POSITION MEASUREMENT SYSTEM, INTERFEROMETER AND LITHOGRAPHIC APPARATUS

摘要

There is provided a position measurement system (200) comprising an object (202) having a reflective surface (204), and an interferometer (206) for determining a position of the object. The reflective surface has a first area (210a), a second area (210b) and a third area (210c). The interferometer is arranged to generate a first signal representative of the position by irradiating the first area. The interferometer is arranged to generate a second signal representative of the position by irradiating the second area. The interferometer is arranged to generate a third signal representative of the position by irradiating the third area. Along a line (212), the first area and the second area are at a first distance (214a) relative to each other. Along the line, the second area and the third area are at a second distance (214b) relative to each other. Along the line, the first area and the third area are at a third distance (214c) relative to each other. The interferometer is arranged to provide a rotation signal representative of a rotation of the object along an axis based on the first signal, the second signal and the third signal. The axis is parallel to the reflective surface and perpendicular to the line.