Sputter-deposited Ti-Ni Thin Films Research Articles

MODELING AND SIMULATION OF MAGNETRON-SPUTTERRED NiTi THIN FILM DEPOSITION BY SRIM/TRIM

Nickel-Titanium (Ni-Ti) thin films have gained a lot of attention due to their unique features, such as the shape memory effect. Micro-actuators, micro-valve, micro-fluid pumps, bio-medical applications, and electronic applications have a lot of interest in these smart thin films. Sputter-deposited NiTi thin films have shown the potential to be very useful as a powerful actuator in micro-electro-mechanical systems (MEMS) because of their large recovery forces and high recoverable strains. Despite the advancement of improved deposition methods for the NiTi thin films, there are still certain unsolved challenges that impede accurate composition control throughout the deposition process. Many applications, spanning from the aerospace industries to a range of nanotechnologies, require knowledge of the sputtering characteristics of the materials that are subjected to bombardment, ejection, and deposition of ions. In recent decades, atomic scale modeling has been given a high emphasis in ion sputtering research, providing an adequate and precise description of collision cascades in solids using the Stopping and Range of Ions in Matter (SRIM) and Transport of Ions in Matter (TRIM). In this paper, SRIM is used to address how the heavy ions interact with the target materials. A variety of ion-solid interaction characteristics, including the sputter yield, have been determined by simulating collision cascades in the solids. On the other hand, TRIM was used to describe the range of ions that enter into the matter and cause damage to the target throughout the process. The simulation was carried out to compare the sputtering yield of Ni and Ti by varying the energy input (from 300[Formula: see text]V to 1300[Formula: see text]V). SRIM simulation was conducted by varying the thickness of the film, the angle of incidence of ions, and the energy involved in the sputtering process. The characterization of the films has been carried out using Field Emission Scanning Electron Microscopy (FESEM) to comprehend the surface and interface morphologies of the films and to validate the simulated results. With an increase in energy input (target voltage), the sputtering yield increased. The sputtering yield of the Ni target was higher than the Ti target indicating that Ni can be removed relatively easier than Ti.

A survey on crystallization kinetic behavior of direct current magnetron sputter deposited NiTi thin films

The purpose of this work is to assess the crystallization kinetic properties of the direct current (DC) magnetron sputtered NiTi thin films. NiTi thin films were deposited on NaCl substrate using a magnetron sputtering method. Differential scanning calorimetry (DSC) was employed at different heating rates of 5, 20, and 30 K min−1 to investigate the crystallization kinetic behavior. Only one dominant peak emerged during amorphous-crystalline transformation at different heating rates. The activation energy values during crystallization were 321.81, 288.86, and 234.56 kJ mol−1 at heating rates of 5, 20, and 30 K min−1, respectively. The average value of the Avrami exponent for the studied films was 2.2. The predominant mechanisms governing the nucleation and growth processes were continuous nucleation and diffusion-controlled two-dimensional growth. We envisage that the results of this systematic work will create new paradigms and opportunities in industrial-scale applications of NiTi films in microelectrochemical systems.

Effects of Cu on the microstructural and mechanical properties of sputter deposited Ni-Ti thin films

The microstructure of sputter deposited Ti-rich Ni-Ti thin films doped with Cu in the range 0–20.4at.% and annealed for 1h at 500 and 600°C has been investigated and correlated with the mechanical properties of the films measured by depth-sensing nanoindentation. X-ray diffraction analysis showed the microstructural evolution of Ni-Ti thin films when doped with Cu and annealed at different temperatures. Heat treatments promoted the nucleation and growth of Ti2Ni precipitates in Ti-rich Ni-Ti thin films, which affected the stability of austenitic and martensitic phases at ambient temperature. Doping with Cu caused the formation of Ti(Ni, Cu)2 plate precipitates, which became more finely and densely dispersed in the grains with increasing Cu content. TEM analysis showed a columnar grain morphology extended through the whole films thickness, while with increasing Cu content a noticeable lateral grain refinement was induced by segregation of a (Ni, Cu)-rich phase to grain boundaries. The nano-hardness increased almost linearly with increasing Cu content owing to this grain refinement, though differences between samples annealed at different temperatures were found which could be related to the evolution of Ti(Ni, Cu)2 plate precipitates with annealing temperature and Cu content. The Young's modulus exhibited a similar dependence on Cu content as nano-hardness, though no significant differences were observed with increasing annealing temperatures.

