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Monolithic LED Microdisplay on Active Matrix Substrate Using Flip-Chip Technology Zhao Jun Liu, Ka Ming Wong, Chi Wing Keung, Chak Wah Tang, and Kei May Lau, Fellow, IEEE
Abstract—A monolithic high-resolution (individual pixel size 300 × 300 µm2 ) active matrix (AM) programmed 8 × 8 microLED array was fabricated using flip-chip technology. The display was composed of an AM panel and a LED microarray. The AM panel included driving circuits composed of p-type MOS transistors for each pixel. The n-electrodes of the LED pixels in the microarray were connected together, and the p-electrodes were connected to individual outputs of the driving circuits on the AM panel. Using flip-chip technology, the LED microarray was then flipped onto the AM panel to create a microdisplay. Index Terms—Active arrays, displays, flip-chip devices, gallium compounds, LEDs. Fig. 1. Cross-sectional view of neighboring LED pixels on completed LED microarray.
I. INTRODUCTION ESEARCH in solid-state lighting has placed much emphasis on improving the brightness and efficiency of III-nitride-based LEDs to increase the luminous flux of power LED chips. Great success has been achieved using various techniques such as adopting flip-chip configuration and indium–tin oxide transparent contacts [1]–[3] to maximize light extraction efficiency. LEDs with a high degree of brightness and a wide wavelength spectrum are suitable for many new applications such as illumination and displays. One application that has received less attention is LED microdisplays. III-nitride-based microdisplays have the potential to enhance and improve the present capabilities of miniature display systems such as liquid crystal displays (LCDs) and organic LEDs (OLEDs) displays with its ultimate performance in many different aspects, particularly observation under bright daylight [4]. Matrix-addressable microdisplays [5] are more attractive because of the reduction of contact pads. Passive matrix-addressable LED microdisplays on sapphire substrates have been reported [6], [7]. The dimensions and pixel brightness, in passive addressable LED microdisplays, are limited by the loading effect in the same row or column. LED array addressed by a single transistor with epitaxial stack method was also reported [8]. For single-transistor-addressable
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Manuscript received December 10, 2008; revised December 30, 2008 and January 30, 2009. This work was supported by the Innovation and Technology Commission (ITC) of Hong Kong Special Administrative Government (HKSAR) through NAMI, ITP/025/07NP. Z. J. Liu, K. M. Wong, C. W. Tang, and K. M. Lau are with the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]). C. W. Keung was with the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong. He is now with the Chip Development Department, Advanced Photoelectronic Technology Ltd., Kowloon, Hong Kong (e-mail: siuginger@ hotmail.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTQE.2009.2015675
Fig. 2.
Solder bumps on the top of AM panel.
LED arrays, the pixel brightness is still limited by the current driving capability of the signal source. Therefore, a new addressing scheme and fabrication technology is needed to improve the operating effectiveness of monolithic LED arrays. In this paper, a GaN-based, active matrix (AM) programmed 8 × 8 LED microdisplay using flip-chip configuration is reported. The AM panel consists of 8 × 8 driving circuits that were fabricated on a single crystal silicon wafer, and in the configuration of two p-type MOS (PMOS) transistors and one capacitor. The LED microarray includes 8 × 8 blue LED (emission wavelength = 440 nm) pixels with the n-electrodes connected together and individually separated p-electrodes. The LED microarray was then flipped onto the AM panel using standard flip-chip technology, and the p-electrodes of the LED pixels are
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Fig. 3.
Fig. 4.
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS
Fig. 5.
Turn-ON voltage distribution of eight LEDs in the same row.
Fig. 6.
Configuration of driving circuit on AM panel.
Fig. 7.
Operation points of LED and driving transistor.
Completed AM LED microdisplay panel.
