Differential Sputtering Yield Research Articles

Absolute doubly differential angular sputtering yields for 20 keV Kr+ on polycrystalline Cu

We have measured the absolute doubly differential angular sputtering yield for 20 keV Kr+ impacting a polycrystalline Cu slab at an incidence angle of θi = 45° relative to the surface normal. Sputtered Cu atoms were captured using collectors mounted on a half dome above the sample, and the sputtering distribution was measured as a function of the sputtering polar, θs, and azimuthal, ϕs, angles. Absolute results of the sputtering yield were determined from the mass gain of each collector, the ion dose, and the solid angle subtended, after irradiation to a total fluence of ∼1 × 1018 ions/cm2. Our approach overcomes shortcomings of commonly used methods that only provide relative yields as a function of θs in the incidence plane (defined by the ion velocity and the surface normal). Our experimental results display an azimuthal variation that increases with increasing θs and is clearly discrepant with simulations using binary collision theory. We attribute the observed azimuthal anisotropy to ion-induced formation of micro- and nano-scale surface features that suppress the sputtering yield through shadowing and redeposition effects, neither of which are accounted for in the simulations. Our experimental results demonstrate the importance of doubly differential angular sputtering studies to probe ion sputtering processes at a fundamental level and to explore the effect of ion-beam-generated surface roughness.

Differential sputtering yield measurement of ion grid materials by novel arch array system

In this experimental study the differential sputtering yield and total sputtering yield of ion thruster grid materials (Mo and C/C composite) under low-energy ion bombardment has been investigated. A novel combinable aluminum arch array is developed to determine the spatial distributions of sputtered atoms in different azimuthal angles. The quartz crystal microbalance (QCM) method is used to validate the typical distribution profile of sputtered atoms at the 180° azimuthal angle. The distribution characteristics of sputtered atoms as well as the specific regions of maximum differential sputtering yield under different incident energies, incident angles and ion species (Xe, Kr, Ar) have been discussed. The sputtering resistance performance of C/C composite materials have been compared with Mo metal by measuring the total sputtering yield. The spatial distributions of C/C differential sputtering yield are found highly anisotropic even under normal incidence, a multi-region discontinuous distribution characteristic of maximum differential sputtering yield have been observed in C/C composite material. The experimental results about the spatial distribution of sputtered atoms are of great importance for establishing the sputter-redeposition models of grid apertures to accurately predict the evolution of grid morphology and lifetime of ion thruster.

Angular distribution of particles sputtered from the vanadium target by 5 keV Kr ions: simulation study

The sputtering of vanadium particles at normal incidence was simulated. The SRIM-code combined to a new ANGULAIR and SDTrimSP simulation was employed to obtain the sputtering yields and the angular distribution of the atoms. The simulation was made for a large number of incident Kr+ ions with 5 keV energy, letting the computer count the number of emitted particles in the solid angle. The angular distribution of differential sputtering yields of vanadium shows an over-cosine tendency.

Stoichiometric influences on ion beam nanopatterning of CoSi binary compound

Impact of initial stoichiometry of binary compound on surface nanostructure formation with low energy ion irradiation has been studied. Different stoichiometric CoSi surfaces are irradiated by Ar+ ion beam with energy of 700 eV, fluence of 7.5×1018 ions cm−2 and angle of incidence 67∘. Within a narrow window of stoichiometric variation, self-organized nanoripples have been observed. The ripple structures are well formed for stoichiometric ratios of 40:60 for Co:Si. Nanoscale ripples start growing for a concentration of about Co22Si78. The root mean square (rms) roughness initially decreases and then increases slightly as Co increases from low to medium concentrations. The evolution of different morphologies has been corroborated from the behavior of power spectral densities (PSD). Correlation lengths are extracted from atomic force microscopy (AFM) images to corroborate the ripple formation region only within a specific stoichiometric range. Differential sputtering yields provide a rationale for the observed pattern evolution.

