Abstract
Ultra-precision products which contain a micro-hole array have recently shown remarkable demand growth in many fields, especially in the semiconductor and display industries. Photoresist etching and electrochemical machining are widely known as precision methods for machining micro-holes with no residual stress and lower surface roughness on the fabricated products. The Invar shadow masks used for organic light-emitting diodes (OLEDs) contain numerous micro-holes and are currently machined by a photoresist etching method. However, this method has several problems, such as uncontrollable hole machining accuracy, non-etched areas, and overcutting. To solve these problems, a machining method that combines photoresist etching and electrochemical machining can be applied. In this study, negative photoresist with a quadrilateral hole array pattern was dry coated onto 30-µm-thick Invar thin film, and then exposure and development were carried out. After that, photoresist single-side wet etching and a fusion method of wet etching-electrochemical machining were used to machine micro-holes on the Invar. The hole machining geometry, surface quality, and overcutting characteristics of the methods were studied. Wet etching and electrochemical fusion machining can improve the accuracy and surface quality. The overcutting phenomenon can also be controlled by the fusion machining. Experimental results show that the proposed method is promising for the fabrication of Invar film shadow masks.
Highlights
Invar is a 36% nickel iron alloy which has the lowest thermal expansion among all metals and alloys in the range from room temperature up to approximately 230 ◦ C
Advanced hole machining methods are being studied by many researchers for maintaining Invar thin film arresting feature, such as electro-discharge machining (EDM), laser beam machining (LBM), electron beam machining (EBM), and electrochemical machining (ECM)
The proper pulse duration depending on the different machining condition is a very important factor in ECM
Summary
Invar is a 36% nickel iron alloy which has the lowest thermal expansion among all metals and alloys in the range from room temperature up to approximately 230 ◦ C. This value is 10 times lower than that of stainless steel and 100 times lower than that of iron, which makes Invar appropriate for application in various precision fields. Mechanical processes, etching, and electroforming are currently the most-used machining methods for Invar micro-feature devices, other methodologies have been suggested [3,4]. The formation of heat-affected zones and micro-cracks on the workpieces could occur, because EDM, LBM, and EBM are all thermal processes [5,6,7]
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