本研究將在可撓式基板上製造導電微結構之雷射直析技術(Laser direct synthesis and patterning, LDSP)與雷射微鑽孔技術進行結合，以開發雙面軟性電路板製程。製程分為兩大步驟，先將聚醯亞胺薄膜(Polyimide film, PI)基板透過雷射定點照射生成微鑽孔後，再以雷射直析技術生成微導電結構，包括填孔及雙面平面電路。雷射直析技術係經由雷射熱源對含有金屬離子之反應溶液進行加熱，促使金屬離子發生還原反應，以振鏡系統控制雷射之掃描路徑，位於雷射路徑上之反應溶液會還原出金屬原子並沉積於基板上。透過低功率雷射微鑽孔、金屬填孔及基板雙面雷射直析金屬微線路成型，實現以經濟型的單一設備完成雙面軟性電路製造，期能降低製程複雜度及成本。本研究內容包括多種製程參數及製程改良方案的嘗試實驗，以及針對雷射高分子薄膜微鑽孔製程中的光、熱、流、固耦合傳遞現象進行深入的分析，並結合理論模型及模擬，深入探究熱毛細力現象(Thermocapillary)對於鑽孔區域表面形貌之影響。
在雷射微鑽孔部分，本研究首先探討在兩種製程環境下以綠光連續式雷射對聚醯亞胺薄膜進行微鑽孔尺寸之影響。第一種環境為：薄膜上表面與空氣接觸、下表面與載玻片相接；第二種環境為：薄膜上下表面皆懸空，與空氣接觸。接著在懸空架構中，加入使用光學斬波器週期性地截斷雷射光束，以模擬中低頻率脈衝式雷射之間歇性，觀察雷射能量間歇參數對微鑽孔之影響。實驗結果顯示，相較於玻片承載基板架構，懸空架構能獲得較小的鑽孔孔徑。此外，為進一步降低雷射微鑽孔時聚醯亞胺薄膜表面之熱效應，本研究嘗試以固定流量之氮氣與氦氣對鑽孔點吹氣，觀察鑽孔尺寸與表面高度變化，結果顯示氮氣與氦氣吹氣能有效縮小熱影響區域，並在較高的雷射斬波器頻率時能顯著降低鑽孔區域的表面熱變形量達52%。參考經典的噴流(impingement flow)強制熱對流分析，理論上預期氦氣噴流應有較強的熱對流。然而實際實驗結果顯示，氮氣吹氣相較於氦氣吹氣，能夠獲得略佳的鑽孔區域熱移除效果。推測造成此結果的主要因素為由於衝量的差異，於目前的噴流流量及流速下，在大氣環境中實際抵達基板鑽孔區域表面的冷卻氣流，以氮氣噴流較為集中強勁，因此有較佳的強制熱對流效果。
在雷射直析成形部份，由於銀離子溶液在雷射加熱下啟動還原反應時會生成副產物二氧化碳氣泡，氣泡若停留於基板上，會阻隔基板與溶液接觸，並且造成入射雷射光的偏折以及局部溫度急遽升高，進而影響金屬導線之表面形貌、結構完整性與導電性。因此，本研究自行設計改良版的雷射直析製程加工治具，將基板置於以懸臂樑形式固定之壓電片上，再利用對壓電片輸入交流電壓，以發生逆壓電效應而作為振盪源，期能透過壓電片之振盪促使停留於基板上之二氧化碳氣泡迅速離開基板表面，以改善金屬導線之斷線率與電阻值。實驗結果顯示，壓電片通電引致基板微振盪的條件下，生成之銀線整體而言線寬較無基板振盪時寬，於特定的振幅位置及雷射製程參數下，斷線率較無基板振盪時改善49%。同時，因壓電片上下振盪使得原本雷射直線掃描路徑投影於基板表面後，銀線結構出現類似弦波之曲線。
在雷射直析填孔方面，由於雷射鑽孔過程中照射表面之升溫理論上可用於還原銀反應溶液生成金屬銀，本研究首先嘗試使鑽孔與填孔兩製程一步完成，實驗結果顯示雷射直析過程中所生成之副產物二氧化碳會導致雷射發生偏折，而定點填孔之銀還原反應尤其劇烈，導致大尺寸二氧化碳氣泡生成，造成雷射偏折，使得同步進行之雷射鑽孔製程無法持續，基板無法鑽穿。因此，分別嘗試了定點及畫圈填孔方式，對經鑽孔製程之鑽孔孔口周圍與孔壁進行雷射直析。實驗結果顯示，同功率下以畫圈填孔方式進行雷射直析可有效降低生成大尺寸二氧化碳氣泡之機率。透過將定點填孔、畫圈填孔與雷射直析生成銀線進行多種順序組合嘗試，歸納目前在製程上還存在多個尚待解決之問題，包括孔口周圍之表面變形會影響雷射之光路、雷射直析製程中生成之二氧化碳氣泡影響後續銀沉積、填孔後生成之銀結構與基板之接合強度不足。需解決以上問題，方能此混合製程得以實行。
由於雷射高分子薄膜微鑽孔的加工品質以及鑽孔區域的表面形貌對於雙面軟板製程的良率至為關鍵，本研究特別針對聚醯亞胺薄膜之間歇性雷射微鑽孔製程進行詳細深入的模擬及理論分析，探討雷射斬波器頻率與鑽孔時間對鑽孔孔徑與孔徑周圍表面形貌高度變化之影響。透過光、熱、流、固多重物理耦合數值模擬得到暫態的薄膜溫度場以及動態鑽孔孔徑與鑽孔深度，並將溫度場資訊代入熱毛細力效應之理論模型，計算聚醯亞胺薄膜經過雷射微鑽孔後的孔口周邊表面形貌。經與實驗觀測結果相互印證，本研究提出一個雷射光束能量衰減的假設模型，以綜合考量在中低雷射脈衝頻率下，於高分子基板表面附近及上方的煙塵(soot plume)對流及其濃度分佈對於入射雷射光束的衰減效應。經過衰減效應模型修正後的模擬結果顯示，隨斬波器頻率提升，孔口周邊之凸起區域範圍因熱擴散特徵長度的縮減而逐漸縮小，但孔口表面變形高度之極大值則因溫度梯度加大而逐漸上升，並於斬波器頻率約60~100Hz之間趨於動態穩定，與實驗結果趨勢相符，證實本研究所提之脈衝雷射燒蝕製程中，煙塵散射所導致的入射雷射光束能量衰減以及熱毛細力導致孔口表面形變的分析模型，具備極佳的合理性及適用性。

