電容式耦合射頻矽烷/氫氣電漿之數值模擬研究 -電壓波型影響

Simulation Study of Capacitively Coupled Radio Frequency Silane/Hydrogen Plasma Discharges -Effect of Tailored Voltage Waveforms

黃宣文(Syuan-Wen Huang)

指導教授 ： 柳克強

國立清華大學/原子科學院/工程與系統科學系/碩士(2018年)

摘要

電容式耦合電漿(Capacitively Coupled Plasma, CCP)被廣泛應用於蝕刻與沉積製程中。本研究旨為探討輸入特製電壓波型(Tailored Voltage Waveforms, TVWs)對SiH4/H2電漿特性之影響，採用數值模擬計算分析，計算二維流體模型模擬電漿，探討分析電漿中之物理與化學機制。本研究TVWs分為兩類分別為疊加兩種頻率與四種頻率。
第一類TVWs由兩個電壓相等的正弦波組成，其頻率分別為13.56 MHz與其第一個諧波頻率27.12 MHz，改變兩頻率間的相位差以產生出不同波型，當TVWs正負電壓的極值不對稱時，兩邊電極的鞘層電壓也會隨之改變，而相位角θ為45°與135°時，正電壓與負電壓的絕對值相差最大。令相位角θ為45°與135°時波型為V45與V135。模擬結果顯示，當波型為V45時，相較於V135與單頻13.56 MHz，接地端鞘層電壓分別低了63與49 %，這是因為V45之供壓電極端的鞘層電壓較大，此特性同時造成電子溫度與功率密度在施加電壓端較高。且由於V45之接地端鞘層電壓較小，電位梯度較小，因此加速離子的電場也較小，可預期在此波型下基板之離子轟擊效應較低。除了電漿放電特性的分析，此研究也分析到達基板之SiH2/SiH3¬與H/SiH3通量比[1, 2]，而模擬結果顯示，電壓波型V45相較於V135，接地電極端的SiH2/SiH3通量密度比值下降了約50%，這是由於波型為V45時接地電極端的電子溫度較低，進而降低SiH2¬的生成速率，而到達基板之H/SiH3通量比相較於V135與單頻13.56 MHz相差在10 % 之內。由於沉積薄膜的缺陷密度與結晶率分別與SiH2/SiH3¬, H/SiH3通量比通量比成正相關，因此模擬結果顯示V45 TVWs可降低薄膜中缺陷密度但並不影響結晶度。
第二類TVWs由13.56MHz與其第二、三、四諧波頻率組成，其各諧波之相位角相同，相位角為0與π時之波型分別為”peak”與”valley”。此時相較於V45與V135可以產生正電壓與負電壓的絕對值比例相差更大的波型，模擬結果顯示在peak波型下離子能量最小，且到達基板之SiH2/SiH3通量密度比相較於V45與單頻13.56 MHz分別低了10 %與40 %，而H/SiH3通量相差3 %之內。因此模擬結果顯示”peak”波型相較於V45與單頻13.56 MHz可降低薄膜中缺陷密度但並不影響結晶度。最後，”peak”波型模擬結果SiH2/SiH3通量密度下降可對文獻實驗結果[3] 微結構比率下降進行解釋，因此在peak波型下薄膜品質提升。

