A Study on Predicting Material Properties Based on Defects in Fine Metal Mask with Electroplating Process
Authors: Pyo Changmin , Shin Jihoon , Kim Jaewoong 
Published: December 2024

OLED manufacturing relies on the use of master masks, which serve as the basis for producing panels for devices such as smartphones, laptops, and TVs, using deposition processes. To enhance productivity and cost-effectiveness, the technology has advanced towards larger master masks, with the latest generation, the 10.5th generation (2,940 X 3,370mm), being approximately 100 times larger in area compared to the 1st generation (270X360mm). Additionally, high-resolution technology is also in demand as pixel density increases. Although new technologies such as laser patterning and inkjet are continuously being developed, the deposition process remains the universally applied method for mass production due to its precision and ability to pattern on a microscale.

In OLED manufacturing, the components of the three primary colors must be deposited in their respective positions, and any interference in the deposition locations can lead to deterioration in OLED quality. To address this, Fine Metal Masks (FMMs) are necessary. FMMs facilitate sequential deposition processes for the selective deposition of red, green, and blue components, ensuring patterning at desired locations. Invar, with its low coefficient of thermal expansion of 36%, is primarily used as the material for FMMs due to the temperature rise inside the vacuum chamber during deposition processes.

To improve FMM technology, it is advantageous to reduce thickness and approach a taper angle of 90 degrees. However, the current rolling-etching process has limitations, typically achieving a thickness of around 25um. Furthermore, improving taper angles also faces challenges. To overcome these limitations, various studies have been conducted in both the industry and academia, utilizing electroplating technology. Electroplating allows for the production of thinner FMMs compared to the conventional rolling method, with thicknesses reaching levels as low as 10um.

Despite advancements, FMMs produced using electroplating methods are not yet widely used in practical applications. This is because attaching multiple or single FMMs to frames for OLED mask fabrication does not demonstrate their feasibility. Gravity-induced sagging during frame attachment requires tensioning of the FMM, and the OLED mask must be manufactured with minimal deformation, considering factors such as wrinkles. When attaching multiple FMMs, the attachment positions and sequence can induce frame deformation, necessitating a systemic approach to address these issues. Finite element analysis (FEA) and other simulations are required to predict deformation accurately and perform design and manufacturing accordingly. Accurate simulation requires precise material properties, and since FMMs produced by electroplating exhibit slightly different properties from conventional rolled invar, it is necessary to derive accurate material properties.

This study focuses on the degradation of material properties due to internal defects and voids. Although voids are one of the factors that degrade material properties, their impact has not been adequately analyzed. Therefore, this study utilizes the Representative Volume Method (RVE), a homogenization method, to model voids and analyze their effects. RVE estimates the mechanical properties of the material when subjected to a certain strain by calculating the average stress in the entire unit cell based on the stress and volume of each mesh, assuming the unit cell is a perfect homogeneous material.

The study confirmed the deterioration of material properties with various porosities in the unit cell. The results showed that at a porosity of 17.1%, the Young's modulus decreased by 25.9%, while at a porosity of 55.1%, it decreased by 70.3%. Thus, it was observed that voids cause more significant degradation in material properties than just their volume.

This work was supported by Technology Innovation Program (project name : Equipment technologies for 50cm2 all solid state battery cell, project Number : 20012349) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

This research was supported by National R&D Program through the National Research Foundation of Korea(NRF) funded by Ministry of Science and ICT(2021M3H4A3A02098099)