Preferential Sputtering Effects Research Articles

Effect of nitrogen pressure on the fabrication of AlCrFeCoNiCu0.5 high entropy nitride thin films via cathodic arc deposition

In the past two decades, high entropy alloy (HEA) coatings have attracted great attention due to their superior mechanical properties, outstanding corrosion and oxidation resistance, and exceptionally high thermal stability. In comparison to HEA thin films, high entropy nitrides (HENs) exhibit higher mechanical strength and chemical inertness. In this work, AlCrFeCoNiCu0.5 HEA and HEN thin films were fabricated using a filtered cathodic arc. By regulating the deposition pressure from 0.0005 Pa (HEA thin film) to 0.05 Pa, the nitrogen concentration in each thin film was precisely controlled to tune the mechanical properties. Scanning transmission electron microscopy-energy dispersive spectroscopy revealed that the nitrogen concentration of the films was up to 21.2 at. % at the pressure of 0.05 Pa. The reduced effect of preferential sputtering increased aluminum concentration from 8.3 ± 1.5 to 12.9 ± 2.2 at. % as pressure increased up to 0.05 Pa. X-ray photoelectron spectroscopy further confirmed the formation of AlN and CrN at pressures of 0.01–0.05 Pa. The highest hardness and elastic modulus of the HEN film were 12.4 ± 0.6 and 347.3 ± 17.7 GPa, respectively, which were 84.8% and 131.4% higher than those of the HEA thin film.

Variations in composition and structure occurring in multicomponent alloy targets during their service term

The composition and microstructure of the targets used in the magnetron sputtering (MP) method may change during its operation, especially if the target is a multicomponent alloy. Therefore, the information on the composition and microstructure of the sputtered region of the target, namely, the groove, which forms on the target over time, makes it possible to predict changes in the properties of the film. The targets for our experiments were fabricated from CoCrCuFeNi, CoCrCuFeMnNi and AlCoCrCuFeNiV high-entropy alloys (HEAs). Scanning electron microscopy (SEM) and chemical microanalysis were utilized to study the structure and composition of the grooves on the targets at the beginning and at the end of the targets service life. The results of the investigations allowed to derive the following conclusions: (1) signs of melting found on the surface of the grooves indicate to high temperatures arising on the targets during their sputtering; (2) a new type of preferential sputtering effect has been established, the main condition of which is the presence in the target composition of inclusions of a component whose sputtering coefficient is noticeably higher than that of the others; (3) the formation of a specific microrelief on the target surface is a consequence of the combined action of two factors: high temperature and sputtering.

Quantification of Ag/Ni Auger electron spectroscopy depth profiles upon preferential sputtering with non-stationary roughness

The measured Auger electron spectroscopy (AES) depth profiles of the as-deposited Ag/Ni multilayers demonstrate a strong preferential sputtering effect. In the framework of the Mixing-Roughness-Information depth (MRI) model, the influence of preferential sputtering on the “measured” depth profile is quantitatively evaluated. The mass conservation upon AES preferential sputtering is verified and applied to obtain the true layer thickness from the measured depth profiling data (i.e. intensity or composition as a function of sputtering time). Meanwhile the influences of preferential sputtering and non-stationary roughness effects on the depth resolution are also quantitatively evaluated. Based on the developed model, the measured AES depth profiles of the Ag/Ni multilayers as-deposited and annealed at different temperatures are fitted and the interdiffusion coefficients are obtained accordingly.

