GaN-based passive matrix LED has been studied intensively for the last decade due to its potential applications. In this study, passive matrix GaN-on-Si LEDs with backside via holes were fabricated. The LED matrix was fabricated on a commercial GaN-based LED epitaxial wafer grown on a Si substrate. Then the backside of the Si substrate was selectively removed using deep reactive ion etching (DRIE) to form backside via holes. Further etching was also done to remove undoped GaN, then metal layers were deposited subsequently to form backside n-metal. Solder pastes and solder balls were used individually to fill via holes to bond on a Si wafer that has n-metal lines for passive matrix function and will be integrated with a Si-based driving/control circuit. In this way, LED wafer did not need to be flipped for bonding. Flux and silver epoxy were applied on the backside of the LED wafer to enhance the bonding strength.
The first trial of passive matrix LED fabrication did not work well with solder pastes because the air gaps or voids were formed in between solder pastes and n-metal inside the via hole. The wafer with no etching of undoped GaN had high resistance. In addition, LED devices were easy to break due to lack of mechanical support after substrate removal, so we re-designed the photomasks. Moreover, the LED emission spectrum showed that the peak wavelength and the bandwidth remained the same, indicating the spectral characteristics of GaN active layers after partial substrate removal was not changed.
In the second trial of passive matrix LED fabrication, solder balls were used to fill via holes instead of solder pastes. However, there occurred some issues that failed the experiments, such as oil contamination during DRIE, poor adhesion of solder balls with the last metal layer (Ti), and insufficient solder ball volume leading to insufficient contact area to metal layer in the via hole. We resolved the oil contamination problem by applying topside photoresist prior to backside processing to protect the topside LEDs from any contamination during the process. Suggestions for future work were proposed at the end of this thesis.

GaN-based passive matrix LED has been studied intensively for the last decade due to its potential applications. In this study, passive matrix GaN-on-Si LEDs with backside via holes were fabricated. The LED matrix was fabricated on a commercial GaN-based LED epitaxial wafer grown on a Si substrate. Then the backside of the Si substrate was selectively removed using deep reactive ion etching (DRIE) to form backside via holes. Further etching was also done to remove undoped GaN, then metal layers were deposited subsequently to form backside n-metal. Solder pastes and solder balls were used individually to fill via holes to bond on a Si wafer that has n-metal lines for passive matrix function and will be integrated with a Si-based driving/control circuit. In this way, LED wafer did not need to be flipped for bonding. Flux and silver epoxy were applied on the backside of the LED wafer to enhance the bonding strength.
The first trial of passive matrix LED fabrication did not work well with solder pastes because the air gaps or voids were formed in between solder pastes and n-metal inside the via hole. The wafer with no etching of undoped GaN had high resistance. In addition, LED devices were easy to break due to lack of mechanical support after substrate removal, so we re-designed the photomasks. Moreover, the LED emission spectrum showed that the peak wavelength and the bandwidth remained the same, indicating the spectral characteristics of GaN active layers after partial substrate removal was not changed.
In the second trial of passive matrix LED fabrication, solder balls were used to fill via holes instead of solder pastes. However, there occurred some issues that failed the experiments, such as oil contamination during DRIE, poor adhesion of solder balls with the last metal layer (Ti), and insufficient solder ball volume leading to insufficient contact area to metal layer in the via hole. We resolved the oil contamination problem by applying topside photoresist prior to backside processing to protect the topside LEDs from any contamination during the process. Suggestions for future work were proposed at the end of this thesis.

[1] S. Guha and N. A. Bojarczuk, “Multicolored light emitters on silicon substrates,” Appl. Phys. Lett., vol. 73, no. 11, pp. 1487–1489, 1998, doi: 10.1063/1.122181.
[2] S. Guha and N. A. Bojarczuk, “Ultraviolet and violet GaN light emitting diodes on silicon,” Appl. Phys. Lett., vol. 72, no. 4, pp. 415–417, 1998, doi: 10.1063/1.120775.
