Laser direct synthesis and patterning technology for fabricating transparent and flexible electronics: technology development and analysis of the microscale photo-thermal-fluids transport phenomena

本研究成功開發利用雷射直析技術(Laser direct synthesis and patterning, LDSP)於聚對苯二甲酸乙二酯薄膜(Polyethylene Terephthalate, PET)上製造微米尺寸的導電結構。製程中使用透明無粒子的銀離子反應溶液為製程所需之金屬源(precursor)，為成功於透明基板上實施此雷射輔助製程技術，根據雷射波長規格以及材料光學特性評估後選擇於離子反應溶液中分別加入紅色色素或醋酸銅作為雷射光的吸收介質，並各別使用連續式綠光(λ= 532 nm)或紅光(λ= 658 nm)雷射為加工光源。雷射直析製程基本原理為由反應溶液吸收雷射光的能量，於基板表面的雷射聚焦點附近選擇性地快速升高反應溶液的局部溫度，進而快速地於反應溶液與基板交界處還原析出金屬的微結構，直接沉積於基板表面。紅色色素染色的反應溶液微觀上為異質(固-液)混合，雷射能量仍由混合於溶液中的固體色素吸收。而以醋酸銅作為吸收介質的反應溶液則為一均質的吸光物質，因此這種反應流體在雷射光所引致的微觀熱流傳遞現象上具有根本上的差異。此雷射直析技術在常溫、常壓下即可作用，過程中不需使用有毒溶劑或者是蝕刻液，亦不需使用昂貴的黃光微影設備與材料，例如曝光機與光罩等。雷射的侷域性快速加熱特性可大幅降低對一般可撓基板(例如本研究所使用的PET)的熱影響，雷射直析製程所使用的金屬離子反應溶液製備程序快速簡便，更可以節省製程所需的材料與時間成本，符合綠色精密製造技術的精神。 本研究經由實驗結果探討製程參數(吸收介質、雷射掃描速度、雷射掃描次數)對於成形之銀微結構的電阻值、導電率、結構樣貌以及成分的影響。同時，本研究利用多重物理耦合數值模擬分析加工區域因雷射加熱時的準穩態(quasi-static)與暫態(transient)光、熱、流耦合傳遞現象，探討微小尺度下的溫度場及流場與銀結構析出成型情況的關係。為了更深入地分析了解形成凹口型金屬微結構的原因，本研究根據模擬分析所獲得的熱流場，搭配金屬熔池的微觀表面流動理論，結合無因次物理參數分析以及積分法(integral method)技巧，成功建立了一套分析模型，可以計算得知熔融銀金屬熔池與反應溶液的交界面上因為表面張力(Marangoni effect)與黏滯力效應所導致的微觀流動，進而可評估並說明形成金屬微結構的主要物理機制，此實驗(經驗)-理論分析模型亦有助於後續更加深入地了解雷射直析與雷射直寫製程中的微觀傳遞現象，以及提供製程參數優化的分析模型基礎。 本論文研究的最後一部份嘗試使用脈衝式雷射(λ= 1064 nm)作為加工源，於有色聚酰亞氨薄膜(Polyimide)上製作微米銀金屬結構，主要目的為初步探討脈衝雷射的脈衝加熱特性是否有助於縮小雷射直析技術的線寬、製程穩定性以及製程效率。實驗依據田口方法進行實驗參數(雷射功率、掃描速度、脈衝頻率、掃描次數)的規劃，系統化地探討各製程參數對於製程(銀線導電率、線寬)的影響，以進行製程參數優化。In this study, a novel laser direct synthesis and patterning (LDSP) technology was modified and applied to fabricate conductive silver microstructure on inexpensive transparent polyethylene terephthalate (PET) substrate. Transparent and particle-free silver ion solution with ethylene glycol was prepared and utilized as the precursor in the LDSP process. In order to elaborate the LDSP technique on the transparent PET substrate with visible light lasers (λ= 532nm and 658nm, respectively), a red-dye or cupric acetate was used as the absorbing material. The absorbing material was dissolved in the precursor solution solvent to complete preparation of process solution. It is noticed that the solution dissolved of red-dye is a heterogeneous solution while the one dissolved of cupric acetate appears to be a homogeneous solution. In the process, continuous wave laser as heat source was focused and conduced on the surface of the PET substrate. The precursor solution deposited on the surface of the PET substrate absorbs the laser energy, transforms to heat and elevates the local temperature of the solution near the laser focal point. Silver pattern was then reduced and deposited in-situ on the PET substrate rapidly. This