Tantalum-base Thin Film Resistors

Abstract

This paper reports the first phase of a programme to develop tantalum integrated circuits. With tantalum, both resistors and capacitors are formed in the same process; they are stable and reliable. They possess compatibility for integration with silicon chips; tantalum active elements are also feasible.

Basic steps in the fabrication of resistors are: (a) sputtering of tantalum, (b) pattern generation using photolithographic process, (c) thermal evaporation of gold for contacts, and (d) anodic oxidation. Facilities for all of these steps have been indigenously created. A diode sputtering unit has been constructed using a 100 mm square tantalum plate as a cathode; it gives a deposit thickness within ± 6 per cent over a central area of 80 mm square. Line widths up to 100 microns have been precisely etched through the photolithographic process.

Resistor values are adjusted by anodic oxidation using 0·01 per cent citric acid solution. With the present set-up using a d.c. Wheatstone bridge, values with deviations within ±0·2 per cent have been trimmed. The temperature coefficient of resistance and the stability of these resistors under different preageing conditions have been studied and the feasibility of preparing stable resistances within near zero TCR has been indicated.