用混合氮氧氣體及電漿氟化與氮化處理技術對矽鍺奈米線於生物感測試劑之靈敏度研究與特性

Abstract

當金氧半場效半導體電晶體(MOSFET)的線寬不斷的微縮之下來獲取更高的驅動電流之下,相關半導體製程技術遇到不同層別的技術瓶頸。因此利用半導體材料上的應變作用(strain effect)來改善元件的驅動電流成為解決半導體製程技術瓶頸的方法之一。絕緣層上矽鍺(SiGe-on-insulator, SGOI)屬於全面應變矽(或雙軸應變,biaxial-strain)的製程方式,全面應變矽具有同時提升 N型與P型-MOSFET之驅動電流的特性。本論文中藉由矽鍺半導體具有提升電洞載子遷移率的特性下,並藉由側牆隔離蝕刻法(side-wall spacer etch)形成絕緣層上矽鍺(SGOI)之矽鍺奈米線(SiGe nanowire) 。 文中除經由電性分析方法比較矽鍺奈米線(SiGe nanowire)與傳統低溫多晶矽之奈米線(Poly-Si nanowire)在電導特性上的差異之外,還經由滴定不同生物感測試劑來討論矽鍺奈米線(SiGe nanowire)之可用性(functional)及對感測試劑之靈敏度。另外由於矽鍺半導體具有在氧化過程中有所謂鍺緻密化效應(Ge condensation),使得大量的鍺元素累積在矽鍺薄膜的表面,隨著鍺元素比例的增加,電洞的載子遷移率也獲得更進一步的提升。但是在鍺緻密化效應(Ge condensation)過程中,矽鍺奈米線的表面會產生因氧化過程中之表面缺陷以及當更多鍺元素累積在矽鍺薄膜的表面所形成的應力之下所形缺陷密度增加。而造成矽鍺奈米線的電導性(conductance)與靈敏度(sensitivity)衰減。 因此本論文改善因鍺緻密化效應(Ge condensation)過程中所造成矽鍺奈米線的電導性與靈敏度衰減,例如氧化製程過程中氣體條件的最適化,退火溫度最佳化,調變式矽鍺薄膜與矽緩衝層厚度藉此提升表面體積比(surface-to-volume ratio),表面護層沉積法來降低氧化速率減緩缺陷密度形成,表面電漿氟化處理技術強化及修補矽鍺奈米線之表面缺陷,最後藉由電漿氮氣鈍化處理技術來降低退火溫度,避免因高溫退火條件下,使得原本藉由鍺緻密化效應(Ge condensation)在矽鍺奈米線表面累積之鍺元素形成擴散而降低鍺元素比例,進而再次衰減矽鍺奈米線的電導性(conductance)與靈敏度(sensitivity)。

Nanowires are widely used in highly sensitive sensor for electrical detection of biological and chemical species. Modifying the band structure of strained-Si metal-oxide-semiconductor field-effect transistors by changing the in-plane tensile strain reportedly improves electron and hole mobilities. For maximum hole mobility, the fraction x of Si1-xGex should approximate 0.3. Thus, the SiGe-on-insulator (SGOI) process has potential applications for fabricating high-sensitivity nanowire sensors. The SiGe oxidation that is caused by Ge condensation increases the Ge fraction and substantially increases hole mobility. However, oxidation increases the number of surface states, which degrades hole mobility. This study used different process treatment and device’s scheme to improve the electrical properties of SiGe nanowires. Firstly, the effect of oxidation ambient was investigated. Mixed N2/O2 oxidation process with optimization ratio can be an effective technology to improve the sensitivity of SGOI nanowires. Secondly, Si1-xGex nanowires using a low-temperature Si (LT-Si) buffer layer with moderate thickness was exhibited to increase surface-to-volume ratio and condense the transport carriers at SiGe which has high conductance. Top-surface passivation by a barrier oxide was deposited to go a step further to reduce oxidation. Passived oxide layer not only reduced oxygen penetration rate, but also the density of free surface states was suppressed which was also effective to reduce oxidation rate and lower defect density formation. Finally, fluorine plasma pre-treatment and nitrogen plasma post-treatment were utilized to enhance conductance and sensitivity of SiGe nanowires. Pre-oxidation treatment using fluorine plasma; improved the conductance of SiGe nanowires because the Si-F binding energy created a more stable interface state than bare nanowire on the surface of SiGe. Since the reparation of surface defects by nitrogen post-plasma treatment is valid, the high post-annealing temperature to reduce defect by re-crystallizing can be reduced. Hence, Ge diffusion at low post-annealing temperature did not reduce the high concentration of Ge at the SiGe nanowire surfaces to sustain high conductance and sensitivity.