The surface of complex oxides; ion beam based analysis of energy materials

LEIS depth profiles, obtained by low energy (0.5 keV) Ar+ sputtering, have been analysed for the mixed conducting oxide material La0.6Sr0.4Co0.2Fe0.8O3-δ. Samples have been examined after differing thermal treatments to examine the sub-surface reorganisation of the cation species. The profiles have shown considerable changes, but these are not strongly correlated with the thermal treatments. The similarity between the profiles suggests that preferential sputtering effects can dominate the sub-surface region (~1–3 nm) where sputtering has not reached equilibrium. Preferential sputtering of oxygen in oxide materials is well known, but here we provide evidence of the preferential sputtering of the cationic species in a complex multicomponent oxide. Of note is strong enrichment (~30%) of the sputtered surface with the heaviest of the elements, La. Simulations using the code TRIDYN have confirmed the observations, in particular, La surface enrichment and the fluence needed to achieve steady state sputtering of > 3 × 1016cm−2.

Formation of multilayer structure and evolution of shape recovery characteristics in proton irradiated TiNi alloy thin films

TiNi thin films have attracted wide attentions as actuator materials in the micro-electro-mechanical systems (MEMs) for aerospace craft. However, multiple irradiation, such as the proton, was generally considered as an essential expired factor for the materials used in the aerospace. The present paper focused on the microstructure, martensitic transformation and shape recovery property of TiNi thin films after being irradiated by proton. A single layer structure B19′ phase to a dual-phase (B2 and B19′) with multilayer structure after proton irradiation have been revealed through a combination technique of grazing incidence (GI) XRD and TEM. As-formed irradiated films with amorphous layer and B2 austenite phase displayed a two-step martensitic transformation behavior, which occurred in the irradiated and the unirradiated layer. In contrast, the unirradiated counterparts just exhibited a single-step martensitic transformation. In addition, some defects could be seen after the TiNi films being irradiated. Such defects could induce a stress field that produces the R phase in the B2 phase. Furthermore, the synergistic effect of preferential sputtering effect and inverse Kirkendall effect induced the GP zones in the irradiated layer after 120 keV proton irradiation. The existence of amorphous phase and precipitation significantly deteriorated the shape recovery property.

Preferential sputtering effects in depth profiling of multilayers with SIMS, XPS and AES

Multilayer profiles are studied using the MRI model with extension to preferential sputtering. The results show a clear distinction with respect to the main contributions to the depth resolution, namely surface and interface roughness, and atomic mixing. For dominating roughness, the effect of preferential sputtering is more pronounced for the residual surface composition profile (detected by XPS and AES), but it is zero for the sputtered matter composition profile (detected by SIMS). Dominating mixing results in a strong effect of preferential sputtering on the shape of the profile. While in that case its effect on the XPS or AES depth profile is relatively moderate, its effect on the SIMS depth profile is surprisingly strong and appears to be in contradiction to textbook statements. Interface width, location and layer thickness are always affected by preferential sputtering in XPS and AES depth profiles, but SIMS depth profiles are only affected if the contribution of atomic mixing to depth resolution cannot be ignored. In conclusion, a new definition of multilayer resolution is proposed which is based on the normalized amplitude of the wave-like profiles. The successful application of MRI model extended for preferential sputtering is given for fitting an experimental AES depth profile of an Ag/Ni multilayer.

Peculiar properties of preferential sputtering of PbTe, SnTe, and GeTe by Ar+ ion plasma

Sputtering of PbTe, SnTe and GeTe compounds having both small and large differences in the individual components’ masses by Ar+ plasma under Secondary Neutral Mass Spectrometry (SNMS) conditions are investigated. The effect of preferential sputtering and its peculiar features caused by the different ratios between masses of the IVB atoms and tellurium were observed. The method and the empirical relationships for determination of the relative detection factor (RDF) of the intrinsic components of the compounds are suggested. It is shown that RDF of Te and metal species of the investigated samples isn’t a constant, but depends on sputtering energy. These dependences are explained by the changes of the sputtering surface morphology and the impact of mass of the sputtered species on the fraction of the species flux ejected into the solid angle collected by the spectrometer mass analyzer. The limits of applicability of SNMS for quantitative analysis of composite samples with big differences in masses of their constituents are established. A possible solution for such cases is proposed. It is shown that in the sequence of PbTe-SnTe-GeTe compounds the surface binding energy of the metal components decreases in the direction from lighter to heavier atom.

