Joseph Fine

Kinetics of Cs+ Desorption from Tungsten

The mean adsorption lifetime τ3 of Cs+ on tungsten has been measured in the 1000°—1200°K temperature range, using a pulsed beam technique. Under conditions of low surface coverage and with either Cs or CsI as beam materials τ3=(1.0±0.5)×10−12exp[(23 600±500)/T]sec was obtained. The heat of desorption can be calculated as the energy required to remove an isolated Cs+ ion from the surface of an electrical conductor. The presence of an adsorbed contaminating layer, arising from residual vacuum gases, decreased the Cs+—W binding energy by 0.5 eV and increased the pre‐exponential factor by about two orders of magnitude. Anomalous results were obtained when CsCl was used for a beam material, suggesting a reaction between the surface tungsten atoms and atomic chlorine.

Interface depth resolution of Auger sputter profiled Ni/Cr interfaces: Dependence on ion bombardment parameters

Interface broadening which often results as a consequence of sputter profiling can make it difficult to assess the structure of an original interface. There are a number of factors involved in this broadening which are associated with the parameters of the ion bombardment and which have not previously been evaluated. Sputter profile measurements obtained on a set of similarly fabricated Ni/Cr multilayered thin‐film structures have shown that it is practical to systematically examine this interface broadening dependence on ion beam energy, ion current density, and angle of incidence, all as a function of sputtered depth. Results are presented of such a set of Auger sputter depth profile measurements and indicate that there can be dramatic changes in sputtered interface widths depending on the ion bombardment parameters used.

Characterization of NBS Standard Reference Material 2135 for sputter depth profile analysis

A Ni/Cr multilayered thin‐film standard reference material (SRM) for sputter depth profile calibration has been developed jointly by the National Bureau of Standards, the Jozef Stefan Institute, and the American Society for Testing and Materials (ASTM) Committee E‐42 on Surface Analysis. This perodically modulated structure can be effectively used to calibrate sputter erosion rates and depth of erosion scales in surface analysis as well as to monitor ion beam stability and to optimize sputtering conditions so as to achieve maximum interface resolution. Characterization results obtained on this first SRM for surface analysis to be issued by NBS indicate that the accuracy of its structure is known to better than 6% and that its sputter profiles are well defined and reproducible. Results of the calibration and compositional analysis of this SRM are presented regarding uniformity and periodicity of thin film layers, absolute film thickness, sputtered interface depth resolution, and relative Ni/Cr sputtering r...

Entropies, Heats of Sublimation, and Dissociation Energies of the Cesium Halides

The vapor pressures of the crystalline cesium halides have been measured in the 700–900°K temperature range. The salt vapor effusing from a Knudsen effusion cell was accurately collimated into a molecular beam with a well‐defined cross‐sectional area. Beam intensities were determined from the positive ion currents produced by dissociation of the halide and ionization to Cs+ on a tungsten surface at 1700°K. From the experimental vapor pressures and heats of sublimation, the crystal entropies, dissociation energies, and heats of sublimation at 298°K were computed using available spectroscopic and thermal data.

Electron stimulated desorption of neutral species from (100) KCl surfaces

Abstract Composition changes of a (100) KCl surface bombarded by 1 keV electrons have been studied by Auger electron spectroscopy. Intensity ratios of characteristic alkali and halogen Auger lines were monitored as a function of target temperature and beam current density. In addition, for the first time angle-resolved energy distributions of electron desorbed K and Cl atoms were measured using mass-analyzed time of flight techniques. For temperatures higher than about 100°C, a near-stoichiometric surface composition was obtained and a significant non-thermal component was observed in the kinetic energy distributions of Cl atoms emitted normal to the (100) surface. These results can be interpreted in terms of new concepts regarding the excitonic mechanism of electron stimulated desorption (ESD).

Auger electron emission from the decay of collisionally-excited atoms sputtered from Al and Si

Abstract Atom collisions of several keV may result in inner-shell excitations. The energy spectra of Auger electrons from excitations induced by ion bombardment of solid materials are different from those stimulated by X-rays or electrons. Auger electron spectra produced by ion bombardment of solids contain features similar to spectra obtained from atoms undergoing Auger transitions in the gas phase, i.e., atomic-like spectra. An interpretation of the atomic-like spectra from ion-bombarded solids is that a significant portion of the atoms undergoing Auger de-excitation have previously been sputtered from the solid. Auger decay in the gas phase can occur if the inner-shell lifetime is sufficiently long for the excited atom to escape. Results from our Monte Carlo calculations of the origin, movement, and decay of ion-bombardment induced 2p inner-shell excitations of Al and Si will be presented. These calculations indicate that a significant portion of the Auger emission originates from sputtered atoms; the kinetic energy of atoms sputtered while experiencing inner-shell excitation far exceeds the average kinetic energy of sputtered atoms, and so, Auger electron emission may constitute a probe of the high energy collision cascade. Calculated dependence of the Auger electron intensity on the incident angle of the ion beam will be compared with measurements, and the effect of inner-shell lifetime on the calculated Auger electron intensity will be discussed.

