Leti: Challenges and Opportunities of Micro-LEDs in Wearables

(Authors: Philip Chang, Analyst, LEDinside, and Judy Lin, Chief Editor, LEDinside)

Ivan-Christophe Robin, Strategic Marketing Manager at Leti, spoke to LEDinside recently about the French research institute’s outlook for micro-LEDs in various end-market applications, especially the wearable sector.

Defined by a diameter of less than 10 microns, micro-LEDs have attracted attention from LED manufacturers due to their outstanding luminous efficiency compared to OLEDs, their self-illumination properties that reduce backlight demands, and the ability to fabricate them on flexible substrates.

Leti’s roadmap for micro-LED applications starts with automobile HUD display projectors, before moving onto HMD and larger display applications, such as augmented reality glasses and smartphones. Following the maturation of the miniaturized LED technology, the micro-LEDs eventually will be found in large TV displays.

Ivan-Christophe Robin, Strategic Marketing Manager at Leti. (Photo courtesy of Leti)

The new display technology rumored to be adopted in Apple Watch 3 is suitable for wearable devices because it is brighter, and uses less electricity than traditional LED-backlit LCD screens, explained Robin. He also noted that micro-LEDs are suitable for smartphone applications.

Monochromatic and color micro-LED displays used for different applications would also require manufacturers to adapt different manufacturing processes. “For smart watches or smartphones the best option would be to do RGB (micro-LED) transfer separately, but if you want to have a 1 cm micro-LED display it is going to be difficult because the LEDs will be next to each other,” said Robin. “In this case, I would suggest using monochromatic or color micro-LED arrays and transfer (the micro-LEDs) together.”

A particular challenge in wearables and head-mounted displays is making the micro-LED display small enough, but still capable of delivering high-resolution performances, while limiting the sacrificed field of view, Robin added.

“For a small micro-LED display with good resolution, basically you want below 5-micron pitch for each color, so basically you’re going to want to have 10-micron pitch full color RGB pixel…so you need to basically work on the fine pitch and the color conversion of the fine pitch,” said Robin, who has accumulated nearly 12 years of experience in industry research and optoelectronic engineering.

In addition to its leadership in micro-LEDs, Leti was a forerunner in micro-OLED technology, which is targeting augmented reality and virtual reality wearable device markets that present complete immersion. The technology has better color-rendering performance, and is well suited for indoor display applications.

Leti partnered with MicroOled to develop 3.8-micron meter-sized micro-OLEDs that are directly disposed on CMOS backplane. Leti has been able to achieve high pixel resolution using a color filter design. Micro-OLEDs are complementary to micro-LEDs because OLEDs are more mature but limited by brightness, whereas micro-LEDs’ high brightness works well for outdoor augmented-reality applications.

Leti’s micro-LED chip technology

The two primary micro-LED chip structures on the market include flip chip LEDs and vertical LED chips. CEA-LETI develops both kind of products depending on the application. “In the case of inorganic micro-displays, prior to hybridization, you want to get all of the micro-LED contacts on the same side of the  micro-LED wafer,” said Robin.

Hybridization is defined by Leti as the process of placing the micro-LED chip on CMOS, and is a process of bonding micro-LED chip array on CMOS. Leti has considered TFT as a substrate for the micro-LEDs, but claims equipment on the market and outsourcing to manufacturers make it difficult to grow TFT smaller than 20 microns. Moreover, driver IC efficiency using TFT is in general lower than CMOS.

Transfer of micro-LEDs, which remains challenging for manufacturers, involves at Leti bonding using microtubes, and mechanical pick and place transfer. As for the micro-LED bonding material, Leti uses µtubes and not Anisotropic Conductive Film (ACF) because of uncertainties in the materials bonding strength.

As for the general outlook for micro-LEDs, aside from growing interest in wearables, Robin believes Taiwanese and Korean manufacturers are well positioned in developing TFT-based micro-LED technologies. European semiconductor and LED manufacturers on the other hand will be more suited for developing CMOS backplane based micro-LEDs, he said. Micro-LEDs present considerable market opportunities, and future technology developments will depend on industry collaboration and companies’ ability to overcome challenges in adapting the technology in end-product applications.

About Ivan-Christophe Robin

Ivan-Christophe Robin is a strategic marketing manager at Leti with an expertise in photonic devices. He has served on roughly 30 conference science committees and holds a portfolio of 20 patents. Dr. Robin joined Leti’s Optronics Department, Grenoble, in 2006, where he worked on IR detectors, solar cell structures, ZnO- and GaN-based LED systems, organic/inorganic heterojunctions, GaN/ZnO heterojunctions, InGaN/GaN nanowires and LED structures.

He received his undergraduate degree in physics and “aggregation” from Ecole Normale Superieure, Lyon, in 2001, and his master’s degree from the Pierre-et-Marie-Curie University, Paris-VI in 2002. After being awarded a PhD degree in 2005 from the University Joseph Fourier, Dr. Robin worked as a post-doctoral fellow at the University of British Columbia, where he studied oxide growth for infrared (IR) solid-state laser fabrication.

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