Fabrication of Mesa Structures by X-Ray Lithographycsrri.iit.edu/~segre/phys570/13F/presentations/saglam.pdf · Hello everyone, I am Hilal Saglam. These are our group memebers; Yasemin
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Fabrication of Mesa Structures by X-Ray Lithography Hilal SAGLAM 1 1 Illinois Institute of Technology, Chicago, U.S.A PHYS 570-Introduction to Synchrotron Radiation Fall 2013
Fabrication of Mesa Structures by X-Ray Lithography
Hilal SAGLAM1
1Illinois Institute of Technology, Chicago, U.S.A
PHYS 570-Introduction to Synchrotron Radiation Fall 2013
Presenter
Presentation Notes
Hello everyone, I am Hilal Saglam. These are our group memebers; Yasemin Demirhan is a PHd student at IZTECH, Prof. Ozyuzer is my advisor and Prof Kadowaki is providing us Bi2212 crystals from Japan. And I will talk about are dependence of Josephson Criitical current density in superconducting Bi2212 mesas for terahertz emission.
Outline Terahertz Radiation
Applications
High Temperature Superconductors
Bi2Sr2CaCu2O8+δ (Bi2212)
Objective of This Work
Mesa Fabrication
Self Heating Problem
X-Ray Litrography
Conclusion
Presenter
Presentation Notes
Here is my outline. i am going to talk about THz radiation (sources and applications), High Temperature superconductors (AC Josephson Effect, Josephson junctions, Bi2212 single crystal and Doping dependence). Then i will briefly explain objective of this work. After that i will mention about the mesa fabrication process ant results. Finally I will finish my presentation by conclusion.
Terahertz Radiation
Non-ionizing Chemical and biological fingerprint Penetrates fabrics, plastics, cardboard Absorbed by water …..
0.1THz – 10 THz 3 mm – 0.03 mm λTHz > λX-Ray
There is still lack of compact solid state THz sources whose emission properties; Coherent Continuous, powerful output Tunable frequency
Presenter
Presentation Notes
Terahertz radiation is a part of the electromagnetic spectrum lying between microwaves and the far-IR. This region has frequencies between 0.1-10 THz and wavelengths from 3 mm to 0.03 mm. Unlike high energetic X-rays, T-rays are non-ionizing radiation, like radio waves or visible light, so people exposed to terahertz radiation will not suffer. THz matches in frequency the motion of an enormous range of chemical and biological molecules, so it can be used as chemical and biological fingerprint. Furthermore, it penetrates through many common materials, such as, fabric, plastics, cardboard, thin walls and paper. Although terahertz radiation does not penetrate through water, this is important for some certain applications. But still there is lack of THz compact solid state sources whose emission properties, coherent continious and powerfull output and tunable frequency.
Applications of Terahertz Radiation
Biology Materials characterization Medicine Security
Presenter
Presentation Notes
Because of the important properties, there are wide application areas of THz radiation such as biology, material characterization, medicine and security. it can be used to detect chemical and biological terror materials in package, to find hiden weapons, and detect the human body diseases with a safer system by using imaging method.
Bi2Sr2CaCu2O8+δ crystal
Generally formulated Bi2Sr2Can-1CunO2n+4+δ
Three phases well known Tc
1. Bi2Sr2CuO6+δ (Bi2201) ≤ 20 K
2. Bi2Sr2CaCu2O8+δ (Bi2212) ≈ 95 K
3. Bi2Sr2Ca2Cu3O10+δ (Bi2223) ≈ 110K
Layered structure
Intrinsic Josephson Junction with thickness of 1.5 nm
High critical temperature Tc ≈ 95 K
Large crystals are available
High anisotropic electrical behavior
Wide oxygen doping range
1 mV = 0.483 THz
Presenter
Presentation Notes
The main principle of generation of terahertz radiation is ac Jpseohson effect. And two superonductors are seperated by a thin insulating layer is called Josephson junction and when we applid voltage to the junction, the junction radiaite at a certain frequency which is called as Josephson effect and here this frequency is also called as Josephson frequency. Here the supercondcutor is Bi2212 single crystal. Bi2212 cryatal has some advanyages such that they are layer structure, large crystal is availabele ant have Htc at about 95 K. Here the thichkness of superconducting CuO2 layer is about 0.3 nm while the thickness of the insulating layer is about 1.2 nm so it means that each junction has 1.5 nm thichness.
