imec uses photolithography to boost OLED display manufacture

07 Dec 2020

Researchers achieve “breakthrough” in reliability with potential to cut need for fine metal masks.

Using photolithography in OLED manufacture permits greater design freedom.

This week, between 9-11 December, at the International Display Virtual Workshop 2020, imec, a Belgium-based research hub in nanoelectronics, presents its latest achievements in the application of photolithography to pattern organic semiconductors and the cathode in an OLED-display stack.

The researchers say they have made a “breakthrough” in device reliability, which has the potential to eliminate the use of fine metal masks. They say that the demonstration is important for display manufacturers as photolithography enables higher resolutions or an increased transparency of the cathode to accommodate for display-integrated sensors.

In the domain of mobile displays, the demand to increase the screen-to-body ratio pushes manufacturers to enable new form factors and hide an increasing amount of functionality, such as front-side camera or fingerprint sensor, inside the screen without degrading the image quality.

But also in high-end mobile, TV and PC or laptop applications, manufacturers are craving for active-matrix OLED (AMOLED) screens to offer unique properties such as vivid colors and flexible and ultrathin format to enable the innovative form factors of future product designs.

At present, the use of fine metal masks (FMM) is the dominant manufacturing route to create red, green and blue light-emitting-layer (EML) patterns in AMOLED displays. Alternatively, multiple companies have shown AMOLED displays surpassing 150ppi resolution via the alternative of solution-printing processes such as inkjet printing (IJP).

Yet, the use of FMM does not apply to very large area glass substrates, says imec, and IJP results in lower reliability compared to evaporated OLEDs.

Photolithography benefits

By using photolithography processes, imec now provides an alternative manufacturing route that allows for higher design freedom and shows promising results in terms of reliability. For example, imec can create patterns on top of organic semiconductors with 1µm pitch (line and space), which corresponds to resolutions around 12,000ppi.

Also, the transparency of the OLED frontplane at higher wavelengths increased from 20 to 70% after cathode patterning. Most importantly, the results of the reliability testing show T95>200hours and no measurable difference in OLED lifetime at 1000 nits for patterned OLEDs compared to reference un-patterned OLEDs.

Tung Huei Ke, R&D project leader for sensor technology, at imec, said, “Photolithographic patterning of OLED was extremely challenging because of the chemical incompatibility of the photolithography chemistry to the OLED materials. Also, the sensitivity of OLED materials to water and oxygen in a conventional photolithography environment was a major difficulty.

“As the first OLED subpixel needs to be patterned at least three times to realize full-color AMOLED array, it was considered that the degradation after multiple photolithographic patterning steps would accumulate. This assumption, confirmed by earlier experiments, discouraged the industry to go for this route.”

He concluded, “This is an important milestone to bring photolithography as patterning technology for OLEDs to the next level. We continue working with the industrial partners to mature the technology for high-volume production and invite interested display manufacturing companies to join this endeavor.”