A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a scanning type lithographic apparatus, it is desirable to control the position of the patterning device and the substrate with high accuracy. To this end, it is desirable to determine the actual position in six degrees of freedom with high accuracy. Interferometer or encoder type position measurement systems might be use to perform these measurements.
An encoder type position measurement system may include an encoder head mounted on the movable object and a reference target mounted on a reference object. The reference target includes a grid or grating arranged on a planar surface. A measurement beam of the encoder head is directed at right angles on the planar surface. 1st and −1st order reflections of the measurement beam on the grating are combined in the encoder head. On the basis of intensity difference in this combined signal a movement of the movable object with respect to the reference object in a direction substantially perpendicular to the measurement beam can be determined.
U.S. Pat. No. 7,573,581, the contents of which is herein incorporated by reference in its entirety, discloses a two dimensional (2D) encoder head capable of measuring a position of an object in two directions, i.e. movements substantially parallel to the planar surface of the reference object and movements substantially perpendicular to the planar surface of the reference object. In the encoder head, a single light beam is split in two measurement beams directed at opposite angles to the grid plate. The +1st and −1st order of diffraction reflected beams are brought back together to form a combined signal. On the basis of intensity difference in this combined signal which is measured by detectors arranged in the encoder head, signals representative for position changes of the movable object with respect to the reference object may be determined. These signals may contain information with respect to movements of the movable object substantially parallel to the planar surface of the reference target, but also with respect to movements substantially perpendicular to the planar surface of the reference target.
Since the measurement beams are directed at opposite angles, i.e. mirrored with respect to a main axis of the encoder head, the components of the signals substantially parallel and substantially perpendicular to the planar surface of the reference target may be calculated by subtracting and adding the measurement signals.
US 2003/0169434 discloses another embodiment of an encoder-type displacement detection apparatus, including a light emitting/receiving unit, which includes a light source for emitting a light, an optical splitting unit for splitting returning lights into a plurality of lights, the light from the light source going to and returning from an external optical system to be the returning lights, a polarizing unit for increasing and decreasing quantity of lights transmitted therefrom corresponding to polarization state of the returning lights, and an optical receiving unit which has a plurality of photo-detecting elements for detecting the lights transmitted through the polarizing unit. The light source, the optical splitting unit, the polarizing unit, and the optical receiving unit are unitedly integrated into one unit.