Development of electronic material and device for high resolution vertical MicroLED display

Abstract

With the advent of the future Internet of Things (IoT), visual IoT platforms have attracted remarkable interest, offering sennsing, collecting, and processing of optical data in hyperconnected society. Displays are essential electronic technology for bilateral visual communication, as they can be installed anywhere, such as TV, signage, automobile, healthcare devices, and smart system. For example, active-matrix-organic light-emitting diodes (AMOLEDs) have been widely commercialized in mobile optoelectronics. However, organic light-emitting diodes have critical drawbacks of low power efficiency, slow response time, and short lifetime. Inorganic-based micro light-emitting diodes (μLEDs) have been considered key technology to replace AMOLEDs and liquid crystal displays (LCDs) due to their excellent light properties (e.g., hue, contrast, and brightness), fast response time, high power efficiency, and operation stability. The μLED display with chip-based μLEDs are already on the verge of commercialization by successfully demonstrating millions of LED pxiels in single panel. However, chip-based μLEDs has limitation in mass production because of handling mil-lions of individual transfer. On the contrary, thin-film μLEDs are considered to be powerful solution for low-cost μLED display due to rapid and cheap mass transfer over thousands of μLEDs in one time. For the fabrica-tion of thin-film μLED display, there are still some challenges to be soloved from the perspective of material and device. In chapter 2, thin-film μLEDs have been attracted as powerful technology for mass production of low-cost μLED display due to its capability of mass transfer in short time and simple process. Although, many re-searchers have studied the transfer and packaging of thin-film μLEDs, fundamental material research is re-quired to lower the high production cost of the μLED display. Cu is an attractive electrode material due to its cheap price, high conductivity, and high robustness. However, its poor adhesion to glass induces the breakdown of current driven μLED under environmental stress of temperature and humidity. Metal adhesion layers such as Mo, Cr, and Ti have been introduced but, they have critical issue of galvanic reaction during Cu patterning. Here, we report highly robust Cu electrode on glass substrates via flash-induced chemical and physical inter-locking. Under the flash illumination, the CuO Nps on display substrates converted into a conductive Cu film. At the same time. Cu2O adhesion layer was chemically formed between photoreduced Cu and glass, and na-noscale interloed structure was formed at Cu/glass interface to improve interfacial fracture energy of Cu elec-trode. The adhesion energy of flash-induced Cu electrode was 5 times higher than that of vacuum deposited Cu film via DCB peel test. Owing to this high robostness, AlGaInP thin-film VLED was electrically intercon-nected to flash-induced Cu electrode, presenting consistent electrical/optical performance during the environ-mental tests such as high temperature storage test, temperature humidity test, and thermal shock test. 50x50 vertical μLEDs were massively transferred onto the flash-inducced Cu electrode, exhibiting uniform distribution of forward voltage, peak wavength and device temperature. In chapter 3, μLEDs have been considered as powerful next generation display technology due to their low power consumption and outstanding stability. Especially, thin-film μLEDs have a great potential to realize low cost flexible μLED displays via the mass transfer of 10000 LED chips in one time. Although many re-searchers have studied in the thin-film μLED transfer, little efforts have been devoted in the thin-film μLED interconnection. Current thin-film μLED interconnection technologies including flip chip bonding and wafer bonding have utilized eutectic metal and solders. However, the high temperature condition for processing metal adhesives have damaged the plastic substrates for flexible μLED displays. By contrast, anisotropic conductive film (ACF) have an advantage of low temperature capability, and have been utilized to fabricate flexible μLED arrays for neural interface. However, ACF should be capable of interconnecting μLED chips miniatur-ized as 10 μm dimension to realize the high-resolution flexible μLED displays for smart phones, tablets, and seamless virtual reality. Herein, we reported the ACF-based interconnection of 10 x 10 μm$^2$ sized AlGaInP ver-tical thin-film μLEDs (VLEDs) on a plastic substrate, and the monochromatic 640 ppi flexible VLED arrays. Owing to the small conductive particles with radius of 2. 75 μm, miniaturized thin-film μLED chips successfully formed electrical connection to flexible bottom electrodes. Simultaneously, the μLEDs over embedded in the ACF exhibited stable electrical/optical performance regardless of mechanical, thermal, and chemical stresses in various reliability tests such as fatigue test, high temperature storage test, 85 ℃/ 85 % test, and thermal shock test. 10 x 10 μm$^2$ sized VLED presented high optical power density of 70 mW mm$^{-2}$ due to its small optical loss and low junction temperature. 100 x 100 array of 10 x 10 μm$^2$ sized VLEDs were successfully transferred, and interconnected to the flexible bottom electrodes without serious epitaxial damage, as confirmed by photolumi-nescence and cathodoluminescence inspection. The flexible VLEDs exhibited the average luminance of was 553 cd m$^{-2}$ with the uniform distribution of dominant wavelength to demonstrate the feasibility of ACF inter-connection technology for high resolution flexible μLED displays.