bluuebunny/crossref_metadata_2025 huggingface dataset:
A 0.5 Terawatt, 125 Femtosecond Ti:Sapphire Laser
Authors: Kmetec J. D. , Macklin J. J. , Lemoff B. E. , Young J. F.
Published: February 2023

Broadband, high-energy storage, solid state amplifiers and chirped-pulse amplification further extend the peak power obtainable from a laser source. Laser-generation of x-rays and strong-field experiments require focusable beams at terawatt power levels. We have constructed a system at 802 nm using Ti:sapphire amplifiers and the technique of chirped-pulse amplification. In our system, a dye laser produces a train of 85 fsec seed pulses. A positively-dispersive grating pair expander chirps these pulses, stretching the pulsewidth to 180 psec while preserving the femtosecond bandwidth. The stretched pulses seed a Ti:sapphire regenerative amplifier, producing 8 mJ, 180 psec output. Further amplification in a triple-passed Ti:sapphire amplifier increases the pulse energy to 120 mJ. A negatively-dispersive parallel grating pair removes the chirp, recompressing the pulse to 125 fsec. The final output contains 60 mJ of energy in a 125 fsec pulsewidth. The output beam is Gaussian and measured to be 1.2 times diffraction limited. Focusing with an f/6 lens achieves 87% transmission through a 10.8 micron pinhole. Hence, f/6 focusing will produce 1018W/cm2. Passage through a saturable absorber reduces the amplifier noise to signal power ratio to 10−9 in the leading edge of the short pulse, with a large signal, whole beam transmission of 70%. Both the regenerative and the final amplifier are pumped by the 532 nm frequency-doubled output of a single commercially available 10 Hz Nd:YAG laser.
Photon accelerator: A novel method of frequency upshifting sub-picosecond laser pulses
Authors: Wilks Scott C.
Published: February 2023

Anovel method of frequency upshifting subpicosecond pulses of laser light, which makes use of relativistic plasma waves, is described in this paper. This method makes use of the fact that the response of a plasma to either a short laser pulse or a short relativistic electron beam is nearly identical. Thus, the wakefields, or electric fields, that are left behind in the plasma as either of these drivers pass, are also very similar. Since electron acceleration in plasma waves can effectively be thought of as the simple superposition of wakes, this analogy brings about the interesting question of whether the superposition of short laser pulse wakes would produce “acceleration” (or equivalently, frequency upshifting) of a trailing laser pulse. We estimate the rate of frequency upshift possible, and discuss some important physical limitations.
Recent advances in soft-x-ray laser research
Authors: MacGowan Brian J.
Published: February 2023

Soft x-ray laser experiments at the Lawrence Livermore National Laboratory, Nova laser will be described. Much of the work involves the study of collisionally pumped Ne-like and Ni-like lasers1 although some studies of recombination pumped and resonantly photo-pumped x-ray lasers will be discussed. These experiments have produced nickel-like collisionally pumped x-ray lasers at wavelengths near to, and below that of the K absorption edge of carbon (43.76-Å)2. Recent work has concentrated on the development of the Ni-like Ta amplifier as a high power source at 44.83- Å. Amplification occurs in a laser produced plasma created by irradiating a thin foil of Ta with two beams of the Nova laser. Up to 8 gainlengths have been demonstrated so far, with a gain coefficient of 3 cm–1 and a gain duration of 250 psec. The Ni-like Ta amplifier at 44.83- Å lies at the edge of the water window, and provides the basis for the design of an x-ray laser source that should fulfill some of the requirements for holography of living cells. It remains to optimize the coherent output power of the amplifier to use it as a source for future x-ray holography experiments. Experiments utilizing normal incidence x-ray optics to enhance the power output from this laser will be described.
Designs for Coherent X-Ray Lasers
Authors: London R. A. , Amendt P. , Feit M. D. , Fleck J. A. , Matthews D. L. , Maxon S. , Stewart R. E. , Trebes J. E.
Published: February 2023

The property of coherence is important for many suggested applications of x-ray lasers. For x-ray holography, the longitudinal coherence length is required to be larger than the pathlength difference between the object beam and the reference beam, estimated to be about 5 µm in a Fourier transform holography system. The longitudinal coherence length is related to the line width by Lℓ = λ2/Δλ, where λ is the laser wavelength. Using the thermal Doppler width for an estimate we get Lℓ = 37 µm for a 45Å laser. This estimate is supported by a measurement at 206Å. We therefore expect that longitudinal coherence will not be a problem. The transverse coherence length must be larger than the extent of the sample, typically ≈ 10 µm, and a certain amount of energy must irradiate the sample in order provide a high signal to noise hologram. The coherence length is obtainable straightforwardly by placing a pinhole in front of the laser, or by placing the object far from the laser. The combined requirement of energy and coherence is, however, much more difficult, requiring efficient production of coherent radiation.
Progress in the Development of a Compact Capillary Discharge Soft X-Ray Laser
Authors: Rocca J.J. , Marconi M.C. , Szapiro B.T. , Meyer J.
Published: February 2023

We have proposed compact and efficient soft X-Ray recombination lasers could be demonstrated by direct discharge excitation of a capillary plasma [1,2]. In this laser scheme a dense and nearly totally ionized plasma core with a large length-to-diameter ratio (L/d = 10-100) is created from a fast discharge by ionizing material from the capillary walls. Subsequent rapid plasma cooling by heat conduction to the capillary walls and radiation results in recombination of highly ionized species into ions of lower charge, creating population inversions and amplification of soft X-Ray radiation.
Real World Issues for Synchrotron Radiation Sources
Authors: Kincaid B. M.
Published: February 2023

