Proximity sensor, lithographic apparatus and device manufacturing method

Apparatus comprising an array of gas gauge level sensors each comprising:

• a measurement gas flow channel having a measurement outlet, the measurement gas flow channel being configured to output a first gas flow towards a measurement object;
• and
• an optical pressure sensor for comparing a pressure of the first gas flow towards the measurement object with a reference pressure, the optical pressure sensor comprising a first optical cavity fluidly connected to the measurement channel and a second optical cavity fluidly connected to the reference pressure, the first optical cavity being configured to output first reflected electromagnetic radiation and the second optical cavity being configured to output second reflected electromagnetic radiation, the optical pressure sensor being further configured to combine the first reflected electromagnetic radiation with the second reflected electromagnetic radiation and determine, based on the combined first reflected electromagnetic radiation and the second reflected electromagnetic radiation, a pressure difference between the pressure of the first gas flow and the reference pressure and determine, based on the pressure difference, a distance between the measurement outlet and the measurement object, wherein the second optical cavities of the gas gauge level sensors in the array are fluidly connected to a common reference pressure.

A method comprising the steps of:

• determining a height map of the substrate using an optical pressure sensor to compare a pressure of a first gas flow towards the substrate with a reference pressure, the optical pressure sensor comprising a first optical cavity fluidly connected to the measurement channel and a second optical cavity fluidly connected to the reference pressure, the first optical cavity being configured to output first reflected electromagnetic radiation and the second optical cavity being configured to output second reflected electromagnetic radiation, the optical pressure sensor being further configured to combine the first reflected electromagnetic radiation with the second reflected electromagnetic radiation and determine, based on the combined first reflected electromagnetic radiation and the second reflected electromagnetic radiation, a pressure difference between the pressure of the first gas flow and the reference pressure and determine, based on the pressure difference, a distance between the measurement outlet and the substrate, the height map being based at least in part on the determined distance between the measurement outlet and the substrate;
• and projecting a patterned beam of radiation onto a substrate based at least in art on the height map.