微晶矽薄膜製程於高頻電漿反應器之電漿診斷與模型研究

Plasma Diagnostics and Kinetic Modeling on VHF PECVD of Microcrystalline Silicon

蔡旺霖(Wang-Lin Tsai)

指導教授 ： 魏大欽

中原大學/工學院/化學工程研究所/碩士(2011年)

https://doi.org/10.6840/cycu201100655

摘要

本研究以高頻電漿化學氣相沉積法(VHF-PECVD, 40.68MHz)針對微晶矽薄膜太陽能電池製程中，以四極柱質譜儀(Quadrupole Mass Spectroscopy, QMS)結合電漿放射光譜儀(Optical Emission Spectroscopy, OES)作為即時分析工具，探討改變操作參數(功率、壓力、氣體配比)對電漿性質及鍍膜速率與結晶率之影響，並進行電漿活性物種濃度之全定量分析。
實驗結果發現在微晶矽製程中提高電漿功率有助於提升解離率與鍍膜速率及膜材結晶率。在固定總流量提高SiH4配比時解離率雖然下降，但因為SiH4流量提高而總解離量提升而鍍膜速率升高，然而氫氣的減少卻導致結晶率下降。壓力增加時，電子平均自由徑下降，各物種解離量也隨之下降，但氫原子在電漿中濃度提高可增加對膜面的蝕刻效率使解離率增加。而在結合質譜儀與光譜儀分析結果發現，利用OES之H/SiH強度來估算微晶矽薄膜表面結晶度雖可得到相符合之趨勢，若是利用氫原子與SiHx物種之通量比值來計算其結晶率能更具有代表性。
本研究亦建立了微晶矽之電漿反應數學模型，包含monosilane與disilane相關的氣相及表面鍍膜反應。改變操作參數探討其對模型之影響發現，SiHx自由基物種濃度以SiH3最高、SiH2次之，Si2Hx自由基物種濃度則以Si2H5最高。利用電漿模型與實驗值比較電漿主要物種組成及解離率，發現在高解離率條件下兩者物種密度最為接近。以電漿模型進行鍍膜反應之動力學分析，發現電子之形成與消耗分別為與H2及H2+碰撞而反應為主，SiH4生成為SiH3與膜表面H反應為主，其消耗則與氫原子反應生成SiH3較多。另外在低功率時SiH4易解離成SiH3而貢獻於鍍膜，高功率下之鍍膜機制則以Si2H5物種占多數。

關鍵字

電漿診斷 ； 電漿放射光譜 ； 四極柱質譜儀 ； 太陽能電池 ； 微晶矽 ； 電漿輔助化學氣相沉積法 ； 電漿模擬

並列摘要

In this study, the plasma diagnostics tools (including actinometrics OES and QMS) were used to investigate the influences of operating parameters on intrinsic layer deposition process of μc-Si:H thin film solar cell in parallel-plate PECVD system with an excitation frequency of 40.68 MHz (VHF). The diagnostics results were correlated to deposition rate and crystalline fraction of deposited silicon films. A combination of experimental diagnostics and computational modeling was utilized to understand the main reaction mechanism in the plasma.
The plasma diagnostic results show that when plasma power is increased, both the silane dissociation efficiency and deposition rate are increased. Meanwhile, it also benefits to film’s crystallinity. Higher silane flow rate leads to a slightly decreased in silane dissociation efficiency, however there are much more dissociated radicals contributed to film growth. When operating pressure is increased, both the silane dissociation efficiency and deposition rate are decreased. Since higher pressure corresponds to higher species collision frequency, the H atoms get higher probability to etch silicon surface, resulting in better film’s crystallinity. By comparing OES and QMS results, quantitative analysis of the main species in the plasma can be achieved. On the basis of theoretical deposition model, we have proposed a flux ratio of H atom to SiHx radical as a new indicator of crystallinity during film growth of μc-Si:H.
Finally, a global model of SiH4/H2 plasma is developed. Both gas phase plasma chemistry and silicon film deposition process are included in the model. The model is found to capture the trend in experiment results on the effect of operating parameters on film deposition rate and active species concentrations in the plasma.

並列關鍵字

μc-Si:H solar cell ； modeling ； QMS ； OES ； plasma diagnostics

參考文獻

【20】 潘彥妤, 「微晶矽薄膜製程之電漿放射光譜分析與其在太陽能電池之應用」, 私立中原大學碩士論文 (2008).

【30】 許聖鑫, 「電感偶合式氬氣及四氟甲烷電漿反應器之二維模型研究與電漿診斷」, 私立中原大學碩士論文 (2004).

【2】 I. Langmuir, “Oscillations in ionized gases”, Proceedings of the National Academy of Sciences of the United States of America, 14, 628(1928).

【3】 彭思君，「電感耦合式六氟化硫電漿反應器之電漿診斷與模型研究」，私立中原大學碩士論文 (2006)。

【4】 H. Curtins, N. Wyrsch and A. V. Shah, “High-rate deposition of amorphous hydrogenated silicon: effect of plasma excitation frequency”, Electronics Letters, 23, 228(1987).