R. Fabbro

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.

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.

Importance of two‐dimensional effects for the generation of ultra high pressures obtained in laser colliding foil experiments

A 12 μm polyester foil is accelerated by a 0.26 μm wavelength laser and collides with a 15 μm thick molybdenum foil. The accelerating pressure is 45 Mbar (laser intensity≊3– 4×1014 W/cm2) and gives to the polyester foil a velocity of about 160 km/sec. The measurement of the shock pressure induced in the impacted foil is made with an improved step technique. When the initial spacing between the two foils is too large compared to the focal spot radius, i.e., larger than 20–30 μm, the different experimental results cannot be reproduced with one‐dimensional simulations; this is only possible by using a two‐dimensional Lagrangian code that has been developed and that takes into account the strong deformation of the accelerated foil. Finally, even with the low level of x‐ray heating due to the ablation plasma, multihundred megabar pressures can be obtained within a very short time.

Measurements of laser shock pressure and estimate of energy lost at 1.05‐μm wavelength

Laser‐driven shock pressures at 1.05‐μm wavelength have been evaluated from measurements of shock transit time through aluminum foils by streak camera records of shock luminosity at the back face of the foil. An ablation pressure of 0.3 TPa is obtained for 1.2×1014 W/cm2 laser pulses focused on 300‐μm spot diameter and 0.55 TPa for 3.5×1015 W/cm2 laser pulses focused on 60‐μm spot diameter. These results, compared with theoretical values, show an important loss of energy, attributed to two‐dimensional effects. The ratio of effective energy for compression to incident energy is estimated to be 12% for 1.2×1014 W/cm2 experiments and only 1% for 3.5×1015 W/cm2 experiments.

Modeling of high‐pressure generation using the laser colliding foil technique

An analytical model describing the collision of two foils is presented and applied to the collision of laser‐accelerated foils. Numerical simulations have been made to verify this model and to compare its results in the case of laser‐accelerated foils. Scaling laws relating the different parameters (shock pressure, laser intensity, target material, etc.) have been established. The application of this technique to high‐pressure equation of state experiments is then discussed.

Two‐dimensional study of shock breakout at the rear face of laser irradiated metallic targets

The two‐dimensional propagation dynamics of laser‐driven shock waves in solids is studied through the analysis of the shock breakout at the rear face of the target for a set of materials and laser intensities. The laser shock simulations were carried out by means of a two‐dimensional hydrodynamics code in which the laser‐ablation pressure is replaced by an equivalent pressure pulse. It is shown that the two‐dimensional code is a very useful tool to analyze laser‐shock experiments where two‐dimensional effects arise from a finite laser‐spot size or a heterogeneous energy deposition.

Laser shock experiments at pressures above 100 Mbar

Abstract Laser shock experiments are performed in planar geometry at 0.26 μm wavelength and 10 15 W/cm 2 irradiance. Two techniques of pressure enhancement are used. In impedance-match experiments, a laser-induced shock of 5 TPa in aluminum is transmitted in gold. From shock velocity measurements, the pressure inferred in gold is about 10 TPa. The flyer plate technique is used with thin aluminum foils accelerated by laser-driven ablation. In the same irradiance conditions, pressures of more than 15 TPa are generated in aluminum targets impacted by the accelerated foil.