1700 pixels per inch (PPI) Passive-Matrix Micro-LED Display Powered by ASIC

Wing Cheung Chong*, Wai Keung Cho, Zhao Jun Liu, Chu Hong Wanga and Kei May Lau

Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong

a3C Limited, 481 Castle Peak Road, Kowloon, Hong Kong

*Email: eeeddie@ust.hk, Phone: +852 2358 8843

Abstract — We report the first 1700 pixels per inch (PPI)

passive-matrix blue light-emitting diodes on silicon (LEDoS) micro-displays. By flip-chip bonding a micro-LED array onto an ASIC display driver, we successfully fabricated a 0.19-inch display with a resolution of 256 x 192, the highest ever reported in LED-based micro-display. In addition, the LEDoS micro-display can deliver brightness as high as 1300 mcd/m2 and render images in 6-bit grayscale. The remarkable performance suggests the tremendous potential of LEDoS micro-displays for portable display applications which require high performance, small size and low power consumption.

Index Terms — Passive-matrix, light-emitting diodes on Silicon (LEDoS), micro-display, micro-LED array, ASIC, high resolution, flip-chip.

I. INTRODUCTION

LED micro-display has received attention recently because of its great potential to augment other existing micro-display technologies in the market [1]-[8]. Unlike liquid crystal display (LCD), liquid crystal on silicon (LCoS), and digital light processor (DLP), LED micro-display is a self-emissive device which can generate bright images efficiently without external light sources and lossy optical components. While organic LED (OLED) is an attractive alternative for micro-display applications, semiconductor-based LED is more advantageous in terms of brightness, lifetime, thermal stability and robustness in extreme conditions. Development of inorganic LED micro-displays is thus highly desirable.

Despite all the attractive advantages, it is challenging to achieve high-resolution inorganic LED micro-displays with high pixel yield. An et al. demonstrated a passive-matrix micro-LED array structure by flip-chip bonding of a gallium nitride (GaN) micro-LED array on a silicon submount with common p-electrode stripes [7]. In their design, each pixel in the micro-LED array is connected to the common p-electrode stripes via an individual solder bump. Due to the large thermal mismatch between GaN and silicon, severe bonding failures occur in the closely spaced bumps. Many pixels are physically disconnected

from the p-electrode line, leading to numerous dead pixels in the display. Similar issues also appear in active-matrix InGaN micro-display in which the LED array was bonded onto the silicon side by high-density indium bumps [8]. To improve the integrity of LED-based micro-displays, an alternative bonding scheme which can reduce the solder bump density is of paramount importance.

In this paper, we report, to our best knowledge, the first 1700 pixels per inch (PPI) blue passive-matrix light-emitting diodes on silicon (LEDoS) micro-displays powered by ASIC with 6-bit grayscale, as shown in Fig. 1. This is realized by flip-chip bonding of a micro-LED array onto a CMOS-based ASIC display driver. The LEDoS micro-display consists of 256 x 192 pixels within a display area of 0.19 inch in diagonal. In our design, all passive-matrix interconnects are implemented on the LED side, in sharp contrast to previous reports in which the interconnects were done on the driver side [7]. With this special architecture, all the solder bumps can be relocated to the peripheral areas of the micro-LED array where the bumps can be bigger and more spread out. At the same time, only 448 bumps are needed for our display with almost 50000 pixels. The huge reduction in bump density significantly improves the bonding reliability. This novel passive-matrix display design and bump arrangement make high resolution and high-yield LEDoS micro-display achievable for a variety of applications.

Fig. 1. Passive-matrix LEDoS micro-display wire-bonded on a flex cable.

978-1-4799-3622-9/14/$31.00 ©2014 IEEE

II. MICRO-LED ARRAY FABRICATION

The blue passive-matrix micro-LED arrays were fabricated on blue epi-wafers with peak wavelength 460 nm, respectively. The chip size was 5.1 x 4.5 mm2 with a display area of 3.8 x 2.9 mm2, consisting of 256 x 192 pixels. The fabrication process of micro-LED arrays is shown in Fig. 2.

Fig. 2. Schematic of passive-matrix micro-LED array in each processing step. (a) Formation of isolation trenches; (b) Patterning of pixels and evaporation of p-type ohmic contacts; (c) N-electrode stripes definition; (d) Patterning of transparent polyimide; (e) P-electrode stripes definition.

LED pixels of the micro-LED array in the same column share a common electrode of the n-type GaN. Thus it is necessary to isolate all column stripes of micro-LED array. This is realized by creating isolation trenches via dry etching of GaN down to the sapphire substrate (Fig. 2(a)). Individual pixels were defined by standard photo-lithography. The photoresist pixel patterns were then transferred to GaN by dry etching down to the n-type GaN layer. After that, tin-doped indium oxide (ITO) was selectively deposited on top of the pixels to form p-type ohmic contacts, as shown in Fig. 2(b). Ohmic characteristic was achieved after rapid thermal annealing. As shown in Fig. 2(c), n-electrode stripes were defined on the n-type GaN layer. Passivation and isolation were done by conformal coating of a thick photo-patternable transparent polyimide over the whole epi-wafers, as shown in Fig. 2(d). Contact holes were opened later to expose the p-type ohmic contacts for subsequent wiring of

p-electrode stripes. The final structure of the passive-matrix micro-LED array is illustrated in Fig. 2(e). The p-electrode stripes were defined on top of the transparent resist and connected to all the pixels in the same row.

