R. L. Webb

Interactions of wide band‐gap single crystals with 248 nm excimer laser radiation. II. NaCl

The emission of charged and neutral particles from single‐crystal MgO irradiated with pulsed 248 nm excimer laser light is studied by means of quadrupole mass spectrometry, time‐resolved emission spectroscopy, luminescence spectroscopy, and scanning electron microscopy (SEM) observations. The role of the initial distribution of near‐surface defects plus defects which result from repeated application of laser pulses is explored. This increase in defect density eventually leads to formation of a visible plume and rapid material vaporization. SEM observations after irradiation indicate that substantial surface fracture is present prior to the onset of rapid vaporization. Defect production during irradiation is attributed to mechanical processes involving deformation and fracture with accompanying dislocation motion. The accumulation of these defects increases laser absorption in the near‐surface region resulting in rapid thermal etching and cluster emission.

Interactions of wide band gap single crystals with 248 nm excimer laser radiation. III: The role of cleavage-induced defects in MgO

When single‐crystal, arc‐fused MgO is cleaved and exposed to single pulses of 248 nm, 3–10 J/cm2 excimer laser light (in vacuum), micron‐sized holes are produced in the irradiated area. Cleavage produces micron‐sized sites which exhibit highly localized absorption, resulting in decomposition, melting, and vaporization of the crystal at these sites. At fluences between 1 and 2 J/cm2, single laser pulses can produce localized, superficial melting and decomposition, leaving clusters of Mg‐rich droplets on the surface. The mass‐resolved charged and neutral particle emission produced by irradiation of the cleaved MgO surface with single laser pulses are examined. There are considerable variations in these emissions when single laser pulses strike different regions of the cleavage surface; these variations are strongly correlated with the production of conical holes in the irradiated region. A phenomenological model for the creation of these sites involving dislocation driven processes during cleavage and the r...

Dopant induced ablation of poly(methyl methacrylate) at 308 nm

Poly(methyl methacrylate) (PMMA) is highly resistant to laser ablation at 308 nm. Either very high fluences or absorbing dopants must be used to ablate PMMA efficiently at this wavelength. We investigate two dopants, pyrene and a common solvent, chlorobenzene, using time-of-flight mass spectroscopy. Both compounds improve the ablation characteristics of PMMA. For both dopants, the first step in ablation is an incubation process, in which absorption at 308 nm increases due to the production of C=C bonds along the polymer backbone. Incubation at 308 nm is similar to that observed for shorter ultraviolet wavelengths in previous studies. The principal ablation products and their corresponding temperatures are consistent with a photothermal ablation mechanism.

Neutral atom and molecule emission accompanying 248-nm laser irradiation of single crystal NaNO3

Abstract We report quadrupole mass-selected measurements of neutral emissions produced by exposing single crystal sodium nitrate to 248-nm radiation from a pulsed excimer laser. The intensity and identity of the observed products are a strong function of laser fluence as well as previous exposure. In particular, a strong incubation effect is observed, with weak emission intensities during the early stages of irradiation which grow dramatically with continued irradiation. At low fluences ( 2 on laser-damaged surfaces), NO is the only neutral product observed. Thus it appears that defects created by NO emission on more or less perfect NaNO 3 surfaces strongly enhance subsequent emission. At higher fluences (> 100 mJ/cm 2 on laser-damaged surfaces), a visible fluorescent plume is observed and other neutral products (primarily O 2 and Na) are emitted along with NO. Time-of-flight measurements of the anion-derived species (NO, O 2 , NO 2 ) show unusually long emission tails which are inconsistent with emission during the laser pulse only. We attribute this “delayed” emission to dissociative electron attachment due to charge transport from the bulk to the surface similar to electron beam induced emission.

CHARACTERIZATION OF PARTICULATES ACCOMPANYING LASER ABLATION OF NANO3

We present observations of submicrometer- to micrometer-sized particles generated by high-fluence (≥10 J/cm2) 248-nm laser ablation of single-crystal NaNO3 in vacuum and at atmospheric pressure. Small particles (50–200 nm in diameter) are ejected by hydrodynamic sputtering. Larger particles (1–20 μm in diameter) are produced by cavitation and spallation in the melt. Many particles formed in air carry electric charge, with roughly equal numbers of positively and negatively charged particles. The particle composition is consistent with substantial nitrate decomposition. The implications of these observations with respect to laser-based chemical analysis are discussed.