[3] A. Strittmatter et al., “Low-pressure metal organic chemical vapor deposition of GaN on silicon(111) substrates using an AlAs nucleation layer,” Appl. Phys. Lett., vol. 74, no. 9, pp. 1242–1244, 1999, doi: 10.1063/1.123512.
[4] C. A. Tran, A. Osinski, R. F. Karlicek, and I. Berishev, “Growth of InGaN/GaN multiple-quantum-well blue light-emitting diodes on silicon by metalorganic vapor phase epitaxy,” Appl. Phys. Lett., vol. 75, no. 11, pp. 1494–1496, 1999, doi: 10.1063/1.124733.
[5] T. Egawa, B. Zhang, N. Nishikawa, H. Ishikawa, T. Jimbo, and M. Umeno, “InGaN multiple-quantum-well green light-emitting diodes on Si grown by metalorganic chemical vapor deposition,” J. Appl. Phys., vol. 91, no. 1, pp. 528–530, 2002, doi: 10.1063/1.1408264.
[6] M. Umeno, T. Egawa, and H. Ishikawa, “GaN-based optoelectronic devices on sapphire and Si substrates,” Mater. Sci. Semicond. Process., vol. 4, no. 6, pp. 459–466, 2001, doi: 10.1016/S1369-8001(02)00003-3.
[7] A. Dadgar, A. Alam, T. Riemann, and J. Bla, “Crack-Free InGaN / GaN Light Emitters on Si ( 111 ),” vol. 158, no. 1, pp. 155–158, 2001.
[8] A. Dadgar, M. Poschenrieder, O. Contreras, and J. Christen, “Bright , Crack-Free InGaN / GaN Light Emitters on Si ( 111 ),” vol. 313, no. 2, pp. 308–313, 2002.
[9] A. Dadgar, M. Poschenrieder, J. Bläsing, K. Fehse, A. Diez, and A. Krost, “Thick, crack-free blue light-emitting diodes on Si(111) using low-temperature AlN interlayers and in situ SixNy masking,” Appl. Phys. Lett., vol. 80, no. 20, pp. 3670–3672, 2002, doi: 10.1063/1.1479455.
[10] Lattice Power Corporation, “Compound Semiconductor, Silicon-based LEDs leap from lab to fab.,” 2010.
[11] OSRAM OS, “Press release “Success in research: first gallium-nitride LED chips on silicon in pilot stage",” 2012.
[12] B. W. Lin, N. J. Wu, Y. C. S. Wu, and S. C. Hsu, “A stress analysis of transferred thin-GaN light-emitting diodes fabricated by Au-Si wafer bonding,” IEEE/OSA J. Disp. Technol., vol. 9, no. 5, pp. 371–376, 2013, doi: 10.1109/JDT.2012.2225824.
[13] K. Wang, K. Ruan, H. Bai, W. Hu, S. Wu, and H. Wang, “Investigation on the bonding quality of GaN and Si wafers bonded with Mo/Au nano-layer in atmospheric air,” Mater. Sci. Semicond. Process., vol. 114, no. November 2019, p. 105069, 2020, doi: 10.1016/j.mssp.2020.105069.
[14] K. Tsuchiyama, K. Yamane, H. Sekiguchi, H. Okada, and A. Wakahara, “Fabrication of Si/SiO2/GaN structure by surface-activated bonding for monolithic integration of optoelectronic devices,” Jpn. J. Appl. Phys., vol. 55, no. 5, 2016, doi: 10.7567/JJAP.55.05FL01.
[15] T. Kamei, T. Kamikawa, M. Araki, S. P. DenBaars, S. Nakamura, and J. E. Bowers, “Research Toward a Heterogeneously Integrated InGaN Laser on Silicon,” Phys. Status Solidi Appl. Mater. Sci., vol. 217, no. 7, pp. 1–8, 2020, doi: 10.1002/pssa.201900770.
