The present work aims to know how the properties of co-sputtered Ni-Ti thin films deposited over a wide range of composition differ from pure Ni and pure Ti thin film, all deposited under varying substrate bias voltage. Ni thin films revealed strong <111> fiber texture irrespective of substrate bias voltage, while film surface roughness as revealed by atomic force microscopy was found to significantly vary with the substrate bias. A change in texture of Ti thin films was observed with increase in negative substrate bias. On the other hand Ni-Ti thin films exhibited amorphous nature irrespective of substrate bias voltage. The stability of crystal structure of pure Ni and pure Ti films under high ion bombardment could be predicted by classical molecular dynamics based on embedded atom method potential.
Keywords: Sputtering, Thin film, Nanocrystalline, Amorphous, Texture, Molecular dynamics.
Ni-Ti alloys are excellent shape memory alloys (SMA), known to demonstrate the ability to return to some previously defined shape or size when subjected to appropriate thermal procedure (cycling). Fabrication of thin Ni-Ti based shape memory alloys for Micro-Electro-Mechanical System (MEMS) based Microactuator applications has been challenged due to difficulty in controlling the composition. The principal requisite for realization of shape memory behavior in Ni-Ti alloy thin film is the near-equiatomic composition of the film . The most widespread techniques employed to fabricate Ni-Ti alloy thin films include (i) deposition either by sputtering from Ni-Ti alloy targets or by alternate multilayer deposition of Ni and Ti [2-5] and (ii) co-deposition from elemental Ni and Ti targets [6-9]. However, the latter one (co-deposition) is regarded as the most feasible route to achieve near-equiatomic composition in Ni-Ti thin film, since individual target power can be readily controlled during the sputter deposition process. Miyazaki and Ishida  pointed out that the transformation and shape memory characteristics of Ni-Ti thin films are strongly dependent on sputtering conditions (gas pressure, substrate temperature, and substrate bias voltage) apart from the other metallurgical factors.
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Application of substrate bias voltage during deposition is known to improve the quality of many sputter deposited metal and alloy thin films. Recently, Almtoft et al.  reported degradation in pure Ag film crystallinity at bias of -150 V without change in film thickness and suggested that Ar ions, instead of promoting high surface diffusion tend to etch away the material from the film surface. On the contrary, application of moderate bias of -90 V was found to improve the crystallinity of Pt films deposited on Si substrates in addition to imposing obstruction to the formation of platinum-silicide products at the interface . However, the scenario is quite different when concerning evolution of crystallinity in thin film alloy systems. In case of Ni-Pt films, preferential re-sputtering of Ni during ion-bombardment causes shift in the lattice parameter due to increase of Pt content in the film even at moderate bias voltage of -80 V . Besides, at high ion-energy regime, Ni-Pt film crystallinity was found to degrade due to excessive incorporation of crystal defects during deposition due to enhanced re-sputtering of Ni. The same group had observed  improvement in the crystallinity of Co-Pt film and increase in lattice parameter of Co-Pt phase with increase in bias voltage up to -150 V. These results  suggested that the enhanced surface diffusivity of adatoms and preferential re-sputtering of Co from the film leads to the increase in Pt content without damaging the film crystallinity. In particular, the various reports on re-sputtering effects due to high ion-bombardment emphasize on decrease in the film deposition rate , degradation of film crystallinity , and variation in the composition of alloy films . Ljungcrantz et al. observed the change in residual stresses from tensile to compressive upon increasing the negative bias voltage from 0 to -300 V in Ti films .
In recent years, there have been numerous studies that demonstrated the effect of substrate bias voltage on the structural and morphological properties of Ni [18-20] and Ti thin films [21-22]. However, according to the authors' knowledge, only limited work has been carried out correlating the influence of ion-bombardment on Ni-Ti films (having a wide range of composition) with the properties of the films. One of such few recent studies shows that ion bombardment on the growing film affects the texture evolution in Ni-Ti films grown on SiO2/Si substrates . Therefore, the present paper intends to investigate the effects of ion-assisted bias sputtering on the properties of magnetron-sputtered Ni, Ti and Ni-Ti films (over a wide range of composition) on Si (100) substrate. The foremost emphasis is laid on the variation in crystallinity, morphology-grain structure, and surface topography of individually deposited Ni, Ti and co-deposited Ni-Ti films in presence of varying substrate bias voltage. Effect of ion-bombardment on the crystallinity of the films has been explained in terms of excess vacancy formation leading to destabilization of the crystalline structure using classical molecular dynamics simulations.
