Method and apparatus for determining the height of a number of spatial positions on a sample defining a profile of a surface through white light interferometry

An apparatus for determining the height location of a number of spatial positions on a sample defining a height map of a surface through white light interferometry with a broadband light source, the apparatus comprising:

• means for positioning an object having a surface to be measured;
• a broadband white light source;
• a reference mirror;
• an optical detector adapted to convert the received light into electrical signals;
• optical means for directing light from the light source to the surface and to the reference mirror and for directing the light reflected by the mirror and reflected by the surface to the optical detector;
• scanning means for amending at least the distance of the optical paths between the surface and reference mirror;
• and
• a processing unit adapted to control the scanning means to perform a scanning action and to receive the signals from the optical detector, wherein the processing unit is adapted to convert the signal received from the optical detector into a correlogram and to estimate the point of the correlogram where an amplitude of the correlogram is at its maximum, thus determining an approximation of the height of the spatial position on the sample represented by said correlogram, wherein the processor is adapted to subject the correlogram to a Fourier transform, subject the Fourier transformed signal to a filter, subject the filtered signal to an inverse Fourier transform, and calculate the location of the centre of mass of the inversed Fourier transformed signal.