Surface Activated Bonding -from the Standard SAB to Modified SAB

The surface activated bonding (SAB) was proposedin thelate 1980’s for bonding of metal to metal and to ceramics at room temperature. The standard SAB method is based on surface activation process by bombardment of the surfaces by Ar ions or neutral atoms in ultra-high vacuum to clean and activate them so that they can be bonded spontaneously only by contact without heat treatment.The term of the SAB was used for the first time in the article of F. S. Ohuchi and T. Suga (1994) [1],to describe the bonding process which “utilizes the fundamental nature of atomically clean surfaces created by energetic particle bombardment leading to bond formation when two surfaces are brought into contact.” The standard SAB process for Al-Al and Al-Si3N4 was described in 1992[2],and Cu-Cu for micro-bonding in 1993[3]. The standard SAB method was applied successfully to the direct wafer boding of Si-Si in 1996[4]and then expanded to heterogeneous bonding between semiconductors and metals at room temperature. The SAB method has attracted increasing interest due to its simple process flow, no need for additional intermediate materials for bonding, and compatibility with CMOS technology. Simulations of the electronic structure of direct bonded interfaces reveal that certain electron transfer may occur across the bonded interface depending on the nature of the activated surfaces, resulting into covalent, metallic, or ionic bonding at the interfaces. The surface activation process of the ion bombardment induces various effects on the activated surfaces. For metals, the native oxide is removed and point defects are induced with nano-grain formation observed in some cases. For semiconductors, dangling bonds are formed at the surface which however sometimes contains certain electronic defects and becomes amorphous structure. For ionic materials such as oxides, carbides, and nitrides, certain preferential sputtering effect is observed: for instance, SiC surface after the surface activation shows a Si depletion layer. Those effects could contribute the strong bonding formation at the bonded interface, or sometime cause some restriction in the utilization of the bonded interface. The standard SAB method has failed in bonding SiO2, carbon materials and organic polymers because SiO2 and carbon materials may hardly form dangling bonds, the later becomes amorphous, and polymer materials are carbonized and inert to bond. To overcome the limit of the standard SAB, modified SAB method has been developed in which metal nano-intermediate layer is inserted into the bonded interface so that the electronic polarization is sealed by the metallic layer and the week adhesion is enhanced by boding to the metal layer. By the modified SAB, bonding of glass, SiC, GaN, diamond, and polymer films as well as their combinations have been demonstrated. The conventional wafer bonding technique which is called as fusion bonding, oxide bonding, or hydrophilic bonding, requires a reaction with water and OH- group formation as a prior condition and belongs to a different category of the SAB. However, its combination with the modified SAB provides also a new possibility for low temperature bonding.

Effects of proton irradiation on the microstructure and shape recovery characteristics of a NiTi alloy

Herein, we investigate the effects of proton irradiation on the structure and shape recovery characteristics of a NiTi alloy and provide a mechanistic explanation of the observed phenomena. Detailed analysis revealed that bombardment with 3 MeV protons induced the formation of a complex multilayer structure comprising TiH2, Ti2Ni, austenite and austenite-martensite layers along the irradiation direction with increasing distance and the origin of the above structure was shown to be closely related to the preferential sputtering effect and irradiation-induced segregation. Finally, the shape recovery characteristics significantly deteriorated with increasing irradiation fluence, since the multilayer structure hindered the martensitic transformation.

