C. E. Otis

On single-photon ultraviolet ablation of polymeric materials

The nature of uv ablation of organic polymers is discussed in terms of a pseudo‐zeroth‐order rate law of the form dx/dt = k0e−(Eact/kT), where Eact is assumed to be the strength of the weakest bonds in the polymer and T is the local temperature increase from the incident laser pulse. Equations derived from previous models that assumed nonthermal photodecomposition were duplicated from this photothermal model. Even for the simple case of single‐photon absorption, nonideal behavior is affected by radiationless decay, pulse length, and thermal diffusion. These effects were probed. Results indicated that thermal diffusion may have a significant effect on the threshold fluence and to some degree on the shape of the etch depth versus fluence curve. Absorption dynamics (saturation and radiationless decay) appear to be the dominant factor in determining the functional dependence of etch depth on fluence. As a result of competition between absorption saturation and radiationless decay, the penetration depth is int...

Photothermal description of polymer ablation: Absorption behavior and degradation time scales

A photothermal model of ablation is used to investigate the time scales for polymer degradation by UV laser light. In the absence of a significant incubation effect, strong absorbers (e.g., polyimide) are expected to decompose roughly three orders of magnitude faster than weak absorbers (e.g., polymethylmethacrylate), owing to the higher surface temperature attained during the absorption. This difference in the time scales reflects processes taking place at significantly different temperatures and should extrapolate to the overall ablation process. The very short calculated time scales (femtoseconds for polyimide and picoseconds for polymethylmethacrylate) indicate that polymer decomposition occurs rapidly compared to the actual ejection of material. For multipulsed ablation experiments, significant incubation modification tends to increase the absorbance of weak absorbers, making the effect less marked than in the ideal case. Incubation effects are attributed to nonablative decomposition that occurs at c...

Temperature measurements of polyimide during KrF excimer laser ablation

The temperature at the interface between a thin polyimide film and a quartz substrate was monitored as a function of time during KrF (248 nm) laser‐induced heating and ablation using thin film NiSi thermistors. These experimental temperature measurements were coupled with heat flow simulations to obtain time‐resolved temperature profiles in the polyimide. Thermal properties of the polyimide were estimated by requiring that the simulations reproduce experimental temperature profiles. The peak surface temperature of the polyimide at the onset of ablation was subsequently estimated from these constrained simulations and a value of 1660±100 K was obtained for the observed ablation threshold fluence of 36 mJ/cm2.

Internal energy distributions of laser ablated species from YBa2Cu3O7−δ

The first simultaneous measurements of the ground‐state translational, rotational, and vibrational energy distributions of a laser ablated diatomic molecule (YO) liberated during a 351 nm excimer laser ablation of YBa2Cu3O7−δ, are reported. In addition, the translational energy distributions of Y, Y+, Ba, Ba+, and Cu are reported. Measurements made at low and high fluences on YO indicate that there are very large differences in the ‘‘temperatures’’ inferred between the translational and the vibrational/rotational energy channels, implying that energy is partitioned in a very nonequilibrium fashion in the expanding plume. A measurement of the rotational and vibrational temperatures shows that the internal degrees of freedom are equilibrated in the expansion plume (Boltzmann distributions) with a temperature of approximately 1000 K being observed at normal processing fluences. The translational energy distributions of YO are observed to fit a shifted Boltzmann distribution at high fluences. The atomic speci...

Gas‐phase oxidation of copper during laser ablation of YBa2Cu3O7−δ in different oxidizing ambients

The gas‐phase production of CuO during ablation of a YBa2Cu3O7−δ target in the presence of three different oxidizer background gases: oxygen (O2), nitrous oxide (N2O), and nitrogen dioxide (NO2), has been monitored using laser‐induced fluorescence. The oxides of nitrogen are found to be much more efficient than oxygen in the production of CuO, with the order of reactivity being O2<N2O<NO2. In general for all the reactant gases, and independent of the ablation laser wavelength (193 and 308 nm), the CuO concentration has been observed to initially increase with increasing background gas pressure to reach a maximum before decreasing again to negligible levels at higher pressures. The rate of increase in the CuO concentration and its subsequent decrease with pressure, while being dependent on the nature of the oxidizer gas, is also very sensitive to the laser fluence used for ablation. The results are discussed in terms of the reaction energetics and the known dynamics of other gas‐phase metal oxidation react...

