Patent ID: 8947632
Filing Date: 2015-02-03
Classification: G03F

Abstract:
1. A lithographic apparatus comprising: a patterning subsystem for transferring a pattern from a patterning device onto a substrate, the patterning subsystem being controlled in accordance with measurements of level variations across a surface of the substrate; a level sensor for projecting a patterned level sensing beam of radiation to reflect from a location on the substrate surface and for detecting the reflected sensing beam to record a height of the surface level at said location, wherein said level sensor incorporates at least one moving optical element and whereby said level sensor is arranged for optically scanning the substrate surface by the level sensing beam in at least one dimension to obtain measurements of the height of the surface level at different locations without corresponding mechanical movement between the level sensor and the substrate; and a focus control arrangement for maintaining focus of said patterned beam on said substrate during said level sensing beam scan as an indication of focus error; wherein said focus control arrangement is responsive to height variations measured during at least one previous sensing beam scan as a predictor of focus error in a current level sensing scan of the level sensing beam of radiation.