Analysis of OLED Fine Metal Mask Cleaning Process Using a Microstructure
Authors: Lee Sang-Ho , Jeong Yong-Cheol , Yu Jun Ho , Shin Kwon Yong , Yoon Chang-Ro 
Published: February 2020

In the manufacture of flat panel displays for television screens, cell phone displays, computer monitors, and so on, organic electroluminescent displays (OLEDs) have attracted attention due to their large angle visibility, high brightness, wide range of working temperature, fast response time, high contrast and vivid color compared with traditional flat panel displays such as liquid crystal displays. A typical OLED display structure consists of multi-organic layers such as an electron injection layer, an electron transport layer, a hole transport layer, and a hole injection layer which are sandwiched between a transparent indium-tin-oxide anode and a reflective metallic cathode. To form pixels emitting red(R), green(G) and blue(B) colors, pixels patterns are selectively deposited onto a thin film transistor (TFT) backplane panel with pixel bank array by evaporation of organic light emitting materials through a fine metal shadow mask (FMM) with tiny pixel-shaped apertures after highly precise alignment of the FMM to the TFT backplane.
The FMM is typically fabricated by laser welding of a thin metal sheet onto a stainless steel frame. The thin metal sheet includes tiny openings formed by electroforming or micro photo etching of stainless steel or Invar (64% Fe-36% Ni alloy). When an FMM is used repeatedly during the continuous manufacturing of a full color OLED, evaporated organic materials accumulate on the surface and interface of the FMM as well as inside the gap between the metal sheet and the stainless steel frame. The long time usage of the FMM causes the blocking of the tiny apertures and the distortion of the FMM, and eventually, the organic material cannot be accurately patterned to form an organic light emitting layer. Therefore, the FMM should be regularly cleaned to avoid patterning error caused by the accumulated materials, however, cleaning can lead to the damage of the FMM.
To find suitable cleaning conditions for minimizing FMM damage, it is important to analyze the efficiency of the FMM cleaning process. In general, the residual contaminant on the surface of an FMM can be analyzed by traditional analytical techniques such as VPD/ICP-MS, TXRF, XRF, AES, XPS and SIMS which are used to detect ionic and organic contamination on the wafer surfaces. The particle contamination on the surface can also be measured by light-scattering-based surface scanners used for measuring particle in the efficiency evaluation of wafer cleaning systems. However, it is impossible to investigate the residual contaminant inside the gap between the welded thin metal sheet and the frame because the metal sheet is opaque and the separation of the welded metal sheet is impossible until the last stage of FMM recycling.
Our study was aimed to find a unique method to analyze the residual contaminants inside the gap as well as on the surface of an FMM. Instead of an FMM, we developed a FMM-mimic microstructure that can be used for the evaluation of cleaning efficiency after FMM cleaning process. A 1.4 mm-thick Tris-(8-hydroxyquinoline) aluminum (Alq3) film was deposited on the FMM-mimic microstructure as a contaminant by vacuum thermal evaporation. We analyzed the residual contaminants inside the gap using an optical and fluorescence microscope after the Alq3-deposited FMM-mimic microstructure was cleaned by N-methly-2-pyrrolidone.

Funding

This study was supported by Grant from the Ministry of Trade, industry & Energy (MOTIE, Korea) under Industrial Technology Innovation Program. No. 10063277, "Development of pattern deposition system based on roll to roll processing under low temperature and atmospheric pressure condition for smart thin film device fabrication."