以偏壓輔助微波電漿化學氣相沉積成長奈米晶鑽石薄膜之研究

Studies of bias enhanced growth of nanocrystalline diamond films by microwave plasma chemical vapor deposition

曾冠欽(Kuan-Chin Tseng)

指導教授 ： 林諭男

淡江大學/理學院/物理學系碩士班/碩士(2011年)

https://doi.org/10.6846/TKU.2011.00984

摘要

因為鑽石與矽基材的晶格常數不匹配，所以在成長鑽石薄膜之前必須要先預孕核或是破壞矽基材表面降低鑽石與矽基材的表面能差，一般常用的方式是將矽基材放入添加鑽石粉與鈦粉的溶液中以超音波震盪45分鐘，其缺點就是成核時間長而且成核密度低。成核完後利用微波電漿化學汽相沉積法(Microwave Plasma Chemical Vapor Deposition, MWPCVD)以甲烷(CH4)與氬氣(Ar)電漿成長超奈米晶鑽石(Ultrananocrystalline Diamond, UNCD)薄膜。本研究開發利用甲烷(CH4)與氫氣(H2)，以負偏壓成核(Bias Enhanced Nucleation, BEN)及負偏壓成長(Bias Enhanced Growth, BEG)製程，合成奈米晶鑽石薄膜(nanocrystalline diamond, NCD)。
在第一部份，我們首先是固定成長時間60分鐘，改變負偏壓(Negative Bias Voltage)、微波功率(Power)、氣體壓力(Pressure)與甲烷濃度 (Methane Concentration)，並且找出一組可以獲得最佳場發射NCD薄膜的製程條件。接著在第二部份，我們固定一組場發射最佳條件，成核10分鐘後改變不同偏壓成長60分鐘，去探討不同偏壓對微結構的影響。在第三個部份，我們比較BEN-BEG製程與「二階段製程」的不同。所謂「二階段製程」是利用超音波震盪後在成長一UNCD薄膜當作孕核層，之後再成長一層微晶鑽石(Micro-crystalline Diamond, MCD)。我們在施加偏壓成核與成長過程中，會利用光放射光譜 (Optic Emission Spectroscopy, OES)與偏壓電流(Bias Current)監測製程，並利用拉曼光譜、SEM分析表面形貌、電子場發射量測(EFE)鑽石薄膜的特性。
研究發現不同的製程參數對於鑽石薄膜的成分、表面形貌電子場發射特性有很大的影響。而甲烷(CH4)與氫氣(H2)電漿BEN-BEG成長出來的鑽石薄膜場發射特性優於一般UNCD。最後利用穿透式電子顯微鏡(Transmission Electron Microscopy, TEM)分析BEN-BEG與超音波震盪成核成長出來的薄膜，BEN-BEG的晶粒尺寸約50nm，與一般甲烷(CH4)與氫氣(H2)電漿成長出來的MCD 晶粒尺寸約500nm很明顯變小，但尚未達到UNCD晶粒尺寸等級，另一方面再去分析基材與鑽石的介面成核機制，則可以發現偏壓輔助成核與超音波震盪成核兩者的機制有很大的不同；偏壓輔助成核可以避免在孕核層型成非晶形碳(amorphous carbon)，且在NCD薄膜中會形成石墨絲(graphitic filament)，這或許可以解釋BEN-BEG場發射起始電場比UNCD低的原因。

關鍵字

偏壓輔助成核 ； 偏壓輔助成長 ； 奈米晶鑽石薄膜 ； 微波電漿化學汽相沉積

並列摘要

Due to the mismatch in the lattice parameters of diamond and silicon, prenucleation process is necessary to grow diamond on the Si-substrate. The conventional way of nucleating the Si-substrates, the ultrasonication in diamond and Ti-powder solution, requires long processing time (i.e., 45 min) and imposed significant damage on the Si-substrates. Once prenucleated, the diamond films are readily grown on the Si-substrates by using the Microwave Plasma Chemical Vapor Deposition (MWPCVD) process in CH4/Ar or CH4/H2 plasma. In this study, we developed an bias-enhanced nucleation and bias-enhanced growth (BEN-BEG) process for synthesizing the nanocrystalline (NCD) diamond films.
In the beginning, we systematically adjust the processing parameters, including bias voltage, microwave power, total pressure and methane concentration, so as to optimized the electron field emission properties of the diamond films (BEN for 10 min and BEG for 60 min in CH4(5-7%)/H2 plasma). Then we investigated the effect of bias voltage on the characteristics, microstructure and EFE properties, of the diamond films. In the third part of research, we examined the evolution of microstructure and EEE properties during the BEN-BEG process. In the final part of research, we compared the characteristics of the NCD films prepared by BEN-BEG process with those of the two-step processed UNCD films. The two-step process includes the formation of UNCD layer to serve as nucleation layer, followed by the growth in CH4/Ar/H2 plasma. We used the optical emission spectroscopy (OES) and the bias current to monitor the BEN-BEG process and used Raman spectroscopy, SEM/TEM and EFE to characterize the diamond films.
We observed that the processing parameters imposed significant change on the morphology and EFE properties of the diamond films. The NCD films prepared by BEN-BEG process exhibit superior EFE properties to the UNCD films prepared by the CH4/Ar plasma (or the two-step MCD/UNCD process). Transmission Electron Microscopy (TEM) examination revealed that the diamond films prepared by BEN-BEG process have grains about 50 nm in size, which is markedly smaller than those prepared by conventional MPECVD (CH4/H2) process (~500 nm), but is not as small as those synthesized in CH4/Ar-MPECVD process (~5 nm). Moreover, the diamond nucleated on Si-substrates without the formation of amorphous carbon (a-C) phase and there formed a graphitic filament inside the NCD films, which can account for the lower EFE turn-on field with higher EFE current density for the BEN-BEG derived NCD films, as compared with the two-step processed UNCD films.

並列關鍵字

BEN ； BEG ； NCD ； MPCVD

參考文獻

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