SmartKem has developed an organic display backplane solution that to date, has focused on LCD and OLED display support. But a new demonstration of a microLED device reveals a novel approach that may be able to solve one of big pain points in microLED device fabrication.
According to Ian Jenks, CEO and Chairman of SmartKem, one of the key yield loss steps in microLED device fabrication is the bonding of the microLED to the driving backplane, typically and LTPS-based glass substrate. “Getting the right bond pressure over the entire substrate has proven difficult and requires rework to properly affix the microLEDs, driving up costs. Our answer is to not try to attach the microLED to the backplane, but rather, grow the backplane on top of an array of known good microLED die.”
Grow the backplane? Yes, that’s exactly the technology that SmartKem offers. The 12-year-old company has developed a series of solution-based polymer organic materials that can be used to fabricate the transistors needed to drive displays. These materials have achieved performance (mobility) levels that are between amorphous silicon (~1 cm2/Vs) and IGZO (~10 cm2/Vs). In a 2 transistor/1 capacitor configuration, SmartKem can achieve a mobility of 3 cm2/Vs. And just as importantly, they can accomplish this in a short channel length, which is important for obtaining the higher current levels needed to drive OLEDs and LEDs.
Growing a-Si or LTPS transistors on top of the microLEDs is not possible because the high temperature processing will destroy the microLEDs. SmartKem’s low temperature Organic Thin Film Transistor (OTFT) processing an be achieved at less than 80-degrees C, thus enabling this architecture.
As a proof of concept, Jenks says the device they recently demonstrated is a 2.2” diagonal display with a 25.4 ppi designed as a local dimming backlight for an LCD with 1296 zones, targeted at a VR application. It is blue monochrome with microLED sized at 300 x 300 microns. The device is designed for about 300K nits and the 2T1C drive circuits can deliver 30 mA per microLED. Other designs on the wafer were produced with 254 ppi with microLEDs of 50 x 20 microns. When asked who the microLED supplier was, Jenks said it was a commercial lighting supplier in China. He noted that such suppliers don’t normally make LEDs that small, but they see opportunity in this market and have developed a mask set to support this size.
The fabrication process is also very interesting. For this demonstrator, they purchased the 4” GaN wafer with the patterned display already fabricated. SmartKem that starts with a planarization process using their special sputter resistant material. This is spin coated over the GaN wafer and has been engineered to create a very flat surface without any chemical-mechanical polishing. In fact, all of their materials are spin coated as solutions and they have demonstrated this on substrates as large as Gen 3.5. All are UV curable in seconds as well. That’s impressive.
The next step is to dry etch vias in the planarization, sputter metal and pattern the anode and cathode contacts to the microLED. A further planarization step is applied and then source drain metal electrodes for the OTFT are patterned. The organic semiconductor layers are spin coated next followed by an organic insulation layer. More planarization follows along with another metalization layer and photolithography to define the via for connection to the transistors and LEDs (detail not shown in graphic). It is a 6-mask process that only requires spin coating, photolithography, sputtering and dry and wet etching. Such processes are fully compatible with existing a-Si lines.
The company’s via technology is also very interesting. Working with Taiwan-based ITRI, they developed the process to create 3-micron vias in their planarization material. This was initially developed for connection of high-performance computer chips, but it spurred the idea for the microLED demo just developed.
SmartKem is now starting to reach out to microLED developers to brief them on this new demo and see if their solution can help speed commercialization of the microLED market. In fact, they are already working with a company to grow the backplane over a substrate of discrete RGB miniLEDs for digital signage applications. We hope to know more soon.