熱處理對於磊晶矽鍺薄膜材料及機械特性之影響

Abstract

矽鍺合金由於擁有比矽更優異的電子特性，至今為熱門研究的半導體材料之一。因此，矽鍺合金可以取代傳統矽材料應用於異質接合雙極性電晶體以及互補金屬氧化物半導體。然而矽鍺合金與矽基材之間存在著晶格不匹配，致使成長之薄膜在半導體後續熱處理過程中易導致不平坦的表面、鍺原子擴散及缺陷產生等。這些現象會降低矽鍺原有之特性。故如何成長良好磊晶品質之矽鍺薄膜以及相對應熱穩定性是需要被研究探討。 首先在生長薄膜部分，矽鍺薄膜藉由超高真空化學氣相沉積法於低溫(500 °C)成長方式，以及矽鍺組成比例為4:1條件下，沉積不同厚度(200、300及500奈米)之矽鍺薄膜於矽基材上，進而探討其相對應磊晶之品質。由於所成長薄膜之厚度均超過理論之臨界厚度，故為亞穩(metastable)的狀態。經由X光繞射儀、原子力顯微鏡以及穿透式電子顯微鏡之定性及定量分析後，發現薄膜厚度在高達500奈米時仍然沒有應變鬆弛及缺陷之現象產生。因此得知藉由低溫下之低磊晶速率製程可以在成長高薄膜厚度時，依舊維持良好之磊晶品質。 在低溫熱處理研究中，發現當薄膜厚度超過300奈米時應變鬆弛開始發生。甚至於500奈米厚度時，低於製程溫度(400 °C)即會發生應變鬆弛並且伴隨著大量差排缺陷產生。故得知越厚之薄膜其組成及品質越不穩定。此外，結構機械性質利用奈米壓痕技術量測，發現硬度及彈性係數隨著熱處理溫度上升而增加，說明了熱處理所造成微結構缺陷產生與相對應提升之機械特性現象。 在探討矽鍺/矽多層薄膜結構強度中發現，其硬度與彈性模數高於相同厚度之單層矽鍺薄膜，得知多層薄膜確實能有強化整體結構的作用。在熱處理的過程中也發現，矽鍺薄膜與矽薄膜間產生之差排缺陷能使整體結構的強度上升。 在虛擬基材應用方面，使用高溫氧化處理可得到應變鬆弛且表面平坦之單層矽鍺薄膜。但此過程中高含量的鍺會累積在薄膜表層，致使在接近薄膜表面所量測之機械強度有弱化的趨勢。但隨著探針持續下壓至接近基材時，所量測之結構機械強度相對於未處理之薄膜條件下會有增加之趨勢。此現象是由於薄膜與基材間介面差排缺陷產生所致。 因此，熱處理對於不同製程參數的薄膜會造成程度不同的組成改變和缺陷產生。這些改變可間接藉由奈米壓痕技術量測其機械特性之變化來得到證實。

Silicon-germanium (SiGe) alloy is one of the most attractive semiconductor materials because of its outstanding behaviors. Since then, SiGe have been applied to heterojunction bipolar transistor (HBT) as well as complementary metal-oxide-semiconductor (CMOS). However, due to lattice mismatch between SiGe alloys and Si substrate, several phenomena may occur in their growth and post-treatment including roughed surface, interdiffusion and partial strain relaxation of SiGe thin films. Therefore, investigation on the growth and thermal stability of high quality SiGe thin films is highly required. First, ultra-high vacuum chemical vapor deposition (UHVCVD) was employed to deposit silicon-germanium (SiGe). The SiGe thin films with different thickness (200, 300, and 500 nm) were deposited on the Si substrate at low deposition temperature (500 °C) and the ratio of Si to Ge for all samples is 4:1. Subsequent qualitative and quantitative analyses such as x-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) measurements exhibited a good epitaxial quality, indicating that the thin films with large thickness deposited at low temperature can avoid the occurrence of strain relaxation. All the thin films were in the metastable condition because their thicknesses were above the critical thickness. Therefore, during the low-temperature thermal treatment, the strain relaxation occurred while the thickness was greater than 300 nm. As the film thickness reached 500 nm, even a large amount of dislocations occurred with treatment temperature below the deposition temperature. It is suggested that the lager the thickness of the films was, the more unstable the structure exhibited. Besides, from the nanoindentation test it can be found that the hardness and elastic modulus increased with increasing treatment temperature, demonstrating a relation between thermal treatment-induced unstable microstructure and structure strength. Besides, SiGe/Si multilayer films was deposited successfully by UHVCVD method and exhibited enhanced hardness and elastic modulus compared with that of single layer films. It was also found that the generation of dislocations at the SiGe/Si interface with thermal treatment increased structure strength. High-temperature oxidation was executed on SiGe single layer can obtain an appropriate virtual substrate. This led to Ge pileup at the surface of the SiGe thin films, which caused slightly reduced mechanical properties near the surface, while relaxation-induced defects caused enhanced mechanical properties near SiGe/Si interface. Therefore, these researches reveal that thermal treatment can lead to different influences on composition and defects, which can be proved indirectly by nanoindentation technique.