Processing and Performance of Tantalum Nitride Thin Film Resistor Networks with ±50 Ppm/°C TCR

Abstract

Vacuum annealing has bean used to routinely manufacture tantalum nitride resistor networks with temperature coefficients in the range of ±50 ppm/°oC. The as-sputtered temperature coefficient of resistance (TCR) of tantalum nitride films with nitrogen content set for optimum stability is approximately --130 ppm. Although near zero TCR's can be obtained by reducing the nitrogen content during sputtering, such films exhibit substantial negative changes when dc loaded. Networks were fabricated on aluminum oxide substrates using in-line sputtered tantalum nitride films and vacuum evaporated chrome-gold conductive termination films. Anneal schedules (to remove structural defects) in the range of 460°C to 510°C for 4 h were found suitable for producing films with near zero TCR over a sheet resistance range of 30-75 \Omega /square. Conventional photolithography techniques were used to delineate resistor networks with up to 20 resistors per network. After a thermal stabilization of 16 h at 250°c, the resistors were anodized to ratio tolerances as close as 0.05%. Resistances of 50 \Omega to 20 k \Omega were manufactured on a single network. Load life and thermal aging results indicate no degradation in performance as a result of shifting the TCR by vacuum annealing. Temperature cycle changes were less than 0.01%. Resistor TCR tracking was less then 7 ppm/°C on 0.700 X 0.700 network. Resistor noise was measured and found not to differ from unannealed resistor films. By incorporating the vacuum annealing step into the standard tantalum nitride manufacturing process, resistor networks with near zero TCR can be produced with the same excellent stability possessed by unannealed films.