高功率氮化鋁鎵/氮化鎵高電子遷移率電晶體之金屬接觸製程研究

Abstract

氮化鎵具有高電子遷移率、高載子濃度和高崩潰電壓等優點，使它適合用在高功率和高頻元件上。但是製作氮化鎵元件的成本太高使它無法與傳統矽元件競爭，主要的原因為採用金的金屬化製程，因為金價格太貴而占據了大部份製造成本。如果我們可以用其他金屬取代金，氮化鎵元件在市場上將更具競爭力。所以我們可以藉由發明新的接觸來達到這個目標。 鎢是一種高熔點和高密度的金屬，它不需要擴散阻擋層因為它的化學安定性非常好。在我的研究中使用鈦/鋁/鎢做為歐姆接觸來製造無金元件，並藉由原子力顯微鏡和電性量測來和傳統元件作比較。結果顯示他它們具有絕佳的表面起伏和高壓可靠度。 另外我採用鎢、氮化鎢、氮化鈦、氮化鎢/銅和氮化鈦/銅做為蕭基接觸來取代傳統鎳/金蕭基接觸。結果顯示出和鎳金相比他們具有較低的漏電流和較高的蕭基障礙。最重要的是經過長時間高壓和高溫測試後它們特性幾乎不受影響。這些研究成果將有助於解決高功率元件中自體發熱和漏電流等問題。

GaN has high electron mobility, high carrier concentration and high breakdown field. These advantages make GaN suitable for high power and high frequency applications. However, the cost of GaN device processing is too high to compete with traditional Si-based devices. The main reason is the gold metallization. The price of gold is so high that it constitutes to a major portion of the fabrication cost. If we can replace gold with other kinds of metal, GaN devices will become more competitive in the commercial market. So a lot of new contact metals are being investigated to achieve this goal. Tungsten is a metal which has the highest melting point and high density. Its chemical stability is great so that diffusion barrier layer is not needed. In this study, Ti/Al/W is used as a new Ohmic contact for gold-free devices. Atomic force microscopy and DC measurement are used to compare traditional devices and gold-free devices. The results show that they have excellent surface morphology and stress reliability. For Schottky contact, W, WN, TiN, WN/Cu and TiN/Cu are adopted to replace Ni/Au Schottky contact. The results show they all have lower gate leakage currents and higher Schottky barrier heights than Ni/Au. Most importantly, they stay nearly unchanged after long-term stress period and high temperature test. These results are helpful to solve self-heating and leakage current occurred in GaN high power devices.