A. Wucher

Sputtered neutral silver clusters up to Ag18

Abstract Neutral silver clusters Agn with n ≤ 18 sputtered from a polycrystalline silver surface under bombardment with 5 keV Ar+ ions were detected by means of energy resolved time-of-flight mass spectrometry. Postionization of the ejected neutral particles was performed by an excimer laser operated with ArF (hv = 6.4 eV). The kinetic energy of the detected particles was determined from their flight time interval between the sputter pulse and the ionizing laser pulse. By saturating the ionization of the sputtered neutrals, relative cluster sputtering yields Y(Agn) were determined as a function of the cluster size. For n ≤ 6, the value of Y(Agn) is found to drop by approximately one order of magnitude if n is increased by one atom, whereas for cluster sizes larger than six atoms the decrease becomes significantly less pronounced and an odd-even intensity alternation similar to that observed for sputtered ion clusters becomes visible. The kinetic energy distributions of silver atoms, dimers, trimers, tetramers and pentamers determined from the time-of-flight method all look very similar and, in particular, show essentially the same high-energy dependence for sputtered atoms and molecules. In comparison, corresponding data taken by electron impact ionization with subsequent electrostatic energy analysis exhibit a significantly different asymptotic behaviour, the high energy slope of the energy spectra becoming steeper as the cluster size increases. From a detailed analysis of the measured signals, it is concluded that due to the concurrence of single- and multiphoton absorption processes the laser postionization results may be corrupted by photo-frag- mentation.

The formation of clusters during ion induced sputtering of metals

Abstract The formation of neutral clusters during sputtering of polycrystalline Ag, Al, Nb and Ta surfaces was studied experimentally by non-resonant single photon post-ionization using a UV or VUV laser beam and time-of-flight mass spectrometry. From the laser intensity dependence of the resulting mass spectra, photoionization cross-sections as well as the yields of sputtered clusters are determined as a function of the cluster size. It is shown that in all cases investigated so far the cluster yields roughly exhibit a power law dependence on the cluster size, the exponent of which is found to be inversely correlated with the sputtering yield of the sample. This finding is of particular importance, since it rules out simple statistical combination models to describe the formation of large sputtered clusters. From the yield distributions it is inferred that — depending on the sputtering conditions — up to 46% of the sputtered atoms may be emitted in a bound state. The experimental results are briefly compared with theoretical model descriptions of the cluster emission/formation process available in the literature. It is found that only Molecular Dynamics computer simulations using modern many body potentials are able to reproduce the experimental findings.

VUV photoionization of sputtered neutral silver clusters

Abstract We report on a study of neutral Ag atoms and Ag n clusters sputtered from a polycrystalline silver sample by keV rare gas ion bombardment. The ejected neutral species were photoionized by a pulsed laser and detected by time-of-flight mass spectrometry. In contrast to previous studies of sputtered neutral silver clusters, the ionization was performed by a VUV laser ( λ = 157 nm ) which, due to its high photon energy of 7.9 eV, permits nonresonant single photon ionization (SPI) of all investigated species. It is shown that the corresponding SPI ionization cross sections do not vary dramatically between silver atoms and the different clusters. As a consequence, fragmentation influences encountered in previous studies with longer wavelength lasers are practically eliminated from the determination of yields and kinetic energy distributions of the sputtered clusters. The resulting relative cluster sputtering yields (normalized to the yield of silver atoms) exhibit a power law dependence on the cluster size n according to n − δ with exponents δ ranging from 4.3 to 7.4 depending on the nature and the bombarding energy of the primary ions. The kinetic energy distributions of the sputtered neutral atoms and clusters are evaluated up to clusters containing seven atoms. It is found that the asymptotic decay of the energy distribution towards high emission energies becomes steeper from Ag to Ag 3 and remains practically constant for larger clusters.

Cluster formation in sputtering: A molecular dynamics study using the MD/MC-corrected effective medium potential

We report on a molecular dynamics simulation of cluster emission during sputtering of metals using a new many‐body potential developed by DePristo and co‐workers. For the specific case of silver as a sample target material, it is shown that this potential allows a much more realistic description of small clusters than the EAM potential used in our previous work. While this has a relatively large effect on the relative abundance of clusters within the total flux of sputtered material, other cluster properties like kinetic energy distributions and internal excitation are found to be less affected. By comparison with corresponding experimental data, we conclude that the formation of sputtered silver clusters can now be almost quantitatively modeled by the simulation.

The mass distribution of sputtered metal clusters: I. Experiment

Abstract The sputtering of silver clusters Ag n under 5 keV Ar + ion bombardment was studied by time-of-flight mass spectrometry of secondary cluster ions Ag + n and their neutral counterparts. The sputtered neutral clusters Ag 0 n were ionized by a combination of electron impact for n = 1,..., 4 and single photon absorption from an ArF excimer laser beam for n = 3, …, 18. By comparing ion and neutral cluster signals, the ion fraction of sputtered silver clusters was determined for clusters up to n = 15. It is found that the preferred charge state changes as a function of cluster size from predominantly neutral at n n > 10. From the ion fraction data, partial cluster yields were evaluated which are independent of the charge state of the detected particles. It is seen that the yield Y ( n ) of sputtered silver clusters follows a power law dependence according to Y ( n ) ∝ n −6.5 . From the resulting mass distribution of sputtered particles, it is estimated that around 17% of the sputtered atoms leave the surface in a bound state.

