TiAl Films Research Articles

Mobility Enhancement Induced by Oxygen Gettering of TiAl for Metal Gated NMOSFETs

In this work, the influence of TiAl gate electrodes fabricated by physical vapor deposition (PVD) and atomic layer deposition (ALD) on electron mobility for metal-gated NMOSFETs is respectively investigated. Due to the specific production chemistry, the ALD TiAl film inherently contains lots of C atoms, which further suppresses its O-gettering capability. Compared to the ALD TiAl device, about 15% higher peak mobility for the PVD TiAl device is demonstrated, which is attributed to the lower bulk trap density in gate dielectric layers as revealed by the 1/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}$ </tex-math></inline-formula> noise characteristics analysis. By means of the energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) analysis, the reduced trap density is related to the fact that PVD TiAl can extract more O atoms from gate dielectric layers than ALD TiAl, which weakens the electron trapping/de-trapping process and correspondingly enhances the mobility.

Structural evolution of amorphous and nanocrystalline TiAl films under helium implantation

The structure evolution and mechanism of the material during helium ion implantation are directly related to their performance and service life, towards the nuclear application. Despite the growing interest in amorphous and nanocrystalline films for a range of nuclear applications, many specifics about how their structure evolves during helium implantation are unclear. Here, the amorphous and nanocrystalline TiAl films produced by magnetron sputtering were in situ observed in a helium ion microscope to further understand the mechanism of microstructural evolution under severe ion implantation of two distinct structured nanomaterials. The result shows that a blister suddenly appeared on the surface of the nanocrystalline TiAl film, caused by a laterally ordered distribution of bubbles along the 〈111〉 crystal direction when the implantation dose reached 3.5 × 1017 ions cm−2. Instead, significant swelling of the whole implantation region of amorphous TiAl film was observed. At the final dose (9 × 1017 ions cm−2), the swelling thickness of the amorphous film was 200 nm and no exfoliation and cracking occurred. Due to long-range disorder, the degradation processes of amorphous TiAl film significantly differ from those of nanocrystalline TiAl film. This research has the potential to classify applications of materials in the nuclear field while providing a new viewpoint on the material structure design.

Me-Doped Ti-Me Intermetallic Thin Films Used for Dry Biopotential Electrodes: A Comparative Case Study.

In a new era for digital health, dry electrodes for biopotential measurement enable the monitoring of essential vital functions outside of specialized healthcare centers. In this paper, a new type of nanostructured titanium-based thin film is proposed, revealing improved biopotential sensing performance and overcoming several of the limitations of conventional gel-based electrodes such as reusability, durability, biocompatibility, and comfort. The thin films were deposited on stainless steel (SS) discs and polyurethane (PU) substrates to be used as dry electrodes, for non-invasive monitoring of body surface biopotentials. Four different Ti–Me (Me = Al, Cu, Ag, or Au) metallic binary systems were prepared by magnetron sputtering. The morphology of the resulting Ti–Me systems was found to be dependent on the chemical composition of the films, specifically on the type and amount of Me. The existence of crystalline intermetallic phases or glassy amorphous structures also revealed a strong influence on the morphological features developed by the different systems. The electrodes were tested in an in-vivo study on 20 volunteers during sports activity, allowing study of the application-specific characteristics of the dry electrodes, based on Ti–Me intermetallic thin films, and evaluation of the impact of the electrode–skin impedance on biopotential sensing. The electrode–skin impedance results support the reusability and the high degree of reliability of the Ti–Me dry electrodes. The Ti–Al films revealed the least performance as biopotential electrodes, while the Ti–Au system provided excellent results very close to the Ag/AgCl reference electrodes.