A new generation of synchrotron radiation light sources covering the VUV, soft-x-ray, and hard-x-ray spectral regions is under construction in several countries. They are designed specifically to use periodic magnetic undulators and low-emittance electron or positron beams to produce high-brightness, near-diffraction-limited synchrotron radiation beams. Some of the novel features of the new sources will be discussed along with the characteristics of the radiation produced, with emphasis on the Advanced Light Source, a third-generation, 1.5-GeV storage ring optimized for undulator use. A brief introduction to the physics of high-brightness undulator radiation will be presented. This will be followed by a discussion of some of the unique challenges faced by users of these new sources, including high beam power, partial coherence, harmonics, optics contamination, and the unusual spectral and angular properties of undulator radiation.
The Physics of X-Ray Multilayer Structures
Authors: Windt David L. , Hull R. , Waskiewicz W. K.
Published: February 2023

The development of multilayer coatings for soft x-ray optics has matured in the last decade, resulting in numerous applications in many fields including x-ray astronomy, x-ray microscopy, x-ray lasers, and projection lithography. High performance multilayer-coated optics have been realized for normal incidence applications at wavelengths longer than the silicon absorption edge near 124 Å and for grazing incidence applications at shorter wavelengths, yet the challenge remains to develop high-reflectance normal incidence coatings above the silicon edge.
Soft X-ray Optic For An Efficient Laser Plasma Spectrometer
Authors: Davey S.C. , Freeman R.R. , McIlrath T.J. , vanWoerkom L.D. , Waskiewicz W.K. , Lucatorto T.
Published: February 2023

A soft x-ray figured optic has been incorporated into a spectrometer with a multi-channel detector to enhance signal levels from short pulse laser produced plasmas.
Possibilities for Projection X-Ray Lithography Using Holographic Optical Elements
Authors: Howells Malcolm R. , Jacobsen Chris
Published: February 2023

Although there is a considerable literature on proximity x-ray lithography [1], the interest in projection x-ray techniques has arisen fairly recently, mainly due to the efforts of the AT and T group [2]. The advantages of the projection approach seem to be the elimination of transmission masks and other problems of close proximity, the opportunity for demagnifying a mask of a coarser spatial scale, and, given a rigid, reflection mask, the possibility of cooling. The assumption behind the pursuit of these advantages is that the technical challenges of making the optical system that will perform the projection and the x-ray source that will illuminate it are tractable on some timescale. The optical fabrication tolerances and other design issues involved in carrying out projection using a conventional reflection system have been investigated by Rogers and Jewell [3]. These authors find that the needed tolerances are well beyond the present state of the art [4] even for spherical surfaces and the required surfaces in these schemes are aspheric. In this presentation we present an alternative approach to the process of projection which, of course, has its own, different challenges. It is a matter of one's judgement and technological starting point to decide which one of these (or other) approaches will provide the easiest path to a working x-ray projection lithography.
Transient Effects in Four-Wave Mixing
Authors: Schreiber W. M. , Chencinski N.
Published: February 2023

A formalism for four-wave mixing in the transient region, applicable to strong fields of short pulse duration, is developed. It is shown that transient effects can broaden resonance peaks and contribute to the appearance of extra resonances.
Stabilization of Two-Electron Atoms in Ultrastrong Fields: Preliminary Results from Fermi Molecular Dynamics
Authors: Lerner P. B. , Cohen James S.
Published: February 2023

In this paper we discuss the stabilization of two-electron atoms in ultrastrong, ultrashort laser pulses. We try to judge stabilization from the study of energy spectra of the bound states surviving after the pulse. These survivors include the ground and highly excited states of helium atoms and the ground and Rydberg states of He+ ions. We interpret the absence of the low-lying excited states of He+ as evidence of one-electron stabilization. Qualitative arguments are given to justify this conclusion. The distinction is made, however, between stabilization, as a one- electron phenomenon of prolonged lifetime in the ultrastrong laser field and real diminishing of total probability of ionization with growing intensity. The latter can be affected in multielectron systems by electron-electron collisions and autoionization, while both processes are modified by the strong field.
Dynamics of Short-Pulse Multiphoton Ionization
Authors: Gibson G. N. , Freeman R. R. , McIlrath T. J.
Published: February 2023

We investigate the relative importance of resonant enhancement and real population transfer in short-pulse multiphoton ionization. Total ionization and excited state population after the laser pulse are calculated numerically by integrating the time dependent Schödinger for a two level atom in a short-pulse high-intensity laser field.
Quantum and Quasi-Classical Analyses of High Order Emission Processes
Authors: Schäfer Kenneth J. , Kulander Kenneth C.
Published: February 2023

High-order harmonic generation from atoms and ions cuts off in accord
with the simple rule:
E
c
~
I
p
+
3U
p
. This cutoff rule as well as
the lack of a cutoff in high-order electron energy
spectra can be understood via a simple extension of existing two step
quasi-classical models.
High-Order Harmonic Generation in Helium
Authors: Tisch J. G. W. , Smith R. A. , Ciarrocca M. , Marangos J. P. , Hutchinson M. H. R.
Published: February 2023

