利用SF6/O2電漿對氮化鋁鎵/氮化鎵高電子遷移率電晶體閘極掘入區域進行後處理之探討

Abstract

閘極掘入是改變閘極與電流通道的距離而使截止電壓改變，進而使電晶體的表現最佳化，閘極掘入的技術已經廣泛的應用在三五族半導體裡。在三五族半導體砷化鎵的元件，閘極掘入的蝕刻多半使用濕式化學蝕刻；但是由於氮化鎵材料本身化學惰性大，沒有辦法使用一般濕式化學蝕刻，於是電漿乾式蝕刻就被應用在蝕刻氮化鎵材料上，同時也帶來在主動區電漿損害的問題。 此論文主要提出利用SF6/O2電漿的後處理，對氮化鋁鎵/氮化鎵高電子遷移率電晶體閘極掘入區域進行後處理，目的在於減輕乾式蝕刻所帶來的損害。 本實驗利用蕭基二極體去模擬元件閘極經過後處理過後的表現。觀察在不同電漿處理下逆向漏電流以及截止電壓的變化，了解電漿後處理對蕭基特性影響程度，進而尋找一個最佳化的後處理條件。 將最佳化的後處理條件應用在氮化鋁鎵/氮化鎵高電子遷移率電晶體，並且製作SF6/O2電漿後處理閘極掘入之氮化鋁鎵/氮化鎵高電子遷移率電晶體，其最大互導係數為160 mS/mm且具有高於100 V的崩潰電壓。在高頻特性上展現了23 GHz的截止頻率(fT)與62 GHz的最高震盪頻率(fmax)。與沒有處理過的元件相比，閘極掘入的高電子遷移率電晶體經過電漿後處理，具有較高的崩潰電壓以及良好的電流回復特性。

Gate recess is widely used in III-V compound transistors to modify the threshold voltage and optimize the device performance through adjusting the gate to channel distance. The recess in GaAs based HEMTs is done using wet chemical etching. On the other hand, recess of GaN based devices is usually achieved using dry etching due to the chemical inertness of the material to the chemicals. However, this leads to the problem of plasma damage in the active region of the device and this degrades the device performance. In this thesis, a post-treatment method using SF6/O2 plasma is proposed and applied for the fabrication of BCl3 recessed AlGaN/GaN high electron mobility transistors (HEMTs) to alleviate the damages related to the dry etch. The condition of the SF6/O2 plasma post-treatment was first investigated using Schottky diodes. Reverse leakage current and the threshold voltage shift were monitored to understand the effect of different plasma treatments. An optimized post-treatment condition was found to have decreased the Schottky reverse leakage current by three orders of magnitude and minimized the threshold voltage shift. This condition was then applied to the AlGaN/GaN HEMTs. The fabricated 0.8μm ×200 μm BCl3 recessed AlGaN/GaN HEMT with SF6/O2 post-treatment exhibited a maximum transconductance of 160 mS/mm with an off-state breakdown voltage of more than 100 V. The cutoff frequency fT and maximum oscillation frequency fmax were 23 GHz and 62 GHz respectively. Compared to the untreated HEMTs, the recessed HEMT devices with the proposed post-treatment have demonstrated higher breakdown voltage and better pulse current recovery.