A Semi-Empirical Model for Tilted-Gun Planar Magnetron Sputtering Accounting for Chimney Shadowing

Integrated computational materials engineering (ICME) approaches to composition and thickness profiles of sputtered thin-film samples are the key to expediting materials exploration for these materials. Here, an ICME-based semi-empirical approach to modeling the thickness of thin-film samples deposited via magnetron sputtering is developed. Using Yamamura’s dimensionless differential angular sputtering yield and a measured deposition rate at a point in space for a single experimental condition, the model predicts the deposition profile from planar DC sputtering sources. The model includes corrections for off-center, tilted gun geometries as well as shadowing effects from gun chimneys used in most state-of-the-art sputtering systems. The modeling algorithm was validated by comparing its results with experimental deposition rates obtained from a sputtering system utilizing sources with a multi-piece chimney assembly that consists of a lower ground shield and a removable gas chimney. Simulations were performed for gun-tilts ranging from 0° to 31.3° from the vertical with and without the gas chimney installed. The results for the predicted and experimental angular dependence of the sputtering deposition rate were found to have an average magnitude of relative error of $$ 4.14\% \pm 3.02\% $$ for a 0°–31.3° gun-tilt range without the gas chimney, and $$ 2.12\% \pm 1.71\% $$ for a 17.7°–31.3° gun-tilt range with the gas chimney. The continuum nature of the model renders this approach reverse-optimizable, providing a rapid tool for assisting in the understanding of the synthesis-composition-property space of novel materials.

Ion fluence dependence of the total sputtering yield and differential angular sputtering yield of bismuth due to 50 keV argon ion irradiation

The dependences of the total sputtering yield of Bi and the differential angular distribution of these sputtered Bi atoms on the fluence of 50keV Ar+ ions at normal incidence have been experimentally measured. Polycrystalline Bi targets were used for these purposes. The collector technique and accurate current integration methods were adopted for the determination of angular distributions of sputtered Bi atoms. The ion fluence was varied from 1.9×1019 to 3.1×1020ions/cm2. The sputtered atoms were collected on high purity aluminum foils under ultra-high vacuum (∼5×10−9Torr). The collector foils were subsequently analyzed using heavy ion Rutherford backscattering spectroscopy. The shape of the angular distribution of sputtered atoms was found not to change significantly with the fluence, but the sputtering yield increased significantly from 2.2±0.2 to 9.6±0.6atoms/ion over the fluence range studied.

Anisotropic dewetting of ion irradiated solid films

Experiments of irradiation with 30keV Ga ions were conducted on ultrathin chromium films on rippled silicon substrates. The evolution of their surface morphology, as detected by real time scanning electron microscopy, shows an apparent differential sputtering yield for regions of positive and negative curvature which is in contrast with the standard theory for curvature depending sputtering yield. In particular, at the end of the irradiation process, chromium wires are left in the valleys of the substrate. This result was explained in terms of local melting caused by the ion impact and of a process of dewetting under the concurring actions of surface tension and Van der Waals forces while ion sputtering is active. The interpretation of the reported experimental results are fully supported by numeric simulations implementing the same continuum model used to explain ion induced spinodal dewetting. This hierarchical self-organization process breaks the symmetry of previously demonstrated ion induced dewetting, making possible to create new structures by using the same fundamental effects.

Angular distribution of sputtered alloy. Experimental and simulated study

Angular distribution of Cu and Be atoms sputtered from Cu98 Be2 alloy under 5 Kr+ ion bombardment were measured at different angle of incidences (0, 30, 70 and 80○). The sputtered material is collected on a MylarTM foil surrounding the target. The used collector was mounted at different ejection angles and analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES). The SRIM-code program was employed to obtain the sputtering yield of Cu and Be from simulated Cu98 Be2 target. The experiment was combined with simulations and revealed that both of them were in a fairly good agreement. Furthermore, angular distribution of differential sputtering yields of both Cu and Be showed over-cosine tendency.

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.