III. ASIC ARCHITECTURE

Fig. 3. Block diagram of the ASIC display driver for driving the passive-matrix micro-LED array.

ASIC display drivers were fabricated at SMIC with

0.18 µm CMOS technology. Fig. 3 shows the architecture of the ASIC driver. The data processing unit reads data and control signals from an external microcontroller and writes the data into the RAM. The pulse frequency modulation (PFM) generator then reads the data from the RAM in sequence and generates 256 PFM driving signals to the 256 columns of the micro-LED array through the data driver. Finally, data driver and scan driver power the passive-matrix micro-LED array by giving enough control voltage to turn pixels on and off.

Fig. 4 Timing diagram of PFM signals generated by ASIC.

Fig. 4 shows the waveforms of different gray levels generated by the ASIC display driver. Symbol f is the frame rate. The pulse frequency of each pixel depends on its grayscale. The PFM generator can create 6-bit grayscale to provide more satisfactory display quality. The whole system operates at 48 MHz and can display 16 frames per second at maximum.

Fig. 5. Schematic of interconnects between ASIC display driver and micro-LED array.

Fig. 5 displays the schematic of interconnects between the ASIC display driver and micro-LED array. In this passive-matrix driving scheme, the LEDs on the same column are driven at different times. Data driver provides 256 PFM signals and the scan driver selects each row in sequence by providing high voltage (>3V) to LED pixels in the same row through the p-electrode stripes. Top views of ASIC display driver and micro-LED array are shown in Fig. 6. A total 448 electrode pads were allocated near the periphery of the micro-LED array for flip-chip bonding.

Fig. 6. Top views of ASIC display driver (left) and 256 x 192 micro-LED array (right) before flip-chip bonding.

IV. INDIUM BUMPING AND FLIP-CHIP PROCESS

Indium is used as a medium to electrically connect all electrode pads between the ASIC display driver and micro-LED array. As shown in Fig. 7(a), indium plates were deposited and formed on electrode pads of the micro-LED array by thermal evaporation and lift-off process. To form ball-shaped indium as shown in Fig. 7(b), the micro-LED array with indium plates were put into a reflow furnace at 220ºC for 1 minutes in a formic acid ambient. After the indium bumping process, the micro-LED array was flip-chip bonded onto the ASIC display driver to form passive-matrix LEDoS micro-displays as shown in Fig. 7(c). The alignment accuracy of the flip-chip bonder is 1µm.

Fig. 7. Microscopic images of micro-LED array with (a) indium plates; (b) indium balls after reflow; (c) pressed indium after flip-chip bonding onto the ASIC display driver.

V. RESULTS

The brightness of blue LEDoS micro-displays was measured by a Spectrascan colorimeter. The maximum brightness of the LEDoS micro-displays was measured to be 1300 mcd/m2, at an average injection current of 20mA to the passive-matrix micro-LED array. The total power consumption of the micro-display is only 0.6W. The operating temperature of the LEDoS micro-display ranges from -55 to 125 °C, tested in Votsch thermal cycling chamber, demonstrating the robustness in harsh environment.

Fig. 8 shows the display quality of the blue LEDoS micro-displays. Images can be clearly rendered in the LEDoS micro-display. Advantages of 1700 PPI and 6-bit grayscale make the display more powerful in high

resolution image reconstruction. It demonstrated that the proposed passive-matrix LEDoS micro-display has tremendous potentials for portable display applications which require high performance, small size and low power consumption.

VI. CONCLUSION

1700 PPI passive-matrix blue light-emitting diodes on silicon (LEDoS) micro-displays were fabricated by flip-chip bonding of micro-LED array onto ASIC display driver chips. Outstanding performance of LEDoS micro-displays demonstrated that the novel passive-matrix design and bump arrangement can make high resolution and high-yield LEDoS micro-display achievable for a variety of applications.

ACKNOWLEDGMENT

This work was supported in part by a grant from the Research Grants Council (RGC) of the Hong Kong Special Administrative Government (HKSAR) under the heme-based Research Scheme (T23-612/12-R). The authors want to thank the HKUST Nanoelectronics Fabrication Facility (NFF), Electronic Packaging Laboratory (EPACK), Suzhou Institute of Nano-tech and Nano-bionics (SINANO), Chinese Academy of Science, Prof. Anthony H.W. Choi, and Prof. Chun-Sing Lee for their facilitation and EPISTAR Corporation for the epi-wafers. Special thanks to Dr. K. W. Ng and R. Q. Zhu for useful discussions.

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Fig. 8. Source files (top) and its corresponding display images (bottom) shown in the blue LEDoS Micro-displays.