Sputtering Yield Ratio Research Articles

Segregation effects of doped VN and ZrN on magnetic and microstructural characteristics of FePt-based films

Segregants are essential materials for forming the complexation or granular microstructure in thin films for magnetic recording media. One such film, FePt-BN, has a metallic L10 FePt main phase as its matrix, in addition to having BN and C segregants at the grain boundaries. Amorphous BN has sufficient mechanical strength to support the columnar growth of FePt grains, however, it has a lower deposition rate resulting in columnar grains with a nonuniform aspect ratio. Moreover, some B diffuses in the interstitial site of FePt lattices. To overcome these limitations, this study added the transition nitride VN and ZrN with equal volume percentages to replace some BN segregants in FePt(BN, Ag, C) films: substantially different segregation behaviors were observed. Microstructural imaging mapping revealed that although VN segregated the FePt grains boundary, ZrN was diffused in the lattice. The FePt-BN-ZrN film thus had more disordered grains contributing to a higher in-plane magnetization and wider hard axis magnetic hysteresis loops than did FePt-BN-VN. The FePt-BN-VN film has the highest perpendicular magnetic anisotropy constant (Ku= 2.17 × 106 J/m3) and coercivity (Hc= 4 T); moreover, parallel remanence magnetization was minimal with a linear in-plane magnetization loop. The FePt-BN-VN film was sputtered directly on MgO(100) but depositing the FePt-BN-ZrN film required a 2-nm-thick FePt nucleation layer. The high atomic number of Zr and high sputter yield ratio (YN/Ymetal) of ZrN may increase the sites occupied by N, potentially affecting ZrN segregation.

Sputtering and structural modifications induced in silicon dioxide (SiO2) thin films under (10–40 MeV) Auq+heavy ion irradiation

Surface sputtering and structural modifications induced in silicon dioxide thin films (SiO2/Si) deposited on silicon substrates and irradiated by swift (10–40 MeV) heavy Auq+(q = +4, +6, +7, and +9) ions were investigated by grazing‐incidence X‐ray diffraction (GIXRD) spectroscopy, Rutherford backscattering (RBS) spectrometry and time‐of‐flight elastic recoil detection (ToF‐ERDA) technique. The GIXRD analysis of the as‐deposited and irradiated samples revealed increasing structural modifications of the SiO2thin films under Auq+ion impacts with increasing ion‐beam energy. The changes consisted of decreased grain sizes with increased strain accompanied by a phase transformation from crystalline to amorphous films. RBS analysis showed a decrease in the mean stoichiometric (O/Si) ratio from (2.2 ± 0.1) to (1.7 ± 0.1), due to preferential sputtering of oxygen, as the incident ion energy increased. The obtained RBS‐results were then completed by those of ToF‐ERDA analysis technique using a 40 MeV Au9+heavy ion beam. The preferential sputtering yield ratios (YSi/YO) were determined experimentally both versus electronic stopping power and ion fluence. The obtained results were then compared to numerical values derived from the inelastic thermal spike (i‐TS) model, Sigmund's analytical formula and SRIM simulation code. A good agreement was observed between the measured preferential sputtering data and the i‐TS calculated values, when considering both nuclear elastic and electronic inelastic collision mechanisms. Besides, a close correlation is observed between the electronic stopping power dependent measured sputtering yields and the XRD peak intensity degradation per unit fluence. These observations suggest that the same mechanism of MeV heavy ion‐irradiation induced extended atomic disordering, occurs both in the case of structural modifications and surface sputtering. Finally, the obtained experimental results are discussed on the basis of the i‐TS model.