Layout of the LED microarray.
connected to the output of the driving circuits on the AM panel. The fabricated prototype can be individually lighted up or fully powered with excellent emission uniformity. II. DEVICE FABRICATION A. Fabrication of LED Microarrays A standard multiple quantum well (MQW) blue LED wafer (emission wavelength = 440 nm) grown on a sapphire substrate was used for fabrication of the LED microarray. Plasmaenhanced CVD (PECVD) grown SiO2 masks were used for inductively coupled plasma (ICP) etching. The LED wafer was etched all the way down to the sapphire substrate. Rows of the array were defined and isolated in this step. PECVD SiO2 mask and ICP were again used to define the mesa structure of each LED pixel, with individual device size of 300 × 300 µm2 . A thin Ni/Au (5/5 nm) current spreading layer was deposited onto
the p-GaN surface by electron beam evaporation. Annealing in the atmospheric ambient at 570 ◦ C for 5 min was performed. Then, a Ti/Al/Ti/Au (30/120/10/30 nm) multilayer metal was evaporated to form the n-contact and as reflective layer on the p-contact simultaneously. Finally, SiO2 passivation was applied onto the wafer. Openings in the SiO2 were defined, and Ni/Au
LIU et al.: MONOLITHIC LED MICRODISPLAY ON ACTIVE MATRIX SUBSTRATE USING FLIP-CHIP TECHNOLOGY
Fig. 8.
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AM LED microarray was (a) fully lit up and (b) individually lit up.
(500/30 nm) contact pad was formed in the opening for flip-chip bonding. The cross-section of the completed LED microarray is shown in Fig. 1.
B. Fabrication of AM Panel The AM panel was fabricated with standard CMOS process on a (1 0 0) single crystal silicon wafer. After cleaning, well regions and body connection was defined by ion implantation. Then, a layer of silicon nitride was deposited and patterned. Field oxidation was performed to define the active area of the transistors using silicon nitride as a hard mask. Then, a thin layer of thermal oxide was grown as gate oxide. After poly-Si deposition and gate patterning, source/drain region was formed by ion implantation with standard self-alignment technology. Then, low temperature oxide (LTO) was deposited, and the wafer was annealed to densify the LTO and to activate the implanted dopants simultaneously. After opening of contact holes on the LTO layer, Al–Si alloy was deposited, and patterned for source/drain electrodes and interconnections.
C. Flip-Chip Process After the CMOS process, a layer of PECVD SiO2 was deposited on the AM panel for passivation and holes were opened. A TiW/Cu (30/500 nm) seed layer was deposited by sputtering and photoresist AZ4903 was coated and patterned by photolithography. A thick Cu layer (8 µm) and solder layer (22 µm) was deposited by electrical plating. After reflow in the annealing furnace, excellent solder bumps were formed in ball shape, as shown Fig. 2. The LED microarray wafer was thinned and diced. After flipping the diced LED microarray onto the AM panel, the finished device is shown in Fig. 3. The completed device was packaged in a dual in-line package (DIP) 40 socket and electrically connected by wire bonding.
III. RESULTS AND DISCUSSION Fig. 4 shows the layout of the LED microarray. In our design, all individual LEDs (emission wavelength = 440 nm) on the same column are connected by the n-contacts at the end of each row as cathodes. The current passes through the n-GaN layer and the n-metal bus line to reach the n-contact. Individual LEDs on the same row are connected to the output of the AM panel through the solder bumps as anodes. The LED microarray has identical electrical properties as a commercial 8 × 8 LED dot array. As shown by commercial discrete power LED manufacturing, flip-chip technology can improve heat dissipation, reliability, and manufacturability. Silicon has a larger thermal conductivity (150 W/m·K) than sapphire substrate (46 W/m·K), and well-developed flip-chip technology has been used in silicon for decades. In the epi-down (bottom emitting) configuration, the p-contact itself can be made reflective, thus eliminating any absorption of the current spreading layer and metal pads. In this way, the light output power and efficiency will be improved. The turn-ON voltages of LEDs, under the same 20 mA current injection, were strongly dependent on the distance between each LED and the n-contact. Series resistance of the bus bars to the n-GaN contact strip resulted in increased turn-ON voltage with longer distance of dies from the contacts at the end of each column [7]. For high-performance microdisplays, the variation of turn-ON voltage might cause a different junction temperature and/or a compensation of piezoelectric field between the individual LED pixels, and hence, a variation in lifetime and emitting wavelength. The later one would result in a poor angular homogeneity of color purity. In this study, the turnON voltage uniformity was greatly improved in a design with 40-µm-wide one-side n-metal bus lines on each row. The turnON voltages varied only from 3.30 to 3.70 V over the whole row under the same current injection, as shown in Fig. 5. Improved designs of the bus bars can be implemented in future versions to eliminate the variation completely.
