Vacuum-deposited metal film resistors

Abstract

Although vacuum-deposited thin metal films have been studied for many years, it is only recently that their anomalous electrical properties have been turned to practical account. In this work the most important requirements for the deposition of metal film resistors are discussed, and an annealing cycle up to 300-350° C in air is proposed to stabilize the resistance of nickel-chromium alloy films, preventing further large irreversible change within the temperature range of normal use. The stability of annealed unprotected films within the range 10-400 Ω/square was determined over a period of 2000 h on load (1 w/in2) and 8000 h at no load. Frequency diagrams show the number of resistors measured, and their percentage resistance change. Resistors at no load increased by an average of 1% compared with a very small mean decrease in the same resistors on load. The temperature coefficient of resistance of films in the range 100-150 Ω/square is shown to vary between 0.002 and 0.1% deg-1C and is related to the rate of film deposition. Starting with an alloy of 80/20 nickel-chromium, the lowest values of temperature coefficient of resistance are obtained for rates of deposition lower than 5 Ås-1 and this effect is discussed in terms of the oxidation of the film and the separation of the alloy during evaporation.