Orthogonal Photolithography makes High-resolution OLED Microdisplays Possible

The realization of OLED technology is contingent upon the development of appropriate materials, processing strategies and device structures which are scalable in terms of their cost, capacity, and capability. During the past decade significant progress has been achieved in the synthesis and characterization of OLED materials. Better understanding of material properties and interfaces between organic semiconductors and other materials has enabled the achievement of superior device performance. Although organic devices have exhibited significant improvement over the last decade in terms of their end performance and stability, there still many technological issues to be solved in order to fully facilitate organic device manufacture on the same scale as e.g. the silicon industry. Whereas standard lithographic processes are already available for the patterning of inorganic circuit components, the micro-patterning, processing and integration of organic materials destined for electronic and optoelectronic systems remain among the most challenging issues to be addressed in the field of organic electronics.

Fraunhofer COMEDD in collaboration with its partner Orthogonal Inc. (Rochester, NY) explores novel approach for OLED micro-structuring, which is called Orthogonal Photolithography. “Orthogonal Photolithography is a patented technology, which allows direct patterning of organic material on CMOS-backplanes.”, explains Dr. Alexander Zakhidov, leader of the development group. “So, high-resolution OLED microdisplays for head-mounted displays and data eye-glasses with brightness of 5000 cd/m² are now possible.” Such high brightness micro-displays are required for augmented reality applications where the virtual image has to blend in smoothly with the daylight environment.

The limit for direct RGB-pixel patterning is given by conventional technologies like shadow masking, enabling pixel pitches of about 50 μm. Today smaller pixel pitches can be realized only by covering all sub-pixels with a white OLED and adding a color filter. This filter is able to separate red, green and blue sub-pixels. The disadvantage is, that the color filter causes enormous losses in the light brightness and brilliance. The reason is, that approximately 2/3 of the spectral range of the white pixels are always cut off for the colors that are not needed, and white OLED is itself less efficient than monochrome, therefore just 10 to 20 % of the emitted light could be used.

The new developed technology takes advantage of the fact that the vast majority of organic materials are either oleophilic or hydrophilic and are hence orthogonal (insolvable and stable) to highly fluorinated chemicals. Therefore, appropriate fluorinated photoresists can be used to pattern organic layers without compromising of performance of the organic device. The availability of such orthogonal photoresists promises to enable the fabrication of complex device structures, expanding the range of possibilities for organic electronics.

Fraunhofer COMEDD established Orthogonal Photolithography in its clean room and incorporated it into 200 mm wafer micro display pilot fabrication line. The scientists offer this technology for joint developments of OLED microdisplays and OLED micro-signage applications together with industrial partners.

Fraunhofer COMEDD capabilities for 200 mm wafer processing:

• 300 m2 clean room class 100

• Resist deposition/processing with uniformity deviation <1%

• Resolution 1 μm

• Alignment precision 1 μm

• Wet bench for cleaning/etching

• Dry RIE etching via Ar ion mill and O2 plasma

• Batch vacuum/N2 oven, hotplates Air/N2

• Optical inspection, particle control

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