滾輪式UV微奈米壓印技術應用於連續製作圖案化藍寶石基板

Abstract

高功率藍光GaN基 LED搭配黃色螢光粉發出白光之裝置，已經廣泛被使用於戶外照明及LCD背光模組，並快速朝室內照明發展。先前研究指出在藍寶石基板上製作微米甚至奈米級週期性陣列，有助於GaN磊晶及光線散射，能夠大幅提升LED發光效率，故現行LED多生長於圖案化藍寶石基板(Patterned Sapphire Substrate, PSS)上。藍寶石硬度極高加工不易，欲製作微米級圖案化基板，通常使用曝光顯影或微奈米壓印製作高深寬比光阻遮罩，再進行ICP-RIE於藍寶石上蝕刻出尖錐狀結構。由於藍寶石基板本身以多層磊晶方式製作，其表面常有彎曲或部分不平整現象，使得以曝光顯影方式製作，尤其於大尺寸基板時備受挑戰。反之，以軟性模具壓印藍寶石基板製作所需遮罩之製程，因其低成本及高適用性得到廣泛關注。 本研究提出以滾輪式UV微奈米壓印技術製作上述光阻遮罩，期許能以滾輪連續快速壓印、低成本、高複製率之優勢，將現行批量式壓印之圖案化藍寶石製程持續向前推進。研究分成三個主要部分：一為滾輪模具製作；二為滾輪式UV壓印；三為反轉式UV滾輪壓印。 第一部分滾輪模具製作將矽晶圓上平面微柱陣列光阻，經電鍍、熱壓印成型、微澆注等製程後，成功以高複製率製作具整齊微米凹洞之PDMS軟滾輪，其中詳細探討熱壓印參數與現象，並以富士感壓軟片針對軟滾輪物理特性進行分析；第二部分以此軟滾輪為模具、量產化之曝光顯影用光阻作為膠體，於藍寶石基板上進行滾輪式UV壓印製作所需微柱光阻遮罩。經過系統化探討光源設計、平台移速、施加壓力等參數後，成功以1.25mm/s滾壓速度，連續於兩吋藍寶石基板上成型微柱陣列。以雷射共軛焦顯微鏡觀察，最佳成型區域結構高度2.01μm，相對於滾輪模具，結構複製率達95.2%。惟基板上成型結構高度並不均勻，呈現順著壓印方向由高至低分布現象，且SEM斷面結果顯示殘留層厚度稍厚，亦針對此兩項缺陷提出相關成因及改良方法。第三部份針對上述製程之成型不均及殘留層厚度等問題提出改良製程：以反轉式UV滾輪壓印技術結合滾輪式液態膠體轉印技術(Roll-typed liquid transfer imprint lithography, R-LTIL)，製作圖案化藍寶石基板所需微柱陣列光阻遮罩。找到最佳化參數後，成功以1.68mm/s滾壓速度成型，以SEM及雷射共軛焦顯微鏡觀察，發現整面兩吋基板均具有高均勻性之微米柱狀陣列。其平均結構高度2.07μm、圓柱直徑1.69μm、週期3μm，相較於母模結構高度複製率達88.8%、相較於PC環型子模結構複製率達98.1%，結合R-LTIL技術也成功使殘留層厚度顯著下降。 本研究成功開發兩套滾輪式UV微奈米壓印製程應用於製作圖案化藍寶石基板，證實自製PDMS軟滾輪結合UV壓印技術能連續於藍寶石基板上製作高深寬比微柱光阻陣列。將目前曝光顯影及平面式壓印等批量式生產，提升至連續式滾輪生產，於未來圖案化藍寶石基板朝快速大量製造、大面積晶圓量產等發展上，相當具有潛力。

White LEDs have been widely used for light source and backlight in LCDs. It has been reported that micro-cone-pattern on sapphire substrates can improve GaN crystalline quality and the light scattering to enhance the output power of GaN-based LEDs. Therefore, LEDs are commonly grown on patterned sapphire substrates(PSS). To fabricate PSS, conventional photolithography and imprinting lithography are used to form high aspect ratio pillar array, which is served as the mask of subsequent ICP-RIE. Because a multi-layered sapphire substrate for LED is often warped, the imprinting methods with soft mold are attracted attention for their low cost and suitability. In this study, two continuous UV roller-type approaches to fabricate micro-pillar array on sapphire substrate have been developed. We expect that by using roller-imprinting, PSS can be fabricated with lower cost and higher throughput. There are three main topics are researched and discussed: fabrication of a soft roller mold, roller-type UV imprint and roller-type reverse imprint. To begin with the fabrication of the roller mold, by electroplating and hot embossing, we can replicate the pattern from flat resist master to a flexible circular mold with high fidelity. Then a roller with PSS cavity made of PDMS is fabricated by casting from the circular mold.The roller is integrally molded with simple process, high fidelity and low-cost. The second, with the conformal contact of the soft roller, we successfully form micro-pillar array on sapphire substrates by roller-type UV-imprinting at high feeding speed 1.25mm/s. The height of pattern in terms of applied pressure, feeding velocity and initial photoresist thickness were investigated. In the best condition, the average height can reach 2.01μm and 95.2% fidelity. Lastly, to enhance the uniformity of pattern and reduce the residual layer thickness (RLT), we develop another roller-type reverse imprinting process by the same soft roller mold. The pattern is formed on almost whole the 2-inch sapphire substrate at 1.68mm/s feeding speed with high uniformity. The height, top diameter and period are 2.07μm, 1.69μm and 3μm, respectively. In this study, two processes have been successfully developed to form micro-pillar array for the PSS mask by UV roller-type imprinting. These roller-type methods can be easily implemented and fabricate PSSs at high feeding speed. Therefore, they have a great potential in fabricating larger size PSS with low cost and high throughput in the future.