Patent ID: 7705995

Claim:
A method of determining etch depth in a process sample made of a sample material, comprising the steps of: a) placing a process sample and a witness sample into a chamber for performing sample etching, said witness sample comprising a surface with a patterned layer present thereupon of a material different from the sample material which is common to both said process sample and witness sample, said witness sample material being directly accessible where the material of the patterned layer is removed to form said pattern; b) causing a beam of electromagnetic radiation to reflect from said witness sample while etching of said process sample and witness sample is being performed, such that a plot of reflectance or an ellipsometric parameter vs. photon energy or wavelength or wavenumber or mathematical equivalent is effectively produced, said plot demonstrating interference between electromagnetic radiation rays which reflect from the surface of the patterned layer and from the witness sample material; c) performing a frequency domain transform of said plot obtained in step b with the result of said frequency domain transform being that at least three identifiable frequency domain peaks result, one of said peaks being proportional to the depth of the patterned layer material, the second of said peaks being proportional to the depth of the atmosphere in the region of the patterned layer which is removed to provide access to said witness sample material including the witness sample etch depth, and the third peak being located at a position determined by the difference between the positions of the first and second peaks along the photon energy or wavelength or wavenumber or mathematical equivalent axis; d) using said frequency domain transform plot to perform a mathematical calculation which quantifies etching of the witness sample material; and e) accepting the results obtained in step d only if said third peak is present at the appropriate position.