We report the observation of harmonics up to 119th in helium, generated using a 1.053μm. 1ps CPA laser at intensities up to 3×1014 W/cm2 and have measured their spatial distribution. Complex spatial distributions are found for harmonics in the plateau region and near the cut-off, their angular distribution narrows to values similar to that predicted by lowest order perturbation theory.
Measurement of the Angular Distribution of High-Order Harmonics Emitted from Rare Gases
Authors: Peatross J. , Meyerhofer D. D.
Published: February 2023

We have measured the far-field angular distributions of high harmonics (orders 11-39) generated by a 1μm, 1ps laser pulse (0.3-3×1014 W/cm2) in low density Ar, Kr, and Xe targets (≤ 1 Torr, 1mm thick). Low-density targets were used to minimize phase mismatch in the case of strong ionization. The 1.2-times-diffraction- limited laser beam was weakly focused to minimize geometrical phase mismatch. The far-field harmonic distributions show a narrow central structure similar to that expected from lowest-order perturbation theory. They also display broad shoulders which appear to indicate an intensity-dependent phase of the atomic harmonic emission.
Propagation of High-Order Harmonics through an Ionizing Gas
Authors: Burnett K. , Rae S. C.
Published: February 2023

We present numerical simulations of the interaction of intense, ultrashort laser pulses with gases, at intensities of 1014–1016 W/cm2, which are currently of particular interest for extremely high-order harmonic generation. We treat the atomic dynamics and the plasma propagation aspects self-consistently, which is vital if the effect of free-electron phase matching is to be correctly included. Single-atom harmonic spectra show that extremely high-order harmonics can be generated at or near the point of ionization. In an ionized medium, however, these will not be able to propagate efficiently, and the plateau cutoff will be determined largely by plasma dispersion. The harmonic radiation will then also be spatially defocused and spectrally blueshifted as a result of this plasma formation, and accurate prediction of these effects requires an ‘integrated model’. Our approach is based on solutions to a one-dimensional Schrödinger to describe the atomic behavior, coupled with plane-wave propagation.
High-Order Harmonic Generation in Laser-Product Ions
Authors: Kubodera Shoichi , Midorikawa Katsumi , Nagata Yutaka , Akiyama Yasuhiro , Obara Minoru , Tashiro Hideo , Toyoda Koichi
Published: February 2023

We have demonstrated high-order harmonic generation using ions in laser-produced plasmas. Ions of various elements in laser-produced plasmas have been used as nonlinear media for production of coherent short wavelength radiation.
Two-Color Harmonic Generation in the Rare Gases
Authors: Crane John K. , Perry Michael D. , Ditmire Todd , Nguyen H. , Herman S.
Published: February 2023

We observe very high-order sum and difference-frequency components when 1053 and 526 nm fields are mixed in a high density gas. We describe how the relative intensity and polarization of this XUV light is affected by the relative polarization of the input fields. Even harmonics are produced due to the breaking of radial symmetry in the atom by the additional field.
Super-Dressed Two-Level Atom: Very High Harmonic Generation and Multi-Resonances
Authors: Kaplan A. E. , Shkolnikov P. L.
Published: February 2023

We show that a simple nonperturbative two-level model of an atom driven
by a very strong periodic field, results in a rich picture of very
high harmonic generation and related phenomena. It reproduces
experimentally observed plateau, yields for the first time simple
analytic formulas for the plateau cutoff frequency, critical driving
intensity, and saturation, and predicts intensity-induced
multi-resonances. One of the most fascinating phenomenon discovered
recently in nonlinear interaction of light with atoms and ions, is
very high-order (up to 135) odd harmonic generation (HHG) by intense
(∼ 1013 W/cm2 and higher) optical laser
radiation in rare gasses and some ions [1], The spectra of generated
harmonics drasticly deviate from the perturbation theory predictions
[2], In particular, intensity of harmonics, falling monotonically with
their orders only up to a certain point, levels off forming a
so-called “plateau", and falls monotonically again beyond it.
Generally, harmonte generation depends on phase-matching conditions
and nonlinear response of individual atoms. It has recently become
clear however [3,4] that the major features of HHG, in particular the
plateau, result mainly from general properties of atomic nonlinear
response. The most direct and apparently successful way so far to
approach the problem theoretically has been numerical simulation of
the Schrödinger [2,5-7] (including an empiric rule [6]) for
many-electron atoms using Hartry-Slater approximation. This approach
requires, however, tremendous amount of calculations and involves many
processes, making it difficult to gain simple insights. An interesting
simplified model [8] based on 3-D delta-potential with a single
(ground) level [9] produces results in integration form. The idea of
retaining a single energy scale (ionization energy) brings one close
to an even simpler system: a two-level model atom. A two-level model
of HHG [10], however, due to various complications introduced into it,
failed to generate simple results, whereas an analytic solution [11]
holds for a virtually degenerate two-level model only (see below)
which is unapplicable to the experimental conditions [1-4,7,9];
besides, no relaxation was considered in [10,11].
Femtosecond Variable-Angle Spectro-Ellipsometry of a Rarefaction Wave Front
Authors: Ahn H. , Wang X. Y. , Downer M. C.
Published: February 2023

