The Revolution is Coming!

The Revolution is Coming!
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OLED displays are exceptional in that they have infinite contrast, the ratio between the blackest black and the whitest white.  They typically have a wide viewing angle, averaging ~140⁰, while LCD displays have a more narrow viewing angle of ~80⁰, meaning the contrast is reduced less as you move away from the center of the screen (Figure 1).  The colors tend to be ‘saturated’, meaning OLED materials have narrow color ‘peaks’ (Figure 2).  Phosphorescent OLED materials are more efficient than LCD displays that need a backlight, they are thin and flexible, and OLED materials have a very rapid response time (typ. 0.01ms for OLED vs. 1 – 16ms for LCD).

Off-Axis contrast (left images) vs. On-Axis Contrast - Source: Best Buy Blog

​OLED & LCD Color Filter Spectral Comparison - Source: Transmission Spectra – Journal of Display Technology

Based on the above, every display used in consumer devices should be an OLED display, providing the best possible image to customers, but there are issues, the largest of which is cost.  While in theory, OLED displays require no backlight as they are self-emitting, and for smaller OLED displays, do not require a color filter, both of which are necessary for LCD displays, the BOM should be lower for OLED display products, but that is not the case.  In a typical OLED fab, OLED materials are vaporized in a heated chamber and pass through a Fine Metal Mask, essentially a very fine screen, that ‘places’ the OLED material in precise positions to form an RGB pixel on a substrate.  Masks must be very thin and very rigid in order to avoid ‘shadows’ that will cause the materials to be misplaced, but as more holes are added to produce higher resolution displays, the masks become more expensive and subject to gravity, causing misplaced pixels, leading to low display yields and an overall higher display cost.

Fine Metal Mask - Source: Toppan

​Slot & Slit Type FMM detail - Source: Toppan

Over the years display equipment engineers have come up with some alternatives to mask-oriented OLED deposition, particularly ink-jet printing, and while there have been a relatively small number of IJP OLED displays made available commercially, the process, in terms of directly printing OLED materials, has its own limitations.  In order for OLED materials to be printed, they need to be dissolved in a solvent, and that can change the properties of some OLED materials, and as the materials must remain in liquid form in order not to clog the ~50,000 nozzles that are small enough to drop 6 pico-liters of material for each sub-pixel (a pico-liter is equal to 1 trillionth of a liter), with a typical 4K RGB OLED display requiring 24,883,200 droplets.   In high resolution displays, where the pixel density is high, a drop that is even a bit too large can cause the material to migrate into another pixel, which leaves IJP to some of the less precise deposition layers, such as encapsulation materials, rather than OLED material deposition itself.
There is an alternative process that is currently being developed by a number of display producers that is based on photolithography, rather than mask-based deposition.  Japan Display (6740.JP) has been developing a maskless photolithography process called eLEAP (Environment positive Lithography with Maskless Deposition, Extreme long-life, low power, & high luminance, Any shape Patterning) that promises 2x the brightness of mask-based deposition displays, 3x current display lifetimes, while reducing 150,000 tons of CO2 emissions/year.  JDI has plans to commercialize the process by 2025 in partnership with China’s HKC (248.HK), and rumors that Samsung Display (pvt) has decided to test JDI’s process. 
A typical (not that there is a typical process) OLED display being processed without a mask would go through the following steps:

1. Clean the substrate.
2. Coat the substrate with OLED material (one color)
3. Apply resist and cure.
4. Pattern vis plasma etch.
5. Strip resist
6. Repeat steps 2 – 5 for each color.

With semiconductor photolithography stepper tools, theoretical line width down to 1um could be patterned, which would lead the way to higher resolutions that would prove extremely challenging for mask-based deposition, but there are drawbacks, a number of which need to be solved before the process becomes scalable or cost effective.  As the process indicated above uses an open-mask or sputtering system, the cost/m2 should be a bit lower than a FMM system, but deposition tools are only produced by two or three manufacturers and can cost upwards of $100m, depending on size and complexity.  Add to that the cost of an i-line or DUV stepper, reticles, and photomasks, and you have added anywhere from $25m to $120m to the start-up cost of such a line, all of which goes into the panel cost.
There are other issues that need to be addressed, particularly the potential effect of the resist on what are typically very sensitive OLED materials, and the effects of UV curing radiation.  There are also questions concerning how the plasma etch process itself might compromise the integrity of the material stacks and a host of other questions that would influence the commercialization of such a process.  So it comes down to physics and chemistry, and then a hefty dose of process engineering to make this concept into a viable display that can compete with other OLED deposition methods, and other existing or potential display modalities.

OLED lithography Patterning Steps - Source: Metal Halide Functionalized by Patterning Technologies DOI:10.1002/admt.202000513

​One point that will help is that display manufacturers are at least familiar with the photolithography process, as they produce the thin-film transistor backplanes that drive all displays, but those processes are well-known and mature, while photolithographic deposition is relatively new.  The good news is that this month Chinese OLED panel producer Visionox (002387.CH) announced that it is introducing ViP (short for Visionox Intelligent Pixelization), what the company says is the world’s first metal-mask free RGB self-alignment pixelation technology, and while that ‘first’ may be contested by JDI, the Visionox process claims to be able to increase the brightness of a ViP display 4 times over metal-mask OLEDs, increase the device life by 6x, increase the light-emitting area from 39% (mask) to 69%, and increase the pixel density to over 1,700 ppi, with flagship smartphone displays at between 400ppi and 500ppi.
Visionox has indicated that it has produced medium sized samples based on the technology and is ‘rapidly advancing the work related to mass production’, which, when completed will be applied to AR/VR, wearables, phones and even TVs, and has even begun to build a ViP batch production line in Hefei.  While there is still much uncertainty regarding the development and production timeline, the fact that two panel producers are seriously considering the technology is likely to expedite R&D efforts and possibly overcome some of the existing obstacles, and then it will become a direct cost issue if it is to become a practical and profitable process for high-resolution displays.  A few years to go, but the fact that it would not require building a new display infrastructure certainly gives one hope that photolithographic deposition can join the other display processes and technologies that are in operation or on the horizon over the next few years.