矽(001)基板上之奈米孔洞在超高真空熱處理下的形貌變化

Morphological evolution of nanoscale pit fabricated on Si(001) substrate during ultrahigh vacuum heat treatment

姚俊宇(Chun-Yu Yao)

指導教授 ： 管傑雄

國立臺灣大學/電機資訊學院/電子工程學研究所/碩士(2011年)

https://doi.org/10.6342/NTU.2011.10858

摘要

本研究團隊致力於鍺量子點的研究，由於量子點的自聚(self-assembly method)特性，我們事先利用微影與蝕刻製程，在矽基板上製作具有週期結構陣列的樣品，接著採用化學氣相沉積(chemical vapor deposition)的製程方式，利用晶格不匹配(lattice mismatch)的原理，在矽基板上生長鍺量子點。我們已成功的實現在具有一維陣列結構與二維孔洞陣列結構之矽(001)基板上成長有序排列及大小一致性的鍺量子點。
在製作鍺量子點的過程中，我們發現事先所設計之奈米孔洞結構，經過同步熱處理(in-situ heat treatment)步驟時，孔洞形貌會發生變化。從許多文獻上已發現，奈米孔洞的形貌會主導整個鍺量子點的排列位置。原因來自於，在矽鍺介面中的化學能(chemical potential)，會影響鍺量子點的定位與排列形式，而化學能是曲率(Curvature)的函數，曲率是由孔洞的表面形貌所決定。因此，研究熱處理對孔洞形貌的變化有著基礎且重要的意義，可以使我們對於鍺量子點的研究領域有更深入且全面的了解。
此篇論文致力於研究在真空下熱處理，奈米孔洞形貌變化的過程，並定義其變化結果之特徵。此外，造成奈米孔洞形貌變化的物理機制也是我們欲討論的重點。首先，我們分別在不同熱處理壓力與溫度下，藉由輕敲式原子力顯微鏡(AFM)量測奈米孔洞的表面形貌，我們發現熱處理環境必需在低壓而且高溫的條件下，奈米孔洞形貌才會發生形變。接著，我們在低壓而且高溫的熱處理環境下，觀察奈米孔洞隨不同時間的形貌變化，我們發現奈米孔洞隨時間變化的特徵有以下幾項，孔洞深度變淺，脊狀結構(Ridge)的寬度變窄，孔洞內部的側壁角度變緩，孔洞底部的距離變小。同時發現一個令人更感到有趣的現象是，原本我們利用微影設計，經由蝕刻(RIE)得到的圓形與方形奈米孔洞，原本底部形貌分別為圓形與方形，在隨著熱處理長時間下，孔洞內部形貌最後均變化為底部是方型且側壁形成四個面。
在探討奈米孔洞隨時間變化的過程，我們製作了一系列不同尺寸大小的孔洞，從實驗結果可以發現，原本未經熱處理的圓形(或方形)孔洞 ，在過程會先變化成不規則的多邊形，最後皆變化成底部為方形且側壁四個面。為了進一步觀察此結果，我們將孔洞形貌繪製成3D結構圖，可以明顯的發現此底部方形且側壁四個面的孔洞形貌，為一倒截頭金字塔形(truncated-inverted pyramid)，它重要特徵有以下幾項，底部為方型且四個邊均沿著<110>方向，側壁形成四個方向為<11n>的面。以200奈米的原形孔洞為例，在熱處理長時間下，孔洞內部形貌最後變為倒截頭金字塔形，而孔洞深度從原本未經熱處理時的58奈米變淺為20奈米，脊狀結構(Ridge)的寬度從原本58奈米變為0奈米(即脊狀結構變化為頂部圓弧形之山丘形貌) ，孔洞內部的側壁角度從原先58度變緩為12度，孔洞底部距離從原先162奈米變小為78奈米。
在探討造成孔洞形貌變化的物理機制上，我們從溫度壓力之相位圖上發現，造成孔洞形貌變化的條件為低壓且高溫，此現象與熱脫附所發生之條件相符合，熱脫附是在低壓且高溫的環境下，藉由矽與二氧化矽反應成一氧化矽氣體(Si + SiO2 → 2SiO (g)↑ ) 使得二氧化矽脫附矽基板表面，反應的過程中，矽原子是藉由表面自我擴散(surface self-diffusion)到有二氧化矽處，參與一氧化矽氣體的生成反應，因此我們可以得知，是矽原子表面自我擴散造成了孔洞形貌的變化。最後，我們回顧可能使矽基板表面生成二氧化矽的製程步驟，經蝕刻製作完成的奈米孔洞，在成長鍺量子點前，會先經過RCA清洗，此道清洗步驟最後會使用雙氧水，此強氧化劑會使得矽基板表面生成1至2奈米的原生氧化層(native oxide)，而之後再經過稀釋(50:1)氫氟酸清洗，將此原生氧化層去除，並且在矽表面形成氫鍵阻擋再次氧化，此隔絕時效一般為1至2個小時。然而矽基板在等待熱處理時，抽真空至超高真空等級(UHV)需要3至4個小時，因此我們推測在抽真空的過程，矽基板表面再次與腔體內的氧氣生成原生氧化層，而熱處理時發生熱脫附現象，原生氧化層脫附過程中，矽原子會發生表面自我擴散，最後造成孔洞形貌產生變化。
一般無論是藉由分子束磊晶(MBE)或是化學氣相沉積(chemical vapor deposition)製程來製作量子點，皆須要經過此道同步高溫熱處理(in-situ heat treatment) ，目的就是為了藉由熱脫附機制，獲得表面潔淨的基板以提升磊晶的品質與均勻度，熱脫附過程伴隨矽原子表面自我擴散與我們觀察到孔洞形貌發生變化的現象相符合。