Sputter deposited Ni–Ti thin films on polyimide substrate

In this paper, we report on the feasibility of depositing Ni–Ti, with Ni-rich composition, thin films at 450°C on polyimide. The films were magnetron co-sputter deposited from Ni–Ti and Ti targets on to three different substrate types: a) polyimide (PI) foils, b) SiO2/Si and c) PI spin coated Si wafers. The crystal structure, surface morphology and microstructure were studied using X-ray diffraction, atomic force microscopy and transmission electron microscopy. A difference in thin film growth mode was observed depending on the substrate. Ni–Ti films deposited on polyimide foil did not show any phase transformation upon thermal cycling. Ni–Ti films deposited on polyimide spin coated wafers showed the expected phase transformation upon thermal cycling.

Surface and interface reactions of sputtered TiNi/Si thin films

We present the surface and interfacial chemical analysis of sputter deposited TiNi thin films over Si (100) substrates using Rutherford backscattering spectrometry (RBS), secondary ionization mass spectrometry (SIMS), and grazing incidence x-ray diffraction analysis. Upon annealing to high temperatures, significant diffusion of Ni into the Si substrate was observed. The analysis of the RBS and SIMS spectra confirm the net movement of the film species into the substrate. Further, we discuss the formation mechanism and the composition distribution surrounding the TiNi/Si interface that proves conducive for the nucleation and growth of the detected nickel silicide compound.

TiNi-Base and Ti-Base Shape Memory Alloys

The basic characteristics of TiNi-based and Ni-free Ti-based shape memory alloys are reviewed. They include the crystal structures of the parent and martensite phases in both the alloys, the recoverable strain associated with the martensitic transformation, the transformation temperatures, the temperature and orientation dependence of deformation behavior, etc. The sputter-deposited Ti-Ni thin films are also reviewed briefly because of their possibility of expanding into micromechanical system applications as the most powerful microactuator.

Nanoindentation of Ni–Ti Thin Films

Ni–Ti thin films of various compositions were sputtered-deposited on silicon substrates. Their mechanical properties (hardness and Young's modulus) were then determined using a nanoindenter equipped with a Berkovich tip. This paper examines the effects of composition on the mechanical properties (hardness and Young's modulus) of the sputter deposited Ni–Ti thin films. This is of particular interest since the actuation properties of these shape memory alloy films are compositionally sensitive. The surface-induced deformation is revealed via Atomic Force Microscopy (AFM) images of the indented surfaces. Which show evidence of material pile-up that increases with increasing load. The measured Young's moduli are also shown to provide qualitative measures of the extent of stress-induced phase transformation in small volumes of Ni–Ti films.

Oxidation behavior of sputter-deposited Ti–Ni thin films at elevated temperatures

The isothermal oxidation kinetics of sputter-deposited equiatomic Ti–Ni thin films in pure oxygen from 823 to 923 K is studied using thermo-gravimetric analysis. The structure, composition-depth distribution and surface morphology of oxidized Ti–Ni thin films are investigated by X-ray diffraction (XRD), Auger electron spectroscopy (AES) and atomic force microscope (AFM), respectively. The results show that the oxidation kinetics of Ti–Ni thin films obeys a near-parabolic law. TiO 2, TiNi 3 and parent B2 phase are the compositions of oxidized Ti–Ni thin films. A double-layered scale including the outermost layer and the Ni-rich layer is formed outside the B2 matrix of oxidized Ti–Ni thin films. Moreover, thermal oxidation induces a surface smoothening of Ti–Ni thin films and surface roughness of oxidized Ti–Ni films decreases with the increasing oxidation temperature.

Effect of annealing temperature on surface morphology and mechanical properties of sputter-deposited Ti–Ni thin films

51.39 at.%Ti–48.61 at.%Ni thin films were prepared by magnetron sputtering and post-annealed at 450, 500, 550 and 600 °C, respectively. Crystallization kinetics of as-deposited Ti–Ni thin films was analyzed using differential scanning calorimetry (DSC). The evolution of structure, surface roughness and mechanical properties of Ti–Ni thin film annealed at different temperature were investigated by X-ray diffactometer (XRD), atomic force microscope (AFM) and nanoindention test. The results show that amorphous Ti–Ni thin films were crystallized at 506.3 °C with 18.6 J/g reaction enthalpy. As annealing temperature increased from 450 to 500 °C, crystallization degree of Ti–Ni thin film rose and surface roughness increased as well. As for Ti–Ni thin film annealed above 500 °C, Ti 2Ni phase precipitated and surface roughness first increased and then decreased with increasing annealing temperature. Moreover, amorphous Ti–Ni thin films are softer than crystalline ones. Microhardness and Young's modulus of crystalline Ti–Ni thin films increased with increasing annealing temperature.