關鍵字

矽烷/氫氣電漿 ； 電壓波型 ； 電容式耦合電漿

並列摘要

Capacitively coupled plasma (CCP) has been widely employed for etching and deposition processes. The purpose of this study is to investigate the effect of Tailored Voltage Waveforms (TVWs) on the plasma characteristics of SiH4/H2 plasma, a fluid model based numerical simulation analysis is employed to investigate the fundamental discharge characteristics of the basic physical and chemical mechanisms occurring in the plasma reactor. The TVWs in this study divided into two categories: consist of two frequencies and four frequencies.
The first type of TVWs are consist of two equal-voltage sine waves, which are the fundamental frequency 13.56 MHz and the second harmonic frequency 27.12 MHz. Different waveforms can be generated by tuning the relative phase between two frequencies. When the absolute values of positive and negative extremes in TVWs are different, the voltages across the two sheaths are different. At the phase θ = 45° and θ =135°, the difference between the absolutes of positive and negative extreme is maximum. Define these waveforms are V45 and V135, when phase angle are 45 and 135 degree. Simulation results show that when the waveform is V45, the sheath voltage in front of ground electrode is 63 and 49% lower than V135 and single frequency 13.56 MHz, respectively. It is because of the voltage in front of powered is bigger, resulting in higher electron temperature and power density at the side of power electrode. Due to the sheath voltage in front of ground electrode is smaller, thus decrease the potential gradient and ion bombardment effect. In addition to the analysis of the plasma characteristics. This study also analyzes the flux ratio of SiH2/SiH3 and H/SiH3 reaching the substrate. Simulation results showed that the SiH2/SiH3 ratio decreased approximately 50% when the waveform is V45 compared to V135. This is because the electron temperature at the ground electrode is lower when the waveform is V45, which in turn reduces the rate of formation of SiH2. However, the H/SiH3 ratio reaching the substrate is within 10% of the V135 and the single frequency 13.56 MHz. Since the defect density and crystallinity of the film directly proportional to flux ratio of SiH2/SiH3 and H/SiH3, respectively, the simulation results show that TVWs V45 can reduce the defect density in the film without affecting the crystallinity.
The second type of TVWs consists of 13.56MHz and its second, third and fourth harmonic frequencies. When θ is 0 and π, the waveform define as “peak” and “valley”. Comparing with V45 and V135, these waveforms having a larger difference ratio between the absolute values of the positive voltage and the negative voltage. Simulation results show that the ion energy is the smallest in waveform “peak”, and the SiH2/SiH3 flux ratio reaching the substrate is 10% and 40% lower than V45 and single frequency 13.56 MHz, respectively. Therefore, the simulation results show that TVWs "peak" can reduce the defect density in the film but does not affect the crystallinity compared to V45 and single frequency 13.56 MHz. Finally, TVWs "peak" simulation results show that the decrease of flux ratio SiH2/SiH3 can explain the microstructure ratio decrease in the literature [3], so the film quality improve under the TVWs “peak”.

並列關鍵字

Silane/Hydrogen Plasma ； Tailored Voltage Waveforms ； CCP

參考文獻

[1] H. Kawasaki, H. Ohkura, T. Fukuzawa, M. Shiratani, Y. Watanabe, Y. Yamamoto, et al., "Roles of SiH3 and SiH2 radicals in particle growth in rf silane plasmas," Japanese Journal of Applied Physics Part 1-Regular Papers Short Notes & Review Papers, vol. 36, pp. 4985-4988, Jul 1997.

[2] B. B. Sahu, J. G. Han, K. S. Shin, K. Ishikawa, M. Hori, and Y. Miyawaki, "Plasma diagnostic approach for high rate nanocrystalline Si synthesis in RF/UHF hybrid plasmas using a PECVD process," Plasma Sources Science & Technology, vol. 24, Apr 2015.

[3] E. V. Johnson, S. Pouliquen, P. A. Delattre, and J. P. Booth, "Hydrogenated microcrystalline silicon thin films deposited by RF-PECVD under low ion bombardment energy using voltage waveform tailoring," Journal of Non-Crystalline Solids, vol. 358, pp. 1974-1977, Sep 2012.

[4] Z. H. Bi, Y. X. Liu, W. Jiang, X. Xu, and Y. N. Wang, "A brief review of dual-frequency capacitively coupled discharges," Current Applied Physics, vol. 11, pp. S2-S8, 2011.

[5] Z. Donko, J. Schulze, B. G. Heil, and U. Czarnetzki, "PIC simulations of the separate control of ion flux and energy in CCRF discharges via the electrical asymmetry effect," Journal of Physics D-Applied Physics, vol. 42, Jan 2009.