Xe laser-produced plasma with a gas-jet target is considered a promising λ = 11.2-nm radiation source for a possible extension of the EUV (Extreme UltraViolet) lithography. EUV spectra of the plasma radiation obtained with the aid of both a grating spectrograph and Bragg mirrors are presented. The absorption of the EUV radiation in the cold peripheral gas has been eliminated in another experiment by means of irradiating the target with a wide defocused laser beam that resulted in an increase of the EUV output by an order of magnitude. In that experiment, the conversion efficiency at λ = 11.2 nm amounted to 3.9%.

Subject of study. The study focuses on laser plasma excited by a Xe gas-jet target. Aim of study. The aim of this study is to increase the output of extreme ultraviolet radiation from such a plasma to a level that meets the requirements of industrial production, specifically for use as a radiation source in a new branch of lithography with a wavelength near 11.2 nm. Method. The primary method used involves changing the diameter of the laser beam by moving a Xe gas-jet target along its axis. This adjustment leads to a change in the interaction area between the beam and the target, which in turn alters the size of the laser spark. The intensity of plasma radiation at wavelengths of 11.2 nm and 13.5 nm was measured using a surface-barrier Si photosensor and a Bragg mirror. Additionally, the energy of the laser radiation absorbed by the plasma was measured. Main results. The results show that, when the diameter of the laser beam illuminating the target increases from 46 mu m to 344 mu m, the energy emitted in the extreme ultraviolet range increases by approximately 5 times. In the identified irradiation mode, the efficiency of converting laser radiation into radiation with a wavelength of 11.2 nm was 3.9%. Recent measurements of the plasma lifetime have shown that it depends on the size of the plasma and, in several experiments, is significantly shorter than the laser pulse duration. This finding suggests that the plasma lifetime can be used as an optimization parameter when selecting the laser pulse duration. Practical significance. A record-high efficiency is obtained for the conversion of laser pulse energy into extreme ultraviolet radiation by a laser-plasma radiation source with a gas target. This achievement opens up the prospect of using such sources in the industrial production of microcircuits. (c) 2024 Optica Publishing Group

The described method is intended for application as a diagnostic tool for a nonstationary, short-lived plasma (in particular, for the laser-produced plasma). It is based on taking into account the lifetime of a laser-produced plasma, which is so short (several nanoseconds) that it is not enough for the ionization equilibrium to be established. Among mechanisms leading to appearance of an ion with a given charge Z in the plasma, only the electron-collisional ionization is considered, because contributions of other phenomena turn out to be negligible. The method is discussed as an example of a plasma excited on the Xe gas-jet target. The necessary collisional cross sections of ions from+7Xe to+16Xe have been calculated specifically for this study using a quantum-mechanical numerical simulation, with its principles and features being also presented in the paper. To demonstrate capabilities of the method, it has been applied to one of the experimental cases when the plasma was produced by the laser beam focused on the Xe gas-jet target. The time-integrated energy of laser radiation absorbed in the plasma was measured, and the absorption coefficient, μ, was derived from it with a correction for the plasma lifetime, which was several times shorter than the laser pulse. Using the method described here, the values of ⟨Z⟩ and then μ were calculated as a function of temperature. The time-averaged plasma temperature, T, in the above-mentioned experiment was believed to be equal to that at which the calculated and experimentally determined values of μ coincided. The following results were obtained: T = 42 eV, ⟨Z⟩ = 10.2.

In this paper, experiments on measuring absorption of infrared laser radiation in the laser-produced plasma of Xe are described. An absorbed fraction of up to 65% was obtained when the gas-jet target was illuminated by a wide, defocused beam, whereas it barely reached 8.5% in the case of a sharply focused beam. The phenomenon is explained on the basis of a hypothesis of the plasma’s hydrodynamic expansion according to which the plasma leaves the illuminated area faster the smaller its size. This explains the similarity of extreme ultraviolet output and laser energy absorption as functions of the laser beam diameter. Based on the experimental results, an attempt to estimate the plasma parameters ( N, T, ) is undertaken, with the mean ion charge, , being calculated using ionization cross-sections for ions from +7 Xe to +14 Xe, which were obtained by means of a quantum-mechanical numeric simulation especially for the present work.

