臨場超高真空穿透式電子顯微鏡觀察矽鍵結雙晶上金奈米結構之成長動力學研究

Dynamic Investigation of Au Nanostructures on Silicon Bicrystal by In Situ Ultrahigh Vacuum Transmission Electron Microscope

劉建宏(Chien-Hung Liu)

指導教授 ： 陳力俊

國立清華大學/工學院/材料科學工程學系/博士(2006年)

摘要

本研究在於提出以矽鍵結雙晶 (silicon bi-crystal) 為基材，在其上生長金 (Au) 的低維度奈米結構。除了觀察基材本身因扭轉螺旋差排 (twist screw dislocation) 所產生的表面規則應力場 (regular surface strain field) 對金奈米結構生長的影響之外，亦針對熱效應及應力效應對於金奈米結構在此特殊基材上行為做動力學的探討。
在金奈米點成長於矽鍵結雙晶方面，由於表面規則應力場的作用，在150 °C 溫度時沈積之金原子傾向於堆積在正交旋轉螺旋差排的交點 (node point) 形成與矽基材有磊晶 (epitaxial) 關係的奈米點陣列 (nanodot array)。規則應力場間距隨著螺旋差排間距而改變，直接影響了金奈米點的生長特性。應力場間距越大 (7nm-45nm)，金奈米點的平均生成數量隨之下降 (767-142個/平方微米)；相對應的是，金奈米點的尺寸一致性卻隨之驟升 (21%-62%)。另一方面，若奈米點尺寸超過應力場間距，正交應力場提供的侷限效應 (confinement effect) 使得金奈米點基底呈現正方形而非一般最低能量之圓形。此一特殊現象提供了低維度奈米材料生成形狀的其他可能。
熱效應對於金奈米點在矽鍵結雙晶上之行為方面，我們以臨場超高真空電子顯微鏡 (in situ UHV-TEM) 觀察到金原子團在遠低於金-矽共晶溫度 (Au-Si eutectic temperature = 363 °C)時，以應力場間距為基本單位做集體快速遷移 (collective movement) 的現象。配合高解析度電子顯微鏡的分析，我們提出在應力場侷限下金原子團傾向以接近黏滯流體的方式 (viscous flow) 集體移動，而非完全以奧斯華方式 (Oswald ripening) 遷移的理論。矽鍵結雙晶的表面有序應力場除了改變原子團移動的機制，也是提供辨識遷移路徑的有力工具。
在金-矽共晶系統的研究已趨成熟的今天，矽鍵結雙晶表面應力場加入了值得研究的變數。我們以臨場超高真空電子顯微鏡觀察了 “金-矽鍵結雙晶”系統的共晶反應 (eutectic reaction)。矽鍵結雙晶之表面應力不但易於誘發金-矽共晶溶液的產生，並且具有表面橫向誘導共晶溶液遷移的現象，迴異於單純的金-矽共晶系統。金-矽鍵結雙晶共晶溶液的形成是基於矽晶體內部含有大量螺旋差排的能量差異所致，利用”溶解-擴散-沈積(dissolution-diffusion- deposition)”的機制降低晶體自由能。同時間造成的基材表面能不均(heterogeneous surface energy) 對液體產生不對稱表面應力 (asymmetric surface tension)，造成溶液的遷移。除此之外，共晶液體的移動方向亦受其表面應力場所控制。液體移動波前 (front edge) 再度受到應力場的影響形成與 [220] (或[2-20]) 方向平行的直線波前，並沿 [2-20] (或[220]) 方向移動。移動速度方面，表面應力場對液體產生的「接觸角滯變效應」(contact angle hysteresis, CAH)，使得液體在應力間距密度較大的表面移動較為緩慢，本研究亦將針對其表現之行為做機制探討。
另外針對溫度效應對金-矽鍵結雙晶共晶液體的移動速度做分析。發現以溫度做唯一變因時，溫度越高(500-700 °C)，共晶液體移動速度越大(4.5-13 nm/s)。但是過快的移動速度將導致”溶解-擴散-沈積”的機制無足夠時間完成，在移動途徑留下過多殘餘的金矽化物 (gold silicides)，也因金消耗太快使得移動路徑大為縮短。在奈米微流體的應用上，金-矽鍵結雙晶系統提供了可引導奈米液滴移動的能力，將是值得持續投入的研究。

關鍵字

臨場超高真空穿遂式電子顯微鏡 ； 矽鍵結雙晶圓 ； 扭轉差排網 ； 週期性表面應力場 ； 金奈米結構 ； 熱動力行為

並列摘要

Formation of Au nanoparticles on Si bicrystal has been investigated by transmission electron microscopy. The Au nanoparticles were found to form preferentially at the node points of the dislocation network to relieve the strains induced by the dislocations. Surface strain field of substrate with smaller dislocation spacing provides higher particle distribution density and better size uniformity. After annealing at 150 °C, the Au nanoparticles were found to be confined by the dislocation grids. Upon further annealing at 250 °C, small nanoparticles shrank in favor of the large nanoparticles, with a size larger than the critical size, as a manifestation of the classical Oswald ripening process. The shrinkage of Au nanoparticles exhibited a stepped behavior in that clusters shrank by steps of dislocation grids [Chapter 4].
The movement of nanosized Au clusters on Si bicrystal was found to be critically influenced by strained fields of the buried twist-dislocation network by in situ ultrahigh vacuum transmission electronic microscopy (in situ UHV TEM). Collective movement of Au atoms was observed. Most strikingly, clusters of more than three million atoms move concertedly by one dislocation spacing (7–45 nm) within 1/30 s at a substrate temperature of 250 °C. The “jumping” mechanism is attributed to the viscous flow. The observation shall serve as a good reference to refine the theory to realize the control of self-organized nanoparticles on silicon bicrystals [Chapter 5].
Directed movement of Au–Si alloy droplets towards buried dislocation grids on a Si bicrystal has been observed. It was found that once the underlying dislocation structure was dissolved, the movement of Au–Si droplets was directed to the region with remaining dislocation network. The migration of Au–Si droplets is driven by the energy difference between the strained bicrystal and nonstrained single-crystal silicon. The directed movement by the buried dislocation network is potentially significant in a wide range of technologies [Chapter 6].
Influence of heating temperature on the behavior of Au-Si eutectic alloy droplet on silicon bicrystal has been investigated. It was observed that under high heat temperature the droplet had conspicuous high moving velocity, which enhanced by the lower active energy of eutectic reaction. High velocity of droplet leaded to silicon precipitation without enough time, and left a lot amount of gold silicide behind the droplet [Chapter 7].

並列關鍵字

In situ UHV TEM ； Silicon bicrystal ； Twist dislocation network ； Periodic surface strain field ； Au nanostructures ； Thermal dynamic behavior

參考文獻

Chapter 1

[1.1] Introduction to Nanotechnology, http://www.nanotech-now.com/introduction.htm

[1.2] E. T. Kisak, B. Coldren, C. A. Evans, C. Boyer, and J. A. Zasadzinski, “The Vesosome - A Multicompartment Drug Delivery Vehicle,” Current Med. Che. 11, 199-204 (2004).

[1.3] J. D. Meindl, Q. Chen, and J.A. Davis, “Limits on Silicon Nano- electronics for Terascale Integration,” Science 293, 2044-2049 (2001).

[1.5] The National Technology Roadmap for Semiconductors, Semiconductor Industry Association, CA, 2005.