NiTi Shape Memory Thin Films Research Articles

CO2 laser annealing of sputtering deposited NiTi shape memory thin films

NiTi shape memory thin films are potentially desirable for MEMS actuators. Traditionally, there are two approaches to obtain NiTi shape memory thin films. One is to anneal amorphous NiTi films in a vacuum chamber and the other is to deposit NiTi on a high temperature substrate (about 500 °C). Both approaches have difficulty in terms of compatibility with traditional IC techniques, and are not applicable for annealing a prescribed local area of a NiTi film at the micron scale. In this paper, a new approach, CO2 laser annealing, which can overcome both problems mentioned above, is presented. Results of optical microscopy, x-ray diffraction (XRD) and atomic force microscopy (AFM) reveal the crystalline structures and phase transformation in annealed NiTi films. Furthermore, a CO2 laser annealed line, about 100 µm in width, demonstrates the capability of a CO2 laser for local annealing of NiTi thin film.

Microstructure Evolution of On-substrate NiTi Shape Memory Alloy Thin Films

ABSTRACTWhen deposited at room temperature, sputtered NiTi thin films are amorphous and need to be crystallized before they can be used as a functional material. We present the results of an annealing study on substrate-constrained NiTi shape memory thin films. Amorphous films of a NiTi shape memory alloy were deposited by UHV sputtering. Films of thickness 1.0 μm were grown on (100) Si wafers both with and without an LPCVD SiNx barrier. The as-deposited films were annealed in vacuum at temperatures ranging from 500°C to 800°C. The microstructure of the annealed films was characterized using transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and Rutherford back scattering (RBS).

Ti-Ni SMA thin film microactuators made on a Si substrate

Among several types of performance materials proposed for fabricating microactuators, Ti-Ni shape memory alloys have great advantages such as large deformation and recovery force. The intrinsic disadvantage of shape memory alloys, the slow response due to the limitation of cooling rate can be greatly improved in micro size actuators, because the surface/volume ratio of materials will drastically increase in thin films so that cooling becomes efficient. Therefore, the development of Ti-Ni shape memory thin films has been proposed in the field of micromachines. In the present investigation, Ti-Ni shape memory alloy thin films were deposited by sputtering on Si substrates covered by SiO2 layer. The Ti-Ni thin films were crystallized by heat-treatment in order to achieve high transformation temperatures. The Ms and As temperatures were 340K and 365K,respectively. This Ms is the highest temperature achievable in the binary Ti-Ni alloy and about 50K higher than room temperature, enabling that a high frequency response is achieved. Square diaphragm type microactuators were fabricated by the photolithography method with anisotropic etching applied to the Si substrate. The dimension of the sides of the squares ranges between 100 and 1,000 micronmeters. The structure of the diaphragm consists of a 1.5 micronmeters thick Ti-Ni film and a 1.0 micronmeter thick SiO2 layer. Joule heating and natural cooling methods were applied to move the actuators. The microactuators moved up and down upon cooling and heating, respectively. The highest position at the center of the actuator was about 30 micronmeters, while the lowest was 0 micronmeters. The highest frequency detected was 100Hz.

A new fabrication process for Ni–Ti shape memory thin films

Abstract A new fabrication method for Ni–Ti thin films showing martensitic phase transformation is presented. A multilayer of up to 100 alternating pure Ni and pure Ti layers is deposited in an automated DC magnetron sputter system optimized for homogeneous deposition on large substrates. The single layer thickness is in the range of 10–20 nm. The multilayer is subjected to different heat treatments in order to allow for interdiffusion of Ni and Ti and subsequent recrystallization as a Ni–Ti intermetallic compound. The martensitic and austenite phase in the resulting films could be identified by means of X-ray diffraction. Differential scanning calorimetry (DSC) measurements monitor the transformation behavior of films with different composition and indicate the existence of an R-phase. The microstructure of the annealed films is investigated by transmission electron microscopy (TEM). The composition and transformation temperatures can be adjusted by varying the thickness ratio of the initial Ni and Ti layers. These films show a well pronounced shape memory effect for certain annealing procedures.

Mechanical properties of Ti–Ni shape memory thin films formed by sputtering

Abstract The stress–strain curves of Ti–48.3, 50.0, 51.5at.%Ni thin films were measured. The results showed that sputter-deposited thin films possess sufficient ductility and strength for practical use. Particularly it was noted that a Ti–48.3at.%Ni thin film annealed at 773 K for 300 s shows a large elongation of 20% contrary to bulk specimens. However, the ductility of Ti–48.3at.%Ni thin films was found to be very sensitive to annealing conditions. Ti–48.3at.%Ni thin films annealed at 873 K for 3.6 ks do not show any ductility. The difference in the ductility of the Ti–48.3at.%Ni thin films can be explained by grain-boundary precipitates of Ti 2 Ni.

Dynamic thermo-mechanical properties of evaporated TiNi shape memory thin film

The shape recovery and re-deflection responses of shape memory alloy (SMA) thin film to thermal cycles were investigated using the bulge method. It was deposited by flash evaporation and had a nominal composition of 50 at.% Ti–50 at.% Ni and a thickness of about 6 μm. After being released from silicon substrates and undergoing vacuum-annealing to obtain memorisation of an initial flat shape, it was deformed into a cap shape of 5 mm in diameter by pressurisation at 400 kPa. Then, by applying 100 ms voltage pulses, its initial shape was recovered by resistive heating at various energies. During these shape recovery and re-deflection cycles, change in displacement with time was measured continuously using a laser displacement meter. The thin film exhibited shape recovery at energies for heating of more than 1 J due to reverse martensitic transformation. Displacement due to shape recovery increased with increasing energy for heating, reaching saturation at around 100 μm at energies of more than 2 J. After heating was completed, the thin film deflected again due to martensitic transformation under pressure. The period for each shape recovery and re-deflection cycle was about 600 ms at an energy of 2.1 J. It exhibited stable shape recovery and re-deflection properties at up to 1000 cycles, which was the maximum number of thermal cycles tested. Finally, the pumping pressures and flow rates which might be expected with such an SMA micropump were also roughly estimated.

