Dependent Tunnelling Devices”

New Generation Magnetic Media storage devices that rely on Magneto-resistance (MR) instead of magnetic-induction in the read/write coil ofthe magnetic head in disk drives.

Processed Spin-Dependent Tunneling (SDT) Junctions using Dry etch byion-milling technique. This method offers greater directionality than wete tch techniques , and can be used for making devices wi th smal lerdimensions than before.

Concept: electron tunneling through an insulating barrier (Al2O

3) can be

controlled by applying an external magnetic f ield that independentlyswitches the polarization states of the top and bottom magnetic layers inthe SDT junction, giving rise to MR = ∆R/R.

Cross-section and top view showing processingof SDT Junction

SDT Junction (Junction Area = 50x50 µm2)

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REU Intern:Chih-Wei Maria Huang Califonia Institute of Technology

Principal Investigator:James Harris, Jr. Stanford University

Contact: harris@snowmass. stanford.edu

“The Optimization and Fabrication of Fabry-PerotVertical Cavity Phase Flip Modulators”

Studied the physics behind a Fabry-Perot Cavity.

Developed a set of computer programs that automated the process ofusing an existing set of numerical tools to optimize a modulator design.

Illustrated new model “Gain Quantum Well Phase Flip Modulators” thatuses a gain quantum well layer to amplify output.

Demonstrated a 180˚ phase change and 80% reflectivity at a bias voltageof 8 Volts.

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“Microfabricated Check Valves for IntraocularPressure Stabilizer for Glaucoma Therapy”

REU Intern:Nicole Jantzi Binghamton University

Principal Investigator:Dr. Mark Zdeblick Ophthalmology, Stanford University


Objective: To create a microfabricated check valve to be used in apressure controlling system for glaucoma treatment.

Approach: Use standard photolithographic processes to create a deeptrench mold for silicone rubber patterning.

Accomplishments : Developed a new process for s i l icone molding.Found a couple of methods for stripping photoresist without affectingthe silicone mold structures.

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REU Intern:John Kim Yale University

Principal Investigators:Prof. James Harris, Barden Shimbo Electrical Engineering, Stanford University


“Fabrication of a Single Electron Transistor Usingan Atomic Force Microscope C-V Profiling of InAs

Quantum Dots in a GaAs Substrate”

Top picture: Single Electron Transistor

Performed capacitance-voltage device simulations using delta-doped,homogeneously doped, and heterojunction samples.

Investigated the discrepancies with the simulator and tried to reconcilethese problems.

At tempted to use a capaci tance microscope to ga ther e lec t r ica linformation about quantum dots.

Bottom picture: AFM images of quantum dots

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“Variances in Hillock Formation on PassivatedAluminum Due to Grain Growth”

REU Intern:Brittany Mitchell Clarkson University

Principal Investigators:Dr. James Plummer, Mike Deal Stanford University


Thermal cycles cause grain growth and stress (which is released by theformation of hillocks) in aluminum thin films.

Al grains grow to a maximum point before stagnating.

An SiO2 passivation layer (such as found in integrated circuits) affects

both the grain growth and hillock formation.

We studied the relationship between hillock formation and grain growthon passivated Al by attempting to grow grains to their maximum sizeunder passivation and then study hillock formation in the absence ofgrain growth.

This information about interconnects will be integrated into the BackEndSimulation Tool (BEST).

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“AFM Tip-induced Oxide Formation on Silicion”

REU Intern:George Paloczi UC Santa Barbara

Principal Investigator:Kathryn Wilder Stanford University


We grew thin lines of silicon dioxide on silicon using the Atomic ForceMicroscope (AFM) with a method called field-enhanced oxidation.

We were able to write oxide l ines with t ip scan speeds of up to 5.4millimeters per second.

We found relationships between the oxide thickness and scanning speed,as well as the dependence of oxide thickness on tip/sample bias voltage.

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“Dopant Redistribution in Polysilicon During TiSi2

Formation”

REU Intern:Adam Pyonin Washington University

Principal Investigator:Prof. Krishna Saraswat Stanford University


Silicide formation affects equilibrium defect concentration.

Defec t concent ra t ions wi th in polys i l icon ga tes affec t e lec t ronicproperties of transistors.

Built polycide gate test structures.

Integrated into Back End Stimulation Tools.

