新穎場發射材料之製程及特性研究

Abstract

在本論文中，我們製作及合成不同的陰極材料並研究探討這些材料特性以改善場發射元件(Field Emission Devices) 的操作電壓。此外，我們利用簡單的IC製程將這些新穎場發射材料製成二極式場發射元件並探討其特性。這些新材料包括偏壓輔助碳化矽尖端陣列、超微細碳管(CNT, carbon nanotube)及準直性(well aligned) 超微細碳管陣列、矽碳氮奈米柱等。對一場發射陰極來說，具有均勻(uniformity)及穩定的發射電流特性是應用在場發射顯示器上一項很重要因素。所以我們結合了薄膜電晶體(TFT)和CNT製成了主動控制場發射CNT，這對CNT在場發射顯示器應用有很重要貢獻。 我們利用半異向性的活性離子蝕刻技術在矽基板上製作了一系列擁有高長-寬比(Aspect Ratio)的場發射尖端陣列(FEA, field emission array)。經過氧化削尖處理之後，可以得到非常均勻且每一陣列內含50´50個尖端的場發射陣列，為了進一步地增進其低電壓場發射的能力與穩定性，偏壓輔助碳化(Bias assisted carburization)微波電漿化學氣相沉積法(MPCVD)，於低溫下(<550℃) 將矽尖端碳化後，可以得到一種極尖銳的場發射碳化結構。偏壓輔助碳化除了可在低溫得到尖銳的場發射碳化尖錐外，我們還可發現此碳化薄膜還是選擇性的鍍覆在尖錐表面，而不會鍍覆在平面基板上。這項結果可以經由Auger電子能譜儀得到證實。當我們將製程偏壓增高時，會在原本的尖錐周圍形成更多的尖錐，稱之為複發射尖錐(multitip)。再分析其材料與電流電壓的特性之後發現複發射尖錐其場發射電流遠較未經塗佈之純矽晶尖端與鉻金屬塗佈之矽尖端優越。此新技術將可滿足平面顯示器之低溫與大面積的製程需求。 奈米碳管其結構是由碳原子以SP2鍵向相鄰同平面的三個碳原子鍵結;就像是石墨中碳等邊六邊形，而以此六邊形一封閉柱狀面形成CNT它的結構具有高度對稱性，不同的晶格結構安排，也影響到它的特性，就以單壁CNT來說，它表現可為半導體、金屬性等。奈米碳管亦像鑽石一般有很好物理、化學穩定性也具有極佳的機械特性。當然它還有另一項神奇特性。就是有很好的”場發射特性”。因此我們只利用了很簡單的IC技術成功地製作出奈米碳管的FEA，並探討了它們的場發射特性。同樣的，奈米碳管的成長是用MW-CVD來達成，反應氣體為CH4、N2、H2，成長時間約為10分鐘。CNT會選擇性成長在已圖樣化的金屬催化層上。每一陣列內含50´50個方塊(cell)，而每個方塊邊長為3 µm。在分析其與電流電壓的特性之後，其場發射電流密度在10伏特/微米(V/µm)的電場下就可達10 mA/cm2，和以前摻磷鑽石塗佈之矽尖端陣列去比較，鑽石陣列所需電場必須加高至20 V/µm 才能達相當的電流密度。由於CNT有相當高的電子發射效率，而且製程簡單又具有大面積製程的能力，使得以CNT為主的場發射顯示器有很大商業化潛力。 此外我們也成功製造出高準直性(well-aligned CNT) 場發射陣列。在此我們製作高準直性CNT陣列更為簡單，首先將催化金屬層(Fe、Co、Ni)用電子束熱蒸鍍透過shadow mask，使圖樣化金屬直接鍍在基板上，厚度約為7 nm。同樣用MW-CVD成長高準直性CNT。成長條件同上所述。經由10分鐘的成長可以得到密度非常高CNT且完全垂直基板方向的CNT，高度可從7 µm至30 µm長。經由穿遂電子顯微鏡的觀察CNT的頂端為封閉的且頂端管內包覆有金屬顆粒。在場發射電流電壓的特性方面，電流密度在15.7伏特/微米(V/µm)的電場下就可高達90 A/cm2，場發射電流的穩定度在550及650伏特二種電壓下進行，在1 小時的測試時間內電流沒有很明顯的退化(degradation)。 在本論文中還探討另一種新的奈米結構材料—SiCN半導體化合物。此新材料是利用兩階段的成長方法，先用ECR-CVD成長矽碳氮緩衝層，使用的基板為拋光矽基板Si(100)及Si(111)，之後再把成長好的矽碳氮緩衝層拿到MW-PECVD系統繼續成長，以H2、N2、CH4、SiH4為反應氣體，如此兩階段成長，即可長出矽碳氮柱狀奈米結構(nanorod)。此奈米結構材料在30萬倍Field Emission SEM下其形貌為直徑20-60nm，長度1-1.5μm的多角形柱狀材料，其結構於XRD、TEM探測知結果幾近α-Si3N4，其中部分Si被C取代，在HRTEM下可看出整個nanorod為單晶結構。經過XPS及EDX則鑑定其矽、碳、氮的成分比例。而光激發螢光光譜PL (Photo-luminescence)測知其為寬能隙特性(4.13eV)。這種奈米柱狀結構具有大的高長-寬，比在分析其與電流電壓的特性之後其場發射電流效果非常特殊可達9mA/cm2，且場發射電流很穩定，其穩定性優於CNT，推測此優點可能和nanorod堅硬結構有關。根據本研究，SiCN nanorod有機會能成為藍光或紫光光電材料和場發射顯示器(FED)的明日之星。 鑽石相關化合物具有很多優良的特性，因此被工業上大量應用，關於鑽石薄膜及相關化合物的場發射研究相當多且完整，而SiCN薄膜也像鑽石一樣屬於寬能隙半導體但對於此化合物的場發射研究卻很少，因此在本論文中還對SiCN薄膜的場發射特性也有較多的探討，SiCN薄膜的成長是用電子回旋共振化學氣相沉積系統(ECR-CVD)，反應氣體為H2、N2、CH3NH2、SiH4，反應溫度550∼800 oC。所沉積出來的薄膜結構包含二種SiCN相(phase)為，非晶系相(amorphous)及奈米晶體(nanocrystalline)相的混和。我們利用拉塞福光譜儀(RBS)來判定薄膜的組成，其矽碳(Si;C)比氮(N)的比例為0.75和氮化矽(Si3N4)的組成比例很接近。另外還用了X光電子光譜儀(X-ray spectroscopy)及拉曼(Raman)光譜儀來分析SiCN薄膜的鍵結結構。研究分析其場發射與電流電壓的特性之後得知二層結構的SiCN薄膜其啟始電壓(Turn-on voltage)比二層結構的Si3N4薄膜來的低，顯示二層結構的SiCN有較好的場發射電流特性，這可能和薄膜結構中含有sp2 CN鍵的關係，類似的結果也發現在碳薄膜上。此外二層的特殊結構對場發射效應也有相當的助益。 對一場發射陰極來說，具有均勻(uniformity)及穩定的發射電流特性是應用在場發射顯示器上一項很重要因素。雖然CNT具有非常優越的場發射特性，能在很低的電壓下就能得到很高的場發射電流，但CNT不易有均勻及穩定的發射電流特性，為了改善這項缺點，所以我們首次結合了薄膜電晶體(TFT)和CNT製成了主動控制場發射CNT。主動控制的場發射極除了具有均勻及穩定的電流。還可省去將CNT製造成三極場發射元件的複雜步驟。我們只用了一些簡單的IC製程將(TFT)與CNT結合。研究其場發射與電流電壓的特性展示了CNT的場發射電流的確受到TFT良好的控制著，也有較好的發射電流穩定性，這對CNT實現在場發射顯示器應用有很重要貢獻。

