Ti Alloy Films Research Articles

Structural properties and sensing performance of Ni–Ti alloy films for extended-gate field-effect transistor pH sensors

This study presents a facile Ni–Ti alloyed film grown on the n+-type Si through a DC sputtering technique for electrochemical pH sensing and its comprehensive structural, morphological, compositional, profiling, microstructural, and elemental characterization. In addition to pH sensing measurements, the properties of the sensing layer were analyzed using X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy analysis. The impact of rapid thermal annealing (RTA) treatment (500–700 °C) on the alloyed Ni–Ti sensitive films was extensively studied. The structural feature of the alloyed Ni–Ti sensitive films is more closely related to their relative sensing performances. The performance of the alloyed Ni–Ti sensitive film at the RTA temperature of 600 °C showed a higher pH sensitivity of 60.73 mV/pH in a wide pH range of 2–12 in comparison with other RTA temperatures. Moreover, the stability of this 600 °C-annealed EGFET sensor exhibited a small drift rate of 0.21 mV/h in pH 7 and a low hysteresis voltage of 1 mV in a loop of pH 7 → 4→7 → 10→7. The alloyed Ni–Ti sensitive film is fully compatible with a standard CMOS process and demonstrates better sensing performance than other metal nitride sensitive films (e.g. RuN, TiN), suggesting it may be a strong candidate for future sensor and biosensor applications.

The structure and mechanical properties of Cr-based Cr-Ti alloy films

Previous studies have dealt with Cr and its alloy films that exhibit promising characteristics as surface modification layers for antiwear, anticorrosive, and decorative applications. However, the effect of Ti alloying on the structure and mechanical properties of Cr films has not been studied. This work aimed to the structure and mechanical properties of Cr-Ti alloy films in the Cr-rich side. To this end, pure Cr, Cr-6 at.% Ti, Cr-11 at.% Ti, Cr-16 at.% Ti, and Cr-21 at.% Ti alloy films were prepared by magnetron sputtering, and the structure and mechanical properties of the films were evaluated. The results indicated that all the films exhibited a Cr-based growth with body-centered cubic structure, and increasing the Ti content decreased the (110) orientation growth of Cr basis. Ti alloying increased the hardness of the films, while leaded to a monotonic decrease in the modulus of the films. The first-principles method was employed to demonstrate that the reduced modulus was determined by the Ti alloying degree, rather than the orientation evolution of the films. The analysis of H/E value suggested that the wear resistance of the films was improved by Ti alloying. The mechanical properties of present Cr-Ti alloy films, and other Cr-based alloy films or metallic glasses in publications were compared and discussed. We proposed that Ti alloying is a considerable way to explore advanced mechanical properties of Cr-based alloy films.

Synthesis of co-sputter deposited Ni–Ti thin alloy films and their compositional characterization using depth sensitive techniques

Ni–Ti (Nickel–Titanium) thin alloy films of different concentration and thickness were synthesized using magnetron co-sputtering process utilizing separate Ni and Ti targets. Concentrations of Ni and Ti in the deposited films were optimized by controlling the power ratio of individual Ni and Ti targets. Thickness and density of the layered structures present in the films were evaluated by X-ray Reflectivity (XRR). Atomic concentrations of Ni and Ti were determined by Energy Dispersive X-ray Spectroscopy and matched with the concentrations obtained through XRR measurements. Secondary Ion Mass Spectrometry and Rutherford Backscattering Spectrometry were utilized to investigate the elemental depth distributions of Ni, Ti and O content in the films. Depth distribution studies confirmed that oxygen as a surface contaminant was present in all the deposited films, although films having thickness in the range of 21.5 nm −22.7 nm, were found to contain oxygen within the films too. Thicker Ni–Ti alloy films, in the range of 51.3 nm −52.3 nm, did not reveal any oxygen contamination in the bulk of the films. X-ray Photoelectron Spectroscopy (XPS) was used to determine elemental and molecular content of the deposited films. XPS analysis confirmed the presence of NiO at the surface and intermetallic Ni–Ti within the deposited films.

