Resist-based measurement of the contrast transfer function in a 0.3 numerical aperture extreme ultraviolet microfield optic

Abstract

In order to meet the high-resolution printing potential of extreme ultraviolet (EUV) lithography, the projection optics must be of very high quality. The contrast transfer function (CTF), a measure of the aerial-image contrast as a function of pitch, describes one key aspect of projection optic quality. In order to support research into EUV lithography, a static microfield exposure tool (MET) based on a 0.3 numerical aperture optic and operating at a wavelength of 13.5nm has been developed at the Advanced Light Source, a synchrotron facility at the Lawrence Berkeley National Laboratory. This work presents the results of resist-based measurements of the CTF for the MET optic. Although the resist is not an ideal aerial-image detector due to its nonlinear response, it is still possible to study some key characteristics of the optics using such methods. These measurements are based on line/space patterns printed in several different EUV photoresists. The experimental CTF results are compared with the CTF from aerial-image simulations including the aberrations measured in the projection optic using interferometric and lithographic techniques. The measured CTF values are found to be significantly lower than predicted from the aerial-image simulations. CTF measurements are presented for both bright-field and dark-field mask patterns in order to investigate the effect of flare. Finally, the orientation dependence of the CTF is studied in order to evaluate the effect of nonrotationally symmetric lens aberrations. Neither flare nor nonrotationally symmetric aberrations were found to have a significant effect on the measured CTF. Photoresist resolution is believed to be the limiting factor in the observed contrast transfer function. These measurements, taken as a whole, provide valuable information about the imaging performance of the MET optic and aid in interpreting the results of other experiments performed using the MET and similar systems.