Patent ID: 7589843

Claim:
A method for measuring a thickness parameter comprising: measuring a heterodyne phase shift, comprising: providing 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: providing a single frequency, amplitude modulated beam; detecting a second reference signal from the single frequency, amplitude modulated beam; propagating the single frequency, amplitude modulated beam to the target; receiving a reflected single frequency, amplitude modulated beam from the target; detecting a second measurement signal from the reflected single frequency, amplitude modulated beam; and measuring a self referencing phase difference between the second reference signal and the second measurement signal for the amplitude modulated beam; finding a phase difference for the target from the heterodyne phase shift and the self referencing phase shift; and calculating a thickness parameter for the target from the phase difference for the target.