There is a need for lithographic technologies that can produce features on semiconductors that are below half a micron in dimension. Currently, there are three competing technologies that may be used to accomplish this: X-ray, deep-uv (ultraviolet), and phase-shifting masks. Compared to the conventional lithography for semiconductor processing, the X-ray and deep-uv technologies are conceptually distinct and practically in need of new steppers, elements, resists, etc. In addition to escalating costs, these technologies are often plagued with reliability issues and, in particular, with respect to lasers and synchrotrons as the light source.
Phase-shifting techniques show considerable promise for extending the submicron performance of the state-of-the-art optical lithographic tools. The prospect of avoiding the costs and complexity of developing new lithographic processes and new capital equipment has in recent years provided great incentives for investigation into phase-shifting techniques. In phase-shifting technology, a layer of appropriate width, thickness, and shape is added to each element of a conventional transmission mask. The phase shift caused by this layer, as well as the subsequent interference between the phase-shifted light and the nonphase-shifted light transmitted through the respective parts of the mask for each element, greatly improves the contrast of the projected image of that element. Phase shifting results in enhanced optical resolution which allows smaller elements to be projected. However, one or more appropriately shaped phase-shifting components has to be added for every element to be projected. For example, to produce a 64-megabit DRAM, tens of millions of phase-shifting elements have to be incorporated in the phase-shifting masks used to produce this device. Being extremely complex, these masks are difficult to design, fabricate, inspect, and repair. For general references on conventional phase-shifting techniques see, for example, Levenson et al., "Improving Resolution in Photolithography with a Phase-shifting Mask", IEEE Transactions On Electron Devices, Vol. ED-29, No. 12, pp. 1828-1836, December 1982; and Levenson, "Phase-shifting Mask Strategies: Line-space Patterns", Microlithographic World, Vol. 1, No. 4, pp. 6-12, September/October 1992, which references are incorporated herein by reference.
An example of a prior art lithography system is disclosed in U.S. Pat. No. 4,947,413. This patent discloses a lithography system capable of doubling the spatial frequency resolution associated with conventional systems. An aperture filter is positioned to intercept the Fourier transform of the mask being exposed. The filter is configured to block certain orders of the diffraction pattern from reaching the wafer. The remaining orders reaching the wafer will produce a cosine-type interference pattern with a period half of the period. There still is a need in the art for a lithographic imaging system which produces enhanced images of intricate patterns.
It is an object of the present invention to provide a lithographic imaging system which produces enhanced images.
Other objects and advantages will be apparent from the following disclosure.