Table-top Extreme Ultraviolet Research Articles

Tabletop extreme ultraviolet reflectometer for quantitative nanoscale reflectometry, scatterometry, and imaging.

Imaging using coherent extreme-ultraviolet (EUV) light provides exceptional capabilities for the characterization of the composition and geometry of nanostructures by probing with high spatial resolution and elemental specificity. We present a multi-modal tabletop EUV imaging reflectometer for high-fidelity metrology of nanostructures. The reflectometer is capable of measurements in three distinct modes: intensity reflectometry, scatterometry, and imaging reflectometry, where each mode addresses different nanostructure characterization challenges. We demonstrate the system's unique ability to quantitatively and non-destructively measure the geometry and composition of nanostructures with tens of square microns field of view and sub-nanometer precision. Parameters such as surface and line edge roughness, density, nanostructure linewidth, and profile, as well as depth-resolved composition, can be quantitatively determined. The results highlight the applicability of EUV metrology to address a wide range of semiconductor and materials science challenges.

Generation of collimated extreme ultraviolet radiation by single-photon process

Generating laser with short wavelength is a bottleneck problem in laser technology. The current applicable table-top extreme ultraviolet (EUV) lasers are all generated by multi-photon process with low efficiency. By utilizing metastable helium atoms excited by microwave and irradiated by a resonant laser, here we report the development of 58.4 nm collimated radiation by single-photon-excitation related anti-Stokes Raman scattering (ASRS). The conversion efficiency is much higher than that of high harmonic generation (HHG). The same divergence of 1.4 mrad as that of excitation laser indicates its stimulating character. Our results show an applicable path towards up-conversion by single-photon process to generate table-top EUV lasers.

Material-specific high-resolution table-top extreme ultraviolet microscopy

Microscopy with extreme ultraviolet (EUV) radiation holds promise for high-resolution imaging with excellent material contrast, due to the short wavelength and numerous element-specific absorption edges available in this spectral range. At the same time, EUV radiation has significantly larger penetration depths than electrons. It thus enables a nano-scale view into complex three-dimensional structures that are important for material science, semiconductor metrology, and next-generation nano-devices. Here, we present high-resolution and material-specific microscopy at 13.5 nm wavelength. We combine a highly stable, high photon-flux, table-top EUV source with an interferometrically stabilized ptychography setup. By utilizing structured EUV illumination, we overcome the limitations of conventional EUV focusing optics and demonstrate high-resolution microscopy at a half-pitch lateral resolution of 16 nm. Moreover, we propose mixed-state orthogonal probe relaxation ptychography, enabling robust phase-contrast imaging over wide fields of view and long acquisition times. In this way, the complex transmission of an integrated circuit is precisely reconstructed, allowing for the classification of the material composition of mesoscopic semiconductor systems.

Multi-shot damage on Mo/Si multilayer induced by nanosecond EUV radiation

As a vital optical element working in the extreme ultraviolet (EUV) region, the damage mechanism on a Mo/Si multilayer has caught great attention. In this paper, using a nanosecond table-top EUV source, an S-on-1 EUV damage experiment was performed on a Mo/Si multilayer mirror with shot numbers S = 1, 2, 5, and 10. It was demonstrated that the multi-shot damage thresholds follow an inverse-power-law as a function of the pulse number. The incubation effect is explained based on the measured damage morphologies by an atomic force microscope. When the Mo/Si multilayer mirror was exposed to the EUV fluence above the single damage threshold, compaction of the layer structure and melting and resolidification processes are dominating the damage mechanism. It was also found that the crater depth and FWHM at high fluence are independent of the shot numbers, which is explained by the formation of the MoSi2 spacing layer.

Coherent Tabletop EUV Ptychography of Nanopatterns

Coherent diffraction imaging (CDI) or lensless X-ray microscopy has become of great interest for high spatial resolution imaging of, e.g., nanostructures and biological specimens. There is no optics required in between an object and a detector, because the object can be fully recovered from its far-field diffraction pattern with an iterative phase retrieval algorithm. Hence, in principle, a sub-wavelength spatial resolution could be achieved in a high-numerical aperture configuration. With the advances of ultrafast laser technology, high photon flux tabletop Extreme Ultraviolet (EUV) sources based on the high-order harmonic generation (HHG) have become available to small-scale laboratories. In this study, we report on a newly established high photon flux and highly monochromatic 30 nm HHG beamline. Furthermore, we applied ptychography, a scanning CDI version, to probe a nearly periodic nanopattern with the tabletop EUV source. A wide-field view of about 15 × 15 μm was probed with a 2.5 μm−diameter illumination beam at 30 nm. From a set of hundreds of far-field diffraction patterns recorded for different adjacent positions of the object, both the object and the illumination beams were successfully reconstructed with the extended ptychographical iterative engine. By investigating the phase retrieval transfer function, a diffraction-limited resolution of reconstruction of about 32 nm is obtained.

