Patent ID: 8543353

Claim:
Apparatus for determining information relating to a surface characteristic of a sample in the form of a thin film structure comprising one or more thin film layers on a substrate, the apparatus comprising: a light source; a light director to direct light from the light source along a sample path towards a surface region of a said sample and along a reference path towards a reference surface such that light reflected by a sample surface region and light reflected by the reference surface interfere; a mover to effect relative movement along a measurement path between the sample and the reference surface; a sensing device operable to sense light representing the interference fringes produced by the sample surface region; a controller operable to carry out a measurement operation by causing the sensing device to sense light intensity at a number of locations along the measurement path to provide, for each location, a corresponding intensity value; and a data processor to receive first intensity data comprising a first series of the intensity values resulting from a measurement operation on a surface area of a substrate and second intensity data comprising at least a second series of the intensity values resulting from a measurement operation on a surface area of a thin film structure, the data processor comprising: a surface characteristic determiner to determine a substrate surface characteristic on the basis of the first intensity data, to determine an apparent thin film structure surface characteristic on the basis of the second intensity data, and to: (a) determine a frequency transform ratio corresponding to a ratio between the second intensity data comprising the second series of intensity values resulting from the measurement operation on the thin film layer structure surface area and the first intensity data comprising the first series of intensity values resulting from the measurement operation on the surface area of a substrate, (b) determine a mean of said frequency transform ratios for a number of different surface regions of the thin film structure, and (c) fit a layer structure model having variable model parameters related to the layer thicknesses of the layers of the thin film structure to data based on the values determined in steps (a) and (b).