Using Oxygen Partial Pressure to Improve Chromium Thin Film Adhesion to Alumina Substrates

Radio frequency (RF) hybrid microcircuits, manufactured at Bendix Kansas City for the Department of Energy, have a chromium-gold ground plane which is vacuum deposited on the backside of an alumina substrate. Production and processing environments for thin films require the backside metallization to have a minimum adhesion of 2.2 N as measured by a 90-deg peel test of thermocompression-bonded copper lead frames. The introduction of Oxygen into tile evaporation system during the inital phase of chromium dep6sition was explored as a possible method of increasing chromium adhesion to the alumina substrate. Oxygen backfill levels were investigated over a pressure range of 2.67 x 10
^-4
to 1.33 x l0
^-2
Pa ar a substrate temperature of 200°C. The optimum lead frame pull test results were obtained at an oxygen backfill level between 4.0 x 10
^-3
and 1.07 x 10
^-2
Pa; the minimum and average pull strengths were 12.5 and 17.8 N, respectively.