Image Number 2 for United States Patent #5337144.
An etch rate monitor for use with semiconductor wafer etching processes includes a source of light of normal incidence to the wafer surface through a window in the etching chamber. In a first embodiment, a Fresnel or positive lens is used to collect some of the diffraction orders caused by the repetitive patterns on the wafer surface which merge from the window. In alternate embodiments, a concave spherical mirror and/or a photodetector system are used to collect the diffraction orders. A collimating lens applies these diffraction orders of normal incidence to interference filters which reject plasma and ambient light and pass the diffraction orders to a photodetector to monitor etch rate as a function of the cycle period between interference minima or maxima caused by the difference in path length between the etched and not etched surfaces of the wafer.