Film thickness and boundary characterization by interferometric profilometry

An interferometric method for measuring the thickness of a film, the method comprising the following steps:

• scanning the film at a predetermined plan location on the film to obtain a correlogram produced by 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 the 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;
• comparing said first and second modulation envelopes to a second threshold, wherein the second threshold is a function of a model modulation envelope;
• 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.

An interferometric device for measuring the thickness of a film, comprising:

• means for scanning the film at a predetermined plan location on the film to obtain a correlogram produced by 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;
• means for calculating a modulation profile from said correlogram;
• means for identifying, as valid modulation values, modulation values from said profile that are above a first threshold;
• means for sorting the 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;
• means for 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;
• means for calculating a film thickness at said plan location on the basis of said first and second scanning locations;
• and
• means for establishing said first threshold by identifying modulation values known to be outside at least some of said regions of high contrast, and by setting the first threshold as a function of the modulation values so identified.

An interferometric device for measuring the thickness of a film, comprising:

• means for scanning the film at a predetermined plan location on the film to obtain a correlogram produced by 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;
• means for calculating a modulation profile corresponding to said correlogram;
• means for identifying, as valid modulation values, modulation values from said profile that are above a first threshold;
• means for sorting the 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;
• means for comparing said first and second modulation envelopes to a second threshold, wherein the second threshold is a function of a model modulation envelope;
• means for 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
• means for calculating a film thickness at said plan location on the basis of said first and second scanning locations.