A table-top EUV focusing optical system with high energy density using a modified Schwarzschild objective and a laser-plasma light source.

For investigating extreme ultraviolet (EUV) damage on optics, a table-top EUV focusing optical system was developed in the laboratory. Based on a modified Schwarzschild objective with a large numerical aperture and a laser-plasma light source, this optical system can generate a focusing spot with the maximum energy density of 2.27 J/cm2 at the focal plane of the objective at the wavelength of 13.5 nm. The structures and the characterized properties of this optical system are presented in this paper. For testing the performance of this setup, single-shot EUV damage tests were carried out on an optical substrate of CaF2 and a gold thin film. The damage thresholds and morphologies of these samples were given and discussed with former research studies.

Development of extreme ultraviolet scatterometer using multiple orders of high-harmonic generation

We developed an extreme ultraviolet (EUV) scatterometer equipped with a table-top EUV source for the characterization of nanoscale grating lines. Appropriate orders of high-harmonic generation at wavelengths ranging from 25 to 35 nm are selected as the coherent light source for high-resolution spatial performance. It is shown that the grating surface profile significantly affects the scattered diffraction intensities and can be retrieved by the structure reconstruction algorithms using inverse modeling by rigorous coupled wave analysis.

Ptychographic hyperspectral spectromicroscopy with an extreme ultraviolet high harmonic comb.

We report a proof-of-principle demonstration of a new scheme of spectromicroscopy in the extreme ultraviolet (EUV) spectral range, where the spectral response of the sample at different wavelengths is imaged simultaneously. This scheme is enabled by combining ptychographic information multiplexing (PIM) with a tabletop EUV source based on high harmonic generation, where four spectrally narrow harmonics near 30 nm form a spectral comb structure. Extending PIM from previously demonstrated visible wavelengths to the EUV/X-ray wavelengths promises much higher spatial resolution and a more powerful spectral contrast mechanism, making PIM an attractive spectromicroscopy method in both microscopy and spectroscopy aspects. In addition to spectromicroscopy, this method images the multicolor EUV beam in situ, making this a powerful beam characterization technique. In contrast to other methods, the techniques described here use no hardware to separate wavelengths, leading to efficient use of the EUV radiation.

Tabletop nanometer extreme ultraviolet imaging in an extended reflection mode using coherent Fresnel ptychography

We demonstrate the first (to our knowledge) general purpose full-field reflection-mode extreme ultraviolet (EUV) microscope based on coherent diffractive imaging. This microscope is capable of nanoscale amplitude and phase imaging of extended surfaces at an arbitrary angle of incidence in a noncontact, nondestructive manner. We use coherent light at 29.5 nm from high-harmonic upconversion to illuminate a surface, directly recording the scatter as the surface is scanned. Ptychographic reconstruction is then combined with tilted plane correction to obtain an image with amplitude and phase information. The image quality and detail from this diffraction-limited tabletop EUV microscope compares favorably with both scanning electron microscope and atomic force microscope images. The result is a general and completely extensible imaging technique that can provide a comprehensive and definitive characterization of how light at any wavelength scatters from a surface, with imminent feasibility of elemental imaging with

Imaging at the Nanoscale With Practical Table-Top EUV Laser-Based Full-Field Microscopes

The demonstration of table-top high average power extreme-ultraviolet (EUV) lasers combined with the engineering of specialized optics has enabled the demonstration of full-field microscopes that have achieved tens of nanometer spatial resolution. This paper describes the geometry of the EUV microscopes tailored to specific imaging applications. The microscope illumination characteristics are assessed and an analysis on the microscope's spatial resolution is presented. Examples of the capabilities of these table-top EUV aerial microscopes for imaging nanostructures and surfaces are presented.

