High-brightness displays made with micro-transfer printed flip-chip microLEDs

Abstract

The incumbent flat-panel technologies, liquid crystal display (LCD) and organic light-emitting diode display (OLED), are ill-suited to produce compact, efficient, and robust high-brightness displays. LCDs are very inefficient, only a small fraction (~5%) of the generated light exits the display. To achieve highbrightness LCDs, practitioners create extremely bright back-light units using inorganic LEDs which require expensive and unreliable active cooling solutions. OLEDs use organic molecules to form light emitting diodes within each display pixel. The lifetime of the organic light emitters is inversely proportional with the display brightness; therefore, OLEDs are not suitable for highbrightness applications. In sharp contrast, inorganic LEDs made using wafer-level semiconductor technology are long-lived, even when operating at high luminance. Displays that use inorganic LEDs as the light-emitters within each display pixel already dominate the giant video walls that increasingly decorate our highways and streetscapes. Today, there are many efforts around the world aimed at making highly miniaturized inorganic LEDs, called microLEDs, and developing methods to transfer those microLEDs from their native substrate to the destination display substrate. Effective techniques to produce microLED displays must have the capability to quickly and accurately transfer millions of microscale devices and are called "mass transfer" technologies. Micro-transfer-printing using elastomer stamps is one such "mass transfer" technology that has been used to produce prototype microLED displays. Here, we will describe how micro-transfer-printing combined with wafer-level packaging techniques can produce highbrightness displays. We will provide fabrication details and characterization results of various 5.1" 70 PPI microLED displays. In one example, we produced a monochrome green display using 8 μm x 15 μm flip-chip InGaN microLEDs with a maximum brightness in excess of 30,000 nits. We will highlight application opportunities and remaining challenges for high-brightness displays using microLEDs.