新穎矽奈米線元件之研製與應用

Abstract

本文提出一新穎多晶矽奈米線電晶體之結構與製作。其製備之特點為利用一般製作側壁邊襯(sidewall spacer)之概念，來形成多晶矽邊襯，此邊襯之直徑可以準確地控制至20 nm ~ 90 nm，故可巧妙地將之作為多晶矽奈米線通道。此外，相較於一般奈米線製備方式(top-down和bottom-up)，本文所提出新結構的主要優越之處在於不需昂貴的設備和高門檻的技術，便可以簡易製程步驟來達到：(1) 直接控制奈米線直徑和長度，(2) 精準定義奈米線位置，(3) 定義汲極和源極之同時，可自我對準形成奈米線通道，(4) 再現性及可靠度高之多晶矽奈米線電子元件。 首先，藉由基本電性之探討結果，發現此多晶矽奈米線元件之開狀態(on-state)性能受到某種程度之限制，以及關狀態(off-state)時展現特殊的漏電流現象。這些問題主要來自於固相結晶所形成之多晶矽具有相當多之缺陷，並且原本所設計之製程和結構有所缺失，而造成此獨特之漏電機制。此漏電流表現，可藉由本研究所提出之結構和製程修正方式明顯得到改善。此外，為了改善多晶矽之結晶品質，本研究使用電漿氫化處理來修補不完整鍵結之缺陷，以及利用金屬誘發側向結晶(MILC)或/和快速熱退火(RTA)的方式來增益奈米線本身的結晶特性。使用MILC製程的過程中，我們發現奈米線的長度、寬度以及結晶引洞的排列，對所形成的薄膜結晶品質有很大之影響。再者，經由結合MILC和RTA處理之多晶矽奈米線，可以得到非常優異之元件性能。由TEM圖分析得知，在此條件下可以獲得具有單晶品質之矽奈米線，此良好結晶性質之矽奈米線可顯著地反映至元件的性能表現上。值得注意的是，在本研究中，元件可以得到高達550 cm2/V-sec之電子遷移率和230 cm2/V-s之電洞遷移率，這些特性為目前奈米線相關文獻中的最佳成果。 除了改善薄膜結晶品質外，本研究更採用多重閘極結構來提升對奈米線通道之控制能力，進而增進元件性能表現。基於本研究電晶體巧妙之架構，多晶矽奈米線可精準地與側、下和上閘極排列。由於奈米線本身體積小，因此可以增強多重閘極之間的偶合作用，故在多重閘極的操控下，元件的性能可以大幅提升。傳統以雜質摻雜調整啟始電壓的方式，易導致奈米線中雜質分佈不均，而本結構的多重閘極可單獨施壓以調控通道之啟始電壓，此調控方式相當可靠且穩定，能避免雜質分佈不均之問題，因此本研究之多重閘極奈米線結構相當具有實用性。 最後，本研究探討多晶矽奈米線用於感測應用方面之可行性。當今大部份的生化感測反應皆於水溶液中進行。而本研究發現，在水溶液中，此多晶矽奈米線元件之表現可以提升至與單晶矽相當，所以其感測靈敏度也會隨之升高。其可能的原因來自於水中的氫(H+)或/和氫氧離子(OH–)對於奈米線內的晶體缺陷，具有明顯且完整之修補作用，並且此修補作用具有再現性。因此，我們將表面具有特定接受器(receptor)之奈米線，進行pH值和生化分子之感測實驗，此奈米線展現相當不錯之感測能力和選擇性。

A novel field-effect transistor (FET) using sidewall-spacer polycrystalline silicon nanowire (poly-Si NW) channels is disclosed and investigated. This scheme features well-controlled dimension, accurate positioning and alignment of NWs as well as reliable source/drain (S/D) contacts. In addition, the approach is reproducible and suitable for low-cost manufacturing. From the examination of the basic electrical properties, an anomalous leakage behavior and suppressed on-state performance are found. These issues are identified to be due to a deficiency in the original device design and the inherent defective crystallinity of solid-phase crystallized (SPC) poly-Si. With the structural and process modifications proposed in this study, the unsatisfactory off-state characteristics could be effectively eliminated. To improve the poly-Si crystal property, plasma hydrogenation, metal-induced lateral crystallization (MILC) and/or rapid thermal annealing (RTA) techniques are employed to passivate the microstructural defects and enhance the crystallinity of the NWs, respectively. In the implementation of MILC process, it is shown that the arrangement of seeding window and the dimension of the NWs play an important role in affecting the resulting film quality. Furthermore, MILC coupled with RTA results in excellent device performance. With the TEM analyses, it is found that single-crystal-like NWs are achieved, leading to significant performance improvement. It is noteworthy that superior field-effect motilities up to 550 cm2/V-sec for electrons and 230 cm2/V-s for holes recorded in this study are among the best reported results for NW devices in the literature. In addition to improving the film crystal property, an alternative strategy is to uplift the gate controllability over the channel with the adoption of multiple-gated (MG) configuration. Based on the proposed device structure, poly-Si NWs can be precisely positioned with respect to the side-gate, bottom-gate and top-gate. The strong gate coupling effect of the MG operation, which is ascribed to the tiny body of NW channels, thus dramatically promotes the device performance. Moreover, another great benefit of such scheme is that the independently applied gate bias can be employed to regularly adjust the threshold voltage (Vth) of NW channels in a reliable manner. The experimental results indicate that the Vth could be modulated by the gates with both positive and negative biases, making it suitable for practical applications. Finally, the capability and feasibility of poly-Si NWs for sensing applications is investigated. In this work, the device performance is dramatically improved to a level comparable to the monocrystalline-Si counterparts, as poly-Si NW channels are exposed to the aqueous environment. Passivation of defects in the poly-Si NW by H+ and/or OH- contained in the aqueous solution is proposed to explain the phenomenon. Moreover, this passivation effect is stable and reproducible. This finding is especially important as it implies that poly-Si NWs can be cleverly operated in aqueous solutions to take advantage of the performance improvement. Consequently, by functionalizing specified receptors on their surface, poly-Si NWs exhibit good sensibility and selectivity for pH-value and biomolecules detections.