A compound Kinoform/Fresnel zone plate lens with 15 nm resolution and high efficiency in soft x-ray

Abstract

X-ray microscope as an important nanoprobing tool plays a prevailing role in nano-inspections of materials. Despite the fast advances of high resolution focusing/imaging reported, the efficiency of existing high-resolution zone plates is mostly around 5% in soft X-ray and rapidly goes down to 1-2% when the resolution approaches 10 nm. It is well known that the rectangular zone shape, beamstop, limited height/width ratios, material absorption of light and structural defects are likely responsible for the limited efficiency. Although zone plates with Kinoform profile are supposed to be efficient, progress for achieving both high resolution (<30 nm) and high efficiency (>5%) have hardly been addressed in soft X-ray. In this work, we propose a compound Kinoform/Fresnel zone plate (CKZP) by combing a dielectric Kinoform zone plate with a 15 nm resolution zone plate. Greyscale electron beam lithography was applied to form the 3D Kinoform zone plate and atomic layer deposition was carried out to form the binary zone plate. Optical characterizations demonstrated 15-nm resolution focusing/imaging with over 7.8% efficiency in soft X-ray. The origin of the efficiency improvement behind the proposed compound lens is theoretically analyzed and discussed.

Refractive X-ray optics: towards wavelengths of 13.5 nm and beyond

“…The use of the above types of mirrors can help increase the spectral range to the level of ~10% dE E , which, together with the use of the effect of total external reflection instead of diffraction from periodical multilayers, will make it possible to collect the radiation dose required for exposure of X-ray resists several times faster. Combining composite lenses and Fresnel zone plates with a relatively small effective aperture (up to 10 -20 μm ) into a single system opens up the possibility of effectively controlling not only the focusing characteristics of X-ray optics, but also its contrast and resolution [21]. In this case, instead of struggling with aberrations and distortions over a sufficiently large image field, it becomes possible to form the final image projected on a substrate covered with a resist from small fragments with controlled optical characteristics.…”

“…Let us assume that a wave 1 ( ) f x whose spectrum is equal to the Fourier image 1 ( ) F u is incident on the lens, and then find the field distribution ( ) g x in the focal plane of the lens. If we use the frequency response ( ) H u in the Fresnel approximation (21) for the space behind the lens, we can obtain the following formula for the field in the focal plane:…”

Refractive X-ray optics: towards wavelengths of 13.5 nm and beyond

“…The use of the above types of mirrors can help increase the spectral range to the level of ~10% dE E , which, together with the use of the effect of total external reflection instead of diffraction from periodical multilayers, will make it possible to collect the radiation dose required for exposure of X-ray resists several times faster. Combining composite lenses and Fresnel zone plates with a relatively small effective aperture (up to 10 -20 μm ) into a single system opens up the possibility of effectively controlling not only the focusing characteristics of X-ray optics, but also its contrast and resolution [21]. In this case, instead of struggling with aberrations and distortions over a sufficiently large image field, it becomes possible to form the final image projected on a substrate covered with a resist from small fragments with controlled optical characteristics.…”

“…Let us assume that a wave 1 ( ) f x whose spectrum is equal to the Fourier image 1 ( ) F u is incident on the lens, and then find the field distribution ( ) g x in the focal plane of the lens. If we use the frequency response ( ) H u in the Fresnel approximation (21) for the space behind the lens, we can obtain the following formula for the field in the focal plane:…”