| 摘要: | 摘 要 本論文利用微波電漿化學氣相沉積(MP-CVD)法以Fe、Co、Ni為觸媒，可以在基板溫度400℃得到密度高且垂直基板方向成長的奈米碳管。利用掃描式電子顯微鏡(SEM)、穿透式電子顯微鏡(TEM)、原子力顯微鏡(AFM)、拉曼光譜儀(Raman Spectroscopy)和I-V量測儀分析奈米碳管特性。 由穿透式電子顯微鏡之高解析觀察得知，本實驗所成長的奈米碳管屬中空竹節狀之多壁奈米碳管，中空竹節形狀為圓錐形，管徑約30~100nm，在奈米碳管頂部及底部都可發現觸媒金屬，頂部的觸媒為不規則形，而底部的觸媒為圓錐形，和中空結構相似，推測其成長機制為碳原子沿著圓錐形觸媒表面往上析出堆積而成奈米碳管。在前處理製程下觸媒薄膜經分裂成核，形成奈米級的顆粒，並非圓錐形，而在成長過程中才會變形為圓錐狀，文中對以微波電漿化學氣相沉積法成長奈米碳管之機制做探討。 先利用單因子實驗探討製程參數對奈米碳管成長速率的影響，製程參數包括：電漿功率、前處理電漿功率、成長電漿功率、基板溫度、工作壓力、N2流量、CH4比例、前處理時間、及成長時間。再應用田口實驗計劃法，分別以長速率及前處理後表面粗糙度為評估製程之輸出回應，探討各因子對奈米碳管成長特性的影響，找尋最佳之成長參數。以奈米碳管成長速率為實驗輸出回應的望大分析，顯示成長時間、前處理時間、氮氣流量、工作壓力為較重要之一半控制因子，而確認實驗，逹到製程最佳化的目標。而以前處理後表面粗糙度為實驗輸出回應的望小特性分析顯示，重要參數望依序為電漿功率、工作壓力、前處理時間，望小特性分析之確認實驗，並未逹到預期目標，主要原因為在前處理過程中，會有少數奈米碳管的生成，導致原子力顯微鏡之量測產生偏差。 奈米碳管可選擇性成長在邊長5μm的方形陣列，量測場發射特性，起始電壓為130伏特，在3伏特/微米(V/μm)的電場下可以得到4.5mA/cm2的電流密度。利用半導體製程將奈米碳管製成三極場發射元件，可利用閘極控制場發射電流密度。另外已成功在0.1μm 的觸媒金屬點上成長單根奈米碳管。 將奈米碳管做抗氧化及高溫退火處理，在高溫氧化的處理過程中，雖可將非晶質碳除去，得到純化的奈米碳管但是也會消耗部份奈米碳管。對奈米碳管做高溫退火，其直徑會隨溫度升高而變大，在頂端之觸媒顆粒有膨脹現象，推測原因為奈米碳管在高溫下碳原子會有重組的效應。 ABSTRACT In this study, high-density vertical carbon nanotubes were grown on Si substrate at 400℃ by microwave plasma chemical vapor deposition with Fe, Co, and Ni as catalyst. The characterization of the carbon nanotubes were carried out by SEM, TEM, AFM, Raman, and I-V measurement. By the observation with high resolution TEM, the appearance of carbon nanotubes were bamboo-shaped and range from 30-100nm in diameter. The bamboo-shaped carbon nanotubes were in a shape of cone and were multi-walled. The catalyst metals were on both top and bottom of the carbon nanotubes. The top metal was with irregular shape and the bottom metal was cone-like similar with the hole-bamboo structure. Base on the above results, we can suppose the grown mechanism of carbon nanotubes achieved by the pill up of carbon atoms with upward movement along the surface of the cone-like catalyst. The catalyst film were separated into nucleation in the pretreatment step, the nanoparticle catalyst were transform into cone-like shape during grown process. In this study, we well focus on the influence of MP-CVD on the grown mechanism of carbon nanotubes. Single factor experiment was used to study the effects of process parameters on the grown rate of carbon nanotubes. These process parameters includes: source power, pretreatment source power, growth source power, growth temperature, working pressure, flow rate of N2, content of CH4, pretreatment time and growth time. Beside, Taguchi method was used for the optimization of process conditions by the study of the influences of various factors on the growth of carbon nanotubes. The growth rate and the roughness of pretreatment sample were evaluated and served as output response. The larger-the-better analysis which uses growth rate of carbon nanotubes as experiment output response indicates that growth time, pretreatment time, flow rate of N2, working pressure were important half control factors. The confirmation experiment results are as expected. The smaller-the-better analysis which uses the roughness after pretreatment as output response indicates that plasma source power, working pressure, pretreatment time which are in the order of importance. The results of the smaller-the-better analysis are not as expected. This maybe caused by the formation of carbon nanotubes during pretreatment process, which results the error of the measure by AFM. Carbon nanotubes also can selectively grow in the array of 5μm square. The measured characteristics of field emission of carbon nanotubes shows the threshold voltage is 130V, and a current density of 4.5mA/cm2 can be obtained under a field of 3V/μm. These pole field emission devices formed by carbon nanotubes can deliver different current density by the control of the gate. Besides, a single carbon nanotube was successfully grown on the catalyst metal with dimension of 0.1μm. The oxidation process at high temperature can exclude amorphous carbon and purify the carbon nanotubes, but some part of the carbon nanotubes were also consumed. The diameter of the carbon nanotubes will become larger after higher temperament annealing. The swelling catalyst metal particles on the top of the grown carbon nanotubes is due to the reorganization of carbon atoms at high temperament. |
