S.-K. Pan

Low‐noise 115‐GHz receiver using superconducting tunnel junctions

A 110–118‐GHz receiver based on a superconducting quasiparticle tunnel junction mixer is described. The single‐sideband noise temperature is as low as 68±3 K. This is nearly twice the sensitivity of any other receiver at this frequency. The receiver was designed using a low‐frequency scale model in conjunction with the quantum mixer theory. A scaled version of the receiver for operation at 46 GHz has a single‐sideband noise temperature of 55 K. The factors leading to the success of this design are discussed.

Fabrication of Nb/Al-Al/sub 2/O/sub 3//Nb junctions with extremely low leakage currents

Nb/Al-Al/sub 2/O/sub 3//Nb trilayer films were deposited using DC magnetron sputtering guns in a UHV (ultrahigh vacuum) system which is capable of 5*10/sup -10/ Torr. SIS (superconductor-insulator-superconductor) junctions as small as 3.2*3.2 mu m/sup 2/ were isolated from the trilayer by standard photolithography. The junctions typically have V/sub m/=70-90 mV at 4.2 K, while at 2.0 K, V/sub m/ is as large as 1 V. This corresponds to a subgap current of 0.15% of the quasiparticle current rise. The subgap leakage current is compared to the predictions of the BCS (Bardeen-Cooper-Schrieffer) theory. The specific capacitance is preliminarily measured to be 45+or-5 fF/ mu m/sup 2/. >

Infinite available gain in a 115 GHz SIS mixer

Abstract We report the observation of infinite available conversion gain at 115 GHz with an SIS mixer. The output impedance can be smoothly changed from positive, through infinite, to negative. The mixer consists of a series array of N = 14 thermally recyclable Pb-alloy tunnel junctions in a reduced-height waveguide mount. The best mixer noise temperatures measured are consistent with a quantum noise temperature of Nħω/k.

SIS mixer analysis using a scale model

In this paper measurements on a 115 GHz SIS mixer are shown to be in good quantitative agreement with the predictions of the quantum theory of mixing. A 40 × scale model of the mixer block was used to determine the experimental embedding impedances.

Fabrication of micron size Nb/Al-Al/sub 2/O/sub 3//Nb junctions with a trilevel resist liftoff process

A trilevel resist, consisting of polyimide planarization, SiO/sub 2/ barrier, and photoresist, was used to pattern junction areas in Nb/Al-Al/sub 2/O/sub 3//Nb trilayer films. After reactive ion etching to define the junction areas, the perimeter of the junction was revealed, and excellent liftoff structures were defined with an oxygen plasma shrink of the exposed polyimide sidewalls. A subsequently deposited insulation layer seals the sides and the top surface along the perimeter of the Nb counter electrode button. High-quality superconductor-insulator-superconductor (SIS) junctions with diameters as small as 1.2 mu m and V/sub m/ as large as 1500 mV at 2.0 K have been fabricated. An SIS receiver using these junctions with integrated tuning elements has a DSB noise temperature of 58 K at 230 GHz. This is believed to be the lowest receiver noise temperature ever reported at this frequency.

Nb/Al-AlOx/Nb edge junctions for distributed mixers

We have fabricated high quality Nb/Al-oxide/Al/Nb edge junctions using a Nb/SiO/sub 2/ bi-layer film as the base electrode, suitable for use as traveling wave mixers. An edge is cut in the bi-layer with an ion gun at a 45 degree angle using a photoresist mask. The wafer is then cleaned in-situ with a physical ion gun clean followed by the deposition of a thin Al (a1) film, which is then thermally oxidized, an optional second Al (a2) layer, and a Nb counter electrode. It was found that devices with an a2 layer resulted in superior electrical characteristics, though proximity effects increased strongly with a2 thickness. The counter electrode is defined with an SF/sub 6/+N/sub 2/ reactive ion etch, using the Al barrier layer as an etch stop. The Al barrier layer is then either removed with an Al wet etch to isolate the individual devices, or the devices are separated with an anodization process. Various ion gun cleaning conditions have been examined; in addition, both wet and plasma etch bi-layer edge surface pre-treatments were investigated. It was found that edge junctions with large widths (i.e., those more suitable for traveling wave mixers) typically benefited more from such treatments. Initial receiver results at 260 GHz have yielded a DSB noise temperature of 60 K.