Intense femtosecond (fs) irradiation of a solid surface creates transient (~ Mbar) internal pressure in the absorption region because the lattice is heated faster than thermal expansion can occur. Before the pressure is released (Δt ~ 1-2 ps) the heated surface remains optically sharp, so that time-resolved ellipsometric measurements can be analyzed with Fresnel’s equations, revealing the optical properties of novel pressurized fluids such as liquid carbon1,2 At later times (2 ps < Δt < 20 ps), a rarefaction wave front develops at the surface, as the laser induced strong compressional shock wave acts on the solid density plasma. The expansion dynamics can provide diagnostic information on initial pressure-temperature conditions. This expanding rarefaction wave front can also be probed by time-resolved ellipsometry measurements, provided they are analyzed by generalized Helmholtz wave equations3. We recently presented preliminary time-resolved reflectivity measurements of hydrodynamically expanding metal surfaces using a single probe wavelength (620 nm) and incidence angle (70°)4. In this paper, we report comprehensive measurements on free electron metals (Al, Ag), semiconductor (Si), and semimetal (C, graphite) targets using variable probe incidence angle, wavelength, and polarization. Such measurements permit our theoretical model of rarefaction wave structure, dielectric properties, and expansion dynamics to be defined much more quantitatively.
Large Aperture 95 fs Terawatt CrrLiSAF Laser
Authors: Beaud P. , Richardson M. C. , Miesak E. , Chai B. H. T.
Published: February 2023

Ultrashort 1.25 TW Fourier-transform limited puises are generated using chirped pulse amplification in a Cr:LiSAF laser system incorporating a regenerative amplifier and two additional double-pass amplifiers. The aperture of the final amplifier (25 mm) can support pulse energies up to the Joule level. Incorporating an additional preamplifier into the system will lead to the generation of 10 TW pulses in the near future.
Startup Experiments using the High-Contrast FCM CPA Laser at Toronto
Authors: Marjoribanks R. S. , Budnik F. W. , Zhao L. , Kulcsár G. , Stanier M. , Mihaychuk J.
Published: February 2023

The FCM–CPA laser system at the University of Toronto uses a
feedback-controlled Nd: glass oscillator to directly generate
high-contrast 1-ps pulses at 1.054µm—ideal for an all-glass, no-fibre
chirped-pulse amplification and compression laser system. Recompressed
pulse contrast is nearly 108, final energy is >1J in a
1-ps pulse, in a relatively simple-to-operate system. With this
system, and theoretically, we have been studying the use of
isoelectronic line ratios in determination of temperature and electron
energy distributions in picosecond laser-plasmas. We find that this
class of spectroscopic line ratios is especially well-suited to
highly transient plasmas, because the similarity of the lines leads to
a diagnostic which is quasi-steady-state—more nearly ‘local’ in its
time response. Also described are investigations in soft x-ray
photopumped XUV laser systems, using the emission from travelling-wave
laser-plasmas formed on self-healing mercury-wetted targets; these
targets are particularly useful for high-repetition-rate
applications.
Generation of 55 TW Femtosecond Pulses in a Ti: Sapphire-Nd: Glass System
Authors: Rouyer C. , Mazataud É. , Allais I. , Pierre A. , Rossage R. , Seznec S. , Sauteret C. , Migus A. , Mourou G.
Published: February 2023

We present a Ti:Sapphire-Nd:glass system which generates pulses of duration 440 fs and 55 TW peak power at 1064 nm. We discuss the implementation of a technique in order to prevent narrowing allowing to generate pulses with duration below 250 fs. The contrast ratio after the Ti:Sapphire regenerative amplifier has been measured to be less than 10-7. We determine the temporal aberrations induced by the different kinds of misalignments of the gratings and we emphasize the importance of spatially transverse effects. The basic principle of self compensation is then introduced. Finally, a new compressor and focusing system allowing to reach intensities in the range 1019 - 1020 W/cm2 is described.
Nonlinear Interactions of Laser Generated X-Rays
Authors: Kyrala G. A. , Fulton R. D. , Schappert G. T.
Published: February 2023

We are investigating the nonlinear interaction of laser generated x-rays with material. The goal is to observe one of the simplest nonlinear processes, i.e., the two-photon ionization of the K-shell of an atom. We intend to use the H- and He-like aluminum x-ray lines (1.5-1.8 keV) generated by the interaction of ultrashort laser pulses with aluminum targets. A preliminary estimate of the two photon ionization cross section indicates that an experimental observation of this process is feasible.
NIST Metrology for Soft X-Ray Multilayer Optics
Authors: Watts R.N. , Ederer D.L. , Lucatorto T.B. , Isaacson M.
Published: February 2023

As part of its commitment to support the microelectronics industry, NIST has begun a metrology program in die area of normal incidence, soft x-ray optical systems for use in both basic and applied research. At present, this program consists of dual efforts in the fields of surface figure and surface finish characterization and optical component soft x-ray reflectometry. In this paper, we will focus our discussion on the work performed at NIST using the existing soft x-ray reflectometer, the design characteristics of a new reflectometer which will replace the present instrument, and our plans to build an optical characterization facility based on a real time, two dimensional, soft x-ray imaging system with an ultimate resolution of a few tens of nanometers.
Illumination Requirements for Optical Projectors: and how to think about them
Authors: Goodman Douglas S.
Published: February 2023

Illumination systems are often given too little thought, since the main lens in a projector is usually much more difficult, and because many of the effects of non-optimimum illumination are too small to be noticed in many applications. For modern chip lithography, however, nothing can be assumed negligible.
Development of Reflective Optical Systems for XUV Projection Lithography*
Authors: Viswanathan V. K. , Newnam Brian E.
Published: February 2023

