Sputter-deposited NiTi Research Articles

Inverse problems in resonant ultrasound spectroscopy based on spectra perturbation

In RUS, the fit between the experimental resonant spectrum and the result of the inversion procedure is usually worse than the inaccuracy of the experimental measurement of the resonant frequencies. Thus, it can be problematic to extract any valuable information from slight perturbation of experimental spectra, where the experimental frequency shift falls well below the accuracy of this fit. However, the inverse procedure can be reformulated and instead of fitting the resonant frequencies itself, it can be based on the direct fitting of the frequency perturbations. In this contribution, we show two examples of the use of this approach (i) quantitative RUS evaluation of the elasticity of micrometric and submicrometric surface layers from frequency differences measured before and after layer deposition [1] and (ii) in situ characterization of the thermal degradation of the bonding layer in a bimetallic system [2]. [1] Thomasová M. et al. Young's moduli of sputter-deposited NiTi films determined by resonant ultrasound spectroscopy: Austenite, R-phase, and martensite (2015) Scripta Materialia, 101, pp. 24–27. [2] Janovská M. et al Characterization of bonding quality of a cold-sprayed deposit by laser resonant ultrasound spectroscopy (2020) Ultrasonics, 106, art. no. 106140.

Structural, Optical and Microstructural Properties of TiNi Thin Films before and after Oxidation

The deposition of composites with tailored optical properties is investigated. This would employ structures consisting of combined metallic and oxides nature. A thin layer of TiNi is obtained by using RF magnetron sputtering on a stainless-steel substrate, followed by oxidation at 400°C and 800°C for four and one hours, respectively. The optical properties of the thin films were characterized by optical spectrophotometer, and Fourier Transform Infrared Spectroscopy (FTIR). The morphology, topography, and structure were studied by scanning electron microscope (SEM), atomic force microscope (AFM), and X-ray diffraction (XRD). The results show that TiO2 has been produced through the oxidation process of the sputter-deposited TiNi thin film at high oxidation temperature. The TiNi thin films showed a significant improvement in optical properties after oxidation, as the absorbance increased, and the emittance was reduced. This work introduces oxidized TiNi thin films as candidates for solar selective absorber.

Rapid and low power laser actuation of sputter-deposited NiTi shape memory alloy (SMA) MEMS thermal bimorph actuators

NiTi SMA thermal bimorph actuators have potential as high-force, high-displacement MEMS actuators. Historically, even microscale SMA actuation response has been limited to a maximum of ˜100 Hz. As NiTi film and device dimensions are scaled down into the micro and nano scales, heat transfer, and thus device cycling speeds can be significantly improved upon. We have used an in-situ annealed nickel-titanium (NiTi) shape memory alloy (SMA) sputter deposition process to sputter equiatomic NiTi films at 600 °C. We characterized our thin film (270 nm NiTi – 1.6 μm NiTi) material and verified its reversible shape memory effects (SME) using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and wafer bow versus temperature measurements. Upon release, the NiTi material exhibited a reversible phase change around 60 °C with a hysteresis of ˜3 °C. For the substrate confined case (i.e. NiTi on Si or Pt), hysteresis was much larger (i.e. ˜40 °C) with the phase change completed at ˜80 °C. In addition to SMA material characterization, we fabricated NiTi/Pt bimorph actuators at several (100 nm to 1.2 μm) NiTi and Pt thicknesses. Free-standing bimorph actuators were produced via a dry etch release, and temperature dependent curvature of these cantilevers was investigated. To address the low power, and high response time aspects, we performed a dynamic characterization using a 440 mW, 532 nm “green” laser to irradiate devices from 2 to 24 W/cm2 while measuring actuation response times that varied from 3 ms at the highest irradiation fluxes to 240 ms at the lowest. Our results showed decreased actuation powers and faster heating or actuation times compared to past works with NiTi microactuators. The NiTi films with 600 nm thickness on top of 20 nm Pt films exhibited the greatest change in curvature from 200 μm to flat states, and actuated in under 3 ms due to the very small volume of SMA requiring to be heated. These results suggest that NiTi/Pt bimorphs have potential applications as lower-power, faster-response, laser-activated micro shutters or thermal switches without needing a traditional wired power source.

Characterization of Sputter-Deposited NiTi Thin Film by Nanoindentation

NiTi shape memory alloy thin film sputter-deposited on a large scale silicon wafer was characterized by means of instrumented (depth-sensing) indentation technique. Thickness of deposited thin film was measured by calotest device. Microstructure of thin film was observed using differential interference (Nomarski) contrast. It was shown that the local mechanical properties are different in areas containing different phases (austenite and martensite) according to different deposition conditions (kinetic energy of deposited atoms when impacting the substrate surface).