Figures from JW Mayer, K Tu, and SS Lau

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“Use of the AFM and NIH Image for RapidStatistical Characterization of Grain Size in

Polycrystalline Silicon Films”

REU Intern:Andrej Sierakowski Cornell University

Principal Investigator:Dr. James Plummer Stanford University


• Statistically characterized grain size, for a givensample, independent of scan size or rate.

• Filtered data from AFM for appropriateapplication of NIH Image.

• Developed algorithm, using existing fi lters, toprocess AFM height image data into two relativelydistinct regimes: Grain and Grain boundaryregions.

• Establish NIH as a Relaible Imaging tool fortraced images.

• Compared NIH Image’s ability to determineaverage grain size to ASTM Standard Methods forDetermining Average Grain Size.

• Resolution is lost for smaller grains which therebyprevented direct application of a normal fit to thegrain size data collected with NIH Image.

• Look for trends in the 2nd, 3rd, and 4th momentsof each distribution.

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For those Stanford REU Interns who did not turn in a written report,their Convocation Abstract is included below:

“Scanning Capacitance Microscopy for Two-Dimensional Dopant Profiling in CMOS Devices”

REU Intern:Brandon Fleming Case Western Reserve University

Principal Investigator:Dr. Peter Griffin Stanford University

The purpose of this project was to explore alternatives for mapping dopant contourlines present in polysilicon wafers. Unlike its predecessor which relied on observing thecontour lines from a profile view, we utilized Scanning Capacitance microscopy toobserve the lines from a surface perspective. This is thought to yield data of value toindustry in manufacturing electronic devices (transistors, etc). Sample wafers wereimplanted with varying concentrations of arsenic and phosphate, and a comparison will bedrawn between the two.

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University of California

at Santa Barbara



Charlie Hammer Middlesex County College Prof. Joe Zasadzinski

Kevin Ness Univ of California at Santa Barbara Dr. Umesh Mishra

Jennifer Mei-An Popham Alabama A & M University Dr. Evelyn Hu

Brett Sackett Univ of California at Davis Dr. Mark Su

Paul Molnar Cornell University Dr. Jorge M. Garcia

John Fuhrman Univ of California at Santa Barbara Dr. Steve DenBaars

Craig Maloney University of Toledo Dr. Atac Imamoglu

Joshua Stamp Hanover College, IN Dr. David Awschalom

Sandra Sattler Columbia University Dr. Mark Sherwin

From Left to Right:

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UCSB Participant

Semiconductor technologies have become aninternational focus of scientific inquiry in recent years.Despite the recent progress in semiconductor technology,blue laser diodes have yet to be developed by Americanresearch facilities. As a summer NNUN REU intern atthe University of California at Santa Barbara (UCSB), Iplayed a role in the search for a blue laser diode. Mygoal was to characterize the optical properties ofsemiconductor wafers made by UCSB researchers.

The motivation behind these efforts includes practicalapplications of the semiconductor blue laser that mayaffect millions of people and open lucrative markets.One example is the use of a blue laser in a computer’scompact disc reader. Switching from the currently usedinfrared lasers to blue lasers might lead to a fourfoldincrease in packing density on disc media.

Motivated by the anticipated demand for semi-conductor blue lasers, researchers at UCSB are workingto develop a blue laser diode principally based on twocompounds: gallium nitride (GaN) and indium galliumnitride (InGaN). These compounds were selected fortheir predicted optical properties: when indium, gallium,and nitrogen compounds are arranged in specificconfigurations and proportions in a semiconductor wafer,the device can be stimulated to emit blue light. Lowtemperature photoluminescence experiments were chosenas the characterization method. Photoluminescence wasachieved by shining an ultraviolet laser at the sample.Cooling the sample to low temperatures allowed a moredetailed analysis of the properties of the sample. Com-pared to room temperature photoluminescence, lowtemperature photoluminescence produces a richer, morecomplete representation of the sample’s optical character-istics. Due to the brevity of the internship, I was unableto complete the study and arrive at significantconclusions.

As an inexperienced undergraduate, working for tenweeks in the UCSB research environment gave me aglimpse of how graduate school works and whatscientific research is l ike. Aside from the hands-onexposure, I also learned much about semiconductorproperties. My mentors taught me a great deal about thephysics and engineering that go into the development ofsemiconductor devices. My internship experience atUCSB was wonderful, and I hope to one day offer asimilar opportunity to an ambitious undergraduate once Iam in graduate school.

