1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means can comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
It will be appreciated that, whether or not a lithographic apparatus operates in stepping or scanning mode, it is desired that the patterned beam or beams is/are directed onto the appropriate target portion of the substrate surface. In many circumstances, multi-layer structures are built up on the surface of the substrate as a result of a sequence of lithographic processing steps. It is of course desired that the successive layers formed in the substrate are correctly in register with each other. Thus, great care is taken to ensure that the position of the substrate relative to the beam projection system is accurately known.
Various techniques are used to determine the position of a substrate relative to the beam projection system. These generally rely upon the substrate having formed upon it alignment marks that are arranged around the periphery of areas of the substrate onto which active circuit components or the like are to be formed. These marks are located to provide reference points relative to which the position of target portions on the substrate are determined. The alignment marks can be detected optically using the beam projection system, which is also used to project patterns onto the substrate. Such a “through the lens” or TTL approach to the problem of locating alignment marks allows for the position measurement location to be the same as the image formation location. Thus, “Abbe” errors are minimized. In other systems, the alignment mark detectors and the main beam projection system have different optical axes, in which case some means can be provided for compensating for relative movement of these axes.
In an example of a scanning-based system, as the frame supporting the beam projection system and the alignment mark detectors is scanned across the surface of a substrate by moving the substrate, the position of the substrate in the direction perpendicular to the scan direction and the scan speed is adjusted in dependence upon the measured positions of the alignment marks. Alternatively, or additionally, the digital image to be projected can be adjusted. In the case where the patterning means comprises an array of individually controllable elements, this can involve translating or otherwise adjusting the digital pattern applied to the array. The height of the substrate can also be controlled using some type of level sensor arrangement.
Typically, a lithographic apparatus of a given production facility is designed (or configured in a relatively inflexible way) for use with substrates of a fixed size. In the case of flat panel displays or color filter plates, the substrate dimensions can be of the order of several meters, with multiple panels being formed on each substrate. The alignment marks are provided around the periphery of the substrate, as well as between panels. However, there is a desire to introduce flexibility in the layout of panels on the substrates, such that a given production facility can be used to produce panels with different dimensions. This has meant that, for non-standard panel layouts, either only alignment marks around the periphery of the substrate can be used (as the periphery is the only “blank” region common to all substrate layouts), or some manual realignment of the detectors must be carried out While a continuous array of alignment mark detectors positioned across the substrate to detect alignment marks in various intermediate positions can be desirable, this is not practical due to the very high resolution required and the number of detectors that that would necessitate.
A number of factors, for example thermal effects, can cause local variations in the shape of a substrate. It is therefore desirable to provide alignment marks at relatively small intervals across the surface of the substrate. However, the approach described in the previous paragraph mitigates against this as the entire central area of the substrate is effectively unusable for the placement of alignment marks.
Therefore, what is needed is a lithographic apparatus and method that allow for the flexible positioning of alignment marks within the central area of a substrate to be patterned.