奈米碳管場發射特性之研究與應用

Abstract

奈米碳管場發射特性之研究與應用 Study of Carbon Nanotubes for Field Emission Display Application 研究生:林家彬 指導教授：鄭晃忠 國立交通大學 電子工程學系 電子研究所碩士班 摘要 近年來，由於真空微電子元件同時具有真空管及半導體元件的優點，所以引起極大的注意。而在此領域中，最重要的關鍵技術就是高效率冷陰極之製造。 而奈米碳管(Carbon Nanotube)，因為具有極好的高寬比、良好的化學及物理穩定性，加上其有良好的場發射特性所以已經被大量應用於場發射元件之製程上。奈米碳管的成長是主要是先用雙電子槍蒸鍍系統(Dual E-Gun)度上一層15nm的催化金屬層(Fe)，經過lift-off圖形化催化金屬層後，再利用MW-CVD 解離CH4、N2 、H2等反應氣體，成長時間約為10分鐘。奈米碳管會選擇性的沈積在已經圖樣化的催化金屬層上。然而一般CVD法沈積的奈米碳管會因為沈積密度太高導致了奈米碳管尖端所受到的有效電場會受到隔鄰的奈米碳管的電場遮蔽效應的影響而遠比單一奈米碳管所受電場的理論值要低，所以需要較高的電場才能達到理論上的場發射電流密度，所以如何降低奈米碳管的工作電壓以及提高場發射電流密度是亟需解決的問題；此外，奈米碳管的場發射電流穩定性與均勻度會因為奈米碳管表面狀態以及表面附著物等等因素而受到相當大的影響，所以研發出具有均勻且穩定的場發射電流特性是目前奈米碳管應用在場發射陰極的重要課題。 在此研究中，我們對以上幾個奈米碳管的課題提出解決方法。在提升奈米碳管之場發射電流密度以及降低工作電壓方面，我們運用了準分子雷射對奈米碳管做破壞性掃瞄，使得奈米碳管的分佈密度改變使電場遮蔽效應降低。主要的原理在於利用奈米碳管被照射到的準分子雷射能量會隨著準分子雷射光束本身能量密度分佈而變化，而達到選擇性的破壞奈米碳管進而改變奈米碳管分布密度的目的。在調整過不同雷射能量(energy density)與不同的雷射光束重複率(overlap)後，發現當準分子雷射能量密度為(100mJ/cm2)且光束重複率50%時有較大的場發射電流密度、較低的起始電場。在分析其電流電壓的特性之後，發現其場發射電流密度可以由5伏特/微米（V/μm）的電場下的5.73 mA/cm2顯著增加至87.15 mA/cm2；而起始電場 （電流密度達 1 μA/cm2）由原先的3伏特/微米（V/μm）下降至 2.1伏特/微米（V/μm）。而經過一小時 5伏特/微米的場發射電流穩定性測試中發現經過準分子雷射處理的奈米碳管的場發射電流穩定度並未因此而下降。同時在以SEM、SNOM等材料分析儀器分析下，證實了奈米碳管的分佈與表面型態的確因準分子雷射處理而改變了。所以準分子雷射或許是目前唯一確定可以以簡單、容易、有效且大面積、大幅度的增進奈米碳管的場發射電流密度及降低工作電壓的方法。 對於一個場發射陰極而言，是否具有穩定(stability)的場發射電流特性是應用在場發射顯示器上的一個重要考量因素。雖然比起其他材料，奈米碳管( CNTs )具有相對優越的場發射電流密度的特性，但是根據研究奈米碳管不易有穩定 (stability)的場發射電流特性。為了改善這項缺點，我們首次結合了薄膜電晶體（TFT）和奈米碳管(CNTS) ，製造了主動控制的場發射元件。主要原理是透過TFT中受閘極電壓(Gate Voltage)控制的穩定飽和（saturated）汲極電流(Drain Current)，提供奈米碳管一個穩定的電子流來源，進而達到控制奈米碳管場發射電流穩定性的目的。而主動控制的場發射極除了具有均勻及穩定的場發射電流外，還可以省去將奈米碳管 ( CNTs ) 製造成三極場發射元件的複雜步驟。我們只需用簡單的IC製程將TFT與CNTs結合。而量測其電壓電流關係，可以確知CNTs 的場發射電流的確受到TFT的良好控制，也有較好的發射電流穩定性。 此外，我們也同時探討傳統結構的長通道(Long-Channel)的LC-TFT與具有off-set 結構設計的Offset-TFT，何者在與CNTs製造成主動控制場發射元件(TFT-controlled CNTs)時有較好的特性。發現用LC-TFT 結構之元件雖然可以將電流波動控制在2%以內，但是其閘極控制電壓卻需要25V以上，並且主動區域(Active region)面積為(250um x 100um)，並不太符合我們低壓控制與高解析度的顯示器目標。於是我們採用Offset-TFT的新結構來取代LC-TFT。從電性量測結果我們可以知道用Offset-TFT控制的元件也可以有效的控制場發射電流之波動率在3%，而閘極控制電壓更可以有效的降低至10V以下，其主動區面積更是只有20um x 100 um. 在這篇論文中，我們首先利用準分子雷射處理方式去增加奈米碳管之場發射電流密度與降低其工作電壓。其次，我們更利用不同的TFT結構，以低壓就達到控制奈米碳管之場發射電流波動在2~3%以下。