Sub-10 nm patterning using EUV interference lithography

Extreme ultraviolet (EUV) lithography is currently considered as the leadingtechnology for high-volume manufacturing below sub-20 nm feature sizes. In parallel,EUV interference lithography based on interference transmission gratings hasemerged as a powerful tool for industrial and academic research. In this paper,we demonstrate nanopatterning with sub-10 nm resolution using this technique.Highly efficient and optimized molybdenum gratings result in resolved line/spacepatterns down to 8 nm half-pitch and show modulation down to 6 nm half-pitch.These results show the performance of optical nanopatterning in the sub-10 nmrange and currently mark the record for photon-based lithography. Moreover,an efficient phase mask completely suppressing the zeroth-order diffraction andproviding 50 nm line/space patterns over large areas is evaluated. Such efficient phasemasks pave the way towards table-top EUV interference lithography systems.

Efficient extreme ultraviolet transmission gratings for plasma diagnostics

We report on a theoretical study of free-standing phase transmission gratings for high-resolution extreme ultraviolet (EUV) and soft x-ray spectroscopy and investigate their properties. Designed for wavelengths between about 2 and 40 nm, the devices may provide a first order diffraction efficiency beyond 30%. We use rigorous coupled wave analysis methods in order to optimize the grating design parameters and discuss features of segmented grating arrays. Elemental, as well as compound, materials such as Be, Mo, LiF, and poly-(methylmethacrylate) are considered with respect to their potential and practical limitations in terms of feasibility and sensitivity to radiation damage. Simulations are performed for several samples on the radiation produced by a table-top EUV plasma source and applications to astrophysical problems are considered.

A study of the mechanical vibrations of a table-top extreme ultraviolet interference nanolithography tool

A prototype low cost table-top extreme ultraviolet (EUV) laser source (1.5 ns pulse duration, lambda=46.9 nm) was successfully employed as a laboratory scale interference nanolithography (INL) tool. Interference patterns were obtained with a simple Lloyd's mirror setup. Periodic structures on Polymethylmethacrylate/Si substrates were produced on large areas (8 mm(2)) with resolutions from 400 to 22.5 nm half pitch (the smallest resolution achieved so far with table-top EUV laser sources). The mechanical vibrations affecting both the laser source and Lloyd's setup were studied to determine if and how they affect the lateral resolution of the lithographic system. The vibration dynamics was described by a statistical model based on the assumption that the instantaneous position of the vibrating mechanical parts follows a normal distribution. An algorithm was developed to simulate the process of sample irradiation under different vibrations. The comparison between simulations and experiments allowed to estimate the characteristic amplitude of vibrations that was deduced to be lower than 50 nm. The same algorithm was used to reproduce the expected pattern profiles in the lambda/4 half pitch physical resolution limit. In that limit, a nonzero pattern modulation amplitude was obtained from the simulations, comparable to the peak-to-valley height (2-3 nm) measured for the 45 nm spaced fringes, indicating that the mechanical vibrations affecting the INL tool do not represent a limit in scaling down the resolution.

Holographic imaging with a nanometer resolution using compact table-top EUV laser

AbstractHolographic 2D/3D imaging with nanometer resolution using short wavelength extreme ultraviolet (EUV) light is presented in this paper. Gabor’s holograms were recorded with a highly coherent table top EUV laser with different numerical apertures demonstrating ultimately a spatial resolution of 46+/−2 nm, comparable with the illumination wavelength, in 2D holographic imaging. Three dimensional images were obtained from a single high numerical aperture hologram recorded in a high resolution photoresist and numerically reconstructed at different image planes, allowing numerical optical sectioning with a lateral resolution ∼170 nm and depth resolution of 2.4 µm. The holograms were recorded in a high resolution photoresist and digitized with an atomic force microscope. To assess the spatial resolution of the numerical reconstructions of the holograms a correlation method was used. The algorithm allows for simultaneous estimation of the resolution and the feature size of the image under analysis.

High resolution full-field imaging of nanostructures using compact extreme ultraviolet lasers

Recent advances in the development of high peak brightness table-top extreme ultraviolet (EUV) and soft x-ray (SRX) lasers have opened new opportunities for the demonstration of compact full-field EUV/SXR microscopes capable of capturing images with short exposures down to a single laser shot. We demonstrate the practical application of table-top zone plate EUV microscopes that can image nanostructures with a spatial resolution of 54 nm and below and exposure times as short as 1.2 ns, the duration of a single laser shot.

Nanopatterning and Nanomachining with Table-top Extreme Ultraviolet Lasers

ABSTRACTNanopatterning and nanomachining of PMMA coated wafers was performed using table top extreme ultraviolet lasers. Features below 100 nm were imprinted with short (50-60 s) exposure times.