Natale M. Ceglio

Time-resolved x-ray transmission grating spectrometer for studying laser-produced plasmas.

The development of a new time-resolved x-ray spectrometer is reported in which a free-standing x-ray transmission grating is coupled to a soft x-ray streak camera. The instrument measures continuous x-ray spectra with 20-psec temporal resolution and moderate spectral resolution (deltalambda >/= 1 A) over a broad spectral range (0.1-5 keV) with high sensitivity and large information recording capacity. Its capabilities are well suited to investigation of laser-generated plasmas, and they nicely complement the characteristics of other time-resolved spectroscopic techniques presently in use. The transmission grating spectrometer has been used on a variety of laser-plasma experiments. We report the first measurements of the temporal variation of continuous low-energy x-ray spectra from laser-irradiated disk targets.

Multipass amplification of soft x rays in a laser cavity

We report the first demonstration to our knowledge of multipass amplification of soft x rays. A gain medium of neonlike selenium ions was placed within a resonant cavity composed of a multilayer mirror and a beam splitter designed for normal-incidence use at the 20.63- and 20.96-nm laser lines of the neonlike selenium. The laser-cavity output was time resolved and exhibited three distinct temporal components identifiable as the single-, double-, and triple-pass amplified emission. In these experiments, multipass amplification was limited by the finite duration of the gain medium.

X-ray laser research at the Lawrence Livermore National Laboratory Nova laser facility

We describe our optical-laser-pumped x-ray laser program. Our long-term goal is to develop and utilize a fully coherent, gigowatt-power-level sub-44-A laser. To this end we have been studying the characteristics of the exploding-foil amplifier coupled with various inversion schemes: Ne-like and Ni-like collisional excitation as well as H-like three-body recombination. Most of our experimental results to date are for the Ne-like schemes; we have observed ~15 laser transitions in Se, Y, and Mo having wavelengths from 26.3 to 10.6 nm. Output power to at least 1 MW has been observed for the Se J = 2 to 1 lines at 20.6 and 20.9 A along with geometrical divergence patterns for the beam. We have also observed time-dependent beam refraction from these amplifiers and have been able to demonstrate double-pass amplification by using a multilayer mirror operated at normal incidence. Future plans for improving beam coherence and producing lasing at wavelengths shorter than 44 A are discussed.

X-ray zone plates fabricated using electron-beam and x-ray lithography

Fresnel zone plate patterns, free of spherical aberration, with diameters of up to 0.63 mm and linewidths as small as 1000 A were fabricated on polyimide membrane x‐ray masks using scanning electron beam lithography. Distortion of the electron beam scan raster was reduced to ?2500 A over a 2×2 mm field by applying deflection corrections, while viewing the distortion using a Moire method. CK x‐ray lithography was used to replicate the zone plate pattern in thick PMMA over a 100 A thick plating base on a glass substrate. Zones plates in 1.3 μm thick gold were fabricated by plating, and made free‐standing by removal of the plating base and the supporting glass substrate. Zone plates were tested as imaging elements with visible light and soft x‐rays.

Undulator radiation for at-wavelength interferometry of optics for extreme-ultraviolet lithography

Techniques are described for at-wavelength interferometry of multilayer coated optics designed for use in extreme-ultraviolet lithography. Broadly tunable undulator radiation, which covers the spectral region from 45 to 400 A, is described. The coherent power available at these wavelengths is described, and several types of interferometer that might be suitable at these short wavelengths are also described.

Soft X-ray laser source development and applications experiments at Lawrence Livermore National Laboratory

Recent progress in experimental laboratory soft X-ray laser research at Lawrence Livermore National Laboratory (LLNL) is reviewed. Research at LLNL in this area has concentrated on further characterising and understanding neon-like X-ray laser plasmas, investigating soft X-ray amplification at shorter wavelengths, and demonstrating examples of X-ray laser applications. For the standard 200 AA neon-like selenium collisional excitation laser, the output source size as well as the beam time history, divergence, energy and spatial profile have been measured. Gain has been demonstrated at wavelengths as short as 50.3 AA in nickel-like ytterbium. Several recombination X-ray laser schemes have also been investigated. X-ray laser holography, cavity operation of an X-ray laser, and the capability to point and focus the output laser beam have been demonstrated.

Front-end design issues in soft-x-ray projection lithography

We present a protocol for the design of an illumination system (front end) for a soft-x-ray projection lithography tool. The protocol is illustrated by specific front-end designs. The most complete design analysis is for a laser-driven system. Other drivers; undulator, synchrotron orbital radiation, and plasma discharge, are also discussed.

Wavelength considerations in soft-x-ray projection lithography

The choice of the operational wavelength for a soft-x-ray projection lithography system affects a wide variety of system parameters such as optical design, sources, resists, and multilayer mirrors. Several system constraints limit the choice for the operational wavelength. In particular, optical imaging requirements place an upper limit and throughput issues place a lower limit on the wavelength selection. We have determined that there are several discrete wavelength regions between 10 and 25 nm that satisfy the system-imposed constraints of high resolution, large depth of focus, and high throughput.

Soft x‐ray projection lithography

Recent advances in x‐ray optics have made possible the practical consideration of soft x‐ray projection imaging for the fabrication of high density integrated circuits.

Gold transmission gratings with submicrometer periods and thicknesses ≳0.5 μm

Gold gratings with spatial periods of 0.3 and 0.2 μm have been fabricated in thicknesses of 0.6 and 0.25 μm, respectively, and used in x‐ray spectroscopy and spatial‐period‐division. Fabrication techniques included: holographic lithography, shadowing, x‐ray lithography, and gold microplating. Control of linewidth to tolerance of the order of 10 nm has been demonstrated for gratings of 0.2 μm period. A high resolution imaging spectrometer, composed of a 22× Wolter x‐ray microscope in conjunction with a gold transmission grating, was tested. At a wavelength of 0.69 nm, a resolving power, λ/Δλ, of 200 was demonstrated. Resolution in this case was source‐size limited. Gratings of 99.5 nm period were exposed in PMMA by x‐ray (λ = 4.5 nm) spatial‐period‐division.