In this study, laser micro-drilling process is combined with laser direct synthesis and patterning process(LDSP), to fabricate double-sided flexible circuits on polyimide films. A main target for the development of this novel process is that the micro-drilling, circuits patterning and through hole (via) filling could be done efficiently on one laser system equipped with a mid-low power (less than 500 mW) continuous wave (CW) laser. The hybrid process is divided into two steps. Laser micro-drilling(ablation) with a fixed-point laser irradiation on the surface of a polyimide film to generate a microhole and fabricate microscale conductive metal structures as well as filling the through holes on the film. In the laser direct synthesizing and patterning process, the polyimide film surface is heated by a focused laser irradiation in a liquid reactive metallic ion (ex. silver) solution environment. Metal ion is then reduced and deposited on the film surface as well as on the inner wall surface of the microhole once the local temperature reaches above the reaction temperature. The LDSP technique could be applied for both the circuit patterning and through hole filling. To improve the completeness and conductivity of the silver lines fabricated with LDSP, a processing platform with micro-vibration was designed and tested to remove bubbles formed in the reactive liquid solution in-situ during the LDSP process. It was concluded that the micro-vibration could reduce the possibility of forming discontinued silver line by 10~50%, depends on the laser parameters. The through hole filling investigated more intensively in the current study since there has been no report yet for its feasibility. It was found that the properties of the silver film coated by LDSP on the laser-drilled microhole surfaces strongly depend on the surface morphology of the microholes. Therefore, the laser-mircodrilling of polyimide film with using the mid-low power CW laser was investigated in more details through experiments and numerical analysis.
