The effect of surface oxidation status on 0.35‐μm HIGFET characteristics

Abstract

AbstractThe paper discusses the effect of the oxide film formed on the surface of GaAs substrate of the gate metal of 0.35‐μm WSi gate HIGFET (Heterostructure Insulated Gate FET) on its characteristics. An optimized SiON film (refractive index 1.58) deposited by plasma CVD method onto the entire HIGFET surface was used as a passivation film to protect the gate metal substrate [1]. In order to modify oxidation conditions on the GaAs surface, two oxidation methods were utilized: low‐temperature oxidation (further referred to as UV/O3 oxidation), which utilizes O3 generated from the atmospheric O2 by applying ultraviolet irradiation produced by a mercury lamp (184.9 and 253.7 nm), and high‐temperature thermal oxidation performed at 400 °C in air. Based on the results of spectroscopic ellipsometry, Auger spectrometry, and XPS analyses, it was the UV/O3 method that led to formation of thermally unstable SiON/GaAs oxide film on the GaAs interface layer due to higher concentration of As oxide. During the heat treatment, involved in the FET fabrication process, As oxide would react with SiON resulting in its thermal decomposition and formation of free As. On the other hand, the oxide film formed during thermal oxidation at 400 °C led to formation of more thermally stable SiON/GaAs oxide film/GaAs interface layer due to higher concentration of Ga oxide via decomposition of thermally unstable As oxide during the heating process. In either case, the presence of GaAs oxide film along with free As in the SiON passivation film deposited onto GaAs substrate is believed to be a major reason for declining FET characteristics. This suggests that removal of the oxide film from the surface prior to depositing a passivation film is required. The improved stability of the FET characteristics was achieved by removing the GaAs oxide film using wet etching with BHF during the FET fabrication process. © 2001 Scripta Technica, Electron Comm Jpn Pt 2, 84(7): 46–57, 2001