“Optical Characterization of InGaN Quantum Wells”

REU Intern:John Fuhrman Harvey Mudd College

Principal Investigator:Steve DenBaars Materials Dept., UCSB

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UCSB Participant

Cadmium Sulfide nanocrystals may be grown using theconcept of reaction compartmentalization. A singlecrystal is grown inside a vesicle, a small aqueouscompartment that is encapsulated by a l ipid bilayer.These bilayers are composed of a class of lipids callphospholipids.

Phospholipids are amphiphilic molecules which containa hydrophilic portion and a hydrophobic portion. Whenphospholipids are introduced to an aqueous solution, it isinherent in their nature to minimize the amount ofhydrophobic portions that are exposed. Consequently,the l ipids will form bimolecular sheets which willeventually self-assemble into vesicles. A portion of theaqueous solution is encapsulated during vesicleformation. This results in a tiny spherical reactor.

Depending on the solution encapsulated by thevesicles, and the solution added to this vesicledispersion, certain crystals of controlled and consistentsize can be synthesized. The size of the crystals isdirectly related to the number of ions that were originallyencapsulated. This is determined by the size of thevesicle and the concentration of the solution.

The crystals made this summer were of CadmiumSulfide (CdS) and were synthesized by encapsulatingCadmium Chloride (CdC

12) solution and then adding

Ammonium Sulfide ((NH4)2S) to the resulting vesicle

solution. These semiconductor nanocrystals exhibitinteresting electronic properties and can potentially havemany applications. Tiny semiconductor materials can beused in devices such as lasers and light-emitting diodes.

In experiments this summer, it was discovered thatmixtures containing different anionic lipids resulted inmulti-lamellar vesicles (see figure). Future work will beto adjust the anionic lipid concentration in the hope ofproducing unilamellar, size monodisperse, mixed lipidvesicles which may alter the crystallization process.

“Preparation of CdS Nanocrystals by ReactionCompartmentalization”

REU Intern:Charlie Hammer Middlesex County College

Principal Investigators:Joseph Zasadzinski, Michael Kennedy Chemical Engineering, UCSB

100 nm

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UCSB Participant

“0-D Polaritons”

REU Intern:Craig Maloney University of Toledo

Principal Investigator:Dr. Atac Imamoglu Electrical & Computer Engr, UCSB

Recently, much interest has focused on the opto-electronic properties of semiconductor microcavitiesenclosed by a pair of Distributed Bragg Reflectors(DBRs). The evolution of these cavities was driven, inpart , by the promise of higher efficiency ofsemiconductor lasers through “engineering” or“tailoring” of the optical environment in which thecharge carriers reside.

One l ine of inquiry deals with the excitonicinteraction with the local electromagnetic field. In theregime in which the excitonic recombination energy isresonant with photonic cavity energy, it is found that theexciton - photon system is best described by a set ofsuper-modes known as “polaritons.” We explore theeffects of lateral confinement on the quasi-particlepolaritons through the altering of the optical cavityenvironment.

Our processing technique in confining the photonmodes relies in combination on circular mesa formationthrough Reactive Ion Etching (RIE) followed by thelateral oxidation of AlAs layers in the top DBR of themesa structure. The high discontinuity in refractiveindex between the oxidized and non-oxidized regions inthe layer provide an effective reflective boundary. Thefigures below are micrographs of the processedstructures.

As one might infer, this project is very much work inprogress. Some test processing runs were made andsamples were cleaved to give the above images. Theseresults are consistent with our oxidation scheme andstrongly suggest that reflectance spectra of thestructures would be modified accordingly.

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UCSB Participant

I have spent this summer helping Dr. Jorge Garcia studyquantum dot resonant tunneling diodes. Resonant tunnelingdiodes have many applications including frequencymultiplication, infrared light detection, and memory cells.If we use quantum dot confinement in the resonanttunneling diodes, we get a more desirable current-voltagecharacteristic which makes it work better in the aboveapplications.

The purpose of my project was to build and test quantumdot resonant tunneling diodes (RTD’s) contained in mesasas small as 6 microns wide. My research mentor, Dr. JorgeGarcia, has been building mesas as small as 100 micronswide. Because the wire contact to the mesa is 100 micronswide, it is not possible to connect the wire to a mesa smallerthan 100 microns. Therefore, by the old method, itwouldn’t be feasible to build mesas smaller than 100microns.

