Joseph W. Burnett

Yields and kinetic energy distributions of sputtered neutral copper clusters

Abstract Neutral copper clusters up to Cu 15 were observed from the sputtering of polycrystalline copper by normally incident 3.6 keV Ar + . This is the first observation of sputtered neutral clusters larger than Cu 5 . The sputtered neutral species were detected by laser post-ionization time-of-flight mass spectrometry. The absolute sputtering yield of Cu 2 was determined to be 0.05 Cu 2 /Ar + . Sputtering yields of the larger clusters relative to the atom were estimated from their postionized concentrations and found to range from 10 −4 for Cu 3 to 10 −9 for Cu 11 . The kinetic energy distributions of Cu, Cu 2 and Cu 3 were also measured. The energ distribution of the atoms agreed very well with the expected distribution from the Sigmund-Thompson model. The maxima in the energy distributions were in the order Cu 3 > Cu 2 > Cu. All three distributions had approximately the same high-energy dependence. Neither the single nor the multiple collision models that have been proposed to describe the cluster energy distributions appear to agree with our measurements.

Depth of origin of sputtered atoms: Experimental and theoretical study of Cu/Ru(0001)

The depth of origin of sputtered atoms is a subject of considerable interest. The surface sensitivity of analytical techniques such as secondary ion mass spectrometry and surface analysis by resonance ionization of sputtered atoms (SARISA), and the sputtering properties of strongly segregating alloy systems, are critically dependent on the sputtering depth of origin. A significant discrepancy exists between the predictions of existing analytical sputtering theories and computer sputtering models; in general, the computer models predict a much shallower depth of origin. The existing experimental evidence suggests that most of the sputtered atoms originate in the topmost atomic layer, but until recently, the results have not been definitive. We have experimentally determined the depth of origin of atoms sputtered from surfaces consisting of Cu films of <2 monolayers on a Ru(0001) substrate. The Cu/Ru target was statically sputtered using 3.6‐keV Ar+. The sputtered neutrals were nonresonantly laser ionized a...

Molecular product formation from the charge-transfer state of C6H6I2

Abstract The C 6 H 6 and I 2 molecular product fragments of the C 6 H 6 I 2 charge-transfer state have been investigated. Wavelength resolved studies show considerable rotational excitation of the benzene fragment and corresponding velocity spectra indicate a degree of translational excitation as well, consistent with rapid I 2 bond dissociation in a bent complex. Another product channel active in the gas phase complex that forms molecular iodine was identified from measurements of kinetic energy release in the I 2 fragment. The reaction mechanism is pictured as a charge-recombination step in the initially excited charge-transfer state, yielding I 2 ( X 1 Σ g + ) and C 6 H 6 (a 3 B 1u ). Comparisons are made to corresponding observations for arene-I 2 species in the condensed phase.

Effect of sputtering gas on the surface composition of an Al-Pd-Mn quasicrystal

Abstract We demonstrate the effects of different inert sputtering gases on the surface composition of icosahedral (i-) Al–Pd–Mn. The sputtering gases used include He, Ne, Ar, Xe, and Kr. We demonstrate that the steady-state composition is independent of the inert gas chosen. This steady-state composition falls outside of the quasicrystalline region of the Al–Pd–Mn phase diagram. However, the fluence (ions/cm 2 ) to achieve steady-state depends on the inert gas. Helium requires a much higher fluence than the other gases. Because of this, He can be used to clean the surface, if the time (fluence) for sputtering is kept to a minimum, without causing the large compositional shift induced with a relatively small fluence by the most common sputtering gas, Ar.

