Total Sputtering Yield Research Articles

The effect on the lunar exosphere of a coronal mass ejection passage

Solar wind bombardment onto exposed surfaces in the solar system produces an energetic component to the exospheres about those bodies. The solar wind energy and composition are highly dependent on the origin of the plasma. Therefore, using the measured composition of the slow wind, fast wind, solar energetic particle (SEP) population, and coronal mass ejection (CME), we have estimated the total sputter yield for each type of solar wind. We show that the heavy ions, especially the He++ and O+7, can greatly enhance the total sputter yield during times when the heavy ion population is enhanced. Folding in the flux with the yield of individual ions, we compute the source rate for several species during different types of solar wind. Finally, we use a Monte Carlo model developed to simulate the time‐dependent evolution of the lunar exosphere to study the sputtering component of the exosphere under the influence of a CME passage. We simulate the background exosphere of Na, K, Ca, and Mg. Simulations indicate that sputtering increases the mass of those constituents in the exosphere more than ten times the background values. The escalation of atmospheric density occurs within an hour of onset. The decrease in atmospheric density after the CME passage is also rapid, although takes longer than the increase. Sputtered neutral particles have a high probability of escaping the Moon, by both leaving the Hill Sphere and photoionization. Density and spatial distribution of the exosphere can be tested with the LADEE mission.

Dynamics of large Ar cluster bombardment of organic solids

Molecular dynamics computer simulations are used to investigate the ejection process of molecules from a benzene crystal bombarded with keV large Ar clusters. The effect of projectile size, the kinetic energy and the impact angle on the sputtering efficiency is investigated. The results show that although the sputtering yield depends on all projectile parameters, this dependence can be greatly simplified if the sputtering yield per projectile nucleon is expressed as a function of projectile kinetic energy per nucleon. A different dependence of the total sputtering yield on the impact angle has been observed for small and large projectiles. This effect is attributed to a ‘washing out’ mechanism. For large projectiles most of the organic molecules are ejected by gentle collective action of argon atoms. Proper selection of the projectile parameters allows for achieving conditions where only intact molecules are emitted. Copyright © 2012 John Wiley & Sons, Ltd.

Sputtering of tungsten by N+ and N2+ ions: investigations of molecular effects

Total sputtering yield of a polycrystalline tungsten surface during bombardment with both atomic N+ and molecular N2+ ions is determined using a highly accurate quartz crystal microbalance technique. The results thus obtained compare well with existing experimental data. At ion energies below 1 keV, an unexpected pronounced molecular effect is found, i.e. the sputtering yield per nitrogen atom for N2+ molecular ions is up to 25% higher than that of a single N+ ion with the same velocity. A qualitative explanation for this enhancement based on a simple energy transfer model is presented.

Secondary Ion Emission from Water Ice at 10–130 K Induced by MeV N2+ Ions

Secondary ion emission from vapor-deposited water ice has been investigated in exploring the influence of phase transitions on the desorption yield of H2O ions, particularly, the hydronium H3O+ and the cluster ions (H2O)nH3O+. For 1.5 MeV N2+ ion bombardment on water ice at a temperature of ∼80 K, the sum of the yields of H3O+ and of the cluster ions represents 80% of the positive ion yield (i.e., 0.23 ions/impact) and 0.33% of the total H2O molecular sputtering yield. When the beam energy of N2+ is varied between 0.5 and 1.7 MeV, the shape of the cluster mass distributions does not change and the total cluster ion yield scales with the third power of the electronic energy loss. Time-of-flight mass spectra were taken during the warming up of the ice from 10 to 216 K. Up to 30 K, the cluster ion yields are approximately constant, whereas between 30 and 75 K, they decrease gradually by a factor 6.5 and remain constant until 130 K. This is in analogy with modifications in the ice structure: in the 38–78 K re...

Erosion of Ag surface by continuous irradiation with slow, large Ar clusters

Molecular dynamics computer simulations are employed to probe processes taking place during continuous irradiation of Ag(111) surface by keV Ar872 projectiles. Surface modification, the total sputtering yield, and the angular distributions of ejected species are calculated at fluences ranging from 0 up to ∼6×1013impacts/cm2. It has been shown that two trends can be identified in the development of surface roughness. At the beginning surface roughness increases fast. This fast increase terminates around 1×1013impacts/cm2 and is followed by a slow increase that finally saturates. The effect of the surface roughness on the sputtering yield depends on the impact angle. At normal incidence the sputtering yield is rather insensitive to the development of the surface topography. Modification of the surface morphology has, however, a significant influence on the total sputtering yield at large impact angles. Both the shape of the sputtering yield dependence on the impact angle and the angular spectra of ejected particles are sensitive to the surface roughness.

