Ni-Ti Alloy Thin Films Research Articles

Depth distribution analysis of alloying and impurity elements in Ni-Ti alloy thin films using secondary ion mass spectrometer

Ni-rich Ni-Ti thin films of various compositions and thickness (69 nm - 141 nm) were deposited on Si (111) substrate by magnetron sputtering system using equiatomic NiTi alloy sputtering target. X-ray reflectivity analysis of the thin films deposited at room temperature showed the presence of a layered structure as: NiO/Ni-Ti/NiO/SiO2 on Si substrate. Secondary ion mass spectrometry (SIMS) was used to determine the qualitative (intensity) depth distribution of elements Ni, Ti, O and Si in the films using MCs+ approach. Relative sensitivity factor values were utilized to convert the qualitative depth profiles to concentration depth profiles. Surface and depth distribution of alloying elements in the deposited films were investigated. It was observed that the films deposited at high sputtering power and working pressure resulted in more non-uniform depth distribution of alloying elements compared to films deposited at low sputtering power and working pressure. SIMS depth profile data showed the presence of high oxygen contaminations at the surface and at the interface of the films. The Ti concentration in the deposited films depends upon the background oxygen contamination present in the deposition chamber during sputtering process. The concentration of Ni in the films deposited under similar working pressure increases with increase in the sputtering power. However, with increase in working pressure during deposition process results in decrease in the concentration of both Ni and Ti in the deposited films. In order to study the effect of insitu substrate heating on the distribution of alloying and impurity elements, one of the film was deposited on heated Si substrate. The film deposited on heated substrate represents the layer structure as NiTiO3 on Si substrate. The concentration of oxygen in the film deposited on heated substrate is comparatively higher than those deposited at room temperature.

Comparison of quantitative elemental depth distribution analyses of Ni and Ti in co-sputtered Ni-Ti alloy thin films using MCs+ and M+ secondary ions

Compositional uniformity in Ni-Ti thin alloy films, both at the surface and in the depth, significantly enhances their performance in various engineering applications. Quantitative depth distributions of Ni and Ti in magnetron co-sputtered Ni-Ti thin alloy films of various composition and thickness, were determined through an effective depth profiling technique. Secondary ion mass spectrometry (SIMS) was utilized to determine the qualitative elemental depth distribution of Ni and Ti in the films using MCs+-SIMS (Cs+ primary ion beam; MCs+ secondary ions, 'M' is the element of interest) and M+-SIMS (O2+ primary ion beam) approaches. The qualitative depth profiles were converted to quantitative depth profiles using the relative sensitivity factor values determined from the elemental concentrations obtained through energy dispersive x-ray spectroscopy analysis of one of the Ni-Ti thin film taken as reference. Thickness of the deposited films determined by x-ray reflectivity was used for depth scale calibration of SIMS raw profile. A comparison of the analytical capabilities of MCs+-SIMS and M+-SIMS approaches for determining the mean concentration and quantitative depth distribution of elements Ni and Ti in deposited Ni-Ti thin films has been presented.

Synthesis of Ni/Ti thin film by magnetron sputtering to study the effect of annealing time on microstructure and mechanical properties

Thin films composed of a single layer of nickel and titanium were fabricated using magnetron sputtering at room temperature on a Si substrate and the as-deposited films were subsequently annealed to achieve nickel-titanium (NiTi) shape memory alloys. In this paper, we have reported a simplistic way that can be applied to achieve the near equiatomic Ni-Ti alloy thin films with precise controllability of chemical compositions. The compositions were prepared by taking the weight of individual elements in the proper stoichiometric proportions maintaining near equiatomic ratio of Ni and Ti. The surface-interface studies have been performed focusing on interlayer diffusion over the nano-level structure at 500°C for different holding times. The X-ray diffraction patterns of the films annealed at 500°C for different annealing times revealed various kinds of intermetallic compounds and second phase particles along with silicides. Nanoindentation tests on the single bi-layer (two alternative layers) of Ni-Ti thin films were conducted to evaluate the mechanical properties. Subsequent investigation revealed diffusion and migration of atoms between Ni and Ti layers by post-deposition heat treatment consequently have a profound impact on mechanical properties of these films.

