氮化鎵元件之歐姆與蕭基接觸的銅製程材料研究

Abstract

本篇論文主旨在於研究以銅製程取代原歐姆以及蕭基接觸的金材料之氮化鎵元件；隨著國際石油價格波動，黃金價格也不斷攀升，為求符合經濟效益，銅製程的使用將會是未來的趨勢，而本實驗的目的就是於氮化鎵的歐姆以及蕭基接觸結構中做改變；藉由銅沉積以取代金成為歐姆接觸的導電層，並且改變其金屬層結構，此論文中包含了鈦/鋁/鎳/銅，鈦/鋁/鎳/鈦/銅以及鈦/鋁/銅金屬結構，再由電性量測以及X-Ray繞射、歐傑電子、原子力學顯微鏡分析其結構變化以得出結論。 實驗結果顯示，於歐姆接觸中，使用了銅取代金並且加入薄層鎳作為銅與基層金屬鈦/鋁的界面緩衝層，再經過900℃的高溫退火之後，形成了極佳的歐姆接觸，目前實驗結果可達1.67*10-6 Ω-cm2且相較於傳統的歐姆接觸金屬8.7*10-7 Ω-cm2 幾乎無差異；而蕭基接觸的部分，為有效阻止銅擴散進入基板，則是以氮化鎢作為蕭基接觸的第一層接面，然後再以銅取代原導電層金，其結果以I-V參數計算理想因子以及能障高度並與傳統蕭基接觸鎳金做比較。實驗結果發現，氮化鎢之閘極漏電特性相似於鎳金，且不論蕭基接觸材料與上方導電層金屬為銅或者金，其皆具有不錯的電性結果。 最後，一個由鈦/鋁/鎳(50Å)/銅-歐姆接觸以及氮化鎢/銅-閘極製造的元件具備低接觸電阻和表面粗糙度為7.6nm 以及電性相近的崩潰電壓136V、轉導128ms/mm以及飽和電流470mA/mm的氮化鎵元件並達到了銅取代金的元件製程。 隨著金價波動逐年上升，銅的引用將會是未來不可或缺的一環；雖然目前仍有氧化與擴散造成元件可靠度的相關問題，但隨時代演進，不具金的元件將會於工業應用以及經濟中擁有一定程度以上的競爭力。

With the fluctuation of global oil price, the gold price has become more expensive than before. Conventional GaN device with Au-based metal contacts is a expensive approach for future technology. This thesis focuses on the GaN device with copper process, which is intergrated to semiconductor process to replace gold in Ohmic and Schottky contacts, in order to reduce the cost and improve DC characteristics. To achieve Au-free GaN device, Ohmic contacts of Ti/Al/Ni/Cu, Ti/Al/Ni/Ti/Cu and Ti/Al/Cu were investigated in this study. X-ray diffraction, Auger electron depth profile and atomic force microscopy were used for material analysis. Cu-based Ohmic contact with Ti/Al/Ni(50Å)/Cu metallization shows low specific contact resistance of 1.67*10-6 Ω-cm2 which is comparable with conventional Au-based Ohmic contacts of 8.7*10-7 Ω-cm2. The result indicates that metal stacks forming good Ohmic contact with copper and thin Ni as internal buffer layer on Ti/Al after heat treatment of 900℃for 30sec. To prevent the Cu diffusion, a well known diffusion barrier WxN is used as the first layer of gate rateing and the top layer Au is replaced with Cu as conduction layer. Device characteristics were measured by DC measurement. The results of ideal factor and barrier height of WxN/Cu will be calculated with I-V curve obtained and compared with the conventional Schottky contact Ni/Au. The gate leakage current of tungsten nitride is similar to conventional Schottky contact. Besides, with the copper layer on the top of Schottky contact, the electrical characteristics of WxN/Cu metallization is as good as Ni/Au. Finally, a fully Cu-based GaN HEMT device with low specific contact resistance and smooth surface roughness of 7.6 nm was fabricated. The electrical characteristics with breakdown voltage of 136V, transconductance of 128ms/mm and saturation current of 470mA/mm were obtained with Ti/Al/Ni(50Å)/Cu Ohmic contact and WxN/Cu gate, which is similar to the conventional Au-based GaN HEMT device. With the rise of gold price, copper process becomes more important. Although it still has some issues need to be solved such as oxidation, diffusion and reliability verification, the lower cost of copper make it more competitive for industry and applications.