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Investigation and fabrication of GaN-based micro LED displays
|關鍵字:||微型發光二極體顯示器;無電極遮蔽;光阻塗佈;micro-LED display;electrodeless shield;photoresist coating||引用:|| J. H. Lee, D. N. Liu, and S. T. Wu, 'Introduction to Flat Panel Displays,' Hoboken, New Jersey, John Wiley & Sons., 2008.  K. Nishiyama, and M. Okita, S. Kawaguchi, '9.3: 32' WXGA LCD TV using OCB Mode, Low Temperature p-Si TFT and Blinking Backlight Technology,' Society for Information Display, vol. 36 , No. 1, 2005.  B. W. Lee, K. Park, A. Arkhipov, and S. Shin, ' L-2: Late-News Paper: The RGBW Advantage for AMOLED,' Society for Information Display, vol. 38 , No. 1, 1386-1389, 2007.  Transparency Market Research, ' Wearable Tchnology Market - Global Industry Analysis, Size, Share, Growth, Trends and Forecast, 2012 - 2018', 2014  IDC Worldwide Quarterly Wearable Device Tracker, 'Forecasts Shipments of Wearable Devices to Nearly Double by 2021 as Smart Watches and New Product Categories Gain Traction ', 2017  Y. C. Lin, J. P. You, N. T. Tran, Y. He, and F. G. Shi , 'Packaging of Phosphor Based High Power White LEDs: Effects of Phosphor Concentration and Packaging Configuration,' JournaL Of Electronic Packaging., vol. 133, No. 1 , 2009  Zhao Jun Liu, Ka Ming Wong, Chi Wing Keung, Chak Wah Tang, and Kei May Lau, 'Monolithic LED Microdisplay on Active Matrix Substrate Using Flip-Chip Technology ', Digital Object Identifier Vol. 15, No. 4, pp. 1298-1302, 2009  Zhao Jun Liu, Wing Cheung Chong, Ka Ming Wong, and Kei May Lau, '360 PPI Flip-Chip Mounted Active Matrix Addressable Light Emitting Diode on Silicon (LEDoS) Micro-Displays ', Digital Object Identifier,678 - 682,2013  Wing Cheung Chong*, Wai Keung Cho, Zhao Jun Liu, Chu Hong Wanga and Kei May Lau, '1700 pixels per inch (PPI) Passive-Matrix Micro-LED Display Powered by ASIC ', IEEE, pp. 1550-8781 ,2014  Huang-Yu Lin, Chin-Wei Sher, Dan-Hua Hsieh, Xin-Yin Chen, Huang-Ming Philip Chen,Teng-Ming Chen, Kei-May Lau, Chyong-Hua Chen, Chien-Chung Lin, And Hao-Chung Kuo, 'Optical cross-talk reduction in a quantumdot-based full-color micro-light-emitting-diode display by a lithographic-fabricated photoresist mold ', Photonics Research, Vol. 5, No. 5, pp. 411-416, 2017  Chang-Mo Kang, Duk-Jo Kong, Jae-Phil Shim, Sanghyeon Kim, Sang-Bae Choi, Jun-Yeob Lee, Jung-Hong Min, Dong-Ju Seo, Soo-Young Choi, And Dong-Seon Lee, 'Fabrication of a vertically-stacked passivematrix micro-LED array structure for a dual color display ', OPTICS EXPRESS ,Vol. 25, No. 3 , pp. 2489-2495, 2017  Sze, S. M./ Lee, M. K 'Semiconductor Devices: Physics and Technology ', John Wiley & Sons Inc,2012  H. L. Hartnagel, A. L. Dawar, A. K. Jain and C. Jagadish, 'Semiconductoer Transparent Thin Films',IOP Publishing Ltd., p.1-4, 1995.  C. H. Kuo, S. J. Chang, 'Nitride-based near-ultraviolet LEDs with an ITO transparent contact', Materials Science and Engineering: B, Vol. 106, pp. 69-72, 2004.  M.D. Benoy, E.M. Mohammed, S.M. Babu, P.J. Binu, B. Pradeep. 'Thickness dependence of the properties of indium tin oxide (ITO) FILMS prepared by activated reactive evaporation', Brazilian Journal of Physics , vol. 39, no. 4, pp. 629-632, 2009  A. Schmitz, A. Ping, M. Khan, Q. Chen, J. Yang, and I. Adesida, 'Metal contacts to n-type GaN,' Journal of Electronic Materials, vol. 27, pp. 255-260, 1998.  郭浩中,賴芳儀,郭守義, LED原理與應用Principles and Applications of Light-emitting Diode(第二版), 台灣:五南圖書, 2012, 頁73-97  Z. L. Li, X. D. Hu, C. Ke, R. J. Nie, X. H. Luo, X. P. Zhang, T. J. Yu, B. Zhang, C. Song, Z. J. Yang, Z. Z. Chen, and G. Y. Zhang, 'Preparation of GaN-based cross-sectional TEM specimens by laser lift-off', Micron, vol. 36, pp. 281-284, 2005  史光國, 半導體發光二極體及固態照明(初版), 台灣:全華, 2005年  LEDinside , a Business Division of TrendForce Corp. (Taiwan)  國科會精密發儀器發展中心, 材料電子顯微鏡學, 台灣:全華圖書, 2004年3月  陳隆建,發光二極體之原理與製, 台灣: 全華圖書,2006年8月。  Rapitech, CAS140B, 2012。||摘要:||
