Study of rigorous effects and polarization on phase shifting masks through simulations and in-die phase measurements

Abstract

As lithography mask process moves toward 45nm and 32nm node, phase control is becoming more important than ever. Both attenuated and alternating PSMs (Phase Shift Masks) need precise control of phase as a function of both pitch and target sizes. However conventional interferometer-based phase shift measurements are limited to large CD targets and requires custom designed target in order to function properly, which limits phase measurement. Imaging simulations, both, in a rigorous and a Kirchhoff regime, show the dependency of the phase in the image plane of a microlithography exposure tool on numerical aperture, polarization, and on the so-called balancing of the mask for features close to the size of the used wavelength. For these feature sizes, the image phase does not coincide with the etch depth equivalent phase calculated from the nominal depth and optical constants of the shifter material. Additionally, for PSMs generating phase jumps deviating from 180°, the resulting phase in the image plane of a microlithography exposure tool depends on the transmitted diffraction orders through the aperture of the imaging system. Consequently Zeiss, in collaboration with Intel, has started the development of a laterally resolving Phase Metrology Tool (Phame) for in-die phase measurements. In this paper we present this optical metrology tool capable of phase measurement on individual line/spaces down to 120nm half pitch. Alternating PSM, Attenuated PSM, Cr-less masks were measured on various target sizes and simulations were performed to further demonstrate the capability and implication of this new method to measure the scanner relevant phase in-die, taking into account NA, polarization, and rigorous effects.