A desktop extreme ultraviolet microscope based on a compact laser-plasma light source

Appl. Phys. B (2017) 123:25 DOI 10.1007/s00340-016-6595-5 A desktop extreme ultraviolet microscope based on a compact laser‑plasma light source P. W. Wachulak1 · A. Torrisi1 · A. Bartnik1 · Ł. We˛grzyński1 · T. Fok1 · H. Fiedorowicz1 Received: 6 September 2016 / Accepted: 14 November 2016 © The Author(s) 2016. This article is published with open access at Springerlink.com Abstract A compact, desktop size microscope, based on laser-plasma source and equipped with reflective condenser and diffractive Fresnel zone plate objective, operating in the extreme ultraviolet (EUV) region at the wavelength of 13.8 nm, was developed. The microscope is capable of capturing magnified images of objects with 95-nm full-pitch spatial resolution (48 nm 25–75% KE) and exposure time as low as a few seconds, combining reasonable acquisition conditions with stand-alone desktop footprint. Such EUV microscope can be regarded as a complementary imaging tool to already existing, well-established ones. Details about the microscope, characterization, resolution estimation and real sample images are presented and discussed. 1 Introduction Recent developments in nanoscience and nanotechnology require nanoscale imaging tools. For that, electromagnetic radiation in the extreme ultraviolet (EUV) spectral range (λ = 10–121 nm wavelength [1]) allows shifting the diffraction limit into a nanometer range [2, 3]. Much work has already been done developing different photon-based imaging techniques and schemes, according to the Rayleigh criterion, which states that the light of shorter wavelength improves the diffraction-limited spatial resolution. Some examples of this demonstrate the use of synchrotronbased sources [4] reaching spatial resolution of ~10 nm [5] or using 13.5 nm wavelength for lithography-related * P. W. Wachulak 1 Institute of Optoelectronics, Military University of Technology, Kaliskiego 2 Str., 00‑908 Warsaw, Poland research, such as mask inspection [6, 7]—reaching 22-nm half pitch resolution or lithography [8], as well as free electron lasers [9] for coherent diffraction imaging (CDI) schemes [10]. These facilities, although state of the art and dedicated to cutting-edge science experiments, are not “user friendly,” with limited user access and require high maintenance costs, because of their scale and complexity. Another approach is to use tabletop high-order harmonic (HHG) sources [11] for sub-100-nm spatial resolution imaging [12]; however, typical 10−6–10−5 HHG conversion efficiency is very low and often does not allow for a proper reconstruction [13], the system is very complicated, and typical CDI requires time-consuming numerical data processing. Ptychographic schemes, although providing very high spatial resolution, are serial in nature, extensively time-consuming and computationally demanding. To partially overcome these limitations, other compact EUV sources, such as discharge [14], Z-pinch [15] or laserproduced plasma sources [16], coupled to zone plates or Schwarzschild mirrors, were used. The first one is compact and shows very good spatial resolution, but requires often (~30 k pulses) capillary replacements, the second one demonstrates quite low performance in terms of spatial resolution and field of view exploiting inadequate mode of imaging for lithographic mask inspection, while the last one requires debris mitigation schemes. The use of compact, short-wavelength sources often does not allow for high signal-to-noise ratio image acquisition. An example of that are recent developments in soft X-ray (SXR) microscopy in so-called water window, such as a compact soft X-ray microscope based on a single nitrogen gas jet, capable of resolving features ~100 nm later improved to ~50 nm in size providing high spatial resolution; however, the exposure time for Siemens star test pattern was equal to 1–2 h, limiting the usability of 13 25 Page 2 of 5 P. W. Wachulak et al. Fig. 1  Scheme of the EUV microscope. Small insets show plasma image in the visible light wavelength range (top), SEM image of the test object (Cu mesh) and scintillator images of intensity distribution of the EUV radiation in (middle–bottom inset) and out (right–bottom inset) of the focal plane of the condenser mirror. Spectrum of the Ar plasma emission (b) (blue-dashed line). The condenser reflectivity is depicted in solid gray line. The green line spectrum shows radiation reflected from the condenser, used for subsequent imaging such system to a few images per day [17, 18]. Much more rapid exposures of 60 s were required to image objects with 40-nm spatial resolution, employing a high average power laser system for plasma generation, occupying, however, several optical tables [19], which in turn limit future possibility of commercialization. Generally speaking, a tradeoff in the short-wavelength EUV and SXR imaging can be seen between the performance, complexity and compactness of the system, which is still a major obstacle in widespread short-wavelength photon-based microscopes. Thus, in this work, we try to partially overcome presented limitations, demonstrating a simple, very compact full-field EUV microscope, which is capable of resolving sub-100-nm features, requires short exposure time and has a desktop footprint. The use of a laser-plasma EUV source based on a gas-puff target [20, 21] eliminates debris production problem of solid targets. The source, which was already successfully employed in the SXR microscopy [22], is simple in construction and was sufficiently bright to be a driver for the EUV microscope that is user friendly and can be operated just by one person. The microscope requires no sample preparation and offers high reproducibility of images and preservation of the sample integrity. Moreover, it is already well known that the water-window radiation is dedicated for imaging of biological samples, due to a natural contrast between carbon and water constituents of the living cells. It is true for imaging internals of the cells. Herein, we propose that the EUV microscopy can be used for imaging biological samples as well; it is more adequate, however, for imaging thin external cell features, such as morphology of the cellular membranes or other external features, such as flagella, which produce high contrast in the EUV range, while it may be overlooked in the water window. 2 EUV microscope construction 13 A scheme of the EUV microscope is shown in Fig. 1a. An Nd:YAG laser pulse (NL302, Eksma), λ = 1064 nm, 500 mJ/4 ns, is focused using a lens onto a double-stream Ar/He gas-puff target [23, 24], produced by an electromagnetic double-nozzle valve [25] resulting in formation of a plasma. The optimum Ar/He pressure for efficient EUV emission from such plasma was found to be 10 and 6 bar, respectively. In order to collect the radiation emitted from the Ar plasma, Fig. 1b—dashed line, and to spectrally narrow the emission, an ellipsoidal off-axis mirror with Mo/ Si multilayer coating (...truncated)