Effects of mirror surface roughness on exposure field uniformity in synchrotron x-ray lithography

Abstract

In this paper we discuss the surface roughness tolerances for mirrors used in synchrotron x-ray lithography beamlines. These requirements are different for lithography beamlines than they are for traditional beamlines, which are imaging systems. For imaging optical systems it is desirable to minimize the surface roughness of the mirror in order to reduce the intensity of the scattered light. Proximity print x-ray lithography requires nonimaging optical systems designed to deliver a uniform power distribution at the exposure field. In such applications, the scattering of x rays is less important, provided that the total flux is not too severely degraded. From this point of view it is important to notice that scattering removes power from the central, coherent, beam and redistributes it over a wider angle. The loss in reflectivity is only apparent because energy is conserved in the process. Furthermore, the presence of scattered x rays can be used to improve the uniformity of the power distribution. The requirement on the surface quality of mirrors used for scanning x-ray illumination is thus relaxed with respect to that of typical imaging systems, providing considerable cost savings and simplifying manufacturing. The effect of the scattered light on the uniformity of illumination is dependent on intensity and angular distribution of the scattering. Experimental results will be compared to the results predicted by calculations.