Microstructure and nanomechanical properties of co-deposited Ti-Cr films prepared by magnetron sputtering

Pure Ti and Ti-Cr films of various Cr content (<40at.% Cr) were prepared on the surface of aluminum alloy by using DC magnetron sputtering. Ti-Cr alloy films exhibited a bcc structure (β phase) stabilized with Cr addition. Pure Ti and Ti-Cr alloy films displayed (002) and (110) preferred orientations, respectively. The denser morphologies and finer grains were achieved for Ti-Cr films with increasing the Cr content. Compared with that of pure α-Ti film, Ti-Cr films (β type) showed remarkable increase in hardness (~6.0GPa). The further increasing Cr content (>10at.% Cr) slightly changed the hardness and elastic modulus of Ti-Cr films with β structure.. The analysis of H-E ratio (H3/E2and H/E) indicated the better plastic and wear resistance of Ti-Cr films than that of pure Ti films.

(Zr,Ti,O) alloy films with enhanced hardness and resistance to cracking prepared by magnetron sputtering

The article reports on the effect of (i) the ion bombardment and (ii) the addition of small amount of oxygen into Ar sputtering gas on the structure, microstructure, mechanical properties and macrostress of the (Zr,Ti) alloy films prepared by DC magnetron sputtering. Properties of the (Zr,Ti) alloy films with three different elemental compositions – (1) Zr95Ti5, (2) Zr30Ti70 and (3) Zr5Ti95 were investigated in detail. It was found that (1) the (Zr,Ti) alloy films sputtered at a low value of the negative substrate bias Us=−50V are well crystalline, (2) the (Zr,Ti) alloy films sputtered at high values of the negative substrate bias | Us |≥150V are nanocrystalline, (3) the (Zr,Ti) alloy films, sputtered at all substrate biases Us used in our experiments and ranging from −50 to −250V, exhibit low hardness H<10GPa, low ratio H/E⁎<0.1, low elastic recovery We<60% and low resistance to cracking and (4) the addition of small amount of oxygen in Ar sputtering gas makes it possible to sputter nanocrystalline (Zr,Ti,O) alloy films with high hardness H>10GPa, high ratio H/E⁎≥0.1, high elastic recovery We≥60% and enhanced resistance to cracking; here E⁎ is the effective Young's modulus. The main result of the presented investigation is the demonstration that the incorporation of a small amount of O in a (Zr,Ti) alloy film is a very effective way to increase its hardness and to form the flexible (Zr,Ti,O) alloy films with enhanced resistance to cracking.

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.

Comparative study of the structure and corrosion behavior of Zr-20%Cr and Zr-20%Ti alloy films deposited by multi-arc ion plating technique

The primary focus of the present work was to perform comparative study of the structure as well as corrosion behavior of two Zr-rich alloy films, i.e. Zr-20%Cr and Zr-20%Ti, as well as metallic Ti, Cr and Zr films, formed by multi-arc ion plating technique. The required alloy film composition was obtained by co-deposition with proper choice of current for the targets of the constituent metals. The deposited alloy film composition was determined by energy dispersion X-ray spectroscopy, X-ray fluorescence and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) techniques, which were in close agreement with each other. The film thickness lied in the range of 550-620nm. The crystal structure was studied by X-ray diffraction, which revealed the formation of nanocrystalline and semi-amorphous structures. The corrosion rates of the films were determined through weight loss measurement in 1M, 6M and 12M hydrochloric acid (HCl) by ICP-AES analysis of the solution after immersion for 200-350h. Anodic (potentiodynamic) polarization was also performed. Zr-20%Cr alloy film exhibited the best corrosion resistance, and its dissolution rate was less than 0.5μm/year in 6M HCl and about 4μm/year in 12M HCl.

Cell Proliferation Ability of Mouse Fibroblast-Like Cells and Osteoblast-Like Cells on a Ti-6Al-4V Alloy Film Produced by Selective Laser Melting

Successful regeneration of tissues and organs relies on the application of suitable substrates or scaffolds in scaffold-based regenerative medicine. In this study, Ti-6Al-4V alloy films (Ti alloy film) were produced using a three-dimensional printing technique called Selective Laser Melting (SLM), which is one of the metal additive manufacturing techniques. The thickness of produced Ti alloy film was approximately 250 μm. The laser-irradiated surface of Ti alloy film had a relatively smooth yet porous surface. The non-irradiated surface was also porous but also retained a lot of partially melted Ti-6Al-4V powder. Cell proliferation ability of mouse fibroblast-like cells (L929 cells) and mouse osteoblast-like cells (MC3T3-E1 cells) on both the surfaces of Ti alloy film was examined using WST assay. Both L929 and MC3T3-E1 cells underwent cell proliferation during the culture period. These results indicate that selective laser melting is suitable for producing a cell-compatible Ti-6Al-4V alloy film for biomaterials applications.