However, we wanted to make the diodes as small aspossible to allow for more pronounced tunneling in thecurrent-voltage characteristic of the RTD. We used amethod suggested to us by Dr. Peter Wellman of the UCSBMaterials Department. The method calls for us to build themesas as small as we want, then coat the entire sample witha layer of non-conducting Silicon Nitride, and finally builda metal bridge from the mesa to a 200 micron metal contact.

The first step of my project was to design aphotolithography mask to build mesas ranging in size from600 microns down to 6 microns with these metal bridges.Next, after the mask was printed, I used this mask tofabricate the diodes by photolithography. Finally, I testedthe diodes for current-voltage characteristics. We wereaiming to produce RTD’s with a distinguishable region ofnegative resistance in their current-voltage characteristics.

This summer’s research experience has been a positiveone for me. Because of my electrical engineeringbackground, I have been curious to see how micro and evennano devices are produced. I had always had theimpression that someone used a fine soldering iron under avery high magnification microscope to produce smalldevices. After a summer working on the fabrication ofRTD’s, I have satisfied my curiosity. It has been a real eye-opener to see how such small devices are actually made andto have the opportunity to design and fabricate them myself.

“Quantum Dot Resonant Tunneling Diodes”

REU Intern:Paul Molnar Cornell University

Principal Investigators:Pierre Petroff, Jorge Garcia Materials Dept, UCSB

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UCSB Participant

“Native Oxides as Insulators for Gallium Arsenide”

My research project’s focus was to find a native oxidefor use as an insulator for Gallium Arsenide (GaAs). Thisinsulator needs to have electronic characteristicscomparable to that of Silicon Dioxide (SiO

2), the insulator

used for Silicon (Si), but which will be used on GaAs.A general overview of a basic MIS (metal-insulator-

semiconductor) transistor will help in understanding howthis insulator is made and the reason it is so important todiscover. A MIS transistor consists of a semiconductor,insulator, and three metal contacts known as the source,gate, and drain. The insulator is absolutely critical to theperformance of an MIS transistor so it will be the mainarea of my research. The way an MIS transistor isnormally used is by applying a bias between the source anddrain inducing a current to flow in the semiconductor.This current flow can be regulated by varying the voltageat the gate. If a negative voltage is applied at the gate itwill deflect electrons from the interface. This deflection ofelectrons will decrease the number of electrons flowingfrom the source to the drain. Therefore, the voltageapplied at the gate will greatly affect the current from thesource to the drain: this, however, will only work as longas the material under the gate is a good insulator.

To create an insulator on GaAs one must go about it inan indirect way. A sample grown out of GaAs has a layerof Aluminum Arsenide (AlAs) about 2000 A thick which isthen covered with a 500 A GaAs cap. This GaAs cap isthere to prevent contamination because the AlAs is veryreactive with oxygen. The sample is then processed usingphotolithography to expose the AlAs from the side. Theoxidation of AlAs to Al

2O

3 takes place in a furnace where

steam moves across the sample.Now, it has been determined that leakage current is not a

problem with Al2O

3 so tests need to be done to determine

the trap density at the semiconductor-insulator interface.To accomplish this, capacitance-voltage (C-V)measurements will be taken to determine the trap density.Unfortunately there was not enough time to complete thismeasurement because I burnt up all the good samples.

I will be continuing my research for the Mishra groupduring the school year. My future plans are to completelyprocess the samples again with a few variations and takethe C-V measurements on them.

REU Intern:Kevin Ness UCSB

Principal Investigators:Umesh Mishra, James Ibbetson Electrical & Computer Engr, UCSB

Source

Gate

Drain

Insulator

e -e -e -

e -e -

e -e - e -e -e -e -

e -e -e -

e -

Electron Travel

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UCSB Participant

****flash, f lash, f lash**** What is that annoyingblinking that you see at the corner of your eye youwonder as you drive home from work. Since you arestopped at a red light, you look to your right to observethe irri tat ing flashing of an electroluminescentadvertisement wanting you to fund the national sciencefoundation. On your way home you stop at the videostore to rent that all time favorite movie, Contact. Finallyreaching home, the lights come on as soon as you unlockyour door with your key card and your computer says,“Welcome home, Sandy. You have e-mail.” Fabricationof GaAs Terahertz Fabry-Perot Resonators

You slip in the rented CD to start watching the moviepreviews as you check your e-mail. You are happy to seethat your friend from overseas has finally responded. Asyou hit print and sit down to watch the movie, you can’thelp thinking what a nice night this is going to be. Littledo you know that most of the technology that youinteracted with this evening had to do with gall iumnitride (GaN), a semiconductor that emits blue light.

