Bright and Tiny: Mojo Vision won’t say exactly what its superdense microLED display (top) is for—perhaps iPhones for insects? A close-up shows the 14,000-pixel-per-inch display (bottom) in monochrome green. Photos: Mojo Vision
One of the most striking things about the prototype microLED display that Silicon Valley startup Mojo Vision unveiled in June was its size. At half a millimeter across, it’s barely bigger than a single pixel from the microLED TV prototype Samsung showed off in 2018. That both use versions of the same technology is remarkable, and it portends big potential for screens made of superefficient and bright micrometer-scale gallium nitride LEDs. Impressive prototypes have proliferated during the past year, and now that companies are turning to the hard work of scaling up their manufacturing processes, displays could appear in some products as soon as late next year.
“We’re seeing really good progress on all fronts, and we’re seeing more and more companies coming out with prototypes,” says Eric Virey, an analyst who follows the nascent industry for Yole Développement.
The driving force behind microLED displays remains a combination of brightness and efficiency that LCD and OLED technology can’t come close to. One demo of a smartwatch-size display by Silicon Valley–based Glo shines at 4,000 nits (candelas per square meter) while consuming less than 1 watt. An equivalent LCD display would burn out in seconds trying to meet half that brightness.
The companies involved broadly fit into two categories. Some are making monolithic displays, where the gallium nitride pixels are made as a complete array on a chip and a separate silicon backplane controls those pixels. And others are using “pick and place” technology to transfer individual LEDs or multi-microLED pixels into place on a thin-film-transistor (TFT) backplane. The former is suited to microdisplays for applications like augmented reality and head-up displays. The latter is a better fit for larger displays.
For those in the first camp, a pathway to a high-throughput, high-yield technology that bonds the backplane to the microLED array is key. The United Kingdom’s Plessey Semiconductors demonstrated a throughput-boosting technology recently, by bonding a wafer full of Jasper Display Corp.’s silicon CMOS backplanes to a wafer of its microLED arrays.
New York City’s Lumiode is founded on the idea that such bonding steps aren’t necessary. “When you have to bond two things together, yield is limited by how that bonding happens,” says Vincent Lee, the startup’s CEO.
Glo showed this 1.6-inch full-color display for a smartwatch at the Society for Information Display conference (top). Plessey's monolithic gallium nitride LED array was bonded to a Jasper Display Corp. silicon backplane to make this monochrome HD display (bottom). Photos, top: Glo; bottom: Plessey Semiconductors
Instead Lumiode has been developing a process that allows it to build a TFT array on top of a premade GaN microLED array. That has involved developing low-temperature manufacturing processes gentle enough not to damage or degrade the microLEDs. Much of the work this year has been translating that process to a low-volume foundry for production, says Lee.
For Glo, the work has been more about making the microLED fit the current-delivering capabilities of today’s commercial backplanes, like those driving smartphone displays. “Our device design is focused on low current—two orders of magnitude lower than solid-state lighting,” on the order of nanoamps or microamps, explains Glo CEO Fariba Danesh. “This year is the first year people are talking about current. We’ve been talking about that for five years.”
Glo takes those microLEDs and places them on either a CMOS backplane for microdisplays or on a TFT backplane for larger displays using the same technology for any resolution or display size. The company doesn’t talk about its pick-and-place technology, but it is key to commercial products. “Our transfer yields are right now high enough to make some parts; now we are focused on making thousands and then millions of zero-defect panels,” says Danesh.
Others are looking to simplify production by changing what gets picked up and placed down. X-Celeprint’s scheme is to place an integrated pixel chip that contains both CMOS driver circuits and red, green, and blue microLEDs. It’s a multistep process, but it means that the display backplane now needs to be only a simple-to-manufacture network of wires instead of silicon circuitry. Engineers at CEA-Leti, in Grenoble, France, recently demoed a way to simplify that scheme by transferring the microLEDs to the CMOS drivers all at once in a wafer-to-wafer bonding process.
“It’s really too early to pick a winner and know which technology is best,” says Yole’s Virey. “Some of the prototypes are quite impressive, but they are not perfect yet. A lot of additional work is required to go from a prototype to a commercial display. You need to improve your cost, you need to improve your yield, and you need to make an absolutely perfect display each time.”
This article appears in the August 2019 print issue as “MicroLED Displays Expected in 2020.”