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

Abstract

The phase-shifting point-diffraction interferometer (PS/PDI) was recently developed and implemented at Lawrence Berkeley National Laboratory to characterize 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 major classes of errors affect the accuracy of the interferometer: systematic effects arising from measurement geometry and systematic and random errors due to an imperfect reference wave. To characterize these effects, and hence to calibrate the interferometer, a null test is used. This null test also serves as a measure of the accuracy of the interferometer. We show the EUV PS/PDI, as currently implemented, to have a systematic error-limited reference-wave accuracy of 0.0028 waves (lambda/357 or 0.038 nm at lambda = 13.5 nm) within a numerical aperture of 0.082.

Manufacturing and Investigating Objective Lens for Ultrahigh Resolution Lithography Facilities

“…The data obtained with the interferometer installed on the synchrotron ALS in Berkeley, USA, are marked by stars. The interferometer using a pin-hole in an opaque screen as a reference spherical wave source, operates at the wavelength =13.5 nm and is considered as a reference for the measurement accuracy among the point diffraction interferometers (Naulleau et al, 1999). It is clearly seen from the pictures that wave front aberrations of the source, developed in this work, are even less as compared with the ALS-sources despite a longer operating wavelength.…”

At-Wavelength Interferometry of High-NA Diffraction-Limited EUV Optics

“…The PS/PDI [1,[10][11][12] has become the de facto accuracy standard for optical system measurements conducted in our laboratory. Pinhole diffraction is used to create spherical reference waves of exceptionally high quality.…”

Tunable coherent radiation at soft X-ray wavelengths: Generation and interferometric applications

“…The availability of high power, spectrally and spatially coherent Extreme Ultraviolet (EUV) radiation has facilitated a wide variety of experiments [1][2][3][4][5][6][7].…”

“…From equation (1)(2), it is here NA is the numerical aperture of the system evident that better resolution will be achieved with decreasing wavelength.…”

“…The visibility can be described as the sharpness of the fringes or [33] (3-15) where I (1) is the intensity on the CCD due to pinhole diffraction, x is the axis on the CCD that is parallel to the pinhole separation, z is the distance from the pinhole pair to the CCD and φ is the fringe shift relative to the geometric center of the interference pattern. Taking a two-dimensional Fourier transform of equation…”