1. Field of the Invention
The invention relates to a projection exposure method for exposing a radiation-sensitive substrate, arranged in the region of an image surface of a projection objective, with at least one image of a pattern of a mask arranged in the region of an object surface of the projection objective, and to a projection exposure system suitable for carrying out the method.
2. Description of the Related Art
Microlithographic projection exposure methods are currently used for fabricating integrated semiconductor devices and other finely structured components. Use is made of masks (reticles) that bear the pattern of a structure to be imaged, for example a line pattern of a layer of a semi-conductor component. A mask is positioned in a projection exposure system between an illumination system and projection objective in the region of the object surface of the projection objective, and illuminated with the aid of an illumination radiation provided by the illumination system on the mask. The radiation varied by the mask and the pattern runs as projection radiation through the projection objective, which images the pattern of the mask onto the substrate to be exposed, which normally bears a radiation-sensitive layer (photoresist).
Improving the spatial resolution of the projected images of masks having increasingly fine patterns is possible by both increasing the image-side numerical aperture (NA) of the projection objective and employing radiation having short wavelength. Currently, ultraviolet radiation having wavelength less than about 260 nm is frequently used. Many projection exposure systems using either refractive or catadioptric projection objectives use primary radiation provided by ArF excimer lasers having a center wavelength λ˜193 nm.
The optical performance of projection systems including refractive optical elements, such as lenses, is predominantly influenced by the refractive index difference occurring at optical interfaces, such as a lens surface adjacent to a gas filled space. Other characteristic design elements of an optical system, such as radia of optical surfaces, distances between optical surfaces, aspherical shape of optical surfaces etc. are optimized for a specific set of design parameters. Each variation of refractive index differences leads to additional imaging aberrations.
If one optical material adjacent to an optical surface is a gas, then pressure changes may lead to significant variations in the refractive index differences at optical surfaces since the density of the gas and, along therewith, the refractive index of the gas changes with gas pressure. Likewise, temperature variations will influence the refractive index differences. While pressure changes predominantly affect the gaseous material in an optical train, temperature changes normally affect gases as well as solid transparent materials, such as glass or crystalline fluorides, or the optical properties of optical liquids, such as immersion liquids optionally used in an optical train.
The refractive index difference at optical interfaces is also influenced by the wavelength of the radiation used. The optical dispersion dn/dλ responsible for this effect is usually larger in solid materials and in liquids than in gaseous materials. For example, the dispersion dn/dλ near λ=193.368 nm is about −1.577×10−6 pm−1 for fused silica, about −0.989×10−6 pm−1 for crystalline calcium fluoride and −2.139×10−6 pm−1 for water (sometimes used as immersion liquid). Due to dispersion, slight changes of the center wavelength of radiation causes discernible refractive index variations within an optical system.
The U.S. Pat. No. 5,838,426 discloses a projection exposure system and a method where the wavelength of illumination radiation provided by a KrF excimer laser emitting a center wavelength λ=248.4 nm is changed in accordance with pressure changes occurring around the projection exposure system.