Differential sputter yields in Si1−xGex

Sputter yields for ion bombardment of Si1−xGex alloys are computed using molecular dynamics simulations with the target material modeled using the Stillinger–Weber empirical potential. The results show that Si atoms are preferentially sputtered relative to Ge atoms for all x. The parameters of the Stillinger–Weber potential are altered to investigate the relative importance of atomic mass, bonding energy, atomic number, and atomic density leading to the observed differential sputter yields. The results explain an experimentally observed nonlinear dependence of total sputter yields on composition in Si1−xGex [Tuboltsev et al., Phys. Rev. B 72, 205434 (2005)].

Differential sputter yield profiles of molybdenum due to bombardment by low energy xenon ions at normal and oblique incidence

We report differential sputter yield measurements for Xe+ ions incident on molybdenum for ion incidence angles of 0°, 15°, 30° and 45°, and ion energies of 250, 350, 500 and 750 eV. The angularly resolved profiles are obtained with a quartz crystal microbalance that is moved to a series of locations above the target. Total sputter yields found by integrating the differential sputter yields are in accord with the literature values. For normally incident ions the profiles are under-cosine, while for obliquely incident ions the profiles show azimuthal variation with pronounced forward sputtering lobes. We discuss the use of published expressions from Zhang and Zhang (2004 Radiat. Eff. Defects Solids 159 301–7) for describing the measured profiles. We find that the profiles can be reasonably described with these expressions by using two free parameters: the total sputter yield (Y) and a characteristic energy (E*). We compare changes in angular sputtering profiles from lightly versus heavily sputtered targets.

Differential sputter yield measurements using cavity ringdown spectroscopy

The first use of cavity ringdown spectroscopy (CRDS) to measure differential (angular) sputter yield profiles of sputtered particles is reported. Owing to the path-integrated nature of CRDS, inversion techniques are required. Our approach is to scan the optical axis relative to the source of sputtered particles and to measure the spatial profile of the CRDS signals. Modeling is then used to determine the differential sputter yield profile from the measured CRDS spatial profile. Demonstrative measurements are made with a Nd:YAG pumped optical parametric oscillator laser system for 750 eV argon ions normally incident on a molybdenum target. At these conditions we find an under- cosine sputtering distribution characterized by alpha = 0.22 +/- 0.07 in good agreement with past quartz crystal microbalance measurements (alpha = 0.19).

Effect of grain orientation on tantalum magnetron sputtering yield

The differential sputter yields of tantalum grains during argon ion magnetron sputtering were measured and found to correlate with the atomic packing density in the uppermost atomic layer. Grains with their (110) planes aligned within 15° of the sputtering surface had the highest sputtering yield (0.95at./ion at 400eV). Simultaneously, grains oriented so that their (111) planes were within 15° of the sputtering surface had sputtering yields of around two times lower (0.47at./ion at 400eV). These results are consistent with the surface sputtering rather than the channeling model. A correlation was found between the number of free atoms on the surface as determined using a facetted surface model and the measured sputtering yield.

Deposition of silicon carbon nitride films by ion beam sputtering

Silicon carbon nitride films have been successfully synthesized at a temperature below 100°C from an adenine(C5N5H5)-silicon-mixed target sputtered by Ar ion beam. The effects of Ar ion sputtering voltage, area ratio of Si to adenine in the mixed target and nitrogen atom addition during deposition on the film growth are investigated. XPS, XRD, and ellipsometry were employed to characterize the composition, chemical bonding, structure, and optical property of the films. The growth characteristic and film properties of the silicon carbon nitride films are also compared with those of the carbon nitride films deposited from an adenine target to elucidate the effect of silicon incorporation. The silicon content of the resultant films increased significantly as the area ratio of Si to adenine increased, whereas a higher Ar ion voltage led to a lower level of silicon incorporation, presumably due to differential sputtering yield of Si and adenine. XPS chemical state analysis revealed multiple bonding structures for every element in the SiCN films, of which possible implications are given. XRD studies showed that only amorphous films for Si-rich SiCN were obtained, while the films with low Si incorporation and deposited at high Ar ion beam voltage contained nanocrystallites. Furthermore, the refractive index of the SiCN films increased with increasing silicon content. The appearance of the nanocrystalline structure in the films led to a reduction in the refractive index.