We discuss the performance requirements for reflective imaging systems which will be needed to obtain ~0.1-μn resolution via projection lithography in the extreme-ultraviolet (XUV) wavelength region ≤100 nm. Designs are considered that use centered, non-tilted, annular optical elements. The inter-relationships between the illumination system, degree of spatial coherence, and the reduction system are evaluated, and the tradeoffs in performance for the total system between using an optimized annular illumination system and full illumination are analyzed. It will be shown that for our designs, annular illumination with XUV wavelengths does not significantly improve system performance. This result simplifies the illumination system design.
Reflection Masks for Soft X-Ray Projection Lithography
Authors: Hawryluk Andrew M.
Published: February 2023

Soft X-ray projection lithography (SXPL) may be used to fabricate high resolution structures for future integrated circuit devices but will require an al1-reflecting optical system with < 100 nm resolution and < 10 nm image distortion over large fields-of-view. In conventional designs, the lithographic, tool for SXPL is envisioned as a "ring-field" scanning system with multiple (3-5), possibly aspheric, imaging optics fabricated to ~<1 nm figure precisian. Conventional system designs will use a reflection mask.
Surface Figure Metrology for X-ray Optics
Authors: Estler W. Tyler , Evans Chris J.
Published: February 2023

A soft-X-ray projection lithography system will require diffraction limited performance at wavelengths near 13 nm. A typical conceptual design for such a system consists of an x-ray source, a reflecting mask, and a series of normal incidence, multilayer coated mirrors used to image the mask upon a resist-coated wafer. System throughput and image field flatness demands will require aspheric mirrors with figure accuracies of λ/200 or better at 633 nm. Commercial phase measuring interferometers (PMIs) offer λ/1000 resolution and, with care, λ/300 repeatability. For flats and spheres, absolute figure accuracy is limited by one’s knowledge of the reference optics, since such PMIs are differential devices. At the National Institute of Standards and Technology (NIST) our present absolute figure uncertainty is about λ/20 for flats up to 150 mm in diameter and no better than λ/10 or so for spheres. For an asphere, compared with a reference sphere of no more than a few waves of figure departure, we can perhaps attain an absolute accuracy of λ/5 or so, but we are fundamentally limited by errors in the interferometer system due to non-common mode optical paths of the test and reference beams.
Multilayer Period Uniformity and Performance of Soft X-ray Imaging Systems
Authors: Kortright J. B. , Watts R.
Published: February 2023

Because multilayer x-ray structures are bandpass reflectors, the variation in multilayer period across the surfaces of near normal-incidence soft x-ray focussing optics has implications for performance of imaging systems. We have investigated these issues as they relate to Schwarzschild objectives coated with Mo/Si multilayers for use at wavelengths ranging from roughly 12.4 to 21.0 nm. The results can be generalized to other types of compound reflective imaging or projection systems and wavelength ranges.
Tungsten/Carbon Multilayers Prepared By Ion Beam Sputtering
Authors: Murakami Katsuhiko , Nakamura Hiroshi , Oshino Tetsuya , Nikaido Hideo
Published: February 2023

W/C multilayers, well known as good X-ray reflecting mirrors, were prepared by ion beam sputtering. The properties of the multilayers were investigated by low-angle X-ray diffraction, transmission electron microscopy (TEM) observation of the cross section and Auger electron spectroscopy.
Resist Characterization and Lithography Simulation
Authors: Neureuther A.R. , Oldham W.G.
Published: February 2023

The properties of important high-resolution resist systems will be briefly reviewed. Techniques for resist characterization will be discussed including experimental techniques, models for exposure (latent image formation), modifications of the latent image including diffusion and amplification, and development.
Effects of Absorption on Resist Performance in Soft X-Ray Projection Lithography
Authors: Mansfield W.M. , Wood O.R. , Bjorkholm J.E. , Bokor J. , Freeman R.R. , MacDowell A.A. , Szeto L.H. , Taylor G.N. , Tennant D.M. , Waskiewicz W.K. , Windt D.L. , White D.L. , D’Souza R. , Neureuther A.R.
Published: February 2023

Projection x-ray lithography has been proposed as a possible candidate for the delineation of integrated circuit features below 0.25µm (1). Recently it was demonstrated that near diffraction limited imaging could be achieved at several soft x-ray wavelengths (2) (3). Using 0.06pm thick films of PMMA, features of 0.2µm nd 0.1µm were printed at 37.5 and 14nm respectively. Figure 1 shows a scanning electron micrograph of these features. Note that the resist sidewalls exposed at 14nm are significantly more vertical than those printed at 37.5nm. The purpose of this work is to investigate the reasons for this difference by developing a simulation tool that would accurately model resist profiles in the soft x-ray regime (5 to 40nm).
Soft X-ray projection optics
Authors: Shafer David
Published: February 2023

There is a surprising variety of interesting optical designs for possible use in soft X-ray lithography. The requirements for the projection system are more difficult than those for the beam handling and mask illuminator optics, and it is only the projection optics that will be discussed here.
Saturation and Kinetic Issues for Optical-Field-Ionized X-Ray Lasers
Authors: Eder D. C. , Amendt P. , Rosen M. D. , Nash J. K. , Wilks S.
Published: February 2023