Thermal stability and electrical properties of Ag–Ti films and Ti/Ag/Ti films prepared by sputtering

Ag–Ti (100 nm) alloy film, and Ti/Ag (100 nm) double-layer and Ti/Ag (100 nm)/Ti triple-layer films were prepared by rf sputtering to investigate the effect of Ti on suppression of agglomeration of the Ag thin film caused by thermal treatment. Scanning electron microscopy revealed that the Ag–Ti and Ti/Ag/Ti films had high thermal stability. X-ray photoelectron spectroscopy analysis showed that the surfaces of both kinds of films were covered with a TiO2 layer after annealing, which was considered to be the key factor for improvement of the thermal stability of the films. In addition, scratch tests indicated improvement of the adhesive strength of the Ti/Ag/Ti film to the SiO2 substrate due to the underlying Ti film layer, which effectively promoted suppression of Ag agglomeration. However, the resistivity of the Ag–Ti films increased abruptly with increasing Ti content due to the impurity scattering effect, and minimum usage of the alloying element was required to achieve low resistivity. In contrast, the Ti/Ag/Ti film exhibited both low resistivity and high thermal stability.

Structure Analyses of Ti-Based Self-Formed Barrier Layers

Self-formed Ti-based barrier layer using Cu(Ti) alloy seed applied to 45-nm-node dual-damascene interconnects was reported to have sufficient barrier strength to prevent Cu diffusion into dielectrics. The constituent Ti compounds in the self-formed Ti-based barrier layers and the barrier structures in Cu(Ti)/dielectric samples were identified by X-ray photoelectron spectroscopy (XPS) analyses. Two types of SiOC with low dielectric constants, SiO2, and SiCN were used as dielectrics. The Ti-based barrier layers consisted mainly of amorphous Ti oxides such as TiO2, Ti2O3, and TiO, regardless of the dielectric. In addition to Ti oxides, barrier layers containing TiC, TiSi, and TiN were observed, depending on the dielectric. TiC and TiSi were in crystalline state. They were formed beneath the Cu(Ti) alloy films, and had orientation relationship with the crystalline Cu(Ti) alloy films. The amorphous Ti oxides were formed above the amorphous dielectric layers. The amorphous Ti oxides are believed to be formed continuously above the dielectric layers and prevent Cu diffusion into the dielectric layers.

Structure Analyses of Ti-Based Self-Formed Barrier Layers

Self-formed Ti-based barrier layer using Cu(Ti) alloy seed applied to 45-nm-node dual-damascene interconnects was reported to have sufficient barrier strength to prevent Cu diffusion into dielectrics. The constituent Ti compounds in the self-formed Ti-based barrier layers and the barrier structures in Cu(Ti)/dielectric samples were identified by X-ray photoelectron spectroscopy (XPS) analyses. Two types of SiOC with low dielectric constants, SiO2, and SiCN were used as dielectrics. The Ti-based barrier layers consisted mainly of amorphous Ti oxides such as TiO2, Ti2O3, and TiO, regardless of the dielectric. In addition to Ti oxides, barrier layers containing TiC, TiSi, and TiN were observed, depending on the dielectric. TiC and TiSi were in crystalline state. They were formed beneath the Cu(Ti) alloy films, and had orientation relationship with the crystalline Cu(Ti) alloy films. The amorphous Ti oxides were formed above the amorphous dielectric layers. The amorphous Ti oxides are believed to be formed continuously above the dielectric layers and prevent Cu diffusion into the dielectric layers.