Two-way Shape Memory Surfaces

AbstractRecently, we discovered that a two-way shape memory effect can be realized on NiTi shape memory alloy surfaces by spherical indentation. Either thermomechanical cycling or slip-plastic deformation under contact loading conditions is effective in inducing reversible indent depth changes: deeper indents in the low temperature martensitic phase and shallower indents in the high temperature austenitic phase [1, 2]. By planarization of these spherical indents to restore a flat surface, circular reversible surface protrusions appear upon heating and disappear upon cooling. Nano-protrusions are also realized in sputter deposited NiTi thin films. Furthermore, two-way reversible line protrusions are made by planarizing scratch tracks on a NiTi shape memory alloy. Shape memory surfaces can be exploited for a wide range of potential applications, including information storage, optical devices, and smart tribological surfaces.

The effect of two types of precipitates upon the structure of martensite in sputter-depositedTi–Ni thin films

In sputter-deposited Ti-rich Ti–Ni shape memory alloy thin films, there are two peculiartypes of precipitates: plate-like GP zones formed along directions of the Ti–Ni phase and sphericalTi2Ni precipitates homogenously distributed within grains. The effect of these precipitates ontwinning in stress-induced martensite were studied with the use of a transmission electronmicroscope and compared with previous observations of twinning in thermallyformed martensite of Ti–48.9 at.% Ni and Ti–48.5 at.% Ni thin films. The mainsubstructure of thermally formed martensite in the thin films containing GP zones orTi2Ni precipitates consists of three orientations of fine (001) twins. On the other hand, thestress-induced martensite consists of domains of lamellar type II twins and fine (001) twins. While the domains of type II twins are preferably oriented in a way characteristic of stress-induced martensite,the (001) compound twins are not. The type II twins are introduced under stress as a lattice invariant shear, in preference to(001) compound twins.

Ti-Ni sputter-deposited SMA thin films and their applications to microactuators

The motivation for fabricating sputter-deposited TiNi-base shape memory alloy thin films originates from the great demand for the development of powerful microactuators which can drive micromachines, because actuation force and displacement are greatest in the shape memory alloys among many actuator materials. Besides, even an intrinsic disadvantage of the shape memory alloy such as slow response due to the limitation of cooling rate can be greatly improved in micro size actuators, because the surface/volyme ratio of materials will drastically increase in thin films so that cooling rate becomes efficient. The present paper will give an outline of the recent development and characterization of sputter-deposited Ti-Ni based shape memory alloy thin films and their applications for microactuators. Some prototypes of microstructures consisting if Si warfare and sputter-deposited Ti-Ni thin films have been fabricated by utilizing silicone-based micromachining techniques. The fabrication processes and actuator characteristics of the microactuators will be presented.

Structure of martensite in sputter-deposited Ti-Ni thin films containing homogeneously distributed Ti 2 Ni precipitates

Abstract The morphology of martensite in sputter-deposited thin films of Ti-48.5 at.%Ni was studied. After annealing at 873 K for 1 h, fine Ti2Ni precipitates formed in the matrix of a TiNi phase. Electron diffraction verified that (001) compound twins are dominantly observed as internal defects, which lie in three orientations. Morphologically it was found that there are three kinds of boundary among martensite domains of (001) twinning, namely {113} type, (001)//{111} type and {111} type. In fact, the wave-like {113}-type boundary is also the (001)//{111} type from the atomic viewpoint. These matching planes beside the boundaries derive from the same close packed planes in a bcc structure, and produce low energy boundaries. The formation of (001) twinning is attributed to the impingement of growing martensite variants with the Ti2Ni precipitates during martensitic transformation. In some local regions, (111) type I or 〈011〉 type II twinning can be found owing to the relatively inhomogeneous distribution...

Structure of martensite in sputter-deposited Ti-Ni thin films containing homogeneously distributed Ti2Ni precipitates

Abstract The morphology of martensite in sputter-deposited thin films of Ti-48.5 at.%Ni was studied. After annealing at 873 K for 1 h, fine Ti2Ni precipitates formed in the matrix of a TiNi phase. Electron diffraction verified that (001) compound twins are dominantly observed as internal defects, which lie in three orientations. Morphologically it was found that there are three kinds of boundary among martensite domains of (001) twinning, namely {113} type, (001)//{111} type and {111} type. In fact, the wave-like {113}-type boundary is also the (001)//{111} type from the atomic viewpoint. These matching planes beside the boundaries derive from the same close packed planes in a bcc structure, and produce low energy boundaries. The formation of (001) twinning is attributed to the impingement of growing martensite variants with the Ti2Ni precipitates during martensitic transformation. In some local regions, (111) type I or 〈011〉 type II twinning can be found owing to the relatively inhomogeneous distribution of these precipitates.

