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 so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, 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.
Many semiconductor fabrication plants, known as “fabs”, use a variety of different lithographic apparatus. The operators of such a fab will often desire to be able to print the same pattern using two different apparatus and obtain consistent results. Where the different apparatus are the same or similar models, straightforward calibration processes to a common reference can match the apparatus sufficiently closely to enable consistent results. However, where the apparatus are different models, or made by different manufacturers, such matching is difficult and may require many trial exposures for each pattern.
The most important parameter of an exposure that must be matched between different apparatus is the critical dimension (CD) as a function of pitch. Since a given pattern may include areas of several different pitches, it is necessary to ensure that each apparatus prints the same CD over a range of pitches, rather than just at a single pitch value. Conventionally, the coherence of the illumination of the mask has been adjusted to attempt to match CD vs. pitch functions for two different apparatus but this often does not provide a sufficiently close match.