Full colored electroluminescent displays replacingbillboards, traffic lights, full length movies on CD’s, bluelight emitt ing diodes, better l ight sources, opticalcommunication undersea, and laser printers are a few ofthe applications that GaN has in store for us with itsshorter wavelengths. With i ts unique properties ofhardness and chemical inertness, GaN is stable in hightemperatures and in caustic environments, but theseproperties also cause problems in the processing of GaNinto useable devices. There are ways to etch down onGaN creating raised patterns of mesas; however, there isno proven way to etch laterally to remove GaN from thesubstrate.

Presently GaN is grown on sapphire which is a poorthermal conductor at room temperature. In order for theheat tolerant GaN devices to be effectively used, theymust be removed from the heat holding sapphire thatwould roast the different machines GaN could be used in.Because sapphire is very hard and chemically inert, it isdifficult to get r id of this substance throughdecomposition or etching. Since sapphire is one of theonly substrates that GaN can be grown on due to itscrystal lattice, scientists must either discover a way toetch GaN off of the substrate or look for a different

“Photoelectrochemical Etching of GaN”

REU Intern:Jennifer Mei-An Popham Alabama A & M University

Principal Investigators:Evelyn Hu, Darron Young Electrical & Computer Engr, UCSB

material to grow GaN on.The answer to the problem of etching GaN off of

sapphire may be the process of photoelectro-chemical(PEC) etching. I t does not take big machines orcomplicated computer programs to carry out this etching.All you need is light, electrolyte, an electrode, and thesample. Because PEC etching has a unique property ofbeing a selective etch, one would be able to decomposethe layer of GaN nearest the substrate lifting off thedevice so that it could be used in different high power,high temperature, and high frequency deviceapplications.

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UCSB Participant

“Fabrication of GaAs Terahertz Fabry-PerotResonators”

REU Intern:Brett Sackett UC at Davis

Principal Investigators:Mark Sherwin, Mark Su Physics Dept, UCSB

Rather than spend yet another summer surfing the cablechannels or gett ing lost in the infinite world of theinternet, I decided to sample life after graduation tosatisfy my curiosity about both graduate school and thefield of experimental physics. The Research Experiencefor Undergraduates (REU) program enabled me to fulfillboth of these goals. The internship allowed anopportunity for me to pretend to be a graduate studentwhile participating in real research.

The Quantized Electronic Structures (QUEST) group atthe University of California at Santa Barbara sponsoredmy internship. I worked for Professor Mark Sherwin, thehead of an experimental physics group which studiesnonlinear effects in confined quantum systems. In thisgroup I worked with a graduate student, Mark Su. Underhis guidance I not only expanded my physics knowledge,I learned a great deal about graduate research. My taskscovered an entire spectrum from reading and performingcalculations, making tools in a machine shop, toprocessing samples in a clean room environment andrunning experiments in the lab. In addition to my dailyresearch, the REU group held meetings twice a week forinterns to share their current progress in work. Thisreally benefited me because I learned from the otherinterns and participated in their learning too.

In the Free Electron Laser (FEL) lab I began myinternship by helping my mentor, Mark Su, set up anoptical experiment. The purpose of the experiment wasto study the optical properties of an AlGaAs/GaAs/AlGaAs quantum well by focusing light onto a samplethat was set into a cryostat. This experiment provided anexcellent opportunity to simultaneously learn about theequipment used in the experiment, and the physics of thesample. I also understood some aspects of theexperiment from previous physics coursework; hence,feeling comfortable seeing the use of such information.

Throughout the internship I felt like both a scientistand a graduate student. I enjoyed the benefit of learningthat which you cannot learn in the classroom, thus addingenthusiasm to further motivate me in school. In the end,this experience proved to be a vital element in mydecision to continue the long journey towards a Ph.D. inPhysics.

