Design of Active Matrix Micro-LED Display with CCCS Pixel Circuits

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Design of Active Matrix Micro-LED Display with CCCS Pixel Circuits Ke ZHANG 1, 2, Zhaojun LIU* 1, 2 and Hoi-Sing KWOK* 1 1 State Key Laboratory on Advanced Displays and Optoelectronics Technologies,

1 Design of Active Matrix Micro-LED Display with CCCS Pixel Circuits Ke ZHANG 1, 2, Zhaojun LIU* 1, 2 and Hoi-Sing KWOK* 1 1 State Key Laboratory on Advanced Displays and Optoelectronics Technologies, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong 2 Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen, China Abstract Micro-LED display is a quite new technology which has developed only about a decade and been attracting tremendous attention in recent years. It was regarded as a great candidate for many applications, especially a new generation display technology. Here we report a design of active matrix Micro-LED display including Micro-LED array fabrication, AM driving circuit and flip-chip integration. 4T2C (4 transistors and 2 capacitors) driving circuits with a configuration of current control current source (CCCS) was used for better pixels uniformity and display quality Introduction Flat panel displays (FDPs) such as liquid crystal displays (LCDs) and organic light emitting diodes (OLEDs) are the major display technologies and dominate the market of TVs, cellphones, desktops/laptops, and wearable devices. While now high pixel per inch (PPI) and high resolution Micro- LED (µled) displays are attracting more and more attentions, because of its superior properties such as self-emission, high brightness and efficiency, low power consumption, long lifetime, and wide operating environment 1,2. The spontaneous properties of GaN material and devices enable Micro-LED great potential in many applications. For example, it has very high and stable brightness and can achieve high quality display even in strong ambient light, and consequently it is very promising for outdoor/indoor large screen displays, especially for AR which is very hot now and suffered disturbance of sunlight. Besides, Micro-LED display can be applied for BLU-free projectors as well. With much fewer optical components and significantly higher light utilization efficiency (LUE), Micro-LED projectors is more portable and effective. Moreover it is also strongly expected for wearable devices, such as smart watches, head mount display and VR devices. In the meanwhile, some additional functions like visible light communication (VLC) and biomedical detection can be also added in those devices with particular Micro-LED array, and achieve integrated system eventually. What s more, the good stability and reliability in many extreme operating environments also makes Micro-LED competitive and promise it a very bright future 3, 4. The applications of Micro-LED are shown in Figure. Fig. 1. The applications of Micro-LED. Here we report a design of active matrix Micro-LED display with 4T2C circuits. The GaN Micro-LED array will be fabricated on sapphire and the 4T2C driving circuits array will be fabricated on silicon wafer. Then they will be integrated together by flipchip boing technology. Here we also analyze the operation principle and advantages of 4T2C circuits so that a better uniformity display can be achieved. 2. Design of Micro-LED Display 2.1 Comparison with Conventional Display Technology Among flat panel displays, LCD is the most popular and mature technology, however with low efficiency. While organic light-emitting diodes (OLEDs) has been introduced to the market gradually owing to their high luminance, large view angle and good color rendering. But the short lifetime and unsteadiness of organic material still brings some problems to be solved. Comparing with mainstream display technologies as OLED and LCD, Micro-LED has its pros and cons. Obviously, LCD and OLED have better pixel per inch (PPI) and

resolution as high as 8K in the latest developed products 5, 6. However, Micro-LED can save more energy with higher light output power.

Besides, Micro-LED display can be applied for BLU-free projectors as well, with much fewer optical components and significantly higher light utilization efficiency (LUE) compared to conventional

2 Micro-LED Pixels Array The GaN Micro-LED array including 400 240 pixels with a pixel size of 30 30 μm 2 was implemented on a single sapphire substrate using LED fabrication process.

The fabrication process of the Micro-LED array includes 5 photography steps which are MESA structure (MS), current spreading layer (SL), p and n electrode layer (EL), passivation (PS) and contact