Surface oxidation of as-deposit uranium film characterized by X-ray photoelectron spectroscopy

Uranium film is a main functional component to realize the high efficiency conversion of laser to X-ray in the study on laser inertial confinement fusion. It also has important applications in relevant physics experiments. Due to its active chemical properties, the metal uranium film is extremely difficult to preserve in the atmosphere. A variety of methods may help to avoid the oxidation of uranium film, such as coating protective layer, vacuum or inert atmosphere protection. But under these conditions, the oxidation property of uranium film still needs experimental investigation. In this paper, the oxidation processes of uranium films under different atmospheres are studied by X-ray photoelectron spectroscopy (XPS) and depth profiling. Firstly, uranium films and gold-uranium multilayer films are prepared by ultra-high vacuum magnetron sputtering deposition, and then they are exposed to atmosphere, high purity argon and ultrahigh vacuum for a period of time. Finally, the distributions and valence states of oxygen and uranium elements are investigated by XPS depth profiling. The oxidation mechanism is analyzed according to the oxidation products and the microstructure characteristics of samples. The results show that the oxygen element is undetectable in the initial films. In the Au-U multilayer film, the protective ability of Au layer is greatly weakened by the micro-defects. The defect is not only the micro channel of oxygen entering into the sample directly, but also the origin of the interlaminar cracks at the Au/U interface. In about three weeks, the uranium layer is severely oxidized and large area lamination occurs. The oxidation products consist of a dense oxide thin film on uranium surface and corrosion pitting around the defects, which are mainly UO2. For the pure uranium film, the surface of the film is completely oxidized when it is exposed to high purity argon only for 6 h. The UO2 layers with different thickness values are formed on their surface, which is due to the rapid diffusion of oxygen atoms at the columnar grain boundaries of the film. After the sample is exposed to the ultra-high vacuum for 12 h, UO2 layer with a thickness of less than 1 nm is generated on the surface of the pure uranium film. In the etching of oxide by argon ion beams, the preferential sputtering effect of O is produced, and UO2 is reduced into non-stoichiometric UO2-x. The effect of preferential sputtering is weakened with the decrease of oxide content. When the oxide content is less than 10%, the reduction of UO2 can hardly be detected.

Preferential sputtering on multicomponent optical surfaces

Abstract In this study, we report the effect of preferential sputtering during ion beam machining multicomponent optics. A binary continuum model is developed to investigate the surface preferential sputtering mechanism. Diverse mechanisms, such as sputtering, Fick diffusion, Mullins diffusion, ion-enhanced viscous flow, and ballistics drift, are integrated to characterize the surface evolution. Model simulations and experiments on ultra-smooth SiO 2 substrates with the roughness of several nanometers are performed and compared. Both results demonstrate a consistent trend that surface composition is preferentially re-organized during ion beam sputtering. As ions impinge, surface Si concentration increases from initial 32.5% of un-sputtered surface to about 50% and then falls and eventually stabilizes at 41 ± 1.5%, both in experiments and model simulations. Furthermore, preferential sputtering enhances surface reflectivity (>45%) while scarcely changing surface roughness (

Niobium/Vanadium doped TiO2 multilayered sol-gel films: Structure, surface chemistry and optical properties

In this work multilayered TiO2 films deposited on glass and Si substrates by the sol-gel – dipping method were studied, focusing on the influence of Niobium/Vanadium doping on the structure, surface chemistry and their optical properties. The TiO2 films doped with 1.2at% Nb(V) crystallized in anatase phase as evidenced by XRD analysis. It was found that thicker films are obtained in the case of V-doped TiO2 films, while the values of the optical band-gap show similar values. Bands related to the vibration of oxygen in the Ti–O, Nb–O and V–O bonds were identified based on Infrared Spectroscopic Ellipsometry, proving the incorporation of the dopants in the matrix of the TiO2 films. Both types of doped films exhibit around 80% transparency on a wide spectral range (300–1700nm). Niobium and Vanadium have been identified in the surface / subsurface region in the 5+ oxidation state (although a high sensitivity of the Vanadium oxidation states during profiling experiments was found due to the preferential sputtering effect). From a morphological point of view, all titania films show a very uniform and compact microstructure, with a homogeneous superficial structure of small grains with diameters in the range of 10–20nm.