Molecular Dynamics Study of the Effect of Surface Topography on Sputtering Induced by 20 keV Au3 and C60 Clusters

Molecular dynamics computer simulations are employed to investigate, at the atomic scale, the process of Ag(111) surface erosion stimulated by continuous bombardment by 20-keV Au3 and C60 clusters at 0° and 70° impact angles. The surface modification, the total sputtering yield, and the kinetic energy and angular distributions of ejected species are calculated at fluences ranging from 0 up to ∼3 × 1013 impacts/cm2. It is shown that Au3 irradiation leads to a more corrugated surface as compared to C60 bombardment. The development of the surface topography and aggregated alteration of the bulk have a significant influence on the total sputtering yield and the angular spectra, whereas the shape of the kinetic energy distributions are not sensitive to the modification of the sample morphology.

Investigation of sputtering of thin diamond-like carbon (DLC) target foils by low energy light ions

Along with use for stripping and timing of high energy ions in accelerator experiments, ultra-thin DLC foils are being successfully applied to the instrumentation for fusion and space plasma research. In the latter case, the foils are exposed to light ions and neutral atoms in the keV energy range. Unlike high energy ion irradiations, the foil lifetime is determined by thinning of the foil due to sputtering by particles that transit the foil. In this work, sputtering of thin (1–3 μg/cm 2) DLC foils produced by two different techniques such as modified glow discharge deposition and laser plasma ablation has been investigated using high intensity He + beams at 4 keV. A loss of foil material under ion impact was estimated from on-line measurements of energy loss of transmitting ions by means of an electrostatic analyzer. For comparison, samples of arc-deposited carbon foils were also evaluated. To avoid any carbon build up on the foils under ion irradiation, the measurements were carried out in an oil-free vacuum environment. Calculated from the measurements, results on both threshold fluence and total sputtering yield of the foils for the incident He + ions are presented together with some extrapolations to other low energy projectiles of fusion plasma interest. This enables an estimate for operational lifetime of the DLC foils determined by sputtering.

Quartz crystal microbalance-based system for high-sensitivity differential sputter yield measurements

We present a quartz crystal microbalance-based system for high sensitivity differential sputter yield measurements of different target materials due to ion bombardment. The differential sputter yields can be integrated to find total yields. Possible ion beam conditions include ion energies in the range of 30-350 eV and incidence angles of 0 degrees-70 degrees from normal. A four-grid ion optics system is used to achieve a collimated ion beam at low energy (<100 eV) and a two-grid ion optics is used for higher energies (up to 750 eV). A complementary weight loss approach is also used to measure total sputter yields. Validation experiments are presented that confirm high sensitivity and accuracy of sputter yield measurements.

Total and differential sputter yields of boron nitride measured by quartz crystal microbalance

We present differential sputter yield measurements of boron nitride due to bombardment by xenon ions. A four-grid ion optics system is used to achieve a collimated ion beam at low energy (<100 eV). A quartz crystal microbalance is used to measure differential sputter yield profiles of condensable components from which total sputter yields can also be determined. We report total and differential sputter yields of three grades of boron nitride due to bombardment by xenon ions for ion energies in the range 60–500 eV and ion incidence angles of 0°, 15°, 30° and 45° from the normal. Comparisons with published values are made where possible.

Simulation of sputtering and desorption of gold nanoclusters under bombardment with 180-eV/atom Au400 nanoclusters using the classical molecular dynamics method

Computer simulation of the interaction of an Au400 nanocluster (the total energy E = 72 keV) with free spherical AuN nanoclusters (6 and 12 nm in diameter) and Au6051 clusters deposited on the (111) surface of an Al substrate is performed by means of the classical molecular dynamics method. The distributions of the absorbed energy (e) converted to one atom of the bombarded nanocluster and the sputtering yield are analyzed. It has been ascertained that the most probable values are either the small (e ≪ emax = E/N) or the maximum possible (e ∼ emax) values of absorbed energy. The total sputtering yield and the absorbed energy decrease with increasing impact parameter. It has been demonstrated that, with a probability of ∼10%, a direct impact can lead to ejection of the entire bombarded nanocluster from the substrate. This event occurs in the case where an incident cluster initiates the secondary emission of target-cluster atoms mainly in the direction of the substrate. As a result, the nonsputtered part of the target cluster acquires the momentum in the opposite direction. This recoil effect can be regarded as one of the possible mechanisms by which nanoclusters deposited on substrate surfaces desorb under ion and cluster bombardment.

