氮化鋁鎵/氮化鎵高電子遷移率電晶體無金金屬化製程之研究

Abstract

氮化鎵元件在高功率和高頻的應用一直備受矚目。一般氮化鎵元件是利用金來做金屬接觸，然而近年來黃金的價格持續飆升，導致元件的製程成本大幅上升。除此之外，以金為基底之元件具有可靠度的問題，長期處於高溫環境之下可能會使金穿過阻擋層並擴散到半導體基板中而使得元件失效。本實驗即研究利用無金金屬化製程取代傳統使用金的製程，來降低製造成本並且增加元件的可靠度。 本論文中，我們使用鈦/鋁/鎢取代傳統的鈦/鋁/鎳/金歐姆接觸，並對此金屬層結構進行高壓穩定度的測試。實驗結果顯示，即使處於100伏特的高壓之下長達15小時，鈦/鋁/鎢金屬層結構的源極-汲極電流仍能穩定地保持在62mA/mm；反觀傳統的鈦/鋁/鎳/金金屬結構在高壓持續44分鐘後，元件因耐不住高壓而損壞。此結果顯示鈦/鋁/鎢金屬結構具有較佳的耐壓性。蕭基接觸的金屬則是利用氮化鎢來取代鎳/金。氮化鎢閘極展現了良好的熱穩定度，即使元件於700℃中進行退火5分鐘，仍能保持極低的閘極漏電流。 這些結果顯示無金金屬化的製程可以應用在氮化鎵高電子遷移率電晶體上並展現了良好的可靠度。

GaN has drawn much attention for high power and high frequency applications. Au is usually used as Ohmic and gate metal in conventional GaN HEMT. However, as the price of Au getting higher and higher, the processing costs of GaN HEMTs were increased massively. Besides, long-term Au diffusion has been proposed as a serious problem. The penetration of Au into semiconductor subtract would cause device failure. To replace traditional gold metallization on GaN, the Au-free metallization was studied for lower costs and better reliability. Instead of the traditional Ti/Al/Ni/Au, Ti/Al/W was used for ohmic contacts in this study, and the bias stress test has been done for the Ohmic structure. The result shows that the drain-source currents remain at about 62mA/mm even stressed under 100 V after 15 hours. However, the conventional Ti/Al/Ni/Au Ohmic structure devices crashed after 44 minutes stressing. This result indicates that the Ti/Al/W Ohmic structure shows better resistance to high voltage. Instead of Ni/Au, WNx was used for Schottky contact metal. The WNx Schottky contact shows good thermal stability. The devices have low leakage current even after annealing at 700℃ for 5 minutes. These results indicate that Au-free metallization process can be used on GaN HEMT and show good reliability performance.