R. W. Dreyfus

On the effect of Knudsen-layer formation on studies of vaporization, sputtering, and desorption

Abstract We are here concerned with experiments in which time-of-flight (TOF) measurements are made with particles which are vaporized, sputtered, or desorbed due to a pulsed heat source. If the emitted particle number density is low enough, the particles will disperse collisionlessly. Provided the emission is truly thermal, the velocities will then be described by a “half-range” Maxwellian, i.e. a Maxwellian with only positive velocities normal to the target, for which it is well known that the surface temperature, T s , and the energy, E , defined by the peak position of the TOF spectrum are related by kT s = E /2 . More commonly the emitted particle density is high enough that near-surface collisions occur. For as few as 3 collisions per particle a Knudsen layer forms, i.e. there is a layer within a few mean free paths of the target surface in which the distribution function evolves to a “full-range” Maxwellian in a center-of-mass coordinate system . We show that for on-axis measurements the relation kT s = E /2 is replaced by kT s = E /η K , with η K ranging from 2.52 for a monatomic species to 3.28 for a species with many accessible internal degrees of freedom. Failure to recognize the formation of a Knudsen layer thus leads to a severely overestimated value for T s , at least for an-axis measurements.

Reconsidering the mechanisms of laser sputtering with Knudsen-layer formation taken into account

Abstract Efforts to establish the mechanisms of laser sputtering have been severely hampered because of a number of inconsistencies. These include time-of-flight (TOF) temperatures which were too high for normally emitted particles, were too low for obliquely emitted particles, and increased with the mass when several species were co-sputtered. At the same time the angular distributions were rarely of the expected form, cos θ , but rather spanned the interval cos 4 θ to cos 10 θ . We argue that the common feature in all cases is the known or suspected occurrence of near-surface gas-phase collisions, also termed Knudsen-layer formation. To this end we establish in the present work analytical solutions for the TOF signals for both a one and two-component system when there are sufficient collisions for a fully developed Knudsen layer, insufficient collisions for adiabatic expansion, and the detector is permitted to be off-axis. The properties of the solutions include (i) a shifting of the TOF spectrum to higher velocities for normal emission and (to a limited extent) to lower velocities for oblique emission, (ii) a temperature which appears to increase with the mass when several species are co-sputtered, as well as (iii) angular distributions similar to cos 4 θ .

Ultraviolet laser ablation of polyimide films

Pulsed laser radiation at 193, 248, or 308 nm can etch films of polyimide (DuPont KaptonTM). The mechanism of this process has been examined by the chemical analysis of the condensible products, by laser‐induced fluorescence analysis of the diatomic products, and by the measurement of the etch depth per pulse over a range of fluences of the laser pulse. The most important product as well as the only one condensible at room temperature is carbon. Laser‐induced fluorescence analysis showed that C2 and CN were present in the ablation plume. At 248 nm, even well below the fluence threshold of 0.08 J/cm2 for significant ablation, these diatomic species are readily detected and are measured to leave the polymer surface with translational energy of ∼5 eV. These results, when combined with the photoacoustic studies of Dyer and Srinivasan [Appl. Phys. Lett. 48, 445 (1986)], show that a simple photochemical mechanism in which one photon or less (on average) is absorbed per monomer is inadequate. The ablation proces...

Cu0, Cu+, and Cu2 from excimer‐ablated copper

Bulk copper is laser etched with 193‐ and 351‐nm excimer radiation. The transition from the thermal to the plasma etch region is studied by measuring the densities and kinetic energies of three copper species (Cu0, Cu+, and Cu2 ) in the etch plume. A unique laser‐induced fluorescence experiment allows these three species to be followed essentially simultaneously as a function of fluence. Three separate types of etching behavior are clearly evident (even within the small fluence range of ∼1–12.5 J/cm2 ); i.e., thermal vaporization of Cu, multiphoton ionization of the Cu vapor, and electron‐atom collision‐induced ionization (breakdown) and dissociation (of Cu2 ).

Laser‐induced fluorescence studies of excimer laser ablation of Al2O3

We have used laser‐induced fluorescence to measure the energy distributions of Al atoms and AlO molecules produced by excimer laser ablation of Al2O3. Excimer laser fluences close to the threshold for ablation were used to minimize the effects of gas phase collisions. The kinetic energies of both species were high, ∼4 eV for Al and ∼1 eV for AlO, but the AlO rotational and vibrational energies were quite low, corresponding to a temperature of ∼600 K. These results rule out thermal vaporization and provide indirect support for an electronic ablation mechanism.

