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imec Sees New Justification for Its Lithographic Patterning of OLED Pixels
 
FMM have served the OLED industry well as it reached pixel densities of 500 to 600ppi. Recently, the Korean government has funded efforts to local suppliers of FMM to compete with the Japan’s Dai Nippon Printing (DNP). DNP uses the etching method of production for its FMM. Japan’s Hitachi Metal is also the exclusive supplier of invar used to manufacture FMM.

However, with the trend of putting sensors under the display (fingerprint, selfie camera…) a new requirement is being formulated, which is an increase in the display transparency, which currently hovers at <40%.  A high resolution 2D display equipped with a lenslet array produces satisfactory imaging cubes, using scaled pixel sizes, generating densities of 3000 to even 6000ppi. Specifically, it would mean bringing the typical ~70µm smartphone pixel size down to ~7µm or even down to the one-micron scale. New patterning techniques are under development for 2000ppi directly patterned OLED pixels that avoid the use of fine metal masks.
 
--Nonetheless, some drawbacks prevent the technology from moving towards higher aperture ratios at high resolution and it is not possible to create a hole structure via FMM methods. Further, the cleaning steps and mask replacements make it hard to reduce this manufacturing running cost. The industry is investigating manufacturing using solution printing processes and inkjet printing (IJP), but pixel densities have peaked at ~200 ppi
 
Lithography was not considered a viable alternative, because of the complex and harsh processing conditions that were not compatible with OLED stacks. For example, the sensitivity of OLED materials to humidity, oxygen, UV radiation, organic solvents, and plasma treatments. These are all elements that are part of a standard lithography flow that could cause OLED degradation. FMMs produced sufficient pixel density for the high resolution smartphone application.  But two requirements have been added to the mix, micro displays using OLEDs could not supply the 30,000 nit AR requirement using a color filter that reduced luminance by >67% and the under display sensors needed more transparency for full operation.
 
Recently, eMagin reported on a photo lithography methodology to produce 2000 ppi on OLED micro displays. Imec has also been developing a process that resulted in a 365nm i-line lithography process allowing the creation of OLED-pixel stacks. No fundamental limitations are expected regarding substrate size for mobile displays or towards 200mm and 300mm wafer sizes for micro displays. These achievements result from the combined optimizations in the device structure, photoresist, and process environment.
 
Figure 1: Patterning Sub-pixels w/Lithography

imec demonstrated some critical results at the International Displays Workshops (IDW) conference in December 2020[v].

• The creation of patterned devices with 10µm size and 20µm pitch, showing no degradation in their electroluminescence (EL) spectra and lifetime compared to non-patterned devices. The only degradation measured that occurred after consecutive lithography steps was an increase in drive voltage. Being in the order of 6.6V, this must be lowered to a more acceptable range of 3.8V at 1000nit, which is the FMM benchmark. 
• This drive voltage that increases after the first lithography step and remains stable after the second may indicate that degradation is due to the direct exposure of photoresist products on top of the organic semiconductor during the litho process, which gives the researchers valuable insights and a feasible route for further optimization.
• The demonstration of high-resolution hole arrays through functional OLED stacks, an essential enabler for in-display sensing. In a first step, a test structure with a full array of holes was created, resulting in a surface with an aperture ratio of 81% that showed a transparency increase from 20-70% depending on the recipes that were used. In a second stage, holes were created within functional OLED devices themselves. The device characteristics before and after hole opening showed no considerable degradation, except for again a slight increase in drive voltage. However, with only 0.6V increase, this was much lower than  in the context of the OLED pixel creation itself.

 
In terms of industrial relevance, the drive voltage is not an issue, when a patterning step is needed to create a single hole array or a single color. For full-color mobile applications where drive voltage should be low or for scenarios in which more complex hole structures are needed, further development is required. 3,000ppi displays are already possible with color-by-white. To enable the 6,000ppi displays and beyond, which the industry is looking for, pixel sizes of around one micron need to be achieved. The further and joint development of photolithography for the domain of OLED displays could make this goal closer to reality than ever expected.
A. Ghosh et al., “Directly patterned 2645 PPI Full color OLED micro display for head mounted wearables,” Dig. Tech. Pap. -SID Int. Symp., vol. 47, no. 1, pp. 837–840, 2016.
[ii] C. Hwang et al., “Novel Plane Source FMM Evaporation Techniques for Manufacturing of 2250ppi flexible AMOLEDs,” SID Symp. Dig. Tech. Pap., vol. 49, no. 1, pp. 1003–1006, 2018.
[iii] Z. Wu et al., “Development of 55-in. 8K AMOLED TV based on coplanar oxide thin-film transistors and inkjet printing process,” J. Soc. Inf. Disp., vol. 28, no. 5, pp. 418–427, 2020.
[iv] T. W. Lee et al., “Characteristics of solution-processed small-molecule organic films and light-emitting diodes compared with their vacuum-deposited counterparts,” Adv. Funct. Mater., vol. 19, no. 10, pp. 1625–1630, 2009.
[v] Tung-Huei Ke et al., “Island and Hole Fabrication on OLED Stack for High-Resolution Sensor in Display Application”, IDW2020.
 
Written by Tung-Huei Ke, R&D project lead at imec.
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