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Synthesis of the films
All Ni, Ti and Ni-Ti films were deposited using a RF-DC magnetron sputtering system (KVT Ltd.) which is a three gun sputter down setup. The sputter targets of Ni and Ti, each with 3 inch diameter and 99.9% purity were employed. The films were deposited on 3 cm x 3 cm p-type Si (100) substrates which were maintained at room temperature during the depositions. Prior to the depositions, substrates were etched with 2 % HF solution in order to remove the native oxide layers deposited on them. The deposition details for Ni, Ti and Ni-Ti films are tabulated in Table I.
Characterization of the films
The film thickness was measured using step height on a masked silicon substrate with the
help of a Veeco-Dektak 150 surface-profilometer. Surface topography and roughness studies were carried out using Nanonics Multiview- 1000TM Atomic force microscope (AFM) under tapping mode. Studies on the microstructure and film growth morphology were done on both planar and cross-sectional films by Carl Zeiss-SUPRA40 Field Emission Scanning Electron Microscope (FE-SEM) with an accelerating voltage of 5 kV and magnification up to 200 kX. The compositional analyses of Ni, Ti, and Ni-Ti films were carried out using OXFORD instruments INCA Energy Dispersive Spectroscopy (EDS) attached with the FE-SEM. In order to identify the crystalline phases formed and the preferred orientations in Ni, Ti, and Ni-Ti films, Grazing Incidence X-ray diffraction technique (GI-XRD) was employed. The diffraction data were collected using Philips 6 X'Pert diffractometer with Cu-target (λ=1.54056 Å), where the grazing angle was fixed at 0.5°. Different 2ï±ï€ angle ranges were used for different films (Ni - 40°-100°, Ti-30°-55°, and NiTi-30°-80°) during scanning. The scan step size was maintained at 0.05°.
3. CLASSICAL MOLECULAR DYNAMICS SIMULATIONS
Classical molecular dynamics (MD) simulations were carried out to understand the effect of vacancy creation on the relative stabilities of crystalline and amorphous phases. MD simulations were carried out on LAMMPS (Large Scale Atomistic/Molecular Massively Parallel Simulator)  platform in which equations of motion are numerically integrated using Velocity-Verlet Algorithm. Embedded Atom Method (EAM) potential developed by Lai et al.  was used in the present study.
In the above expression, ï¡ï€ and ï¢ï€ indicate atomic species, rij is the distance between ith and jth atoms, and parameters A, p, d, ï¸ï€ and q are constants depending on the specific material systems.
MD simulations were carried out to study the effect of vacancy creation (occurred during bombardment of high energy atoms) on relative stabilities of crystalline and amorphous structures. Ni-Ti crystals with different compositions viz. pure Ni, Ni0.8Ti0.2, Ni0.7Ti0.3, Ni0.6Ti0.4
and pure Ti were generated and equilibrated at 300 K. The simulation box, having dimension of
100 Å -100 Å - 100 Å with approximately 62500 atoms, was subjected to periodic boundary condition. Different percentages of atoms were removed from the crystals resulting in creation of vacancies. The crystals were subsequently annealed at 300 K under pressure of 1 bar using NPT ensemble scheme to study the resulting structural changes. The vacancy concentration was varied from 2.5 % to 10 %. The radial distribution function (RDF) was generated from the phase space at specified intervals of time in order to analyze the structural changes.
RESULTS AND DISCUSSION
Figure 1 shows the plot for film thickness versus substrate bias voltage for Ni, Ti and Ni-Ti thin films. In particular it is observed that there is no appreciable variation in the Ni film thickness (~225 nm) with increase in ion-bombardment. In the case of Ti deposition by RF sputtering, significant decrease in film thickness from 425 to 280 nm (34 % reduction in the deposition rate) was observed when the bias voltage was increased from 0 to -100 V. When a small negative bias voltage is applied to the substrate end during the deposition, it is assumed that a small secondary sputtering (re-sputtering) takes place near the substrate surface. In addition, RF sputtering is known to induce self-bias at the substrate end which could lead to the increase in ion-current near the substrate end. Therefore, in the case of RF sputtering of Ti, due to the combined effect of substrate bias voltage and substrate self-bias, the extent of re-sputtering is expected to be much higher resulting in decrease in the effective deposition rate. The composition of Ni-Ti films, obtained by co-sputtering of Ni and Ti, were varied by changing the titanium target power while keeping the Ni target power fixed. Earlier, the authors  have reported that maintaining Ni : Ti the deposition rate ratio of Ni : Ti as 1 : 1 yielded Ni rich Ni-Ti films (70 at. % Ni). Therefore, in present work, to obtain near-equiatomic Ni-Ti thin films, the ratio of deposition rate of Ni and Ti were fixed to 2 : 3 by varying the Ti target power (RF power), resulting in net deposition rate of 17 nm/min during co-sputter deposition of Ni-Ti.
