奈米碳管應用於場發射元件之研究

Abstract

奈米碳管(CNTs)具有高深寬比、高傳導性、與高機械強度等特性，被視為應用於場發射元件最適合的材料之一。而以奈米碳管作為發射源的應用之中，奈米碳管場發射顯示器(CNT-FED)最受青睞，為了達到低製造成本，可大尺寸化等要求，搭配網印技術厚膜成型於玻璃基板，成為量產的最可能方式之一。受限於一般玻璃基板耐溫低於550℃，故以往技術是先合成奈米碳管後，調配成漿料方式，再網印於電極之上，但此法需再額外作活化處理，才能提升奈米碳管的場發射特性。 故在此篇論文中，主要目的就是要在低溫下合成或沈積奈米碳管於網印厚膜結構之中，並得到合適的場發射特性運用於場發射元件。著重討論奈米碳管合成與沈積技術以及沈積之後的特性。首先提出兩種觸媒成型方式-溶液沈積法與溶膠-凝膠法，可搭配漿料與網印技術，製作於厚膜結構之中，加上微波電漿化學沈積法(MPCVD)，以CH4 /H2氣體系，低溫選擇性成長於玻璃基材上，實驗結果顯示兩種方式所成型的觸媒，經合成後之奈米碳管，其型態為不規則捲曲，中間帶有魚骨狀結構，且為多層壁。觸媒成型為溶液沈積法，合成之碳管其發射源啟動電場(turn-on electric field)約3.2 V/μm(電流為10 μA/cm2)。另一藉由溶膠-凝膠法成型之觸媒，其合成之碳管其發射源啟動電場約5.4 V/μm。 為了在低溫下大面積合成奈米碳管，本論文也嘗試使用易於大尺寸化的熱化學沈積法(thermal CVD)，同樣採用溶膠-凝膠法成型觸媒，搭配漿料與網印技術，以C2H2 /H2氣體系成長於厚膜銀電極之上，研究結果顯示此法可於480℃之下合成奈米碳管，其型態為不規則捲曲的多層壁結構，在二極與三極結構中量測，呈現均勻場發射影像與高亮度，發射源啟動電場約3.85 V/μm(電流為10 μA/cm2)，當電流達到1 mA/cm2時，電場強度為5 V/μm。後續也探討陰極結構的形狀效應，對奈米碳管合成的影響，為了搭配三極結構製作的便利性，觸媒改採濺鍍方式，藉由控制反應氣體氣流的變化，成功於三極結構中合成奈米碳管。 最後基於化學氣相沈積法低溫合成時，會有碳管品質不佳的問題，著手採用電泳法(EPD)直接於三極結構中擇區沈積奈米碳管，藉由兩台電壓源分別於閘極與陰極提供不同的電壓，隨著電場分佈，奈米碳管會被閘極排斥，而被陰極吸引而沈積。此電壓控制的設計，具有不需要使用光阻或犧牲層覆蓋電極的優點，並可在常溫下進行。

Carbon nanotubes(CNTs) exhibit excellent field emission characteristics due to their high aspect ratios, small tip radius of curvature and low work function. A low cost process combines arc-discharge producing CNTs as a paste and screen-printing technologies which was developed to prepare field emitters on the glass substrate. Unfortunately, using CNTs paste by screen-printing technology has a relatively low resolution and needs a surface rubbing technology or other activated steps to enhance field emission characteristics. By using chemical vapor deposition (CVD) process, CNTs can directly grow on the predefined catalyst layer, and also achieve high yield and uniformity. Therefore, the development of direct growth CNTs on the glass substrates at low temperature by a low cost technology will play an important role in practical field emission display (FED) applications. In this thesis, we successfully combined the advantages of screen printing process, microwave plasma chemical vapor deposition (MPCVD) and thermal CVD process to pattern catalyst/Ag cathode by screen printing technology and grow CNTs on patterned Ni-catalyst of cathode electrode on glass substrates below 550℃. Two novel ways can be applied to coat catalyst-Ni with Ag powders in a solution phase by the solution deposition method and sol-gel method. However, the mass flow transport behaviors of precursor gas play important roles for CVD technology especially in the stepped electronic structures. The depth dimension of dielectric holes in thick-film normal triode structures may range from 10μm to 50μm. In order to solve the gas transport problem in such narrow and deep dielectric holes, we controlled the pumping system and precursor concentration during the CVD process. The relationship between CNTs synthesis and aspect ratio of dielectric holes was investigated in this work. The cathode with high aspect ratio dielectric holes for field emission devices was properly produced. Another novel way applied to deposit CNTs in triode structure was the electrophoretic deposition (EPD) method. Two power supplies were used to provide the gate electrode and cathode electrode with different voltages, respectively. Following the electric field distribution, CNTs were forced to be dragged toward the cathode electrode but repelled out of the gate electrode. By conducting this designed voltage-controlling method, CNTs were able to be deposited in the selected-area without any photo-resist or sacrificial layer at the room temperature.