Patent ID: 7545503

Claim:
A method for measuring a thickness parameter comprising: measuring a heterodyne phase shift, comprising: receiving a split frequency, dual polarized beam; detecting a reference signal from the split frequency, dual polarized beam; propagating the split frequency, dual polarized beam to a target; receiving a reflected split frequency, dual polarized beam from the target; detecting a measurement signal from the reflected split frequency, dual polarized beam; and measuring a phase difference between the reference signal and the measurement signal for the reflected split frequency, dual polarized beam; measuring a self referencing phase shift, comprising: receiving a split frequency, p-polarized beam; detecting a reference signal from the split frequency, p-polarized beam; propagating the split frequency, p-polarized beam to a target; receiving a reflected split frequency, p-polarized beam from the target; detecting a second measurement signal from the reflected split frequency, p-polarized beam; and measuring a self referencing phase difference between the reference signal and the measurement signal for the split frequency, p-polarized beam; calibrating the phase difference for the target with the self referencing phase difference; and finding a thickness parameter from the phase difference for the target.