本論文成功地開發藍光與綠光小間距的微型發光二極體(LED)顯示器，其解析度為64 × 32，單顆像素尺寸為50 μm × 50 μm，像素間距為100 μm。以ITO透明導電膜作為P-type氮化鎵(GaN)的歐姆接觸層同時幫助電流擴散，在製程設計上採覆晶式LED製程方法達到無電極遮蔽的效果，正負電極使用鈦/鋁/鈦/金(Ti/Al/Ti/Au)作為電極金屬，最後以異方性導電膠與IC對位貼合。顯示器以被動式的IC電路作驅動，以定址的方式控制矩陣的單點像素，達到影像的顯示。
實驗結果顯示，不論藍光或綠光單顆LED，第1列與第64列之順向電壓在直流1 mA的電流驅動下僅有小於8%的變化。藍光LED在3 mA電流注入下具有847 μW的光輸出功率，其中在0.3 mA時，其外部量子效率為12.1 %，在頻率105 Hz且duty cycle為1/32的脈波寬度調變電壓源下，其亮度為430 nit；綠光LED在3 mA電流注入下具有449 μW的光輸出功率，其中在0.3 mA時，其外部量子效率為8.0 %，在頻率105 Hz且duty cycle為1/32的脈波寬度調變電壓源下，其亮度為590 nit。本研究在元件製程完成後，其單點像素在 -5 V的電壓源下，其逆向電流約為10 nA~60 nA。根據熱影像量測結果，當藍光LED顯示器與綠光LED顯示器切換至全點亮模式，經過點亮30分鐘後，其元件表面溫度僅有小幅度從28 C增加至33 C，且溫度上升的趨勢隨著時間的增加而趨緩，最終達到穩定。
In this thesis, blue and green micro-LED displays have been successfully developed. These two micro LED displays with 64 × 32 pixels both possess the pixel size of 50 μm × 50 μm and pixel pitch of 100 μm. For fabricating the LED device, ITO is used as both the ohmic contact layer on the p-GaN and the transparent conducting film to improve the current spreading. To solve the light-shading problem caused by the metal electrodes, electrodeless shielding of flip-chip LEDs were prepared in this study. Moreover, Ti/Al/Ti/Au was deposited as P-pad electrode and N-pad electrode. The micro LEDs were bonded to integrated circuits by using the anisotropic conductive film. The display is driven via the passive integrated circuit. Single pixel on matrix display is controlled through the mix multi-electrodes addressable method, and the image can be shown on the display successfully.
During the fabrication process of micro LED, photoresist coating was employed to fill the 6-μm-deep trench. Using the two-step spin coating method, the surface tension of photoresist on the sample can be reduced. Via the increment of the soft-baking time, the motion of photoresist is increased. Through the optimization of photoresist coating process, the increase of electrical resistance and the circuit-broken problem occurred in the evaporation process for p-pad electrode are avoided. Additionally, the deposition of Ti/Al/Ti/Al contact metal on the N-GaN layer can prevent the increase of forward voltage with increasing the pixel distance, and the high consistency of optoelectronic performance of each pixel on the display is achieved.
When blue or green LEDs was operated at the direct current of 1 mA, the variation of forward voltage between the 1st column and 64th column was less than 8%. At an injection current of 3 mA, the outputpower were achieved to 847 μW, the external quantum efficiency of blue LED was determined to be 12.1 % at 0.3 mA. Meanwhile, as the current of 3 mA was injected into green LED, the output power was achieved to 449 μW and its external quantum efficiency was 8.0% at 0.3 mA. On the other hand, as the pulse-width modulated voltage was biased at frame rate is 105 Hz and duty cycle is 1/32 , the luminance of blue and green LEDs was 430 and 590 nit, respectively. The leakage current of single pixel was ranging from 10 to 60 nA (@ -5 V). Based on the results of thermal infrared imaging measurements, when the micro LED displays were turned to the full-light mode, the surface temperature was increased slightly from 28 to 33 C after operating for 30 min. Moreover, the increased tendency of surface temperature was slowed down to a stable state with increasing the test time.
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