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19 th International Symposium on Space Terahertz Technology P3-2 Formation of High Quality AlN Tunnel Barriers via an Inductively Coupled Plasma Thomas W. Cecil, Arthur W. Lichtenberger, and Anthony R. Kerr Thomas Cecil and Arthur Lichtenberger are with the Charles L. Brown Department of Electrical and Computer Engineering at the University of Virginia. Anthony Kerr is with the National Radio Astronomy Observatory Increasing the operating frequency of SIS receivers requires junctions that can operate at higher current densities. A major limiting factor of higher current density junctions is the increase in subgap leakage that occurs in AlO X barriers as current densities approach and exceed 10kA/cm 2 . AlN insulators are a promising alternative due to their lower leakage current at these high current densities. However, these barriers are more complicated to fabricate – requiring a plasma to crack the diatomic nitrogen molecules - and problems with achieving reproducible current densities have been reported. In this paper we present on the synthesis of AlN barriers using an inductively coupled plasma (ICP) source. The ICP allows for independent control of ion energy and current density in the plasma. Additionally, plasmas with very low ion energy (~20eV) and a high degree of dissociation (~80%) can be achieved. This improved control allows for the repeatable synthesis of high quality barriers. We report on the relationship between barrier thickness and plasma conditions as determined by in-situ discrete ellipsometry. Ellipsometry results were verified by fabricating Nb/Al-AlN/Nb junctions and measuring current-voltage curves. Curves for a range of current densities are presented. 118
19 th International Symposium on Space Terahertz Technology Design of finline SIS mixers with ultra-wide IF bands P3-3 Paul Grimes, Ghassan Yassin Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH,UK. pxg@astro.ox.ac.uk The instantaneous bandwidth of heterodyne receivers is defined by their IF bandwidth. This bandwidth limits the sensitivity of SIS based receivers to continuum emission and limits the instantaneous frequency range for spectroscopic observations. New telescopes and backend processing technologies have led to demands for ever wider IF bandwidth SIS mixers. We present the design of 230 GHz finline SIS mixers with a 2-20 GHz IF band. These mixers are intended for use in a prototype high brightness sensitivity, low spatial resolution interferometer for Sunyaev-Zeldovich effect measurements that is currently under construction in Oxford. The first batches of these devices have recently been fabricated at KOSMA, University of Cologne, and will soon be undergo initial testing in Oxford. The finline coupled mixer design is particularly suitable for use with very wide IF bandwidths, as the finline transition provides a clean transition from waveguide to microstrip, and allows all of the RF tuning and IF circuits to be fabricated in planar circuit. This removes many of the problems of grounding the mixer and allows relatively large IF contacts and circuits to be included on the mixer chip. The first wide IF band finline SIS mixers use the antipodal finline transition (as featured on our previous finline mixers and detectors) on quartz substrate to couple the input waveguide to the mixer's microstrip circuits. RF bandpass filters made up of microstrip and anodised niobium capacitors are used to provide isolation between the finline and the IF circuits. Two RF tuning designs have been used. In the first design a single SIS junction is tuned by a single series stub, terminated by a stepped microstrip RF choke. In second design, two SIS junctions are used in a single-ended dual junction tuning circuit, in conjunction with the same stepped microstrip choke. The 16 Ohm output impedance of SIS mixer is matched to the 50 Ohm input impedance of the cryogenic IF amplifier using a 5-stage stepped microstrip transformer fabricated on Rogers/Duroid 6010LM. The design of this transformer is also used to tune out the inductance of the bondwires connecting the mixer chip to the transformer. We are currently designing a finline SIS mixer utilising SOI substrate technology and beam lead connections to the IF circuit, with greatly reduced parasitic inductance in the IF connection. We have also developed a transition direct from unilateral finline to microstrip or coplanar waveguide transmission line that will allow the design of 50 Ohm characteristic impedance finline mixers, removing the need for IF matching transformers, as well as being significantly shorter than the current antipodal finline transition. Designs for these mixers will be also be presented. 119
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