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IEEE Transactions on Magnetics

Improved performance of tin variable-thickness superconducting microbridges

Miguel Octavio, William J. Skocpol, M. Tinkham

Published Jan 1, 1977• 39 citations
Publisher

Abstract

We have fabricated tin variable-thickness microbridges with which we have observed the ac Josephson effect to as high as 3.7 mV, corresponding to the 187th harmonic of X-band radiation. The dc I-V curves of the bridges show reduced hysteresis and strong subharmonic gap structure to very low temperatures. We use the gap structure as a local thermometer in order to demonstrate that the improved high-frequency performance correlates with the more efficient removal of Joule heat from these bridges.

DOI
10.1109/tmag.1977.1059325

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We have fabricated tin variable-thickness microbridges with which we have observed the ac Josephson effect to as high as 3.7 mV, corresponding to the 187th harmonic of X-band radiation. The dc I-V curves of the bridges show reduced hysteresis and strong subharmonic gap structure to very low temperatures. We use the gap structure as a local thermometer in order to demonstrate that the improved high-frequency performance correlates with the more efficient removal of Joule heat from these bridges.
The temperature and field variation of the photoresponse of tin Josephson film microbridges are studied in experiments in which the microbridges are irradiated in the centimeter and millimeter ranges. Experiments have been carried out under conditions that correspond to narrow-band and wide-band detection. Certain features of the response which are due to stimulation of superconductivity in the microbridges by the microwave radiation and a direct current are studied. In heterodyne detection these features disappear at a high power level of the heterodyne oscillator, and a substantial increase is observed in the response, this is caused by a higher differential resistance of the microbridge. The maximum sensitivity (approx.10/sup 4/ V/W) is achieved with selective detection.
All-NbN nanobridges with gap structure in I-V curves have been reproducibly constructed using RIE and lift-off techniques. The nanobridges had a width of 2 μm, and a thickness ofcRNproducts of ∼3 mV, and low excess current. Small-area dc SQUIDs were made using the nanobridges, and analyses of the response to magnetic flux were performed. The current-phase relationship of the nano-bridges was found to be close to sinusoidal. The maximum LC resonant voltage was about 1.2 mV, corresponding to a frequency of 580 GHz. The IF peak was obtained up to the bias voltage of about 4 mV in 101 GHz Josephson mixing.
Several reactive dry etching methods; ion etching, sputter etching and plasma etching, were studied in order to fabricate a niobium variable-thickness type thin film bridge (VTB). Niobium was etched by CF4 reactive sputter etching with submicron accuracy and no significant influence on the superconducting transition temperature except very near the surface (<200 Å). The niobium VTB behaved as an S-S'-S bridge, and a reduced self-heating effect was realized by a variable-thickness geometry. The DC Josephson effect was observed in the temperature range without thermal hysteresis. The quality factor of the I c R n product obtained was at least 0.42 mV.
A Nb superconducting microbridge is fabricated, 40-µm long, less than 0.25-µm wide and 0.1-µm thick. Four-terminal measurement of the I-V characteristics reveals hysterisis in the I-V curve. The microbridge has an excess current over its current in its normal state. Frequency resonance appears over the range of a few hundred kiloherz by electromagnetic wave radiation. The I-V characteristics show that critical current I c is depressed, and as radiation power increases I c decreases. This phenomenon is thought to be due to resonance of quasiparticles in the microbridge, and to resonance restricting the supercurrent.
Abstract We report the fabrication and properties of titanium nitride (TiN) nanobridge Josephson junctions (nJJs) and nanoscale superconducting quantum interference devices (nanoSQUIDs) on SiN-buffered Si substrates. The superior corrosion resistance, large coherence length, suitable superconducting transition temperature and highly selective reactive ion etching (RIE) of TiN compared to e-beam resists and the SiN buffer layer allow for reproducible preparation and result in long-term stability of the TiN nJJs. High-resolution transmission electron microscopy reveals a columnar structure of the TiN film on an amorphous SiN buffer layer. High-resolution scanning electron microscopy reveals the variable thickness shape of the nJJs. A combination of wet etching in 20% potassium hydroxide and RIE is used for bulk nanomachining of nanoSQUID cantilevers. More than 20 oscillations of the V(B) dependence of the nanoSQUIDs with a period of ∼6 mT and hysteresis-free I(V) characteristics (CVCs) of the all-TiN nJJs are observed at 4.2 K. CVCs of the low-I c all-TiN nJJs follow theoretical predictions for dirty superconductors down to ∼10 mK, with the critical current saturated below ∼0.6 K. These results pave the way for superconducting electronics based on nJJs operating non-hysteretically at 4.2 K, as well as for all-TiN qubits operating at sub-100 mK temperatures.