Study of an ECR sputtering plasma source

The characteristics of a sputtering plasma source using electron cyclotron resonance, operating on the ERIC facility dedicated to isotope separation by the plasma process, are described. A simple model based upon the particle conservation equation allows us to derive the ionization probability and the reflux fraction that govern the source operating conditions, both with and without carrier gas. Other data such as sputtering yields of Ar+ and Kr+ ions on Ni, Cu, Pd and Gd targets as well as their self-sputtering yields have been also determined for the first time in the case of gadolinium. The axial deposit profile of metal atoms on the lateral screen of the source is in accordance with the Yamamura semi-empirical differential sputtering yield, and the ionization coefficient of GdI is deduced.

Particle energy and angle distributions in ion beam sputtering

Abstract We performed investigations on energy distributions of ions and neutrals generated during argon and krypton ion beam sputtering of titanium and boron nitride using a modified energy selective mass spectrometer. During the process, we measured the secondary particle energy and angle distribution and estimated a “differential sputter yield” which gives a semi-quantitative, angle-resolved number of sputtered particles per incident ion. Sputtering was performed in the low-energy range between 20 eV and 1200 eV at a 90° fixed angle between the sputtering source and the detector system. In addition, a detector–tilt system was developed for angle-resolved measurements. Dependent on the primary ion energy, backscattered and resputtered process gas ions with clearly different energy distributions were detected.

A PRINCIPLE FOR MICRO-REGION SUPERPOSITING OF ION SPUTTERING

Solid surfaces subjected to energetic ion bombardment generally develop chara-cteristic structures, which may significantly change total and differential sputtering yields. The change is due to several competing effects, for instance, change of the effective projectile incidence angle, recapture of obliquely ejected particles (shadow effect), and element local-enrichment in different micro-region on the alloy surface. From these, we have now developed an efficient new theoretical method for covering all of the above effects exactly. This method and its application to the analysis of experimental results of element (Cd) and alloy system (ＡｌxSn１００－x)targets are de-scribed in detail.

Angular distributions of sputtered atoms for low-energy nitrogen irradiation of silicon

We studied the angular distributions of silicon and nitrogen atoms emitted from a Si target subjected to reactive sputtering by N 2 + ions at primary energies of 0.5 and 2keV. The composition of the deposited material does not depend strongly on the substrate position. From a comparison with nonreactive sputtering, we show that the observed shift of the Si angular distribution is mainly due to the contribution of collision events occurring in the first monolayer. Contrary to the case of noble gas ions, the sharpness of the Si distribution depends on the N 2 + energy. The behavior of the differential sputtering yield of silicon indicates that this effect is likely to be due to a loss of recoil atoms out of the preferential direction. A possible explanation of the observed phenomena consists in assuming an anisotropic emission of Si x N y radicals. This hypothesis is very attractive as it could satisfactorily explain the similarity we observed between the angular distributions of silicon and nitrogen.

Investigation of angular distributions of ion-induced atomic emission produced under various ion incidence angles

Angular atomic emission distributions of sputtered neutral particles indicate the angular momentum distribution of collision cascades modulated by lattice induced anisotropies of the ejection mechanism. The differential sputtering yield was measured in the angular range between 3° and 80° to the target normal using the collector method: after condensation of the sputtered target material on cooled hemispherical collectors the thickness distributions of the sputtered films were measured in an electron backscattering device. In this way, undistorted angular sputtering yields produced by ion bombardment of monocrystalline Au under various incidence angles were investigated. The results are presented for incidence directions previously checked by ion-induced secondary electron emission. For incidence angles up to 75° to the target normal no deviation from a symmetric emission relative to the surface normal is obvious, whereas the distributions at 80° and 85° show a preference for emission back to the direction of the incident ions.