In this thesis, various cathode materials are synthesized and the field emission characteristics are investigated to improve the turn-on voltage. In addition, by using simple IC processes, the diode-type field emission devices were achieved easily with those cathode materials, including bias assisted carburization-clad Si tips arrays, carbon nanotubes emitter arrays. In FED application, the problems of non-uniformity and instability of electron emissions are important issues. A new cathode technology of active-controlled diode emitters monolithically integrated with carbon nanotube emitters was also designed in this thesis. Various types of ultra sharp Si microtips and multitips with carbon-clading films were fabricated by microwave plasma chemical vapor deposition (MWCVD). The radii of these Si tips prepared by bias assisted carburization (BAC) can be reduced below 300 Å under a low deposition temperature (<550oC). The advantage of BAC is that a film coated selectively only on the tip structure of a triode emitter but not on the insulator or gate layer can be realized. Field emission characterization was performed in a high vacuum environment. With an applied anode voltage of 1100 V, emission currents of 169 µA, 198µA, and 385 µA can be achieved from an array of 50x50 BAC-clad Si monotips, Si multitips via high bias, and Si multitips via the Ar presputtering technique, respectively. Both the auger electron spectroscopy (AES) and X-ray photo-electron spectroscopy (XPS) studies of the C 1s peak suggest that the BAC-cladding is more likely to be a carbon-rich SiC layer or a SiC layer mixed with a small amount of diamond nuclei. This BAC-carbon can be used as an effective nucleation layer for further diamond nuclei. Due to the low field emission, low temperature, and large area growth capability, the sharp BAC-clad Si multitip field emitter arrays are attractive for flat panel display applications. Arrays of carbon nanotubes (CNTs) and diamond-clad Si tips were grown by microwave plasma-enhanced chemical vapor deposition. The former ones were grown directly on pre-patterned cobalt-coated silicon substrate while the latter ones were grown on Si-tip arrays. Each array contains 50x50 emitting cells and each individual cell is 3 µm in width. A macroscopic emission current density of 10 mA/cm2 with operating fields around 10 V/µm can be routinely achieved from an array of CNTs emitters. In contrast, operating fields above 20 V/µm were needed to draw a comparable emission current density from all of the diamond-clad Si tips arrays. Emission stability test performed at 40 mA/cm2 for CNTs arrays also showed little sign of degradation. Due to the high efficiency of electron emission, simple sample process, and large area growth capability, field emitter arrays based on CNTs are attractive for flat panel display applications. The fabrication processes of well aligned-carbon nanotubes emitter arrays are successful by simple steps without using porous Si as the growth substrate. The substrate was patterned with iron (Fe) films (7 nm thick) by electron beam evaporation through shadow masks, containing circle openings with diameter 80 mm. The density of CNT was very high and the CNT was oriented vertically to the substrate. Beside, the well aligned-CNTs were 7 µm in length and 20~60 nm in diameter with only 10 min deposited time. TEM image reveals that the round tip is closed its inside is hollowed with iron particles filled at ended of well aligned-CNT. The field emission characteristics of the well-aligned CNT also exhibited high emission current. At an applied anode voltage 1100V (i.e. an applied field of 15.7 V/ µm), the emission current of about 8 mA was achieved from emission area of 0.086 cm2, the maximum emission current density Js (A/cm2) which was calculated from the measured current dividing the measured sample area approached to 90 A/ cm2 at the