Level sensor, lithographic apparatus, and substrate surface positioning method

A level sensor for measuring a position of a surface of a substrate comprising:

• a projection unit;
• and
• a detection unit;
• wherein the projection unit comprises:
• an emitter for emitting a radiation beam towards the substrate;
• a projection grating positioned in between the emitter and the substrate, said projecting grating comprising a measurement grating and an aperture, such that the radiation beam from the emitter incident on the projection grating is divided by the projection grating into a measurement radiation beam associated with the measurement grating and a capture radiation beam associated with the aperture;
• wherein the detection unit comprises:
• a first and second measurement detector;
• a first and second capture detector;
• a detection grating;
• and
• an optical system with a first optical unit and a second optical unit;
• wherein the detection grating comprises a ruled grating with multiple rules,
• wherein each rule of the multiple rules of the ruled grating comprises a first measurement facet associated with the first measurement detector and a second measurement facet associated with the second measurement detector;
• wherein the first measurement facet is configured to refract radiation of the measurement radiation beam incident on said first measurement facet towards the first optical unit,
• wherein the first optical unit is configured to divert radiation refracted by the first measurement facet to the first measurement detector;
• wherein the second measurement facet is configured to refract radiation of the measurement radiation beam incident on said second measurement facet towards the second optical unit;
• wherein the second optical unit is configured to divert radiation refracted by the second measurement facet to the second measurement detector;
• wherein the detection grating comprises a first capture element having a first capture facet associated with the first capture detector and a second capture facet associated with the second capture detector, said second capture facet being adjacent to the first capture facet;
• wherein the first capture facet is configured to refract radiation of the capture radiation beam incident on said first capture facet towards the first optical unit,
• wherein the first optical unit is configured to divert radiation refracted by the first capture facet to the first capture detector;
• wherein the second capture facet is configured to refract radiation of the capture radiation beam incident on said second capture facet towards the second optical unit;
• and
• wherein the second optical unit is configured to divert radiation refracted by the second capture facet to the second capture detector.