Nanostructure of Rutile TiO 2 Thin Films Prepared on Unheated Substrate by Dual Cathode DC Unbalanced Magnetron Sputtering

In this work, structural and optical properties of the TiO2 films deposited on unheated substrates by dual cathode dc unbalanced magnetron sputtering at long substrate-target distance (ds-t) were studied. Titanium dioxide (TiO2) thin films were deposited on unheated Si (110) wafers, glass slides and carbon coated copper grids at different substrate to target (ds-t) distances. The structural properties of TiO2 thin films were characterized by X-ray diffraction (XRD) and transmission electron microcopy (TEM) with selected-area electron diffraction (SAED), surface morphology using atomic force microscopy (AFM) and optical transmission spectra using a spectrophotometer. XRD results show that TiO2 films deposited at various ds-t distances have only rutile crystal structure. The crystallinity and thickness of the films increased while the roughness decreased with decreasing ds-t distance. The refractive indices of the deposited films were found to be in the range of 2.51 2.82 and increased with decreasing ds-t distance.


Introduction
During the past two decades, nanocrystalline titanium dioxide (TiO 2 ) thin films have attracted a great deal of interest due to their photocatalytic properties and photoinduced superhydrophilicity [1] [2] [3] [4].
In general, TiO 2 exists in both crystalline and amorphous forms.Crystalline TiO 2 exists in three different phases: anatase, rutile and brukite.Anatase and W. Kongsri  brookite are stable at the temperature below 600˚C and the anatase phase completely transforms to the rutile phase at the temperature above 750˚C [5].The anatase phase is suitable for photocatalytic activities and superhydrophilic properties, white the rutile phase is widely used for optical applications [1]- [8].
TiO 2 thin films can be prepared by many methods, such as chemical vapor deposition, electron beam evaporation, ion beam assisted deposition, spray pyrolysis, sol-gel process and sputtering [1]- [14].However, most of the TiO 2 films prepared by the above-mentioned methods are either amorphous or anatase.
Among these methods, the reactive magnetron sputtering is one of the most widely used methods for the deposition of TiO 2 thin films.As reported in the literature, the TiO 2 films deposited on unheated substrates by dc reactive magnetron sputtering often exhibit an amorphous phase.Therefore, to obtain the rutile phase, the films have to be heated during or after the deposition at a high tem- Very few studies report on the deposition of TiO 2 thin films on unheated substrates by dc reactive magnetron sputtering [12] [13] [19].In addition, the deposited TiO 2 films reported by those investigators were carried out at a relatively short-substrate distance in the range of 15 to 40 mm.
In this work, the rutile TiO 2 films were prepared on unheated substrates by a home built dual cathode dc unbalanced magnetron sputtering.The effect of the substrate-target distance (d s-t ) on the structural and optical properties of the TiO 2 films was studied.

Sputtering Apparatus
Figure 1 shows the schematic diagram of dual-cathode dc unbalanced magnetron sputtering constructed in our laboratory.The magnetron sputtering cathodes consist of a coating cathode and an enhancing plasma cathode.The vacuum chamber has a diameter of 35 cm and a height of 38 cm.Two separated dc power supplies were used to supply each cathode independently.However, in this work only coating cathode was used for sputtering the Ti target and, hence, enhancing plasma cathode was closed by the shutter.
The maximum distance from the substrate to the target of coating cathode (d s-t ) was about 14 cm.The substrate holder can be rotated with an angular velocity from 10 -200 rpm.The base pressure of the vacuum chamber was 1.0 × 10 −5 mbar.
Figure 2 shows the schematic diagram of each magnetron sputtering cathode.
A magnetron sputtering cathode consists of the magnetron body with two holes and two permanent magnets inside the magnetron body.The poles of both magnets were attached with the iron steel plate which hold on the magnetron body surface.The central magnet has a cylindrical shape with a diameter of 15 mm and a thickness of 15 mm.The outer magnet is a ring magnet with the inner and outer diameters of 53 and 83 mm, respectively.The magnetic field strength  Both cathodes are the same and installed in the vacuum chamber (Figure 1).
However, in this work only the coating cathode was used and; hence, the other one behaves as enhancing plasma cathode.The study allows explanation into the possible effect of magnetic field on the rate of deposition and the phase formation of the deposited thin film, since the enhancing cathode alters the magnetic field strength of the system.
Figure 3 shows the magnetic field lines of dual-cathode unbalanced magnetron sputtering before it was installed in the vacuum chamber.The magnetic field lines are a combination of transverse and perpendicular components of the magnetic field.The magnetic field strength was also measured at different d s-t distances from coating cathode using a teslameter (PHYWE).The magnetic field strengths at the center and the side of target surface were measured to be 1700 and 1330 G, respectively.

Sample Preparation
The metallic titanium disc with a purity of 99.97% (Kurt J. Lesker) and 54 mm diameter was used as sputtering target.The Ar (99.999%,TIG) and O 2 (99.999%,TIG) were used as sputtering gas and reactive gas whose flow rates were controlled with the mass flow meters (MKS) at 1 and 4 sccm, respectively.Prior to sputtering, the chamber was evacuated to a base pressure of lower than 1.0 × 10 −5 mbar and pre-sputtered for 5 min to clear out impure gases in the chamber The substrates used were glass slides for surface roughness analysis and optical transmission measurement, carbon coated copper grids for microstructure analysis and silicon (110) wafers for crystal structure analysis.

