Patent ID: 7591923

Claim:
A plasma processing system comprising: a chamber having an opening and containing a plasma processing region; a chuck, constructed and arranged to support a substrate within the chamber in the processing region; a plasma generator in communication with the chamber, the plasma generator being constructed and arranged to generate a plasma during a plasma process in the plasma processing region; and an optical system in communication with the chamber through the opening and having a window with one side facing the plasma processing region, the optical system being constructed and arranged to detect a plasma process condition through the window and a window transmission condition, wherein the window is configured to transmit a plasma optical emission formed into an incoming light beam by the optical system, the plasma processing condition being detected using the plasma optical emission, the optical system further comprising: a transmission input lens mounted on the side of the window facing the plasma processing region, and configured to transmit a transmission detection beam from a light source through the window; a transmission output lens mounted on the side of the window opposite the plasma processing region, and configured to receive the transmission detection beam for detecting the window transmission condition; and a shutter valve positioned between the transmission input lens and the window along the optical axis of the transmission detection beam, the shutter valve being configured to protect the transmission input lens, but not the window, from contamination by the plasma when closed, and allow passage of the transmission detection beam when open, wherein an optical axis of the transmission detection beam is positioned at an angle of less than 90° with respect to an optical axis of the incoming light beam and such that the transmission detection beam does not traverse the plasma processing region, so that interference of the transmission detection beam and the incoming light beam is minimized allowing detection of the window transmission condition during detection of the plasma optical emission.