Tantalum Thin Film Capacitors

Abstract

This paper reports the second phase of our programme to indigenously develop hybrid integrated circuits based on tantalum thin films. (Report on the first phase, viz. development of resistors, has appeared in the September 1971 issue.) These IC's provide quality performance for linear applications right from audio/radio to microwave frequencies and are increasingly incorporated in recent versions of communication and radar equipment.A tantalum film capacitor is fabricated by anodizing the upper portion of the film to serve as a dielectric, the lower portion serving as a bottom electrode. Gold layer deposited on the dielectric constitutes the top electrode. The presence of pinholes, however, does not permit one to adopt such a simple approach. The problem has been successfully solved by adopting ‘a double layer approach’. It consists in once again depositing tantalum on the anodized film and anodizing it completely. Since the probability of pinholes in the two layers coinciding is very rare, this approach has been found to provide 100 per cent yield. Advantages that no additional materials and processes are involved and that large area density of capacitance is still maintained are obvious.Capacitances with an area density of 0·1 μF/cm2 have been prepared following this approach. Each layer is anodized at 100 V. The breakdown voltages in the forward and reverse directions are around 50 and 25 V respectively. At 800 Hz, the dissipation factor is around 0·001. They have also been tested for variations against temperature and frequency. The temperature coefficent of capacitance is +270 ppm/°C and increase in the dissipation factor is negligible up to 5 MHz.These capacitances have been integrated with resistances for coming up with hybrid IC's. A twin-T selective amplifier is under investigation.