Patent Number: 054882290
Section: summary

This invention relates to high resolution deep ultraviolet photolithography systems for exposing material surfaces to a high energy deep ultraviolet radiation. The processing of very high density integrated circuits requires high resolution systems capable of resolving and exposing portions of the circuit substrate, with the use of a mask in the optical path. In the process of making advanced integrated circuits, it is conventional to transfer a circuit pattern image from a mask or reticle onto a layer of photosensitive material called photoresist. The wavelengths used in optical lithography onto silicon wafers have gone from 436 nanometers (G line) to 365 nanometers (I line) using mercury lamp sources. Future wavelengths include 248 nanometers from a krypton fluoride excimer laser (or 250 nanometers from an intense mercury lamp) 213 nanometers from a solid state (5xYAG) laser and 193 nanometer from an argon fluoride excimer laser. In accordance with one aspect of the invention, there is provided a high resolution, deep UV photolithography system for exposing a surface, directly or through a mask, the photolithography system including a deep UV radiation source for generating a beam of narrow wavelength deep ultraviolet radiation along a path, a first optical system in the path for homogenizing and shaping the deep UV energy in the path; mask receiving structure in said path; and a second optical system in the path for directing radiation energy onto the surface of a substrate to be processed, the second optical system including large area mirror structure having a numerical aperture of at least 0.3 and a plurality of refractive elements disposed between said mask receiving structure and said substrate for compensating (reducing) image curvature introduced into the system by the large area mirror structure. Preferably, the deep UV radiation source is an excimer or other laser of wavelength in the 150-250 nanometer range (although it will be appreciated that other types of deep UV sources such as gas fill lamps and microwave energized sources may be employed in particular embodiments). The large area mirror structure includes first mirror structure that is disposed on the system axis, and second mirror structure of concave spherical configuration with an aperture portion disposed on the system axis for passing a beam of radiation to the first mirror structure. In a preferred embodiment, the optical compensation assembly includes a distributed array of refractive elements designed for a particular wavelength. The system provides a microlithography system with a numerical aperture (N.A.) of at least 0.3 with a pair of reflective elements and a group of refractive elements with substantial positive power (which flattens the field distorted by the spherical mirror). The system preferably allows the operator to view the imaging process at a visible 633 nanometer wavelength while the substrate processing operative at a wavelength such as 193 nanometers or 248 nanometers is in progress. In accordance with one aspect of the invention, the optical system relies principally on mirrors to transmit the integrated circuit pattern information from the mask or reticle to the photoresist coated silicon wafer or other substrate with refractive elements (all made of the same refractive material--fused silica) to compensate for field curvature produced by the mirror structure, as well as other aberrations introduced into the system. The invention provides improved performance in the form of a very flat field, necessary for microlithography, as well as giving a high numerical aperture (or fast speed of the lens) of 0.6 N.A. The correcting refractive elements have substantial positive power (which flattens the field). The image formed by the two mirrors thus is essentially completely flattened just before it reaches the photoresist coated wafer where the aerial image is translated into a latent image in the photoresist coating. Having strong lens power would normally cause substantial chromatic (or color) variations in aberrations, as well as both longitudinal chromatic focus shift (each change in wavelength or color would have a corresponding change in the location of focus of the image). In a particular embodiment, such aberrations are corrected and a broad deep ultraviolet spectral region can be imaged with diffraction-limited performance, by using only fused silica lenses. Normally, color correction is made by using two different types of glass such as flint glass and crown glass, each with different refraction characteristics. Another way to correct for color aberrations in the deep ultraviolet is to use calcium fluoride or magnesium fluoride lens elements. These types of elements have problems of poor homogeneity (material inconsistency), cannot be easily polished or fabricated (not hard enough) into high quality surfaces, and add complexity to the design by adding several additional optical elements. Color correction is achieved in a particular embodiment by using shaped refractive elements, and all the image correcting (refractive) elements are made of the same material (fused silica). Conventional deep-UV lenses are of the all-refractive type (no mirrors). They require that the light source be highly spectrally narrowed, down to less than one picometer. The result of this spectral narrowing is loss of 80-90 percent or more of the light energy. Such `line-narrowed` laser exposure systems, currently experimental for 248 nanometers and 193 nanometers lithography, have very low wafer exposure throughput, a key parameter in IC manufacturing economics. The mirror-based catadioptric embodiment of the invention allows for unnarrowed laser light, and uses most of the available power.