Design considerations for multilayer-coated Schwarzchild objectives for the XUV

Abstract

ABSTRACT The performance of multilayer—coated Schwarzschild objectives, and similar nearnormal incidence soft xray focussing systems, can be affected by the changing anglesof incidence for different rays and possible variations in multilayer period acrossthe curved mirror surfaces. A design analysis which considers these issues ispresented, using as an example a 20 times demagnifying Schwarzschild objective coatedwith molybdenum/silicon multilayers for operation at roughly 76 eV. The largebandwidth of these multi].ayers eases requirements on control of the variation of thed—epacing for the system considered. Implications for extension to similar systemsoperating with different magnifications, sizes, and photon energies are discussed. 1. INTRODUCTION The advent of multilayer reflective coatings for extreme ultraviolet to x—ray (XUV)wavelengths in the last decade has extended the spectral range of significant near—normal incidence reflectance into these regions. Especially exciting among the manyapplications made possible by these nanometer—scale interference structures are thosein which multilayers are applied to curved optics for focussing. Such applicationsinclude, for example, experiments utilizing a single curved multilayer mirror forimaging1'2'3 and for gain enhancement in multipass x—ray laser experiments.4'5Compound multilayer-coated focusing optics, such as Schwarzschild objectives, arebeginning to be utilized in microscopy6'7'8'9 and astronomy, and these and similaroptics are being investigated for use in soft x-ray projection lithography.10'11'12It is this later class of compound multilayer-coated, near—normal incidence opticsthat are the subject of this paper.This paper investigates issues relating the areal distribution of multilayer periodon the curved surfaces of compound reflective optics to the performance of theseoptics. We present an analysis combining the geometrical optical considerations ofSchwarzschild objectives with the physical optical considerations relating tomultilayer reflective structures and the deposition system dependent considerationswhich affect the variation of multilayer period on curved surfaces. Discussion isbased on experience gained in the course of the design and coating of severaldifferent Schwarzschild objectives (SO) for soft x-ray magnifying and demagnifyingapplications. To illustrate the analysis we examine a 20X reduction system whichdemagnifies a beam from an undulator source at the Synchrotron Radiation Center(University of Wisconsin, Madison) to form a sub—micron spot primarily for use inphotoelectron spectro—microscopy studies of various surfaces. Some details thisproject, called MAXIMUM, can be found in ref. 8.