The Study of Copper Metallization for AlGaN/GaN High Electron Mobility Transistors
Sze, Simon M
Chang, Edward Yi
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。
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.
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