Study of TiAl thin films on piezoelectric CTGS substrates as an alternative metallization system for high-temperature SAW devices

Ti/Al multilayer films with a total thickness of 200 nm were deposited on the high-temperature (HT) stable piezoelectric Ca3TaGa3Si2O14 (CTGS) as well as on thermally oxidized Si (SiO2/Si) reference substrates. The Ti–Al films were characterized regarding their suitability as an alternative metallization for electrodes in HT surface acoustic wave devices. These films provide the advantage of significantly lower costs and in addition also a significantly lower density as compared to Pt, which allows a greater flexibility in device design. To realize a thermal stability of the films, AlNO cover as well as barrier layers at the interface to the substrate were applied. The samples were annealed for 10 h at up to 800 °C in high vacuum (HV) and at 600 °C in air and analyzed regarding the γ-TiAl phase formation, film morphology, and possible degradation. The Ti/Al films were prepared either by magnetron sputtering or by e-beam evaporation and the different behavior arising from the different deposition method was analyzed and discussed. For the evaporated Ti/Al films, AlNO barriers with a lower O content were used to evaluate the influence of the composition of the AlNO on the HT stability. The sputter-deposited Ti/Al films showed an improved γ-TiAl phase formation and HT stability (on SiO2/Si up to 800 °C in HV and 600 °C in air, on CTGS with a slight oxidation after annealing at 800 °C in HV) as compared to the evaporated samples, which were only stable up to 600 °C in HV and in air.

Evolution of the mechanical properties of Ti-based intermetallic thin films doped with different metals to be used as biomedical devices

This study reports the assessment of the mechanical properties of intermetallic titanium thin films, doped with different amounts of aluminium, copper, silver, and gold, aiming their use as biopotential electrodes for non-invasive physiological monitoring. The four binary thin film systems, Ti-Me (Me = Al, Cu, Ag, Au), were prepared by DC magnetron sputtering, placing different number of Me pellets on a pure Ti target. The use of a Ti-composed target gave rise to a wide range of compositions, resulting in three distinctive zones of (micro)structural features, identified in all the prepared systems. In the first zone, a Ti-rich one, the films behaved like solid solutions, developing Ti-like microstructures. As the Me/Ti atomic ratio increased, the formation of intermetallic phases played the leading role and it became possible to observe two different microstructural trends, clearly related to the Me type. This zone was identified as an intermetallic region. In the third zone, a Me-rich one, the microstructures displayed by the different films (Me/Ti > 1.0) showed to be dependent on the Ti solubility into Me. The assessment of the mechanical properties revealed an improved hardness and stiffness with the Me addition, directly related to the formation of intermetallic compounds in different degrees of crystallinity. Moreover, the adhesive strength between the substrate and the coating was higher for the films deposited in the intermetallic zone, more evident in the films prepared with Au, Cu and Ag, in this order. The hardness enhancement was especially evident for the thin films presenting microstructures typical of thin film metallic glasses (TFMGs), Ti-Au and Ti-Cu, about twice the values exhibited by the Ti-Al and Ti-Ag ones. Furthermore, the toughness of these metallic glass-like thin film systems was remarkable, more evident within the Ti-rich zone, presenting H/E ratios close to 0.1 and good elastic recoveries. In contrast, the typical columnar morphologies, combined with the brittle intermetallic structures of the Ti-Ag and Ti-Al films, proved to be less resistant to the plastic deformation (H/E ≤ 0.04), despite the improved elasticity presented by the Ag-rich films.

Electrical and structural properties of direct current magnetron sputtered amorphous TiAl thin films

Structural and electrical properties of amorphous TiAl films with 50 at.% Ti and 50 at.% Al deposited with a compound target by dc magnetron sputtering were characterized by X-ray diffraction (XRD), atomic force microscopy, scanning electron microscopy, and electrical measurements. We found that as-deposited Ti0.5Al0.5 films with a thickness below 1000 nm were amorphous, whereas small hexagonal crystallites could be detected in thicker films. Such layers exhibited a compressive stress which increased with decreasing the thickness of TiAl. The behaviour of stress with the temperature was qualitatively similar in films independently on their thickness. Thermal annealing at 300 °C reduced the initial compressive stress and no changes of the phase of TiAl were observed. We correlated these observations with annihilation of point-like defects created by the deposition in thin Ti0.5Al0.5 films or with appearance of nanocrystallites in the amorphous matrix of TiAl. These effects were also responsible for the changes of the resistivity observed at elevated temperatures. We showed that a heat treatment induced by an electronic current with a density of about 2 MA/cm2 led to the crystallization of TiAl films independent on their thickness. A similar effect could be achieved by annealing the samples at 520–550 °C in air or N atmosphere. A significant drop of the resistivity down to 90 μΩcm was detected in the annealed films at room temperature. XRD confirmed that two different phases of TiAl (γ and α2) with randomly oriented crystallites appeared in the annealed films.