關鍵字

奈米孔洞結構 ； 形貌變化 ； 倒截頭金字塔形 ； 磊晶同步熱處理 ； 熱脫附 ； 矽原子表面擴散

並列摘要

The research team has continued to dedicate ourselves to the Germanium Quantum Dots (Ge QDs) field. Due to the self-assembly characteristic of quantum dots, firstly, we have employed Lithography and Etching process to fabricate patterned silicon substrate with periodic array structure on it. Then applied Chemical Vapor Deposition process, the growth of germanium quantum dots on patterned silicon substrate would be achieved by utilizing the characteristic of Lattice mismatch,. We have successfully demonstrated well-aligned Ge dots with uniform size distribution on Si (001) grown on one-dimension grating array structure and two-dimension hole array structure respectively.
During the production of Ge QDs, we found that the originally designed nanoscale pit revealed morphological evolution after in-situ heat treatment. Recently, many literatures have indicated that the growth of Ge QDs on pattered substrate will be significantly dominated by the morphology of nanoscale pit. Because of the existing Chemical Potential between the silicon-germanium interface, which affect the positioning and formation of Ge QDs. Furthermore, the chemical potential is a function of Curvature, and the curvature is determined by the surface morphology of nanoscale pit. Therefore, the study of nanoscale pit morphological evolution during heat treatment has a fundamental and significant meaning. It might provide us a detailed and thorough understanding in Ge QDs research field.
This thesis is mainly focused on discussing the morphological evolution of nanoscale pit during ultra-high vacuum heat treatment and defines each characteristic of the evolution results. In addition, we would like to discuss the mechanism which leads to the morphological evolution. First, we processed heat treatment on the sample at different pressure and temperature. We measured the surface morphology of nanoscale pit by tapping-type atomic force microscope (AFM) and found that only in low pressure and high temperature environment, the morphological evolution of nanoscale pit would be observed. Then, we observe the morphological evolution of nanoscale pit during different time in low pressure and high temperature condition. The important characteristics of the morphological evolution results as the following, the depth decreased, the ridge width narrowed, the sidewall angle became smaller, the bottom distance decreased. In the same time, an extremely interesting result is observed. That is, the origin designed patterns (using E-beam lithography and RIE) of nanoscale pits are circle and square (both top and bottom) respectively, however; after long time heat treatment, the bottom shape of each pits turned to square and the sidewall was formed as four facets.
In order to observe more details of the morphological evolution, we fabricated a series of nanoscale pits with different sizes. The experiment results reveals that both circle and square nanoscale pit remain their original morphology before heat treatment, as the heating time increased, the morphology began to change. We found in the middle step of the change, the transient morphology of the nanoscale pits (circle and square) are multi-line shapes (irregular polygon). However, in the final step, they will both become to square bottom shape and four-facet sidewall. All those changes have no regard with the pit size. Furthermore, we draw the 3D structure diagram for easier understand the whole surface morphology of the pit. This kind of morphology (square bottom shape and four-facet sidewall) is defined as truncated inverted-pyramid. It significant characteristics as follows, the four sides of the square bottom are along with <110> and the four facets of the sidewall are <11n>. Taking 200nm a circle pit for example, after long time heat treatment, the morphology became to truncated inverted-pyramid. And the pit depth decreased from the original 58nm to the final 20nm, ridge width decreased from the original 58nm to the final 0nm (two edges merged), the sidewall angle decreased from the original 58 degree to the final 12 degree, and the bottom distance decreased from the original 162nm to the final 78nm.
