Extreme ultraviolet (EUV) rays can be used for EUV lithography (EUVL), EUV spectroscopy, and EUV microscopy. Of particular interest is the EUV lithography at wavelengths near 13.5 nm for the manufacture of the next-generation integrated circuits, where objectives can be used to transfer or print de-magnified images of circuit components from a mask onto semi-conductor surfaces with a spatial resolution of better than 45 nm. The optimization of EUV lithography for the manufacture of the next-generation integrated circuits is a subject of intense research in industry and laboratories worldwide (Bollanti, Dilazzaro, et al., 2006; Budano, Flora, et al., 2006; Wu and Kumar, 2007).
EUV lithography poses many challenges. EUV is absorbed by all matter, thus requiring EUV lithography to take place under vacuum. Furthermore, EUV radiation requires multilayer reflectors in order to focus the rays for lithography. Due to these multilayer reflectors, the Bragg condition needs to be satisfied for a given reflector. Moreover, the components of a EUV system that are directly exposed to the EUV light source, which is usually a plasma produced by directing a powerful laser beam onto a solid or liquid target, are vulnerable to damage from the simultaneously emitted high-energy ions and other debris.
Schwarzschild objectives and their modifications are the leading candidates to transfer or print de-magnified images of circuit components from a mask onto semi-conductor surfaces. The conventional Schwarzschild objectives consist of two concentric, concave and convex, spherical mirrors or reflectors and were originally developed for use in astronomy, where the rays are all paraxial. Schwarzschild objectives are therefore not optimized for the non-paraxial rays encountered in lithography, since image distortions, such as spherical aberrations, coma, and astigmatism occur if the rays deviate from near-normal incidence.
For EUV lithography at wavelengths near 13.5 nm, which will be used for the manufacture of the next-generation integrated circuits, the reflectors must be spherical multi-layer structures with a certain distance, d, between the consecutive Bragg reflecting layers of those structures. Therefore, the Bragg condition must be simultaneously satisfied on the reflectors, which comprise the EUV objective. This latter condition is a challenging requirement for all the Schwarzschild designs, which have been considered for lithography at 13.5 nm so far, since with those designs this condition can be satisfied only locally and cannot be met over the entire area of the reflectors. Therefore, an image of the entire object (mask) can only be obtained in multiple exposures by moving both mask and wafer synchronously (scanning) through an EUV beam of a small cross-section.
Attempts have been made to increase the Bragg-reflecting area of the reflectors by laterally grading the d-spacing of the multi-layer structures over the area of the two reflectors (Foltyn, Bergmann, et al., 2004). Aside from the technical challenges in the manufacturing of such multilayer structures, a lateral grading of the d-spacing will also cause additional imaging errors, since the spherical symmetry of the two concentric reflectors is thereby destroyed.
Another problem with the presently used Schwarzschild systems is that even small deviations from normal incidence lead to severe image distortions due to astigmatism and spherical aberration (Bollanti, Dilazzaro, et al., 2006; Budano, Flora, et al., 2006). Attempts to deal with these faults have resulted in an increase in the number of optical elements in the optical chain, which reduce the throughput, and require longer exposure times or more powerful and expensive EUV sources. In this case the lifetime of the EUV source and damage of the collector mirror near the source become significant problems, and can cause costly downtime for cleaning or replacing parts.
U.S. Pat. No. 8,217,353 describes an imaging arrangement applicable to EUV and x-ray imaging where the Bragg condition is satisfied on a pair of matched spherical concave reflectors. This design is, however, impractical for EUV lithography because the optics and object to be imaged would be on opposite sides of the vacuum chamber required for EUVL and EUV imaging. Furthermore, since the image would be projected on a point on yet another end of the chamber, the size of the imaging arrangement would be prohibitively large.
A two-dimensional, stigmatic x-ray imaging system, which consists of two concentric, convex and concave, spherically bent crystals has been proposed (Bitter, Hill, et al., 2012). This x-ray imaging system, which was designed for the x-ray diagnosis of hot plasmas at x-ray energies in the range from 3 to 13 keV or wavelengths in the range from 1 to 4 Å, has the unique property that the Bragg condition is simultaneously fulfilled at each point on two crystal surfaces. However, in this imaging system layouts were only considered for a particular ray pattern, which limited the system to a certain Bragg angle pair and certain de-magnification.