Michael E. Cyberey

Optical Spectroscopic Analysis of ICP Nitridation for SIS Junctions With AlN Barriers

High quality, low-leakage superconductor-insulator-superconductor (SIS) junctions with Nb electrodes and aluminum oxide barriers are widely reported in the literature, and have become integral in the design and fabrication of various superconducting circuits. However, as current densities are increased, aluminum oxide based tunnel barriers show excess leakage currents due to defects and pinholes in the barrier layer. First reported by our group in 2007, AlN tunnel barriers grown via inductively coupled plasma (ICP) nitridation of Al overlayers are a promising alternative, producing low-leakage Nb/Al-AlN/Nb SIS junctions, with current densities in excess of 30 kA/cm2. A correlation between junction quality and plasma dissociation has been reported for Nb/Al-AlN/Nb junctions produced via an electron cyclotron resonance (ECR) plasma. In this work, a quantitative measure of the relative dissociation (RD) of an ICP is determined through the use of a commercially available Ocean Optics USB4000 Optical Spectrometer. The effects of various ICP parameters on the RD, and a correlation between the RD and quality of the resulting Nb/Al-AlN/Nb junctions are reported.

Development of Nb/Al-AlN/NbTiN SIS Junctions With ICP Nitridation

Increasing the operating frequency of SIS receivers requires a shift from Nb/Al-AlOX/Nb junctions to new material systems. Two major limiting factors of higher frequency operation are the increase in subgap leakage that occurs in AlOX barriers as current densities approach and exceed 10 kA/cm2 and the increased loss in Nb electrodes above ~ 700 GHz. A promising alternative structure is the hybrid Nb/Al-AlN/NbTiN junction. Realization of these devices has been difficult due to the challenge of fabricating devices with repeatable current densities and electrical characteristics. We present on the fabrication and dc testing of Nb/AIN/NbTiN junctions. The A1N barrier is formed using our inductively coupled plasma (ICP) technique which allows for independent control of both ion energy and current density. This improved control enables the repeatable synthesis of high quality and high Jc barriers. Nb and NbTiN electrodes are deposited by unbalanced dc magnetron. A new fabrication process was developed to enable fabrication of junctions with area as small as 0.28 mum2. The relationship between barrier thickness and plasma conditions is determined by in-situ discrete ellipsometry. Ellipsometry results were verified by comparison with measured current-voltage characteristics. I(V) curves for a range of junction sizes are presented. Plans for in-situ Faraday monitoring of the energy and current density of the ICP nitridation plasma will also be discussed.

A micromachined differential probe for on-wafer measurements in the WM-1295 (140–220 GHz) band

This paper describes the first-reported development of a micromachined differential probe for direct on-wafer measurements operating in the WM-1295 (140–220 GHz) frequency band. Design and fabrication of the probe, which includes integrated circuitry for converting the input of a single-ended vector network analyzer to differential mode, are described and an on-wafer calibration procedure for extracting the probe mixed-mode scattering parameters. is detailed.

An Epitaxy Transfer Process for Heterogeneous Integration of Submillimeter-Wave GaAs Schottky Diodes on Silicon Using SU-8

This letter describes a new approach for fabricating quasi-vertical submillimeter-wave GaAs Schottky diodes heterogeneously integrated to high-resistivity silicon substrates. The new method is robust and eliminates previous processing steps that were prone to result in wafer fracture and delamination. Diodes fabricated with the new process and measured in the 325–500 GHz range using on-wafer RF probes exhibits low parasitic capacitance and series resistance, achieving device characteristics comparable to the prior state-of-the-art submillimeter-wave diodes.

Cryogenic temperature, 2-port, on-wafer characterization at WR-5.1 frequencies

We demonstrate 2-port, on-wafer S-parameter characterization at WR-5.1 frequencies and cryogenic temperatures. CAD-based simulation optimized the thermal pathways for the probe station, waveguide and probe assembly.