In the micro-drilling process, an optical chopper was utilized to modulate the CW laser irradiation on the polyimide film surface with pulsate intervals. The effects of the chopper frequency, i.e. the pulse and cooling duration of the laser irradiance, on the inlet and outlet diameters of the resulted microholes were discussed. The motivation of using an optical chopper inconjuction with the CW laser is to imitate the intermittent characteristic of pulse laser to hopefully reduce the thermal impact to the polymer while keeping the cost-effectiveness of the proposed hybrid laser process. It was found that the hole diameters and surface morphologies at the inlet can be significantely improved by chopped laser irradiation in comparision to the original CW laser. However, a volcano shape porous structure still exsists at the hole inlet, which severly affects the following LDSP process. Thus, a numerical and theoretical model was developed to reveal the formation of the volcano structure at the hole inlet mainly resulted from the thermocapillary force that originated from the temperature gradient on the polyimide surface above its pyrolysis temperature during the laser drilling. In the simulation, an Gaussian type radial function was propsed for the attenuation factor to consider the extinction of laser intensity through the upper and lower soot plumes observed in the laser-drilling experiments. Transient simulation results of laser drilling and the surface morphology evolved during the drilling process with different chopping frequency agreed well with the experimental results correspondingly. Both the experimental and simulated results suggested that the dynamic temperature variation and the surface change approached quasi-static for chopper frequencies the inbetween 60 to 100 Hz. It is proved that the analytical model developed in this study is effective and can be used to assist understaning the complicated pulsed laser ablation of polymer films with long pulses.

[1] P. Buffat and J. P. Borel, "Size effect on the melting temperature of gold particles," Physical review A, vol. 13, no. 6, p. 2287, 1976.
[2] B. J. Kang, C. K. Lee, and J. H. Oh, "All-inkjet-printed electrical components and circuit fabrication on a plastic substrate," Microelectronic Engineering, vol. 97, pp. 251-254, 2012.
[3] M. Tavakoli et al., "EGaIn‐Assisted Room‐Temperature Sintering of Silver Nanoparticles for Stretchable, Inkjet‐Printed, Thin‐Film Electronics," Advanced Materials, vol. 30, no. 29, p. 1801852, 2018.
[4] A. Mahajan, C. D. Frisbie, and L. F. Francis, "Optimization of aerosol jet printing for high-resolution, high-aspect ratio silver lines," ACS applied materials & interfaces, vol. 5, no. 11, pp. 4856-4864, 2013.
[5] F. Cai et al., "Aerosol jet printing for 3-D multilayer passive microwave circuitry," in 2014 44th European Microwave Conference, 2014: IEEE, pp. 512-515.
[6] K.-S. Kim, K.-H. Jung, and S.-B. Jung, "Design and fabrication of screen-printed silver circuits for stretchable electronics," Microelectronic engineering, vol. 120, pp. 216-220, 2014.
[7] A. Eshkeiti et al., "Screen printing of multilayered hybrid printed circuit boards on different substrates," IEEE transactions on components, packaging and manufacturing technology, vol. 5, no. 3, pp. 415-421, 2015.
[8] R. Abbel, Y. Galagan, and P. Groen, "Roll‐to‐Roll Fabrication of Solution Processed Electronics," Advanced Engineering Materials, vol. 20, no. 8, p. 1701190, 2018.
[9] B. Dou et al., "Roll-to-roll printing of perovskite solar cells," ACS Energy Letters, vol. 3, no. 10, pp. 2558-2565, 2018.
[10] R. Srinivasan, "Etching polyimide films with continuous‐wave ultraviolet lasers," Applied Physics Letters, vol. 58, no. 25, pp. 2895-2897, 1991, doi: 10.1063/1.104714.
[11] M. Himmelbauer, E. Arenholz, and D. Bäuerle, "Single-shot UV-laser ablation of polyimide with variable pulse lengths," Applied Physics A, vol. 63, no. 1, pp. 87-90, 1996.
[12] K. Yung, D. Zeng, and T. Yue, "High repetition rate effect on the chemical characteristics and composition of Upilex-S polyimide ablated by a UV Nd: YAG laser," Surface and Coatings Technology, vol. 160, no. 1, pp. 1-6, 2002.
[13] O. Yalukova and I. Sarady, "Investigation of interaction mechanisms in laser drilling of thermoplastic and thermoset polymers using different wavelengths," Composites science and technology, vol. 66, no. 10, pp. 1289-1296, 2006.
[14] B. Shin, J. Oh, and H. Sohn, "Theoretical and experimental investigations into laser ablation of polyimide and copper films with 355-nm Nd: YVO4 laser," Journal of materials processing technology, vol. 187, pp. 260-263, 2007.
[15] N. C. Nayak, Y. Lam, C. Yue, and A. T. Sinha, "CO2-laser micromachining of PMMA: the effect of polymer molecular weight," Journal of micromechanics and microengineering, vol. 18, no. 9, p. 095020, 2008.
