Invited Article: Progress in coherent lithography using table-top extreme ultraviolet lasers.

Compact (table top) lasers emitting at wavelengths below 50 nm had expanded the spectrum of applications in the extreme ultraviolet (EUV). Among them, the high-flux, highly coherent laser sources enabled lithographic approaches with distinctive characteristics. In this review, we will describe the implementation of a compact EUV lithography system capable of printing features with sub-50 nm resolution using Talbot imaging. This compact system is capable of producing consistent defect-free samples in a reliable and effective manner. Examples of different patterns and structures fabricated with this method will be presented.

Single-shot soft x-ray laser linewidth measurement using a grating interferometer.

The linewidth of a 14.7 nm wavelength Ni-like Pd soft x-ray laser was measured in a single shot using a soft x-ray diffraction grating interferometer. The instrument uses the time delay introduced by the gratings across the beam to measure the temporal coherence. The spectral linewidth of the 4d1S0-4p1P1 Ni-like Pd lasing line was measured to be Δλ/λ=3×10(-5) from the Fourier transform of the fringe visibility. This single shot linewidth measurement technique provides a rapid and accurate way to determine the temporal coherence of soft x-ray lasers that can contribute to the development of femtosecond plasma-based soft x-ray lasers.

Defect-tolerant extreme ultraviolet nanoscale printing.

We present a defect-free lithography method for printing periodic features with nanoscale resolution using coherent extreme ultraviolet light. This technique is based on the self-imaging effect known as the Talbot effect, which is produced when coherent light is diffracted by a periodic mask. We present a numerical simulation and an experimental verification of the method with a compact extreme ultraviolet laser. Furthermore, we explore the extent of defect tolerance by testing masks with different defect layouts. The experimental results are in good agreement with theoretical calculations.

Sequential single-shot imaging of nanoscale dynamic interactions with a table-top soft x-ray laser.

We demonstrate the first real-space recording of nanoscale dynamic interactions using single-shot soft x-ray (SXR) full-field laser microscopy. A sequence of real-space flash images acquired with a table-top SXR laser was used to capture the motion of a rapidly oscillating magnetic nanoprobe. Changes of 30 nm in the oscillation amplitude were detected when the nanoprobe was made to interact with stray fields from a magnetic sample. The table-top visualization of nanoscale dynamics in real space can significantly contribute to the understanding of nanoscale processes and can accelerate the development of new nanodevices.

Microscopy of extreme ultraviolet lithography masks with 13.2 nm tabletop laser illumination.

We report the demonstration of a reflection microscope that operates at 13.2 nm wavelength with a spatial resolution of 55+/-3 nm. The microscope uses illumination from a tabletop extreme ultraviolet laser to acquire aerial images of photolithography masks with a 20 s exposure time. The modulation transfer function of the optical system was characterized.

Patterning of nano-scale arrays by table-top extreme ultraviolet laser interferometric lithography.

Arrays of nanodots were directly patterned by interferometric lithography using a bright table-top 46.9 nm laser. Multiple exposures with a Lloyd's mirror interferometer allowed to print arrays of 60 nm FWHM features. This laser-based extreme ultraviolet interferometric technique makes possible to print different nanoscale patterns using a compact table-top set up.

Volume extreme ultraviolet nano-holographic imaging with numerical optical sectioning.

Three dimensional images were obtained using a single high numerical aperture hologram recorded in a high resolution photoresist with a table top lambda = 46.9 nm laser. Gabor holograms were numerically reconstructed over a range of image planes by sweeping the propagation distance in the numerical reconstruction algorithm, allowing numerical optical sectioning. A robust three dimension image of a test object was obtained with numerical optical sectioning, providing a longitudinal resolution of approximately 2 mum and a lateral resolution of 164 nm.

Continuous high-repetition-rate operation of collisional soft-x-ray lasers with solid targets.

We have generated a laser average output power of 2 microW at a wavelength of 13.9 nm by operating a tabletop laser-pumped Ni-like Ag laser at a 5 Hz repetition rate, using a solid helicoidal target that is continuously rotated and advanced to renew the target surface between shots. More than 2 x 10(4) soft-x-ray laser shots were obtained by using a single target. Similar results were obtained at 13.2 nm in Ni-like Cd with a Cd-coated target. This scheme will allow uninterrupted operation of laser-pumped tabletop collisional soft-x-ray lasers at a repetition rate of 10 Hz for a period of hours, enabling the generation of continuous high average soft-x-ray powers for applications.

Sub-38 nm resolution tabletop microscopy with 13 nm wavelength laser light.

We have acquired images with a spatial resolution better than 38 nm by using a tabletop microscope that combines 13 nm wavelength light from a high-brightness tabletop laser and Fresnel zone plate optics. These results open a gateway to the development of compact and widely available extreme-ultraviolet imaging tools capable of inspecting samples in a variety of environments with a 15-20 nm spatial resolution and a picosecond time resolution.

Sub 400 nm spatial resolution extreme ultraviolet holography with a table top laser.

We report sub-400 nm spatial resolution with Gabor holography obtained using a highly coherent table top 46.9 nm laser. The hologram was recorded in high resolution photoresist and subsequently digitized with an atomic force microscope. The final image was numerically reconstructed with a Fresnel propagator. Optimal reconstruction parameters and quantification of spatial resolution were obtained with a wavelet analysis and image correlation.

Plasma conditions for improved energy coupling into the gain region of the Ni-like Pd transient collisional x-ray laser.