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Figures 2 (a-i) shows the FE-SEM micrographs of Ni, Ti, and Ni-Ti films on Si (100) substrate. The presence of grains is more apparent in the case of pure Ni and pure Ti films. It can be seen that with increase in bias-voltage the microstructure tends to become more compact and refined (except for Pure Ni at -50 V substrate bias voltage). A transition from columnar to fibrous structure was also observed. Above trend can be explained in terms of higher kinetic energy of the deposited atoms in the case of higher bias voltage, leading to higher diffusivity and thus more compaction. Thus, the reason for the appearance of porous structure in unbiased films is the insufficient atomic mobility on the substrate surface during the nucleation and growth of the films. This causes the adatoms to deposit where they land, leading to a fine-grained porous film structure which lies in the zone I (columnar) of Thornton's Structural Zone Model (SZM) . According to revised Structural Zone Model large substrate bias changes microstructure from zone I to zone T (fine-fibrous columnar) structure . There is an exception to this trend as previously mentioned; Ni film deposited at -50 V bias voltage showed more compact structure than the Ni film deposited at higher substrate voltage of -100 V. At this stage the reason for this exception is unknown. With the increase in bias voltage the kinetic energy of the striking adatoms will increase which will lead to higher kinetic energy of atoms or higher temperature at the surface of the deposited film. Enhanced temperature will lead to higher diffusivity as suggested and thus higher adhesion of grains leading to decrease in porosity.
Figure 3 shows the AFM image of film surface and the roughness values of the surfaces for different substrate bias. With an exception viz. deposition of pure Ni under substrate bias of -50 V, the roughness was found to decrease with increase in the substrate bias voltage.
GI-XRD peaks obtained from Ni films were indexed to face-centered cubic (fcc) Fm3m phase irrespective of applied bias voltage (Figure 4). In the case of the depositions carried out at room temperature (30 °C) on Si (100) under different bias voltages, Ni films exhibit good crystallinity with strong <111> fiber texture. It has been reported that even at low temperature (below -270 °C), formation of amorphous Ni films are not feasible under clean conditions (without any presence of impurities) even by sputter deposition process, since it is difficult to seize the Ni adatom movement at that temperature .
GIXRD peaks obtained from the Ti films were indexed to hexagonal closed pack (hcp) phase, where preferential orientations of the grains were found to change as the bias voltage was increased from 0 V to -100 V (Figure 5a). In addition, it has also been observed that increase in negative bias voltage resulted in damage of crystallinity. The ratio of relative intensities of various diffraction peaks are calculated and compared to ascertain whether a lattice plane is preferentially parallel to the film surface (Figure 5b). The texture co-efficient (T) is calculated using the following equation :
where (hkl) are (100), (002) or (101) orientations and I is the relative intensity.
At lower bias voltage or without bias condition the grains are preferentially oriented in (100) plane which are the most favorable planes due to their minimum surface energy. As ion-bombardment was introduced by increasing the bias voltage to -50 V, grains of (002) orientations at 2θ = 38° were developed along with (100) oriented grains at around 2θ = 35°. This suggests that the mixed orientation ((100) and (002)) exists in Ti films at substrate bias voltage of -50 V. However, the texture coefficient of (002) is the highest amongst all the planes. Further, increase in ion-bombardment causes complete change of orientation of grains along (101) plane (dominant peak at 2θ = 40°) suggesting that enhanced re-sputtering of the films causes removal of those grains which are unfavorably oriented along (002) plane.
On the other hand, DC sputtered Ti films show a dominant (002) orientation in unbiased condition, upon increasing the bias to -100 V the preferential orientation was found to change from (002) to (101). Upon increase in the bias voltage to -150 V the preferential orientation completely changes from (101) to (100) .
The transition in texture with increase in substrate bias appears to be due to variation in film thickness with the application of substrate bias voltage. At the Ti-Si (100) interface, the (002) plane is being preferred, possibly due to lower interface energy with the Si (100) and thus is preferentially formed. As the Ti film grows (101) an orientation which has possibly lower surface energy predominates. It may be noted that the interfacial energies between Ti and Si is not known and therefore deeper insight into the variation of texture with film thickness is not possible at this stage.
