photolithographic feature fabrication in ltcc · can show that it is possible to fabricate devices...

© International Microelectronics And Packaging Society

The International Journal of Microcircuits and Electronic Packaging, Volume 23, Number3, Third Quarter 2000 (ISSN 1063-1674)

Intl. Journal of Microcircuits and Electronic Packaging

286

Photolithographic Feature Fabrication in LTCCPatricio Espinoza-Vallejos and Jorge Santiago-Avilés200 South 33rd StreetMoore Building, Electrical Engineering DepartmentUniversity of PennsylvaniaPhiladelphia, Pennsylvania 19104Phone: 215-898-1858Fax: 215-573-2068e-mails: [email protected], and [email protected]

Abstract

The authors have developed a patterning technique for the fabrication of meso-scale (100 µm to centimeters) three-dimensionalstructures utilizing Low Temperature Cofired Ceramics (LTCC). A dry negative resist film (Riston®) is used to pattern partiallysintered LTCC. The researchers also have analyzed the surface of tapes treated from 550oC to 900oC using x-ray photoelectricspectroscopy. In addition, they studied the different stages of the process: partial sintering of the glass-ceramic composite tape, dryresist lamination to the partially sintered tape, UV exposure of the dry resist, developing, and etching using buffered hydrofluoric acid.The authors have also demonstrated the suitability of the technique by fabricating three-dimensional structures such as an electro-phoretic channel and thick-conductor coils. Stacking and fully firing the patterned partially fired tapes yields homogeneous bodieswhere the former layers are indistinguishable. Note that the technique is compatible with thick film technology.

Key words:

Low Temperature Cofired Ceramics, LTCC, Photolithography, andPatterning.

1. Introduction and Background

The LTCC system consists of a cast tape, thick film pastes forconductors, resistors, and the recently introduced technology ofDiffusion Patterningä and Photo Imagable dielectrics1. Thesesystems have been in use since the early 80’s and lately havefound new impetus from high reliability applications to wirelesstelecommunications, automotive electronics, mass data storage,and medical devices2 . The LTCC is a glass ceramic compositeof Pb, Ca, Al borosilicate glass, alumina, and an organic binder.These glass-forming oxides have high electrical resistivities andfrequency-independent dielectric constants3.

With this work, one can attempt to further extend the capa-bilities and use of these systems. One of the main advantages ofLTCC is that it permits the fabrication of 3D structures that canbe used for a multiplicity of fluidic applications, sensing4, and

actuation5. However, the researchers found difficulties duringthe fabrication of 3D-devices. One of the problems was the sag-ging of suspended structures. The other, that there is no suitablebatch technique for the patterning of the LTCC tapes. The re-searchers, as well as others, has addressed the first issue usingsacrificial materials6,7,8. This paper presents a solution to thesecond problem, one that is based on a technology originallydeveloped for the PCB industry.

This technology can be considered an extension of MEMS tothe meso-scale. By increasing the minimum feature size, onecan show that it is possible to fabricate devices that are as usefulin the meso-scale as those in the micro scale at a lower cost.LTCC can yield hermetic devices allowing the fabrication ofmyriad fluidic systems. Although biocompatibility of the tapehas not been reported, its major constituents are biocompatible9,10.Additionally, the material’s electrical properties do not dramati-cally change with temperature as happens with silicon.

The technique for patterning this material utilizes the sameinstrumentation as in silicon lithography. One of the advantagesof this approach is that the process operates in the batch mode.Chemical etching allows partial etching of the substrate, which ismuch harder to achieve with traditional techniques like punchingor drilling. In addition, one does not need high resolution whenpatterning LTCC, an inexpensive version of the silicon photoli-thography can be used.