Microstructure and mechanical behavior of nanotwinned AlTi alloys with 9R phase

Nanotwinned metals have shown high strength and ductility. However, metals with high stacking fault energy, such as Al, typically do not contain high-density growth twins or stacking faults. Here we show that by using a small amount of Ti solutes, 0.15–5.1 at.%, fine grains with a large population of extended incoherent twin boundaries bounding broad 9R phase form in sputtered AlTi films. These nanotwinned AlTi films have hardness as high as 2 GPa. This study provides an alternative approach to design high-strength Al alloys by grain refinement and introducing high-density twin boundaries and 9R phase.

Formation of intermetallic compounds and their effect on mechanical properties of aluminum–titanium alloy films

AbstractAl–Ti alloy films with various Ti contents were prepared through magnetron co-sputtering with Al and Ti targets and treated with vacuum annealing at 400 °C. Energy dispersive spectroscopy, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and nanoindentation were used to determine the composition, microstructure and hardness of the films to reveal the existence of intermetallic compounds and their effects on the microstructure and mechanical properties. The results showed that Al–Ti films with lower Ti contents formed nanocrystalline structures of highly supersaturated solid solution. With the increase of Ti content, various types of Al–Ti intermetallic compound bonds formed. The existence and increasing concentration of these compound bonds gradually transformed the films into amorphous structures and supported the continual increase of the film hardness, reaching a high value of 8.8 GPa at 36.3 at.% Ti.

Aluminum–Titanium Alloy Back Contact Reducing Production Cost of Silicon Thin-Film Solar Cells

In this study, metal films are fabricated by using an in-line reactive direct current magnetron sputtering system. The aluminum–titanium (AlTi) back contacts are prepared by changing the pressure from 10 mTorr to 25 mTorr. The optical, electrical and structural properties of the metal back contacts are investigated. The solar cells with the AlTi had lower contact resistance than those with the silver (Ag) back contact, resulting in a higher fill factor. The AlTi contact can achieve a solar cell conversion efficiency as high as that obtained from the Ag contact. These findings encourage the potential adoption of AlTi films as an alternative back contact to silver for silicon thin-film solar cells.

Optical and electrical properties of polycrystalline and amorphous Al-Ti thin films

The structural, optical, and transport properties of sputter-deposited Al-Ti thin films have been investigated as a function of Ti alloying with a concentration ranging from 2% to 46%. The optical reflectivity of Al-Ti films at visible and near-infrared wavelengths decreases with increasing Ti content. X-ray absorption fine structure measurements reveal that the atomic ordering around Ti atoms increases with increasing Ti content up to 20% and then decreases as a result of a transition from a polycrystalline to amorphous structure. The transport properties of the Al-Ti films are influenced by electron scattering at the grain boundaries in the case of polycrystalline films and static defects, such as anti-site effects and vacancies in the case of the amorphous alloys. The combination of Ti having a real refractive index (n) comparable with the extinction coefficient (k) and Al with n much smaller than k allows us to explore the parameter space for the free-electron behavior in transition metal-Al alloys. The free electron model, applied for the polycrystalline Al-Ti films with Ti content up to 20%, leads to an optical reflectance at near infrared wavelengths that scales linearly with the square root of the electrical resistivity.

Neutron diffraction of titanium aluminides formed by continuous electron-beam treatment

Ti-Al-based alloys were produced by hybrid electron-beam technologies. A composite Ti-Al film was deposited on a Ti substrate by electron-beam evaporation (EBE), followed by electron-beam treatment (EBT) by a continuously scanned electron beam. The speed of the specimens motion during the EBT were V1 = 1 cm/sec and V2 = 5 cm/sec, in order to realize two different alloying mechanisms -- by surface melting and by electron-beam irradiation without melting the surface. The samples prepared were characterized by XRD and neutron diffraction to study the crystal structure on the surface and in depth. SEM/EDX analysis was conducted to explore the surface structure and analyze the chemical composition. Nanoindentation measurements were also carried out. No intermetallic phases were registered in the sample treated at velocity V1, while the sample treated at V2 exhibited a Ti3Al/TiAl structure on the surface, transformed to Ti/TiAl in depth. The nanoindentation test demonstrated a significant negative hardness gradient from the surface to the depth of the sample.

