(Author: Judy Lin, Chief Editor, LEDinside)

X-Celeprint, a XTRION owned company, is working with potential partners to adopt its elastomer stamp micro-assembly technology for commercialization, the company CTO Christopher Bower told LEDinside during LEDforum 2016, which took place in late September in Taiwan.

The company is best known in the micro-LED industry for its elastomer stamp transfer printing technology, which was developed by Professor John Rogers and colleagues at the University of Illinois in U.S.

Positioning itself as a technology supplier in the micro-LED industry, X-Celeprint aims to find potential clients interested in utilizing its micro-assembly technology, explained Bower.

“X-Celeprint is focused on micro-assembly technology, we are excited to see that micro-LEDs seem to be gaining momentum,” said Bower. “Our core technology, micro-transfer printing, has been in development for more than 10 years now, and we are optimistic that 2017 will be an important year for commercialization of our technology, not just in micro-LEDs, but also in other applications.”

The company intends to license its intellectual property, including patents, processes and know-how, to interested technology companies. The company currently offers a line of micro-transfer-printing tools aimed primarily for research and development, and expects that mass production equipment will be available in the near future. The company has engineering teams located at its headquarters in Cork, Ireland and also at its wholly owned subsidiary in Research Triangle Park, North Carolina. Both sites serve as application laboratories, where potential customers can work jointly with X-Celeprint to prove feasibility and develop working prototypes. 

X-Celeprint’s scalable micro-transfer printing technology

Confident in its mass transfer technology, X-Celeprint offers a scalable transfer method, which enables it to transfer large volumes of micron sized devices economically.

The micro-transfer printing process for inorganic micro-LEDs works by using an elastomer stamp to carry the micro-LEDs from their native wafer substrate to a non-native receiving substrate, for example a display panel. The pick-up and transfer processes are aided by the physics of rate-dependent adhesion between the viscoelastic elastomer stamp and the solid micro-LEDs. The devices attach to the stamp through Van der Waals forces, which is a type of weak intermolecular chemical bond. In practice, the stamps are patterned to allow densely packed devices, such as micro-LEDs, to be dispersed at lower densities onto the receiving substrate, which can be much larger than the native wafer substrate. To date, the company has built printers capable of populating up to GEN2.5 sized panels, but suggest that scaling the hardware to larger format panels is practical. The size of the transfer stamp is dictated by the device wafer substrate, and the company has developed stamps designed to retrieve devices across a 150mm diameter wafer in a single pick-up operation.     

After the micro-LEDs are printed onto the receiving surface the stamp is cleaned by touching a tacky pad.  Once cleaned of potential debris, the stamp returns to the micro-LED source wafer for the next pick-up and print sequence.  This sequence repeats until all of the micro-LEDs have been retrieved from the native source wafer.  With this process, the densely packed micro-LEDs have now been precisely distributed across the receiving substrate. The company has recently demonstrated that stamps still work well after 30,000 transfer cycles, indicating that the elastomer stamps have useful lifetimes.

The company has demonstrated both monochrome and full-color passive matrix displays using printed micro-LEDs. In these passive matrix prototypes, 10,000 micro-LEDs were transferred per print sequence. The company presented a 254ppi RGB PMILED display where each color micro-LED was made on its unique wafer substrate and then transferred to the glass display substrate. The company has also demonstrated plastic passive matrix displays, and Bower suggests that the technology is well suited for printing micro-LEDs onto many different types of substrates.

In addition to passive matrix displays, the company has demonstrated a prototype full color active matrix micro-LED display that utilizes no TFTs, instead it has transfer-printed within-pixel micro-ICs. Bower suggested that these printed micro-ICs, made using a standard CMOS foundry, offer the possibility of new display functions and architectures which can’t be achieved using TFT technologies.

Looking beyond micro-LEDs with X-Celeprint’s elastomer stamp technology

Speaking about a misconception that X-Celeprint’s technology is designed specifically for micro-LED transfer, Bower noted the elastomer stamp transfer technology is useful in many other fields, and that processes have already been developed for transfer-printing of microscale Integrated Circuits (IC), solar cells, lasers and sensors.

X-Celeprint is headquartered in Cork, Ireland at the Tyndall National Institute. Ireland’s strength in photonics is one reason that X-Celeprint is currently based there, where it is receiving some support from the Irish government.

“Part of the reason that X-celeprint is in Tyndall right now is because of its strong expertise in optoelectronics,” said Bower.

X-Celeprint is currently participating in the Irish Photonic Integration Center (IPIC), an EU funded EUR 30 million (US $33.42 million) project rolled out in 2014. “Our technology is currently available to all participants in IPIC,” said Bower.

More recently, X-Celeprint is participating in the EU-funded Transfer-print Operations for Heterogeneous Integration (TOPHIT), a project led by Tyndall National Institute which is worth approximately EUR 5.5 million in total that is divided among all participants, said Bower. TOPHIT participants include Huawei, Seagate, IMEC and others, highlighting that the company’s technology is being developed in sectors beyond micro-LEDs.