Low primary ion fluence dependence of single crystal sputtering: a molecular dynamics study

Abstract In recent experiments, the fluence dependence of the sputtering yield for single crystals was investigated in the submonolayer range. Surprisingly, a 50% decrease of the yield was observed in the case of Ar ion bombardment of a ruthenium single crystal. We have investigated this behaviour by means of a molecular dynamics simulation for 100 to 500 eV Ne, Ar and Xe ion bombardment of a Cu(100) surface. The simulations indicate that changes in the surface structure on an atomic scale can be responsible for this observed decrease in the sputtering yield. Removal of a surface atom due to sputtering will leave a ‘hole’ at the surface. We have calculated how the sputtering yield changes in the neighbourhood of such a missing surface atom. For example, in the case of 500 eV Ar on Cu the yield drops from 3.3 to 2.3 for primary ion impact parameters within half of the lattice constant of the missing atom position and is only slightly above normal outside this area, reaching the value for the undamaged surface (3.3) within a lattice constant. The effect is even more pronounced for lower energies and higher ion masses (Xe). Thus the yield of a virgin surface will decrease with fluence until an equilibrium surface topography has developed. This effect could also explain why molecular dynamics calculations typically give sputtering yields higher than experimentally observed values.

Ion and neutral atomic and cluster sputtering yields of molybdenum

The yield of neutral and ionized Mo, Mo2, and Mo3 sputtered from a Mo target by 4‐keV Ar+ has been measured in the surface analysis by resonance ionization of sputtered atoms (SARISA) machine. Ionization spectroscopy combined with time‐of‐flight (TOF) secondary ion mass spectrometry (SIMS) allowed us to obtain for the first time absolute sputtering yields and ionization fractions of sputtered atoms and metal clusters. Unlike sputtered atomic species, Mo clusters have been found to be sputtered with large ion fractions. The sputtering yield of Mo clusters is very sensitive to oxygen on the surface, i.e., even small amounts of oxygen on the surface identified by Mo+ and MoO+ peaks in the SIMS spectrum, reduce the cluster yield substantially. A broad structureless absorption band was observed for sputtered Mo2 molecules indicating substantial rovibronic excitation as predicted by theoretical models.

Sensitive, low damage surface analysis using resonance ionization of sputtered atoms

It is clear that in order to understand better the detailed mechanisms associated with sputtering it is necessary to develop new measurement techniques which can measure properties of neutral sputtered atoms with primary ion doses which are so low as to be essentially nondamaging. The results of a new surface analysis by resonance ionization of sputtered atoms (SARISA III) are reported here. Fe impurities in Si are measured at sub-ppb levels with removal of less than a surface monolayer.

Rotational excitation as a probe of photochemistry in (HI)n clusters

Abstract Laser excitation in the range 280–285 nm was used to initiate chemistry in small (HI)n clusters formed in a supersonic expansion. The same laser pulse also probed the rotational population of HI monomers by 2+1 REMPI through three different excited states. An increase in the HI rotational temperature above the normally very low temperature of the molecular beam was detected and is indicative of chemistry occurring within the cluster environment. Laser intensity and stagnation pressure dependence studies were conducted to provide further insight into the chemistry. Possible mechanisms that yield product molecules with internal excitation, including H-atom exchange reactions, are discussed.

Depth of origin of sputtered atoms

The depth of origin of sputtered atoms is currently a very important quantity in surface physics. The importance is best observed by examining the literature on computer simulations and theoretical calculations. Despite this interest, it is very hard to find experimental measurements of the depth of origin of sputtered atoms, which is probably due to the difficulty of such experiments. A good determination of the depth of origin should be done on a well characterized system and without damaging it. The results reported here are obtained from 1–2 monolayers of Cu on a Ru(0001) single crystal surface. The first 2 monolayers of Cu evaporated onto Ru(0001) is known to exhibit a layer by layer growth[l]. The sputtering is studied in the Surface Analysis by Resonance Ionization of Sputtered Atoms(SARISA) apparatus[2], where it is possible to do the determination before the overlayer is damaged by the ion beam. One monolayer of Cu on Ru is found to give a ratio of 3.2 between Cu and Ru in the sputtered flux for 4 keV normal incidence Ar ions.

Surface Diffusion of Large Ag Clusters on Ag(100)

Scanning tunneling microscopy has shown that large, two-dimensional Ag clusters undergo observable diffusion on Ag(100). The variation of the diffusion coefficient with cluster size in the range studied, 100 to 540 atoms per cluster, indicates that two-dimensional evaporation-condensation is the main mechanism of cluster diffusion. The experimental data consistently show evidence for a backward-correlation in the cluster motion. The meaning and origin of this correlation is discussed.