Mastering The Manufacture Of MicroLED Micro-displays
A revolution is now underway in micro-displays. Demand for this technology is rising fast, due to a range of emerging applications that include wearable electronics, head-up displays and augmented reality. However, to succeed in the marketplace, these miniaturized displays, which typically have a diagonal screen size of no more than an inch, will have to meet stringent technical requirements that are not fully satisfied by current technologies.
One of the most promising technologies for forming these micro-displays is based around the microLED. This tiny device – the LEDs have dimensions on the order of microns – are directly integrated as pixel elements onto the silicon driver IC backplane.
Taking this approach allows the excellent light emission capabilities of compound semiconductor devices to be paired with the unsurpassed functionalities of the ICs. The displays that result outperform those based on liquid crystal and organic LED technologies, by having the upper hand in terms of brightness/contrast, power efficiency, response time and device reliability. However, their manufacture is far from easy, due to the tremendous challenges associated with the integration of compound semiconductor microLEDs and silicon-based ICs – devices with vastly different material properties and fabrication processes.
Overcoming these obstacles has been the goal of many researchers in academia and industry, who have been spurred on by the alluring market potential of microLED micro-displays. And many have turned their attention to flip-chip technology, which is the most common method for integrating compound semiconductor functional device arrays with siliconbased ICs.
Flipping the chip
The starting point for the flip-chip approach is the design and fabrication of microLED arrays on one substrate, and the design and production of siliconbased pixel driver ICs on another. Crucially, the geometric layouts on both these entities must be identical.
Chip-level fabrication follows, with the individual chips that contain microLED arrays diced and separated from the original wafer. Using a high-precision alignment bonder, microLED arrays are then flipbonded to silicon ICs using an array of solder bumps. This provides an electrical connection to the pixel driver circuit array.
Aided by good progress in flip-chip technology, some groups have recently reported demonstrations of GaN-based active-matrix microLED micro displays. However, one should note that there are inherent drawbacks associated with flip-chip technology – and they limit the performance and cost of microLED micro-displays.
One significant drawback is that as the fabrication process is at the chip level, it requires precise alignment, which hampers throughput and increases cost. In addition, often transparent substrates are essential, because the microLED growth substrate tends to remain after hybrid integration. And yet another issue is that there is a significant thermal mismatch between the compound semiconductor device substrate and the silicon substrate. This difference introduces built-in stress in the microdisplay chips, compromising manufacturing yield and long-term reliability.
What’s more, there is limit to how small the pixel size of the microLED can be. It is determined by the capability of the manufacturing equipment and the flip-chip process. Today, there are efforts in industry to realise a pixel size as small as 20 μm, but there is not a foreseeable, clear path with the flip-chip process to shrink this to less than 10 μm while maintaining a reasonable yield and cost. That’s a big issue, because microLEDs with a pixel size of less than 10 μm are needed to make displays for augmented reality.
At Jade Bird Display (JBD), a start-up founded in 2015 and headquartered in Hong Kong, we are addressing all the drawbacks of conventional flipchip technology for the microLED micro-display with a novel, wafer-scale technology. At its heart is the transfer of entire compound semiconductor epitaxial layers onto a silicon IC wafer. This is accomplished by wafer bonding and substrate removal. By adopting a wafer-level epi-transfer process, we eliminate the need for precise alignment, a pre-requisite for flip-chip technology, and we are ultimately able to turn to a high-throughput, wafer-scale fabrication process.
Figure 1. JBD’s wafer-scale monolithic hybrid integration technology.
Once we have formed our epi-on-IC templates, we use standard semiconductor device fabrication processes to produce our hybrid chips with different functionalities. With high precision photolithography equipment and processes well established in the semiconductor industry, we can fabricate our compound semiconductor functional device arrays on top of pixel driver circuit arrays with sub-micron alignment accuracy. This holds the key to hybrid integration of fine pitch device arrays on silicon ICs with pixel sizes that can be as small as a few microns (see Figure 1).
Another considerable advantage of our technology is that it can utilize well-established infrastructure. Working with equipment sets and semiconductor processes developed by the silicon-based IC industry, we can draw on low-cost mass-production techniques to produce integrated functional hybrid chips, including our high-performance microLED micro-displays.
Figure 2. Packaged red, green, and blue active matrix microLED micro-displays.
Armed with our technology, we have already made significant strides in the development of microLED micro-displays. Effort began in early 2016, and we realised our first milestone later that year: our first micro-displays. They exhibited exceptional device performances, combining excellent brightness/contrast with power efficiency and a small device footprint. Even with our first attempt, our prototype outperformed existing micro-display technologies by a substantial margin (see Figure 2).
These first monochromatic red, green and blue micro-displays were made in a VGA format and had a standard definition display resolution of 640 pixels by 480 pixels. The green and blue microLEDs were made from InGaN/GaN-based materials, while AlInGaP/GaAs-based materials were used for the red. All these LEDs had a circular geometry, with a diameter of just 6 μm, and they formed a microdisplay with a pixel pitch of 20 μm, which corresponds to a resolution of 1,270 pixels per inch. Driven at a peak current per pixel of 20 μA, the brightness of the green LEDs in this display can exceed 5x105 nits – that’s over 500 times that of existing, OLED-based self-emissive micro-displays.