Monte Carlo calculations of the atmospheric sputtering yields on Titan

Context. Sputtering serves as an important mechanism of atmospheric escape in the solar system. Aims. This study is devoted to atmospheric sputtering on Titan, with a special focus on how the N2 and CH4 sputtering yields respond to varying ion incidence energy and angle, and varying ion mass. Methods. A Monte Carlo model was constructed to track the energy degradation of incident ions and atmospheric recoils from which the sputtering yields were obtained. A large number of model runs were performed, taking into account three categories of incident ion with representative masses of 1, 16, and 28 Da, as well as two collision models both characterized by a strongly forward scattering angle distribution, but different in terms of the inclusion or exclusion of electronic excitation of ambient neutrals. Results. Our model calculations reveal substantial increases in both the N2 and CH4 sputtering yields with increasing ion incidence energy and angle, and increasing ion mass. The energy distribution of escaping molecules is described reasonably well by a power law, with an enhanced high energy tail for more energetic incident ions and less massive atmospheric recoils. The CH4-to-N2 sputtering yield ratio is found to range from 10 to 20%, increasing with increasing incidence angle and also increasing with decreasing incidence energy. An approximate treatment of ion impact chemistry is also included in our model, predicting N2 sputtering yields on Titan that are in broad agreement with previous results.

Reference binding energies of transition metal carbides by core-level x-ray photoelectron spectroscopy free from Ar+ etching artefacts

We report x-ray photoelectron spectroscopy (XPS) core level binding energies (BE’s) for the widely-applicable groups IVb-VIb transition metal carbides (TMCs) TiC, VC, CrC, ZrC, NbC, MoC, HfC, TaC, and WC. Thin film samples are grown in the same deposition system, by dc magnetron co-sputtering from graphite and respective elemental metal targets in Ar atmosphere. To remove surface contaminations resulting from exposure to air during sample transfer from the growth chamber into the XPS system, layers are either (i) Ar+ ion-etched or (ii) UHV-annealed in situ prior to XPS analyses. High resolution XPS spectra reveal that even gentle etching affects the shape of core level signals, as well as BE values, which are systematically offset by 0.2–0.5 eV towards lower BE. These destructive effects of Ar+ ion etch become more pronounced with increasing the metal atom mass due to an increasing carbon-to-metal sputter yield ratio. Systematic analysis reveals that for each row in the periodic table (3d, 4d, and 5d) C 1s BE increases from left to right indicative of a decreased charge transfer from TM to C atoms, hence bond weakening. Moreover, C 1s BE decreases linearly with increasing carbide/metal melting point ratio. Spectra reported here, acquired from a consistent set of samples in the same instrument, should serve as a reference for true deconvolution of complex XPS cases, including multinary carbides, nitrides, and carbonitrides.

Study and simulation of the sputtering process of material layers in plasma

Abstract Sputtering is characterized by a sputtering yield ratio which depends on several conditions, in particular the incident ions energy to the cathode, in normal incidence and when considers the angle α of the incident ions. Our investigations may be considered in first step to calculate the sputtering yield of three metals: copper, silver, and aluminum collide with argon, xenon, oxygen and nitrogen ions using highly developed software called SRIM (Stopping and Range of Ions in Matter) with normal incidence, then with varied angles in future works. The results obtained are compared with those obtained using the analytical models based on the Monte Carlo method proposed by researchers as Sigmund and Yamamaura in order to validate models.

Computer modeling of angular emission from Ag(100) and Mo(100) surfaces due to Arn cluster bombardment

Molecular dynamics computer simulations are employed to investigate the effect of projectile size and surface morphology on the angular emission stimulated by impact of Ar gas cluster projectiles. Argon clusters of sizes n = 10–1000 and kinetic energies of 10 and 20 keV Arn aimed at normal incidence are used to sputter Ag(100) and Mo(100) samples. The total sputtering yield is larger for Ag(100) than for Mo(100). The ratio of sputtering yields is inversely proportional to the ratio of sublimation energies of these solids for projectiles between Ar20 and Ar250. In both systems, the angular distributions are sensitive to both the projectile size and the surface roughness. The maximum of angular spectra shifts from direction normal to the surface toward off-normal direction with the increase in the projectile size. An opposite trend is observed with the increase in the surface roughness. Formation of a cloud composed of projectile atoms and the enhanced lateral material relocation caused by projectile lateral expansion upon impact are the main factors responsible for promoting off-normal ejection. On the other hand, material ejection from randomly inclined surface areas and the influence of nearby topography are found to be responsible for enhancing ejection along the surface normal for rough surfaces.