Ultrashort (tp ≤ 100 fs) high intensity (I ≥ 1017 W/cm2) lasers are potential drivers for x-ray lasers with wavelengths below 100 Å. The short wavelengths are obtained by lasing down to the ground state of the ion. We address a number of saturation and kinetic issues that arise from this type of laser. One scheme, that we have consider in detail, is lasing in the 3d - 2p transitions in Li-like Ne at 98 Å. We have found that for a UV driver (λ = 0.25 µm) stimulated Raman heating is reduced sufficiently to obtain high gains and reasonable efficiencies. Raman heating is a potential barrier for extending the scheme to shorter wavelengths due to the required higher intensities and corresponding increase in heating.
Wavelength Issues in Soft-X-Ray Projection Lithography
Authors: Hawryluk A. M. , Ceglio N. M.
Published: February 2023

The operating wavelength for a soft-x-ray projection lithography stepper is of considerable interest. However, the choice of wavelength is not a free variable because it will influence other systems parameters. The ideal system would operate at a wavelength where the optics and multilayer mirrors are easy to fabricate, conventional sources exist, resists are sensitive, and resist processing is viable. Having analyzed the choice of wavelength from a systems point of view, we are reporting our results.
X-Ray Production ~ 130 Å from Laser-Produced Plasmas for Projection X-ray Lithography Applications*
Authors: Kauffman R. L. , Phillion D. W. , Spitzer R.
Published: February 2023

X ray production in the region around 130 Å from laser-produced plasmas has been investigated for using them as a possible source for projection x-ray lithography. The dependence of production efficiency on target material, intensity, and pulse length has been extensively studied using a 0.53 µm wavelength laser with a maximum output of 0.3 J. Production efficiency of 1% into a 3 Å bandwidth has been demonstrated from a Sn target for intensities around 1011 W/cm2 using an 8 ns pulse. Absolute x-ray production in the region around 130 Å is measured using two different techniques for wavelength discrimination. A broad band channel defined by a grazing incidence carbon x-ray mirror and a Be transmission filter coupled with a Si XUV solid state diode measures x-rays in the 177 Å to 111 Å region. We also measure x-ray production in a narrow band around 130 Å defined by a near normal incidence synthetic multilayer mirror. Filters, mirrors, and detectors have been absolutely calibrated at synchrotrons to obtain absolute yields. In addition, we measure spectra using a transmission grating spectrometer. The spectra show that a relatively narrow feature in Sn near 130 Å enhances x-ray production making this element an optimum choice for this wavelength region. The intensity scaling data suggest that laser spot size and two-dimensional expansion of the plasma play an important role in optimizing x-ray production for these sub-joule irradiation conditions. In future experiments we plan to extend these measurements to larger spot sizes using a higher power laser to better understand the effects of two-dimensional plasma expansion on x-ray production. We also plan to extend these studies using a 1 µm laser to investigate the wavelength dependence on conversion efficiency.
Soft X-ray Multilayer Mirrors
Authors: Zhengxiu Fan
Published: February 2023

The present status of studies on soft x-ray multilayer mirrors in our institute are described. These include the optimum design, deposition technique and the preliminary characterizations.
Observation of Influence of Electrical Isolation of Dielectric Substrates on Structure and Reflectivity of Mo/Si Multilayer Coatings
Authors: Gutman Georgy , Watts Richard
Published: February 2023

One of the most important general reguirements for soft X-ray optics, and X-ray lithography in particular, is high reflectivity at normal incidence for long wavelength X-rays. Dependence of the multilayer structure on the deposition parameters and identification of appropriate conditions for the growth of multilayers having optimally smooth and sharp interfaces has been reported elsewhere1,2,3,4.
1× Reflective X-Ray Optics
Authors: Pease R.F.W. , Maluf N.I. , Markle D.A. , Owen G.
Published: February 2023

Using soft X-rays with a wavelength of 13 nm, a diffraction limited projection system with an NA of about 0.09 would have a resolution of about 0.1 μm. The corresponding Rayleigh depth of focus would be about 1.6 μm. Several catoptric reduction designs for use in this regime have been proposed, but they are limited to very small field sizes, because of the difficulty of controlling geometric aberrations. In addition, central obscuration in some designs (e.g. the Schwarzschild), further reduces image modulation. The use of aspheric mirrors can improve the performance, but the figuring and in particular testing of such surfaces at these wavelengths are major challenges. In addition, the alignment of such an aspheric system is a far from trivial undertaking. An alternative approach is to adopt a unit magnification optical system, and make use of the inherent freedom from aberrations such a configuration can posess. Such an approach, using the Offner ring field design, has been proposed by Wood et al [1]. An objection that is often raised to 1× lithography in general is the perceived difficulty of making masks. It is shown that the assumptions customarily made to extrapolate 5× mask specifications down to the 1× level are fallacious. In particular, evidence is presented that error distributions do not, as is commonly assumed, lie on a Gaussian. Even more importantly, the errors are not distributed evenly over the mask: it is shown how this fact, in particular, greatly eases the supposed difficulty of making 1× masks.
Propagation error in precision Fizeau interferometry
Authors: Huang Chunsheng
Published: February 2023

It has long been recognized that aberrations of the optical system comprised of the interferometer cavity and the viewing system can result in distortion of fringes1-5. However, what is less understood is that the aberrations of the optical system, especially the viewing system, can influence the optical path difference (OPD) of interference fringes between the reference wavefront and the test wavefront due to the fact that rays from the test surface may not trace the exact same optical path as those from the reference surface. The propagation error that is produced by the optical system and the aberrated surface or optical system under test is basically pupil shear of the viewing system and is referred to here as the shearing error.
Status of the Soft X-ray/XUV Optical Metrology Program at the National Institute of Standards and Technology
Authors: Watts Richard , Ederer David , Lucatorto Thomas , Tarrio Charles
Published: February 2023

