低表面粗糙度與低接觸電阻之氮化鋁鎵/氮化鎵高電子遷移率電晶體歐姆接觸電極之開發與機制研究

Abstract

氮化鋁鎵/氮化鎵高電子遷移率電晶體的歐姆接觸之特性好壞對於元件的直流和微波特性有顯著之影響。目前氮化鋁鎵/氮化鎵高電子遷移率電晶體歐姆接觸所面臨的最重要議題為高溫熱退火時，金屬間相互擴散所造成之高表面粗糙度。高表面粗糙度歐姆接觸會直接影響電子束微影系統的準確性，並且還會對元件特性以及可靠度造成潛在的影響。 本碩士論文中，利用較厚的阻障金屬層來減緩金屬間的相互擴散，以達到改善歐姆接觸表面粗糙度的目的。當金屬層設計為：鈦/鋁/鈦/鎳/金，厚度為 20/120/40/60/50奈米時，經過高溫攝氏870℃，35秒的快速熱退火後，可以得到方均根粗糙度為27.6奈米，特徵接觸電阻為3.9X10-6Ωcm2之低表面粗糙度與低接觸電阻之歐姆接觸電極。而經過穿透式電子顯微鏡和X光繞射儀等儀器做材料分析後，可以發現大而平緩的鋁鎳合金形成於長條狀的氮化鈦上，而非在鋁金合金當中析出，因此形成了較平緩的歐姆接觸表面。當選用的鈦鎳層厚度得宜時，可以限制含鋁合金的形成。此外，中間層的鈦，也會和氮化鋁鎵層反應，而生成較厚的氮化鈦層，產生高濃度的氮空隙，促進歐姆接觸通道效應的產生，而達到低的接觸電阻。 而此歐姆接觸也被應用在元件製作上，可製作出閘極偏壓於0伏特時，最大電流達490mA/mm，最大轉導值為170mS/mm，以及崩潰電壓大於100伏特之直流特性並可與標準歐姆接觸特性之元件匹配。

The characteristics of Ohmic contacts on GaN high electron mobility transistors (HEMTs) have profound influence to the DC and RF performance. The most critical issue for Ohmic contacts on GaN HEMTs is the roughness of the surface morphology caused by interdiffusion of metals under high temperature annealing. The rough surface morphology will cause difficulty in E-beam lithography aligner and potential degradation in device performance and reliability. In this study, I optimized the surface morphology of the Ohmic contact on GaN HEMTs with improved barrier layer thickness. The Ti/Al/Ti/Ni/Au (20/120/40/60/50 nm) metal scheme shows a low specific contact resistivity (ρc) of 3.9x10-6Ωcm2 and a smooth surface morphology of 27.6nm root mean square roughness (Rrms) after annealed at 870℃for 35 seconds. This metal scheme is then analyzed by transmission electron microscope (TEM) and X-ray diffractormeter (XRD) to figure out the formation mechanism of it. According to the analytic results, formation of large flat AlNi grains that lay on the TiN stripe rather than surrounded by Al-Au might be the reason for improved surface morphology. With moderate Ti and Ni barrier layer thickness, the formation of Al-based intermetallics could be confined. Moreover, the second Ti layer would form thick TiN above AlGaN that might facilitate the tunneling mechanism of Ohmic contact. Finally, the optimized Ohmic contact was applied to make the HEMT devices, and the DC characteristics are as shown below: VDmax about 490mA/mm at Vg =0V, peak gm about 170mS/mm and VBR larger than 100V.