US Patent 11421977 Eliminating internal reflections in an interferometric endpoint detection system

A method is disclosed for operating an endpoint detection system of a processing chamber having a ceiling formed therein, a substrate support located internal to the processing chamber, and a substrate resting on the substrate support. A transparent panel is located in the ceiling of the processing chamber, the panel oriented at a first acute angle relative to the substrate and the substrate support. The transparent panel receives an incident light beam from the endpoint detection system at a second acute angle relative to the panel. The transparent panel transmits the incident light beam to the substrate within the processing chamber at an angle perpendicular to the substrate and the substrate support.