具有光化學氣相沈積及電漿加強化學氣相沈積氮化矽保護層之砷化鎵功率金屬半導體場效電晶體之研究

Abstract

本論文記述在砷化鎵功率金屬半導體場效電晶體上使用光化學氣相沈 積及電漿加強化學氣相沈積氮化矽保護層。在光化學氣相沈積保護層中， 藉由一低壓下的汞燈照射促成汞催化之甲烷和氨分解產生含氫的非晶氮化 矽。文中詳述了元件各個製程步驟的技術，而光化學氣相沈積氮化矽是第 一次使用於此種元件保護層上。 文中研究此二種保護層製程對元件的影 響，經由薄膜和元件特性的探討得到薄膜成份對元件直流和微波特性的關 係。光化學氣相沈積的保護層顯示元件在其他特性不退化下崩潰電壓增加 ， 光化學氣相沈積過程中由元件通道表面產生電荷而形成不穩定的空乏 層效應減少，在沈積過程的氫離子束縛在表面的過量電荷使得非計量氮化 矽薄膜能減少表面電荷，達到我們對元件保護層的要求。實驗結果顯示光 化學氣相沈積在砷化鎵功率金屬半導體場效電晶體保護層應用上有很大的 發展潛力。 This thesis described the passivation of GaAs power MESFETs using either Photo-CVD or PECVD silicon nitride films. In the photo-CVD passivation process, hydrogenated amorphous silicon nitride films were prepared by mercury sensitized decomposition of silane and ammonia using a low-pressure mercury lamp. Detail processing techniques including ion-implantation, annealing, mesa isolation, ohmic contact, gate recess, gate formation, passivation, airbridge and backside processing were reported. The photo-CVD silicon nitride film was first deposited for passivation of GaAs power MESFET. The effect of these twotypes of process on MESFET passivation was studied. Depending upon the device structure, the passivation process needs tobe tailored to achieve minimal drift in device performance. DC and RF performances were relatable to the underlying compositional effects through extensive characterization of these films and devices. The GaAs power MESFET fabricated with photo-CVD passivation exhibited an improvement in the breakdownvoltage without the degradation of the transfer characteristics of the device.The unstable depletion region formed on the channel surface due to the generation of surface charges (during the deposition process) and subsequent trapping of these charges was minimized by using the photo-CVD process.The use of the non-stoichiometric nitride film offered the potential to develop films with minimal surface charges, the excess electronic charges being tied up by hydrogen (H+) ions that are generated during this process. Photo-CVD can be a good candidate for passivation of GaAs power MESFET.