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Etching is either a chemical or physical process to selectively remove the Cr/CrO layer along the pattern formed on the surface. Wet etching (chemical etching) and dry etching (plasma or ion etching, thus, physical etching) are both commonly used methods in the manufacturing of photomask nowadays. Dry etching is preferred over the wet etching when patterns with finer geometries are required.
In general, there are three types of dry etchers, depending upon the mode of creating plasma: (1) PE (Plasma Etching), (2) RIE (Reactive Ion Etching) , and (3) ICP (Inductively Coupled Plasma).
After Develop process, PR pattern is formed on top of the Cr layers of the photomask. The resultant PR layer protects the Cr layer underneath during the etching process, resulting in the removal of Cr layer which is exposed to the etching agent (either wet or dry). Wet etching is preferred for lower grade photomask which can tolerate relative higher etch bias. When finer geometries and tight CD control is desired, dry etching provides superior performance.