精密玻璃模造相關技術於光學元件之應用與研究

Abstract

玻璃材料相較於高分子材料，具有更耐熱、耐候、抗刮、抗腐蝕、較優異的光學特性等，因此在某些特殊環境僅能使用玻璃材料，例如高能雷射系統或是嚴苛氣候條件。精密玻璃模造技術可快速地製作高精度玻璃元件，但因玻璃模造關鍵技術，如模仁之硬膜鍍膜技術及模造製程參數，皆為各廠商的商業機密。且光學玻璃材料種類繁多，在玻璃模造製程參數測試初期，需要耗費許多時間才能獲得適合該材料特性的穩定製程參數，導致後人較難以先前研究墊基。因此，本研究針對各種欲成形之玻璃材料進行實驗分析，以獲得最佳的模造製程參數。後續相關研究學者只要依照元件外形微調本論文之製程參數，即可迅速地完成相似光學元件。 本論文以精密玻璃模造技術為核心，發展開放式及封閉式兩種玻璃模造實驗方法，進行元件成形探討。實驗結果發現開放式模造製程中，因有氮氣的對流熱傳加熱效果，玻璃預形體及模仁升溫效率較封閉式佳，所以適合用來成形微結構，但因其缺乏定位導銷而無法使鏡片之兩面定心，因此若要製作精密曲面光學元件，則需採用封閉式模造製程。本實驗利用開放式製程對微結構元件進行成形機制探討，包括微柱狀透鏡陣列、單面/雙面微透鏡陣列，且利用封閉式製程探討精密曲面元件成形，包括平面積體光學元件與非球面透鏡。最後利用具有微結構與精密曲面的菲涅耳透鏡，來驗證微結構與精密曲面的研究成果。 目前精密玻璃模造技術之模仁多使用碳化鎢材料並透過超精密鑽石輪磨技術製作，然而鑽石輪磨技術針對複雜性或微結構輪廓有其加工限制。因此，本論文提出除了使用鑽石輪磨技術所加工之模仁外，亦透過線切割放電加工技術在導電的碳化鎢材料上製作模仁結構，以及利用雷射加工技術在碳化矽與不□鋼材料上製作微結構陣列，並利用電解拋光技術改變不□鋼微結構輪廓，完成微陣列模仁，本論文所提出之製程方法，使玻璃模造之模仁加工法不必再侷限於精密鑽石輪磨製程。 在模造製程中，本論文發現提高模造溫度、降低模造力量或是減少降溫速率，可有效的解決元件破裂現象，而降低模造溫度可減少沾黏現象。在模造環境方面，充填氮氣的狀態下，模仁以及玻璃預形體的受熱方式較多，因此溫度較高且成形性較佳，然而真空模造環境可避免元件產生氣泡，因此，可藉由增加上下模座之間距、增加保溫時間以及提高模造溫度，使玻璃元件在真空模造環境成形。若玻璃元件尺寸大於15 mm，可增加保溫時間至180秒以上，使玻璃元件溫度更均勻，降低破裂現象的產生，且減緩降溫速率，可減少折射率的變化量，對小尺寸非球面透鏡而言，使用18.5 °C/min之降溫速率可獲得較穩定的鏡片品質。 在開放式模造製程方面，利用線切割放電加工技術可加工高硬度材料，製作微長條形溝槽陣列於碳化鎢模仁，以N-FK5材料完成外徑20 mm、厚度5 mm，其上有高度234 μm、曲率3.42 mm之微柱狀透鏡陣列，並利用雷射光束驗證其經過微柱狀透鏡陣列之後，原為圓形光斑之雷射輪廓轉變為線形雷射輪廓。此外，還利用雷射技術非接觸加工的特性製作微結構陣列於碳化矽模仁，以鈉玻璃完成鏡片外徑20 mm、厚度1.2 mm，其上具有曲率半徑為851 μm，寬度與高度分別為460 μm及52 μm，透鏡間距為700 μm之雙面微透鏡陣列。以及使用雷射加工不□鋼模仁，經電解拋光技術去除再鑄層，以鈉玻璃完成曲率為58.1 μm，高度為39.3 μm之微透鏡陣列以及曲率79.8 μm，高度為37 μm之微柱狀鏡陣列，其陣列範圍皆為10 mm*10 mm。 在封閉式模造製程方面，模仁材料皆使用碳化鎢，且利用超精密鑽石輪磨技術加工，在最佳模造溫度750 ℃時，完成外徑53 mm、高度6 mm，具有拋物面結構之N-BK7平面積體光學元件。並使用K-VC79玻璃材料，於最佳模造溫度585 ℃時，完成外徑4.47 mm，表面精度小於0.5 λ (0.316μm)之非球面透鏡。亦使用K-CSK120玻璃材料，在最佳模造溫度560 ℃時，完成填充率達99 %之菲涅耳透鏡，其外徑為15 mm。

