OPTIMIZATION OF SION FILM COMPOSITIONS FOR ENCAPSULATION OF REFRACTORY-METAL GATE GAAS METAL-SEMICONDUCTOR FIELD-EFFECT TRANSISTOR

The composition of the SiON film were optimized to be used as an annea l encapsulation based on the GaAs metal-semiconductor field effect tra nsistor (MESFET) characteristics. The SiON films investigated in the p resent study were deposited by plasma-enhanced chemical vapor depositi on. The refractive indexes of the SiON films with various compositions were found to be useful indicators of film composition. By varying th e ratio of the source gases, SiH4 and N2O, we could control the compos ition to range from SiO2 to Si3N4 corresponding to refractive indexes from 1.47 to 2.02. MESFET characteristics were evaluated in terms of t he Schottky diode ideality factor n, which was found to be highly depe ndent on the film composition: when we used a SiON film with a refract ive index below 1.53, which corresponds to an O-rich composition, the n value exceeded 1.4. This indicates an unstable interface between the GaAs and the SiON. When we used the N-rich SiON, however, such as SiO N with a refractive index 1.6, the n value was 1.25. These results are discussed from the viewpoint of interface reaction between GaAs and S iON, which we investigated by using Auger electron spectroscopy.