選擇性沉積碳奈米結構圖案之奈米操控與場效發射研究

Abstract

操控碳奈米結構(CNSs)沉積的位置、密度與排列方向為目前碳奈米技術主要的研究課題之一，本研究主要目的為開發在指定的區域沉積具有方向性排列的CNSs製程，並且量測其結構與性質特性，使用的三種製程包含陽極氧化鋁(AAO)模板及觸媒輔助方法、無電鍍觸媒輔助法及電泳輔助沉積法。在CNSs的結構與性質分析方面，包含SEM、EDS、TEM、HRTEM、Raman、AES，及場效發射電性量測；利用紫外可見光吸收光譜(UV-Vis absorption spectroscopy)及界達電位量測(Zeta potential measurements)進行CNSs懸浮液之穩定性分析。 第一種製程是AAO模板及觸媒輔助方法，首先在矽基材上沉積鋁膜，利用兩階段陽極氧化處理製備出具有均勻孔洞大小直徑約80 nm、厚度約550 nm的奈米孔洞作為模板，以電鍍法沉積鈷觸媒於AAO孔洞內，再以電子迴旋共振化學氣相沉積法合成CNSs，為製作場發射陣列，SiO2介電層與Al閘極層分別沉積於CNSs上，經黃光製程後利用乾式與濕式蝕刻方法顯露出場發射區域。在模板輔助成長下，當電鍍鈷觸媒的時間為45 s時(高度約48 nm)，CNSs的平均直徑約22 nm，密度約37 #/□m2；當鈷電鍍時間增加為90 s時(高度約84 nm)，CNSs的直徑約53 nm，密度約162 #/□m2。為了解CNSs在受限制的AAO通道內如何成長，提出一個可能的成長機制。CNSs大部份為頂端成長模式，並且存在一個最佳的鈷觸媒電鍍時間60 s，CNSs的平均直徑約25 nm，密度約45 #/□m2，具有較佳的場發射特性(起始電場為5.1 V/□m)，其相對應製作的CNSs場發射陣列具有起始電壓8.1 V/□m的場發射特性。此方法可以彰顯以AAO做為模板，可以有效的控制CNSs場發射陣列的結構與特性 第二種製程是無電鍍觸媒輔助法，其主要的目的為利用微波電漿化學氣相沉積系統在具溝槽之圖案矽基材上，選擇性成長水平奈米碳管(CNTs)橫跨其溝槽。此製程之選擇性是利用無電鍍法在觸媒於無晶矽層(a:Si)上析出觸媒金屬之化學反應性甚大於氮化矽層(Si3N4)，而Si3N4層設計鍍在a:Si層上，可導引CNTs水平成長方向成長。結果顯示CNTs主要為竹節狀多壁結構，管壁石墨層數約20∼30。CNTs的電導性可經由760oC的熱退火後，得到很大的改善。結果展現水平成長CNT之數目可藉由觸媒電鍍時間與溝槽寬度來調控。本製程亦具有製造奈米元件導線之潛力。 第三種製程是CNSs電泳輔助沉積法，利用電泳沉積法(EPD)及後續的退火處理將單壁奈米碳管(SWNTs)選擇性沉積在各種材料或圖案上。此EPD製程首先將SWNTs與三種分散劑或界面活性劑（SDS、TOPO、CTAB）配製成CNTs懸浮液。結果顯示就SWNTs的分散性及懸浮液穩定性而言，SDS為最好的界面活性劑。EPD製程參數中，除了CNT的濃度之外，對於薄膜場發射與附著性並無顯著的影響。然而要改善薄膜與基材間的附著性需要施予較高的退火溫度加上適當的退火時間，並因而降低電阻，提高場發射特性。高效率的退火薄膜具有小的內應力，其結構往往是由許多微米大小的島狀結構組成，而小島間有許多水平排列的CNTs。另外，就應用EPD製程於不同基材的可行性，例如導電玻璃，其圖案是由具有導電及不導電材料所組成(Al 和SiO2)，或在不同形狀基材之可行性，例如彎曲的鋁箔紙，本製程可以成功的用來選擇性沉積CNTs圖案。

