The assembly of superconducting millimeter and submillimeter-wave circuits often requires RF ground connections. These are usually made by soldering, wire bonding, conductive adhesive or conductive wire gaskets. The difficulty of assembly increases with frequency as chip dimensions and tolerances shrink. The assembly issues, and also the throughput requirements of large radio astronomy projects such as ALMA (Atacama Large Millimeter Array), suggest the need of a beam lead technology for these circuits. Beam lead processes are already established for silicon and gallium arsenide wafers. However, niobium circuits on quartz substrates present unique difficulties. SIS junctions introduce additional thermal and chemical constraints to process development. For quartz, wet etches are isotropic and dry etches with high etch rates require large ion energies. Therefore, it is difficult to develop a conventional process in which gold pads on the substrate surface are formed into beam leads by a backside etch. Instead we have developed a topside process in which, after the mixer circuits are completed, dicing cuts are made at the finished chip dimensions but only partly through the wafer. The dicing cuts are then filled with a sacrificial material in a non-CMP process, and planarized. Gold plated pads are then defined, overhanging the planarized cuts. The sacrificial material is then removed from these cuts, leaving the gold beam leads. The wafer is then backside lapped into the cuts to the desired thickness, separating the individual chips. We discuss the new planarization scheme developed for this beam lead process and compare a variety of sacrificial materials.

High quality, low-leakage superconductorinsulatorsuperconductor (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/cm 2 . 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.Index Terms-Aluminum nitride, inductively coupled plasma, superconductorinsulatorsuperconductor (SIS), spectroscopy. 1051-8223

High quality Nb-based superconductor-insulatorsuperconductor (SIS) junctions with Al oxide (AlOx) tunnel barriers grown from Al overlayers are widely reported in the literature. However, the thin barriers required for high critical current density (Jc) junctions exhibit defects that result in significant subgap leakage current that is detrimental for many applications. High quality, high-Jc junctions can be realized with AlNx barriers, but control of Jc is more difficult than with AlOx. It is therefore of interest to study the growth of thin AlOx barriers with the ultimate goal of achieving high quality, high-Jc AlOx junctions. In this work, 100% O2 and 2% O2 in Ar gas mixtures are used both statically and dynamically to grow AlOx tunnel barriers over a large range of oxygen exposures. In situ ellipsometry is used for the first time to extensively measure AlOx tunnel barrier growth in real time, revealing a number of unexpected patterns. Finally, a set of test junction wafers was fabricated that exhibited the well-known dependence of Jc on oxygen exposure (E) in order to further validate the experimental setup.