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UCSB Participant

The existence of Cosmic Microwave BackgroundRadiation (CMBR) and its blackbody nature stronglysupports the hot Big Bang model of the universe. The BigBang model predicts the CMBR’s existence with i tsdescription of how the universe was formed and why itlooks the way it does today. This model describes theearly universe as a very hot and dense soup of protons,electrons, and photons As the universe expanded it grewcooler and the photons lost energy allowing hydrogen toform. The photons were free to travel far - so far thatthey could reach us today in the form of CMBR. Theradiation left the dense early universe hot, and redshiftedas the universe expanded; photon wavelengths would beinfluenced by the expansion of the universe causing thewave to expand lengthwise with the universe. So the hotradiation of the early universe would reach us today aslower energy radio waves. Arno Penzias and RobertWilson discovered CMBR in 1965 while they wereinvestigating radio emission of the Milky Way atmicrowave wavelengths. They found background noisethat would not go away no matter where they pointedtheir instrument. The noise was equally intense in alldirections. This is the radio wave radiation predicted bythe Big Bang theory, and is the CMBR that we are stillstudying now.

We can investigate the CMBR that reaches us bymeasuring its brightness. The Big Bang theory predictedthat CMBR would have a Planck curve of a blackbody at2.7 K The Cosmic Background Explorer satellite (COBE)measured the intensity of the microwave background atwavelengths around the peak of the blackbody curve,wavelengths of 3-10 mm. It corresponded to a blackbodycurve of 2.728±002 K.

It is important to study the CMBR very precisely tomeasure cosmological parameters (like the geometry ofthe universe and the Hubble expansion rate) which tell usmore about how the universe evolved to i ts presentstructure. The CMBR power spectrum is very smooth, butwith a closer look there are small fluctuations in theradiation corresponding to temperature differences on theorder of 10‘’ The fluctuations in the intensity of theradiation could be evidence of structure (varying density)in the dense soup of the early universe. To investigate

“Testing Millimeter Wave Detectors in LowBackground Conditions”

REU Intern:Sandra Sattler Columbia University

Principal Investigator:John Ruhl Physics, UCSB

this structure, these seeds of galaxies, and to learn howthe universe as we know it formed, we investigate theCMBR to a very high accuracy. One tool that we use toinvestigate the CMBR is a photon detector. Thesedetectors come in many shapes, sizes, and flavors, but forthe application of “looking” at the CMBR, they must bevery sensitive at millimeter wavelengths. This paperdescribes a method of testing various millimeter wavedetectors. To test the sensit ivity of the detectors ablackbody device was built. This device was mountedonto a 4 K stage of a dewar with a heater and athermometer attached. The blackbody load was designedto minimize thermal conduction in a low-temperaturesystem. It was also designed to maximize the blackbody’sability to absorb radiation. The device was then heatedfrom 4 K to 10 K to radiate onto a detector. Comparisonscan then be made between the radiation emitted from theblackbody and the radiation absorbed by the detector.

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UCSB Participant

To understand the significance of Magnetic ForceMicroscopy, i t is beneficial to be reminded thatmagnetism is a large part of commercially availabletechnology. For example, magnetism is used to storedata in computers by aligning magnetic dipoles on astorage disk. Disks are coated with a magnetic thin filmand data is stored in binary language as either a zero or aone. These zeros and ones can be stored by setting aorientation of a magnetic dipole for each number. Thestorage density has limits because of both the recordingprocesses and the film itself. There are limits to the sizeof the dipoles because there are too many magneticinteractions on the film and the recording process takesup a significant amount of room on the recording mediafor markers to track the information

Characteristics of nanometer-size magnets can now beseen with the Magnetic Force Microscope (MFM). Thisgives a more practical insight on small magnets.Providing external magnetic f ields to samples andimaging them shows how they behave. Withunderstanding of magnetism this small there can be newbreakthroughs in magnetically dependent engineering andproducts.

This summer I studied generic behavior of sub-micronmagnets, what the MFM does and how it works, andeventually connected the two and studied a nickel sampleto understand its properties. A variety of samples wereinitially studied with the MFM to determine what wouldbe a good test sample. Eventually, the nickel sample wasdesignated for extensive research. The sample wasimaged in a variety of magnetic fields. The next step wasto damage the sample so to see the impact on domainwall movement in increasing magnetic fields.

It was excit ing to learn of new technology that ispossible due to magnetic force microscopy. Devices suchas a Quantum Magnetic Disk (QMD) use nanoscopicparticles to magnetically store information. A QMD ismade up of nanoscopic magnetic pillars embedded intonon-magnetic material. A standard QMD has a storagedensity a couple of magnitudes higher than the disks usednow because the writing process is quantized. Transitionnoise is minute because there are only two possible statesfor each bit, and tracking is better since there are nowphysically recognizable regions between bits.

