Laser sputtering–part IV: Laser interferometry and laser-induced fluorescence studies of insulators

Abstract

Ultra-violet laser etching (sputtering) of solids has been explained in various terms, including thermal, electronic, hydrodynamic, exfoliational, and even collisional processes. To distinguish between these possibilities, two laser techniques are used here to probe the plume of material expanding away from the etched surface of certain insulators due to 248 nm pulses. The first set of experiments measures the free-electron density in the plume using a Michelson interferometer.The results indicate that a plasma is formed for laser fluences greater than some threshold value (which depends on the material). With ,41203, the plasma threshold is, moreover, similar to but slightly higher than the etching threshold, suggesting that collisional sputtering due to fast ions from this plasma does not contribute substantially to the etching. The second set of experiments, using laser-induced fluorescence, measures the number densities and energy distributions of A1 and A10 in the plume when etching Ah03. Time-of-flight distributions indicate Boltzmann-like behavior with kTequal to -3.5-17eV for A1 and -1.1-0.3 eV for AIO. In contrast, the rotational and vibrational energies of A10 correspond to only 0.04-0.16 eV, suggestive of a low surface temperature during etching. These observations are consistent with an electronic desorption mechanism similar to that found with incident fission fragments, electron pulses, or small accelerated dust particles.

Time-of-flight investigation of the intensity dependence of laser-desorbed positive ions from SrF2

“…With increasing number of desorbed particles a dense expanding plasma is formed at the surface. The free electron density depends on the irradiation energy and can reach high values (>101S/cm 3 [25]) making the plasma reflective. Additionally, a part of the laser light can be lost due to the acceleration of plasma electrons via inverse bremsstrahlung and to secondary ionisation processes in the gas phase [29].…”

“…It is generally accepted that at a certain material-and wavelength-dependent laser energy, the flux of desorbed particles becomes so high that an expanding plasma is formed at the surface [25,28]. The detected positive ions can either be desorbed as ions from the surface or can be ionized in the gas phase.…”

“…At laser intensities in the order of GW/cm 2, CB electrons produced by multiphoton absorption processes are accelerated via absorption of laser photons (inverse bremsstrahlung) [21] and can in this way obtain kinetic energies of more than 12 eV [22]. Despite the possibility of kinetic CB electrons, a single desorption process based on an "intrinsic" KFM remains unlikely and collective processes like dielectric breakdown [23,24] or thermal instability (local melting) [25][26][27] of the absorbing solid will occur. One purpose of this work will be to investigate, if desorption models developed for semiconductors can be adapted to ionic crystals.…”

Electronic temperature and density of the plasma produced by nanosecond ultraviolet laser ablation of LiF

“…8 A limited number of works have been focused to the study of the fundamental plasma parameters in PLD, such as the electron temperature and/or the electron density, that can be of interest for the study of the plasma reactivity, the energy transport into the plasma, the role played by plasma ions ͑whose concentration equal that of the electrons͒ and the plasma formation process when irradiating wide-band-gap materials with pulsed UV radiation from excimer lasers. [11][12][13][14][15] A recent work 16 reports interesting results on the comparison between nanosecond ͑ns͒ and femtosecond ͑fs͒ PLD of LiF at relatively high laser fluences ͑12.5 J / cm 2 ͒ including extensive optical emission spectroscopy ͑OES͒ studies, although no measurements of N e and T e were provided. In this regard, measurements of N e and T e in the near-surface region of the luminous plume formed by laser ablation of wideband-gap materials have only been indirectly or partially reported.…”

“…In this regard, measurements of N e and T e in the near-surface region of the luminous plume formed by laser ablation of wideband-gap materials have only been indirectly or partially reported. [11][12][13][14] Therefore, the aim of the present work is to determine the spatial evolution of N e and T e in the plasma produced by pulsed UV laser ablation of LiF crystals.…”