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26 June 2003 Experimental assessment of pattern and-probe aberration monitors
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First experimental evidence of the high sensitivity of interferometric-probe based aberration targets on phase-shifting masks is presented. Measurements were made on an AIMS tool modified for NA = 0.2 with 150 μm imaging and 300 μm illumination pinholes to match an inadvertent 4× oversizing of the layout dimensions. Calibration of the actual NA (= 0.18) was accomplished through known phase-edge distances and comparison of images of isolated probes and large features with aerial image simulation. Even though only two-ring versions of the targets were measured the peak of the 90 deg. central probe in the defocus target increased linearly with focus at a rate of 47% of the clear field per Rayleigh unit (RU) of defocus when measured over a ±1/2 RU interval about best focus. The focal position can be measured to within 1/40 RU and the prediction of best focus on an absolute basis agrees with that determined by the Strehl ratio to within 1/35 of a Rayleigh focal length. The two-ring spherical and higher-order spherical targets showed decent orthogonality to focus with changes in their central peak intensities of only 0.47 and 0.37 of that of the defocus target even when viewed at an NA 10% smaller than their design.
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Garth C. Robins and Andrew R. Neureuther "Experimental assessment of pattern and-probe aberration monitors", Proc. SPIE 5040, Optical Microlithography XVI, (26 June 2003);

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