The National Institute of Standards and Technology (NIST) has initiated a program devoted to the characterization of soft x-ray optics at the wavelength of use. Although NIST has an operational XUV characterization facility which it is using to make measurements for users across the country, that facility suffers from several deficiencies that will limit its usefulness in the coming years. Therefore, we are constructing an improved monochromator/ reflectometer beamline that will upgrade and extend our XUV measurement capabilities.
Soft-X-Ray Projection Imaging Using a 1:1 Ring-Field Optic
Authors: MacDowell A. A. , Bjorkholm J. E. , Early K. , Freeman R. R. , Himel M. , Mulgrew P. P. , Szeto L. H. , Taylor D. W. , Tennant D. M. , Wood O. R. , Bokor J. , Eichner L. , Jewell T. E. , Waskiewicz W. K. , White D. L. , Windt D. L. , Zernike F.
Published: February 2023

The first demonstration of diffraction-limited imaging at 14 nm in the soft-x-ray region, which resulted in the printing of 0.05 μm wide lines and spaces in a 60 nm thick film of PMMA resist, was produced using a multilayer-coated 20:1 reduction Schwarzschild optic.1 Unfortunately, a Schwarzschild optic possesses a central obscuration and a small image field and, hence, is not a very practical camera. A slightly more complicated optical system, but one that has already been used in a practical camera at visible wavelengths, is the 1:1 Offner ring- field optic.2 In theory a 0.0835 NA ring-field optic should be able to image 0.1 μm lines and spaces in a 100 micron wide 50 mm radius ring-shaped field at high contrast when illuminated with radiation at wavelengths shorter than 15 nm.3 In fact, an iridium-coated Offner 1:1 ring field camera was recently used to carry out projection imaging using 42 nm radiation from an undulator in the vacuum ultraviolet storage ring at Brookhaven National Laboratory.4
Characterization and Control of Laser Plasma Flux Parameters for Soft-X-Ray Projection Lithography
Authors: Richardson Martin C. , Silfvast William T.
Published: February 2023

We discuss particle and plasma emission measurements of, and particle interdiction techniques for, laser plasma sources designed to meet the needs of soft-x-ray projection lithography.
Small Scale Tokamak for X-Ray Lithography
Authors: Suckewer S. , Bromberg L. , Cohn D.
Published: February 2023

A toroidal plasma device (tokamak) with electron temperature in the range of 150-200 eV and density ~1013 particles/cm3 can be built as a very compact and relatively inexpensive machine (~3 M$). A tokamak with a major radius R ≈ 1m, minor radius r ≈ 0.1m, and confining magnetic field ~ 5k Gauss is not a very attractive for fusion research, however it can be an excellent source of soft X-ray radiation. In particular if operated in a steady state regime or at a high repetition rate it can provide several orders of magnitude more soft X-ray radiation than a small synchrotron with an undulator. This can be seen easily by comparing the total radiated power of a small tokamak and a small sychrotron, taking into account the spectral intensity distribution of line radiation from the tokamak plasma and of continuum radiation from synchrotron. We will present related calculations. Based on the calculations we will discuss the usefulness of a small tokamak for X-ray projection and proximity lithography and simple methods to change the dominant lines in the plasma radiation spectrum.
Resist Alternatives for Sub-0.35-μm Lithography Using Highly Attenuated Radiation
Authors: Kunz R. R. , Hartney M. A. , Rothschild M.
Published: February 2023

Traditional optical lithography uses semitransparent single-layer resists where the exposing radiation produces a latent image throughout the thickness of the resist, and an isotropic liquid-based development step is used to create the desired resist profile. However, resolution of 0.25-μm or better, which is required for 256 Mbit DRAM chips and beyond, cannot be obtained with conventional photolithographic technology (250-400 nm wavelength). Shorter wavelengths are required, such as 193-nm (DUV-193), soft-X- ray (10-50 nm) or hard-X-ray (1-2 nm) radiation. Unfortunately, between the wavelengths of 10 and 220 nm few materials are semitransparent For example, at 193 nm novolac resins commonly used at longer wavelengths have an absorption depth of only 40 nm, and at the 13-nm wavelength it is 400 nm. New resist processes capable of accommodating a latent image confined to the near-surface region must therefore be developed. The lithographic process must now include an additional processing step, namely pattern transfer into the bulk of the resist. This step must be highly anisotropic so that the pattern in the surface layer is faithfully reproduced at the resist-substrate interface. These general considerations were outlined already in 1984 by Taylor et al.[1]. They apply also to other forms of strongly absorbed radiation such as ion beams. (For instance, a 30 keV Ga+ beam has a projected range of only 34 nm in photoresist.) Today, surface imaging processes have been demonstrated not only for soft X-ray projection (SXP) [2] and deep ultraviolet (DUV) lithographies [3], but are being considered for manufacturing processes at optical wavelengths as well [4]. In this paper we review some of the recent developments in the areas of surface imaging and multilayer resists, with emphasis on application to 193-nm lithography. It should be noted, however, that some of the reviewed resist concepts may be applicable to SXP lithography as well.
Characterization of Ray-PN Resist for Soft-X-Ray Projection Lithography
Authors: Early K. , Tennant D. M. , Jeon D. , Mulgrew P. P. , MacDowell A. A. , Wood O. R. , Kubiak G. D. , Tichenor D. A.
Published: February 2023

