Quantitative phase imaging of EUV masks

In this paper, we present two methods for directly measuring the effective complex reflectance function of a patterned EUV mask. Obtaining this measurement can provide important insight into a number of key areas in EUV mask development, including obtaining a deeper understanding of mask 3-D effects, and characterizing and quantifying the amplitude and phase generated by attenuated and etched phase shift masks. The first method, Quantitative Zernike Phase Contrast Microscopy (QZPCM), works by modulating the imaging pupil function with several known phase shifts, and obtaining through-focus images of a target area for each pupil setting. The second method, Lateral Shearing Imaging (LSI), works by splitting and interfering two copies of the complex amplitude function in the image plane separated by a distance s. The resulting fringe pattern gives information about the derivative of the complex amplitude in the direction of the shift. We present results from two experiments: the first demonstrates QZPCM at EUV on the Berkeley SHARP microscope, and the second utilizes LSI in an optical prototype using a visible light laser source.