n-型鍺錫反轉式及無接面式鰭式電晶體製作與分析

Abstract

自從1960年代摩爾定律被提出後，電晶體等邏輯元件便跟隨摩爾定律持續微縮至今。為了提升元件效能以延續摩爾定律，以具更高載子遷移率的新材料如鍺或鍺錫，來作為電晶體的通道材料，是一個可行的選項。鍺錫合金的電洞遷移率，可藉由施加壓縮應變來提升；而電子遷移率則可藉由將電子聚集在等效質量較小的Γ能帶來提升。目前為止已有高效能的p-型鍺錫電晶體被發表，然而關於n-型鍺錫電晶體的相關研究至今卻十分稀少，且效能相較p-型鍺錫電晶體偏低。 本論文製作出n-型鍺錫平面電晶體以及n-型鍺錫鰭式電晶體。與平面電晶體相比，鰭式電晶體在導通電流以及次臨界擺幅表現上有顯著的提升。鰭式電晶體的最佳表現為100 μA/μm導通電流(在VOV = 1 V)與170 mV/dec次臨界擺幅，且隨著通道長度與通道寬度微縮，電晶體表現會因更佳的閘極對通道控制能力，而進一步提升。除了反轉式鍺錫鰭式電晶體，論文中也製作出無接面式鍺錫鰭式電晶體。由於鍺錫合金相較於矽、鍺具有較差的熱預算及熱穩定性，降低熱預算是鍺錫電晶體製程的一大重點。無接面鍺錫鰭式電晶體由於可避免載子活化的熱製程步驟，因此整體製程的熱預算得以降低，使得表現優於反轉式鍺錫鰭式電晶體。n-型濃摻雜的鍺錫合金，可藉由化學氣相沉積方式，在鍺錫合金磊晶過程中以內摻雜技術來成長。完成之無接面鍺錫鰭式電晶體，在導通電流、次臨界擺幅與電流開關比表現上，均優於反轉式鍺錫鰭式電晶體，最佳表現為200 μA/μm導通電流(在VOV = 1 V)、90 mV/dec次臨界擺幅與106電流開關比(在VOV = 1 V)，是目前發表之n-型鍺錫電晶體的紀錄。

Logic devices such as MOSFETs have been scaled down since 1960’s by following Moore’s law. To continue Moore’s law for better device performance, new channel materials with high mobility such as germanium or germanium-tin (GeSn) are required. Hole mobility can be boosted in GeSn by compressive strains and electron population in the Γ-band of GeSn can enhance electron mobility due to its smaller effective mass. While high-performance GeSn p-MOSFETs were demonstrated, GeSn n-MOSFETs ween underdeveloped. In this thesis, n-type GeSn planar MOSFETs and FinFETs were fabricated. Significant improvement of drive current and subthreshold swing (SS) for FinFETs were demonstrated compared to planar FETs. The highest drive current is 100 μA/μm with an overdrive voltage of 1 V and the best SS is 170 mV/decade. The drive current and subthreshold swing can be improved as the devices are scaled down due to the great gate control of the FinFET structure. In addition to the inversion-mode devices, junctionless (JL) GeSn FinFETs were also fabricated. For JL FinFETs, the dopant activation process can be avoided to save thermal budget, which is crucial for GeSn due to its poor thermal stability. By in-situ doping technique, n-GeSn films were epitaxially grown by chemical vapor deposition. The JL devices show better drive current, subthreshold swing, and Ion/Ioff ratios compared to the inversion-mode devices. The highest drive current is 200 μA/μm with an overdrive voltage of 1 V, the best SS is 90 mV/dec subthreshold swing, and the highest Ion/Ioff ratio is 106 with an overdrive voltage of 1 V, which are the best reported performance among the demonstrated GeSn n-MOSFETs.