[16] L. Brusberg, M. Queisser, C. Gentsch, H. Schröder, and K.-D. Lang, "Advances in CO2-laser drilling of glass substrates," Physics Procedia, vol. 39, pp. 548-555, 2012.
[17] S. Ahmad Sobri, R. Heinemann, and D. Whitehead, "Development of laser drilling strategy for thick carbon fibre reinforced polymer composites (Cfrp)," Polymers, vol. 12, no. 11, p. 2674, 2020.
[18] C. Zhang, N. R. Quick, and A. Kar, "A model for self-defocusing in laser drilling of polymeric materials," Journal of Applied Physics, vol. 103, no. 1, p. 014909, 2008.
[19] Z. Yu, L. Xu, W. Cao, and J. Hu, "Study on picosecond laser processing of blind holes in carbon fiber-reinforced plastics," Applied Physics A, vol. 126, no. 12, pp. 1-10, 2020.
[20] J. Yeo et al., "Next generation non-vacuum, maskless, low temperature nanoparticle ink laser digital direct metal patterning for a large area flexible electronics," 2012.
[21] S. B. Walker and J. A. Lewis, "Reactive silver inks for patterning high-conductivity features at mild temperatures," Journal of the American Chemical Society, vol. 134, no. 3, pp. 1419-1421, 2012.
[22] Y.-K. Liu and M.-T. Lee, "Laser direct synthesis and patterning of silver nano/microstructures on a polymer substrate," ACS applied materials & interfaces, vol. 6, no. 16, pp. 14576-14582, 2014.
[23] Y. D. Suh et al., "Maskless fabrication of highly robust, flexible transparent Cu conductor by random crack network assisted Cu nanoparticle patterning and laser sintering," Advanced Electronic Materials, vol. 2, no. 12, p. 1600277, 2016.
[24] S.-L. Tsai, Y.-K. Liu, H. Pan, C.-H. Liu, and M.-T. Lee, "The coupled photothermal reaction and transport in a laser additive metal nanolayer simultaneous synthesis and pattering for flexible electronics," Nanomaterials, vol. 6, no. 1, p. 12, 2016.
[25] K. Kordás et al., "Laser-assisted metal deposition from liquid-phase precursors on polymers," Applied surface science, vol. 172, no. 1-2, pp. 178-189, 2001.
[26] S. Y. Chou, C. Keimel, and J. Gu, "Ultrafast and direct imprint of nanostructures in silicon," Nature, vol. 417, no. 6891, pp. 835-837, 2002.
[27] B. Cui, W. Wu, C. Keimel, and S. Y. Chou, "Filling of nano-via holes by laser-assisted direct imprint," Microelectronic engineering, vol. 83, no. 4-9, pp. 1547-1550, 2006.
[28] K. Wang, C. Liao, W. Wang, Y. Xiao, X. Liu, and Y. Zuo, "Removal of Gas Bubbles on an Electrode Using a Magnet," ACS Applied Energy Materials, vol. 3, no. 7, pp. 6752-6757, 2020.
[29] H. Wang, Z. Chen, Y. Chen, M. Xie, and L. Hua, "Mechanism study of bubble removal in narrow viscous fluid by using ultrasonic vibration," Japanese Journal of Applied Physics, vol. 58, no. 11, p. 115503, 2019.
[30] 藍辰睿, "雷射直析技術之微觀流場觀測與製程改良," 國立中興大學機械工程學系研究所, 臺中市, 2018.
[31] 范芝榕, "雷射直析製程改良之研究," 國立中興大學機械工程學系研究所, 臺中市, 2020.
[32] T. Schwarz-Selinger, D. G. Cahill, S.-C. Chen, S.-J. Moon, and C. Grigoropoulos, "Micron-scale modifications of Si surface morphology by pulsed-laser texturing," Physical Review B, vol. 64, no. 15, p. 155323, 2001.
[33] S. Siddharth, S.-L. Tsai, Y.-B. Chen, and M.-T. Lee, "Opto-thermo-fluidic transport phenomena involving thermocapillary flow during laser microfabrication," International Journal of Heat and Mass Transfer, vol. 162, p. 120303, 2020.
[34] "DuPont™ Kapton® Summary of Properties," Internet. [Online]. Available: https://www.dupont.com/content/dam/dupont/amer/us/en/products/ei-transformation/documents/EI-10142-Kapton-Summary-of-Properties.pdf.