In discussion of the mechanism of the morphological evolution of nanoscale pit during heat treatment, we have found that the heating condition leads the morphological evolution are low pressure and high temperature. From the temp-pressure phase diagram, we find that the condition corresponds with the thermal desorption mechanism. Thermal desorption occurs in accordance with the low oxygen pressure and high temperature environment. In this condition, the reaction of silicon and silicon-dioxide will format silicon monoxide, which is a gas molecular. (Si + SiO2 → 2SiO (g) ↑) Through this reaction makes SiO2 desorption from the surface of silicon substrate. Meanwhile, during the process of SiO2 desorption will lead to the surface self-diffusion of silicon atoms, they will diffuse to the SiO2 sidewall and join the formation of SiO.
From the above result, we could reasonably speculate that the surface self-diffusion of silicon atom result in the morphological change of the nanoscale pit. Therefore, we review the process flow and try to find which process step could possibly make the SiO2 formation on Silicon substrate. The silicon substrate with nanoscale pit was fabricated by RIE process, before the growth of germanium quantum dots, the sample will go through the RCA clean. The final step of RCA clean will use Hydrogen Peroxide Solution (H2O2) dip. Since H2O2 is a strong oxidizing agent, so it will make about 1 to 2 nm of native oxide growth on the surface of silicon substrate. After diluted (DI water 50: HF 1) hydrofluoric acid cleaning, thin native oxide will be etched out from the surface of silicon substrate. Meanwhile, silicon surfaces etched in HF become hydrophobic if no oxide is left on the surface and surfaces with hydrogen termination are similar to organic surfaces and do not interact with water. Therefore, it would against the growth of silicon dioxide again and normally the passivation time would last for 1 to 2 hours. In heat treatment process, the silicon substrate will stay in the load-lock chamber until the pressure of main processing chamber lower as ultra-high vacuum. Normally, vacuum pumping to ultra-high vacuum level takes 3 to 4 hours. So we speculate that during the vacuum pumping, native oxide layer would form on the silicon surface again. Because of the ambient of load-lock has sufficient oxygen to react with the silicon surface. During heat treatment, the oxide desorption phenomenon will occur and accompany the surface self-diffusion of silicon atom in the same time. Finally, silicon atom diffusion leads to the morphological change of the nanoscale pit.
No matter we choice MBE or CVD method to fabricate the Ge QDs, there is necessary to go through in-situ heat treatment. The purpose of the process is to obtain a perfectly clean silicon surface by thermal desorption. In order to get better quality and uniformity of the Ge QDs. The phenomenon of thermal the surface self-diffusion of silicon atom accompanied during thermal desorption is in correspondence with the morphological change of the nanoscale pit.

並列關鍵字

morphology of nanoscale pit/hole ； morphological evolution/change ； in-situ heat treatment ； thermal desorption ； silicon surface self-diffusion

參考文獻

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