Contribution to the Quantitative Analysis of Ternary Alloys of Group III-Nitrides by Auger Spectroscopy

Contribution to the Quantitative Analysis of Ternary Alloys of Group III-Nitrides by Auger SpectroscopyFor correct quantitative interpretation of Auger spectra of group III-nitrides and their alloys it is necessary to have the relative sensitivity factors of elements and the sputtering yields measured for the material under analysis. These data are not available in the literature for those materials. In this work, the quantities have been determined experimentally that are needed for reliable and precise quantitative interpretation of Auger spectra of such materials, thus of AlN, GaN and their ternary alloys AlxGa1-xN. Measurements of reference AlN and GaN samples allowed to find the elemental sensitivity factors for these nitrides, and measurements on reference samples of ternary alloys AlxGa1-xN allowed to find the ratio of the component sputtering yields, YGa/YAl. It has been confirmed that if the relative sensitivity factors are obtained from measurements of reference samples of group III-nitrides, thus of compounds, and if in the alloys of such compounds no further change of the shapes of Auger peaks occurs, the both the areas below the Auger peaks in direct spectra and the Auger peak-to-peak heights in differentiated spectra can be used for quantitative analysis.

High-speed processing with Cl2 cluster ion beam

Cluster ion beam processes can produce high rate sputtering with low damage compared with monomer ion beam processes. Cl2 cluster ion beams with different size distributions were generated with controlling the ionization conditions. Size distributions were measured using the time-of-flight (TOF) method. Si substrates and SiO2 films were irradiated with the Cl2 cluster ions at acceleration energies of 10–30keV and the etching ratio of Si/SiO2 was investigated. The sputtering yield increased with acceleration energy and was a few thousand times higher than that of Ar monomer ions. The sputtering yield of Cl2 cluster ions was about 4400atoms/ion at 30keV acceleration energy. The etching ratio of Si/SiO2 was above eight at acceleration energies in the range 10–30keV. Thus, SiO2 can be used as a mask for irradiation with Cl2 cluster ion beam, which is an advantage for semiconductor processing. In order to keep high sputtering yield and high etching ratio, the cluster size needs to be sufficiently large and size control is important.

Selective effect of ion/surface interaction in low frequency PACVD of SiC:H films: Part A. Gas phase considerations

Hard silicon carbide based films (a-SiC:H) are obtained in a capacitively coupled low frequency PACVD device from the decomposition of tetramethylsilane (TMS) diluted in argon (ν=50 KHz, DC bias: ∣Vdc∣=0–400 V). To get a better understanding of the effect of ion bombardment occurring during growth, optical emission spectroscopy was performed near film surfaces to identify energies and relative densities of Si+ and H emitting species, and underline their etching or sputtering from the film surface as it corresponds to strong microstructural modifications of the material. Concurrently, simulation of the effect of ion bombardment onto a film surface has been performed by the use of ©SRIM 2003 to estimate sputtering yields of its different constituting elements. As a general manner, carbon in C–Csp3 form presents the lower sputtering yield relatively to C–Csp2 and Si–C. Ion sputtering can then lead to a relative increase of C–Csp3 at the extreme surface of the target. Such an increase is observed both in experiments, through post-treatments of a film in Ar plasmas (results are exposed in Ref. [A., Soum-Glaude, L., Thomas, E., Tomasella, J.M., Badie, R., Berjoan, Selective effect of ion/surface interaction in low frequency PACVD of SiC:H films: Part B: Microstructural study. Submitted for publication Surf.Coat.Technol.]), and in simulations where this phenomena occurs when ions energy ranges 100–250 eV. For higher energies, sputtering of the surface is no more selective: the simulated sputtering yield ratio [γ(Si–C)/γ(C–Csp3)] is constant versus EAr+ and equals the relative content (%(Si–C)/%(C–Csp3) in the material. Similar results are obtained for C–Csp2 bonds. Such a phenomenon is employed to enhance hardness of films.