Energy and Entropy of Formation and Motion of Vacancies in NaCl and KCl Crystals

Recent experiments on the dc ionic conductivity of NaCl crystals doped with divalent cation impurities led to a re‐evaluation of some of the basic constants for defect formation and motion in these crystals. The key parameter is the energy of motion em of a cation vacancy. This parameter has been determined from the temperature dependence of the conductivity under conditions where the concentration of cation vacancies remains fixed. Such conditions apply (1) in the range just below the intrinsic region, and (2) in samples rapidly cooled to below 0°C such as to prevent the attainment of an equilibrium degree of association. From the present work combined with previous data, the jump rate of the isolated positive‐ion vacancy in NaCl is ν=12ν0 exp(−em/kT) with ν0=1014.1±0.3 sec−1 and em=0.80±0.02 ev. With the aid of earlier measurements in the intrinsic range it is concluded that the energy of formation of a Schottky defect in an NaCl crystal is 2.12±0.06 ev, while the entropy of formation is 6.2±1.8 (in uni...

Characterization of laser vaporization plasmas generated for the deposition of diamond-like carbon

Pulsed laser vaporization of graphite is rapidly emerging as an effective technique for the preparation of high quality diamond‐like carbon films. However, the dynamics of the process and mechanisms by which diamond‐like properties are obtained have not been well understood. The characteristics of the vapor plume generated by 248 nm KrF excimer laser irradiation of a graphite target are investigated using laser induced fluorescence and a Langmuir probe. It is found that the kinetic energy of the C2 molecule increases with laser fluence, reaching a value in excess of 12 eV in the moderate fluence range (3–5 J/cm2) employed for deposition. The Cn+ ions are 5–10 times more energetic and comprise ∼10% of the vapor flux. A notable finding is that irradiation of the surface at an angle of 70° with respect to the target normal increases the ion velocity when compared with 0° laser incidence at the same surface fluence. Analysis of the films prepared under such conditions supports the theory that diamond‐like fil...

Studies of excimer laser ablation of solids using a Michelson interferometer

A Michelson interferometer has been used as a direct quantitative probe for gas phase plasma formation in the UV excimer laser ablation of solids. Excimer laser fluence thresholds for plasma formation are determined and correlated with optical emission from electronically excited ablation fragments.

Electronic probe measurements of pulsed copper ablation at 248 nm

We have used a single wire probe to measure time of flight velocities for copper photoablated with a 248 nm pulsed excimer laser. For the range of fluences, 1.6–12 J/cm2, we find copper ion velocities in the range of 1–2×106 cm/s. We have used Langmuir probe theory to determine ion densities and electron temperatures as a function of fluence and target‐probe separation. Results are consistent with recent kinetic and photoablated plasma theories.

Laser-induced fluorescence study of laser sputtering of graphite

Abstract Pulsed laser irradiation of graphite surfaces has been known for some time to lead to the ejection of C, C2, and C3 neutrals as well as related ions. Since most relevant thermodynamic quantities are known, graphite represents an ideal system for further study. We report on the sputtering of pyrolytic, polycrystalline, and vitreous graphite by 20 ns pulses of laser light at 351 nm. The threshold energy density for sputtering is found to be 0.5 to 0.6 J/cm2. At this fluence, the material removal rate is of the order of a monolayer/pulse. This is consistent with pulsed evaporation provided that the surface reaches a peak temperature of ~ 4000 K. The emitted particles are probed using laser-induced fluorescence (LIF). Kinetic (i.e. translational) energies are obtained by time-of-flight and correspond, for the lowest fluences, to ~ 4600 K. Rotational and vibrational distributions are obtained by analysis of the LIF spectra for the D1Σu+X1Σg+ Mulliken bands of the dimer, C2. Detailed analysis indicates a rotational temperature of 4100 ± 300 K and a vibrational temperature of 3650 ± 350 K. Since the temperatures are all similar at the lowest it is concluded that the laser sputtering of graphite involves thermally activated vaporization, i.e. is what is normally termed “thermal sputtering”. At higher fluences, the time-of-flight information appears to be significantly perturbed by Knudsen layer formation.