電漿輔助化學氣相沉積奈米碳纖維與場發射特性之研究

Study of Growth of Vertically-Aligned Carbon Nanofibers by Plasma Enhanced Chemical Vapor Deposition－Growth Mechanism and Field Emission Characterization

魏鴻文(Hung-Wen Wei)

指導教授 ： 柳克強 ； 蔡春鴻

國立清華大學/原子科學院/工程與系統科學系/博士(2008年)

https://doi.org/10.6843/NTHU.2008.00004

摘要

本論文研究係發展電漿輔助化學氣相沉積奈米碳纖維製程，並探討其成長機制與場發射之特性。首先成功開發以電感式耦合電漿源成長垂直排列的奈米碳纖維之方法，其中基板為鍍有鎳金屬當催化劑顆粒的矽基板。成長製程使用C2H2/H2的電漿，操作於較低氣壓，基板置於具有溫控及可施加直流偏壓的基座上，操作溫度可低於550 oC，基板所加的直流偏壓約為200 - 400 V。為能進一步探討奈米碳纖維在電漿中的成長機制，經由改變電感式耦合電漿源的功率、基板上的直流偏壓、基板的溫度、製程壓力、不同的氣體流量比例等不同的製程參數成長奈米碳纖維，本研究以電漿光譜儀和四極質譜儀分析電漿中的不同氣體或離子成分濃度，即時監測電漿特性。成長後的奈米碳纖維其直徑、長度、成長密度以掃描電子顯微儀（SEM）量測，用穿透式電子顯微儀(TEM) 觀察層狀和竹節狀結構，並以拉曼光譜分析奈米碳纖維石墨化的程度，此外亦以二極結構量測奈米碳纖維的場發射特性。本研究並以實驗結果為依據，建立奈米碳纖維在電漿鞘層中所承受的靜電力模型、在電漿中的成長動力模型、場發射量測時在球狀探針下電場分佈模型，探討奈米碳纖維不同的成長製程參數對奈米碳纖維的特性影響。研究的結果顯示當基板的直流偏壓和電流太高對長度較長及直徑細小的奈米碳纖維都不利其存在基板上。電漿鞘層所形成的靜電場壓力是幫助奈米碳纖維往上長的力量，但也是將奈米碳纖維拉拔離基板表面的力量。藉由理論分析乙炔在電漿的環境下會形成長鏈的碳氫化合物沉積在鎳奈米催化劑的表面上形成非晶系碳膜，阻止催化反應，因此需藉由氫原子與氫離子將非晶系碳移除只留下單一碳原子在鎳催化劑的表面上經由擴散過程形成石墨層，提升氫離子與氫原子的雖可增進奈米碳纖維的成長，但也促進奈米碳纖維本體被腐蝕。

關鍵字

奈米碳纖維 ； 電感式耦合電漿 ； 電漿輔助化學氣相沉積

並列摘要

We have developed the growing carbon nanofibers (CNF) process by plasma enhanced chemical vapor deposition (PECVD) and studied the growth mechanism of carbon nanofibers (CNFs) in the plasma environment and the characterizations of field emission. The vertically aligned CNFs are grown on the Si substrate deposited with nanoparticles of Ni catalyst by the inductively coupled plasma (ICP). A pyrolytic graphite/pyrolytic boron-nitride heater, located beneath the graphite stage, was utilized for substrate heating. A DC electric bias was applied on the graphite stage for conductive substrate. The feed gas was a mixture of acetylene, to provide carbon source for CNFs growth, and hydrogen, to keep the catalyst active by etching away amorphous-carbons. The processing gas was a mixture of acetylene and hydrogen and it was introduced into the process chamber. The inductively coupled plasma was turned on under low gas pressure, ~ 20 mTorr. The CNFs are grown at various process parameters such as the ICP power, the DC bias, the temperature of substrate, the process pressure and the ratio of gas flow. The gas components are monitored in situ by optical emission spectroscopy and quadrupole mass spectrometry. The length, diameter and growth density of CNFs are measured by scanning electron microscopy and the sheets and bamboo-like layers of CNFs are observed by transmission electron microscopy. The graphitized degree of CNFs is characterized by Raman spectroscopy. The field emission of CNFs is also characterized. The growth results show that they are less likely for smaller radius CNFs and longer length CNFs to survive as the DC bias and current applied on substrate are high during the growth process. The electrostatic pressure produced by the plasma sheath electric field is the vertical alignment force on the growth of CNFs, but it is also the force to detach the CNFs from the substrate surface. From the results of theoretical analysis, the long chain hydrocarbon produced with acetylene in the plasma environment deposites on the nanoparticle surface of Ni catalyst to form the amorphous carbon film and prevent from catalytic reaction. Therefore, the amorphous carbon needs to be removed by hydrogen atoms and ions and the single carbon atoms remaining on the surface of Ni catalyst form graphite layer by diffusion process. Although the CNFs growth can be improved by increasing the concentration of hydrogen atoms and the flux of hydrogen ions, the etching rate of CNFs body is also increasing.

並列關鍵字

carbon nanofibers ； inductively coupled plasma ； plasma enhanced chemical vapor deposition

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