利用掃描式超音波噴塗奈米碳管薄膜於微結構化 軟性基板之場發射照明元件之研究

Abstract

近年來，場發射元件優異的特性使其極具潛力在軟性照明，其中以塗佈之奈 米碳管薄膜擁有高機械應力、高化學穩定性、低電阻值之特性，應用於大面積、 低成本之軟性場發射照明元件之陰極具有高競爭優勢。然而，奈米碳管薄膜應用 於場發射照明元件之陰極遭遇到嚴重的屏蔽效應(Screening Effect)、軟性基板之選擇、附著度以及均勻度之挑戰。本論文透過理論模擬與元件實作的驗證，建立奈米碳管薄膜於微結構化軟性基板應用於場發射照明元件之技術。利用氫氧化鉀 溶液非等向性蝕刻 (1 0 0) 矽晶圓為模板，將金字塔形狀之微結構轉移(Transfer)至聚二甲基矽(Polydimethylsiloxanes, PDMS)應用於軟性照明以降低屏蔽效應之影響，並利用氧電漿對PDMS 表面處理增加附著度及使用掃描式超音波噴塗(Scanning Ultrasonic Spraying)技術將奈米碳管溶液噴塗於微結化之PDMS 軟性基板以達到高均勻度之奈米碳管薄膜。 本論文利用Technology Computer Aided Design (TCAD)模擬軟體探討金字塔微結構對場發射特性之影響，我們設計不同的金字塔微結構高度、距離，歸納出”高度效應”與” 距離對高度(R/H)比例效應”對每個金字塔頂端電場之影響，並且成功模擬出最佳電流密度表現之結構為金字塔高度等於30 微米，R/H 比例等於 2 之條件。 根據模擬的結果，我們利用光罩的設計以及氫氧化鉀溶液非等向性蝕刻 (1 0 0) 矽晶圓製備不同R/H 比例之樣品，並將其金字塔微結構轉移至PDMS 軟性基 板，接著利用掃描式超音波噴塗技術將奈米碳管溶液均勻噴塗在金字塔結構化之 PDMS 基板並完成場發射特性之量測。從實驗結果顯示，奈米碳管薄膜於金字塔 高度等於30 微米以及R/H 比例等於的金字塔微結構化PDMS 基板之場發射特 性遠優於平面式奈米碳管薄膜之場發射特性，與模擬所得結果相吻合。此結果顯 著證明經過微結構化之基板可大幅降低屏蔽效應，進而提升場發射照明元件之特 性。 此外，針對附著度的提升，本研究提出利用氧電漿對PDMS 基板表面改質 進而提升奈米碳管薄膜與PDMS基板之間的附著能力。本研究調變偏壓瓦數(Bias Power)控制氧離子轟擊PDMS 基板之能量，在偏壓瓦數為70 瓦特時，成功將去 離子水(DI water)在PDMS 之接觸角(Contact Angle,θ)從θ=105.59゜(斥水性)改質成θ=14.55゜(親水性)，因此奈米碳管薄膜塗佈在金字塔高度為30 微米、R/H比例為2 及氧電漿偏壓瓦數70 瓦特處理之金字塔微結構化PDMS 基板擁有優異的場發射特性，起始電場為1.39 V/μm，並在6000 秒、4 V/μm 的可靠度測試中，仍有穩定200 μA/cm2 的電流密度輸出，相較無氧電漿處理之樣品只剩下約2~3μA/cm2 的電流密度而言，足見氧電漿處理對奈米碳管薄膜與PDMS 基板之附著能力有顯著的提升。 總而言之，以簡易之蝕刻矽所轉移之PDMS 軟板上，加以氧電漿轟擊改善其表面能之簡易方式，利用掃描式超音波噴塗技術噴塗奈米碳管可以形成優良特性之場發射照明元件，為一種具有軟性照明之有潛力技術。

Excellent properties of the field emission devices made them be greatly potential in the flexible lighting recently. The carbon nanotube thin films (CNTFs) had competitive advantages of remarkable mechanical strength, outstanding chemical stability, and high conductivity on the applications of the cathodes in the large-area and the low-cost flexible field emission lighting devices (FELDs). However, applying the CNTFs as the cathodes of the FELDs encountered the challenges of severe screening effect, the choice of the flexible substrate, the adhesion issue, and the uniformity issue. With the theoretical simulation and the device realization in this thesis, the techniques of applying the CNTFs on the micro-structured flexible substrates for the FELDs were established. Using the potassium hydroxide (KOH) solution to etch the (100) silicon wafers as the templates which were used to transfer the pyramid structures to the polydimethylsilicone (PDMS) for the flexible FELDs to reduce the screening effect, applying the O2 plasma treatment on the surface of PDMS for improving the adhesion, and using the scanning-ultrasonic-spray techniques to deposit the uniform CNTFs on the flexible microstructured PDMS, a high-performance flexible lighting devices could be thereby easily fabricated. Technology Computer Aided Design (TCAD) simulation software was applied to investigate the effects of the microstructures on the field emission characteristics. By designing of the height(H) and the interspacing(R) of the pyramid microstructures, we generalized that there were the “height effect” and the “R/H ratio effect” would affect the electrical fields on the pyramid tips. We also successfully found the optimum pyramid microstructure with the pyramid height of 30 μm and R/H of 2. According to the simulation results, we fabricated the samples with different R/H ratios by the mask design and KOH solution anisotropic etching on the (100) silicon wafers, and then transferred the pyramid structures on the etched silicon to the flexible PDMS. The CNT solution was then uniformly scanning-ultrasonic-sprayed on the pyramid-structured PDMS substrates and the measurements of field emission characteristics were completed. The field emission characteristics of the CNTFs on the pyramid-structured PDMS with the pyramid height of 30 μm and R/H of 2 are obviously superior to the CNTFs on the planar PDMS. The experiment results are consistent with the simulation results. It is a significant proof that the microstructured substrates can greatly reduce the screening effect and enhance the characteristics of the FELDs. In addition, for improving the adhesion between the CNTF and the PDMS, the O2 plasma treatment on the surface of PDMS was proposed. The bias powers were modulated to control the ion bombardment on the PDMS. For the bias power of 70 W, the contact angle of DI water on the PDMS changed from 105.59゜(hydrophobic ) to 14.55゜(hydrophilic). Therefore, the adhesion-improved CNTFs on the pyramid-structured PDMS with the pyramid height of 30 μm , R/H of 2 and the bias power of 70 W had the optimum field emission characteristics with the turn on field of 1.39 V/μm. In the reliability test at 4 V/μm for 6000 sec, there is a much stable emission current density of 200 μA/cm2 as compared to the emission current density of 2~3 μA/cm2 without O2 plasma treatment. These results indicated that the O2 plasma treatment on the PDMS will significant improve the adhesion between the CNTFs and the PDMS.