Nb/Al-AlOx/Nb edge junctions for distributed mixers
Amos, R.S.; Lichtenberger, A.W.; Tong, C.E.; Blundell, R.; Pan, S.-K.; Kerr, A.R.
Applied Superconductivity, IEEE Transactions on
Volume 9, Issue 2, Jun 1999 Page(s):3878 - 3881
Digital Object Identifier 10.1109/77.783874
Summary:We have fabricated high quality Nb/Al-oxide/Al/Nb edge junctions
using a Nb/SiO2 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
SF6+N2 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
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