Microstructural evolution of nanometric Ti(NiCu)2 precipitates in annealed Ni–Ti–Cu thin films

A sputter deposited Ni–Ti–Cu thin film was annealed for 1 h at 500 and 600 °C and its microstructure investigated by transmission electron microscopy and focused ion beam. Annealing at higher temperatures caused a remarkable microstructural evolution of the nanometric Ti(NiCu)2 plate precipitates formed inside the grains. Precipitate coarsening occurred in the inner part of the grains according the Ostwald ripening effect, while close to the grain boundaries, solute atoms preferentially diffused towards grain boundaries, thus causing local precipitates depletion. The density distribution of metastable precipitates in the grain interior of annealed Ni–Ti–Cu films depends on the distance from grain boundary.

Young’s moduli of sputter-deposited NiTi films determined by resonant ultrasound spectroscopy: Austenite, R-phase, and martensite

Young’s moduli of 3 μm thick NiTi films sputter-deposited on silicon wafers are determined by resonant ultrasound spectroscopy. Three samples with different transition temperatures are studied; for each sample, the evolution of Young’s modulus is obtained in a thermal cycle covering the austenite → R-phase → martensite transition sequence. The results prove that the smallest Young’s modulus is attained for the R-phase, and both austenite and martensite exhibit pronounced softening towards the transition temperatures.

The relationships between sputter deposition conditions, grain size, and phase transformation temperatures in NiTi thin films

Sputter-deposited NiTi films on full 100mm wafers exhibit different phases, depending on the wafer region. Comprehensive characterization and analysis show that, unlike the common assumption, this effect is not related to the Ti/Ni stoichiometry. Instead, we show that the different phases are related to differences in grain size. Our results indicate the existence of a critical grain size of approximately 50–100nm, below which a significant inhibition of the martensitic transformation occurs. Further, we study the effect of deposition conditions and show that a desired uniform martensitic film can be achieved by decreasing the impact energy of the deposited atoms.

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.

In situ crystallization of sputter-deposited TiNi by ion irradiation

Abstract TiNi is well known as a typical shape-memory alloy, and the shape-memory property appears only when the structure is crystalline. Until recently, the material has been formed as amorphous film by single-target sputtering deposition at first and then crystallized by being annealed at high temperature over 500 °C. Therefore, it has been difficult to make crystalline TiNi film directly on a substrate of polymer-based material because of the low heat resistance of substrate. In order to realize an actuator from the crystallized TiNi film on polymer substrates, the substrate temperature should be kept below 200 °C throughout the whole process. In our previous studies we have found that deposited film can be crystallized at very low temperature without annealing but with simultaneous irradiation of Ar ions during sputter-deposition. And we have also demonstrated the shape-memory effect with the TiNi film made by the new process. In order to investigate what parameters of the process contribute to the low-temperature crystallization, we have focused to the ion fluence of the ion irradiation. Resultantly, it was found that the transition from amorphous structure to crystal one has a threshold range of ion fluence.

Compositional uniformity in sputter-deposited NiTi shape memory alloy thin films

Compositional uniformity in NiTi thin films produced under “cold” and “hot” target conditions using d. c. magnetron sputtering is presented in this article. Film deposited under “hot” target condition shows significantly better compositional uniformity compared to those deposited under “cold” target condition. The Ti composition in the film produced under “cold” target condition showed a 2.5 at.% loss relative to the target composition, whereas film produced under “hot” target condition exhibits composition nearly identical to that of the target. XRD and DSC measurements confirm the presence of martensitic phase at room temperature and the shape memory effect in the film deposited under “hot” target condition.

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.

The effect of crystallizing procedure on microstructure and characteristics of sputter-deposited TiNi shape memory thin films

Two kinds of Ti-rich TiNi shape memory thin films were deposited onto monocrystal silicon substrates at room temperature and high temperature by using the magnetron-sputtering technique. Their crystallizing procedures are different from each other. The first type of film was originally amorphous and post-crystallized at a higher annealing temperature (600 °C) after sputtering, but the second film type was crystallized in situ during sputtering at about 500 °C. It was found that there are clear differences of microstructure and characteristics between both kinds of film, such as grain size, growth texture, stress range, phase transformation behaviors etc. In order to improve and enhance film properties, the objective of the present work is to reveal the reason for these differences occurring and understand the relationship between the crystallizing procedures, microstructures and characteristics of the films.