Multifunctional Ti–Me (Me = Al, Cu) thin film systems for biomedical sensing devices

Ti–Me binary intermetallic thin films based on a titanium matrix doped with increasing amounts of Me (Me = Al, Cu) were prepared by magnetron sputtering (under similar conditions), aiming their application in biomedical sensing devices. The differences observed on the composition and on the micro(structural) features of the films, attributed to changes in the discharge characteristics, were correlated with the electrical properties of the intermetallic systems (Ti–Al and Ti–Cu). For the same Me exposed areas placed on the Ti target (ranging from 0.25 cm2 to 20 cm2) the Cu content increased from 3.5 at.% to 71.7 at.% in the Ti–Cu system and the Al content, in Ti–Al films, ranged from 11 to 45 at.%. The structural characterization indicated the formation of metastable Ti–Me intermetallic phases for Al/Ti atomic ratios above 0.20 and for Cu/Ti ratios above 0.25. For lower Me concentrations, the effect of the α-Ti(Me) structure dominates the overall structure. With the increase in the amount of the Me into the Ti structure a clear trend for amorphization was observed. For both systems a significant decrease of the electrical resistivity with increasing Me/Ti atomic ratios (higher than 0.5 for Al/Ti atomic ratio and higher than 1.3 for Cu/Ti atomic ratio) was observed. Although similar trends were observed in the resistivity evolution for both systems, the Ti–Cu films presented lower resistivity values in comparison to the Ti–Al system.

Compositional Control of Co-deposited TiAl Film using Dual Magnetron System

Titanium Aluminum (TiAl) films have been deposited by physical vapor deposition method using two unbalanced planar magnetrons. Pure Titanium (Ti) and Aluminum (Al) sputtering targets were used for the co-deposition of TiAl films of varying compositions. Precise control of the sputtering yields of target materials were achieved through optimization of applied power. This in turn controlled the composition of the Ti and Al in the grown films. The films were obtained at applied power levels of ‘50W to 900W’ and their compositions were established and affirmed by EDAX measurements. Amorphous TiAl films were formed for a higher Al composition (i.e. from 20 % to 50 Atomic %), while crystalline film was formed for low Al composition (i.e.10%). Crystallinity of the TiAl film was improved at post annealing temperature of 800 C.

Ultrathin amorphous Ti–Al film used as a diffusion barrier for copper metallization

The viability of ultrathin amorphous Ti–Al film (∼4 nm) as a diffusion barrier layer between Cu and Si for the application in Si-based ultra-large scale integration (ULSI) has been investigated. The Cu/Ti–Al/Si heterostructures are annealed in a high vacuum at various temperatures. There is no impurity peaks in the X-ray diffraction patterns for the samples up to annealing temperature of 800 ∘C, although the island-like grains were observed on the surface of the 800 ∘C annealed sample due to dewetting and agglomeration of the Cu film. No inter-reactions can be found from the images of transmission electron microscopy and Ti–Al is still amorphous after high-temperature annealing. These results indicate that Ti–Al film can effectively separate Cu from Si at high temperatures, and that the amorphous ultrathin Ti–Al film can be a very good barrier layer for Cu metallization.

Effect of Sputtering Power on the Properties of Transparent Conducting Ti-Al Co-Doped Zine Oxide Films Prepared by DC Magnetron Sputtering

Transparent conducting Ti-Al co-doped zinc oxide films (TAZO) with high transparency and relatively low resistivity have been successfully prepared by direct current magnetron sputtering. The effect of sputtering power on the structural, optical, and electrical properties of Ti-Al co-doped films were investigated. The XRD patterns show that the thin films were highly textured along the c-axis and perpendicular to the surface of the substrate. The electrical resistivity decreases when the sputtering power increases from 40W to 120W. When the sputtering power is 120w and the target-substrate distance is 60mm, it is obtained that the lowest resistivity is 3.23×10-4Ω·cm.The lowest stress is 0.864Gpa in all the deposited films. All the films present a high transmittance of above 91% in the visible range.