Functional fatigue of submicrometer NiTi shape memory alloy thin films

Submicrometer NiTi alloy thin films prepared using biased target ion beam deposition (BTIBD) recently revealed ultranarrow thermal hysteresis and a B2 ⇌ R-phase transformation path. Here, the authors investigate the influence of thermal cycles on the phase transformation characteristics of near-equiatomic NiTi alloy films with 800 nm thicknesses deposited using BTIBD. Evolution of transformation temperatures, thermal hysteresis, and recovery stress over thermal cycles is tracked using the wafer curvature method, and changes in atomic crystal structures are detected using x-ray diffraction. The authors find that the submicrometer thin films exhibit stabilized transformation temperatures, consistent recovery stresses, and reproducible narrow thermal hysteresis over up to 100 cycles although Ni49.7Ti50.3 films undergo two-stage phase transformation B2 → R-phase → B19′ while Ni50.3Ti49.7 films have one-stage transformation between the B2 and R-phases. The inherent deposition mechanism and transformation-path-related lattice distortion can be responsible for the reduced fatigue of functional characteristics in submicrometer NiTi alloy thin films.

Biased Target Ion Beam Deposition and Nanoskiving for Fabricating NiTi Alloy Nanowires

Nanoskiving is a novel nanofabrication technique to produce shape memory alloy nanowires. Our previous work was the first to successfully fabricate NiTi alloy nanowires using the top-down approach, which leverages thin film technology and ultramicrotomy for ultra-thin sectioning. For this work, we utilized biased target ion beam deposition technology to fabricate nanoscale (i.e., sub-micrometer) NiTi alloy thin films. In contrast to our previous work, rapid thermal annealing was employed for heat treatment, and the B2 austenite to R-phase martensitic transformation was confirmed using stress-temperature and diffraction measurements. The ultramicrotome was programmable and facilitated sectioning the films to produce nanowires with thickness-to-width ratios ranging from 4:1 to 16:1. Energy dispersive X-ray spectroscopy analysis confirmed the elemental Ni and Ti make-up of the wires. The findings exposed the nanowires exhibited a natural ribbon-like curvature, which depended on the thickness-to-width ratio. The results demonstrate nanoskiving is a potential nanofabrication technique for producing NiTi alloy nanowires that are continuous with an unprecedented length on the order of hundreds of micrometers.

The effect of annealing temperature on the structural, morphological, mechanical and surface properties of intermetallic NiTi alloy thin films

In this work, we report our studies on intermetallic NiTi thin films grown on silicon (1 0 0) substrates by glancing angle DC-magnetron sputtering technique using separate sputter targets Ni and Ti. The as prepared intermetallic NiTi thin films were vacuum annealed for one hour at four different temperatures, i.e. 350, 450, 550 and 650°C. The crystallization and surface morphology were studied using X-ray diffraction technique (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM). It was observed that the annealing temperatures had a significant influence on structural, morphological, mechanical and surface properties of intermetallic NiTi thin films. XRD studies revealed that the degree of crystallization increased with the increase in annealing temperature. The surface mean height (Ra), root mean square (RMS) and maximum peak to valley height (P-V) values have been obtained by AFM characterization. Nanoindentation tests on annealed intermetallic NiTi thin films were performed at room temperature. The hardness and elastic moduli increased from 8.32 to 10.24GPa and from 154.60 to 164.49GPa, respectively, with respect to the increase in annealing temperature. X-ray photo-electron spectroscopy (XPS) was used to study the composition and surface chemistry of the annealed films. From HR-XPS investigations, it was observed that intermetallic NiTi thin films had higher affinity to form Titanium dioxide (TiO2) layer on the film surface leaving a Nickel enriched matrix immediately behind the metal oxide layer.

