Microstructure-based analysis of fine metal mask cleaning in organic light emitting diode display manufacturing
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This study proposes the unique method to analyze the cleanliness of the fine metal mask (FMM) used in OLED display manufacturing after FMM cleaning process. We developed a FMM-mimic microstructure as a substitute for the FMM, which can be used for the evaluation of cleaning efficiency. The FMM-mimic microstructure was fabricated using a combination of photolithography, reactive ion etching, anodic bonding and sand blasting processes. To demonstrate the proposed cleanliness analytical method, a 1.4 μm-thick Tris-(8-hydroxyquinoline) aluminum (Alq3) film was deposited on the FMM-mimic microstructure as a contaminant by vacuum thermal evaporation. The Alq3-deposited FMM-mimic microstructure was cleaned by N-methyl-2-pyrrolidone (NMP) with changing cleaning time. We analyzed the residual contaminants on the FMM-mimic microstructure using a fluorescence microscope. The developed FMM-mimic microstructure proves very convenient for inspecting the residual contaminant inside the gap through the transparent glass by general optical and fluorescence microscopy.
KeywordsMicrostructure Analysis Fine metal mask Cleaning Organic light emitting diode Display
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 [1, 2, 3, 4]. 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 [1, 3, 4, 5].
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 [5, 10, 11].
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 [12, 13]. 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 [12, 14, 15]. 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. The residual contaminants inside the gap could take a longer time reaching the base vacuum pressure in the evaporation chamber for subsequent RGB patterning. Recently, Pyo et al. reported their research on chemical analysis of FMM cleaning process with confocal Ramen spectroscopy. They analyzed the cleaning solution used in FMM cleaning and made an analytical model to determine the concentration of residue in the cleaning solution .
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 μm-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-methyl-2-pyrrolidone (NMP).
To test the feasibility of the proposed cleanliness analytical method, we deposited 1.4 μm-thick Alq3 film as a contaminant on the FMM-mimic microstructure by vacuum thermal evaporation. N-methyl-2-pyrrolidone (NMP) was used as a cleaning chemical and iso-propyl alcohol (IPA) and deionized (DI) water were used for rinsing after the NMP cleaning. We analyzed the residual contaminants with an optical and fluorescence microscope after nitrogen gas drying.
The photoluminescent images of the residual emitting materials were recorded by an objective lens coupled with a homemade reflective fluorescence microscope. The samples were excited through an objective lens (Edmund optics, 5×, 0.14NA) by using collimated UV light emitting diodes (Thorlabs, 365 nm). The emission from the sample was collected by the same objective lens and detected by a charge-coupled device.
Results and discussion
The FMM-mimic microstructure was successfully fabricated by the proposed microfabrication process as the designed dimensions and as a result, Fig. 3b shows a photograph of the fabricated FMM-mimic microstructure including a rectangular opening (center), bonding spots (perimeter) and support pads (corner). Figure 3c shows the SEM images of the top and tilt view positions at the corner of a square opening. We also confirmed that the gap was well maintained with 10 μm between the glass substrate and the silicon structure without sink-down phenomenon as shown in the SEM images of Fig. 3d. The main advantage of the developed FMM-mimic microstructure is that we could easily inspect the contaminants inside the gap through the transparent glass by general optical and fluorescence microscopy. Also, during the optimization of the cleaning process, we could inspect the residual contaminant inside the gap without breakage of the sample while changing cleaning conditions.
This study successfully demonstrated the FMM-mimic microstructure based analysis method to evaluate the cleanliness of an FMM after the wet cleaning process used in OLED manufacturing. The FMM-mimic microstructure was developed using a micromachining process, which included photolithography, RIE, anodic bonding and sand blasting processes. To mimic the FMM corner portion, the FMM mimic microstructure was designed to include a rectangular opening, bonding spots, and support pads, and a gap of a 10 μm was formed between the glass substrate and the silicon structure. For the feasibility test of the proposed cleanliness analysis method, a 1.4 μm-thick Alq3 film was deposited on the FMM-mimic microstructure as a contaminant. With varying NMP cleaning time, we effectively analyzed the residual contaminants inside the gap as well as the surface of the FMM-mimic microstructures by the fluorescence microscope. The FMM-mimic microstructure is particularly to monitor the Alq3 contaminant trapped around the bonding spots, which was the main research objective to be solved in this study.
S-HL proposed the main concept for the cleanliness analytical method, designed the FMM-mimic microstructure, and wrote the overall manuscript. Y-CJ performed the cleanliness analysis and assisted in writing the manuscript. J-HY and K-YS performed the device fabrication. SL performed the cleaning experiments. C-RY supervised the research and reviewed the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
The datasets supporting the conclusions of this article are included within the article.
Ethics approval and consent to participate
This work was supported from Advanced Technology Center Program of MOTIE (Ministry of Trade, Industry and Energy) of Republic of Korea (Project Number: 10062356).
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