Growth of Ti-Based Interface Layer in Cu(Ti)/Glass Samples

Cu(Ti) alloy films with low-resistivity and excellent-adhesion have been successfully prepared on glass substrates. To gain further resistivity reduction and adhesion strength, growth of a Ti-based interface layer was investigated using Rutherford backscattering spectrometry (RBS) in the present study. Cu(0� 5 at%Ti) alloy films were deposited on glass substrates and subsequently annealed in vacuum at 400� 600 � C for 0:5� 24 h. Results were compared with those for samples on SiO2 substrate previously obtained. Ti peaks were obtained in RBS spectra only at the interfaces for both Cu(Ti)/glass and Cu(Ti)/SiO2 samples. Molar amounts of Ti atoms segregated to the interfaces (n) were estimated from Ti peak areas. The m values estimated from the slopes of the log n versus log t lines were almost similar for all the samples (m ¼ 0:10� 0:12), suggesting that growth of the Ti-based interface layers was controlled by a similar mechanism. The activation energy of the Cu(Ti)/glass samples was similar to that of the Cu(Ti)/SiO2 samples, while a pre-exponential factor (Z) of the Cu(Ti)/glass samples was approximately half of the value of the Cu(Ti)/SiO2 samples. The Z value shows the frequency with which the Ti atoms meet oxygen in the glass substrates. Impurities in the glass substrates lowered the frequency. These factors lead to the conclusion that growth rate of the Ti-based interface layers on glass substrates was slower than that on SiO2. The Ti-based interface layer growth was also influenced by microstructure of Cu(Ti) alloy films formed on the glass substrates. Columnar grains in the Cu(Ti) alloy films were seen to enhance Ti segregation. However, an equiaxed zone above the interface retarded Ti diffusion to the interface, leading to lack of Ti atoms for the reaction. [doi:10.2320/matertrans.MBW201017]

Effects of Pore Sealing on Self-Formation of Ti-Rich Barrier Layers in Cu(Ti)/Porous-Low-k Samples

To investigate effects of pore seals on self-formation of Ti-rich barrier layers, Cu(1 at. % Ti) alloy films were deposited on porous SiOxCyH (low-k) dielectric layers in samples with and without about 6.5-nm-thick SiCN pore seals. Self-formed Ti-rich barrier layers were formed on the porous low-k layers after annealing in Ar for 2 h at 400–600 °C. The samples without pore sealing had a rough interface, indicating that Ti atoms reacted with the porous low-k layers at the pore surfaces. In contrast, the pore-sealed samples had a smooth interface. The Ti-rich barrier layers in the samples consisted of amorphous Ti oxides. In addition, polycrystalline TiC and amorphous TiN and TiC were observed to be formed beneath the Cu(Ti) alloy films in the annealed samples without pore sealing and pore-sealed samples, respectively. Note that Ti segregation at the interface was divided into two layers, suggesting that the Ti-rich barrier layers self-formed by the reaction of Ti atoms with the pore sealing and porous low-k layers are separated. The resistivity of the pore-sealed samples was lower than that of the samples without pore sealing. This is attributed to lower residual Ti atoms in the alloy films and coarser columnar grains in the pore-sealed samples.

Ti-Rich Barrier Layers Self-Formed on Porous Low-k Layers Using Cu(1 at.% Ti) Alloy Films

To investigate the applicability of the technique of barrier self-formation using Cu(Ti) alloy films on porous low-k dielectric layers, Cu(1 at.% Ti) alloy films were deposited on porous SiOCH (low-k) dielectric layers in samples with and without ~6.5-nm-thick SiCN pore seals. Ti-rich barrier layers successfully self-formed on the porous low-k layer of both sample types after annealing in Ar for 2 h at 400°C to 600°C. The Ti-rich barrier layers consisted of amorphous Ti oxides and polycrystalline TiC for the samples without pore sealing, and amorphous TiN, TiC, and Ti oxides for the pore-sealed samples. The amorphous TiN originated from reaction of Ti atoms with the pore seal, and formed beneath the Cu alloy films. This may explain two peaks of Ti segregation at the interface that appeared in Rutherford backscattering spectroscopy (RBS) profiles, and suggests that the Ti-rich barrier layers self-formed by the reaction of Ti atoms with the pore seal and porous low-k layers separately. The total molar amount of Ti atoms segregated at the interface in the pore-sealed samples was larger than that in the samples without pore sealing, resulting in lower resistivity. On the other hand, resistivity of the Cu alloy films annealed on the porous low-k layers was lower than that annealed on the nonporous low-k layers. Coarser Cu columnar grains were observed in the Cu alloy films annealed on the porous low-k layers, although the molar amount of Ti atoms segregated at the interface was similar in both sample types after annealing. The cause could be faster reaction of the Ti atoms with the porous dielectric layers.