2 resolution as high as 8K in the latest developed products 5, 6. However, Micro-LED can save more energy with higher light output power. Moreover, the high response frequency and bandwidth also bring it a bright future of visible light communication/interconnection, as well as indoor/outdoor localization. Besides, Micro-LED display can be applied for BLU-free projectors as well, with much fewer optical components and significantly higher light utilization efficiency (LUE) compared to conventional projectors as shown in Figure. 2. Fig. 3. The process flow of Micro-LED array fabrication and pattern. Fig. 2. A comparison between Micro-LEDs, LCoS, DLP and 3LCD 2.2 Micro-LED Pixels Array The GaN Micro-LED array including pixels with a pixel size of μm 2 was implemented on a single sapphire substrate using LED fabrication process. 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 substrate. The fabrication process of the Micro-LED array includes 5 photography steps which are MESA structure (MS), current spreading layer (SL), p and n electrode layer (EL), passivation (PS) and contact pads for flip-chip bonding. The pattern on the mask is the same for steps of passivation and contact pads, so the same mask was used but the former step is for wet etching and the later step is for the lift-off. The specific pattern process is shown in Figure. 3. The epitaxy was grew as shown in step 1 including p-gan layer, MQW layer, n-gan layer and sapphire substrate. Then p-gan and MQW were etched to insulate pixels as shown in step 2. Current spreading layer and electrode layer were evaporated then sequently as step 3 and 4. Last step is to build connect pads for subsequent integration process. In this design, we used configuration of common n-electrode and individual p-electrode. More n-pads were used to get better current uniformity here. And with most of the n-gan area covered by n-metal layer, the electrical and optical characteristics of the pixel are with good uniformity Figure 4 shows the layout design of a micro-led array, in which the LED pixel size is μm 2. 3μm design rule was used to give enough tolerance for photolithography and etching steps. The p-electrode and n-electrode were in circular shape and placed onto p-gan and n-gan respectively. The detailed pixel structure is shown in Figure 4 (a) and the schematic of Micro-LED array is shown in Figure 4 (b). (a) (b) Fig. 4. (a) The structure of LED pixel and (b) the schematic diagram of the Micro-LED array 2.3 CCCS Pixel Circuits In this design, we used active matrix circuit to drive the Micro-LED array. Each pixel has its own driving circuit and integrated together so that better control ability and display quality can be achieved. Active matrix displays have been developed for decades

3 such as active matrix liquid crystal displays (AMLCDs), active matrix organic light emitting diode displays (AMOLEDs) and active matrix light emitting diode displays (AMLEDs) 7. Compared with passive matrix (PM) addressing mechanism, active matrix (AM) mechanism has obvious advantages Firstly, active components such as metal-oxidesemiconductor (MOS) and thin field transistors (TFTs) promise superior controllability. So crosstalk issues are practically eliminated. Secondly, active matrix can provide sufficient driving capability for large scale and high resolution displays. Besides, in AMOLEDs and AMLEDs, the driving currents of the pixel flow along a path of VDD, driving transistors, light emitting components, ground. Negligible power is consumed on the rest components and paths, resulting in a high power efficiency; Last but not the least, AM circuits can provide more gray-scales for high quality displays. As Micro-LED is a current-controlled emission device, pixel circuits with configuration of current control current source (CCCS) is the best candidate for better driving capability, display uniformity, and emission stability. The schematic diagram of a 4T2C driving circuit is shown in Fig. 5 (a), here Idata is from current sinking source. T1 and T2 are switching transistor, T4 is driving transistor and T3 is a mirror transistor. C1 and C2 are 2 storage capacitors which is connected between the scan line and VDD in a cascade structure. When scan signal is applied, T1 and T2 will be switched on, leading to a charging period of capacitors. When the voltage of note B meets the requirement of the Vth of T3 and T4, there is a passage for Idata, and ILED will be proportional to Idata (affcted by the W/L of T3 and T4) according to the principle of current mirror. This period is called writing time. When the scan signal passed by, T1 and T2 will be switched off. The voltage memorized in the capacitors will continually keep T4 switching on, leading to a continuous desired current for Micro-LED (corresponding to a desired pixel brightness) throughout a frame period. And this period is called holding time. The Timing diagram of two frames is shown in Fig. 5 (b). The pixel in first frame is ON and in second frame is OFF. And t1 and t3 is writing time, while t2 and t4 is holding time. frames. The advantage of this configuration is that the uniformity and degradation of the Micro-LED pixels would not affect the output current of the driving transistor. As a result, the pixels uniformity and display quality will be improved, better gray scale can be achieved. Moreover, 2 capacitors can be more accurate to adjust the threshold voltage of T 3 and T Flip-chip Integration Actually, there are two kinds of integration method. Wire bonding is usually used for large pixel display and passive matrix driving circuit. To be specific, liquid conductive silver colloid is used to achieve the designated contact of each pixel on PCB. The horizontal Micro-LED devices' electrodes were connected to n- and p-contact pads respectively towed by gold wires using ultrasonic bonding instrument as shown in Figure 6 (a). Although this method is easy to achieve with low cost, it can t be used for high PPI and high resolution display because of the size limitation. Besides the heat dissipation ability is also not good enough. So here we used flip-chip bonding technology. The whole Micro-LED array shares common n- electrodes connected to ground. Then flip-chip bonding was conducted for integration as follow. Firstly, indium plates were deposited and patterned on the connect pads of the Micro-LED array by liftoff process. The Micro-LED array was then put into the reflow furnace at 240 C to reflow all indium plates to become indium balls. Finally the Micro- LED array was flip-chip bonded to the CMOS driver SoC via thermal compression bonding at 180 C with a pressure of 20Newtons. A scanned electron microscope (SEM) image of the indium bumps generated after the reflow process for flip-chip bonding is presented in Fig. 6 (b). Using flip-chip technology, the Micro-LED array was then flipped onto the AM substrate to create a Micro-display. The 3D schematic view of a single pixel structure is shown in Fig. 6 (b). The bottom part is a single-crystal silicon (100) backplane with active matrix circuits fabricated on it. The output of circuits is connected with solder bump pad and solder bump, on which the LED pixel is located. The light of the LED pixel is emitted from the thinned and polished sapphire side (blue color). The scanning electron microscopy (SEM) image of solder bumps before and after the reflow process is shown in Fig. 7 Fig. 5. (a) The schematic diagram of a 4T2C active matrix driving circuit and (b) Timing diagram of two

Fig. 6. 3D structure view of the Micro-LED pixel by (a) wire bonding and (b) flip chip bonding. (a) (b) Fig.7.