Stability analysis of reactive sputtering process with variable sticking coefficients

In reactive sputtering, the introduction of reactive gas would create a hysteresis transition from metal to compound mode in both the target and the substrate. The hysteresis transition is characterized by a sudden change in partial pressure, sputtering rate and fraction of compound formation, etc. Therefore, the stability is an important issue of process control. In this paper, a mathematical model with variable sticking coefficients based on surface kinetics is introduced to study the process stability. The variable sticking coefficient represents different mechanisms for surface reactions from the type of Langmuir to precursors. To facilitate the analysis, several nondimensional parameters are introduced and used for formulation. Results show that, when the chemical reaction on a substrate is moderate, a higher sputtering yield of the compound leads to a more stable steady-state at lower inflow rates. However, when the ratio of sputtering yield (compound/metal) is zero, there is no hysteresis transition because of the deposition of pure metal (single phase) on the substrate. For different sticking mechanisms, the precursor type is found to make the surface easier to saturate with compound due to its higher default sticking coefficient and the lower pressure and inflow rate for the hysteresis transition.

Parameters and Stability Analysis of Reactive Sputtering

Based on Berg's model for reactive sputtering, several nondimensional parameters and ratios are first identified in line with their physical significance. These parameters and ratios are subsequently used to analyze the steady-state solutions of reactive sputtering in detail. Results show that, when the chemical reaction in the substrate is moderate, the lower sputter yield of a compound enhances the hysteresis loop at higher inflow rates. However, when the ratio of sputtering yield (compound/metal) is zero, there is no hysteresis transition because of the deposition of pure metal (single phase) on the substrate. Hysteresis is influenced by pumping speed and a stability criterion is established based on the segment of s curve with negative slope. From the criterion, two turning points for the hysteresis loop can be easily found either graphically or algebraically.

On the formation of concentration profiles by low-energy ion bombardment and sputter depth profiling

An analytical model is developed to describe the generation of concentration profiles in binary solids under low-energy ion bombardment. The model accounts, in particular, for preferential sputtering and radiation-enhanced particle transport in the solid, and is found to apply for a quantitative representation of steady-state concentration microprofiles in the near surface region of initially homogeneous binary targets as Fe-silicides. A model-based evaluation of such profiles delivers absolute values of the corresponding coefficients for radiation-enhanced diffusion. The numerical solution enables to trace the profile evolution in dependence of the bombarding time or ion fluence. When being applied to sputter depth profiling of a pre-existing concentration profile, a severe influence of the actual sputter yield ratio of the target components is found which may completely deteriorate the characteristics of the profile to be analyzed.

Local SiO2 formation in silicon bombarded with oxygen above the critical angle for beam-induced oxidation: new evidence from sputtering yield ratios and correlation with data obtained by other techniques

The first direct comparison of the angular dependence of the sputtering yield of silicon under bombardment with oxygen and inert gas ions of similar mass (neon) is reported. The O2+/Ne+ ratio of the yields, measured at the same energy per projectile atom (5 keV), increases smoothly from 0.33 at 0° to a constant value of 0.89 above ∼65°. This large-angle ratio agrees with the number expected from the projectile mass dependence of sputtering yield theory. The experimental data were analysed on the basis of the assumption that the measured sputtering yield is the sum of contributions from areas composed of either SiO2 or (mostly) Si. The SiO2 fractions thus derived agree quite well with results obtained recently in studies on ion-induced electron emission. Combining the present results with literature data on the angular dependence of the composition of oxygen-bombarded silicon measured by x-ray-induced photoemission spectroscopy, ion-induced electron emission and Rutherford backscattering spectrometry, the SiO2 fractions were found to increase roughly with the square of the O/Si ratio. Data presentation as a function of the atomic fraction of oxygen, on the other hand, gives rise to misleading interpretations. Implications of the results for the interpretation of secondary ion yields and ripple formation are discussed briefly. Copyright © 2000 John Wiley & Sons, Ltd.

