DOI: 10.1007/s00340-006-2405-9 Appl. Phys. B 85, 603–610 (2006) Lasers and Optics Applied Physics B s. bollanti p. di lazzaro f. ﬂora l. mezi d. murra a. torre ✉ Conventional and modiﬁed Schwarzschild objective for EUV lithography: design relations ENEA-UTS Tecnologie Fisiche Avanzate, via E. Fermi 45, 00044 Frascati (Rome), Italy Received: 2 March 2006/Revised version: 5 July 2006 Published online: 16 August 2006 • © Springer-Verlag 2006 ABSTRACT The design criteria of a Schwarzschild-type opti- cal system are reviewed in relation to its use as an imaging system in an extreme ultraviolet lithography setup. Both the conventional and the modiﬁed reductor imaging conﬁgurations are considered, and the respective performances, as far as the geometrical resolution in the image plane is concerned, are com- pared. In this connection, a formal relation deﬁning the modiﬁed conﬁguration is elaborated, reﬁning a rather na¨ ıve deﬁnition presented in an earlier work. The dependence of the geometri- cal resolution on the image-space numerical aperture for a given magniﬁcation is investigated in detail for both conﬁgurations. So, the advantages of the modiﬁed conﬁguration with respect to the conventional one are clearly evidenced. The results of a semi-analytical procedure are compared with those obtained from a numerical simulation performed by an optical design program. The Schwarzschild objective based system under im- plementation at the ENEA Frascati Center within the context of the Italian FIRB project for EUV lithography has been used as a model. Best-ﬁt functions accounting for the behaviour of the system parameters vs. the numerical aperture are reported; they can be a useful guide for the design of Schwarzschild objective type optical systems. PACS 42.15.Eq; 42.82.Cr 1 Introduction Extreme ultraviolet lithography (EUVL) is an ex- tension of the conventional optical lithography that has been used for decades in integrated circuit manufacturing to trans- fer images from a mask onto a semiconductor wafer [1–3]. EUVL is presently considered one of the most promising tech- nologies to support a smaller than 45-nm-resolution printing. It potentially extends optical lithography by using light from the EUV region of the spectrum (10–15 nm wavelength), and hence less than one-tenth the wavelength of the optical al- ternatives. Evidently, working in the EUV spectrum poses challenges, basically due to the fact that the properties of ma- terials in the EUV are very different from their properties in the visible and UV ranges. In fact, since EUV radiation ✉ Fax: +39-06-94005334, E-mail: torre@frascati.enea.it is absorbed by all materials, including nitrogen and oxygen, a system for EUVL must be enclosed in a vacuum chamber. In addition, EUV imaging cannot use refractive optical elements like lenses and transmission masks, entirely reﬂective optical elements being required; in particular, EUV masks are de- signed as high-reﬂectance low-defectivity multilayer-coated substrates with patterned absorbing layers instead of mem- branes as in the conventional optical lithography. The Italian FIRB (Fondo Investimenti per la Ricerca di Base) project on EUVL [4, 5] is aimed at developing the Ital- ian know-how in all aspects of EUVL, i.e. EUV sources, debris mitigation, collector mirrors, multilayer Mo/Si mir- rors, masks, high-resolution optics design and high-precision alignment. All this research should materialize into the real- ization of a microlithography prototype capable of imaging patterns on the mask onto the silicon wafer down to a reso- lution below 100 nm. The optical system speciﬁcally devoted to the imaging of the mask pattern onto the wafer consists of a Schwarzschild objective (SO) based conﬁguration. In particular, the projection optics under implementation at the ENEA Center of Frascati will employ a modiﬁed SO setup, whose design criteria are illustrated below. Due to its simple conﬁguration, consisting of two mirrors only, and its good geometrical aberration characteristics on a limited ﬁeld size, the SO is widely used for EUVL experi- ments and investigations [6, 7]. In order to extend the good performance characteristics of the SO to a larger exposure ﬁeld, modiﬁed conﬁgurations have been proposed. Indeed, in [6] a non-concentric conﬁguration has been proposed and the relative performances have been accurately investigated in regard to both the geometrical resolution and the various aberrations (spherical, coma, astigmatism, ﬁeld curvature and distorsion). Alternatively, the conﬁguration suggested in [7] preserves the concentric position of the two mirrors whilst placing the object and the image planes at positions differ- ing from those conforming to the standard conﬁguration. Such positions are properly chosen in order to minimize the image size for a point-like object. Here, reﬁning the analysis pre- sented in [7], a formal relation deﬁning the modiﬁed SO (MSO) conﬁguration is elaborated, which parallels that rela- tive to the conventional one, while addressing image-plane resolution instead of longitudinal aberration related require- ments. The dependence of the geometrical resolution on the image-space numerical aperture for a given magniﬁcation is