Measuring Geometric Parameters of a High-Power Infrared Laser Beam near the Focus for Applications in a Laser-Plasma Sho
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Measuring Geometric Parameters of a High-Power Infrared Laser Beam near the Focus for Applications in a Laser-Plasma Short-Wave Radiation Source A. V. Belashova, P. S. Butorina, Yu. M. Zadiranova, S. G. Kalmykova, *, V. A. Maximova, M. E. Sasina, and P. Yu. Serdobintsevb, c a Ioffe
b
Institute, St. Petersburg, 197101 Russia Peter the Great St. Petersburg Polytechnic University, St. Petersburg, 197101 Russia c St. Petersburg University, St. Petersburg, 199034 Russia *e-mail: [email protected] Received February 7, 2020; revised April 3, 2020; accepted April 15, 2020
Abstract—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. Keywords: laser plasma, laser, focus, caustic surface, waist, ablation DOI: 10.1134/S0030400X2008007X
INTRODUCTION The present work is undertaken for needs of the nanometric wavelength lithography (Extreme UltraViolet range—EUV) with a Xe laser plasma as the working radiation source. A conception of such a source where the plasma was generated on a gas-jet target had been proposed and realized as early as in 1990s [1, 2]. After approximately 10-year effort to adapt this type of the source to the λ = 13.5 nm lithography, the choice of the target material had been changed in favor of microdroplets of metal tin [3, 4] because of insufficient radiation intensity of the Xe source. However, a proposal [5] to develop a new lithography version with the wavelength of λ = 11.2 nm and with Be-containing interference mirrors [6, 7] induced a new wave of interest in the source with the Xe laser plasma, since its radiation intensity at the new wavelength was expected to be about an order of magnitude higher than at λ = 13.5 nm. First experimental studies of the λ = 11.2 nm source have been published in [8–11]. Development of the lithographer optical system as well as studying a number of physical aspects of the source require a knowledge of geometric dimensions of the plasma emitting in the EUV range. Since the radiation energy loss dominating energy balance of plasmas of many-electron atoms is characterized with energy confinement times significantly shorter than both the laser pulse duration and characteristic times
of diffusion and hydrodynamic processes [10], dimensions of the hot plasma core should coincide with those of a laser-illuminated zone, that is, with the external caustic surface of the beam in a near-focus area (waist). In other words, measuring the laser beam geometry near the focus is an indirect method to define dimensions of the hot, EUV-emitting plasma core that explains the present work motivation. Measurements of the be
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