Damage analysis of benzene induced by keV fullerene bombardment

Molecular dynamics computer simulations have been used to investigate the damage of a benzene crystal induced by 5keV C20, C60, C120 and C180 fullerene bombardment. The sputtering yield, the mass distributions, and the depth distributions of ejected organic molecules are analyzed as a function of the size of the projectile. The results indicate that all impinging clusters lead to the creation of almost hemispherical craters, and the process of crater formation only slightly depends on the size of the fullerene projectile. The total sputtering yield as well as the efficiency of molecular fragmentation are the largest for 5keV C20, and decrease with the size of the projectile. Most of the molecules damaged by the projectile impact are ejected into the vacuum during cluster irradiation. Similar behavior does not occur during atomic bombardment where a large portion of fragmented benzene molecules remain inside the crystal after projectile impact. This “cleaning up” effect may explain why secondary ion mass spectrometry (SIMS) analysis of some organic samples with cluster projectiles can produce significantly less accumulated damage compared to analysis performed with atomic ion beams.

A quartz-crystal-microbalance technique to investigate ion-induced erosion of fusion relevant surfaces

We describe a highly sensitive quartz-crystal-microbalance technique capable of determining erosion as well as implantation and retention rates for fusion relevant surfaces under ion bombardment. Total sputtering yields obtained with this technique for Ar ion impact on polycrystalline gold and tungsten surfaces are presented. The results compare well with existing experimental data as well as theoretical predictions and thus demonstrate the feasibility of the developed technique. Our setup is capable of detecting mass-changes as small as 10−5μg/s, which corresponds to a removal of only 10−4W monolayers/s.

Carbon atom and cluster sputtering under low-energy noble gas plasma bombardment

Exit-angle resolved carbon atom and cluster (C2 and C3) sputtering yields are measured during different noble gas (Xe, Kr, Ar, Ne, and He) ion bombardments from a plasma, for low incident energies (75–225 eV). A quadrupole mass spectrometer (QMS) is used to detect the fraction of sputtered neutrals that is ionized in the plasma and to obtain the angular distribution by changing the angle between the target normal and the QMS aperture. A one-dimensional Monte Carlo code is used to simulate the interaction of the plasma and the sputtered particles in the region between the sample and the QMS. The effective elastic scattering cross sections of C, C2, and C3 with the different bombarding gas neutrals are obtained by varying the distance between the sample and the QMS and by performing a best fit of the simulation results to the experimental results. The total sputtering yield (C+C2+C3) for each bombarding gas is obtained from weight-loss measurements and the sputtering yield for C, C2, and C3 is then calculated from the integration of the measured angular distribution, taking into account the scattering and ionization of the sputtered particles between the sample and the QMS. We observe undercosine angular distributions of the sputtered atoms and clusters for all the studied bombarding gases and a clear decrease of the atom to cluster (C2 and C3) sputtering ratio as the incident ion mass increases, changing from a carbon atom preferential erosion for the lower incident ion masses (He, Ne, and Ar) to a cluster preferential erosion for the higher incident ion masses (Kr and Xe).

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(+).

The effect of incident angle on the C 60+ bombardment of molecular solids

The effect of incident angle on the quality of SIMS molecular depth profiling using C 60 + was investigated. Cholesterol films of ∼300 nm thickness on Si were employed as a model and were eroded using 40 keV C 60 + at an incident angle of 40° and 73° with respect to the surface normal. The erosion process was characterized by determining at each angle the relative amount of chemical damage, the total sputtering yield of cholesterol molecules, and the interface width between the film and the Si substrate. The results show that there is less molecule damage at an angle of incidence of 73° and that the total sputtering yield is largest at an angle of incidence of 40°. The measurements suggest reduced damage is not necessarily dependent upon enhanced yields and that depositing the incident energy nearer the surface by using glancing angles is most important. The interface width parameter supports this idea by indicating that at the 73° incident angle, C 60 + produces a smaller altered layer depth. Overall, the results show that 73° incidence is the better angle for molecular depth profiling using 40 keV C 60 +.

Multiparameter investigation of the sputtering behavior of Ag∕Cu Alloys for low energy argon ion bombardment

