氮化鋁鎵/氮化鎵高電子遷移率電晶體之銅金屬化製程研究

Abstract

氮化鎵元件在高功率和高頻的應用一直備受矚目。然而為了增加市場佔有率，製程成本必須達到最小化，而氮化鎵元件常使用金來做金屬接觸是高製造成本的原因之一。本實驗即研究利用銅金屬化製程取代傳統使用金的製程，來降低製造成本。 首先使用鈦鋁鎢取代傳統的鈦鋁鎳金歐姆接觸，製作不同厚度的鈦然後在不同的溫度下退火，以期找到降低特徵接觸電阻的最佳條件，並利用多重步驟退火的方法進一步降低阻值，接著藉由X光繞射、原子力學顯微鏡、及穿透式電子顯微鏡做材料分析，實驗所得最佳的特徵接觸電阻為2×10-5 Ω cm2，而且有極佳的表面平整度。 蕭基接觸的金屬則是利用氮化鎢/銅來取代鎳/金。氮化鎢/銅閘極的漏電流約為鎳/金閘極漏電流的十分之一，實驗所得之理想係數為1.57，蕭基能障高度為0.67ev。光罩圖形經過特別設計使得進行氮化鎢閘極製作時同時沉積氮化鎢於鈦/鋁/鎢歐姆接觸上，接著製作具有傳統鈦/鋁/鎳/金歐姆接觸以及鎳/金閘極的元件來比較其電性。 銅金屬化的元件的崩潰電壓為125V飽和電流密度為420mA/mm，轉導為190ms/mm，元件特性和傳統使用金製程的元件相當，說明本研究在矽基板上成功開發不使用金而用銅的氮化鋁鎵/氮化鎵高遷移率電晶體之低成本製程。

GaN has drawn much attention for high power and high frequency applications. In order to increase the market share, the costs of process have to be minimized. Gold based metallization is usually used for GaN devices and that is one of the reasons responsible for high processing cost. To replace traditional gold metallization on GaN, copper metallization was studied. The processing cost could be significantly reduced with copper metallization. Instead of the traditional Ti/Al/Ni/Au, Ti/Al/W was used for ohmic contacts. Different thicknesses of Ti layer and rapid thermal annealing temperatures are tested to obtain the lowest specific contact resistivity. The results were further improved with multi-step annealing process. X-ray diffraction, atomic force microscopy and transmission electron microscopy were used for material analysis. A specific contact resistivity of 2×10-5 Ω cm2 was obtained with excellent surface morphology. Instead of Ni/Au, WNx/Cu was used for Schottky contact metal. The leakage current of WNx/Cu gates were about one order lower than that for Ni/Au. An ideality factor of 1.57 and a Schottky barrier height of 0.67ev were obtained. With the special design of mask, WNx/Cu was deposited on the top of Ti/Al/W ohmic contacts the same time with gate patterning. Devices with traditional Ti/Al/Ni/Au ohmic and Ni/Au gate were also prepared for comparison. The HEMT devices with copper metallization exhibited breakdown voltage of 125V. The saturation current is 420mA/mm and the maximum transconductance is 190ms/mm. These results are comparable with gold based devices. Therefore, the low cost process with copper metallization and not involving gold metallurgy for AlGaN/GaN HEMTs are successfully developed.