Thermo-mechanical properties of TiNi shape memory thin film formed by flash evaporation

TiNi thin film with a nominal composition of 50 at.% Ti–50 at.% Ni and a thickness of about 6 μm was deposited onto silicon substrates by flash evaporation, and then released from those substrates to become free-standing film. After vacuum annealing at 500°C for 60 min so that an initially flat diaphragm could be formed, the thermo-mechanical properties of this thin film were evaluated using the bulge test, which causes deformation by pressurisation. Stress–strain curves revealed that it exhibited shape memory at temperatures of less than 60°C and super elasticity at temperatures of more than 80°C. It also showed shape recovery against pressures of up to at least 500 kPa. Under pressurised conditions, the temperature at termination of martensitic transformation, which causes deflection during cooling, increased with increasing pressure, although it was about 30°C without pressure. On the other hand, the temperature at termination of reverse martensitic transformation, which causes shape recovery during heating, remained almost constant at 75°C, independent of pressure. These two specific temperatures indicated that, under loading conditions, thin film could be driven easily, merely by heating and air cooling. At pressures of more than 100 kPa, slip deformation, which cannot be reversed, appeared and increased with increasing pressure. Moreover, displacement of deflection–recovery decreased during the early stages of the thermal cycle, because of work hardening due to slip deformation. After more than 50 cycles, however, displacement of deflection–recovery became constant, indicating that the thin film could give stable actuation, even under high load conditions.

Graphoepitaxial NiTi shape memory thin films on Si

Graphoepitaxial Ni50Ti50 films were grown on Si substrates by sputter deposition of an alloy target. The microstructure evolution of the film was investigated by hot stage atomic force microscopy. The topological features of the martensitic graphoepitaxial Ni50Ti50 films are directly associated with etch pits on the surface of the silicon substrate and exhibit facets with well-defined preferential in-plane orientation. The highly ordered martensitic facets disappear as the film transforms to austenite at elevated temperatures.

Deformation behaviour associated with the stress-induced martensitic transformation in Ti–Ni thin films and their thermodynamical modelling

On one part, the paper presents a phenomenological description allowing the simulation of the mechanical behaviour of shape memory alloys (SMA). Our theoretical work is focused on the pseudoelasticity related to the creation of the martensite phase from the mother phase (austenite). An isothermal tensile loading is performed here to induce this phase transformation. The thermodynamics of irreversible processes framework was used. One internal variable is taken into account: the volume fraction of martensite produced under stress action. On another part, Ti–Ni shape memory thin films are made by r.f. magnetron sputtering. Tensile tests are performed at various temperatures. The parameters of the model have been identified for two particular SMA (Ti-48.0 at.% Ni and Ti-50.3 at.% Ni) and the simulated results show good agreements with experiments.

The reaction between a TiNi shape memory thin film and silicon

The reaction between shape-memory TiNi thin films and silicon has been characterized by conventional, analytical, and high-resolution transmission electron microscopy. A reaction layer is formed during the 525 °C post-deposition crystallization anneal of the sputter-deposited TiNi, and consists of several phases: Ti2Ni, a nickel silicide, and a ternary titanium nickel silicide. The mechanism for the interlayer formation is discussed.

Preparation of TiNi ferroelastic–ferroelectric thin film heterostructures

Active and adaptive thin film materials for `smart' materials will likely be a heterostructure of several types of applications. In this investigation, TiNi shape memory thin films on Si(100) substrates with BaTiO3 and PbZr1−xTixO3 (PZT) as buffer layer were deposited by the pulsed laser deposition method. The critical issues of deposition and processing of shape memory ferroelastic and ferroelectric materials have been addressed to fabricate the thin film sensor–actuator heterostructures. Characterization of the films has been done by using X-ray diffractometer and atomic force microscope techniques. The buffer layer of BaTiO3 and PZT have been beneficial to grow crystalline quality TiNi films on Si(100) substrates.

Structures and defects induced during annealing of sputtered near-equiatomic NiTi shape memory thin films

Structures and defects induced during annealing were studied in free-standing NiTi thin films produced by rf magnetron sputtering. Ni4Ti3 precipitates coherent with the (B2) matrix do not affect the shape memory behavior of the thin films annealed at 550 °C for short times. With the further increase of the annealing time and/or temperature, less coherent Ni4Ti3 precipitates develop and hinder the shape memory behavior. The embrittlement of the annealed films probably results from grain–boundary precipitation, thermal etching, and stress gradients.

Strengthening of Ti-Ni shape-memory films by coherent subnanometric plate precipitates

It has been found that subnanometric thin plate precipitates are formed in sputter-deposited Ti-rich Ti-Ni shape-memory films. This has been achieved by heating the as-sputtered amorphous sample at relatively low temperatures. The precipitates are produced along the {100} planes of the bcc (B2) parent phase and are completely coherent with the matrix. They have a disc-like shape with a thickness of only two to three lattice planes and a diameter of 5-10 nm. The distance between neighbouring precipitates is dependent on the heat treatment temperature, varying in the range 5-30 nm. Owing to the existence of these precipitates in the parent phase, the critical stress for slip in the parent phase is greatly increased and, as a result, excellent shape-memory properties, such as 6% recoverable shape strain and 670 MPa recovery strength, are obtained for Ti-Ni shape-memory thin films. It is concluded that the precipitates have a Ti-rich composition and are produced only in the off-stoichiometric amorphous Ti-Ni ...