Quantitative AES analysis and preferential sputtering of titanium silicide thin films

Quantitative AES analysis of TiSix (1.3 ≤ x ≤ 2.1) thin films deposited on silicon by co-sputtering in a magnetron system has been performed. The time variation of the surface composition during sputtering with Ar+ ions of 3 keV energy has been explained using a kinetic model that allows one to determine the sputtering yield ratio for Ti and Si atoms in the alloy (sputtering corrections) as well as the thickness of the altered layer where the composition is changed by sputtering. In addition to that, both sensitivity factors and matrix corrections are taken into account in order to obtain the alloy composition. Quantitative results obtained using this procedure agree very well with Rutherford backscattering measurements for composition showing that silicon is sputtered preferentially. Copyright © 1999 John Wiley & Sons, Ltd.

Transport theory of sputtering I: Depth of origin of sputtered atoms

Sputter theory employing a sum of two power cross sections has been implemented. Compared with the well known Lindhard power cross section ( V∝ r −1/m), a sum of two such cross sections can give a much better approximation to the Born–Mayer scattering in the low energy region ( m ∼ 0.1). By using both one and two power cross sections, we have solved the linear transport equations describing the sputtering problem asymptotically. As usual, electronic stopping is ignored in the analysis. It has further been proved that Falcone’s theory of the atom ejection process contradicts transport theory. The Andersen–Sigmund relation for partial sputtering yield ratios between two elements in an arbitrary multicomponent target has been derived by both methods. The energy deposited in the target surface layers has been computed for a few typical ion–target combinations. The numerical curves show that both theories generate almost the same results (error <10%) for m=0.2. It is also shown that, if the sputtering yield equals the corresponding one in Sigmund’s theory, the depth of origin of sputtered atoms must be shorter than in Sigmund’s theory for 0.25 > m⩾0. The former even may be only about one half of the latter as long as m=0.

Models for the sputter correction factor in quantitative AES for compound semiconductors

Quantification in Auger electron spectroscopy using the relative elemental sensitivity method with matrix correction factors is a popular method. Matrix factors for bombardment-induced compositional changes and topography development are, however, seldomly used. The main emphasis of this study is to find a sputter correction factor for bombardment-induced surface compositional changes in compound semiconductors after low-energy argon ion bombardment. For this purpose several analytical preferential sputter models are employed. In these models several approximations for the surface binding energy are also used. The calculated factors are compared with experimental averages of 13 binary compound semiconductors bombarded with argon ions. Based on the Sigmund preferential sputter model, a generalized formula for the sputter correction factor for room-temperature AES and XPS analyses with readily available parameters is given by K T AB =(M B ) 0.29 (2X B ΔH B a + X A ΔH A a + X A ΔH B a ) 0.71 (M A ) (2X A ΔH A ° + X B AH A ° + X B ΔH B ) where M i is the atomic mass of atomic species i, ΔH i a is the heat of atomization (which is equal to the heat of sublimation) and X i is the measured surface composition after sputtering. In the context of the presented models, this study also shows that mass effects are more important than chemical binding effects in the bombardment-induced compositional changes in compound semiconductors. The analysis also shows that elemental sputter yield ratio approximation for the sputter correction factor is not valid.

Sputtering of Al-Cr and Al-Ti composite targets in pure Ar and in reactive Ar-N 2 plasmas

Al-Cr-(N) and Al-Ti-(N) coatings were deposited on glass slides by reactive magnetron sputtering of composite concentric Al-Cr or Al-Ti targets in different Ar-N 2 gaseous mixtures (N 2 mass flow <2 sccm). When sputtering occurs in pure argon, the chromium or titanium content increases with the insert diameter of chromium or titanium. For a given insert diameter, the introduction of nitrogen into the reactor yields changes in the Ti/Al or Cr/Al content ratio in the coatings. Both plasma diagnostics, performed by optical emission spectroscopy, and computer simulation allow description of the mechanisms of matter transfer. The composition changes observed can clearly be attributed to a combined effect of differential nitriding of the target with respect to its current density distribution and to the nitride-to-metal sputtering yield ratios involved. However, preferential nitriding of the outer zones of the targets was also observed when the crown was aluminium (with titanium insert) or titanium (with aluminium insert). This behaviour can be related to the sputtering wind (wind effect), which has also been simulated by considering a sinusoidal attenuation of the impinging flux of reactive species from the external zone to the centre of the target.

