Interview with X Display (XDC)
X Display Company (XDC) spun out of X-Celeprint in 2019 to continue the commercialization of Micro Transfer Printing for display manufacturing. This technology has been developed over the last 15 years, which makes XDC one of the first companies to offer a mass transfer technique specifically for microLED displays. According to XDC, the technology is particularly well-suited for micron-scale chips, and they have achieved the highest transfer yields with devices smaller than 100 microns.
Micro Transfer Printing uses an elastomer stamp made of PDMS to transfer LED chips from a source wafer to a target substrate. The target substrate can be a display panel or an intermediate carrier wafer (sometimes referred to as an interposer). XDC introduced the idea of an intermediate wafer early on, as it can reduce the number of transfer steps and speed up the entire manufacturing process. Moving the LED chips from a small epiwafer to a standard size 300 mm (12”) wafer also facilitates further processing in semiconductor fabs.
Aside from Micro Transfer Printing, XDC is also working on a display driving technology that uses semiconductor ICs instead of TFT backplanes. XDC sells a proprietary MicroIC that can drive all three subpixels with 14-bit PWM (Pulse Width Modulation). The company also develops a PixelEngine chip that contains a driver IC and red, blue, green microLEDs, all in one package. Both the MicroIC and the PixelEngine components are designed to be transferred by Micro Transfer Printing.
Last October, it was announced that Lextar licensed XDC’s technology. Lextar is now part of Ennostar, a joint venture with Epistar, and the collaboration with XDC is still active. The development and license agreement between the two companies signaled a significant development in the microLED supply chain. Lextar and XDC previously worked together to make prototypes. At Touch Taiwan in 2019, they demonstrated two 5.1” transparent UltraMatrix microLED displays with MicroIC drivers. One display was monochrome green and the other one was RGB.
XDC currently has 35 people and is growing. In January, the company announced the installation of their first 300mm Micro Transfer Printing tool. Dr Chris Bower, CTO, and Dr Matthew Meitl, VP of Displays, accepted to answer a few questions about the tool and the company.
5.1” UltraMatrix MicroLED Display (Showcased at Touch Taiwan 2019)
Recently Installed 300mm Tool
DSCC: Can you tell us more about the capabilities of the 300mm tool that was recently installed?
XDC: We are excited about this new tool because it is our first tool designed to populate 300 mm diameter wafers. At its heart, the tool is a motion platform with optics. For this tool, the source wafers and destination substrates are placed on a stage with 600 × 800 mm travel. With minor modifications, the tool can support panel substrates as large as 700 mm.
The tool precisely moves an elastomer stamp to retrieve and transfer arrays of microLEDs. The elastomer stamps have surface topography, protrusions we call “posts”, designed to selectively transfer devices. The stamps are made on a rigid glass plate, so the stamps look and feel like a photomask. The stamps are transparent, and the tool has machine-optics that look through the stamp to align to the microLEDs. The throughput of the process is a function of the number of microLEDs transferred by each cycle of stamping and by how quickly the tool completes the full stamping cycle. The stamps can be large and still achieve good contact with large arrays of microLEDs because the stamp is compliant. We have demonstrated stamps with greater than 100mm transfer fields.
To help illustrate how it works, consider that the source of the microLEDs is a wafer with 10 × 10 µm2 microLEDs made on a 15 µm pitch, and we wish to assemble the microLEDs onto a non-native destination substrate at 60 µm pitch (423 PPI). The pitch of the posts on the stamp will match the destination density, in this case 423 PPI, and a 60 × 60 mm2 stamp area would transfer one million microLEDs each cycle. If the tool performs four stamping cycles per minute, that equates to 240 million units per hour. When doing development, we run the tool a little slower, about two cycles per minute. The tool, really the full technology ecosystem, is designed to accommodate all types of microLEDs. We have demonstrated prototype displays using lateral “pads up” microLEDs and also flip-chip “pads down” microLEDs. Beyond microLEDs, our transfer process works with microICs, lasers, etc.
DSCC: Any indication of the country or company where the tool has been installed? Will it be used as part of a pilot production?
XDC: The tool is installed in the US as part of a pilot production effort involving a multi-national supply chain. The tool was originally planned to be in Asia, but the pandemic created some logistical challenges. We ultimately decided to install the tool in the US to keep the program moving. Tools to support the product ramp are still planned for installation in Asia.
DSCC: How should the source LED wafer be processed in order to be compatible with the tool?
XDC: More and more groups are investigating our technology and equipment to handle die in standard singulated formats like dicing tapes, but our partners accomplish the highest levels of placement accuracy and miniaturization with “transfer ready” structures designed into the microLED wafer. These structures are tethers that break away when the stamp retrieves the devices. We offer tether designs that use standard materials and microfabrication techniques that can be readily employed by LED makers. The end result is a wafer with densely packed microLEDs that is not diced, that can withstand shipping and robot handling, and readily supplies the elastomer stamp during the transfer process.
DSCC: What is the current status on transfer yield? How does it compare with others in the microLED industry?
XDC: In 2015 we showed four nines (99.99%) transfer yield in a research lab. Since then, our partners have demonstrated four and five nines, and we are confident that the technology will exceed six nines in mass production. We are confident that mass transfer based on XDC technology will continue to produce industry-leading yields.
DSCC: How many companies have licensed the technology so far? Can the licensee build their own tools for mass production, or do they need to buy from XDC?
XDC: Spanning all fields, including non-display, ten companies have licensed the technology, and our technology is being evaluated by several more. Our licensees can buy tools from XDC or build their own tools for mass production. In one case, a licensee has already worked with a 3rd party on building a tool. Our tool offerings are aimed at giving our partners and licensees development flexibility. It allows our customers to make beautiful transfer printed displays with minimal development time.
DSCC: What is the roadmap for the microIC and PixelEngine? Which application will be the most likely to use this technology?
XDC: Our ambition is to enable display makers with our technology and intellectual property. One aspect of our business model is to develop and sell components through a fabless semiconductor model. The microIC is a good example of a PixelEngine component that is designed and offered by XDC and fully fabricated by a foundry partner. Some companies are already sampling the XDC pixel-driver microIC designed to support the XDC UltraMatrix digital drive architecture. Another type of PixelEngine component in development at XDC is a tiny package that includes a red, green and blue microLED that is ready to be directly integrated with display backplanes. These RGB PixelEngine components allow the display maker to focus their mass transfer efforts on a single device type that includes all the colors, and these components will be delivered on larger format wafers that support larger transfer fields and device throughput.
Our business and technology are aimed at enabling all display applications. Our current focus is on large displays, but this year we will begin a new effort on smaller ones.