Ultra-stable MOS input operational amplifier

It is widely known that the best way to reduce input bias current for integrated amplifiers is to employ p-channel MOS transistors as input devices. In comparison with bipolar amplifiers, however, conventional MOS input amplifiers have a fatal drawback in stability. What is worse, they require high supply voltage, because the threshold voltage of MOS transistors is high. These two major drawbacks have been generally attributed to PSG (phosphosilicate glass) which is commonly utilized as a passivation film in integrated circuits --- the first one being due to its polarization phenomenon, and the second one due to its gettered positive charges. This paper describes a newly developed passivation technique, which replaces the PSG film. The technique is based upon barrier and gettering properties of Ta, Ta oxide and Al oxide with respect to ionic contamination. This structure can be obtained by preferential anodization of successively evaporated Ta and Al films, which realizes "the Planar Interconnection" at the same time. Applying this technique to MOS input operational amplifiers, above-mentioned drawbacks have been eliminated. As a result, the stability of initial and long-term operation and the immunity from the external noise have been improved. Furthermore, the operable supply voltage has been widened to the range of V± = ±3V ------ ±18V.