Nano-crystalline Ni[sbnd]Ti alloy thin films fabricated using magnetron co-sputtering from elemental targets: Effect of substrate conditions

The primary goal of this work is to examine the effect of in situ substrate conditions viz., substrate bias voltage and substrate temperature on the structural, morphological, compositional, and mechanical properties of co-sputtered NiTi alloy thin films. Application of substrate bias voltage during deposition creates defect sites which facilitate the growth of nano-crystalline NiTi films by imposing restriction on the coalescence of neighboring grains which has lead to interesting properties. Glancing Incidence X-ray diffraction studies revealed the co-existence of B2 and B19′ crystal structures and thus possess potential for martensitic phase transformation, which is the prerequisite for shape memory behavior. Thermally-induced martensitic twin bands were observed within the nano-grains with high-resolution transmission electron microscopy. Surface elemental composition was investigated by X-ray photoelectron spectroscopy to understand the chemical state of NiTi thin films. Differential scanning calorimetric measurements confirms the martensitic transformation below room temperatures. Nanoidentation studies have shown higher hardness values for NiTi films processed at high substrate temperature when compared to the film deposited under the influence of bias voltage due to the deformation mechanism governed by grain boundary absorption of the localized strain in nanometer scale.

Resputtering Effect on Nanocrystalline Ni-Ti Alloy Films

We report on the effect of resputtering on the properties of nanocrystalline Ni-Ti alloy thin films deposited using co-sputtering of Ni and Ti targets. In order to facilitate the formation of nanocrystalline phases, films were deposited at room temperature and 573 K (300 °C) with substrate bias voltage of −100 V. The influence of substrate material on the composition, surface topography microstructure, and phase formations of nanocrystalline Ni-Ti thin films was also systematically investigated. The preferential resputtering of Ti adatoms was lesser for Ni-Ti films deposited on quartz substrate owing to high surface roughness of 4.87 nm compared to roughness value of 1.27 nm for Si(100) substrate.

Crystallization and microstructure evolution of nanoscale NiTi thin films prepared by biased target ion beam deposition

NiTi alloy thin films of nanoscale thickness were fabricated using a novel technique known as biased target ion beam deposition (BTIBD). Ni-poor/Ti-rich, near equiatomic NiTi, and Ni-rich film composition ranges were investigated in the as-deposited condition. Heat treatment was necessary to crystallize the otherwise amorphous as-deposited films. Crystallization and microstructure evolution were contrasted with those for nanoscale thickness films fabricated using the more common magnetron sputtering technique. For each composition range, the as-deposited magnetron sputtered films exhibit a columnar-void morphology. In situ transmission electron microscopy heating results show that crystallization requires the morphology to merge, which produces small grains. Larger grains are formed in crystallized BTIBD films, which are attributed to increased adatom mobility facilitated by independent control of low energy ions. This work postulates that enhanced mobility in BTIBD eliminates the columnar-void morphology and reduces crystallization energy barrier that restricts grain size during heat treatment.

Kinetic Monte Carlo simulation of 3-D growth of NiTi alloy thin films

In this paper, a 3-D Monte Carlo model for NiTi alloy thin film growth on square lattice substrate is presented. The model is based on the description of the phenomenon in terms of adsorption, diffusion and re-evaporation of different atoms on the substrate surface. In this article, multi-body NiTi potential is used to calculate diffusion activation energy. The energy which is related to the types of the atoms is equal to the total energy change of the system before and after the diffusion process happens. The simulations serve the purpose of investigation of the role of diffusion in the determination of the microstructure of the alloy clusters. The effects of the substrate temperature and the deposition rate on the morphology of the island are also presented. The island size distribution and roughness evolution have been computed and compared with our experimental results.

