Wavefront measurement interferometry at the operational wavelength of extreme-ultraviolet lithography

Abstract

Two basic types of interferometer, a point diffraction interferometer (PDI) and a lateral shearing interferometer (LSI) suitable for operation in the extreme-ultraviolet (EUV) wavelength region, are described. To address the challenges of wavefront measurement with an accuracy of 0.1 nm rms, we present a calibration method for the PDI that places a mask with two large windows at the image plane of the illumination point light source and a general approach to deriving the phase-shift algorithm series that eliminates the undesired zeroth-order effect in the LSI. These approaches to improving the measurement accuracy were experimentally verified by the wavefront measurements of a Schwarzschild-type EUV projection lens.

Probing multilayer stack reflectors by low coherence interferometry in extreme ultraviolet

“…The consequence of 2) and 3) is that classical beamsplitters are very difficult to manufacture (in some cases impossible), are usually chromatic, and show a relatively low efficiency. Although various interferometric systems based on an amplitude division were performed [12][13][14][15], our goal was to design a general purpose instrument, i.e., weakly chromatic, having the accuracy required to produce tens of fringes, with a spacing in the range of micrometers to tens of micrometers. In addition, it was specified that it should work close to the zero path difference, in order to accommodate the broadest possible spectral band.…”

Estimating transmitted wavefronts in a broad bandwidth based on Zernike coefficients

“…For example, 248 and 363 nm laser interferometers are used to test high-precision ultraviolet transmission systems, 1064 nm interferometers are used for testing military imaging systems and near IR optical systems, and 1550 nm interferometers are used for testing telecommunications optics [16][17][18]. As current Fizeau laser interferometers are typically optimized and built for only one wavelength [19,20], to test different optical systems, whose operating bands vary from extreme ultraviolet to far-infrared, a number of laser interferometers with different design wavelengths are necessary, which largely increases the cost of this testing.…”

Non-contact XUV metrology of Ru/B4C multilayer optics by means of Hartmann wavefront analysis

“…The most used interferometric technique in the X-ray range is the grating interferometry [22,17], mostly used at the beamlines. Grating interferometry requires sources providing high brightness and fast repetition rates [22,23], only possible at large-scale facil-ities [24,25]. Hence, there is no routinely accessible method for such important metrology in a tabletop setup.…”