Height measurement sensor using derivative of periodic points

Abstract

A height measurement sensor comprising projection and detection units. The projection unit comprises a radiation source and a projection grating comprising a projection grating spot having a plurality of grating lines, the projection grating arranged to receive radiation and output a radiation beam onto the surface to create a radiation spot. The detection unit comprises: a detection grating comprising a detection grating spot having a plurality of grating lines; a detector arranged to receive a reflected radiation beam comprising radiation from the radiation spot after passing through the detection grating spot; and a controller configured to (i) obtain a detector output signal comprising a plurality of periodic components; (ii) take a derivative of two points at different locations of the output signal, wherein the two points are separated by a period of the periodic components, and (iii) determine a focus plane of the sensor when the derivative changes sign.