Nanoscale strain fields research of boundaries between B2 matrix and G.P. zone in Ni-Ti alloy thin films.

Ti-47at.%Ni alloy films were prepared by magnetron sputtering followed by 460°C for 40 minutes heat-treatment. The strain fields between B2 phase matrix and G.P. zone were mapped by a combination of high-resolution transmission electron microscopy and geometric phase analysis method. It was found that there is a compressive strain region parallel to the longitudinal axis of G.P. zone with 2 nm in width, −2.2% in average strain at the boundaries between B2 phase and G.P. zone.

An Investigation on Phase Formations and Microstructures of Ni-rich Ni-Ti Shape Memory Alloy Thin Films

Ni-rich Ni-Ti alloy thin films were fabricated by radio frequency (RF)-direct-current (DC) magnetron sputtering using elemental Ni and Ti as sputter targets. Si (100) was chosen as a substrate that was either held at room temperature or at 573 K (300 °C) during the depositions. The 380-nm-thick films were characterized by field emission scanning electron microscopy, energy dispersive spectroscopy, grazing incidence X-ray diffraction, atomic force microscopy, high-resolution transmission electron microscopy, and Vickers microhardness tester. The results suggests that because of the lack of surface mobility of the adatoms at the room temperature, the deposited films were smooth and amorphous, with a crystallite size of 15 nm accompanied with a porous morphology. At higher substrate temperatures, an increase in the surface diffusion leads to the formation of partially crystalline, rougher films with a denser, compact, fibrous grain microstructure. The formation of Ni-rich precipitates such as Ni4Ti3, Ni2Ti, and Ni3Ti along with small amount of the NiTi phase were attributed to the localized heating and cooling within the grains. Few grains exhibited band structures that are believed to be a result of type II twins.

X-ray photoelectron spectroscopy analysis of the effect of thickness on the transformation temperature of NiTi alloy thin films

The effect of thickness on transformation temperature of the NiTi thin films has been studied by X-ray diffraction and X-ray photoelectron spectroscopy. Results show that the crystallization temperature for 3?μm-thick film is higher than that for 18?μm thick film at the same growth temperature and post annealing. With the substrate temperature increasing, the start temperature （As） of austenite phase is lowered after annealing at 763 K for 1?h. There is an oxide layer （TiO2） on the film surface, which prevents the Ni atom from coming onto the surface. There is an oxide layer of a mixture Ti2O3 with NiO on the film /substrate interface. The oxide layers affect the transformation temperature by changing the Ni atomic content in the interior of the film.

Chemical and Structural Characterization of the Ni-Ti Alloy/6H-SiC Contacts

AbstractNi-Ti alloy is a promising candidate for high-temperature contact metallization for SiC electronic devices. In the present study Ni-Ti alloy thin films (100 nm) of two different compositions (Ni90Ti10 and Ni50Ti50) were coevaporated on 6H-SiC substrate. Interfacial reactions, microstructure, compositional changes, and phase formation were investigated as functions of heat-treatments in the range of 400–800 °C. The study was carried out using Auger electron spectroscopy, x-ray diffraction, and analytical transmission electron microscopy. In the case of the Ni90Ti10 alloy the interaction was found to begin at 450 °C. Ni and C are the dominant diffusing species. The reaction zone is divided into three layers. In the first layer, adjacent to the SiC substrate, the presence of a Ni-rich silicide, Ni2Si, and C precipitates, was observed. The second layer is composed mainly of TiC, while the third - of Ni2 Si. In the case of the Ni50 Ti50 alloy the interaction began at 800 °C. Carbon is the dominant diffusant. The reaction zone is divided into two layers. The first, next to the substrate layer is composed of epitaxially grown TiC and the second - of Ni3Ti2Si compound. A thin (∼5 nm) amorphous discontinuous layer was found at the TiC/SiC interface. Factors controlling phase formation in the Ni-Ti/SiC system are discussed.