B. Faral

Laser plasma x‐ray sources for microlithography

A plasma produced by laser irradiation of solid targets is a promising candidate as an efficient x‐ray lithography source. In order to design a practical laser created x‐ray source, it is necessary to study the factors affecting the x‐ray emission. For this purpose, we investigate both theoretically and experimentally the influence of the laser target parameters on the x‐ray emission in different spectral ranges for two laser wavelengths (λ=1.06 μm, λ=0.26 μm). From these results and considering mask transmission, resist sensitivities, and wafer throughput, we establish the characteristics of the laser required as an x‐ray lithography source with performance acceptable for industrial applications.

Preheating study by reflectivity measurements in laser-driven shocks

A study on preheating effects in laser-driven shock waves is presented. Two different diagnostics were used: the color temperature measurement deduced by recording the target rear side emissivity in two spectral bands, and the rear surface reflectivity measurement by using a probe beam. In order to test the response of the two diagnostics to the preheating, three types of targets characterized by different radiative properties were used. The greater sensitivity of the second diagnostic compared with the first was demonstrated. A model which calculates the reflectivity using a one-dimensional hydrodynamic code data was developed. In this model, the wave propagation equations in the expanding plasma using an appropriate model for the electron–ion collision frequency applicable to the cold solid-hot plasma transition were solved. The comparison between the calculated and measured reflectivities allows us to estimate the preheating process.

Temperature and melting of laser-shocked iron releasing into an LiF window

Absolute reflectivity and self-emission diagnostics are used to determine the gray-body equivalent temperature of laser-shocked iron partially releasing into a lithium fluoride window. Pressure and reflectivity are measured simultaneously by means of velocity interferometer system for any reflector interferometers. In the temperature-pressure plane, a temperature plateau in the release is observed which is attributed to iron’s melting line. Extrapolation of data leads to a melting temperature at Earth’s inner-outer core boundary of 7800±1200K, in good agreement with previous works based on dynamic compression. Shock temperatures were calculated and found to be in the liquid phase.

Brominated plastic equation of state measurements using laser driven shocks

In order for brominated plastic (CHBr) to be used in future large lasers, such as the National Ignition Facility, capsule design, and equation of state (EOS) data are needed to address uncertainties in modeling. We have performed CHBr EOS measurements using the impedance matching technique. Laser beams spatially smoothed, and giving a spot size of 400 μm and intensities ⩽5×1013 W/cm2, produced high-quality shock waves allowing the simultaneous measurements of the shock velocities in two materials, one used as reference. Results are compared to other experiments and to EOS calculations. We obtained very good agreement with the theoretical curve for pressures ranging from 1 to 3 Mbar.

The x‐ray emission, ablation pressure, and preheating for foils irradiated at 0.26 μm wavelength

The x‐ray emission, ablation pressure, and preheating for foils irradiated with a 0.26 μm laser at intensities ∼1015 W cm−2 are studied. The foils are Al with various thicknesses, coated or uncoated with CH or Au. The x‐ray emission and conversion efficiency are obtained with a multichannel x‐ray diode spectrometer, the ablation pressures are deduced from shock transit times, and the rear temperatures are inferred from x‐ray pyrometry. For thin foils (≪12 μm), the rear temperatures can be predicted reasonably well with the use of the front x‐ray spectra. For thick foils shock preheating is dominant.

Experimental study of ablation pressures and target velocities obtained in 0.26 μm wavelength laser experiments in planar geometry

In 0.26 μm wavelength laser experiments that were performed in planar geometry with irradiances between 1013 and 1015 W/cm2, the ablation pressure and the target velocity have been measured using a shock‐velocity measurement and the double foil technique, respectively. The conditions are discussed that must be satisfied if the double‐foil technique is to give an accurate measurement of the velocity of the dense part of the target. The rocket model has also been improved using a time‐dependent applied pressure pulse, in order to accurately describe the relation between ablation pressure, target velocity, and ablated fraction. Pressures up to 50 Mbar have been easily generated since lateral energy transport is rather low with a 0.26 μm wavelength laser.

Enhancement of a laser‐driven shock wave up to 10 TPa by the impedance‐match technique

We have used the impedance‐match technique to increase the shock pressure induced in an aluminum‐gold target by a laser of 0.26 μm wavelength and intensity of 1015 W/cm2. With incident pressures of 4.5 TPa in aluminum, transmitted pressures of 10+4−3.5 TPa in gold are inferred from shock velocity measurements. Experiments on gold‐aluminum targets, with the same irradiation conditions, verify the shock pressure decrease due to the reverse impedance‐match effect.

Experimental study of laser acceleration of planar targets at the wavelength 0.26 μm

The main characteristics of accelerated aluminum targets, which are the target velocity, the uniformity of the acceleration and the backside temperature have been studied in laser experiments performed at wavelength 0.26 μm with an absorbed flux of a few 1013 W/cm2, in 400‐ps pulse duration by using the double‐foil technique and an optical pyrometry diagnostic: The ablation pressure was inferred from the velocity measurements. The uniformity of the acceleration was shown to be controlled by the hot spots in the focal spot, and the importance of studying the smoothing of laser inhomogeneities for accelerated targets with large ablated fractions was emphasized. The observed dependence of the backside temperature as a function of the initial foil thickness is discussed in the light of shock wave heating and radiative heating.

Optical smoothing for shock-wave generation: Application to the measurement of equations of state

Experimental results are presented on shock-wave generation in solid samples, irradiated directly by optically smoothed laser beams. Random phase plates and phased zone plates have been successfully used. In particular, the last technique allowed the production of uniform shock fronts that have been used for equation of state experiments at pressures above 10 Mbar. Pressures higher than 35 Mbar were achieved in gold, by using laser pulses with energy E = 100 J, and structured, two-step, two-material targets.

Dynamics of laser produced shocks in foam–solid targets

The influence of foams on laser shocks was studied with ns laser pulses smoothed with phase zone plates and focused onto layered foam–aluminum targets. Foams of 5–200 mg/cm3 density and 60 μm thickness were used. A strong pressure increase was measured with the foam in comparison to focusing the beam directly onto aluminum due to impedance mismatch at the aluminum–foam interface. Below a particular density, the measured pressure decreased as a result of hydrodynamics effects. Results are compared with computer simulations.