Ellipsometry for Pellicle-Covered Surface

The photomask (PM) surface is covered with a pellicle to protect from dust and other airborne particles. However, a defect known as haze appears on the surface of the PM during exposure even with a pellicle cover. As lithography goes into the deeper UV, the photochemical reactions of contaminants become enhanced to form haze on the PM surface. This affects the lithography as haze absorbs or scatters UV. Ellipsometry may be an ideal technique for the early detection of a haze layer. However, when the PM is covered with a pellicle, the ellipsometric data collected from the surface become extremely distorted owing to the non-normal transmission through the pellicle. In this article, we introduce a novel technique so that the conventional ellipsometric data can be obtained without removing the pellicle. If ellipsometry can be used for the inspection of the PM with a pellicle in place, the cleaning frequency of the PM will be reduced significantly.