Photolithographic Feature Fabrication in LTCC


© International Microelectronics And Packaging Society 287

2. Photolithography for the LTCCPatterning

The use of a photolithographic technique has the advantageof allowing batch processing. Therefore, it reduces the cost ofproduction when compared with serial techniques such as punch-ing; routing, ablative lasers, IR (thermal) lasers, and abrasiveblasting. The goal is to present a batch fabrication method basedon a laminable dry resist (instead of spinning on a liquid resist,as in silicon photolithography). The dry resist offers a thick uni-form layer, which is extremely convenient and harmonious withthe LTCC process technology. After patterning partially sin-tered blanks, they can be fully fired forming a three-dimensionalstructure. The option of stacking and fully firing of partiallysintered layers was demonstrated for the fabrication of a pressuresensor by this group of researchers11. This technique has alsoallowed the group to fabricate smooth curves and complex pat-terns by simply changing the photo-mask design. Note that thecurve profiles will be smoother than the one fabricated by, forexample, punching.

3. Definition of Partially Sintered LTCC

In order to achieve the photolithographic process, the authorshave developed a new pre-processing of the tapes: Partial Vis-cous Sintering. The LTCC blanks must be stiff enough to allowhandling; however, they should be suitable for etching. Althoughetching a ceramic would be a very difficult task, the researchersare able to do it since they are dealing with a glass-ceramic com-posite and the glass can be subjected to etching. As shown inFigure 1, the more advanced the viscous sintering process is, themore the alumina grains are covered with a glass coating. Weremove the tapes at an early stage of the viscous sintering pro-cess. Although a small amount of mass transfer has occurred,the material is rigid and manageable. Since the glass frit compo-sition is rich in silica, and silica is readily removed by hydrofluo-ric acid, we were led to experiment with this idea for chemicalmachining.

R2 = 0.9588

R2 = 0.9511

0

10

20

30

40

50

60

70

80

500 550 600 650 700 750 800 850 900 950

Firing Temperature [°C]

Ato

mic

%

Al

Si

Figure 1. Surface concentration aluminum and silicon as afunction of firing temperature. Data obtained using x-rayphotoelectric spectroscopy.

Figure 2 shows primary electron micrographs; they show thechange in structure as the temperature rises. Nominally, thesetapes are fired at 850°C or 870°C; however, we have reduced thetemperature to 810°C. The selection of this temperature is acompromise between mechanical strength, etchant reactivity andresist-adhesion. At this point, a thin glass neck (see Figure 3)that can be readily etched connects the grains.

Figure 2. Composition micrographs of LTCC tapes fired at8 different temperatures. Light tones indicate lead (82) andsilicon (14) rich species. Dark tones indicate Aluminum.The glass flows at a temperature in the 700 to 750°C interval.The bright glassy regions can be etched upon diluted HFreleasing the filler grains.

Alumina Grains

Glass Neck

Figure 3. During sintering, the alumina grains are bondedto each other through a silicate glass neck.




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4. The Use of Riston® Resist

Riston®12 , 13 is a dry resist that can be laminated and thenexposed using UV radiation. It is used to pattern partially sin-tered LTCC. The exposure system has the energy spectrum pro-file characterized by the intensities given in Table 1.

Table 1. Characterization of the exposure system.

Wavelength[nm]

Intensity[mW/cm^2]

310 3.2365 12.5380 5.2400 14.5

Riston® 9015, which behaves as negative resist, is a productof DuPont and has proved to be an excellent solution for the taskof patterning partially sintered ceramic14. This dry photoresist isable to withstand an HF etching solution for several hours.

Two thicknesses of the dry resist were used. The thicker onewas applied to the back side in order to maximize protection ofthe tape surface from the acid. The thinner one was more diffi-cult to apply but yielded better results for small feature sizes.Although DuPont originally developed these resists for pattern-ing copper, they show a good adhesion to green tapes and alu-mina substrates. A peel test was attempted with 99% aluminasubstrates. However, the film-substrate bond was stronger thanthe film tension rupture limit, making it difficult to measure theadhesion due to failure of the film.

