The devices we study involve thin films of superconductors and normal metals deposited with a variety of techniques onto substrates, typically oxidized silicon. The methods for patterning and depositing the films are similar to techniques in the microelectronics industry. For large-scale features, we use photolithography, where a polymer resist is exposed optically to define a particular pattern. This technique can produce features with linewidths down to the micron scale. To transfer submicron patterns, necessary especially for the small-capacitance tunnel junctions in the quantum coherence experiments and narrow channels in vortex dynamics measurements, we use electron-beam lithography, in which a Scanning Electron Microscope (SEM) is employed to write patterns in a polymer resist. With electron-beam lithography, it is possible to produce feature sizes below 100 nm.
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SEM image of Al-AlOx-Al tunnel junctions patterned at the Cornell Nanoscale Facility with electron-beam lithography. |
Atomic Force Microscope (AFM) image of weak-pinning channel ratchet patterned with electron-beam lithography at the Cornell Nanoscale Facility and transferred with reactive ion etching into a bilayer film of weak-pinning a-NbGe and strong-pinning NbN. |
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We perform all of our lithography in the state-of-the-art laboratories at the Cornell NanoScale Facility (CNF), about a one-hour drive from Syracuse. We pattern our large-scale features with photolithography and perform various etch processes also at the CNF. We have constructed a dedicated electron-beam evaporation system in our lab here at Syracuse to deposit the aluminum films for the Al-AlOx-Al tunnel junctions. We also have an optical microscope for sample inspection and a wirebonder for making contacts to the devices.
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| Electron-beam evaporation system. | Westbond 7400A wirebonder. |
Almost all of the measurements in our lab are performed at temperatures close to absolute zero. For any superconducting device, the system must be cooled below the transition temperature of the particular superconductor: 9K for niobium, 6K for a-MoGe, achievable with simple probes immersed in liquid helium-4; 1K for aluminum, achievable with a helium-3 cryostat -- we purchased such a system from Janis and we now have it running in our lab.
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| Helium-3 insert from Janis Research | First cooldown of Helium-3 system -- base temperature = 230 mK |
Many of our measurements, in particular, the quantum coherence studies, require even lower temperatures in order to reduce the thermal energy below the other relevant energy scales. These experiments take place in a dilution refrigerator at temperatures below 50 mK. It is a challenge to measure microfabricated circuits at these low temperatures and to ensure that noise and interference from room-temperature electronics do not travel down the electrical leads in the refrigerator and perturb the circuit. Thus, extensive amounts of filtering, attenuation, and shielding are required. At the end of the summer of 2007, we finished building our new dilution refrigerator, following the highly successful design of John Martinis at UC Santa Barbara. This fridge provides a large region cooled to the base temperature and has substantial space available for many coaxial lines and electrical filters. We had our first successful cooldown in September, reaching a base temperature of 30 mK.
Right: dilution refrigerator with vacuum jacket and heat shields removed. Lower right: fridge running with jackets and shields in place. Lower left: gas-handling system and still pumping line. |
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