[35] A. N. Volkov and L. V. Zhigilei, "Melt dynamics and melt-through time in continuous wave laser heating of metal films: Contributions of the recoil vapor pressure and Marangoni effects," International Journal of Heat and Mass Transfer, vol. 112, pp. 300-317, 2017.
[36] P. Sysel, V. Šindelář, E. Chánová, and B. Wallin, "Preparation of polyimides by using mixtures of tetrahydrofuran and methanol and their properties," Polymer journal, vol. 34, no. 2, pp. 54-56, 2002.
[37] B. B. Sauer and G. T. Dee, "Surface tension and melt cohesive energy density of polymer melts including high melting and high glass transition polymers," Macromolecules, vol. 35, no. 18, pp. 7024-7030, 2002.
[38] "Kapton JP Technical Infromation," Internet. [Online]. Available: https://www.9e.com.tw/tw/product/KAPTON_JP.pdf.
[39] J. Ho, C. Grigoropoulos, and J. Humphrey, "Computational study of heat transfer and gas dynamics in the pulsed laser evaporation of metals," Journal of Applied Physics, vol. 78, no. 7, pp. 4696-4709, 1995.
[40] F. P. Incropera, T. L. Bergman, and A. S. Lavine, Foundations of Heat Transfer. Wiley, 2013.
[41] R. L. Blaine, "Thermal applications note," Polymer Heats of Fusion, 2002.
[42] M. Elomaa et al., "Combustion of polymeric materials," Critical Reviews in Analytical Chemistry, vol. 27, no. 3, pp. 137-197, 1997.
[43] J. Lin et al., "Laser-induced porous graphene films from commercial polymers," Nature communications, vol. 5, no. 1, pp. 1-8, 2014.
[44] R. S. Kappes et al., "A study of photothermal laser ablation of various polymers on microsecond time scales," SpringerPlus, vol. 3, no. 1, pp. 1-15, 2014.
[45] R. Greiner and F. Schwarzl, "Thermal contraction and volume relaxation of amorphous polymers," Rheologica acta, vol. 23, no. 4, pp. 378-395, 1984.
[46] Q. Du, T. Chen, J. Liu, and X. Zeng, "Surface microstructure and chemistry of polyimide by single pulse ablation of picosecond laser," Applied Surface Science, vol. 434, pp. 588-595, 2018.
[47] J. Zou, W. Yang, S. Wu, Y. He, and R. Xiao, "Effect of plume on weld penetration during high-power fiber laser welding," Journal of Laser Applications, vol. 28, p. 022003, 2016.
[48] T. E. Itina, K. Gouriet, L. V. Zhigilei, S. Noël, J. Hermann, and M. Sentis, "Mechanisms of small clusters production by short and ultra-short laser ablation," Applied Surface Science, vol. 253, no. 19, pp. 7656-7661, 2007.
[49] P. Diwakar, S. Harilal, M. Phillips, and A. Hassanein, "Characterization of ultrafast laser-ablation plasma plumes at various Ar ambient pressures," Journal of Applied Physics, vol. 118, no. 4, p. 043305, 2015.
[50] D. Price, G. Anthony, and P. Carty, "Introduction: polymer combustion, condensed phase pyrolysis and smoke formation," in Fire retardant materials: Elsevier, 2001, pp. 1-30.
[51] I. Verdugo, J. J. Cruz, E. Álvarez, P. Reszka, L. F. F. da Silva, and A. Fuentes, "Candle flame soot sizing by planar time-resolved laser-induced incandescence," Scientific Reports, vol. 10, no. 1, pp. 1-12, 2020.
[52] M. Li, K. Sun, and Z. He, "Integrated research of a multi-wavelength method in anisotropic scattering flame on soot temperature and radiative coefficient reconstruction," Applied optics, vol. 57, no. 21, pp. 5899-5913, 2018.
[53] P. Sunderland, J. Quintiere, G. Tabaka, D. Lian, and C.-W. Chiu, "Analysis and measurement of candle flame shapes," Proceedings of the Combustion Institute, vol. 33, no. 2, pp. 2489-2496, 2011.
[54] I. Lakkis, "Grid-free vortex methods for natural convection; Handling source terms and nonlinear diffusion," Numerical Heat Transfer, Part B: Fundamentals, vol. 62, no. 5, pp. 370-398, 2012.