1. Field of the Invention
The invention relates to a method for multiple exposure at least of one substrate coated with a photosensitive layer, a microlithography projection exposure installation for multiple exposure at least of one substrate coated with a photosensitive layer, and a projection system having an illumination system and a projection objective.
2. Description of the Related Art
The efficiency of projection exposure installations for the microlithographic fabrication of semiconductor components and other finely patterned devices is substantially determined by the imaging properties of the projection objectives. Moreover, the image quality and the wafer throughput that can be achieved with the installation are influenced substantially by properties of the illumination system arranged upstream of the projection objective. Said system must be capable of preparing the light of a primary light source, for example a laser, with the highest possible efficiency, and in the process of producing as uniform as possible a distribution of intensity in an illumination field of the illumination system.
Depending on the nature and size of the patterns to be produced on the wafer, suitable exposure parameters can be set on the illumination system and/or the projection objective. For example, conventional illumination with different degrees of coherence and annular field illumination or polar illumination can be set on the illumination system in order to produce an off-axis, oblique illumination. The numerical aperture can be set on the projection objective.
Given a prescribed wavelength of the primary light source, the selection of suitable exposure parameters can serve, inter alia, for imaging structures that it would not be possible to image with satisfactory quality by using other exposure parameters because of their small structural sizes. However, a long exposure time is frequently associated with a selection of exposure parameters for which such fine structures can be resolved, and so the wafer throughput turns out to be low. Such fine structures frequently cannot be correctly resolved given a selection of exposure parameters for which a higher wafer throughput is achieved, although structures with larger structural sizes can be.
Since the structures to be produced on the wafer can frequently be subdivided into fine and coarse structures, it can be favorable to carry out a double exposure of the wafer for which a first set of exposure parameters is used for imaging the coarse structures, while a second set of exposure parameters different from the first one is used for imaging the fine structures. The exposure parameters of the first set can be selected, for example, such that only a short exposure time is required for imaging the coarse structures. The exposure parameters of the second set can be optimized such that only those structures are imaged that are so fine that they cannot be imaged with the first set of exposure parameters. Of course, multiple exposures with more than two exposures are also possible.
In a known type of double exposure, an exposure by means of an amplitude mask is carried out with a first set of exposure parameters. It is possible in this way to make use of oblique illumination such as, for example, annular, dipole or quadrupole illumination in order to increase the resolution. A second exposure with a second set of exposure parameters is carried out with the aid of a phase mask. A coherent illumination with a low degree of coherence σ is normally set thereby at the illumination system. Such a method is described, for example, in the article entitled “Improving Resolution in Photolithography with a Phase-Shifting Mask” by M. D. Levenson, N. S. Viswanathan, R. A. Simson in IEEE Trans. Electr. Dev., ED-29(12), pp. 1828-1836, 1982, and in the article entitled “Performance Optimization of the Double-Exposure” by G. N. Vandenberghe, F. Driessen, P. J. van Adrichem, K. G. Ronse, J. Li, L. Karlaklin in Proc. of the SPIE, Vol. 4562, pp. 394-405, 2002. In the method for multiple exposure that is described there, a first set of exposure parameters is initially set on the projection system for the first exposure. Thereafter, a second set of exposure parameters is set on the projection system for the second exposure, and this requires a reconfiguration of the projection system. This reconfiguration of the projection system from the first to the second set of exposure parameters results in a time loss and in mechanical wear of the parts whose position and/or shape need to be varied in the reconfiguration.