Integrating microLEDs with advanced CMOS

Bonding 300 mm GaN-on-silicon LED wafers to CMOS backplanes of the same size offers the best approach for producing displays that have a microLED pitch of just a few microns.

BY Soeren Steudel FROM MICLEDI Microdisplays

For the last five years or so microLED manufacturing has been a very hot topic within the display industry. It’s been high on the agenda at leading display events, such as Display Week, with the focus on flat panel manufacturing of various displays, ranging in size from smart watches to mobile phones and TVs. For this technology, efforts at scaling manufacture are thwarted by challenges associated with mass transfer of the microLED, as well as the repair of defective die.

A new set of issues are faced when considering microLEDs for microdisplay applications, like augmented-reality (AR) glasses. One approach to making a microLED-based display module for that type of application involves uniting microLEDs and CMOS backplane ASICs, which control and drive the microLEDs. This approach eliminates issues associated with mass transfer, but comes up against a different set of obstacles. Consider the primary application on the horizon for this technology, AR glasses: there’s a need for a brightness exceeding 1 Mnits, pixel pitches below 3 mm, a resolution of up to 2K and beyond, ultra-low power consumption and acceptable cost, all in a light-weight module.

As of today, none of these specifications are being met by existing microdisplay manufacturing methods. But progress is underway. The chipmaker JBD of Shanghai, China, has introduced several impressive prototypes and is delivering modest volumes. However, high-volume manufacturing is elusive. Meanwhile, our company, MICLEDI Microdisplays of Leuven, Belgium, is making good strides on addressing issues that limit the brightness and resolution of microLED displays. To this end, we are developing an approach that’s needed to deliver high-volume, low-cost manufacturing. Read on to discover the details of the challenges we face and our compelling solutions.

Figure 1. Light-outcoupling in a planar microLED with (a) small pixel versus a (b) tight-pitched array.

Brightness and efficiency
One weakness of AR glasses is their very substantial optical losses. The extent of this is governed by the implementation, but often less than just 1 percent of the photons emitted by the display arrive in the eye. Due to these staggering losses, displays have to generate up to 10 million nits of white light to support outdoor usage with high-transparency glasses.

A comparable microOLED display, which is a rival technology to the microLED display, can achieve in the best case 20 knits for green, even though the efficiency number for OLED will be better than for microLED. A summary of the state-of-the-art by Yole Intelligence, the French market analyst, in 2019, showed efficiency numbers for OLED (RGB – 22 percent, 22 percent, 7 percent), versus the 5 mm microLED (RGB – 7 percent, 15 percent, 25 percent). Note that these figures are far below the values for the internal quantum efficiency of the GaN LED, which has a typical value of 85 percent in the blue, 60 percent in the green and less than 30 percent in the red.

Efficiency losses at scaled dimension are mostly due to optical outcoupling and, to a lesser extent, electrical losses due to defects at the mesa sidewalls, leading to a high amount of non-radiative recombination and an increased leakage current. The light-outcoupling in a planar LED is limited by the angle of internal reflection from the high index compound semiconductor material into air. This automatically means that for a GaN LED, less than 10 percent of the light can be extracted with a perfect backside mirror, neglecting interference effects. For larger LEDs with dimensions greater than 100 mm, surface texturing is applied with a very good backside mirror. This enables every photon to have multiple chances of escaping under different angles, leading to a theoretical light extraction efficiency of 75 percent. Surface texturing is not a solution for microLEDs with dimensions below 5 mm since there is no space for the photon to undergo multiple reflections.

Figure 2. ((left) Field-of-view (FOV) plotted as a function of pixel number for different angular resolutions (Mpixel refers to 1 pixel with a red-green-blue sub-pixel with a display ratio 1:1); (right) Die size for different sub-pixel pitches (assuming an advanced node CMOS (<45nm) with a framebuffer and a display ratio 16:9).

It’s also worth noting that the efficiencies provided by Yole are very optimistic, and only apply to individual LEDs spaced very far apart. This is illustrated in Figure 1, which considers different spacing scenarios. In microLEDs, the direct emission through the transparent front-side contact is very low, typically below 10 percent – but extraction can be boosted by adding a sidewall mirror that extracts light beyond the angle of internal reflection. However, when packing microLEDs closer together to ensure a higher pitch, any type of sidewall mirror is less effective. Due to this impediment, the external quantum efficiency of microLEDs in very small displays is expected to be limited to no more than 8 percent, unless there is a shift to a directional emitter architecture.

So, given these low values for microLED efficiency, these devices are still seen as a viable alternative to OLEDs, because they have a vastly superior current handling capability. By being able to sustain a current density that is more than a thousand times higher than an OLED can handle, these GaN-based devices can deliver the target brightness.

Figure 3. Die yield versus field-of-view (FOV) for different sub-pixel pitches. (left) assumes RGB pixel-by-pixel; (right) assumes RGB die-by-die.

Display size and resolution
Two questions for any display based on the microLED are: What is its required size? And what resolution is appropriate? To answer them, it’s imperative to consider the capabilities of the human eye. In both the green and blue spectral domain, the human eye has an angular resolution of 60 pixels per degree. We noted this figure when considering the targeted field-of-view for AR glasses. As one would expect, the number of pixels in a display must increase when increasing the field of view, or the angular resolution (see Figure 2, left). In some current commercial headsets, where the system supports a full high-definition display, there is a limited field-of-view of 50°. One benefit of moving from 5 mm to 1 mm microLEDs is that they can offer a larger field-of-view from the same display size (see Figure 2, right).

While this level of miniaturisation is appealing, it is far from easy to realise with routine success. Even the manufacture of a full high-definition display with 5 mm pitch approaches the limits of the reticule size of the exposure tool. Operating near this limit impacts manufacturability and yield.

We have calculated the impact of yield for different pitches. According to our manufacturing yield model (see Figure 3, left) – that assumes red, green and blue microLEDs co-integrated side-by-side – even when the target resolution is reduced to only 40 pixels per degree, a pitch of less than 3 mm is needed to exceed a 50 percent yield.

It is possible to significantly relax these conditions by manufacturing three different colours of emitter independently, before bringing them together with an optical combiner (see Figure 3, right). This gets far closer to the yield model of an incumbent colour display technology, known as the sequential liquid-crystal-on-silicon display.