Measurements of geometric parameters of a high-power infrared laser beam have been realized by three methods. The obtained results are in a satisfactory agreement with each other. When focusing the beam with wavelength of 1.064 μm with a specially designed non-aberrational objective, a focal spot of approximately 40 μm diameter could be obtained, with the waist length being 230−280 μm. The measured parameters give an idea of the size of a laser plasma generated by this beam on a Xe gas microjet.

Absorption of the laser pulse energy in the plasma has been shown to change from 8.5% at irradiation of a gas-puff target with a beam narrow-focused onto a near-central area of the jet up to 65% at the wide defocused irradiation. An analysis of this phenomenon is based on a hypothesis that plasma density decays significantly during the 10ns laser pulse due to hydrodynamic expansion of the hot plasma. A similarity of the EUV (Extreme UltraViolet) intensity and the laser light absorption as functions of the laser beam diameter suggests a revision of the conventional idea of a strong EUV radiation self-absorption in a cold peripheral shell of the laser plasma whereas the high absorptivity of the laser radiation by the plasma looks like a major feature to gain high efficiency of an EUV source.

Measurements of geometric parameters of a powerful infrared laser beam have been realized by three methods. The results obtained are in a satisfactory agreement with each other. When focusing the beam with wavelength of 1.064 μm by means of a specially designed non-aberrational objective, a focal spot of ≈40μm diameter could been obtained, with the waist length being 230−280 μm. The measured parameters give an idea of the size of the laser plasma generated by this beam on the Xe gas microjet.

AbstractA more than tenfold increase in the intensity of EUV radiation from a laser-produced plasma source is observed experimentally under the illumination of a Xe gas-jet target with a wide laser beam covering its densest part compared with the radiation intensity at the traditional geometry, when the beam is sharply focused on the jet axis. As an explanation, it is assumed that in this case almost the whole dense part of the jet is heated and ionized under the action of laser irradiation and becomes transparent to the quanta of the studied radiation. The thickness of the absorbing peripheral shell of a laser spark decreases accordingly, that leads to the increase in the radiation intensity reaching the observer.

Spectra of Xe laser plasma radiation were measured using Si/Mo and Mo/Be interference mirrors. The intensity within the 11.0-11.8 nm wavelength band has been shown to be 6-7 times as high than that in the 12.5-14 nm range. The spectra obtained by means of the mirrors are compared with those measured earlier using a spectrometer.

A more than tenfold increase in the intensity of EUV radiation from a laser-produced plasma source is observed experimentally under the illumination of a Xe gas-jet target with a wide laser beam covering its densest part compared with the radiation intensity at the traditional geometry, when the beam is sharply focused on the jet axis. As an explanation, it is assumed that in this case almost the whole dense part of the jet is heated and ionized under the action of laser irradiation and becomes transparent to the quanta of the studied radiation. The thickness of the absorbing peripheral shell of a laser spark decreases accordingly, that leads to the increase in the radiation intensity reaching the observer.

With the aid of Mo/Be and Si/Mo interference mirrors, measurements of radiation intensity from laser plasma with Xe gas-jet target have been realized within a wavelength interval of 11–14 nm with a spectral resolution of 3–6 Å. The results are compared with the spectrum formerly measured with the aid of a spectrograph. The ratio of intensities at wavelengths of 11.2 and 13.5 nm has been found to be about 10 under experimental conditions studied.

Поступило в Редакцию 8 сентября 2017 г.В окончательной редакции 6 марта 2018 г.

Emission spectra in the wavelength range of 5–25 nm of the laser plasma produced using a gas jet of Xe and a mixture of Xe + Ar with an atomic density of up to 7 × 1018 cm–3 are described. There are no discrete spectral lines in the xenon spectra, but a wide continuous peak of the radiation is observed within the 9–14 nm wavelength band. At variations of experimental conditions, its maximum demonstrates a regular wavelength shift, which is attributed to the corresponding change in the plasma temperature. Another feature is an only slight decrease in the intensity of xenon emission when the target is strongly diluted with argon.