“Sub-Micron Magnetism”

REU Intern:Joshua Stamp Hanover College, IN

Principal Investigators:Dr. David Awschalom, Mark Field Physics Dept, UCSB

Nonmagnetic

Magnetic

Figure: Quantum Magnetic Disk.

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Index of Interns

and

Mentors

A

Akunyili, Ijeoma ........................................ 31Ast, Dieter ................................................. 15Awschalom, David .................................... 65

B

Batt, Carl ..................................................... 7Beyder, Arthur ........................................... 35Blakely, Jack ............................................. 18

C

Carr, Dustin ............................................... 24Cawood, Benjamin ................................... 29Chandler, Leeann ....................................... 3Chigirinskaya, Yelena ................................. 7Chong, John ............................................... 3Collier, Veronica ........................................ 30Craighead, Harold ............................... 7, 24Cross, John .............................................. 43Culkin, Daniel ............................................. 9

D

Davis, Robert ............................................ 37Deal, Mike ................................................. 50Demaree, Dedra ....................................... 26DenBaars, Steve ...................................... 57

E

Eastman, Lester ....................................... 26Engstrom, James ........................................ 9

F

Fan, Rong ................................................. 44Field, Mark ................................................ 65Finlan, Mark .............................................. 45Fleming, Brandon ..................................... 54Fonash, Stephen ...................................... 36Frame, Mary ............................................. 12Fuhrman, John ......................................... 57

G

Ganem, Bruce ............................................ 3Garcia, Jorge ............................................ 60Giannelis, Emmanuel ............................... 26Griffin, Peter ............................................. 54Grigsby, Franklin ....................................... 10

H

Hadeed, Farouk ........................................ 46Hammer, Charlie ....................................... 58Harris, Gary .............................................. 31Harris, James .................................... 47, 49Harris, Mesha ........................................... 30Hebert, Keith ............................................. 36Heseltine, Mary ......................................... 12Hiller, Laurance......................................... 15Hu, Evelyn ................................................ 62Huang, Chih-Wei Maria ............................ 47

I

Ibbetson, James ....................................... 61Imamoglu, Atac ......................................... 59

J

Jantzi, Nicole ............................................ 48Jones, Kimberly ........................................ 30Joshi, Mayur ............................................. 45

K

Kennedy, Michael ..................................... 58Kim, John .................................................. 49Krusius, J Peter ........................................ 26

M

MacDonald, Noel ................................. 3, 12Maloney, Craig .......................................... 59McVittie, James ................................. 44, 45Mishra, Umesh ......................................... 61Mitchell, Brittany ....................................... 50Molnar, Paul .............................................. 60Moran, Brendan ........................................ 18

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N

Neiman, Brad ............................................ 37Ness, Kevin .............................................. 61Nixon, Rhasheda ...................................... 39

O

Ober, Christopher ..................................... 20Owings, Matthew ...................................... 26

P

Paloczi, George ........................................ 51Petroff, Pierre ........................................... 60Plummer, James ......................... 43, 50, 53Popham, Jennifer Mei-An ......................... 62Pyonin, Adam ........................................... 52

R

Rogosky, Michael ..................................... 39Rouch, Amber ............................................. 3Ruhl, John ................................................ 64

S

Sackett, Brett ............................................ 63Saraswat, Krishna .................................... 52Sattler, Sandra .......................................... 64Sherwin, Mark ........................................... 63Shimbo, Barden ........................................ 49Sierakowski, Andrej .................................. 53Spencer, Michael ...................................... 29Stamp, Joshua .......................................... 65Su, Mark ................................................... 63Sundararjan, Narayan .............................. 20Suzuki, Yuri ............................................... 10

T

Tarng, Bennet ........................................... 26

W

Wang, Shan .............................................. 46Wilder, Kathryn ......................................... 51Winograd, Nicholas .................................. 35

Y

Yoon, Seok-Beom ..................................... 15Young, Darron .......................................... 62

Z

Zasadzinski, Joseph ................................. 58Zdeblick, Mark .......................................... 48Zhou, Xinyi ................................................ 20Zwald, Mark .............................................. 24

REU Intern Photos

The 1997 NNUN REU Convocation ......... iivCornell Nanofabrication Interns .................. 1Howard University Interns ........................ 27Pennsylvania State Interns ....................... 33Stanford Nanofabrication Interns .............. 41UCSB Interns ............................................ 55

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