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The configuration of the driving circuit on the AM panel is shown in Fig. 6. Transistor T1 serves as a switching transistor and T2 serves as a driving transistor. When T1 is switched on by a scan signal, a data signal would switch T2 on and be stored in the capacitor C1. Then, T2 would provide current to light up the LED pixel whose p-electrode was connected to the drain of T2. Driving transistor T2 was designed with a large W /L ratio to warrant enough output current for the LED pixel. Fig. 7 shows a typical I–V characteristic of an individual pixel in the AM LED array. Since the LED and the driving transistor are connected in series, the operating points are determined by the power supply voltage as well as the I–V characteristics of the LED and the driving transistor. From the I–V curve, we could find that the AM panel has sufficient driving capability for the LED microarray. Fig. 8 shows the display results of the AM LED microarray. The LED pixels had high brightness, good luminance uniformity, and individual controllability by the AM panel. IV. CONCLUSION In summary, we have demonstrated a monolithic highresolution AM-programmed LED microdisplay by using flipchip technology. The display has the advantages of high brightness, good luminance uniformity, and individual controllability. This paper shows that integration of GaN-based LEDs with the mature Si manufacturing technology is viable, and different lighting systems on a chip can be developed using similar technology. UV LED arrays are being fabricated for a full 12 × 12 Red, Green, Blue (RGB) AM panel for high-resolution, fullcolor LED microdisplays. ACKNOWLEDGMENT The authors would like to thank the Nano Fabrication Facility (NFF) and Electronic Packaging (EPACK) Laboratory of Hong Kong University of Science and Technology (HKUST) for the fabrication and flip-chip bonding facilities.
[8] K. Chilukuri, M. J. Mori, C. L. Dohrman, and E. A. Fitzgerald, “Monolithic CMOS-compatible AlGaInP visible LED arrays on silicon on latticeengineered substrates (SOLES),” Semicond. Sci. Technol., vol. 22, pp. 29– 34, 2007.
Zhao Jun Liu was born in Hebei, China, on May 1980. He received the B.S. degree in electrical engineering from Heibei University of Science and Technology, Heibei, in June 2003, the M.S. degree in physical electronics from Nankai University, Tianjin, China, in June 2007. He is currently working toward the Ph.D. degree at the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong. From October 2006 to April 2007, he was a Research Assistant in the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology. He is engaged in the development of active matrix LED microdisplay.
Ka Ming Wong was born in Hong Kong in 1986. He received the B.Eng. degree in electronic engineering from Hong Kong University of Science and Technology, Kowloon, Hong Kong, in 2008. He is currently a Research Assistant at the Photonic Technology Center, Hong Kong University of Science and Technology, where he is engaged in the development of active matrix LED microdisplay and materials integrations through wafer bonding.