Resistivity Reduction and Adhesion Increase Induced by Surface and Interface Segregation of Ti Atoms in Cu(Ti) Alloy Films on Glass Substrates

Low-resistivity and excellent-adhesion Cu(Ti) alloy films were prepared on glass substrates. Cu(0.3∼4 at%Ti) alloy films were deposited on the substrates, and subsequently annealed in vacuum at 400°C for 3 h. Resistivity of the annealed Cu(Ti) alloy films was significantly reduced to about 2.8 μΩcm. Tensile strength of the Cu(Ti)/glass interface increased to about 60 MPa after annealing. The low resistivity and excellent adhesion resulted from Ti segregation at the film surface and the Cu(Ti)/glass interface. The segregated Ti atoms reacted with atmospheric oxygen at the surface and with oxygen in glass and/or from atmosphere at the interface, and formed a TiO 2 layer at the surface and a TiO 2 layer with a small amount of Ti 2 O 3 and TiO at the interface. The layers were non-crystalline. Columnar grains in the alloy films were seen to enhance Ti segregation and subsequent Cu grain growth. The Cu grain growth also contributed to low resistivity of Cu(Ti) alloy films.

Combinatorial Optimization for the Anti-Corrosion Properties of Ti Doped Zn-Al Alloy Films

A Zn-Al-Ti material chip was fabricated by combinatorial technology using an ion beam sputtering method. The anti-corrosion properties of the Zn-Al alloy film and Ti doping effects on its corrosion resistance were investigated. High quality alloy films were obtained using low-temperature diffusion at 200 ℃ for 1 h combined with high-temperature crystallization at 370 ℃ for 2 h in Ar+5% (φ, volume fraction) H2 atmosphere. X-ray diffraction (XRD) and scanning electron microscope (SEM) were also used to characterize the structure and surface morphology of typical samples. The anti-corrosion behavior of samples was studied by electrochemical methods. Results indicated that the corrosion rate decreased markedly when a small amount of Ti was doped in the Zn-Al binary compositions. The Zn-Al- Ti alloy film doped with 6.0% (w, mass fraction) Ti (94.0% (w) Zn-Al with wAl∶wZn=55%∶45%) showed the best anti- corrosion behavior mainly because the small amount of Ti dopant developed fine grains, made the surface denser, and enhanced its passivation behavior.

Characterization of Self-Formed Ti-Rich Interface Layers in Cu(Ti)/Low-k Samples

In our previous studies, thin Ti-rich diffusion barrier layers were found to be formed at the interface between Cu(Ti) films and SiO2/Si substrates after annealing at elevated temperatures. This technique was called self-formation of the diffusion barrier, and is attractive for fabrication of ultralarge-scale integrated (ULSI) interconnects. In the present study, we investigated the applicability of this technique to Cu(Ti) alloy films which were deposited on low dielectric constant (low-k) materials (SiOxCy), SiCO, and SiCN dielectric layers, which are potential dielectric layers for future ULSI Si devices. The microstructures were analyzed by transmission electron microscopy (TEM) and secondary-ion mass spectrometry (SIMS), and correlated with the electrical properties of the Cu(Ti) films. It was concluded that the Ti-rich interface layers were formed in all the Cu(Ti)/dielectric-layer samples. The primary factor to control the composition of the self-formed Ti-rich interface layers was the C concentration in the dielectric layers rather than the enthalpy of formation of the Ti compounds (TiC, TiSi, and TiN). Crystalline TiC was formed on the dielectric layers with a C concentration higher than 17 at.%.

Effects of Dielectric-Layer Composition on Growth of Self-Formed Ti-Rich Barrier Layers in Cu(1 at%Ti)/Low-k Samples

