The phase-shifting point diffraction interferometer (PS/PDI) has recently been developed and implemented at Lawrence Berkeley National Laboratory to meet the significant measurement challenge of characterizing extreme ultraviolet (EUV) projection lithography systems. Here progress on the characterization of the PSIPDI accuracy is presented. Two major classes of errors affect the accuracy of the interferometer: the first being systematic effects arising from the measurement geometry, and the second being random and systematic errors caused by an imperfect reference wave. In order to characterize these contributions and calibrate the interferometer, a null test is required. This null test also serves as a measure of the absolute accuracy of the interferometer. Experimental results demonstrating a systematic-enor-limited accuracy of 0.004 waves (21250 or 0.05 nm at X = 13.4 nm) is reported.
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