REFERENCES [1] S. J. Chang, C. S. Chang, Y. K. Su, C. T. Lee, W. S. Chen, C. F. Shen, Y. P. Hsu, S. C. Shei, and H. M. Lo, “Nitride-based flip-chip ITO LEDs,” IEEE Trans. Adv. Packag., vol. 28, no. 2, pp. 273–275, May 2005. [2] J.-O. Song, D.-S. Leem, J. S. Kwak, Y. Park, S. W. Chae, and T.-Y. Seong, “Improvement of the luminous intensity of light-emitting diodes by using highly transparent Ag-indium tin oxide p-type ohmic contacts,” IEEE Photon. Technol. Lett., vol. 17, no. 2, pp. 291–293, Feb. 2005. [3] S. J. Chang, W. S. Chen, Y. C. Lin, C. S. Chang, T. K. Ko, Y. P. Hsu, C. F. Shen, J. M. Tsai, and S. C. Shei, “Nitride-based flip-chip LEDs with transparent ohmic contacts and reflective mirrors,” IEEE Trans. Adv. Packag., vol. 29, no. 3, pp. 403–408, Aug. 2006. [4] H. X. Jiang, S. X. Jin, J. Li, J. Shakya, and J. Y. Lin, “III-nitride blue microdisplays,” Appl. Phys. Lett., vol. 78, no. 9, pp. 1303–1305, 2001. [5] H. W. Choi, C. W. Jeon, and M. D. Dawson, “High-resolution 128 × 96 nitride microdisplay,” IEEE Electron Device Lett., vol. 25, no. 5, pp. 277– 279, May 2004. [6] C. W. Jeon, H. W. Choi, and M. D. Dawson, “Fabrication of matrixaddressable InGaN-based microdisplays of high array density,” IEEE Photon. Technol. Lett., vol. 15, no. 11, pp. 1516–1518, Nov. 2003. [7] C.-W. Jeon, H. W. Choi, E. Gu, and M. D. Dawson, “High-density matrixaddressable AlInGaN-based 368 nm microarray light-emitting diodes,” IEEE Photon. Technol. Lett., vol. 16, no. 11, pp. 2421–2423, Nov. 2004.
Chi Wing Keung received the B.Eng. and M.Phil. degrees in electronics engineering from Hong Kong University of Science and Technology (HKUST), Kowloon, Hong Kong, in 2004 and 2007, respectively. He was engaged in LED fabrication. In particular, he focused on the fabrication of matrix-addressable III-nitride LED arrays on silicon substrates by flipchip technology. Since January 2007, he has been with the Chip Development Department, Advanced Photoelectronic Technology Ltd., where he has been engaged in high-power LED design and fabrication.
LIU et al.: MONOLITHIC LED MICRODISPLAY ON ACTIVE MATRIX SUBSTRATE USING FLIP-CHIP TECHNOLOGY
Chak Wah Tang was born in Hong Kong. He received the M.S. degree in electrical engineering from Taiwan National Cheng Kung University, Tainan City, Taiwan, in June 1996. He is currently with Hong Kong University of Science and Technology, Kowloon, Hong Kong. He is engaged in the development of III–V materials growth and characterization.
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Kei May Lau (S’78–M’80–SM’92–F’01) received the B.S. and M.S. degrees in physics from the University of Minnesota, Minneapolis, and the Ph.D. degree in electrical engineering from Rice University, Houston, TX. She was a Senior Engineer with the M/A-COM Gallium Arsenide Products, Inc., where she was engaged in epitaxial growth of GaAs for microwave devices, development of high-efficiency and millimeterwave impact ionization avalanche transit-time (IMPATT) diodes, and multiwafer epitaxy by the chloride transport process. After two years in the industry, she joined the Faculty of the Electrical and Computer Engineering Department, University of Massachusetts (UMass), Amherst, where she became a Full Professor in 1993, and initiated metal–organic chemical vapor deposition (MOCVD) and compound semiconductor materials and devices programs. Since 2000, she has been the Chair Professor in the Electronic and Computer Engineering Department, Hong Kong University of Science and Technology (HKUST), Kowloon, Hong Kong. She spent her first sabbatical leave at the Massachusetts Institute of Technology Lincoln Laboratory. She developed acoustic sensors at the DuPont Central Research and Development Laboratory, Wilmington, DE, during her second sabbatical leave. She also established the Photonics Technology Center for R&D efforts in wide-gap semiconductor materials and devices. She was engaged in studies on heterostructures, quantum wells, strained-layers, III–V selective epitaxy, as well as high-frequency and photonic devices. Prof. Lau was a recipient of the National Science Foundation (NSF) Faculty Awards for Women (FAW) Scientists and Engineers. She was a member of the IEEE Electron Devices Society Administrative Committee. From 1996 to 2002, she was an Editor of the IEEE TRANSACTIONS ON ELECTRON DEVICES. She was also a member of the Electronic Materials Committee of the Minerals, Metals and Materials Society (TMS) of the American Institute of Materials Engineers (AIME). She is a Croucher Senior Research Fellow (2008–2009).