Optical transmission through inelastically deformed shocked sapphire : stress and crystal orientation effects

Plane shock wave experiments were performed to examine optical transmission in sapphire single crystals (c cut, a cut, and r cut) compressed to longitudinal stresses ranging between 119 and 260kbar. Peak stress and particle velocity values in our experiments were obtained by compiling and analyzing published continuum data on shocked sapphire crystals. Time-resolved measurements, with nanosecond resolution, showed time-, stress-, and orientation-dependent changes in optical transmission beyond the Hugoniot elastic limit. Over the 300–680-nm range examined in our work, no wavelength dependence was observed. Loss of optical transmission in the stress range examined is due to inelastic deformation in shocked sapphire crystals. The present data reveal that inelastic deformation mechanisms are different in r-cut sapphire compared to a-cut and c-cut sapphire, and may be different for all three orientations examined. Although quantitative analysis of the time-dependent optical transmission data is not possible a...

Photoluminescence imaging of mechanically produced defects in MgO

Abstract Deformed MgO displays a strong blue photoluminescence when illuminated by 248 nm excimer laser radiation. This photoluminescence is readily imaged in situ (during laser exposure) using commercial microscope optics. Photoluminescence images of indented, cleaved, abraded, and laser-damaged surfaces, are reported and the observed features are related to the expected deformation patterns. Locally deformed regions on MgO single crystals are sites of localized absorption at 248 nm and serve to nucleate localized damage under high fluence irradiation. Photoluminescence imaging has great potential for the identification of flaws in optical materials prior to, during and after exposure to damaging fluences of laser radiation and for fundamental studies of deformation, fracture and tribology.

The role of defects in the rear side laser ablation of MgO at 308 nm

Over a range of fluences, 308 nm pulsed laser irradiation of cleaved, single crystal MgO can produce a fluorescent plume on the side of the specimen opposite the incident light (rear side ablation). After one or two laser pulses, linear patterns of melted material are often observed. We present evidence that Fresnel diffraction from heterogeneous features on the front surface control the formation of the localized interactions on the rear side. At fluences below the threshold for plume formation on the front surface, large areas of the rear surface can be melted and ablated without fracture and pitting. At higher fluences, defect accumulation on and near the front surface eventually yields a front surface plume and eliminates the rear surface plume. Mechanical treatments strongly affect the evolution of these features due to the production of strongly absorbing point defects.

Sensitization of PMMA to laser ablation at 308 nm

Abstract Pure polymethylmethacrylate (PMMA) is highly resistant to ablation at 308 nm. The value of PMMA in lithography and semiconductor packaging and the availability of reliable 308 nm optics has motivated the development of dopants to facilitate PMMA ablation at 308 nm. We investigate the laser ablation of solvent cast PMMA films with and without pyrene, a typical dopant. The presence of residual solvent is shown to strongly promote laser ablation at low fluences in the case of chlorobenzene (CB), but not in the case of N-methyl 2-pyrrolidinone (NMP). At low laser fluences, many laser pulses may be required before significant neutral particle emissions are observed – an incubation effect. Scanning electron microscope observations indicate that the onset of emission coincides with the rupture of a thin surface layer, presumably depleted of solvent during film manufacture. The depleted layer would be relatively impervious to volatile fragments produced in the bulk. When this layer ruptures, volatile fragments escape and can be detected. Thus, the ablation behavior depends not only on the choice of dopant, but on the choice of solvent and the details of film manufacture.

Ablation of Single Crystal MgO by UV Excimer Irradiation

The ablation of single crystal MgO irradiated with 248 nm excimer laser light is studied by means of time resolved spectroscopy and quadrupole mass spectrometry. Luminescence spectra and SEM observations indicate that repeated laser bombardment gradually increases the density of potentially absorbing defects. In polished samples, this progressive growth is preceded by an initial clean-up (reduction) of surface damage. Unlike many wide band gap materials, defect production in MgO by electronic mechanisms is not likely. Chemical etch techniques indicate the presence of high dislocation densities in regions etched by the laser, suggesting that point defect production by dislocation motion is important. The ablation plume is composed of charged particles, including cluster ions, as well as a high density of excited neutrals. The growth of the plume with repeated bombardment correlates with defect formation as indicated by luminescence intensities.