Characterization
The Instruments Nanoscope IV in a tapping mode for observation of the surface roughness, and film thickness.The optical transmission measurements of the films were carried out at room temperature using a UV-VIS spectrophotometer (Bruker, D8 Advance) in the wavelength range from 400 to 750 nm.Then, the refractive index (n) was calculated from optical transmission of the films using the Swanepoel's method [20].that of single magnetron cathode.The increase in magnetic field strength result in the increase of plasma as shown in Figure 5.

Results and Discussion
The rutile is a result of the reaction between decelerate Ti + species or activated Ti atoms and ions [21].Generally, the electrons generated during the sputtering of coating cathode could be trapped in an unbalanced magnetic field (UMF), which results in a plasma being directed onto the substrate surface and relatively low voltage ion bombardment of the growing film surface due to self-bias potential [22].The number of electrons was increased when the enhancing plasma cathode was installed, due to increasing the UMF [23].The Ti + ions, which ionized by the electron impact on the Ti atoms, were increased and resulting in the increase of the probability of rutile formation [24] [25] [26].The increase of crystallinity of rutile with the decrease of d s-t is the result of increasing the concentration of Ti + species and deposition energy.Normally, in a sputtering process, the sputter atoms are deposited on the substrate with kinetic energy in thermodynamically exception the potential energy [27].Kinetic energy is the reciprocal of d s-t due to diminution of kinetic energy the impacting the sputtered atoms with other atoms in the deposition chamber.Inaddition, the high electron concentration in the magnetron sputtering was exhibited around the target face because of the high magneticfield strength.Thus, the high concentration of Ti + species is probably around the entire target face.
The surface root mean square roughness (R rms ) of the films deposited at different d s-t distances of 6, 8, 10 and 12 cm was determined from AFM images and the results are shown in Figure 6.The thickness of films was measured from the  section analysis of 2D-AFM images and the results are also shown in Figure 6.
The grain size of the films was estimated from 2D-AFM images and the results were found to be 52.0,43.3, 40.1 and 30.4 nm for d s-t distances of 6, 8, 10 and 12 cm, respectively.It is seen that the roughness decreases with increasing distance as the effect of low kinetic energy of Ti particles.Kinetic energy is seen as the reciprocal of d s-t due to diminution of kinetic energy by impacting of the sputtered atoms with other atoms in the deposition chamber.
The high-resolution transmission electron microscopy (HRTEM) images accompanied with the corresponding SAED patterns of films are shown in Figure 7.The observable lattice fringes and the distances of the fringe are measured to be 0.32 nm which corresponds well to the standard data (JCPDS 88-1175) in the (110) plane (d = 0.32 nm) of TiO 2 rutile phase.Moreover, the corresponding SAED patterns of films show the further planes of (101) and (211) rutile in the films deposited at d s-t distances of 6, 8 and 10 cm, whereas only (110) plane is observed in the film deposited at the d s-t distance of 12 cm due to the decrease of deposition energy when the d s-t distance is increased.
The high-resolution transmission electron microscopy (HRTEM) images accompanied with the corresponding SAED patterns of films are shown in Figure 7.The observable lattice fringes and the distances of the fringe are measured to be 0.32 nm which corresponds well to the standard data (JCPDS 88-1175) in the (110) plane (d = 0.32 nm) of TiO 2 rutile phase.Moreover, the corresponding SAED patterns of films show the further planes of (101) and (211) rutile in the films deposited at d s-t distances of 6, 8 and 10 cm, whereas only (110) plane is observed in the film deposited at the d s-t distance of 12 cm due to the decrease of deposition energy when the d s-t distance is increased.
Figure 8 shows the refractive indices in the wavelength from 400 -750 nm of the deposited films as calculated from the optical transmission spectra using Swanepoel's method.The refractive indices were found to be in the range of 2.51 -2.82.The high refractive index was observed on films deposited at a low d s-t distance.Generally, the refractive index is proportional to the crystallinity of films.
Therefore, the higher crystallinity of film deposited at d s-t distance of 6 cm exhibited the high refractive index.

Conclusion
The rutile TiO 2 films were deposited on unheated Si (110) wafers, glass slides
structure of the films was characterized by X-ray diffraction technique in thin film mode (TF-XRD, Rigaku, RINT-2100) adjusted with CuKα radiation, 40 kV, 40 mA at a step of 2θ = 2˚ min-1 and a 3˚ glancing angle against the incident beam in the 2θ range of 20˚ -65˚.The crystal orientation of the films was investigated by transmission electronmicroscopy (TEM, Jeol JEM-2100) working at 160 kV.An atomic force microscopy (AFM, Veeco) was achieved with a Digital

Figure 4 Figure 4 .
Figure4shows the XRD-patterns of TiO 2 thin films deposited at different d s-t distances of 6, 8, 10 and 12 cm.A single phase of rutile was clearly observed and could be indexed as R (110) plane according to JCPDS file No. 88-1175.Moreover, the crystallinity of rutile increased with decreasing d s-t distances.This is because the magnetic field strength of the dual magnetron cathode is higher than

Figure 5 .
Figure 5. Photo of the Ar gas plasma at a pressure of 2.0 × 10 −3 mbar and a sputtering power of 250 W.

Figure 6 .
Figure 6.Thickness and surface root mean square roughness (R rms ) of TiO 2 films deposited by dual cathode at various ds-t distances.

Figure 7 .
Figure 7. (a) HRTEM images and (b) SAED images of TiO 2 films deposited at various d s-t distances.

Figure 8 .
Figure 8. Refractive index (n) of TiO 2 films deposited at various d s-t distances.
et al.