This post was sponsored and authored by Radiant Vision Systems, LLC

Introduction

Emissive OLED, microLED (μLED), and miniLED are emerging as the next wave of technology in the display market. This is exciting because these displays promise improved display performance and visual appearance with greater efficiency than other display technologies, thanks to their individually emitting pixel elements. Both OLEDs and microLEDs have superior contrast ratios and sharper images with deeper blacks and more vibrant colors than traditional LCDs. These emissive displays require no backlight, resulting in thinner, lighter-weight displays that use less electricity. OLEDs also bring a dramatic boost in responsiveness, about 1,000 times faster than existing technologies, virtually eliminating blur on fast-moving and 3D video. MicroLEDs match OLED technology for response time and view-angle performance, but exceed OLED in brightness and ruggedness, with even lower power consumption.

Figure 1 – The 219-inch microLED display “The Wall” by Samsung, shown at IFA 2019, September 6-11 in Berlin, Germany. (Source: Samsung)¹

As manufacturers work to launch commercially viable emissive display products, high costs due to material prices and manufacturing yield issues have hindered widespread technology adoption—most dramatically in large-format implementations, as they drive up end-customer prices. The smartphone market has been the most successful segment for OLED technology to date and will likely be the catalyst that drives long-term adoption of OLEDs and microLEDs for other applications. Display Supply Chain Consultants (DSCC) cites smartphones as the dominant OLED market, accounting for around 91% of units per year with revenue share around 79% by 2022². Yole Développement (Yole) projects a similar trend for microLEDs, with a longer ramp up period, and a market reaching up to 330 million units by 2025³. With this type of growth in demand, improvements in manufacturing efficiency are needed.

Figure 2 – The smartphone market leads the way for OLED adoption. [Source: DSCC’s Quarterly OLED Shipment and Fab Utilization Report]²

Figure 3 – Smartphones are similarly forecast to drive the microLED market. [Source: Yole’s MicroLED Displays report]³

Although the near-term market size is small, analysts at DSCC predict that large-format OLED TVs will take second seat to smartphones, rising to 42% market share by 2022, overtaking LCDs by as soon as 2021² . A limited sample of large-format microLED displays have been showcased by manufacturers like Samsung (The Wall) and Sony (Crystal-LED), whereas small-format displays continue to be more viable for mass production. For both OLED and microLED, low production yields due to manufacturing complexity and—once manufactured—visual quality issues impact the timing of viable market entry and drives up retail prices for finished display panels. Current commercially available, large-format OLED TVs are priced into the thousands of dollars (USD), while microLED screens remain cost prohibitive to most consumers, making lower-cost, high-definition LCD and LED options more appealing to the budgets of price-minded consumers. The price point for volume market adoption of new emissive display types and replacement of current technologies must be significantly lower, demanding greater efficiency and control in manufacture.

Display Manufacturing Challenges

OLED and microLED technologies add several unique challenges to the manufacturing process, regardless of the size of display.

Large-format OLED screen manufacturing has been somewhat costly to date due to inefficiency of deposition methods for adding organic molecules to their substrate (with the exception of the latest efficient inkjet printing methods), which has limited the application of OLED technology primarily to smaller screen sizes like smartphones. Likewise, producing an entire television screen out of microLED chips has so far proven to be challenging. MicroLEDs require new assembly technologies, die structure, and manufacturing infrastructure. For commercialization, fabricators must find methods that yield high quality with microscopic accuracy while also achieving mass-production speeds. As a point of comparison, a miniLED backlight screen may be made up of thousands of individual miniLED units; a microLED screen is composed of millions of tiny LEDs.

To fabricate a microLED display, each individual microLED must be transferred to a backplane that holds the array of units in place. The transfer equipment used to place microLED units is required to have a high degree of precision, with placement accurate to within +/- 1.5 µm. Existing pick & place LED assembly equipment can only achieve +/- 34 µm accuracy (multi-chip per transfer). Flip chip bonders typically feature accuracy of +/-1.5 µm—but only for a single unit at a time. Both of these traditional LED transfer methods are not accurate enough for mass production of microLEDs.

There are also visual quality and performance issues. Inspecting OLED and microLED displays is in many ways more challenging than inspecting LCDs. Traditional LCD displays using LED backlights produce only a mostly uniform luminance (brightness) and color output across the display screen. With emissive display types, however, every individual emitter in the display can be subject to a high a degree of variability. For instance—although microLEDs are LEDs—traditional binning processes are virtually impossible for microLEDs, which can be as small as 1/100th the size of a conventional LED. Without these controls in place, ensuring brightness and color uniformity in microLEDs can be extremely challenging. It is necessary to be able to measure and quantify every pixel and subpixel element of an emissive display to identify defects and ensure uniformity, to produce the level of quality that consumers expect at these displays’ higher price points.

Visual Quality Issues

Line Mura

In the OLED manufacturing process, material is deposited on a substrate to form the individual subpixels, while extremely tiny microLEDs are transferred to backplanes with the goal of achieving extreme precision. If this process is not completely uniform, implications emerge in terms of visual quality. One such issue is line mura, which appears as well-defined horizontal and/or vertical orientation in the display.

Figure 4 – Image of an OLED display with uncorrected line mura.

Subpixel Luminance Performance

Emissive display pixels are composed of red, green, and blue subpixels. When current is applied, each subpixel lights up individually, and output of each subpixel is also individually controlled. The brightness and color of each pixel in the display is determined by combining the subpixel outputs. Due to production discrepancies in the manufacture of emissive display types, there may be variations in luminance for the same electrical signal input throughout the population of same-colored subpixels on the display. This results in differences in brightness from pixel to pixel of the same color. When combined, subpixels of each color together outputting light at various brightness levels produce display pixels that exhibit even more variability, and overall display appearance appears low-quality.

Figure 5 - Subpixels combine to create pixels with various colors and brightness.

Earlier in January, Belgium-based MICLEDI announced that it raised 4.5 million Euro to develop next-generation MicroLED microdisplays for AR applications.

We had a short talk with MICLEDI's CEO, Sean Lord, who helped us understand a bit more about the company's technology and goals. MICLEDI is indeed focused entirely on the AR market, and so aims to provide display engines that will be extremely bright (over 10 million nits), efficient, and with a very small footprint. In addition costs are also important for the future consumer AR market.