Fresnel zone plate telescope for condenser alignment in water-window microscope

Abstract

Microscopes operating at short wavelengths, in the extreme ultraviolet and soft x-ray spectral region, require careful condenser positioning to avoid possible artifacts related to enhancing or diminishing certain spatial frequencies in the image plane. Various methods are often used to visualize the condenser illumination pattern, including direct visualization on a CCD camera; however, these are not always straightforward to use. We present and discuss a novel and convenient method to image a condenser illumination pattern upstream the sample plane, using two zone plates with matched numerical apertures. This imaging system, operating herein in the water-window spectral range in telescope configuration, allows us to change the distance between the conjugated planes, thus overcoming limitations related to the geometry of the vacuum system. This geometry, which is optimized for the highest possible spatial resolution allowed by the zone-plate objective, is not necessarily particularly good for visualization of the condenser illumination pattern. The presented method was demonstrated with a compact, gas puff target source based soft x-ray microscope, which is capable of resolving 60 nm features (half-pitch resolution), requires a few seconds exposure time, and is debris-free due to the gaseous nature of the target for soft x-ray generation. The method, presented herein, may solve mentioned vacuum system geometry limitations. Also, it can easily be extended to other systems and other wavelengths, provided a proper optic is used. Modes of operation and the results are presented and discussed.

A stand‐alone compact EUV microscope based on gas‐puff target source

“…The gas‐puff target approach has been developed for EUV metrology applications in the frame of MEDEA + project (Fiedorowicz et al ., ; Fiedorowicz et al ., ), at the Institute of Optoelectronics, Military University of Technology, Poland. More details about the EUV source based on the gas‐puff target and its optimization for microscopy applications are reported in the literature (Bartnik et al ., ; Wachulak et al ., c).…”

“…This source is very robust, simple in construction, has low maintenance costs and enables the performing of experiments with many advantages, namely a high photon flux, obtaining a monochromatic radiation emission, a change in the emission spectrum simply by changing the gas – target, or tuning the emission to one's needs by applying different thin film filters. The gas‐puff target source has already been proven to be suitable and, recently, it was successfully used in a variety of imaging experiments including SXR full field microscopy obtaining half‐pitch spatial resolution up to 60 nm (Wachulak et al ., b). Additionally, the EUV radiation was already shown to be useful for imaging of nontrivial samples, to demonstrate that it can reveal additional information about the sample compared to other microscopes, such as optical microscope and SEM cannot (Wachulak et al ., ).…”

“…The EUV radiation can be easily absorbed by solid materials with very small thicknesses of the order of about 100 nm (Wachulak et al ., ) and by gaseous materials with thicknesses of the order of a few millimetres. Moreover, such radiation can provide information about such thin materials, depth of the microcracks (Wachulak et al ., ) or directly visualize the gas flow in 2D (Wachulak et al ., ) and 3D mode (Wachulak et al ., ), usually investigated with indirect techniques such as interferometry (Park et al ., ).…”

Development and optimization of a “water window” microscope based on a gas-puff target laser-produced plasma source

“…4a), overcoming geometrical restrictions of the vacuum system and allowing for visualization of the illumination pattern without the need to convert short wavelength radiation to the visible range using scintillators. Much more details about this techniques can be found in [21]. Fig.…”