Novel geometrical effects observed in debris when polymers are laser sputtered

When polymers are sputtered with 248 or 308 nm laser pulses there are two generically different responses. The most straightforward is where the emitted particles expand outwards away from the target surface obeying, in so doing, the laws of one‐dimensional adiabatic flow. The other is where the particles expand both outwards and sideways and, because a certain fraction recondenses on the target surface, there is a prominent deposit of debris lying around the bombarded spot. For spots with other than circular shape the debris show interesting symmetry in which there is rotation with respect to the spot. We show that this rotation occurs wholly through the laws of flow and we conclude, therefore, that the debris phenomenon is a purely gas‐dynamic effect. It follows that the elimination of debris can be approached in gas‐dynamic terms. We also show that the numerical extent of debris formation increases with the complexity of the particles involved, a result which suggests additional methods to control debris.

On the debris phenomenon with laser‐sputtered polymers

The sputtering of polymer films with 248 or 308 nm laser pulses causes two groups of particles to be emitted. Light particles are emitted more rapidly and escape to form a shock wave. Heavy particles are emitted more slowly and under some conditions are impeded by the light ones, expand sideways, and leave debris on the target surface. Debris formation can be understood from ordinary gas dynamics where it is equivalent to recondensation beyond the bombarded spot. This is shown by numerical solution of the flow equations.

Ultraviolet photoablation of p-tetrafluoroethylene : rotational energy distributions of the CF radical and time resolved mass spectra

Gas‐phase products from the nanosecond ultraviolet excimer ablation of polytetrafluoroethylene (Teflon©) have been characterized using mass‐resolved vacuum ultraviolet laser photoionization techniques. Picosecond and nanosecond vacuum ultraviolet (10.5 eV) photoionization mass spectra recorded following the ablation of polytetrafluoroethylene display a significant dependence on the ablation wavelength. Polymer fragments ranging in mass from 31 to greater than 300 amu are observed following the 193 nm ablation of the polymer, whereas using 248 and 308 nm, the mass spectra consist almost entirely of the monomer (C2F4). Rotationally resolved resonant two‐photon ionization (R2PI) spectra of the CF radical, recorded following the 193 nm ablation, were used to estimate the rotational temperature of this species in the ablation plume; temperatures ranged from 500 to 800 K.

H/D isotope effect in the predissociation of C2HD

Resonance‐enhanced multiphoton ionization (2+1 REMPI) spectroscopy near 243 nm was used to detect H and D photofragments of the 193 nm photolysis of C2HD. Measurements were performed in a molecular beam, laser photolysis apparatus, equipped with a time‐of‐flight mass spectrometer. The measured H/D ratio of 2.85±0.3 may indicate nonadiabatic surface hopping occurs between 2 3 A’ and 1 3 A’ and/or 2 1 A’and 1 1 A’surfaces in exit channels leading to ground state C2H(X 2 Σ+ )+H(2 S) fragments in the predissociation of C2H2.

Time‐resolved temperature measurements during pulsed laser irradiation using thin film metal thermometers

In this article, we describe a technique using NiSi and Pt thin film metal thermometers to provide accurate temperature information on a nanosecond time scale during pulsed laser processing of materials. A surface layer of interest is deposited onto the thermometer layer, and temperatures are determined from temperature dependent changes in the metal film’s resistance. Details concerning the design and fabrication of the device structure and experimental considerations in making nanosecond resolved resistance measurements are discussed. Simple analytical estimates are presented to extract quantities such as incident laser energy stored in the sample. Finally, transient temperature data in the thermometer film, in combination with heat flow calculations, allow temperature determination as a function of time and depth into the sample and, additionally, can provide information about material properties of the surface layer.