Glass offers better anti-thermal, anti-environmental, corrosion resistance, and optical properties than polymer materials. Therefore, glass can be used in demanding environments such as high-energy laser systems or severe weather conditions. Precision glass molding (PGM) techniques can be used in rapid fabrication of highly accurate optical components. However, key aspects of PGM techniques, such as the hard coating for mold protection and the operating parameters, are kept confidential by manufacturers. Because various types of optical glass have different characteristics, the initial testing period of the PGM process is time consuming. Therefore, researchers are difficult to follow the optimal molding parameters from previous studies. Consequently, this thesis focuses on the analysis of glass materials and the optimization of molding parameters. In order to understand the molding mechanism for producing microstructures and precision curves components, the open-type and close-type PGM approaches were studied in this thesis. According to experimental results, the thermal absorptivity of glass in open-type PGM processing is better than the close-type due to glass can be heated by thermal convection of nitrogen in open-type PGM processing. Therefore, open-type PGM processing is suitable for forming microstructures. However, it is difficult to align two surfaces of components in this type of processing. To fabricate components with precision curves, closed-type PGM processing is needed. This thesis studies open-type PGM processing of microstructure arrays such as micro cylindrical lens arrays and one- or double-sided microlens arrays and close-type PGM processing of planar-integrated micro-optical components and high-precision aspheric lenses. Finally, a Fresnel lens, which combines microstructure and precision curve, was used to verify the experimental results of above experiments. Precision diamond grinding technique was usually used to fabricate the mold of PGM processing on tungsten carbide (WC) material. But Precision diamond grinding technique was not easily to generate the complex and tiny microstructure. Hence, this thesis used not only diamond grinding technique but also wire electrical discharge machining technique on WC and laser machining on silicon carbide and stainless steel to fabricate the mold of glass molding. Besides, the electro polishing has been used to change the shape of micro structure on mold. The processing methods using in this thesis offers the various ways to manufacture molds. This thesis obtains, the fracture of glass components can be avoid with increasing molding temperature, decreasing molding force and decreasing cooling rate in the PGM processing. Furthermore, decreasing molding temperature can reduce adhesive phenomenon. Besides, the thermal absorptivity of mold and glass in nitrogen atmosphere are better than vacuum environment, the deformable level of glass can be increased. But molding in vacuum environment can avoid bubbles product on glass. Therefore, increasing space of upper and under mold, temperature holding time and molding temperature are effective to form glass components in vacuum environment. If size of component larger than 15 mm, temperature holding time longer than 180 s is useful to uniform temperature and avoids fracture of glass. Moreover, slower cooling rate reduces the variation of refractive index. For small lens, 18.5 °C/min in cooling rate obtains the stable quality. In this thesis, the wire electrical discharge machining process, which is suitable for machining hard materials such as WC, was used to fabricate a microgroove array on a WC mold in the open-type PGM approach. N-FK5 glass was used, and a micro cylindrical lens array 20 mm in diameter and 5 mm thick was successfully obtained. Cylindrical lenses 234 μm in height and 3.42 mm in curvature were fabricated in the array. In addition, a laser ablation approach, which offers the advantage of noncontact machining, was adopted to generate a microstructure array on a silicon carbide mold, and a double-sided microlens array 20 mm in diameter and 1.2 mm thick was obtained on soda-lime glass. The microlens components were 851 μm in curvature, 460 μm in width, 52 μm in height, and 700 μm in pitch. Furthermore, electro polishing was used to remove the recasting layer used in laser ablation, and a stainless steel mold was used. A microlens array 58.1 μm in curvature and 39.3 μm in height on soda-lime glass and a micro cylindrical lens array 79.8 μm in curvature and 37 μm in height were obtained. Both these components were 10 mm × 10 mm in size. WC molds were successfully fabricated by a precise diamond grinding technique using in close-type PGM processing. The optimal molding temperature was 750 □C. N-BK7 glass was used to obtain planar-integrated micro-optical components 53 mm in diameter and 6 mm in height. K-VC79 glass was used to obtain a high-precision aspheric lens with a surface accuracy (peak to valley) of less than 0.5 λ (0.316 μm) at an optimal molding temperature of 585 □C. With an optimal molding temperature of 560 □C, a Fresnel lens 15 mm in diameter was fabricated from K-CSK120 glass, and a fill rate of 99% was achieved.