Atomic-scale study of structural change of TiAl alloy film during the cooling process

Microstructural changes of molten TiAl films are investigated by using molecular dynamics simulations within the framework of embedded atom method in quenching and continuous cooling processes. Atomic local structures in these films are analyzed by using atom average energy, pair distribution functions, and pair analysis technique. The local structure changes in the TiAl films can be divided in to three stages on quenching, and they are divided in to two stages on continuous cooling.

Molecular Dynamics Investigation of Relaxation and Local Structure Changes in a Quenched Molten TiAl Alloy Film

Relaxation and local structure changes of a molten TiAl alloy film during quenching have been investigated by molecular dynamics simulations within the framework of embedded atom method (EAM). The details of atom motions are analyzed using mean square displacement (MSD). Accompanying with massive atom rearrangement at a certain quenched temperature and time, local structural patterns are identified by decomposing peaks of pair distribution functions (PDFs) according to the pair analysis(PA) technique. The relaxation factor clearly reveals two relaxation processes involving in slow relaxation and fast relaxation of the quenched liquid TiAl film. Concerning the studied film, the obtained results reveal how quenched temperatures affect local structure changes.

Development of Y-Shaped Filtered-Arc-Deposition System for Preparing Multielement Composition-Controlled Film

In recent years, multielement films have been required for high-performance cutting tools. In this paper, a Y-shaped filtered-arc-deposition (Y-FAD) system with two vacuum-arc sources was developed. First, an optimum magnetic coil arrangement was experimentally established to transport two plasma beams through the Y-shaped duct at the same time. Since the two plasma beams have the same electrical polarity, they naturally tend to repel each other. Therefore, in the second step, the two plasma beams were combined into one plasma beam through a mixer part by vibrating the plasma beams with a laterally oscillating magnetic field. Third, the electrical bias applied to the duct was optimized to obtain a higher transportation rate of plasma and deposition rate. After these design developments and tuning, titanium-aluminum (Ti-Al) film with a combined deposition pattern was finally obtained with Al and Ti cathodes. The controllability of the composition ratio by the arc current was verified.

Corrosion behavior of Ti–Al–N/Ti–Al duplex coating on AZ31 magnesium alloy in NaCl aqueous solution

In this study, Ti–Al–N/Ti–Al duplex coating was deposited on AZ31 magnesium alloy by magnetron sputtering with a Ti/Al composite target. Scanning electron microscopy and Auger electron spectroscopy were applied to investigate the morphology and elemental concentration of the obtained coating, respectively. The top layer was Ti–Al–N film with a Ti:Al:O:N ratio of 0.32:0.84:0.08:1, and the bottom layer was Ti–Al film with a Ti:O:Al ratio of 1.94:0.12:1. Each layer of this coating presented a developed columnar structure. The polarization test and immersion test were used to investigate corrosion behavior of the coated sample in 3.5 wt.% NaCl aqueous solution. The results showed that this duplex coating could protect the substrate effectively in NaCl aqueous solution. Nevertheless, several through-thickness micropores in the coating finally induced the failure of the coated AZ31 in the immersion test.

Amorphous &lt;inline-formula&gt;&lt;math display="inline" overflow="scroll"&gt;&lt;mrow&gt;&lt;mi&gt;TiAl&lt;/mi&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/inline-formula&gt; films for micromirror arrays with stable analog deflection integrated on complementary metal oxide semiconductors

Large micromechanical mirror arrays (MMA) with analog pixel deflection integrated onto active CMOS address circuitry require both high-quality planar reflective optical surfaces and a stable deflection versus voltage characteristic. However, for implementing a CMOS-compatible surface-micromachining process, certain obstacles such as a restricted thermal budget and a limited selection of suitable materials must be overcome. Amorphous TiAl is presented as a new actuator material for monolithical MEMS integration onto CMOS circuitry. TiAl films may be sputter deposited at room temperature, have an x-ray amorphous structure, and a low stress gradient. The glassy structure and high melting point make TiAl less vulnerable to stress relaxation, which makes TiAl an ideal spring material. One-level actuators with TiAl or Al-TiAl-Al structural layers and two-level actuators with separate TiAl spring and Al-alloy mirror layers were fabricated and tested with respect to their drift stability. The stability of TiAl-based actuators was found to be superior in comparison to one-level Al-alloy actuators. Two-level actuators with TiAl hinges emerge as the most promising design.