To be useful for integrated circuit (IC) manufacturing, a resist process must be capable of high throughput and exhibit good pattern transfer capability, as well as demonstrate high resolution. In 1990, imaging of 50 nm-wide lines and spaces in polymethyl methacrylate (PMMA) resist, using a 20× Schwarzschild optic at λ = 14 nm, was reported.1,2 PMMA has long been known for its extremely high resolution and good process latitude, but its low sensitivity and poor plasma etch resistance make it an unlikely candidate for an IC fabrication process.
Study of Defects in Multilayer Reflective Coatings for Soft X-ray Projection Lithography Mask
Authors: Nguyen Khanh B. , Nguyen Tai D. , Wong Alfred K. , Neureuther Andrew R. , Attwood David T.
Published: February 2023

In a soft x-ray/EUV projection lithography system, multilayer reflective coatings consisting of 30-50 bilayers of alternating high-Z/1ow-Z materials would be used on all reflecting surfaces - collection optics, masks, and imaging optics. A reflection mask would likely be a multilayer coating with a top layer of patterned absorber [1, 2]. Since the mask is to be faithfully reproduced on the wafer, any mask defects and imperfections would also be reproduced.
Masked Deposition Techniques for Controlling Multilayer Period Variation on Short Wavelength Reflective Lenses
Authors: Kortright J. B. , Gullikson E. M. , Denham P. E.
Published: February 2023

Multilayer-coated focussing systems for XUV imaging, by their very nature, generally require laterally graded multilayer period or thickness across the clear aperture of mirrors to optimize performance. This requirement stems from the variation of incidence angles given by the optical design, and the inherint finite bandwidth of x-ray multilayers [1,2]. The ideal period (d) variation at each mirror is prescribed for small-field imaging systems by the variation in incidence angle θ across the clear aperture through the Bragg condition for constructive multilayer interference, λ= 2dcosθ. For large field systems the field size can make an additional contribution to angular variations which can lead to vignetting under extreme cases [3]. The actual d variation should conform to the prescribed ideal d variation to a tolerance set by the multilayer bandwidth. Otherwise, degraded performance may result from unwanted phase and intensity variations across the clear aperture. The phase errors may contribute to limiting imaging performance if wavefront errors resulting from mirror figure and system alignment errors are reduced to much less than λ. The intensity errors result in nonuniform intensity across the wavefront, and will limit the effective aperture if d variation problems are severe. These effects can and should be investigated with appropriate calculations for specific optical systems of interest [4,5].
A New Surface Imaging Resist Technology for Soft-X-Ray Projection Lithography
Authors: MacDowell A. A. , Wood O. R. , Calvert J. M. , Koloski T. S.
Published: February 2023

Sub-quarter-micron features have been produced in a number of organic
resists using soft-x-ray projection lithography (SXPL) at 14 nm,
including a demonstration of 0.05 um lines and spaces in poly
(methylmethacrylate) (PMMA).1 Unfortunately, the opacity of
conventional resists in the soft-x-ray region requires that the resist
thickness be reduced to less than 200 nm;2 in practice, 70
nm thick layers are often used. Such thin layers lead to several
problems. First, spincoating pinhole-free layers becomes increasingly
difficult at thicknesses below 100 nm. Second, the etch resistance of
thin organic layers may not be sufficient for subsequent pattern
transfer operations. Third, they are too thin to function as
planarizing layers over preexisting substrate topography. One solution
has been to use a trilevel resist composed of a thin top PMMA imaging
layer, an intermediate layer of germanium for etch resistance, and a
thick hardbaked photoresist for planarization. While the trilevel
approach has been successfully demonstrated for producing high
resolution features with SXPL at 14 nm,3 the complexity of
this process makes it problematic for rapid acceptance in a production
environment. An alternative approach, which circumvents the resist
opacity issue, involves the use of so-called surfaceimaging
resists.4 In a surface imaging resist, radiation-induced
changes in a thin imaging layer lead to conversion of the exposed
region to an etch resistant material.
High-Resolution Imaging of Laser-Produced Plasmas at a Wavelength of 130Å using a Normal Incidence Multilayer-Mirror Microscope
Authors: Seely J. F. , Holland G. E. , Giasson J. V.
Published: February 2023

Laser-produced plasmas were recently imaged at a wavelength of 44 Å using a spherical multilayer-coated mirror operating near normal incidence, and emission features were recorded with a spatial resolution of 40 μm.1 In order to produce images with better resolution, we have developed a microscope consisting of two spherical mirrors in a Cassegrain-type optical configuration. Spherical optical figure was chosen because of the demonstrated low surface roughness and high optical figure accuracy of previously fabricated mirrors. The spherical aberrations of the concave primary mirror and the convex secondary mirror tend to cancel, and the radii of curvature of the two mirrors and the mirror separation were chosen to minimize the overall spherical aberration of the optical system.
Computational Simulations of a Soft X-Ray Projection Lithography Laser Plasma Source
Authors: Olson R. E. , Sweatt W. C. , Rockett P. D.
Published: February 2023

A Sandia National Laboratories / AT&T Bell Laboratories team is developing a soft x-ray projection lithography tool that uses a compact laser plasma as a source of 14 nm x-rays. Optimization of the 14 nm x-ray source brightness is a key issue in this research. This paper describes our understanding of the source as it has been obtained through the use of computer simulations utilizing the LASNEX radiation-hydrodynamics code.