Imaging of nanostructures with sub-100 nm spatial resolution using a desktop EUV microscope

Przemyslaw W. Wachulak 0 1 Andrzej Bartnik 0 1 Henryk Fiedorowicz 0 1 Dalibor Panek 0 1 Petr Bru za 0 1 0 P. W. Wachulak (&) A. Bartnik H. Fiedorowicz Institute of Optoelectronics, Military University of Technology , ul. gen. S. Kaliskiego 2, 00-908 Warsaw, Poland 1 D. Panek P. Bruza Faculty of Biomedical Engineering, Czech Technical University in Prague , Nam. Stna, 3105 Kladno, Czech Republic Laser-produced plasma sources of short-wavelength radiation offer an interesting alternative to synchrotron and free-electron laser installations. Recently, we reported on a newly developed desktop EUV microscope based on plasma generated from a gas-puff target and diffractive optics. The half-pitch resolution of the microscope approached 50 nm. Compared to analogous microscopes based on synchrotron sources, our system is compact and cost-effective. In this paper, we present the results of imaging experiments on a thin polycrystalline object that was carried out in order to further examine the applicability of the microscope. We have demonstrated here that EUV microscopy can provide structural information that cannot be accessed by conventional optical microscopy or SEM. 1 Introduction Recent rapid developments of nanoscience and nanotechnology require nanometer scale resolution imaging tools and methods. One of the methods, extensively studied for the last few decades, is an extreme ultraviolet (EUV) and soft X-ray (SXR) microscopy, based on Fresnel zone plates [1]. Photon-based EUV/SXR microscopy is capable of reaching resolutions down to 12 nm using synchrotron radiation [2]. However, the complicacy and extreme financial demands associated with measurements on synchrotrons leads to the existing demand for more affordable sources of short-wavelength radiation. The introduction of compact sources of bright EUV and SXR radiation paved the way for the development of tabletop microscopes that can render images of nanoscale objects with exposures as short as a few seconds and spatial resolution approaching that of synchrotron-based microscopes [35]. Many imaging experiments were carried out to date using smaller-scale EUV and SXR sources, such as high-order harmonics [6], SXR lasers [7], and incoherent laser-plasma based sources [8]. A 13.2-nm wavelength radiation from Ni-like Cd EUV laser allowed for a 55 nm in reflection mode [9] and sub-38 nm resolution nano-imaging in transmission mode with standard test objects such as various pitch gratings [3]. A capillary discharge EUV laser was employed for imaging of an entanglement of 50 nm diameter carbon nanotubes with spatial resolution approaching that of wavelength of illumination k = 46.9 nm and temporal resolution of *1 ns [10]. Quasi-monochromatic emission from an incoherent SXR source based on liquid nitrogen, k = 2.88 nm, in the so-called water window range, allowed to demonstrate SXR microscopy with sub-50 nm spatial resolution [11]. Finally, using a xenon-based gas discharge EUV source, a Schwarzschild objective and a Fresnel zone-plate for the second magnification step, EUV imaging was demonstrated reaching the spatial resolution of *100 nm [12]. In this paper, we report on an application of a desktop microscopy using a laser-plasma EUV source based on a gas-puff target for studies of thin silicon membrane and NaCl crystals morphology. Our motivation is to test the recently developed EUV microscopy setup on non-trivial objects. Previously measured spatial resolution of this microscope reaching 50 nm allows for acquisition of images of the membranes and salt crystals with high spatial resolution and field of view approaching 50 9 50 lm2 in a very compact setup. Utilization of the short wavelength EUV radiation allows demonstrating the intrinsic advantage of this radiation for extraction of additional information about the investigated object, which cannot be obtained directly from optical micrographs and SEM images. Moreover, this microscope does not require additional sample modification necessary for SEM microscopy. 2 Experimental setup The EUV microscope was equipped with an ellipsoidal mirror with Mo/Si coating to focus extreme ultraviolet (EUV) radiation onto an object. A Fresnel zone plate (FZP) objective was used to form a magnified image onto a EUVsensitive CCD camera in the transmission mode. The use of the gas-puff target eliminates the debris production problem associated with solid targets. Quasi-monochromatic EUV radiation, which is required for the use of Fresnel optics, was produced by spectral selection of a single line emitting at 13.8 nm wavelength from argon plasma. So far, different test objects (samples) were imaged with this setup: Cu mesh with thickness of *4 lm [4] and carbon foil, 70 nm thick, with holes [5], where the EUV images have been obtained with the half-pitch spatial resolution approaching *50 nm (3.7k) in a very compact setup. The scheme and experimental arrangement are shown in Fig. 1a) and a photograph of the EUV microscope is shown in Fig. 1b). The laser plasma EUV source, used in the experiment, has been developed for EUV metrology applications in the frame of the MEDEA? project [13] and later modified for quasi-monochromatic emission in the 1314 nm wavelength range, described in detail elsewhere [14]. This source has the advantage over other compact sources that simply by choosing a suitable gaseous target it is possible to change both the peak emission wavelength and the inverse relative bandwidth of the emission from the plasma. Moreover, the gaseous target does not produce any debris associated with solid targets. Ar plasma was produced by Nd:YAG laser (Eksma) irradiation of a gaseous target with pulse duration of 4 ns, wavelength 1,064 nm, and energy 0.74 J. The plasma radiates in a very broad range of wavelengths, dominantly in the EUV range (550 nm wavelength). By using additional spectral filtering, it is possible to shape the spectral emission of the source. The source can operate at up to 10 Hz repetition rate. A pressure of 2 9 10-3 mbar was constantly maintained in the chamber during the source operation. EUV radiation from the plasma was both focused and spectrally filtered by a Mo/Si multilayer ellipsoidal mirror (condenser) with 80 mm in diameter. The mirror was developed in collaboration with Reflex s.r.o. (mirror substrate) and Fraunhofer Institut fur Angewandte Optik und Feinmechanik (coating). The multilayer coating was optimized for 13.5 0.5 nm (FWHM) wavelength range and Fig. 1 a Scheme (not to scale) and b photograph of the EUV microscope using a laser-plasma EUV source based on gas-puff target 45 degrees incidence angle. The theoretical reflectivity of the mirror is 37.7 % at 13.5 nm wavelength for an unpolarized light. The laser plasma source was optimized for efficient generation of EUV radiation from Ar plasma. The in band (k = 1314 nm) photon flux was (8.8 0.5) 9 1010 photons per pulse in a horizontally elongated spot with a size 1.09 9 0.39 mm2 (...truncated)