奈米碳管於側向場發射元件之研究

Abstract

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。真空微電子主要區分為兩種不同結構: 垂直式與橫側式場發射元件。垂直式場發射元件主要的潛力是應用於平面顯示器的製造上。而橫側式場發射元件則是希望應用在高頻快速元件上，同時與標準的半導體製程技術相互整合在一起。 在此領域中，最重要的關鍵技術就是高效率冷陰極之製造。奈米碳管(Carbon Nanotubes)，因為具有極好的高寬比、良好的化學及物理穩定性，再加上其有優異的場發射特性，所以已經被大量應用於場發射元件之製程上。然而利用化學汽相沉積法 (CVD) 得到的奈米碳管密度太高(~1012/cm2)導致電場之遮蔽效應(screening effect)使得其場發射特性並不因其具有較高密度之場發射源而變好。為了進一步改善奈米碳管之場發射特性，我們提出了利用高密度氧電漿處理來改變奈米碳管之密度以降低電場之遮蔽效應，實驗結果證實在適當的條件之下，奈米碳管之場發射起始電場可以大幅降低(由 4.8 V/mm 下降至 2.3 V/mm)，而場發射電流亦可大幅增加(由78.7 uA/cm2 上升至 18 mA/cm2 當操作電場在 5.5 V/mm.)。同時在SEM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高密度氧電漿處理而改變了。主要的原因是氧電漿與奈米碳管起化學反應，產生CO、CO2 等揮發性氣體而影響了奈米碳管的密度。 在橫側式場發射元件的應用中，低操作電壓與高密度電流是很重要的因素。利用半導體的蝕刻技術以及奈米碳管可選擇區域成長的特點，可以很容易地製作此種二極結構，不需要很先進的曝光設備就可以將間距縮小到1.84 mm，此元件的啟動電壓可以降低到0.4 V，而當陽極電壓為10 V時就可得到5 mA 的發射電流。而奈米碳管的長度對二極結構場發射特性的影響也有所探討，太短的奈米碳管會使得場發射電流變小，而太長的奈米碳管有短路的問題，所以必須適當地控制奈米碳管的長度。同時此二極元件在短時距1500秒的穩定度電性量測下，其電流波動率小於3.5﹪。最後，我們將高密度氧電漿處理方法應用在橫向式二極場發射元件中，實驗結果表示場發射電性可大幅改善藉由調變奈米碳管之密度而得到。

Recently, vacuum microelectronics have attracted much attention because of combining the advantages of both vacuum tubes and solid-state semiconductor devices. Vacuum microelectronics can be divided into two major groups : vertical and lateral structures. The vertical-type field emission devices are applied to field plat display, while the lateral-type field emission devices are promising in high frequency usage and compatible with IC technology. The most important technology of the field emission devices is the fabrication of high efficiency cold cathode. Carbon Nanotubes (CNTs) have attracted a great deal of attention owing to its advantageous properties, such as high aspect ratio,excellent chemical stability, high Young’s modulus, and mechanical strength, etc. These useful properties of CNTs make themselves good candidates for field emitters. A dense arrangement of CNTs grown by MPECVD results from the local electric field on the tips of CNTs decreases because of the screening effect between the neighbor CNTs. To improve the field emission properties of the CNTs, a novel post treatment process via high-density-oxygen plasma was proposed. The results depicted the field emission properties can be upgraded with proper oxygen plasma treatment conditions. Scanning electrical microscopy (SEM) images show the different morphology of CNTs which oxygen plasma react with carbon nanotubes to form CO,CO2 volatile gas to change the density of CNTs. To achieve a low-voltage and high-eurrent field emission device, a new fabrication method of lateral-type CNT diode has been demonstrated. The CNT diodes can be easily fabricated by combining semiconductor fabrication technology and selective-area growth of CNTs. No advanced lithographic tool is needed and the anode-to-emitter gap can be reduced to 1.84 mm. The device has the turn on voltage of 0.4 V and emission current density of 5 mA/cm at 10 V. The effect of the length of CNTs was also investigated to achieve the low turn-on voltage field emission diodes. The emission current fluctuation is about ± 3.5% for1500 seconds. Finally, oxygen plasma-post-treatment was introduced on the lateral field emission diode to improved the field emission characteristics. Emission current density increased from 1.1 mA/cm to 2.2 mA/cm at applied voltage 10V, because the density of CNTs is optimized with oxygen plasma treatment.