How to deposit or manipulate the carbon nanostructures (CNSs) with the desired orientation, number density and locations to form patterns is one of the key issues for fabricating the nanodevices. The purpose of this research is to develop nanofabrication processes to manipulate the CNSs patterns and to examine their structures and properties. The developed processes may be roughly divided into three categories, including the anodic aluminum oxides (AAO) template-catalyst-assisted, the electroless plating catalyst-assisted, and CNSs-electrophoresis-assisted processes. The structures and properties of the CNSs and their patterns were characterized by SEM. EDS, TEM, HRTEM, Raman, AES, and field emission I-V measurements. The stability of CNSs-suspensions was evaluated by UV-Visible absorption spectroscopy and Zeta potential measurements. For the AAO template-catalyst-assisted process, nanoporous AAO template was first prepared by a two-step anodization process on the Si(100) substrate with 80 nm in diameter and 550 nm in depth, followed by Co catalyst and the vertically aligned CNSs deposition in AAO pore channels by electron cyclotron resonance chemical vapor deposition (ECR-CVD). The field emitter arrays were further prepared by directly depositing SiO2 dielectric and Al gate layer on the CNSs. Reactive ion and wet etches were then used to open the field-emission area. The results show that the average diameters and the corresponding number density of the AAO-assisted CNSs vary from 22 nm, 37 #/□m2 to 53 nm, 162 #/□m2 for Co deposition times of 45 s (~ 48 nm in height) and 90 s (~ 84 nm in height), respectively. A mechanism is first proposed to delineate the CNS growth in a confined pore channel space of the AAO template. The CNSs are mainly the tip-growth type in structure and there is existence of an optimum field emission turn-on field of 5.1 V/□m for AAO-assisted CNSs with ~ 25 nm in diameter and ~ 45 #/□m2, which is corresponding to the turn-on field of 8.1 V/□m for the emitter array. The results had demonstrated that the structure and properties of CNSs emitter array can be effectively manipulated by this AAO template-assisted process. For the electroless plating catalyst-assisted process, the main purpose was to selectively grow the horizontally-oriented carbon nanotubes (CNTs) across the trenches of the patterned Si wafer in a microwave plasma CVD system. The CNT selectivity of the process is based on the greater chemical reactivity of the catalyst with a:Si than with Si3N4, where the Si3N4 barrier layer of the pattern was designed on top of the a:Si layer to guide the growth of CNTs in horizontal direction to bridge across the trenches of the pattern. The CNTs are mainly bamboo-like multiwalled CNTs (MWNTs) with a wall thickness of 20~30 graphene layers. Its electrical conductivity can be greatly improved by subjecting to 760oC heat treatment under nitrogen atmosphere. The results demonstrate that the amounts of the horizontally-oriented CNTs are tunable with the Ni catalyst plating time and the trench width. This process also demonstrates the potential for nano-connecter fabrication in nano devices. For CNSs-electrophoresis-assisted method, electrophoretic deposition (EPD) followed by post air annealing treatment was developed to selectively deposit the single-walled CNTs (SWNTs) on various substrates or patterns. The EPD was conducted from a solution mixture of SWNTs and various dispersants or surfactants, including sodium dodecyl sulfate (SDS), hexadecyl trimethyl ammonium bromide (CTAB) and trictylphosphine oxide (TOPO). The results indicate that the SDS is the best surfactant in terms of SWNTs dispersion and solution stability, and the EPD parameters, except CNTs concentration in the suspension, have no significant effects on their field emission (FE) and adhesion properties of the deposited films. However, a higher post annealing temperature combined with an optimum annealing time is required to improve the film-substrate adhesion, so to reduce its electrical resistance and to enhance FE properties. The high performance annealed films with negligible internal stress are made of the micro-sized islands with the horizontally oriented SWNTs between them. On feasibility to apply the EPD process on other substrate materials such as ITO, the pattern made of conductive and nonconductive coatings (Al and SiO2), or the substrates with different shapes like curved Al-foil, can be successfully used to selectively deposit CNTs pattern by EPD.