Thin film resistors and capacitors for multichip modules

A significant advantage of using thin film, rather than laminate technology, for MCMs is the ability to incorporate passive components, resistors, capacitors and spiral inductors at low cost. Tantalum-silicon alloy resistors and silicon nitride capacitors have been widely used but modifications to traditional processing have greatly improved the robustness of the process. The use of NF₃ gas for tantalum silicide etching provides excellent sidewall geometry and uniformity over the whole of the wafer so that narrow lines can yield high-tolerance resistors. It also fulfils the requirements of the Montreal Convention. Silicon nitride capacitors are formed using plasma enhanced chemical vapour deposition (PECVD) instead of low pressure chemical vapour deposition so as to reduce the temperature of deposition from 785°C to 400°C. This minimises oxidation of the tantalum silicide and the associated resistor drift. The PECVD nitride provides pinhole free capacitors with a yield of >99% up to 3 mm square. Breakdown strength is in excess of 1.7×10^-6 V/cm. PECVD also provides excellent uniformity, <2% over a 150 mm wafer. The values of resistors fall by 5% during polyimide cure at 400°C but there is no widening of the distribution so the tolerance is not affected. The thermal coefficient of resistance is less than 100 ppm/K over the temperature range 25-175°C