5. Process Description

5.1. Partial Sintering

The first step is partial sintering of all the blanks, even thosethat are not going to need patterning. This permits simultaneouspost firing of all the blanks after patterning. Two partially sin-tered profiles have been experienced; 2-min partial sinteringand 10-min partial sintering. In the first case, the sintering fur-nace set point was kept at 850°C for 2 mins; in the second case,it was kept at 850°C for 10 mins. The idea with a longer firingtime is to obtain smoother walls as the glass has flowed to alarger extent. However, the lamination was found to be trouble-some for the 10-min partially sintered tapes.

5.2. Dry Resist Lamination

The most critical step in the process (Figure 4) is laminationof the dry resist film to the glass-ceramic substrate. Lamination

without air bubbles must be achieved; where there is an air bubble,a defect will appear during etching. The lamination temperatureis between 90°C and 130°C.

Stack blanks and fire them

Strip Photoresist

Etch pattern

Post Exposure1000 mJ/cm^2

Dry with hot air

Developing64°F Sodium Carbonate

Exposure 40 to 100 mJ/cm^2

Film laminationTemperature 100°C to 130°C

Clean and dry the partially sintered tapeto allow good adhesion

Figure 4. Cleaning, lamination, and exposure steps weredone in a clean room nominally class 10,000 underlaminar flow hoods. Lamination was done using a 4” GBSlaminator.

When one attempt to laminate 10-min partially sintered tapeblanks, bubbles appear, making it difficult to test resolution as afunction of firing temperature. With more careful control of thetemperature (90°C), it is possible to achieve successful lamina-tion. However, resist delamination occurs consistently duringthe subsequent wet-etching process. Consequently, 2-min par-tially sintered tapes was used for the fabrication of devices.

5.3. Developing

For developing a breakpoint of 65% is preferred, that is, thefeatures must appear developed after the 65% of the developingtime; this assures that there is no residue after the developingprocess has been completed. The developer is dispensed througha showerhead at a pressure of 30 psig and temperatures rangingbetween 21°C and 30°C. An even flat fan nozzle is used in thespray-head. In order to obtain uniform developing, the nozzle isconnected to a moving arm. For development uniformity, thepartially sintered tape was on a rotary station described else-where14. When utilized in the etching of copper PCB, DuPontrecommends rinsing with water after developing with sodiumcarbonate. However, when Riston® is used with partially sin-tered green tapes, water tends to produce delamination andwrinkles on the edges. Hence, one would immediately dry withhot air after developing.

Developing time is important. It should be long enough toallow the removal of the unexposed photoresist but short enoughto avoid peeling. In order to determine the required developingtime, one can measure the thickness of the residual photoresistas a function of time.




Figure 5 clearly shows that the resist removal progression hastwo regimes. In the early stages, the actual removal occurs. To-wards the end, the variation in resist thickness is small due to thefact that most of it has been already washed away. Therefore, itis appropriated to fit one curve to each regimen. The intersectionof these curves is used to determine the time at which all theresist has been removed. The film is nominally of a thickness of38.1µm (1.5 mils). It can be observed that there are no filmresidues for developing times larger than 24 seconds. The aver-age of the last five samples is 38.6 µm, a value that is in confor-mity with the nominal thickness.

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42

Time

[s]

Re

mo

ve

d R

es

ist

Fil

m

[m

]

\

T = 10.164t^0.5 - 16.289

R2 = 0.9599

38.4 µµµµ m

Figure 5. Removed dry resist film as function of thedeveloping time. All the film has been removed after 29seconds.

From the profilometer data, a developing time of 27 secondsappears to be enough to fully remove the unexposed resist. How-ever, Figure 6(a) reveals that a longer time is required. Figure6(b) shows a fully developed pattern after 30 seconds under thesodium carbonate shower. The line width is very consistent withthe photo-mask feature size.

200 µµµµm

200 µµµµm

a) b)

Figure 6. From the profilometer data, 27 seconds appears tobe enough to develop the pattern. However, a photograph(a) reveals that a longer developing time in required. Thesecond picture (b) shows a fully developed pattern after 30seconds under to the sodium carbonate shower.