applied field 15.7 V/ µm. The time stability of the well-aligned CNT at two different applied voltage (550 V and 650 V) was tested for 1 hr. The emission current fluctuated but didn’t exhibit significantly degradation. Nano-rod materials are very attractive candidate for field emitters due to their geometric field enhancement and structural stability. Here we report on the preparation and field emission properties of SiCN nano-rod for the first time. The SiCN nanorods are formed by using a two-stage growth method where in the first stage involves formation of a buffer layer by electron cyclotron chemical vapor deposition (ECR-CVD) and the second stage involves using (MW-CVD) for high growth rate along a preferred orientation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies reveal that nanorods are rod-shaped single crystals with 1 mm~1.5 mm in length and about 50 nm in diameter. EDX studies show the nanorod contains about 26 of Si at. %, 50 of C at. % and 24 of N at. %. Characteristic I-V measurements indicated a low turn-on field of 10 V/µm. Field emission current density of 1.5mA/cm2 was observed at 35 V/µm, which is the maximun accessible field strength in our setup. Beside, SiCN nano-rod exhibit stable emission current. Due to the stable field emission properties, and large area growth capability, field emitters of SiCN nano-rods are also attractive for flat emission devices applications. The electron emission characteristics of two-layer structured silicon carbon nitride (SiCN) films, which were composed of amorphous and nanocrystalline phases, were studied. The SiCN films were deposited in an ECR CVD reactor with the reaction gas H2、N2、CH3NH2、SiH4 and the deposited temperature was 550-800 oC. Rutherford backscattering spectroscopy (RBS) was used to determine the composition of the SiCN film. The ratios (Si;C) / N of the SiCN film was kept around 0.75, which is identical to that of Si3N4 film. High resolution X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to investigate the bonding structures of the SiCN films. In comparison with silicon nitride films, the turn-on voltage (for an emission current of 0.01mA/cm2) of the SiCN films was lower and the emission current densities of the SiCN significantly enhanced. The promising emission properties of the SiCN film could be due to the unique two-layer structure wherein nanocrystalline SiCN was grown on top of the amorphous interlayer with sp2 CN bond in the SiCN film. The stability and controllability of field emission currents are an important issues for the application such as a field emission display. It is well known that the Fowler-Nordheim field emission is very sensitive to the work function of the emitter surface and the nano-meter structure of the emitter. In recent, carbon nanotubes (CNTs) have attracted much attention as field emitters due to their low-electric field emission, high chemical stability, and strong mechanical strength. In FED application, the diode type emitters have serious problems of non-uniformity and instability of electron emissions. We have designed a new cathode technology of active-controlled diode emitters (ACDE) monolithically integrated with CNT to solve the problems of the field emitters in FEDs. This combines diode typed field emitters, instead of the triode type emitters in conventional actively controlled cathodes, and a control devices to regulate electron emissions of the field emitters.