Dependence of surface-compositional and angular distributions on the bulk composition in the sputtering of CuPt and CuNi alloys

Under 1 keV Ar ion bombardment of CuPt and CuNi alloys at high fluence at low temperature, the depletion degree of the segregating species at the second surface layer, and the normalized sputtering yield ratios (non-segregating species/segregating species) in the near-normal direction (15°) and at oblique angles (75°), have been estimated by an improved kinetic BIGS (Bombardment-Induced Gibbsian Segregation) model. For both the bombarded CuPt and the bombarded CuNi alloys, calculations show that the depletion degree of the segregating species at the second surface layer is small when its bulk composition equals the extreme value, but this depletion degree becomes large when its bulk composition equals the intermediate value. Furthermore, the difference of the normalized sputtering yield ratio between 15° and 75° is small when the bulk composition of the segregating species equals the extreme value, but this difference becomes large when that of the segregating species equals the intermediate value.

Rocking-Angle Ion-Milling of Cross-Sectional Samples for Transmission Electron Microscopy of Multi-Layer Systems

AbstractThe cross-sectional transmission electron microscopy (TEM) specimens of Pt/Ti/SiO2/Si, RuO2/SiO2/Si, W/TiN/SiO2/Si, (Pb,La)TiO3/Pt/MgO, Bi4Ti3O12/Lal-xCaxMnO3/MgO, and GaN/Al2O3 were successfully made by the rocking-angle ion-milling technique. The differential thinning problems could be effectively mitigated when the rocking-angle and the ion-beam incidence angle were optimized for each heterostructure. It was found that the sputtering yield ratio between the layer milled most quickly and the layer milled most slowly is one of the important factors which determine the optimum rocking-angle ion-milling condition. The atomic force microscopy study on the surface topography of the cross-sectional Pt/Ti/SiO2/Si TEM sample after ion-milling provided quantitative information about the effects of the rocking-angle variation. A parameter which is the ratio between the layer with a minimum electron transparency and the layer with a maximum electron transparency was suggested.

Transient phenomena and impurity relocation in SIMS depth profiling using oxygen bombardment: pursuing the physics to interpret the data

High detection sensitivity in bulk analysis or depth profiling by secondary ion mass spectrometry (SIMS) can only be achieved, for positively charged ions, if the nearsurface regions of the sputter eroded sample are fully oxidized. Using oxygen primary ions, a stationary oxidation state is established after some time of bombardment during which period the sputtering yield decreases and the ionization probability increases. The physical and chemical processes occurring during the transient period are reviewed with emphasis on the results for impurity analysis in silicon, i.e. the matrix material that has been studied most thoroughly in the past. The transient decrease in sputtering yield gives rise to a depth scale offset and an associated apparent shift of impurity profiles towards the surface. The effect is largest at normal beam incidence, ca. 1 nm /keV (O + 2 ), in which case silicon is fully oxidized. The transition depth, i.e. the depth sputtered before achieving a stable ion yield is about twice as large and increases as the impact angle is turned away from normal. The shift and the depth scale offset can be measured safely using thin (delta) layers of isotopically pure tracers, for example 30 Si in 28 Si. Boron delta layers can serve as secondary standards because this impurity behaves almost the same as the host silicon atoms. The profile shift observed with other common dopants may contain contributions due to unidirectional relocation, often driven by segregation away from the surface. At O + 2 energies below about 0.7 keV the transition depths fall below the thickness of typical native oxides, so that the transient changes in sputtering yield and ionization probability can disappear. The transient phenomena may be described by a simple sputtering-oxidation model that connects the depth scale offset to other observable parameters like the initial and final sputtering yield, the transition fluence and the oxide thickness.