Structure of martensite in sputter-deposited Ti–Ni thin films containing Guinier–Preston zones

It was found that Guinier–Preston (GP) zones (coherent plate precipitates with a thickness of a few atomic layers) have an influence on the twinning modes of martensite in sputter-deposited Ti–48.9at%Ni thin films. In an equiatomic Ti–Ni thin film (without GP zone) or a thin film with limited GP zone, 〈011〉 type II twins were predominantly observed as a lattice invariant shear. With increasing annealing temperature and/or annealing time, GP zones grow in amount and in size. These GP zones influence the martensitic transformation. They change the twin band width ratio as a lattice invariant shear during martensitic transformation and increase the probability of appearance of ( 11 1 ̄ ) twinning. When GP zones are sufficient in amount and size, (001) compound twinning was observed as internal defects in the thin film martensite. In general the (001) fine twinning plates are arranged in three orientations with an angle of about 120° between each other. Though there is only one kind of relationship, i.e., (111) twinning, between these (001) twinning regions, three kinds of boundaries exist, i.e., a (111) twinning plane, an averagely approximate ( 1 ̄ 1 ̄ 3 ) plane, or a ( 11 1 ̄ ) plane. It is believed that these morphologies are also self-accommodated.

Stress-strain curves of sputter-deposited Ti-Ni thin films

Stress-strain curves of sputter-deposited Ti-Ni thin films

Effect of Ti Content on Nanometric Substructure and Shape Memory Property in Sputter-Deposited Ti-Rich Ti-Ni Thin Films

Effect of Ti content on the formation of bct thin plate precipitate in sputter-deposited Ti-rich Ti-Ni thin films has been studied by high-resolution electron microscopy. This bct non-equilibrium precipitate having perfect coherency with the B2 matrix is a key factor to produce very good shape memory properties. The results indicate that the upper limit of Ti content to produce this precipitate is about 53 at. %.

The Interface of Sputter-Deposited TiNi Thin Film on (100) Si Wafer

The sputter-deposited TiNi thin films on (001) Si wafer were annealed at 400°C∼800°C for 30 minutes and their interfaces were studied by using TEM, AES, and XRD analysis. For annealing temperature below 600°C, Ni atoms are the main diffusing species of the film-substrate interface reaction and NiSi 2 compound forms triangularly toward the Si substrate. For annealing temperature higher than 600°C, Si atoms are the main diffusing species and Ti-Ni-Si ternary compounds are observed. A Ti 4 Ni 4 Si 7 layer with NiSi 2 islands inside, and another Ti 1 Ni 1 Si 1 layer in between TiNi thin film and Ti 4 Ni 4 Si 7 layer, coexist at the interface of specimen annealed at 700∼800°C.

Martensitic transformation and shape memory behavior in sputter-deposited TiNi-base thin films

Abstract Since 1990, TiNi and TiNiX (X=Cu, Pd, Hf) thin films have been made by sputtering. The motivation for fabricating sputter-deposited TiNi-base shape memory alloy thin films originates from the great demand for the development of powerful microactuators which can drive micromachines, because actuation force and displacement are greatest in shape memory alloys amongst many actuator materials. Stable shape memory effect and superelasticity, which are equivalent to those of bulk alloys, have been achieved in the sputter-deposited TiNi thin films. Narrow transformation temperature hysteresis and high transformation temperatures were also achieved in TiNiCu and TiNi(Pd or Hf) thin films, respectively. In the meantime, unique microstructures consisting of nonequilibrium nanoscale precipitates and nonequilibrium compositions in the matrix have been found in Ti-rich TiNi thin films which were fabricated from amorphous condition by annealing at a considerably low temperature. Several micromachining processes have been proposed to fabricate some prototypes of microactuators utilizing TiNi thin films. The present paper will review the recent development of the above mentioned topics relating to sputter-deposited TiNi-base shape memory alloy thin films.

Surface configurations during annealing of sputter-deposited NiTi thin films

SPUTTER-DEPOSITED NiTi thin films are becoming more and more important because of their po-tential applications to the intelligent thin-film systems. The as-deposited NiTi thin films pre-pared by magnetron sputtering are generally amorphous. Because the amorphous NiTi thinfilms cannot show shape memory effect, they should be crystallized under appropriate anneal-