[16] S. H. Yuan, S. S. Yan, Y. S. Yao, C. C. Wu, R. H. Horng, and D. S. Wuu, “Process Integration and Interconnection Design of Passive-Matrix LED Micro-Displays with 256 Pixel-Per-Inch Resolution,” IEEE J. Electron Devices Soc., vol. 8, no. March, pp. 251–255, 2020, doi: 10.1109/JEDS.2020.2967476.
[17] W. Guo, J. Tai, J. Liu, and J. Sun, “Process Optimization of Passive Matrix GaN-Based Micro-LED Arrays for Display Applications,” J. Electron. Mater., vol. 48, no. 8, pp. 5195–5201, 2019, doi: 10.1007/s11664-019-07330-3.
[18] C.-M. Kang et al., “Fabrication of a vertically-stacked passive-matrix micro-LED array structure for a dual color display,” Opt. Express, vol. 25, no. 3, p. 2489, 2017, doi: 10.1364/oe.25.002489.
[19] J. Herrnsdorf et al., “Active-matrix GaN micro light-emitting diode display with unprecedented brightness,” IEEE Trans. Electron Devices, vol. 62, no. 6, pp. 1918–1925, 2015, doi: 10.1109/TED.2015.2416915.
[20] M. Y. Soh et al., “Design and Characterization of Micro-LED Matrix Display with Heterogeneous Integration of GaN and BCD Technologies,” IEEE Trans. Electron Devices, vol. 66, no. 10, pp. 4221–4227, 2019, doi: 10.1109/TED.2019.2933552.
[21] J. G. Um et al., “Active-Matrix GaN µ-LED Display Using Oxide Thin-Film Transistor Backplane and Flip Chip LED Bonding,” Adv. Electron. Mater., vol. 5, no. 3, pp. 1–8, 2019, doi: 10.1002/aelm.201800617.
[22] N. Grossman et al., “Multi-site optical excitation using ChR2 and micro-LED array,” J. Neural Eng., vol. 7, no. 1, 2010, doi: 10.1088/1741-2560/7/1/016004.
[23] A. Zarowna-Dabrowska et al., “Miniaturised optoelectronic tweezers controlled by GaN micro light emitting diode arrays,” 2010 23rd Annu. Meet. IEEE Photonics Soc. PHOTINICS 2010, vol. 19, no. 3, pp. 102–103, 2010, doi: 10.1109/PHOTONICS.2010.5698778.
[24] A. H. Jeorrett et al., “Optoelectronic tweezers system for single cell manipulation and fluorescence imaging of live immune cells,” Opt. Express, vol. 22, no. 2, p. 1372, 2014, doi: 10.1364/oe.22.001372.
[25] Z. J. Liu, W. C. Chong, K. M. Wong, K. H. Tam, and K. M. Lau, “A novel BLU-free full-color LED projector using LED on silicon micro-displays,” IEEE Photonics Technol. Lett., vol. 25, no. 23, pp. 2267–2270, 2013, doi: 10.1109/LPT.2013.2285229.
[26] G. Leds et al., “1.5 Gbit/s Multi-Channel Visible Light Communications Using CMOS-Controlled GaN-Based LEDs,” vol. 31, no. 8, pp. 1211–1216, 2013.
[27] D. Tsonev et al., “A 3-Gb/s single-LED OFDM-based wireless VLC link using a gallium nitride μ LED,” IEEE Photonics Technol. Lett., vol. 26, no. 7, pp. 637–640, 2014, doi: 10.1109/LPT.2013.2297621.
[28] J. J. D. Mckendry et al., “Visible-Light Communications Using a CMOS-Controlled Micro-Light-Emitting-Diode Array,” J. Light. Technol., vol. 30, no. 1, pp. 61–67, 2012.
[29] T. Li, M. Mastro, and A. Dadgar, Eds., III-V Compound Semiconductors, Integration with Silicon-Based Microelectronics, 1st ed. CRC Press, 2016.