In our previous studies, Ti atoms in Cu(Ti) alloy films were found to segregate at the film surface and the interface between Cu(Ti) alloy films and dielectric layers after annealing in Ar atmosphere at elevated temperatures. Such self-formed Ti-rich interface layers can act as a diffusion barrier layer. This technique was called ‘‘self-formation of the diffusion barrier,’’ which is attractive for the fabrication of ultra-large scale integrated interconnects. In the present study, we investigated the growth of Ti-rich barrier layers in Cu(Ti)/dielectric-layer samples wit ha low Ti content (1 at%) after annealing in ultra high vacuum (UHV). Ti atoms were found to segregate only to the Cu(Ti)/dielectric-layer interface under annealing in UHV. The microstructures were analyzed by transmission electron microscopy and Rutherford backscattering spectrometry, and correlated with the electrical properties of the Cu(Ti) films. It was concluded that Ti-rich interface layers were formed in all the Cu(Ti)/dielectric-layer samples. The Ti-rich interface layers were identified to consist of TiC or TiSi in addition to Ti oxides. The growth of the Ti-rich interface layers consisting of TiC was faster than those consisting of TiSi. Similarly, the resistivities of Cu(Ti)/dielectric-layer samples in which the TiC formation was observed were quickly reduced and those in which the TiSi formation was observed were gradually reduced. Compositions of the self-formed Ti-rich interface layers were concluded to be determined by the C concentration in the dielectric layers rather than by the enthalpy of formation. The growth of the self-formed Ti-rich interface layers consisting of TiC may be controlled by C diffusion in the Ti-rich interface layer. The composition of the dielectric layers was concluded to play an important role on the growth of the Ti-rich interface layers. [doi:10.2320/matertrans.MAW200809]

Self-formation of Ti-rich Layers at Cu(Ti)/low-k Interfaces

ABSTRACTIn our previous studies, thin Ti-rich diffusion barrier layers were found to be formed at the interface between Cu(Ti) films and SiO2/Si substrates after annealing at elevated temperatures. This technique was called “self-formation of the diffusion barrier,” which is attractive for fabrication of ultra-large scale integrated (ULSI) interconnects. In the present study, we investigated the applicability of this technique to Cu(Ti) alloy films which were deposited on the four low dielectric constant (low-k) dielectric layers which are potential dielectric layers of future ULSI-Si devices. The microstructures were analyzed by transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS), and correlated with the electrical properties of the Cu(Ti) films. It was concluded that the Ti-rich interface layers were formed in all the Cu(Ti)/dielectric-layer samples. The primary factor to control composition of the self-formed Ti-rich interface layers was the C concentration in the dielectric layers rather than the formation enthalpy of the Ti compounds (TiC and TiSi). Crystalline TiC was formed on the dielectric layers with a C concentration higher than 17 at.%.

Characteristic of Copper Films on Molybdenum Substrate by Addition of Titanium in an Advanced Metallization Process

Mo(Ti) alloy and pure Cu thin films were subsequently deposited on <TEX>$SiO_2-coated$</TEX> Si wafers, resulting in <TEX>$Cu/Mo(Ti)/SiO_2$</TEX> structures. The multi-structures have been annealed in vacuum at <TEX>$100-600^{\circ}C$</TEX> for 30 min to investigate the outdiffusion of Ti to Cu surface. Annealing at high temperature allowed the outdiffusion of Ti from the Mo(Ti) alloy underlayer to the Cu surface and then forming <TEX>$TiO_2$</TEX> on the surface, which protected the Cu surface against <TEX>$SiH_4＋NH_3$</TEX> plasma during the deposition of <TEX>$Si_3N_4$</TEX> on Cu. The formation of <TEX>$TiO_2$</TEX> layer on the Cu surface was a strong function of annealing temperature and Ti concentration in Mo(Ti) underlayer. Significant outdiffusion of Ti started to occur at <TEX>$400^{\circ}C$</TEX> when the Ti concentration in Mo(Ti) alloy was higher than 60 at.%. This resulted in the formation of <TEX>$TiO_2/Cu/Mo(Ti)\;alloy/SiO_2$</TEX> structures. We have employed the as-deposited Cu/Mo(Ti) alloy and the <TEX>$500^{\circ}C-annealed$</TEX> Cu/Mo(Ti) alloy as gate electrodes to fabricate TFT devices, and then measured the electrical characteristics. The <TEX>$500^{\circ}C$</TEX> annealed Cu/Mo(<TEX>$Ti{\geq}60at.%$</TEX>) gate electrode TFT showed the excellent electrical characteristics (<TEX>$mobility\;=\;0.488\;-\;0.505\;cm^2/Vs$</TEX>, on/off <TEX>$ratio\;=\;2{\times}10^5-1.85{\times}10^6$</TEX>, subthreshold = 0.733.1.13 V/decade), indicating that the use of Ti-rich(<TEX>$Ti{\geq}60at.%$</TEX>) alloy underlayer effectively passivated the Cu surface as a result of the formation of <TEX>$TiO_2$</TEX> on the Cu grain boundaries.