Etching Rate of Partially Sintered Tape

d = - 2.4198t + 120

y = - 4.9825x + 113.25

- 20 0

20 40 60 80

100 120 140

0 10 20 30 40 50 60

Time [min]

HF: Ammonium Fluoride (1:4) at 90 ° C HF:H2O (1:6) at 60 ° C

Etching Rate of Partially Sintered Tape

-

d = - 4.9825t + 113.25

- 20 0

20 40 60 80

100 120 140

0 10 20 30 40 50 60

Time [min]

Th

ickn

ess

[µ [µ[µ[µm

] ]]]

HF: Ammonium Fluoride (1:4) at 90 ° C HF:H2O (1:6) at 60 ° C

d

t

Figure 7. Note that the diluted solution is more reactive,even though the temperature and concentration aresmaller than for buffer HF.

5.4. Etching

For DuPont series 951-AX tapes (254 µm or 10 mils unfired),the etching rate is parabolic and therefore controlled by diffu-sion11. However, when measuring thinner tapes, 951-AT(114.3µm or 4.5 mils unfired), the relationship between thick-ness and time is linear. When one extend the firing period, theetching rate still has a linear relationship with time as shown inFigure 8.

d = 0.5553t

R2 = 0.8982

0

10

20

30

40

50

60

0 20 40 60 80 100

[m ins]

[mic

ron

s]

Figure 8. Etching rate of 10 min partially sintered tape(850°C). Solution: Buffer HF 1:4 (HF 49%: AF 40%) at58°C. Substrate: 114.3 µm (4.5 mils) tape.

5.5. Stacking and Full Firing

The etched pattern must include four holes for alignment ofthe layers of the three-dimensional structure. Alumina and/orgraphite alignment pins are inserted in each of the alignmentholes. In order to fully fire the partially sintered tapes, it is notnecessary to include the 350°C segment of the nominal heatingschedule. Therefore, the temperature is increased at 10°C/minup to 850°C or 875°C, depending on the conductor paste being




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used, held for 30 minutes and allowed to cool at a 10°C/min rate.The researchers are able to fully sinter tapes after partially

sintering. This allows for the fabrication of three-dimensionaldevices. At first, the authors thought that it would be necessaryto add some adhesion promoter in order to achieve hermetic seal-ing. Therefore, they attempted adding glass frit of the same com-position found in the tape. Optical micrography results showedthat the glass frit produces large gaps between layers. However,if one just used a slight pressure (0.5 ounces per square inch)during firing, the bonding is completed. The interface betweenthe two original layers is indistinguishable by optical micros-copy, as shown in Figure 9.

Figure 9. Optical micrographs of stacked and fully firedstructures (black background has been removed to improvecontrast). On the left micrograph, two blanks of partiallysintered fired tapes have been fully fired using a glass topromote adhesion. Note a circular defect at the right of center.On the right micrograph, the glass was not used yieldingsatisfactory results.

Using a similar procedure we can laminate a glass to a fullyfired LTCC tape. Blanks of the corning 8511 glass and blanks ofthe green tapes can be bonded together due to matching of theirCTE: 5.7 ppm/°C for the glass, and 5.8 ppm/°C for the greentapes. After fully firing the green tapes, the blanks of glass andtapes are stacked and the bonding process is accomplished byraising the temperature at a ratio of 10°C/min and holding it at750°C for 5 minutes, finishing with the usual cooling profile of –10°C/min.

Features with high aspect ratios must be laminated beforepartial sintering (step a) in order to avoid deformations duringthis stage.

6. Applications

6.1. Electrophoretic Apparatus.

The researchers patterned a 100µm electrophoretic channelin an attempt to develop a capillary electrophoretic channel. Inorder to have a significant electro osmotic flow, usually capillarytubes have sizes ranging from 100µm to 20µm. Other advan-tages of using a small diameter in electrophoresis devices are: i)Minimal variation in sample concentration, ii) Possibility of largeratios of capillary length to diameter, and iii) Better heat control.

