Patent ID: 7119909

Claim:
An interferometric method for measuring the thickness of a film at a predetermined plan location on the film, the method comprising the following steps: scanning the film at said predetermined plan location on the film to produce a correlogram corresponding to interference of a reference beam with a test beam reflected at a first interface between the film and a first adjacent medium and at a second interface between the film and a second adjacent medium; calculating a modulation profile from said correlogram; identifying, as valid modulation values, modulation values from said profile that are above a first threshold; sorting said valid modulation values into a first modulation envelope corresponding to a first region of high contrast produced by the first interface and into a second modulation envelope corresponding to a second region of high contrast produced by the second interface; determining a first scanning location corresponding to a peak of the first modulation envelope and a second scanning location corresponding to a peak of the second modulation envelope; and calculating a film thickness at said plan location on the basis of said first and second scanning locations; wherein said first threshold is calculated by identifying modulation values known to be outside at least one of said regions of high contrast, and by setting the first threshold as a function of the modulation values so identified.