Level sensor, lithographic apparatus and device manufacturing method

A level sensor system configured to determine a height level of a substrate, the level sensor system comprising:

• a projection unit configured to project a measurement beam on the substrate, the projection unit comprising a projection grating having a period P, the projection grating configured to provide a patterned measurement beam having a periodically varying intensity distribution in a first direction having the period P;
• a detection unit to receive a reflected patterned measurement beam after reflection on the substrate, the reflected patterned measurement beam having a periodically varying intensity distribution in a second direction, having the period P, wherein the detection unit comprises a sensor array configured to receive the reflected patterned measurement beam having the periodically varying intensity distribution, wherein the sensor array comprises a plurality of sensing elements, the plurality of sensing elements being arranged along the second direction at a pitch that is smaller than or substantially equal to half the period P, and wherein the sensor array comprises a first set of sensing elements and a second set of sensing elements that are alternatingly arranged along the second direction at a pitch p, wherein n*p, n being an even number, corresponds to the period P of the reflected patterned measurement beam and the sensing elements of the first set of sensing elements are connected at a first common output of the sensor array and the sensing elements of the second set of sensing elements are connected at a second common output of the sensor array;
• and
• a processing unit configured to receive, at an input, one or more sensor signals from the sensor array and to determine the height level of the substrate based on the one or more sensor signals received from the sensor array.