The resistivity of YBCO/Ag interfaces has been measured for different
oxygen annealing temperatures for a series of ex-situ fabricated thin-
film contacts having sizes ranging from 16 mu m x 16 mu m down to 4 mu
m x 4 mu m. The interface resistivity began to decrease after anneali
ng at 300 degrees C for 10 minutes in one atmosphere oxygen. After ann
ealing at 400 degrees C, the contact resistivity decreased by several
orders of magnitude to the 10(-7) Omega-cm(2) range. The 500-nm thick
Ag layer showed massive surface diffusion and agglomeration for anneal
ing temperatures above 400 degrees C; this temperature thus represents
a practical limit for oxygen annealing the YBCO/Ag interface system f
or more than 10 minutes. Rapid cooling of the chip after annealing led
to a severe loss of critical current density in the YBCO layer, which
could be restored by reannealing and cooling at a slower rate of 50 d
egrees C/min. The relative shape of the conductance-vs.-voltage charac
teristics of the YBCO/Ag interface were essentially unaltered by oxyge
n annealing; the overall parabolic shape, scattering zero bias anomaly
remained constant, even though the contact conductance increased by s
everal orders of magnitude. These data suggest that the main reduction
in interface resistivity arises from an enhancement of the effective
contact area, not from a change in interface conduction mechanism.