電漿輔助化學氣相沉積製程研究非晶矽碳化物薄膜之放射光譜量測分析與薄膜特性之關聯性

Correlation between Analysis of Optical Emission Spectrum and Properties of a-Si1-xCx:H Thin Film Deposited by Plasma Enhanced Chemical Vapor Deposition System

林敬博(Ching-Po Lin)

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(2011年)

https://doi.org/10.6843/NTHU.2011.00148

摘要

中文摘要
氫化非晶矽碳化物薄膜為高能隙(bandgap)材料，主要應用在異質接面太陽電池的window layer、結合非晶矽薄膜做tandem結構的太陽電池，以及做為智能玻璃窗的TFSC(薄膜太陽電池)元件部份等。為了提高開路電壓及提高穿透率之要求，提升光學能隙及縮小薄膜厚度變為重要的議題，其中薄膜材料的結構、光學特性及電性與沉積薄膜時電漿中重要物種及自由基的相對濃度有重要關聯性，因此研究電漿特性對沉積薄膜的影響為重要的議題。
本研究主旨為利用射頻27.12 MHz電漿輔助化學氣象沉積系統製備氫化非晶矽碳化物薄膜，主要做為TFSC(薄膜太陽電池)的i-layer之應用。並以光學放射光譜儀(OES)量測電漿建立一系列電漿放射光譜分析，研究電漿特性隨不同製程參數之變化(射頻功率26~58 W，CH4/SiH4氣體流量比0.93~2，氣體壓力100~200 Pa，H2/SiH4氣體流量比6~18)。另一方面，量測沉積薄膜的材料性質、光學性質及電特性。最後探討電漿物種改變對薄膜材料、光學、及電特性之影響。
研究發現當增加射頻功率(26 W~58 W)，各特徵粒子放射光強度也增加，其中SiH*的放射光強度比CH放射光強度大(5.8倍~7.75倍)，顯示沉積薄膜SiH*物種扮演關鍵的角色。SiH*放射光強度也證明SiH3前趨物沉積速率影響。另一方面，CH/SiH*放射光強度比值(OES ratio)可視為CH粒子與SiH*粒子的相對濃度比，研究顯示此OES ratio與光學能隙的結果趨勢一致。另外增加CH4/SiH4氣體流量比(0.93~2)，結果顯示甲烷的氣體流量增加，造成薄膜的光學能隙上升，此結果與文獻一致。並發現光敏性隨光學能隙上升而下降，表示光敏感度下降。OES量測結果顯示，當CH4/SiH4氣體流量比增加兩倍，OES ratio顯示CH/SiH*上升約四倍，研究發現在氣相中CH物種與SiH*物種的相對濃度上升，此結果導向薄膜中C-Si比例的增加，並與光學能隙的實驗結果穩合。OES提供了一個以量測相對CH/SiH*粒子濃度，預測光學能隙隨操作參數之變化趨勢的方法。
Abstract
Si-C:H film has attracted a lot of attention recently for application in Si-based thin film solar cells, since its bandgap can be easily tunable over a range of 1.5-2.5 eV by simply varying the Si to C ratio in the film. Capacitively coupled SiH4/CH4/H2 plasmas are often employed for deposition of high quality Si-C:H film. A better understanding of the physical and chemical mechanisms in the plasma discharge is desirable. In this study, optical emission spectroscopy (OES) of five spectral lines, including H (656.2 nm), H (486.2 nm), SiH*(412.8 nm), CH (430.1 nm) and H2 Fulcher band ( 600-630 nm), has been carried out for analyzing the discharge characteristics during the deposition process. The material property, optical property of the film and electrical property are measured after deposition. The H2 Fulcher band intensity, an indicator of plasma density, is found to increase with rf power, as expected. The deposition rate also increases with rf power. Correlation between OES spectral line intensity ratio and bandgap for different operating condition is demonstrated. As the flow rate ratio of CH4/SiH4 increases from 0.93 to 2.00, the CH intensity increases accordingly, as expected, while SiH* intensity decreases little. On the other hand, the H and H2 Fulcher band vary little but both intensities drop as the CH4/SiH4 ratio reaches 2.00. These results indicate that CH4 plays an less important role for the ionization process in the SiH4/CH4/H2 plasma, as a result of its higher ionization potential than that of SiH4.

關鍵字

氫化非晶矽碳化物薄膜 ； 電漿放射光譜 ； 電漿輔助化學氣相沉積

參考文獻

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