Position measurement unit, measurement system and lithographic apparatus comprising such position measurement unit

A measurement system to determine a position of an object in six degrees of freedom, the measurement system comprising three measurement units, each of the three measurement units configured to determine a position in a first and a second dimension and including:

• a diffraction type encoder to determine with a diffraction of an encoder measurement beam on a first and a second diffraction grating the position in the first dimension of the second grating with respect to the first grating,
• an interferometer to determine with an interference between an interferometer measurement beam following a reference path and a measurement path respectively, the position in the second dimension of a mirror in the measurement path, and
• a combined optical unit to transfer the encoder measurement beam as well as the interferometer measurement beam.

A lithographic apparatus comprising a measurement unit. the measurement unit configured to determine a position in a first and a second dimension and including:

• a diffraction type encoder to determine with a diffraction of an encoder measurement beam on a first and a second diffraction grating the position in the first dimension of the second grating with respect to the first grating,
• an interferometer to determine with an interference between an interferometer measurement beam following a reference path and a measurement path respectively, the position in the second dimension of a minor in the measurement path, and
• a combined optical unit to transfer the encoder measurement beam as well as the interferometer measurement beam, the measurement unit being connected to a substrate table to measure a position of the substrate table.

A lithographic apparatus comprising a measurement unit, the measurement unit conflaured to determine a position in a first and a second dimension and including:

• a diffraction type encoder to determine with a diffraction of an encoder measurement beam on a first and a second diffraction aratina the position in the first dimension of the second grating with respect to the first grating,
• an interferometer to determine with an interference between an interferometer measurement beam following a reference path and a measurement path respectively, the position in the second dimension of a mirror in the measurement path, and
• a combined optical unit to transfer the encoder measurement beam as well as the interferometer measurement beam, the measurement unit being connected to a mask table to measure a position of the mask table.