Patent ID: 8441640

Claim:
A substrate processing system comprising: a processing chamber having an exterior wall with a transparent portion and an interior portion; a pedestal to support a work piece disposed within the interior portion, the pedestal comprising a rigidly attached first ring that is nearly concentric with a second ring, the second ring being rigidly attached to the processing chamber; and an optical measurement assembly disposed outside of the processing chamber and coupled to the exterior wall, the optical measurement assembly comprising a laser gauge that supplies light and detects the supplied light after being reflected; wherein the laser gauge, the first ring, and the second ring are disposed to allow light to be transmitted by the laser gauge into the processing chamber through the transparent portion, and to detect reflected light from the first ring and the second ring by the laser gauge; wherein the optical measurement assembly comprises a processor that calculates a height difference between the first ring and the second ring at a plurality of locations around a circumference of the pedestal.