Abstract
The reliability of evaporated thin films of nichrome 1OO to 2,000 angstroms in thickness when deposited onto a ceramic substrate can be related to the physical and electrical properties of the substrate. Through careful selection of materials that are physically and electrically compatible with these extremely thin films, reliability and volumetric efficiency of precision film resistors can be materially improved. Evaluation of commercially available substrate materials resulted in the selection of industrial sapphire as a most superior substrate material. Reliability was improved by one order of magnitude, and volumetric efficiency was increased by a factor of thirty. The evaporated metal film deposited on monocrystalline industrial sapphire exhibited a marked uniformity of characteristics within the lot, as well as between lots of precision film resistors. Temperature coefficient of resistance, voltage coefficient and noise factor characteristics for evaporated nichrome sapphire resistors were found to be superior over alkaline earth and high alumina ceramic precision film resistors produced under identical evaporation conditions. Because of the excellent thermal conductivity of industrial sapphire, electrical ratings ranging between 1/2 watt at 25°C through 1/20 watt at 125°C were achieved for cylindrical resistor rods O.105" long x .050" diameter as compared to MIL-R-10509 resistor styles with rectilinear envelopes four times larger. The industrial sapphire precision film resistors achieve remarkable miniaturization, economical advantages through higher production yields, reduced temperature coefficient of resistance characteristics and substantially greater reli- ability.
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