Bilayer thin film resistor and method for manufacture

A composite thin film resistor is disclosed, including an electrically inert substrate upon which is deposited a nickel-chromium alloy thin film, and an overlying second thin film - which is initially deposited as metallic tantalum. In the final product the tantalum is passivated, as by thermal oxidation, so that the said film is substantially tantalum oxide throughout, except where such film underlies a pair of conductive terminal pads. The terminal pads are spaced on the substrate, so that the resistive path therebetween is defined through the nickel-chromium thin film - via the thin metallic tantalum film interfacing between the nickel-chromium film and conductive pads. The overlying tantalum oxide provides a very high degree of environmental protection with respect to the Ni-Cr film, whereby the product not only displays the desirably low TCR characteristics of Ni-Cr, but is also highly resistant to moisture and other environmental factors. The product also exhibits outstanding resistance to the adverse effects of electrolysis supported by the presence of moisture. The various films, including a gold film from which the terminal pads are derived, may be deposited upon the inert substrate by sequential sputtering during a single evacuation of a vacuum chamber. Desired resistive patterns may then be formed by photo-etching, subsequent to which oxidation of accessible portions of the tantalum film can be effected by heating in an appropriate atmosphere.