Patent ID: 8112246

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 beam splitter configured 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 configured to effect relative movement along a measurement path between a sample and the reference surface; a sensing device configured to sense light representing the interference fringes produced by a sample surface region; a controller configured 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 configured to receive first intensity data comprising a first series of 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 intensity values resulting from a measurement operation on a surface area of a thin film structure, the data processor comprising: a gain determiner configured to determine a gain for the or each thin film of a thin film structure; and a surface characteristic determiner configured 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 modify the apparent thin film structure surface characteristic using the substrate surface characteristic and the gain or gains determined by the gain determiner.