Thermal Processing of Tantalum Nitride Resistors

A study of the thermal oxidation of tantalum nitride (Ta-N) thin films has led to the development of a thermal processing technique by which these metal films can be made to exhibit superior resistor characteristics. The thermal process simply involves heating the Ta-N film resistors in air at some temperature between 450° and 700°C. Radiant heating has been found to be most convenient for this purpose. While the resistor is maintained at the elevated temperature, its resistance is monitored. When the resistor reaches the desired value, the heat source is quickly removed. Resistors can be easily trimmed to O.B%, and by exercising some care, 0.1% and less may be attained. In comparison with the usual "anodic" processing of Ta-N resistors, the thermal technique holds promise of greater economy in manufacture by reducing the number of process steps and by increasing the resistor yield. The latter benefit derives from the fact that the electrical behavior of thermally oxidized Ta-N resistors shows a marked insensitivity to certain deposition parameters which are normally controlled with some care. Accelerated tests of resistors made by this method are being carried out at various load levels and under high humidity conditions. Resistance changes as low as 0.1% per thousand hours have been recorded for nominal 1/2 watt resistors operating at 1 watt. Processing parameters such as substrate material, nitrogen doping level, trim rate, and amount of trim have been investigated. Although continued evaluation will be required before thermal processing is fully qualified, it is already clear that it will be of considerable importance to the Ta-N discrete and integrated resistor technology.