Method and device for measuring the thickness of a layer

Method for measuring thickness of layers during a process in which one or several layers are deposited or adsorbed on an object, comprising:

• positioning a diffraction object, which has at least two opposing diffracting limits spaced by a known distance, in a process chamber where a layer to be measured during a layer deposition process so that the layer is deposited or absorbed at said diffracting limits of said diffraction object, where thickness of layer has a known relationship with the thickness of the diffraction object;
• projecting said diffracting limits of said diffraction object that is located in said process chamber, by a coherent light pencil of a known wave length onto a detection surface, with said light pencil being diffracted at said diffracting limits of said diffraction object;
• detecting a local intensity distribution of said light pencil, which is caused by diffraction, on said detection surface in at least one dimension, and
• computing an actual distance of said diffracting limits of said diffraction object from said intensity distribution and determining the thickness of the layer from the computed actual distance.