Film Thickness Monitor for the Controlled Evaporation of Vacuum Deposited Films

Abstract

<p> A thin film thickness monitor has been designed and constructed based on the "mass loading" effect of a resonant quartz crystal. A 6.0 MHz Y-cut crystal, having a theoretical "mass determination sensitivity" of 8.15x10^7 Hz. - cm^2/gm, serves as the sensor element. This sensitivity can be closely approached in practice if the entire active area of the quartz plate is exposed to the evaporant stream. However, due to source, substrate and crystal geometry the "effective" sensitivity of the monitor is only 0.433 of the above value. </p> <p> Both film thickness and deposition rate can be measured by the monitor in terms of equivalent frequency changes. The actual thickness and rates depend upon the density of the evaporant. In the case of silver (density 10.5 gm/cm^3), the monitor measures average thicknesses from several (oA) to 1.36 microns in one single deposition. Each crystal can be used to monitor a total of 4.5 microns of silver before replacement. Deposition rates for silver can be measured from as low as 0.l (oA)/sec to 1360 (oA)/sec. </p> <p> By combining the thickness monitor with apparatus for controlled evaporation, a system was set up which can control film thickness to within 2% and deposition rate to within 5%. </p>