Experiment and simulation of cluster emission from 5 keV Ar → Cu

The abundance distribution of neutral Cun clusters sputtered by 5 keV Ar impact from a polycrystalline Cu surface is measured using single-photon laser post-ionization. Molecular dynamics computer simulation is used to gain insight into the cluster sputtering process. Three diAerent codes and two potentials are used to check the sensitivity of the results on numerics and physical input. DiAerences in the results obtained by the various codes and the diAerent potentials used are discussed. While the total sputter yield is consistent with experiment, the fraction of atoms bound in clusters, and in particular the dimer fraction, are overestimated by at least a factor of 4. This is also true for a many-body potential which has been fitted to describe both bulk Cu and dimers. In detail, the simulation shows that larger clusters are emitted at later times from the target. Clusters originate mainly from regions of the surface, which are around the melting temperature of bulk Cu. Large clusters are emitted preferably from ion impacts with a high individual sputter yield. Finally, we simulate sputtering from a model Cu material with an artificially decreased cohesive energy. Here, drastic high-yield events (up to Ya 78) can be observed, which produce clusters abundantly. ” 1998 Elsevier Science B.V. All rights reserved.

Sputtering of silver dimers: a molecular dynamics calculation using a many-body embedded-atom potential

The ejection of neutral Ag atoms and Ag2 dimers under 1 keV Ar+-bombardment of (111)-, (110)- and (100)-silver surfaces was studied by a molecular dynamics simulation using a many-body embedded-atom method (EAM) interaction potential. The absolute dimer yields YAg2 as well as the translational and internal energy distributions were calculated using the EAM potential for the description of both the solid and the isolated dimer. Significant differences were found between YAg2 obtained for different crystal faces which, however, do not agree with the results of previous studies using additive pair potentials. The calculated translational energy distributions of both atoms and dimers, averaged over the three crystal faces, show good agreement with the corresponding experimental data obtained for a polycrystalline silver surface. The internal state distributions calculated for the sputtered Ag2 molecules can be fitted by thermal populations revealing vibrational and rotational temperatures of 4200–5800 K and 8500–12300 K, respectively, with the (110)-surface always producing the highest and the (111)-surface the lowest temperatures.

Energy Deposition during Molecular Depth Profiling Experiments with Cluster Ion Beams

The role of the location of energy deposition during cluster ion bombardment on the quality of molecular depth profiling was examined by varying the incident angle geometry. Cholesterol films approximately 300 nm in thickness deposited onto silicon substrates were eroded using 40-keV C60(+) at incident angles ranging from 5 degrees to 73 degrees with respect to the surface normal. The erosion process was evaluated by determining at each incident angle the total sputtering yield of cholesterol molecules, the damage cross section of the cholesterol molecules, the altered layer thickness within the solid, the sputter yield decay in the quasi-steady-state sputter regime, and the interface width between the cholesterol film and the silicon substrate. The results show that the total sputtering yield is largest relative to the product of the damage cross section and the altered layer thickness at 73 degrees incidence, suggesting that the amount of chemical damage accumulated is least when glancing incident geometries are used. Moreover, the signal decay in the quasi-steady-state sputter regime is observed to be smallest at off-normal and glancing incident geometries. To elucidate the signal decay at near-normal incidence, an extension to an erosion model is introduced in which a fluence-dependent decay in sputter yield is incorporated for the quasi-steady-state regime. Last, interface width calculations indicate that at glancing incidence the damaged depth within the solid is smallest. Collectively, the measurements suggest that decreased chemical damage is not necessarily dependent upon an increased sputter yield or a decreased damage cross section but instead dependent upon depositing the incident energy nearer the solid surface resulting in a smaller altered layer thickness. Hence, glancing incident angles are best suited for maintaining chemical information during molecular depth profiling using 40-keV C60(+).

Depth Resolution During C60+ Profiling of Multilayer Molecular Films

Time-of-flight secondary ion mass spectrometry is utilized to characterize the response of Langmuir-Blodgett (LB) multilayers under the bombardment by buckminsterfullerene primary ions. The LB multilayers are formed by barium arachidate and barium dimyristoyl phosphatidate on a Si substrate. The unique sputtering properties of the C60 ion beam result in successful molecular depth profiling of both the single component and multilayers of alternating chemical composition. At cryogenic (liquid nitrogen) temperatures, the high mass signals of both molecules remain stable under sputtering, while at room temperature, they gradually decrease with primary ion dose. The low temperature also leads to a higher average sputter yield of molecules. Depth resolution varies from 20 to 50 nm and can be reduced further by lowering the primary ion energy or by using glancing angles of incidence of the primary ion beam.

Formation of large clusters during sputtering of silver

Abstract We have studied the formation of polyatomic clusters during sputtering of metal surfaces by keV ion bombardment. Both positively charged (secondary cluster ions) and neutral clusters have been detected in a time-of-flight mass spectrometer under otherwise identical experimental conditions, the sputtered neutrals being post-ionized by single photon absorption using a pulsed 157 nm VUV laser beam. Due to the high achievable laser intensity, the photoionization of all clusters could be saturated, thus enabling a quantitative determination of the respective partial sputtering yields. We find that the relative yield distributions of sputtered clusters are strongly correlated with the total sputtering yield in a way that higher yields lead to higher abundances of large clusters. By using heavy projectile ions (Xe+) in connection with bombarding energies up to 15 keV, we have been able to detect sputtered neutral silver clusters containing up to about 60 atoms. For cluster sizes above 40 atoms, doubly charged species are shown to be produced in the photoionization process with non-negligible efficiency. From a direct comparison of secondary neutral and ion yields, the ionization probability of sputtered clusters is determined as a function of the cluster size. It is demonstrated that even the largest silver clusters are still predominantly sputtered as neutrals.