We have directly probed the conditions in which the Ni-like Pd transient collisional x-ray laser is generated and propagates by measuring the near-field image and by utilizing picosecond resolution soft x-ray laser interferometry of the preformed Pd plasma gain medium. The electron density and gain region of the plasma have been determined experimentally and are found to be in good agreement with simulations. We observe a strong dependence of the laser pump-gain medium coupling on the laser pump parameters. The most efficient coupling occurs with the formation of lower density gradients in the preformed plasma and when the duration of the main heating pulse is comparable to the gain lifetime (approximately 10 ps for mid- Z Ni-like schemes). This increases the output intensity by more than an order of magnitude relative to the commonly utilized case where the same pumping energy is delivered within a shorter heating pulse duration (<3 ps) . In contrast, the higher intensity heating pulses are observed to be absorbed at higher electron densities and in regions where steep density gradients limit the effective length of the gain medium.

Nanoimaging with a compact extreme-ultraviolet laser.

Images with a spatial resolution of 120-150 nm were obtained with 46.9 nm light from a compact capillary-discharge laser by use of the combination of a Sc-Si multilayer-coated Schwarzschild condenser and a free-standing imaging zone plate. The results are relevant to the development of compact extreme-ultraviolet laser-based imaging tools for nanoscience and nanotechnology.

Observation of a multiply ionized plasma with index of refraction greater than one.

We present clear experimental evidence showing that the contribution of bound electrons can dominate the index of refraction of laser-created plasmas at soft x-ray wavelengths. We report anomalous fringe shifts in soft x-ray laser interferograms of Al laser-created plasmas. The comparison of measured and simulated interferograms shows that this results from the dominant contribution of low charge ions to the index of refraction. This usually neglected bound electron contribution can affect the propagation of soft x-ray radiation in plasmas and the interferometric diagnostics of plasmas for many elements.

Saturated high-repetition-rate 18.9-nm tabletop laser in nickellike molybdenum.

We report saturated operation of an 18.9-nm laser at 5-Hz repetition rate. An amplification with a gain-length product GL of 15.5 is obtained in the 4d 1S0-4p 1P1 laser line of Ni-like Mo in plasmas heated at grazing incidence with approximately 1-J pulses of 8.1-ps duration from a tabletop laser system. Lasing is obtained over a broad range of time delays and pumping conditions. We also measure a GL of 13.5 in the 22.6-nm transition of the same ion. The results are of interest for numerous applications requiring high-repetition-rate lasers at wavelengths below 20 nm.

Guiding of intense laser beams in highly ionized plasma columns generated by a fast capillary discharge.

We have demonstrated the guiding of laser pulses with peak intensities up to 2.2 x 10(17) W/cm(2) in a 5.5 cm long plasma column containing highly charged Ar ions generated by a fast capillary discharge. A rapid discharge-driven hydrodynamic compression guides progressively lower order modes through a plasma with increasing density and degree of ionization, until the guide collapses on axis. The lowest order mode (FWHM approximately 50 microm) is guided with 75% transmission efficiency shortly before the plasma reaches the conditions for lasing in Ne-like Ar. The subsequent rapid plasma expansion forms a significantly leakier and more absorbent guide.

Two-dimensional effects in laser-created plasmas measured with soft-x-ray laser interferometry.

Soft-x-ray laser interferograms of laser-created plasmas generated at moderate irradiation intensities (1 x 10(11)-7 x 10(12) W cm(-2)) with lambda=1.06 microm light pulses of approximately 13-ns-FWHM (full width at half maximum) duration and narrow focus (approximately 30 microm) reveal the unexpected formation of an inverted density profile with a density minimum on axis and distinct plasma sidelobes. Model simulations show that this strong two-dimensional hydrodynamic behavior is essentially a universal phenomena that is the result of plasma radiation induced mass ablation and cooling in the areas surrounding the focal spot.

Picosecond x-ray laser interferometry of dense plasmas.

We present the first results from picosecond interferometry of dense laser-produced plasmas using a soft x-ray laser. The picosecond duration and short wavelength of the 14.7 nm Ni-like Pd laser mitigates effects associated with motion blurring and refraction through millimeter-scale plasmas. This enables direct measurement of the electron-density profile to within 10 microm of the target surface. A series of high-quality two-dimensional (2D) density measurements provide unambiguous characterization of the time evolution in a fast-evolving plasma suitable for validation of existing 1D and 2D hydrodynamic codes.

Dense plasma diagnostics with an amplitude-division soft-x-ray laser interferometer based on diffraction gratings.

We report the demonstration of an amplitude-division soft-x-ray interferometer that can be used to generate high-contrast interferograms at the wavelength of any of the saturated soft-x-ray lasers (5.6-46.9 nm) that are available at present. The interferometer, which utilizes grazing-incidence diffraction gratings as beam splitters in a modified Mach-Zehnder configuration, was used in combination with a tabletop 46.9-nm laser to probe a large-scale (~2.7-mm-long) laser-created plasma.

Soft-x-ray laser interferometry of a pinch discharge using a tabletop laser.

We have used a tabletop soft-x-ray laser and a wave-front division interferometer to probe the plasma of a pinch discharge. A very compact capillary discharge-pumped Ne-like Ar laser emitting at 46.9 nm was combined with a wave division interferometer based on Lloyd's mirror and Sc-Si multilayer-coated optics to map the electron density in the cathode region of the discharge. This demonstration of the use of tabletop soft-x-ray laser in plasma interferometry could lead to the widespread use of these lasers in the diagnostics of dense plasmas.

Soft-x-ray laser interferometry of a plasma with a tabletop laser and a Lloyd's mirror.

We report what is believed to be the first demonstration of soft-x-ray interferometry of a plasma with a tabletop soft-x-ray laser. A Lloyd's mirror interferometer was used in combination with a very compact lambda = 46.9 nm capillary-discharge-pumped laser to map the electron density in the cathode region of a pinch plasma.