A molecular dynamics simulation of collisional excitation in sputtering from Al

Abstract The molecular dynamics sputtering code SPUT2 was modified to permit investigation of core excitation in Al following bombardment with 5 keV Ar + ions. This code was used to investigate sputtering mechanisms responsible for the ejection of core-excited atoms from solid surfaces. Simulations were carried out with the ion incident along both low- and high-index directions. In contrast to most previous studies, essentially all ejection of core-excited atoms resulted from asymmetric collisions (i.e. collisions between the incident ion and a target atom). When the ion is incident along a low-index direction, core-excited atoms arise almost exclusively from the first layer of the target. When the ion is incident along a high-index direction, core-excited atoms are found to eject from deeper layers as well.

Kinetics of Desorption. II. Cs+ and Ba+ from Rhenium

The mean desorption lifetimes (τ3) of Cs+ and Ba+ on an atomically clean rhenium surface have been measured. Under conditions of low surface coverage τ3=(1.9±0.9)×10−13exp(23 300±500T)sec was obtained for Cs+ in the temperature range 950<T<1080°K. This is in excellent agreement with the data obtained previously for the Cs+–W system. For Ba+ on rhenium τ3=(0.6±0.4)×10−13exp(54 700±2400T)sec was obtained in the temperature range 2100<T<2310°K. The Cs+–Re, like the Cs+–W bonding, can be attributed to the electrostatic image energy of a Cs+ ion on an electrically conducting surface. The Ba+–Re interaction, on the other hand, must include a large exchange force in addition to the electrostatic image force because of the unpaired valence electron remaining in the univalent barium positive ion. The presence of an adsorbed layer of residual vacuum gases decreased the Cs+–Re binding energy by 0.4 eV and increased the pre‐exponential factor by two orders of magnitude. These results are almost identical with that re...

Measurement of time-dependent sputter-induced silver segregation at the surface of a NiAg ion beam mixed solid

Abstract Sputter depth profiling of alloys and interfaces using low energy ion beams can cause in-depth compositional changes to occur. One possible mechanism responsible for such changes is enhanced diffusion occuring along point defects generated by the ion bombardment in the near-surface region. Sputter profiling of a Ni/Ag interface produces a mixed NiAg surface region and we have found that in such a region, bombardment with 1–4 keV argon ions at 20°C, the Ag will segregate to the surface. This segregation can be observed to occur in real time after the ion bombardment has been stopped. Auger spectroscopy was used to obtain a unique set of measurements of the kinetics of surface segregation due to bombardment-enhances near-surface diffusion. The kinetics of this segregation is examined and its influence on sputter depth profiling demonstrated.

Amolecular dynamics simulation of collisional excitation mechanisms in Al

A modified version of the SPUT2 molecular dynamics sputtering code was used to reinvestigate core excitation in Al atoms following bombardment with 1–5 keV Ar+ ions. For all bombarding energies, asymmetric collisions between the incoming ion and target atoms yielded smaller minimum distances-of-closest-approach between the collision partners for hard collisions than did symmetric collisions between pairs of target atoms. Simple critical distance-of-closest-approach models were used to estimate core excitation for both asymmetric and symmetric collisions. A single value of Rc (0.367 A) was used for asymmetric Ar-Al collisions, while two choices of Rc were used for symmetric Al-Al collisions (0.442 and 0.530 A). With the smaller Rc value for Al-Al collisions, we find that core excitation proceeds predominantly by asymmetric collisions at all bombarding energies above threshold. At 5 keV bombarding energy the percentage of sputtered, core-excited atoms originating from asymmetric collisions ranged from 89 to 95% depending on the incident direction of the projectile. With the larger Rc value, core excitation proceeds predominantly by asymmetric collisions at bombarding energies above approximately 3 keV; and at 5 keV asymmetric collisions accounted for ∼ 60 to ∼ 84% of sputtered, core-excited atoms. Lifetime corrections and corrections for Auger neutralization near the target surface had little effect on the ratio of asymmetric to symmetric collisions responsible for atomic-like Auger emission. These simulation results suggest that simultaneous multiple collisions are very important in the initial energy- and momentum-transfer stage which initiates the cascade.