1. Field of the Invention
The present invention relates to a lithographic apparatus, beam delivery systems, prisms and to device manufacturing methods.
2. Description of the Related Art
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 that instance, 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. comprising 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. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and 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.
A lithographic apparatus requires a high-intensity, narrowband light source. Early lithographic apparatus used high-pressure mercury vapor lamps and filters to select a single emission line. However, the drive to fabricate components with ever smaller feature sizes has necessitated the use of shorter wavelengths. Thus excimer lasers, e.g. with outputs at 248, 193 or 157 nm, are now commonly used. Such a laser is a large device with a high power consumption so is commonly located outside of the clean room containing the lithographic apparatus with the laser beam being “piped” to the lithographic apparatus by a beam delivery system. This is possible because the laser beam is well collimated and has a small cross-section. A beam delivery system may involve multiple, for example, at least two and typically 3 to 6, reflections of the beam by high-reflectivity mirrors oriented at 45° to the beam axis.
Recently, improvements of the resolution of lithography apparatus have been made by control of the polarization of the exposure radiation. Although the output of an excimer laser has a high degree of polarization, this is often reduced by the beam delivery system, which may involve multiple mirrors in the p-mode, so that a clean-up polarizer must be provided in the illumination system of the lithographic apparatus to achieve a desired high degree of polarization, e.g. >93%. The clean-up polarizer unavoidably reduces the intensity of the beam of radiation, reducing throughput.