Study of Carbon Nanotubes for Field Emission Display Application Student: Chia-Pin Lin Advisor: Dr. Huang-Chung Cheng Department of Electronics Engineering & Institute of Electronic National Chiao Tung University Abstract The technological advances in the field of microelectronic fabrication techniques have triggered great interest in vacuum microelectronics. The most important technology of the field emission devices is the fabrication of efficiency cold cathode. Since its discovery, the carbon nanotubes (CNTs) have attracted a great deal of attention owing to its advantageous properties, such as high aspect ratio, high Young’s modulus, and mechanical strength, etc. These useful properties of CNTs make themselves good candidates for field emission display. In this thesis, we have grown CNTs selectively on Fe-coated Si substrates using microwave plasma-enhanced CVD (MPECVD), which has been previously reported as a useful tool for the growth of CNTs. Several groups reported the effect of screening the electric field by a dense arrangement of CNTs grown by MPECVD. The local electric field on the tips of CNTs decreases because of the screening effect between the neighbor CNTs. As one can observe that the field enhancement factor (β) value of the CNTs decreases rapidly when the intertubes spacing decreases. We treated CNTs by excimer laser treatments (ELTs). The lower the energy density and the overlap we used, the better the emission properties of CNTs we found. When the CNTs with ELT at 100 mJ/cm2 and 50% overlap has the best field emission characteristics such as the turn-on field decreases to 2.1 V/um, the emission current density increases to 87.5 mA/cm2, and the β value also increases to about 3000. We also demonstrated the morphology difference by scanning electrical microscopy (SEM), near-field scanning optical microscopy (NSOM) and so on. The other important issues of cold emission cathodes are the stability and driving voltage characteristics. In FED applications, the CNTs have serious problems of instability of electron emissions and very high driving/addressing voltage. We demonstrated the CNTs fabricated monolithically with a long channel thin-film transistor (LC-TFT) to solve the problems of the CNTs in FEDs. The CNTs were deposited on the drain region of the long-channel thin-film transistor, and we named this structure as LC-TFT-CNT. This structure has been successfully demonstrated to bring a technological breakthrough in FEDs, including great improvement in the emission performances and low-voltage control irrespective of a required high voltage for field emissions. The gate voltage of LC-TFT controlled the emission current emitted from CNTs. The emission current fluctuations were decreased to 2%, and the gate voltage we used was sweeping from 25 V to 45 V. Although, the emission current stability of the LC-TFT-CNTs is better than the non-controlled CNTs, there are some drawbacks exist such as high gate voltages (25~45 V) and large active area (250 um x 100 um). Therefore, we used a TFT with an offset region between the gate and the drain to solve the problems. The results showed that the Offset-TFT-CNTs also decreases the current fluctuations to 3 %, the threshold voltage to 4.1 V, the active area to 100um x 20 um. In this thesis, we increased the emission current density of CNTs by ELT. We also improved the emission current stability of CNTs by LC-TFTs and Offset-TFTs.