The effect of surface oxidation status on 0.35-mu m HIGFET characteristics

The paper discusses the effect of the oxide film formed on the surface of G aAs substrate of the gate metal of 0.35-mum WSi gate HIGFET (Heterostructur e Insulated Gate FET) on its characteristics. An optimized SiON film (refra ctive index 1.58) deposited by plasma CVD method onto the entire HIGFET sur face was used as a passivation film to protect the gate metal substrate [1] . In order to modify oxidation conditions on the GaAs surface, two oxidatio n methods were utilized: low-temperature oxidation (further referred to as UV/O-3 oxidation), which utilizes O-3 generated from the atmospheric O-2 by applying ultraviolet irradiation produced by a mercury lamp (184.9 and 253 .7 nm), and high-temperature thermal oxidation performed at 400 degreesC in air. Based on the results of spectroscopic ellipsometry, Auger spectrometr y, and XPS analyses, it was the UV/O3 method that led to formation of therm ally unstable SiON/GaAs oxide film on the GaAs interface layer due to highe r concentration of As oxide. During the heat treatment, involved in the FET fabrication process, As oxide would react with SiON resulting in its therm al decomposition and formation of free As. On the other hand, the oxide fil m formed during thermal oxidation at 400 degreesC led to formation of more thermally stable SiON/GaAs oxide film/GaAs interface layer due to higher co ncentration of Ga oxide via decomposition of thermally unstable As oxide du ring the heating process. In either case, the presence of GaAs oxide film a long with free As in the SiON passivation film deposited onto GaAs substrat e is believed to be a major reason for declining FET characteristics. This suggests that removal of the oxide film from the surface prior to depositin g a passivation film is required. The improved stability of the FET charact eristics was achieved by removing the GaAs oxide film using wet etching wit h BHF during the FET fabrication process. (C) 2001 Scripts Technica, Electr on Comm.