Conclusions In this paper, the development and application of Micro-LED technology was briefly introduced. And a comparison between different display technologies was also conducted.

2 kinds of integration method were also introduced and compared together, we choose flip-chip technology eventually to achieve high PPI and high resolution active matrix display.

Acknowledgements The authors would like to thank Prof. Kei May Lau, Mr. Wing Cheung Chong, Mr. Ka Ming Wong.

4 Fig. 6. 3D structure view of the Micro-LED pixel by (a) wire bonding and (b) flip chip bonding. (a) (b) Fig.7. scanning electron microscope image of solder bumps (a) before and (b) after the reflow process. 3. Conclusions In this paper, the development and application of Micro-LED technology was briefly introduced. And a comparison between different display technologies was also conducted. Then a detailed Micro-LED fabrication process was arranged and a design of active matrix Micro-LED display with 4T2C circuits was report. Besides. 2 kinds of integration method were also introduced and compared together, we choose flip-chip technology eventually to achieve high PPI and high resolution active matrix display. Moreover, we adopted an advanced CCCS circuits for better pixels uniformity and display quality. The operation principle and advantages were analyzed respectively. 4. Acknowledgements The authors would like to thank Prof. Kei May Lau, Mr. Wing Cheung Chong, Mr. Ka Ming Wong. Besides, the author also would like to appreciate the support from Nanoelectronic Fabrication Facility (NFF), and Material Characterization & Preparation Facility (MCPF) in Hong Kong University of Science and Technology. References 1. Griffin, Chris, Zhang H, Guilhabert B, M. D. Dawson, et al., "Micro-pixellated flip-chip InGaN and AlInGaN light-emitting diodes," in Conference on Lasers and Electro-Optics (CLEO), 2007, pp K. Zhang, D. Peng, W. C. Chong, K. M. Lau, Z. Liu, "Investigation of Photon-Generated Leakage Current for High-Performance Active Matrix Micro-LED Displays," IEEE Transacion Electron Devices, vol. 63, no. 12, pp , W. C. Chong,K. M. Wong,Z. J. Liu,K. M. Lau, A Novel Full-Color 3LED Projection System using R-G-B Light Emitting Diodes on Silicon (LEDoS) Micro-displays, SID Symp. Dig. Tech. Papers, vol. 44, no. 1, pp , C. W. Jeon, H. W. Choi, and M. D. Dawson, Fabrication of matrix-addressable InGaNbased microdisplays of high array density, IEEE Photon. Technol. Lett., vol. 15, no. 11, pp , Y. Liu, Y. Chen, D. Buso, G. Zissis, Influence of Driving Current on Photometric Performances of a White Light OLED, IEEE Transactions on Industry Applications, vol. 52, no. 6, C. C. Chen, C. Y. Wu, Y. M. Chen, and T. F. Wu, Sequential color LED backlight driving system for LCD panels, IEEE Transactions on Power Electronics, vol. 22, no. 3, pp , May A. Nathan, G. R. Chaji, and S. J. Ashtiani, "Driving Schemes for a-si and LTPS AMOLED Displays," Journal of Display Technology, vol. 1, no. 2, pp , Z. Gong, H. X. Zhang, E. Gu, C. Griffin, M. D. Dawson, "Matrix-Addressable Micropixellated InGaN Light-Emitting Diodes With Uniform Emission and Increased Light Output," IEEE Transactions on Electron Devices, vol. 54, no.10, pp , J. L. Zhao, W. K. Chi, and K. M. Lau, "GaN Based Active Matrix Light Emitting Diode Array by Flip-Chip Technology," in Asia Optical Fiber Communication & Optoelectronic Exposition & Conference, 2008, pp B. Hekmatshoar, A. Z. Kattamis, K. H. Cherenack, K. Long, J. Z. Chen, S. Wagner, et al., "Reliability of Active-Matrix Organic Light- Emitting-Diode Arrays With Amorphous Silicon Thin-Film Transistor Backplanes on Clear Plastic," IEEE Electron Device Letters, vol. 29, no. 1, pp , S. Sambandan and A. Nathan, "Single- Technology-Based Statistical Calibration for High-Performance Active-Matrix Organic LED Displays," Journal of Display Technology, vol. 3, no. 3, pp , 2007.

5 12. S. Ono, K. Miwa, Y. Maekawa, and T. Tsujimura, "VT Compensation Circuit for AM OLED Displays Composed of Two TFTs and One Capacitor", IEEE Transactions on Electron Devices, vol. 54, no.3, pp , 2007.

Overview of All Pixel Circuits for Active Matrix Organic Light Emitting Diode (AMOLED)

Chapter 2 Overview of All Pixel Circuits for Active Matrix Organic Light Emitting Diode (AMOLED) ---------------------------------------------------------------------------------------------------------------