Patent ID: 8092695

Claim:
A method for processing a substrate, comprising: etching a photomask reticle disposed on a substrate support in a plasma etch chamber; obtaining a first signal indicative of light collected through a first window of the substrate support, the first window disposed below a center of the photomask reticle; obtaining a second signal indicative of light collected through a second window of the substrate support, the second window disposed below a first edge of the photomask reticle; obtaining OES information collected through a sidewall of the etch chamber; normalizing at least one of the signals using the OES information; determining from the normalized signal and an optical thin film model at least one of an etch rate or thickness of a chromium layer disposed on the photomask reticle; and determining a parameter of etching the photomask reticle using the OES information and at least one of the obtained signals.