Height measurement sensor using derivative of periodic points

A height measurement sensor comprising:

• a projection unit, wherein the projection unit comprises:
• a radiation source;
• a projection grating comprising a projection grating spot, the projection grating spot comprising a plurality of projection grating lines, the projection grating arranged to receive radiation emitted by the radiation source and output a radiation beam onto a surface of a substrate to create a radiation spot on the surface, the radiation spot comprising a plurality of radiation lines;
• and
• a detection unit, wherein the detection unit comprises:
• a detection grating, wherein the detection grating comprises a detection grating spot, the detection grating spot comprising a plurality of detection grating lines;
• a detector arranged to receive a reflected radiation beam comprising radiation from the radiation spot reflected from the surface after passing through the detection grating spot;
• and
• a controller configured to:
• obtain an output signal of the detector, wherein the output signal comprises a plurality of periodic components;
• 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
• determine a focus plane of the height measurement sensor in response to the derivative changing sign.