Patent ID: 6894261

Claim:
A lithographic projection apparatus comprising: a projection beam illumination system which supplies a projection beam of radiation; a first object table for holding a protection beam patterning device which patterns the projection beam according to a desired pattern; a second object table for holding a substrate; and a projection system which images the patterned beam onto a target portion of the substrate; a reference frame; and a position measuring device comprising: a radiation source mounted on said reference frame; a two-dimensional radiation detector mounted in a fixed position on said reference frame; and a mirroring device mounted on one of said object tables that is moveable relative to said reference frame so as to reflect radiation emitted by said radiation source toward said radiation detector, wherein the radiation source and the two-dimensional radiation detector are mounted to the reference frame so that a radiation beam from the radiation source is incident at a predetermined angle relative to the movable object table and the predetermined angle is 45°.