1. Field of the Invention
This invention relates to lithographic apparatus and methods.
2. Summary of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacturing of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., including part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. 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 the beam of radiation in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Photolithography is widely recognized as one of the key steps in the manufacturing of ICs and other devices and products with small features. However, as the dimensions of features become smaller, photolithography is becoming one of the most, if not the most, critical gating factors for enabling ICs and other devices and products with small features to be manufactured on a massive scale.
Fabrication of these ICs and other devices and products with small features involves the control of space tolerances between such small features, e.g., contact holes and interconnecting lines, as well as the control of the size of these features. The smallest space between two features and/or the smallest width of a feature such as, for example, a contact hole or an interconnecting line, is referred to as the critical dimension or CD. For an array of features, a pitch P of the periodicity may also be defined. The pitch refers to the mutual distance between two corresponding points of two substantially identical, neighboring features.
In order to control the critical dimension of these features during manufacturing, several lithographic responses may be used. These responses generally include the depth of focus (DOF), the exposure latitude (EL), the dense isolated bias (DIB), and the mask error enhancement factor (MEEF). The depth of focus is generally viewed as one of the most critical factors in determining the resolution of the lithographic apparatus. It is defined as the distance along the optical axis over which the image of the pattern is adequately sharp. The mathematical representation of DOF is:
                    DOF        =                              +                          /                              -                                  k                  2                                                              *                      λ                          NA              2                                                          (        1        )            where k2 is an empirical constant, λ is the wavelength of radiation user, and NA is the numerical aperture of the projection system used to make the feature. The exposure latitude describes the percentage dose range wherein the printed pattern's critical dimension (CD) is acceptable, typically 10%. It is used in combination with the DOF to determine the process window, i.e., the regions of focus and exposure that keep the final resist profile within prescribed specifications. As for the DIB, it is a measure of the size difference between similar features, depending on the pattern density. Finally, the MEEF describes how reticle CD errors are transmitted into substrate CD errors. This parameter corresponds to the incremental change in the final feature size on the substrate per unit change in the corresponding pattern feature size (where the pattern dimension is scaled to substrate size by the reduction ratio of the lithographic apparatus). Near the resolution limit of a lithographic apparatus, the MEEF often rises dramatically.
With increasing demands on the number of features per area of die to be printed, there have been tremendous efforts within the industry to lower the CD and the pitch of these features. Typically, the industry has used the Rayleigh criterion to provide a theoretical estimate of the limits of feature printing for a given process. The Rayleigh criterion for resolution CD is shown in equation (2):
                    CD        =                              k            1                    *                      λ            NA                                              (        2        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to image the feature, and k1 is a process dependent adjustment factor, also called the Rayleigh constant. For conventional optical lithography, the ultimate resolution limit of conventional lithographic apparatus is reached at k1=0.5, which corresponds to the state at which only one set of diffracted orders can pass through the projection system. The resolution limit of k1=0.5 stands firm even as exposure wavelengths decrease from 248 nm to 193 nm and then to 157 nm, and numerical aperture increases from 0.5 to 0.75.
The effectiveness of a given lithographic process is generally weighed based on its capability to print arrays of dense features with sufficient latitude. However, any given photolithographic layer may also include small features that are positioned in one or more arrays at a pitch larger than the smallest distance between two features. Therefore, the printing of layers including small features occurring at both minimum pitch, i.e., dense features, and larger pitches, i.e., semi-dense features and isolated features, is of importance. Dense features are commonly known to be separated by a distance that is substantially equal to the target feature dimension, isolated features are commonly known to be separated by a distance that is more than about five times the target dimension, and semi-dense features are spaced apart by a distance ranging between about one and about five times the target feature dimension.
Thus, the printing of features arranged in the full pitch range may be complicated because the requirements for printing dense features generally differ from those for printing isolated features. Finding process conditions that simultaneously satisfy high depth of focus, low mask error enhancement factor, low sidelobe printing, and good pattern fidelity for dense, semi-dense and isolated features may be difficult and may become even more difficult as k1 decreases below 0.4.