Photomask Etching

Abstract

While plasma etching of wafers has been a mainstay of the semiconductor industry for more than 25 years, only in the last decade has dry-etch processing become interesting to mask manufacturers. The reason for the late introduction of this technology becomes obvious when the methodologies used to image silicon wafers are examined. Contact printing, which uses a mercury arc lamp as a light source, satisfied the early needs of the industry when critical dimensions (CDs) werewell over 2 µm. The industry standard for a mask with CDs less than 5 µm was a quartz or glass plate on which a thin 80-100 nm layer of chrome was deposited. Resist was spun onto the mask and exposed using either an optical reduction camera or, as the industry became more sophisticated, an electron beam writing system. Because the smallest features were many micrometers in width, the patterns were defined simply by placing a the mask in a wet etchant such as cerium ammonium nitrate. The resist was resistant to the chrome and etch and could easily be removed chemically or in a plasma asher.