自2000年Wong等人發表以銦钯合金(In-Pd)應用於晶圓鍵結(wafer bonding)整合雷射剝離技術(laser lift-off)以製備垂直氮化鎵基(GaN-based)薄膜發光二極體起，即引起相當廣範的注意與研發能量的投入。在製備垂直元件的過程當中，基板置換技術是相當關鍵且重要的一環。它除實現垂直導通的電流路徑，更是易碎的氮化鎵基磊晶在藍寶石基板以雷射剝離技術移除後的機械支撐提供者。習知的基板技術，以電鍍金屬基板(electroplating metal substrate)與晶圓鍵結最普為使用。不論何者，其製程時間都相當的長，進而降低生產效率。此外，為了提供足夠的基械支撐力，置換基板需與磊晶緊密結合。因此，對於外在的應力鮮具緩衝能力，限制了其應用。
在本論文中，我們展示了採用了錫基(Sn-based)金屬焊球(solderball)用於製備金屬基板之技術。透過對鎳金屬網格的對準，錫基金屬焊球植附於鎳-金溼潤層之上。於280oC迴焊90秒後，錫基金屬基板即製備完成。整合以區塊雷射剝離技術，免切割金屬基板是為實現。經表面粗化與電極成形，所製備之垂直結構元件與傳統藍寶石基板上橫向元件相較，所製之元件在注入電流為350 mA時，順向壓降(VF)為3.46 V同時光輸出增加了145.36%。同時，透過量測峰波長(peak wavelength)與電流相依關係，顯示所提之錫基金屬基板技術對於元件的熱積聚效應提供了顯著的改善效果。
為提升垂直LED元件結構遭遇外應力之光電特性穩定性，本論文另提出一高熱穩定之彈性導電膠體作為晶元鍵結之介質層。此一彈性膠體由鍍鎳樹脂球與可撓式聚脂組成。此膠體於常溫至200oC的作業範圍內皆具備相當的彈性緩衝特性。整以具可撓性之不鏽鋼基板，晶圓鍵結過程在180oC下以15秒完成。經區塊雷射剝離、表面粗化與電極成形，所提之具緩衝外在形變並維持穩定特性之元件是為完成。所製之元件與習之製備於藍寶石基板上水平導通元件相較，在注入電流為120 mA時，順向壓降(VF)下降0.21 V，光輸出增加80%，使得整體光電轉換效率改善幅度達91%。在元件與外力相依性的量測上，我們定義一基板有效長度(effective length, Le)做為所施外力之指標。透過置具施加外力撓曲元件基板改變其有效長度並量測元件之主波長(dominant wavelength)與外力相依性揭示，在有基板有效長度由24 mm減少至5 mm時，其主波長最大位移量為0.3 nm。此一波長位移應來自於外在的應變改變了內在極化電場的狀態而反應於主波長之上。同時，元件之光輸出與電流電壓特性維持相當的穩定無明顯變動。應證所提之元件結構對於外力的響應極小且具穩定之重現性。
於本論文中，我們亦提出有關減緩於晶粒黏著製程中熱應力效應的初步研究成果。錫基金屬焊材目前廣為業界採用於封裝高功率金屬基板垂直結構發光二極體晶粒。然過程中高達280oC的製程溫度與溫度變化，亟易因金屬基板與上層磊晶間熱膨脹系數的差異造成元件結構上的形變、影響發光復合效率及其光輸出特性。透過使用鋁掺雜之氧化鋅薄膜與區塊化的氧化鎳金歐姆接觸做為減緩熱膨脹差異之緩充層併與蕭特基阻障為電流擴散結構，與習之標準垂直結構元件相比較，光電轉換效率改善達21%。同時，在經過280oC、40s的高溫變化後，具所提結構之元件仍保有穩定的光析出特性。

Since Wong et al. demonstrated the worldwide first vertical structured thin-GaN light-emitting diode (LED) using laser lift-off (LLO) with Pd-In bonding for substrate transfer, tremendous efforts have been devoted to the development of vertical thin-GaN LEDs. For the fabrication of vertical thin-GaN LED, substrate transfer techniques play an important role. It not only enables vertical conduction, but also serves as mechanical support for the brittle epi-GaN after sapphire removal with LLO. Electroplating and wafer bonding are the most common two substrate technologies for substrate transfer. In either way, it was time consuming and decided most of the fabrication time. Besides, the stiff substrates were intimately adhered to epi-GaN. Thus there was mere buffer for external stress, and this limited the applications. In this dissertation, we aimed to reduce the fabrication of vertical thin-GaN LEDs by providing time-saving substrate technologies and prompt the feasibility to integrate LEDs with Flextronics.
Commercial available Sn-based solder balls were used to form dicing-free Sn-based metal substrates. The substrates were implemented after a reflow process within 90s. As compared with out previous electroplating nickel techniques, the processing time is substantially reduced. At 350 mA, devices using proposed Sn-based substrate exhibited a forward voltage drop (VF) of 3.46 V and an improvement in light output power (Lop) of 145.36% in comparison with conventional sapphire substrate LEDs. Merits of using metallic substrates were well reserved.
A thermal stable elastic conductive adhesive (ECA) comprised of flexible epoxy resin and nickel micro-spheres was used as bonding agent for the fabrication of vertical thin-GaN LEDs on flexible stainless substrates (namely Flex-LEDs). The wafer bonding process was carried out at 180oC for 15s. It showed that Flex-LEDs have negligible changes in light output intensity-current-voltage (L-I-V) and dominant wavelength-current (WD-I) characteristics confronting external stress from substrate bending. At 120 mA, it was seen that Flex-LED has a VF of 3.3 V and an enhancement in Lop of 80%.
In chapter 5, we reported our recent studies on releasing the strains during die-bonding processes for these high brightness vertical thin-GaN LEDs. Through the use of AZO CTE matching layers with patterned oxidized Ni/Au, SR-LEDs were fabricated. With prompted current spreading in the periphery area from Schottky blocking, the enhancements in power conversion reached 21%. In addition, highly stable EL characteristics confronting high temperatures up to 280oC during die-bonding were attained. Hope this could interest more efforts devoted for the development of better substrate structures considering strain-related phenomena.

Chapter 1
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Chapter 2
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Chapter 3
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Chapter 4
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Chapter 5
[5.1] S. J. Wang, K. M. Uang, S. L. Chen, Y. C. Yang, S. C. Chang, T. M. Chen, C. H. Chen, and B. W. Liu, “Use of patterned laser lift-off process and electroplating nickel layer for the fabrication of vertical-structured GaN-based light-emitting diodes”, Appl. Phys. Lett., vol. 87, p. 011111, 2005.
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