technology is fast, clean, green and economic. The use of laser as the heat source greatly alleviates the thermal impact to common plastic flexible substrates. In addition to experiments, a numerical analysis was also carried out the first time to investigate the coupled opto-thermo-fluidic transport phenomena and the effects on the silver film growth. The characteristics of the temperature field and the thermally induced flow associated with the moving heat source are discussed. The effects from type of absorber, laser scanning speed and number of laser scans on the structure and properties of the resulted silver pattern were discussed based on the results. To further investigate the causes to the formation of the morphology of silver microstructure, a theoretical analysis to the surface tension induced flow of the silver melting pool was performed in conjunction with the numerical analysis. It is shown that the Marangoni effect and viscous force are the dominant kinetics to the movement of the surface of the meting pool. The developed semi-empirical model can reasonably depict the important physics of the thermo-capillary flow during the laser heating of the micro-melting pool, and provides great insights to the microscale transport phenomena as well as the optimization of the LDSP process. Lastly, the feasibility of using pulsed laser as the heating source for LDSP process was investigated preliminarily in the current study. The goal is to further reduce the line width of the metal line fabricated by using LDSP. The Taguchi method was applied to plan the experiments and to evaluate the effects of the process parameters (laser power, laser scanning speed, pulse frequency and number of laser scans). The results of this study help to improve the LDSP technology and the understandings to the physics of the process.摘要 i Abstract iii 圖目錄 viii 表目錄 xi 符號對照表 xii 第1章 緒論 1 1.1 前言 1 1.2 文獻回顧 5 1.2.1 製造電路板金屬線路方法 5 1.2.2 雷射製造金屬微結構 6 1.2.3 雷射加工對截面樣貌影響 9 1.3 研究目的與動機 12 第2章 理論簡介 15 2.1 熱傳遞統御方程式 15 2.1.1 熱傳導 15 2.1.2 熱傳導方程式 [25] 15 2.2 流體力學統御方程式 16 2.2.1 連續方程式 16 2.2.2 動量方程式 16 2.3 表面張力機制 16 2.4 吸收度 17 2.5 田口方法 19 2.5.1 田口方法介紹 19 2.5.2 田口方法步驟 20 2.5.3 直交表 20 2.5.4 S/N ratio 21 第3章 實驗分析 22 3.1 溶液製備 22 3.2 實驗過程 24 3.2.1 含色素顆粒反應溶液吸收雷射光製程 24 3.2.2 無顆粒反應溶液(醋酸銅)吸收雷射光製程 25 3.2.3 以基板吸收雷射光的製程 27 3.3 剖面觀察 28 3.3.1 冷鑲埋 28 3.3.2 研磨拋光 29 3.4 實驗設備 30 第4章 數值模擬 31 4.1 模擬分析 31 4.1.1 幾何模型 31 4.1.2 統御方程式及邊界條件 32 4.1.3 網格收斂性 33 4.2 理論分析 35 第5章 結果與討論 43 5.1 雷射直析實驗結果 43 5.1.1 銀線結構樣貌分析 44 5.1.2 銀線成分分析 48 5.1.3 銀線電阻值分析 49 5.1.4 銀線線寬分析 51 5.1.5 銀線剖面厚度分析 52 5.1.6 銀線導電性分析 55 5.1.7 銀線表面形貌 58 5.1.8 成果展示 59 5.2 模擬熱流場對應銀導線結果 60 5.3 理論分析結果 63 5.4 脈衝雷射實驗結果 70 第6章 結論 76 Reference 7