Figure 10 shows two of the fabricated channels. Figure 10 (b)shows the results of exposing with 100µm feature size mask andunderexposing and overetching, one gets a triangular cross sec-tion feature with base dimensions of 600µm. Figure 11 (a) showsthe stylus profilometer pattern of the 600µm-wide channel. Notethat the channel is partially etched, a characteristic that cannotbe achieved with other available techniques such as punching orrouting. Due to the limited range of the available profilometer, itwas necessary to estimate the actual depth of the channel by ex-tending the side-wall profile, as depicted in Figure 11 (a) and(b). The estimated depth value is 70µm.

600 µm

Figure 10. Two different samples of the electrophrecticchannel, the first (a) is etched all the way through withsomewhat rough walls and the second (b) was etched 70 µm.

a)

b)

600 ìm

70 ìm 13o

100 ìm

Figure 11 (a). The extrapolation of the profilometer scanshow that the channel depth is about 70 µm. The walls areapproximately linear due to the isotropic etching. Theopening is 600 µm wide. (b) Scaled drawings of a transversalcut of the etched pattern: the angle of the etching is 13°; theopening is 6 times larger than the actual mask.

6.2. Patterns for Low Resistance Coils

Low resistance coils are needed in order to achieve efficientelectromagnetic actuators15. One alternative is to fabricate thickconductors. To this end, the authors have etched through 114.3µm (4.5 mils) tapes as depicted in Figure 12. This coil have beenfilled with conductor paste and then stacked and fully fired toform a three-dimensional structure.




Figure 12. A photograph of a spiral etched on a partiallysintered LTCC tape ready to be fully fired. the coil diameteris 2 cm and the smallest feature size (the coil strand) is 400mm.

7. Conclusions and Future Directions

The researchers have developed a novel photolithographicprocess combining various technologies such as photolithogra-phy, LTCC processing, and PCB patterning technology. Theauthors introduced the concept of partially sintered tape and foundthe pertinent processing parameters for the photolithography ofthe LTCC tapes. Several devices were fabricated with the tech-nique. This new technique adds flexibility to the fabrication ofcomplex three-dimensional patterns as it incorporates a myriadof materials and technologies: thick and thin film technologyand glass. One important application is the fabrication of three-dimensional fluidic structures.

The techniques presented in this work form the basis for thefabrication of LTCC meso-scale 3-D structures that can see ap-plications in the field of sensors and actuators, for fluidic sys-tems, and biotechnological devices. The researchers are cur-rently working in the utilization of this techniques for the devel-opment of a flow diverter valve, a pump, a manifold and sensorpackaging for a flow injection analysis system. Moreover, it isclear that similar methodology can be used to pattern other kindof tape cast materials such as those with magnetic, superconduc-tive, dielectric, piezo- and ferro-electric, electrostrictive, or con-ductive properties.

One cannot also forget the purely packaging applications ofthese techniques, such as in three dimensional packaging, MEMSpackaging, and self packaged LTCC based sensors and actua-tors.

References

1. S. Horowitz and C. R. Needes, C. R. S. “Smart Materials forHybrid Circuit and Multi chip Module” Proceedings JapanInternational Electron. Manufacturing Technology Sympo-sium, pp. 199-204, 1995.

2. M. O’Neill and C. Modes, “A Lead Free, Mixed Metals LowTemperature Cofired Ceramic Materials System”, Proceed-ings of SPIE-Int. Soc. Opt. Eng., Vol. 3235, pp. 512-516, 1997.

3. B. Chiou. “Dielectric Properties of Lead Borosilicate GlassesContaining Alumina”, Ts’ai Liao K’o Hsueh, Taiwan, Vol.16A, No. 1, pp. 37-41, 1984.

4. M.R. Gongora-Rubio, L.M. Solá-Laguna, M. Smith, and J.J.Santiago-Avilés, “The Utilization of Low Temperature Co-fired Ceramics (LTCC-ML) Technology for Meso-scale EMS,a Simple Thermistor Based Flow Sensor”, Sensors and Ac-tuators, Vol. 73, pp. 215-221, 1999.