Low‐Energy Ion‐Scattering Spectroscopy of Modified Silicate Glasses

Low‐Energy Ion‐Scattering (LEIS) spectroscopy is a technique with a unique sensitivity to the elemental composition of the top atomic layer of a solid surface. LEIS measurements of ternary silicate glasses modified with Na2O, Cs2O, CaO, and BaO show that the compositions of the as‐cast (melt) surface and the in‐vacuum fracture surface often differ. While the as‐cast surface is usually depleted of alkali ions (Na+ or Cs+) compared to the nominal (batch) glass composition, there is often strong accumulation of the same mobile cations on the fresh fracture surface. Depth profiles obtained by sputter etching reveal elemental concentration gradients normal to the glass surface. The final concentrations often fail to reach the nominal glass composition, suggesting the likely presence of preferential sputtering effects and thereby the distortion of the measured concentration gradient. At present, the lack of reliable standards and preferential sputtering effects in the LEIS of multicomponent glasses limit somewhat the absolute chemical composition and structural information that can be obtained with this otherwise unique and powerful method of surface analysis.

High vacuum arc ion plating NiCrAlY coatings: Bias effect and approach to preparation of functional gradient coatings

The substrate bias effects on high vacuum arc ion plating (HV-AIP) of NiCrAlY coatings on superalloy were investigated at the vacuum level of 10−3–10−4Pa, which are 2–3 orders in magnitude better than that of conventional low vacuum arc ion plating (LV-AIP). The negative bias applied to the substrate was in the range of −50V to −450V. Microstructures, deposition rates, and chemical compositions of the coatings are significantly influenced by the negative bias. Except for commonly observed bias induced densification in microstructure and decrease in deposition rate, strong preferential sputtering effect at high bias was observed. The sputtering yield for Cr was estimated to be ranged from ~0.1 to ~0.2 at bias from −50V to −450V, while those for Ni and Al were from ~0.1 to 1.5. As a result, the major phase constitution of the coatings changed from γ-Ni/γ′-Ni3Al phases at bias of −50V to α-Cr phase at bias of −450V. Such stronger preferential sputtering effect in HV-AIP fosters a strategy for preparing compositionally gradient NiCrAlY coatings just by control of the substrate bias without the need of further treatments or multi-targets. Based on this concept, preparation of several types of gradient NiCrAlY coatings with great flexibilities by one target and one process was demonstrated.

Sputtering of amorphous silicon nitride irradiated with energetic C60 ions: Preferential sputtering and synergy effect between electronic and collisional sputtering

Amorphous silicon nitride films (thickness 30nm) deposited on Si(001) were irradiated with 30–1080keV C60 and 100MeV Xe ions to fluences ranging from 2×1011 to 1×1014ions/cm2. The composition depth profiles of the irradiated samples were measured using high-resolution Rutherford backscattering spectrometry. The sputtering yields were estimated from the derived composition profiles. Pronounced preferential sputtering of nitrogen was observed in the electronic energy loss regime. In addition, a large synergy effect between the electronic and collisional sputtering was also observed. The sputtering yields were calculated using the unified thermal spike model to understand the observed results. Although the calculated results reproduced the observed total sputtering yields with a lowered sublimation energy, the observed preferential sputtering of nitrogen could not be explained. The present results suggest an additional sputtering mechanism related to the electronic energy loss.

Copper thin films by ion beam assisted deposition: Strong texture, superior thermal stability and enhanced hardness

