The Filmetrics F20 is used to measure the thickness and optical constants (n and k) of transparent and semi-transparent thin films. Measured films must be optically smooth and between 100 Å and 50 um thick. Commonly measured films include semiconductor process films such as oxides, nitrides, resists and polysilicon. Films that cannot be measured include very rough films and opaque films. The Filmetrics thin film analyzer is very useful for rapidly fine-tuning resist recipes and measuring etch rates.