L. Ozyuzer et al., Science 318, 1291 (2007)
M. Tsujimoto et al., Journal of Applıed Physıcs 111, 123111 (2012) T. M. Benseman et al. Preprint. 2013
Recent studies…
Presenter
Presentation Notes
Here I would like to mention about some recent studies . Firstly, Ozyuzer et al. showed that rectangular shaped mesa structures Bi2212 single crystal can be used as coherent continues solid state terahertz sources. They achieved to obtain 0.5 microwatt terahertz radiation at 0. 85 teraherz frequency. Then, recently, Tsujimoto showed that terahertz radiation can be used as a source for imaging applications. They have used a razor blade and japanese coin inside a brownish paper and managed to image them by terahertz radiaiton as seen in the figure. After that Benseman et al. obtained 600 microwatt terahertz radiaiton by synchronizing three mesa structures at the same time. So we can state that there is a growing interest in all over the world towards terahertz science ant technology.
Mesa Fabrication Annealing system O2 doping level in Cu-O
planes change: Number of the charge carriers
in HTSs. Critical Temperature Tc. Energy Gap. …
E beam writing
Cleaving
Sapphire substrate
Bi2212 crystal
UV rays
Developer Distilled water 25 sec.
Presenter
Presentation Notes
Oxygen doping level in Cu-O planes change the number of the charge carriers in HtSs, critical temperature and energy gap. So, to obtain doped crystals, we annealed the crystals at 425 C by using furnance under vacuum. Then Bi-2212 is glued onto a sapphire substrate from its smooth a-b surface by silver epoxy . In order to get a fresh and smooth surface on Bi2212, the crystal was then cleaved with an scotch tape. For fabrication by e-beam litrography we design the triple mesa structures wıth various dimensions by e line program. Firstly sample is exposed to 5 kV e-beam to form desired triple mesa structures on the same chip. After that sample is placed under UV ligth for 7 seconds. Then 0.2 M NaOH solution is used to devolop this triple mesa structure. Finally sample is hard baked in oven at 120 C for 30 minutes.
Mesa Fabrication Ar ion beam etching
CaF2 (200 nm)
Gold contacts
Au stripe (200 nm)
Presenter
Presentation Notes
After PR is patterned on 100 nm gold layer of Bi2212, sample is mounted in Ar ion beam etching system that accelerate the ion beam through the surface of sample to etch some unprotected area. So the mesa structure with required dimensions is obtained after the etching. This is the image of the Ion Beam Etching System. After that, because of the difficulties in making a contact on small area of the mesa, firstly CaF2 insulating layer is deposited by evaporation on small area of the mesa then lift-off technique is used with electron beam lithography to form gols stripes, Finally 30 µm width gold strpie is deposited by evaporation. Finally three gold wires are connected on two path and mesa by silver epoxy. The contacts are current and voltage leads. The final measurement configuration of triple mesa structure with 3 point probe can be seen in Figure.
The reasons for self-heating!
The poor thermal conductivity
Very large current density
Close proximity of multiple neighboring junctions
The result of the self-heating! Backbending in the I-V curves . High and sharp peaks in the conductance of intrinsic Josephson junctions in Bi2212.
Kurter et. al. Physical Review B 81, 224518 (2010)
Kurter et al.J Supercond. Nov. Magn. 24: 101–104 (2011)
Solution !
Need to fabricate smaller size mesas to understand the mechanism of
terahertz generation;
Significant reduction in heating
Much lower dissipation Kurter et. al. Physical Review B 81, 224518 (2010)
Method: X-ray lithography !
Short wavelength from X-rays : 0.4 - 4 nm
15 nm high resolution
No diffraction effect
Simple to use
No lens
Faster than e-beam lithography
Uniform refraction pattern
High resolution for small feature size
Conclusion
By e-beam lithography we are able to obtain mesas with different dimensions
on the same crystal.
We had heating effect issues which are not good for terahertz emission.
If we use smaller size mesas which eliminates the self-heating, we would be
able to better understand the mechanism to get high power and high frequency.
To do this: we need to use x-ray litrography since it provides very smaller size