5. M. Gongora, P. Espinoza-Vallejos, and J. Santiago-Aviles,“Overview of Green Ceramic Tape Technology for MST”, Sen-sors & Actuators, Submitted, 2000.

6. P. Espinoza-Vallejos, J. Zhong, M. Gongora-Rubio, L. Sola-Laguna, and J.J. Santiago-Aviles, “The Measurement andControl of Sagging in Meso (intermediate scale) Electrome-chanical LTCC Structures and Systems”, Proceedings of theMRS Symposium, Vol. 518, pp. 73-79, 1998.

7. K. Utsumi, “Development of Multilayer Ceramic ComponentsUsing Green-Sheet Technology”, Ceramic Bulletin, Vol. 70,No. 6, 1991.

8. L. Malatto, D. Filippini, S. Gwirc, and L. Fraigi, “A NovelProcedure to Use Green Tape as Low Thickness CeramicsStructures”, Anales de la Asociacion Quimica Argentina, Vol.83, No. 6, pp. 317-320, 1995.

9. K Lobel, “Ossicular Replacement Prosthesis,” in ClinicalPerformance of Skeletal Prostheses, eds. LL Hench and J Wil-son. New York: Chapman and Hall, 1986.

10. D.C. Greenspan http://www.devicelink.com/mddi/archive/99/03/011.html. An MD&DI March 1999 Column.

11. Jaiyoung Park, Patricio Espinoza-Vallejos, Heather Lynch,Jorge J. Santiago-Avilés, and Luis Sola-Laguna, “Meso-ScalePressure Transducers Utilizing Low Temperature CofiredCeramic Tapes”, Materials Research Society Fall 98 Sympo-sium Proceedings, Vol. 546, pp. 177-182, 1999.

12. DuPont Photopolymer & Electronic Materials, “Riston® 9000Series Data Sheet and Processing Information, TechnicalData,” DS94-02 (7/98) Rev. 2.0, 1998.

13. DuPont Printed Circuit Materials, “Riston® CM206 DataSheet and Processing Information, Technical Data,” DS97-38 (12/97) Rev. 3.0, 1997.

14. Patricio Espinoza-Vallejos, Clara Dimas, and Jorge Santiago-Avilés, “Lithographic Processing of LTCC Tapes”, Proceed-ings of SPIE The International Society for Optical Engineer-ing, Vol. 3906, pp. 664-669, 1999.

15. M.R. Gongora-Rubio, L.M. Solá-Laguna, M. Smith, and J.J.Santiago-Avilés, “A Meso-scale Electromagnetically Actuated




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Normally Closed Valve Realized on LTCC Tapes”, Proceed-ings of SPIE, Conference on microfluidic devices and sys-tems II, Santa Clara, California, SPIE Vol. 3877, pp. 230-239, September 1999.

About the authors

Professor Santiago-Avilés received his B.S. in Physics in hisnative Puerto Rico, and his Ph.D. in Materials Science and Engi-neering at the Pennsylvania State University in 1970. He joinedthe Electrical Engineering Department at the University of Penn-sylvania in 1985 where his research interest has developed alongthe lines of materials and devices for microelectronics, more spe-cifically, metallization schemes for interconnection, rapid ther-mal processing for silicides, and the problem of homogeneousSchottky barriers in epitaxial silicides. Currently, his electronicmaterials interests are meso-scale electromechanical systems uti-lizing low temperature cofired ceramic tapes. During the lastten years, he has been collaborating with scientist from institu-tions in South America in the field of bio-microsensors and meso-scale LTCC tapes based sensing and actuating devices.

Patricio Espinoza-Vallejos received his B.S. Degree in Elec-trical Engineering from The University of Chile in 1995. Afterworking in industry for about a year, he went to the University ofPennsylvania to pursue his Master of Science in Engineering(1998). His areas of interest are fluidic systems, sensors, actua-tors and techniques for patterning of Low Temperature Cofiredceramics. He is currently working in the last stages of his Ph.D.Dissertation at the University of Pennsylvania.

photolithographic feature fabrication in ltcc · can show that it is possible to fabricate devices...

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