The sputtering behavior of Ag∕Cu alloys due to low energy (100–1500eV) Ar+ bombardment as a function of component composition, sample temperature, ion current density, ion dose, and ion energy has been completed. The results indicate that the surface topography development is a strong function of temperature and depends to a lesser extent on ion energy, ion dose, and ion current density. The surface at low temperature coupled with low ion dose and/or low ion current density is flat and faceted with widely dispersed large cones. As the temperature is raised, the rate of Ag surface diffusion increases faster than that of Cu and Ag begins to coat the Cu grains thereby, decreasing the subsequent erosion of the Cu. Selective sputtering of the higher yield Ag and continuous Ag diffusion toward the surface results in a decrease in the Ag concentration in the near surface region as measured by energy dispersive spectroscopy. Auger electron spectroscopy (AES) depth profiles indicate significant diffusion of both Cu and Ag when the electron beam was used in spot mode. Angular distributions of sputtered material indicate that Ag is ejected more preferentially at large angles to the surface normal. The total sputter yield is a function of temperature, ion energy, and dose. These effects are related to the growth of surface topography and the subsequent recapture of ejected material on to the sides of surface features. A method of increasing the thermal conductivity between the sample and holder using graphite colloid was found to affect the surface topography growth and AES profiles. The results of these experiments have implications for depth profiling of multicomponent metallic alloys.

Study of gold nanocluster sputtering under 38-keV Au ion bombardment by a classical molecular dynamics method

The computer simulation of the interaction of 38-keV Au1 ions with isolated spherical Au N nanoclusters of diameters 2.6 and 18 nm is performed in the framework of the classical molecular dynamics (MD) method. The distribution of the absorbed energy e per one atom of the irradiated cluster and the sputtering yields are analyzed for different ratios of the nanocluster diameter D to the average projective range R p of the bombarding ion. It is established that the small values of the absorbed energy (e ≪ emax = E/N) are most probable for D < R p, and either small (e ≪ emax) or the maximum possible (e ∼ emax) values are mainly realized for D ≤ R p. It is shown that the total sputtering yield depends weakly on the impact parameter. It is demonstrated for the first time that the irradiated cluster, as a whole, can be ejected by direct impact with a probability of approximately 6–13%. Such events are realized in the cases where the bombarding ion causes secondary cluster-atom emission in the dominant direction to a substrate, with the result that an unsputtered cluster fraction acquires momentum in the opposite direction. This recoil effect can be one of the mechanisms for desorption of nanoclusters deposited on the surface under ion (or cluster) bombardment.

Annihilation of craters: Molecular dynamic simulations on a silver surface

The ability of silver cluster ions containing 13 atoms to fill in a preexisting crater with a radius of about $28\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ on a silver (001) target has been investigated using molecular dynamics simulations and the molecular-dynamics--Monte Carlo corrected effective medium potential. The largest lateral distance $r$ between crater and ion was about three times the radius of the preexisting crater, namely, $75\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. The results reveal that when $rl20\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and $rg60\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ the preexisting crater is partially filled in, and for other distances there is a net growth of the crater. The lattice damage created by the cluster ions, the total sputtering yield, the cluster sputtering yield, and simulated transmission electron microscopy images of the irradiated targets are also presented.

Investigation of the projectile atomic number influence to the total sputtering yield in the keV energy region

The non-monotonous dependence of the total sputtering yield on the projectile atomic number, which is unexpected in the frame of the Sigmund linear cascade theory, is investigated using Monte Carlo simulations (program SRIM 2003). This effect is studied on the example of aluminum sputtering by six different projectiles (N, Ne, Al, Ar, Kr and Xe) at normal incidence. The incident projectile energy is 2 keV. Investigation consists of the analyses of ASI distributions of sputtered atoms as well as of nuclear energy loss depth distributions of projectiles with fixed number of ejected atoms. The results show that the non-monotonous behavior of Y(Z) is due to the ability of projectiles somewhat lighter than aluminum to efficiently eject large number of atoms by formation of collision cascades in the subsurface region which are directed towards the surface. On the other hand, ions that are heavier or significantly lighter than aluminum cannot form this type of cascades – the heavier ions cannot transfer a lot of energy to recoils in a primary knock-on collision that will move towards the surface, while significantly lighter ions transfer the energy too deep into the target.

Sputtering of ice by low-energy ions

We measured the total sputtering yield Y of amorphous water ice at 80 K for 0.35–4 keV He and Ar ions. We found that Y depends linearly on the elastic stopping cross section at low energies as predicted by the standard linear cascade theory of sputtering. As the energy increases, a quadratic dependence with the electronic stopping cross section arises due to cooperative effects between excitations produced by the projectile. We also studied how sputtering depends on the projectile incidence angle and found that Y follows a cos − f ( θ) dependence with f = 1.45 ± 0.05 for 2 keV He + and f = 1.78 ± 0.08 for 2 keV Ar +. Measurements of Y for 2 keV ions vs. ice temperature between 30 and 140 K show the same temperature dependence as that reported previously for high-energy ions. We introduce a general formula to calculate Y below 100 keV once the ice temperature, projectile type, energy and angle of incidence are known. This formula can be used for modeling the production of extended neutral atmospheres around astrophysical icy bodies subject to ion bombardment.