Determination of the Optical Properties of a Pellicle and Their Effects on High NA Lithography

Abstract

As the critical dimension in memory devices shrinks, various resolution-enhancement techniques are developed in optical lithography. Also, besides shortening wavelength, high numerical aperture NA lens systems and immersion techniques are employed. In these cases, the angle of incidence on the photomask side of the imaging lens increases. Thus, a non-normally transmitted or diffracted beam by the photomask and the underlying pellicle needs to be considered for optimized imaging. That is, non-normal transmission introduces an intensity variation over the incidence angle and a modification of the polarization states of exposure beam. In order to integrate these effects into lithography, we need to characterize the optical properties of the pellicle very well. In this study, we introduce a new method to obtain the optical constants of a thin pellicle membrane by using double transmission ellipsometry. In this method, the ellipsometric spectra are obtained from the transmission through the pellicle by using reflection from a reference sample in a conventional ellipsometry mode. Due to the double transmission and the high intensity signal, this method shows a higher sensitivity than conventional reflection ellipsometry.