M. V. Zorina

Roughness measurement and ion-beam polishing of super-smooth optical surfaces of fused quartz and optical ceramics

The main problems and the approach used by the authors for roughness metrology of super-smooth surfaces designed for diffraction-quality X-ray mirrors are discussed. The limitations of white light interferometry and the adequacy of the method of atomic force microscopy for surface roughness measurements in a wide range of spatial frequencies are shown and the results of the studies of the effect of etching by argon and xenon ions on the surface roughness of fused quartz and optical ceramics, Zerodur, ULE and Sitall, are given. Substrates of fused quartz and ULE with the roughness, satisfying the requirements of diffraction-quality optics intended for working in the spectral range below 10 nm, are made.

Particulars of studying the roughness of substrates for multilayer X-ray optics using small-angle X-ray reflectometry, atomic-force, and interference microscopy

The abilities of standard methods for rough surface investigation are analyzed in case of supersmooth substrates for multilayered X-ray optics. The X-ray specular reflection technique is shown to be the most adequate one. X-ray diffuse scattering and AFM can be applied only in a case of PSD-function calculation and approximation over the entire spectrum range of spatial frequencies.

Ion-beam polishing of fused silica substrates for imaging soft x-ray and extreme ultraviolet optics.

We have studied the surface treatment of polished fused silica by neutralized Ar ions with energy of 500-1500 eV and incidence angles of 0-90°. We found the following regularities: for samples that passed the standard procedure of deep polishing (initial effective roughness σ(eff)∼0.5  nm), the effective roughness decreases to the ultrasmooth level (i.e., σ(eff)∼0.25  nm in the range of spatial frequencies q∈[4.9×10(-2)-63]  μm(-1)). The effect begins to be noticeable at the material removal of 150 nm and reaches saturation at depths of removal greater than 1 μm. For supersmooth samples (σ(eff)<0.3  nm), the effective roughness keeps the initial level at material removal down to tens of micrometers. The optimal ion energy range is 800-1300 eV (maximum smoothing effect); at higher energy some surface roughness degradation is observed. All the smoothing effects are observed at the incidence angle range θ(in)=0-35°. Increasing the ion energy above 1300 eV increases the etching rate by up to 4 μm per hour (J(ion)=0.8  mA/cm2), which allows for deep aspherization of sized substrates. The technique allows for manufacturing the optical elements for extreme ultraviolet and soft x-ray wavelength ranges with a numerical aperture of up to 0.6.

The evolution of roughness of supersmooth surfaces by ion-beam etching

The behavior of roughness on surfaces of Cr/Sc multilayer structure, crystalline silicon [100], and fused quartz upon ion beam etching in the region of middle and high frequencies of the spatial spectrum (10−2–102 μm−1) is studied. The possibility of keeping surface roughness at level σ ∼ 0.3 nm upon etching by Ar atoms/ions to depths of up to 10 μm is demonstrated.

Evolution of the roughness of amorphous quartz surfaces and Cr/Sc multilayer structures upon exposure to ion-beam etching

The effect of ion-beam etching on surfaces of Cr/Sc multilayer structures and amorphous quartz roughness is studied. The dependence of roughness on the depth of etching, incidence angle, and ion energy is considered. Optimal parameters of etching which provide minimal evolution of surface roughness are presented.

Investigation of supersmooth optical surfaces and multilayer elements using soft X-ray radiation

The aim of this work is application of soft X-ray diffuse scattering for certification of diffractiongrade optical elements and their substrates at a working wavelength. A device is suggested that allows certification under laboratory conditions owing to a dynamic range approaching that of synchrotron radiation sources. Experimental data are compared with data of alternative methods.

Problem of roughness detection for supersmooth surfaces

The requirements for multilayered x-ray elements for diffraction quality imaging optics (EUV - lithography, x-ray microscopy) achieves 0.2-0.3 nm roughness in spatial frequency range 10-3 - 103 mcm-1; its also true for the substrates. Although, there are plenty of publications on studying a surface, when it comes to angstrom-quality substrates there is still a problem. In some cases we observe, standard methods like x-ray diffuse scattering (XRDS), atomic force microscopy (AFM) and optical interferometric microscopy (OIM) give notably different results in surface characterization. The goal of the attestation procedure is choosing the sample for sputtering a multilayer coating with better reflection properties, thats why its important to understand the physical causes of the difference and get reliable information about the surface. In this work we discuss the limitation for aforesaid standard methods. OIM is seems to be inapplicable for supersmooth surface investigation because of applying references. Its also shown, that examination substrates with damaged layers in the volume (caused, for example, by ion-beam etching) by XRDS can lead to incorrect results. Imaging systems are composed by nonplanar optical elements with radiuses from 10 mm to 1 meter. That makes impossible using hard x-rays and also limited AFM applicability to high frequencies. Therefore, we propose the diffuse scattering of soft x-rays as an alternative approach. We also describe a new reflectometer, based on soft x-ray and visible light diffuse scattering, which can be used for surface investigation in middle and high spatial frequency ranges for both plane or curved substrates.

Precision aspherization of the surface of optical elements by ion-beam etching

Ion-beam methods are developed for forming supersmooth aspherical optical surfaces of polished fused quartz with a precision on the level of 3 nm relative to the standard deviation and 0.2–0.3 nm for the mean-square surface roughness in the range of spatial frequencies of 0.01–100 μm−1.

On the problems of the application of atomic-force microscopes for studying the surface roughness of elements for imaging optics

The key problems of applying atomic-force microscopes when studying the surface roughness of elements for imaging optics are discussed. Special attention is paid to optics of ultrahigh spatial resolution with an operating wavelength of no shorter than 100 nm. It is proposed that a specialized microscope for optical applications equipped with a tetraaxial goniometer be developed.

A method for measuring the shape of a surface of an ellipsoid of rotation on a Talysurf CCI 2000 interference microscope

A method for measuring the shape of the surface of an ellipsoid of rotation on a Talysurf CCI 2000 white-light scanning interference microscope is presented. The measurement results are compared to alternative measurements. It is shown that the measurement results are quite consistent.