[30] A. Trampert, B. Oliver, and K. H. Poolg, “Crystal Structure of Group III Nitrides,” Semicond. Semimetals. Gall. Nitride I, vol. 167, pp. 167–192, 1998.
[31] P. V. Schwartz, C. W. Liu, and J. C. Sturm, “Semi-insulating crystalline silicon formed by oxygen doping during low temperature chemical vapor deposition,” Appl. Phys. Lett., vol. 62, no. 10, pp. 1102–1104, 1993, doi: 10.1063/1.108755.
[32] M. Takahashi, M. Tabe, and Y. Sakakibara, “I-V Characteristics of Oxygen-Doped Si Epitaxial Film (OXSEF)/Si Heterojunctions,” IEEE Electron Devices Lett., vol. 8, no. 10, pp. 475–476, 1987.
[33] M. A. Mastro et al., “High-reflectance III-nitride distributed Bragg reflectors grown on Si substrates,” Appl. Phys. Lett., vol. 87, no. 24, pp. 1–3, 2005, doi: 10.1063/1.2140874.
[34] R. Ishito, K. Ono, and S. Matsumoto, “Si (100)-GaN/Si (111) low temperature wafer bonding process for 3D power supply on chip,” in 2019 IEEE CPMT Symposium Japan (ICSJ), 2019, pp. 151–152.
[35] M. Hönle, P. Oberhumer, K. Hingerl, T. Wagner, S. Huppmann, and S. Katz, “Mechanism of indium tin oxide//indium tin oxide direct wafer bonding,” Thin Solid Films, vol. 704, no. 137964, pp. 1–7, 2020, doi: 10.1016/j.tsf.2020.137964.
[36] P. C. Liu, C. Y. Hou, and Y. S. Wu, “Wafer bonding for high-brightness light-emitting diodes via indium tin oxide intermediate layers,” Thin Solid Films, vol. 478, no. 1–2, pp. 280–285, 2005, doi: 10.1016/j.tsf.2004.11.070.
[37] X. Zhang et al., “Active Matrix Monolithic LED Micro-Display Using GaN-on-Si Epilayers,” vol. 31, no. 11, pp. 865–868, 2019.
[38] X. Zou, X. Zhang, W. C. Chong, C. W. Tang, and K. M. Lau, “Vertical LEDs on Rigid and Flexible Substrates Using GaN-on-Si Epilayers and Au-Free Bonding,” pp. 1–7, 2016.
[39] C. Mack, Fundamental Principles of Optical Lithography: The Science of Microfabrication, 1st ed. John Wiley & Sons, Ltd., 2007.
[40] Rohm and Haas Electronic Materials, “MICROPOSIT S1800 G2 SERIES: For Microlithography Applications,” 2006.
[41] MicroChem Corp., “LOR Lift Off Resists Datasheet,” 2002.
[42] MicroChemicals, “Microchemicals Application Notes: Exposure - Basics of Microstructuring,” 2000.
[43] KemLab, “K-PRO Series MSDS: Positive Photoresist for Packaging Applications.”
[44] MicroChemicals, “Microchemicals Application Notes: Development - Basics of Microstructuring,” 2000.
[45] SUSS MicroTec, “Manual Mask Aligner.”
[46] SUSS MicroTec, “Manual Mask Aligner: SUSS MJB4 Versatile System for R&D Applications and Low-volume Production.”
[47] SUSS MicroTec, “Semi Automated Mask Aligner: SUSS MA/BA Gen4 Series - Compact Mask Aligner Platform for Research and Low-volume Production.”
[48] V. K. Khanna, “Adhesion-delamination phenomena at the surfaces and interfaces in microelectronics and MEMS structures and packaged devices,” J. Phys. D. Appl. Phys., vol. 44, no. 3, pp. 1–19, 2011, doi: 10.1088/0022-3727/44/3/034004.