40.68 MHz連續式電漿輔助化學氣相沉積之微晶矽薄膜製程開發研究與電漿放射光譜分析

Study of Microcrystalline Silicon Thin Film Deposition by 40.68 MHz In-Line PECVD and Plasma Emission Spectroscopy Analysis

范智翔(Chih-Fan Hsiang)

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(2013年)

摘要

摘要
薄膜太陽能電池因無須使用較昂貴的矽原料，取而代之可沉積於大面積、低成本的玻璃、不鏽鋼和塑膠基板上，因此能大幅降低材料成本。微晶矽(c-Si:H)薄膜太陽能電池擁有更好的光譜響應，且對於紅外與進紅外光區有更好的吸收效率以及具有更佳抵抗因日光照射造成的衰退等優點。
在本論文研究主旨裡，將研究開發利用In-Line PECVD沉積微晶矽薄膜之相關製程技術以沉積出高效率之太陽能電池，並在製程中配合光學放射光譜儀(OES)的使用，來協助監測電漿的穩定度以及建立電漿光譜與製程參數，如射頻功率、氣體壓力以及矽甲烷流量比間之關係，並透過分析薄膜的材料特性以及電特性來建立電漿反應粒子與製程參數以及薄膜三者之間的關聯性。在本論文中監測的粒子除了SiH* (414.2 nm)、H(486 nm)、H(656 nm)與H2 fulcher (600-630 nm)之外，還另外加入了Si* (288 nm)的量測，並使用加入新的OES-ratio(Si*/SiH*、H/Si*、H/Si*)來建立電子溫度與製程參數之間的關係並與原先的OES-ratio(H/H分析比較，希望能夠更進一步的了解電漿內部的反應機制。並使用H2 fulcher來代表電子密度的變化趨勢。
在本論文研究中，使用的電漿源為40.68 MHz，通入的氣體為SiH4與H2。從研究中發現，結晶度與OES-ratio(H/SiH*, H/SiH*)不論是在不同的射頻功率、氣體壓力或是矽甲烷流量比下，皆呈現出相同的變化趨勢，受功率的提升而升高，隨著氣裡壓力及矽甲烷流量增加而下降。而光敏性除了在不同的射頻功率下，其餘呈現相反的變化趨勢。在電子溫度的部分，在提高射頻功率時，OES-ratio(HH、Si*/SiH*、H/Si*、H/Si*)跟著功率的提高而上升。但是在改變壓力時，OES-ratio卻隨著壓力的上升而跟著提高，此部分與本實驗室古馥瑋學長所模擬之結果有所出入，但若使用H/Si*及H/Si*時，其變化趨勢即隨著氣體壓力的上升而下降，此部份即符合古馥瑋學長所模擬之結果。
在改變射頻功率以及氣體壓力時，電漿中特徵物種的強度與電漿密度有相同的變化趨勢，射頻功率提高時，粒子強度因為電子密度的上升而提高，而氣體壓力上升，粒子強度因為電子密度的下降而降低。
而在改變矽甲烷流量比方面，隨著矽甲烷流量的提升，電子密度的變化較無規律性。而電子溫度的部分，OES-ratio(HH)與OES-ratio(Si*/SiH*)的變化則呈現相反的變化趨勢。而H/Si*及H/Si*隨矽甲烷流量比的變化也是不一致的，其中H/Si*還呈現出不規則的變化情形。
經由上述的實驗分析， In-Line PECVD可在高功率1000 W、高壓力5 torr、高矽甲烷流量比為3.3%時，沉積出條件較佳的c-Si:H薄膜，其結晶度為42%，光敏性為8.74×102，此光敏性已達到102以上，以達到製作元件的要求。但其電池效率表先不佳，僅有0.01%，因此降低射頻功率以減少因高功率對Barrier-layer及I-layer間的離子轟擊所造成的接面缺陷增加，當射頻功率降到600 W、4 torr時，效率有大幅的提升，轉換效率為4.05%。若在未來能將P-I-N layer皆在In-Line PECVD成長，預期轉換效率應可再提升。

關鍵字

微晶矽薄膜 ； 電漿放射光譜 ； 薄膜太陽能電池

並列摘要

Abstract
Thin-film solar cells are fabricated by low-cost production processes, and deposited on low-cost substrate like glass, stainless steel or plastics. Therefore, thin-film solar cells are an alternative to conventionally used wafer solar cells based on crystalline silicon. The hydrogenated microcrystalline silicon(c-Si:H) thin film solar cells have better absorption efficiency at near infrared and better resistance to degradation caused by sunlight illumination.
The purpose of this thesis is develop process for c-Si:H thin films by In-line plasma enhanced chemical vapor deposition(In-line PECVD). When in the process, optical emission spectroscopy(OES) be employed to monitor the stability of plasma and to build the relationship between plasma spectrum and process parameters such as RF power, gas pressure and silane concentration. Correlation among plasma reactive species, process parameters and thin film are established by analyzing material property of thin film and electric property. In addition to SiH* (412.8 nm), the H (656.2 nm), the H (486.2 nm) and the H2 Fulcher (600-630 nm), we also interest in Si* (288 nm) and use OES-ratio(Si*/SiH*, H/Si*H/Si*) to analyze connection between electron temperature and process parameter. We also compared new OES-ratio(Si*/SiH*, H/Si*H/Si*) with OES-ratio(H/H to understand the reaction mechanism of plasma further. H2 fulcher is used to represent the trend of electron density.
c-Si:H thin films are prepared by the silane-hydrogen plasma with VHF-PECVD (40.68 MHz). The experiment results found that whether it is varying the silane concentration, RF power or gas pressure, the XC and OES-ratio(H/SiH*, H/SiH*) have the same trend, but the trend of photosensitivity is opposite except for RF power. In the part of electron temperature, OES-ratio(HH, Si*/SiH*) increase with RF power. They also increase with pressure but that are not consistent with the simulation results. But when we replaced OES-ratio(HH, Si*/SiH*) with OES-ratio(H/Si*, H/Si*), the trends of OES-ratio(H/Si*, H/Si*) are consistent with simulation result.
The intensity of plasma characteristic species and electron density have the same trend. When the RF power increases, the emission intensity of plasma species get higher as a result of the increases of electron density. But when the gas pressure increases, the intensity of plasma species get lower as a result of the decreases of electron density..
For the different silane concentration, electron density has irregular trend when the silane concentration increase. In the part of electron temperature, there is opposite trend between OES-ratio(HH) and OES-ratio(Si*/SiH*). The trend of OES-ratio(H/Si*) and OES-ratio(H/Si*) are also different depend on variation of silane concentration. Among of them, OES-ratio(H/Si*) has irregular trend, too.
Currently, In-line PECVD can deposit c-Si:H thin film which XC = 42% and photosensitivity = 8.74 × 102 at high power(1000 W), high pressure(5 torr) and high silane concentration(3.3%). But the cell efficiency is just 0.01%. Therefore we use lower RF power to reduce the defect at junction between Barrier-layer and I-layer by reduced ion bombardment. When the RF power lower to 600 W, gas pressure at 4 torr, the cell efficiency has been improved dramatically to 4.05%. If the N-I-P layer all deposited in In-line PECVD, we expect the cell efficiency will be improved more.

並列關鍵字

microcrystalline silicon thin films ； plasma emission spectroscopy ； thin film solar cells

參考文獻

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