The characterization of crystalline particle growth in TiNi thin films

Small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) have been used to investigate sputter-deposited TiNi films annealed at 773 K for 3, 8, 13, 15, 25 and 60 min. The specific interfacial area of the crystalline–amorphous two-phase system increases at the beginning of annealing, achieves a maximum after about 13 min and decreases on further annealing, whereas the radius of gyration of the crystalline particle increases during the annealing process. The prominent increase of the specific interfacial area and the slight increase of the radius of gyration of the crystalline particle at the beginning of annealing are correlated with the nucleation of the crystalline particle. The subsequent decrease of the specific interfacial area is correlated with the growth of the crystalline particles.

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.

Effect of SiO 2 buffer layer on properties of sputter-deposited NiTi shape memory alloy thin films

NiTi thin films were co-sputtered at 450 °C by using a Ni 50Ti 50 and a Ti targets on 450 μm thick (1 0 0) silicon wafer without or with a SiO 2 buffer layer (Film A/B). The phase transformation characteristics, shape recovery and mechanical property of the deposited films were investigated. Freestanding Film A shows a multi-stage transformation, while freestanding Film B has a two-step transformation during cooling. In the subsequent heating, both freestanding films show a two-step reverse transformation. Under a constant stress of 10 MPa, the shape recovery strain is approximately 1.35% for Film A and 0.25% for Film B. It is also found that the substrate-induced stress reduces the transformation hysteresis. The stress–strain curves of the two freestanding films are similar as determined by using a 6-axis micro-specimen tester.

Direct Measurement of the Nanoscale Mechanical Properties of NiTi Shape Memory Alloy

ABSTRACTThe mechanical properties of sputter-deposited NiTi shape memory alloy thin films ranging in thickness from 35 nm to 10 μm were examined using nanoindentation and atomic force microscopy (AFM). Indents made in films as thin as 150 nm showed partial shape recovery upon heating, although film thickness was found to have a marked effect on the results. A modified spherical cavity model is used to describe the findings, which suggest that the substrate tends block the shape memory effect as film thickness decreases below a threshold level which is specific to applied load. This has the net effect of decreasing the indent recovery below the critical film thickness. The fact that the spherical cavity model predicts the critical film thickness at which the shape memory effect is blocked indicates that the increased recovery of nanoscale indentations is due to a suppression of plastic processes rather than an enhancement of shape memory processes.

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...

Transformation properties and microstructure of sputter-deposited Ni-Ti shape memory alloy thin films

The influence of annealing parameters on the martensitic phase transformation in sputter-deposited Ti rich Ni-Ti films is systematically studied by differential scanning calorimetry and by transmission electron microscopy. The annealing temperature range extends from the crystallization temperature of the films up to 900°C. For increasing temperature, multiple phase transformations, transformations via an R-phase or direct martensite/austenite transformations are observed. A similar behavior is found for increasing annealing time. Related changes of the film microstructure, such as the strongly varying distribution of round Ti2Ni precipitates in the grains, are analyzed. Transformation temperatures could be shifted over a wide range by adjusting the film composition from 48 to 54 at.% Ti. The corresponding transformation curves, grain structure as well as nature and amount of precipitates were investigated. No subsequent annealing process is required for films deposited on substrates heated above about 500°C. In this case, the as-deposited films have a very fine-grained and homogeneous microstructure.

Characterization of shape-memory alloy thin films made up from sputter-deposited Ni/Ti multilayers

A novel fabrication process for Ni–Ti shape-memory alloy thin films is presented. This process is based on the appropriate annealing of sputter-deposited Ni/Ti multilayers. X-ray diffraction shows that interdiffusion of the two constituents results either in the amorphization of the multilayer structure after annealing at 330°C or in the recrystallization as a Ni–Ti intermetallic compound after annealing at temperatures above 400°C. A single 30 min annealing step in the temperature range from 400 to 800°C is sufficient to obtain Ni–Ti films showing martensitic phase transformations and the shape-memory effect. The influence of increasing annealing temperature on the transformation behavior is investigated by differential scanning calorimetry. The evolution of the transformation temperatures is found to be qualitatively similar to conventional sputter-deposited Ni–Ti films. The corresponding microstructure is studied by transmission electron microscopy. A very fine-grained structure is observed even after annealing at 800°C. The film composition can be varied by adjusting the thickness ratio of the individual Ti and Ni layers. Transformation curves of films with nominal compositions of 49.5 and 54.0 at.% Ti are compared. It is demonstrated that Ni–Ti films made up from multilayers may possess an intrinsic “two-way” shape-memory effect, which is a very interesting feature in view of the development of thin film micro-actuators.