Extreme-ultraviolet phase-shifting point-diffraction interferometer: a wave-front metrology tool with subangstrom reference-wave accuracy

The phase-shifting point-diffraction interferometer (PS/PDI) was recently d eveloped and implemented at Lawrence Berkeley National Laboratory to charac terize extreme-ultraviolet (EUV) projection optical systems for lithography . Here we quantitatively characterize the accuracy and precision of the PS/ PDI. Experimental measurements are compared with theoretical results. Two m ajor classes of errors affect the accuracy of the interferometer: systemati c effects arising from measurement geometry and systematic and random error s due to an imperfect reference wave. To characterize these effects, and he nce to calibrate the interferometer, a null test is used. This null test al so serves as a measure of the accuracy of the interferometer. We show the E UV PS/PDI, as currently implemented, to have a systematic error-limited ref erence-wave accuracy of 0.0028 waves (lambda/357 or 0.038 mo at lambda = 13 .5 nm) within a numerical aperture of 0.082. (C) 1999 Optical Society of Am erica.