An upper limit of absorption of the laser radiation in the plasma produced in a gas jet Xe target with the average density of (3–6) × 1018 cm–3 and the effective diameter of 0.7 mm is found. It is equal to ≈50% and remains constant under any variation in this range of densities. This result contradicts both theoretical assessments that have predicted virtually complete absorption and results of earlier experiments with the laser spark in an unlimited stationary Xe gas with the same density, where the upper limit of absorption was close to 100%. An analysis shows that nonlinearity of absorption and plasma nonequilibrium lead to the reduction of the absorption coefficient that, along with the limited size of plasma, can explain the experimental results.

Обнаружен верхний предел поглощения лазерного излучения в плазме, созданной им на Xe газоструйной мишени со средней плотностью (3-6) · 1018 cm-3 и эффективным диаметром 0.7 mm. Он составляет ~ 50% и остается неизменным при любых вариациях плотности в этом диапазоне. Этот результат расходится как с теоретическими оценками, которые предсказывали практически полное поглощение, так и с результатами более ранних экспериментов с лазерной искрой в неограниченном стационарном Xe такой же плотности, где он был близким к 100%. Анализ показывает, что нелинейность поглощения и неравновесность лазерной плазмы приводят к уменьшению коэффициента поглощения электромагнитного излучения в ней, что вместе с ограниченностью ее размера может объяснить полученные экспериментальные результаты.

In experiments with a laser-plasma EUV-radiation source, the main IR Nd: YAG laser pulse was preceded by that of a UV KrF excimer laser. Dramatic modulations of EUV plasma emissivity have been observed at long interpulse times, from hundreds of nanoseconds up to microseconds. To discover the nature of these prepulse-produced long-living perturbations of the target, a fluid dynamics numerical simulation of the Xe gas jet has been carried out. The prepulse has been found to generate a quasi-spherical shock wave with a thin dense front layer and a vast rarefied inside area. In the course of time, the front expands and simultaneously drifts downstream along with the gas. Depending on the interpulse time, the IR laser beam either intersects the dense layer or propagates within the rarefied gas cavity whereby the above-mentioned variations in the plasma emission can be explained. The possibilities of making use of the discovered phenomena to enhance the observed EUV plasma brightness are discussed.

Описываются спектры свечения в диапазоне длин волн 5-25 nm лазерной плазмы, создаваемой на газовой струе из Xe и смеси Xe + Ar с плотностью атомов до 7· 1018 cm-3. В спектрах Xe отсутствуют дискретные спектральные линии, но в полосе 9-14 nm наблюдается широкий непрерывный пик излучения, максимум которого при вариациях условий эксперимента обнаруживает закономерное смещение по шкале длин волн, которое приписывается соответствующему изменению температуры плазмы. Другой особенностью является лишь незначительное уменьшение интенсивности свечения Xe при сильном разбавлении мишени аргоном. DOI: 10.21883/PJTF.2017.22.45255.16957

Based on the results of numerical fluid dynamics simulation, an imitation parameter has been constructed which simulates the observed intensity of the laser plasma emission in a short-wave range. Within the computational model frame, a high-temperature perturbation is created in the jet that generates a strong shock wave. The resultant complicated target structure and its evolution lead to nonmonotonic time variations of the simulation parameter. This result agrees well with the experimentally measured behavior of emission from the laser plasma formed on the target perturbed by an additional laser prepulse.

Previous investigations of the laser plasma at two-pulse mode of its generation revealed long-living perturbations of the gas-jet target by the first pulse, leading to significant modulations of the plasma radiation. In the present paper, results of a numerical hydrodynamic simulation of the gas-jet target are reported which provide explanation of the observed phenomena. An impact of the first pulse (prepulse) upon the target results in formation of a dense quasi-spherical layer in it, with a low-density area inside. This layer expands with the time and drifts downstream with the gas flow. Depending on the time interval between pulses, the second laser pulse can either intersect the dense layer or pass through the low-density gas, whereby the observed modulations of the plasma emission can be explained.