AlN Barrier SIS Junctions in Submm Heterodyne Receivers: Operational Aspects

Th e Atacama L arge Millimeter I nterferometer (ALMA) (1) sit e is an o bservatory which consists of more than fifty 12 meter diameter sub-mm telescopes, locat ed at an altitude of 5000 m in t he Atacama desert in Chile. It covers the 30-950 GHz frequency range which is divided into ten bands following the atmospheric transmission w indows. ALMA fro nt- ends w ill be u sing Superconductor-Insulato r-Superconductor (SIS) heterodyne mixers as the key sensitive elements for a ll of its high frequency bands. The ALMA band 9 receiver which covers 600-720 GHz is being developed in t he Neth erlands. Th e first eight rec eivers h ave already been built base d on AlOx barrier SIS junctions mixers. These mixers have excellent noise temperature but show limited RF and IF ba ndwidth. Using new AlN bar rier highe r c urrent density SIS junctions, it is possible to improve on the RF and IF bandwidth of SIS mixe rs thus makin g this te chnology significantly more attra ctive f or th e u se in l arge s eries of S IS receivers. Curren tly, a good sensitivit y and band coverage has already b een ach ieved for AlN mixers (2). Ho wever, performance of AlN mixe rs at highe r fr equencies de pends highly on Josephso n noise su ppression and i s only optimal if a precise magnetic field is applied on the barrier. Present routines for Jose phson noise suppr ession in AlO x b arrier mixe rs were not s ufficient. I n th is p aper th ese rou tines will b e d iscussed in detail and its a daptation to AlN barrier mix er operation will be reported.