not so micro-LED

While OLED technology is currently enjoying its time in the spotlight, display innovators are already turning their attention to the next big technological shift – micro-LED. Major product companies, including Samsung, Apple, and Facebook’s Oculus, are already looking at this technology for future products, and various manufacturing and research companies are stocking up on patents.

Micro-LED technology was invented in the year 2000 by a research group run by Prof. Hongxing Jiang and Prof. Jingyu Lin of the Texas Tech University. However, the first demonstration of micro-LED in a consumer product can be tracked backback to Sony’s 55-inch FullHD “Crystal LED Display” that was showcased back in 2012. At the time, it boasted a much better contrast ratio and color gamut than rival LCD TVs.

However, Sony’s technology was incredibly expensive, and the manufacturing technique was not commercially viable on a major scale. That hasn’t stopped companies investing in and improving micro-LED manufacturing techniques though, and industry mutterings suggest we are closing in on commercial viable production.

Micro-LED tech explained

Micro-LED shares a number of traits with OLED technology, making comparisons a little easier than LCD vs OLED debates. For starters, you’ll notice that both have LED in their name, meaning that they’re both constructed from light emitting diodes. This means the two are “self-emitting” technologies, so each red, green, and blue sub-pixel produces its own light, unlike LCD, which requires a dedicated backlight. Therefore micro-LED displays will offer very high contrast ratios and deep blacks, just like OLED. There’s also a TFT plane used to power each of the individual pixels.

Where micro-LEDs differ from OLED is in the makeup of their LED materials. The O in OLED stands for organic, and refers to the organic materials used in light producing part of the pixel stack. Micro-LED technology changes this to an inorganic Gallium Nitride (GaN) material, which is typically found in regular LED lighting. This switch also reduces the need for a polarizing and encapsulation layer, making panels thinner. As a result, Micro-LED components are tiny, hence the name, measuring less than 100 µm. That’s less than the width of a human hair.

LEDinside

Another way of looking at this is that mirco-LEDs are simply traditional LEDs shrunk down and placed into an array. The actually LED technology isn’t new, but manufacturing a panel array using such tiny components is the real difficulty. Conversely to other panel technologies, it’s actually easier to build small form factor micro-LED panels, such as smartwatches and smartphones. Scaling up to TV sizes is proving more difficult. However, squeezing very high resolutions into smartphone panel sizes is also tough to achieve, due to the high requirement for soldering accuracy.

Micro-LED isn't reinventing display technology, instead the unresolved problem for panel manufacturers is how to mass transfer and bond million of tiny LEDs.

The unresolved problem for panel manufacturers is how to mass transfer and bond million of LEDs over to the control circuit panel. One potential solution sees the LEDs picked and placed into a larger array, to then be soldered to complete a display. The issue is that the accuracy of current pick and place manufacturing is ±34µm, which doesn’t meet the ±1.5µm accuracy requirements to place these tiny micro-LED components.

Alternative wafer production methods involve either etching LED arrays for bonding to an IC, or transferring a separate TFT layer to the LED array. These etching methods avoid the accuracy problems of chip bonding, but improving the techniques to meet the small component sizes of micro-LEDs and the demand for high resolution displays is expensive and difficult to implement. Manufacturing times are also very slow and refinement is still needed to improve yields.

Wafer bonding appears to be the most viable process in the short term. However it’s only currently suitable for low pixel volume panels, such as a low resolution smartwatch display rather than a QHD smartphone. Pick and place is likely the end goal for high resolution micro-LED displays, but manufacturing accuracy is still being refined.

Yole Développement

Pros and cons Vs. OLED

Despite the manufacturing hurdles, micro-LED technology is still worth pursuing because it offers up a number of improvements over OLED. The first is improved brightness to power (lux/W) efficiency, meaning that the same panel brightness can be achieved for lower power. For comparison, power consumption can be 90 percent lower than LCD and up to 50 percent lower than OLED. This is potentially a major boon for portable technologies like smartphones, as it means much longer screen-on time. Alternatively, manufacturers can increase panel brightness without consuming additional power compared to current OLED and LCD, for better daylight viewing.

Micro-LED displays will also offer a longer lifespan than current OLED panels. OLED burn in is still an issue, due to the limited lifespan of the organic materials used to make the blue OLED. Micro-LEDs don’t exhibit the same problems and could even last longer than LCD displays before color shifting begins to occur.

Yole Développement  The TFT backplane between micro-LED and OLED can be essentially identical, simplifying production techniques somewhat.

The smaller micro-LED sizes also makes the prospect of higher resolution panels in a compact form factor, such as 4K or 8K smartphones or VR displays, more achievable. Speaking of VR, OLED panels already boasts very high response times in the µs (microsecond) range. This makes them ideal for virtual reality applications. However, micro-LED can reduce this down into ns (nanoseconds) or a thousand times faster.

Micro-LED offers all of these benefits, while still retaining the high contrast ratio, wide color gamut, and potential use in flexible displays that we’ve come to associate with OLED. Unfortunately, these next-generation panels are also expected to be considerably more expensive, possibly three to four times higher than current LCD and OLED panels. This will undoubtedly fall in time, but it’s likely to discourage some immediate investments, especially as many panel manufactures are still ramping up meaningful OLED production.

Final thoughts

Micro-LED technology certainly sounds promising, and there are plenty of upsides that are particularly well suited to mobile products. Although many manufacturers are only just tuning into the benefits of OLED, high-end OEMs are almost certainly eyeing up micro-LED as their next-generation display technology.

Exactly when the first products will arrive remains an unknown for now. Some analysts are expecting 2018 or 2019 to yield more talk about production capabilities and potential products, although it will probably take a while longer before panel manufacturers can produce meaningful volume.

However, there are also heavy investments being made into refining flexible OLED and bezel-less displays that might not make micro-LED the highest priority for every manufacturer. Even so, it seems likely that this technology will be the one the industry will eventually adopt in the longer term.

In a future article, we will be touching on another benefit of Micro-LED: the ability to integrate sensors to make “smart displays” a reality. Stay tuned.

Robert Triggs

Lead Technical Writer covering SoCs, displays, cameras, and everything in between. In his spare moments you’ll find him building audio gadgets.