Nanocrystalline metals generally exhibit exceptionally high strength. However, their susceptibility to grain growth restricts their applications in high temperature environments. The current study presents that nanocrystalline Cu thin films produced by ion beam assisted deposition (IBAD) are able to sustain their as-deposited microstructure and high hardness upon annealing at high temperatures. IBAD-Cu films exhibit a strong (111) fiber texture, which is caused by the ion beam induced effects of substrate cleaning, preferential damage and preferential sputtering. The microstructure of the IBAD-Cu films is stable at temperatures up to 800°C (80% of the melting point of Cu). The hardness of the as-deposited IBAD-Cu films can reach a maximum value of 3.85GPa. Even after annealing, their hardness is still much higher than that of the normally deposited (without ion beam) films as well as their bulk nanocrystalline counterparts before heat treatment. The excellent thermal stability of microstructure is attributed to the formation of nanometer-sized voids and their pinning effect on grain boundary migration. The kinetics of void formation, the contribution of twin boundaries and ion beam induced defects to the hardness are analyzed and discussed. The findings in this study demonstrate that IBAD is an effective method for the stabilization of microstructure and mechanical properties of nanocrystalline metal thin films.

A model for quantification of GDOES depth profiles

The MRI (Mixing-Roughness-Information depth) model and the CRAS (Crater-Simulation) model are combined for the quantification of GDOES (glow discharge optical emission spectroscopy) depth profile by taking into account the effects of crater, roughness and preferential sputtering in depth profiling. This combined model is successfully applied for the quantification of the measured GDOES depth profiles of N in a nitride coating and of Ni in a Ni-coated copper substrate.

Compositional depth profiling of TaCN thin films

The composition profiling of thin TaCN films was studied. For the composition profile determination using x-ray photoemission spectrometry (XPS) in combination with Ar sputtering, preferential sputtering effects of N with respect to Ta and C were found to lead to inaccurate elemental concentrations. Sputter yield calculations for the given experimental conditions allowed for the correction of a part of the error, leading to fair accuracy by reference-free measurements. Further improvement of the accuracy was demonstrated by the calibration of the XPS compositions against elastic recoil detection analysis (ERDA) results. For Auger electron spectrometry (AES) in combination with Ar sputtering, accurate results required the calibration against ERDA. Both XPS and AES allowed for a reliable and accurate determination of the compositional profiles of TaCN-based thin films after calibration. Time-of-flight secondary-ion mass spectrometry was also used to assess the composition of the TaCN films. However, the analysis was hampered by large matrix effects due to small unintentional oxygen contents in the films. Energy-dispersive x-ray spectrometry is also discussed, and it is shown that an accurate reference-free measurement of the average film concentration can be achieved.

Ion beam sputtering of multicomponent targets: Surface composition change and cluster emission

Preferential sputtering effect, which occurs during irradiation of a multicomponent target by medium energy ions, was under our investigation. A new term characterizing the preferential sputtering, called “surface sputtering yield” and defined as average number of components i sputtered from a top surface layer per one primary ion, was suggested. A direct proportionality between the dimer emission and surface concentration of the component, forming the dimer, was concluded; this let us estimate the preferential sputtering of any target from the composition of the flux of secondary particles, analyzed directly during the ion sputtering process, and define the composition of the ion sputtered surface.

Influence of the ion type on the ion beam mixing of Cr/Al interfaces

The room temperature ion beam mixing (IBM) of Cr/Al interfaces by 3keV Ar+ and He+ ions has been carried out in order to study the influence of the ion type on the IBM process. A two stages IBM kinetics has been found by application of factor analysis to X-ray photoelectron spectroscopy data. The first stage is characterized by a strong decrease of the metallic Cr species concentration associated with sputtering and formation of a nearly stoichiometric chromium aluminide, CrAl. The CrAl related species concentration reaches a maximum at the end of the first stage, slowly decreasing subsequently during the second stage because of sputtering. The differences observed between the final ion dose and maximum CrAl species concentration at the end of the first stage between Ar+ and He+ IBM have been explained as due to the different sputtering yields and energy deposited per incoming ion. Likewise, the difference observed in the stoichiometry of the CrAl compound formed has been associated with Cr preferential sputtering effects taking place during Ar+ IBM. The disagreement between experimental results and Monte Carlo TRIDYN simulations suggests that radiation-enhanced diffusion processes contribute to the low energy Ar+ and He+ IBM of Cr/Al interfaces at room temperature, especially when He+ is used.