The micro strain (ε) of Ti films were calculated using Equation (3) with standard c/a = 1.587 for Ti (100) peak (a = 2.9512 Å and c = 4.6845 Å)  and are shown in table II.
where α is the calculated lattice parameter of the Ti film and αo is the standard lattice parameter. As-deposited unbiased Ti film was found to exhibit tensile behavior. The moderate bias voltage of -50 V induces compressive stress in the film due to the growth of (002) orientation leading to
expansion of the crystal lattice by 3.8 % as evident from the increase in c/a ratio. It is also observed that further enhancement of ion-bombardment modifies the state of stress in Ti film from compressive to tensile, along with the suppression of (002) orientation and increase in the
fraction of (101) orientation.
Figure 6 shows the X-ray diffraction profiles of Ni-Ti films at glancing angle of 0.5°. There is no clearly distinct peak which represents the crystalline nature of the film. Instead of that, presence of broad peak centered at 2θ = 41.8° suggests that the films deposited under different bias voltage are amorphous. This 2θ (41.8°) value corresponds to (-111) reflection of Ni-Ti martensite phase. However, when the bias voltage is increased to -100 V the intensity of that broad peak slightly improves which reveals that the crystallinity of the films might have increased. In the present case, the improvement in film crystallinity under higher bias voltage cannot be attributed to the variation in film thickness. Alloy films, containing equal number of large and small atoms (near-equiatomic Ni-Ti) with a size difference of 15.6 %, tend to exhibit amorphous films due to atomic mobility constraints .
As stated earlier, classical molecular dynamics simulations were carried out to study the stability of crystalline structure under high energy ion bombardment condition, during which vacancies get generated in the films [33, 34]. The plots of radial distribution functions (RDFs) for different compositions and vacancy concentrations are shown in Figure 7. It can be observed that except the cases of pure elements, the presence of even 2.5% vacancy led to the formation of amorphous phase. In the case of alloy film, even at 0 % vacancy slight deviation from crystallinity can be observed which can be attributed to the size difference of Ni and Ti atoms. The transition to amorphous structure can be understood in terms of distortion of the crystal structure around a vacancy. This distortion is spontaneous and athermal in nature. Due to removal of several atoms the extent of distortion increases leading to the formation of amorphous phase. The transformation is athermal because it takes place within a time span of few picoseconds. To confirm the athermal nature of the transformation some of the simulations were repeated at much lower temperatures. Even at temperatures as low as 100 K, formation of glassy phase was observed, as shown in Figure 8. Thus, according to the simulation results amorphization of pure elements (Ti and Ni) is difficult, as larger concentration of vacancies is required. Similar simulations have been carried out on irradiation of atoms leading to creation of Frenkel defects and consequent amorphization . The results reported in the present work
has also been reported elsewhere .
In conclusion, we have investigated the effect of substrate-bias voltage on the structural, morphological, and surface topographical properties of magnetron sputtered Ni, Ti (individually deposited) and Ni-Ti (co-deposited over wide range of composition) thin films on Si substrates. Increase in the bias voltage hardly influences the thickness and <111> fiber texture of Ni films, but re-sputtering results in the surface modification. Change in preferential orientation of Ti films accompanied with considerable thickness reduction was observed with increase in bias voltage. Further, grain size and surface roughness significantly were reduced due to enhanced re-sputtering from Ti film surface. Upon increasing bias voltage from 0 to -100 V, Ti film exhibit stress reversal from tensile to compressive and again from compressive to tensile under high ion-bombardment due to change in preferential orientation from (100) to (002) and (002) to (101), respectively. Atomic force microscopy studies reveal that the roughness decreased with increase in substrate bias. This is attributed to higher surface diffusivity resulting from ion bombardment. Co-deposited Ni-Ti thin films, over a wide range of composition and for all substrate bias voltage, showed amorphous structure. On the other hand pure Ti and pure Ni thin films exhibited crystalline structure even at high substrate bias. The crystallinity of pure Ti and pure Ni films, even at high substrate bias voltages could be predicted using classical molecular dynamics simulation. The simulations reveal that re-sputtering leading to excess vacancy does not de-stabilize the crystallinity of pure Ti and pure Ni films, whereas, it de-stabilizes the crystallinity in Ni-Ti films.