147

Trans. Mat. Res. Soc. Japan 37[2] 147- 150 (2012)

147

Preparation of a titanium thin film using a sputtering deposition process with a powder material target

Hiroharu Kawasaki*1, Tamiko Ohshima*1, Kento Arafune*1,Yoshihito Yagyu*1, Yoshiaki Suda*1

*1Sasebo National College of Technology, 1-1 Okishin, Sasebo, Nagasaki 857-1193

Fax: 81-956-34-8409, e-mail:h-kawasa@post.cc.sasebo.ac.jp

Titanium (Ti) thin films were produced using a high frequency magnetron sputtering method with a Ti powder material target, and the processing plasma was analyzed via an optical emission method. Atomic force microscopic images of the films prepared using Ti powder targets shownearly the same images as the film prepared using Ti bulk target. XRD and XPS measurementssuggest that titanium oxide thin films can be prepared using Ti powder targets, and their properties depend on the substrate temperature and the argon and oxygen gas mixture. Mono-atomic neutral tungsten atoms and ions were identified by optical emission spectroscopy.

Key words: powder target, sputtering, thin film, titanium

1. INTRODUCTIONPhysical vapor deposition (PVD) is a

common film deposition methods, and its deposition mechanisms have been thoroughly studied [1-12]. In particular, sputteringdeposition and pulsed laser deposition (PLD) have become widely used techniques for the deposition of thin films because of the advantages of a simple system setup, wide range of deposition conditions, wider choice of materialsand higher instantaneous deposition rates.Sputtering deposition is a method for vapor deposition of thin films using an ion sputtered material from a bulk target, that is depositiononto a substrate. Sputtering deposition has been used to prepare large area uniform amorphous and crystalline thin films, for applications such as solar cells, liquid panel displays andphotocatalysts. The PLD method uses a high power pulsed laser beam that is focused inside a vacuum chamber such that it strikes a target of the material. It is well known that this methodshows high reproducibility for the preparation ofcrystalline thin films. Because of its versatility, we have been able to develop several kinds of functional thin films, such as tungsten carbide, silicon carbide, chromium carbide, titanium carbide, cubic boron nitride, carbon nitride and silicon nitride using the PVD method [13-23].

In these PVD methods, high density bulk targets, (>3g/cm3 and >95% in density) are generally used. Therefore, when producing a thin film with certain element ingredients using thismethod, it is necessary to make new targets by other methods, such as spark plasma sintering

(SPS). The target cooling method which raised the hardness using very low temperature liquid nitrogen and liquid helium, has also successfully applied to the preparation of functional thin films, including an organic electroluminescence thin film. However, these target making systems require a lot of time, and they are usually expensive. These PVD film deposition methods;therefore, may become more attractive if powdermaterial targets can be used.

Sputtering deposition using powder targetshas been applied to the preparation of magnet-optics thin films. Kajima et al prepared ferromagnetic nanocomposite oxide sputtered films with a Bi2O3-Fe2O3-PbTiO3 ternary systemusing powder targets [24]. They suggested that the prepared films using the powder targets worked well as thin film capacitors. However, the film properties, such as the crystallinity, composition ratio, hardness, roughness of the film surface, and adhesion between the films and the substrate, were not studied. In addition, important plasma parameters, such as the electron temperature, electron density, and emission species were not investigated. As the result, the mechanisms of thin film deposition using powder targets have not been extensively explored.

In this paper, titanium (Ti) thin films were prepared using a sputtering deposition method with Ti powder and bulk Ti targets, and the film qualities, such as crystallinity, composition ratio and surface roughness were compared. In the sputtering process, the emission spectra were measured using monochromator. Crystallinity, composition ratio and surface roughness of the

148 Preparation of a titanium thin film using a sputtering deposition process with a powder material target148

films were measured by XRD, XPS and AFM. Based on these results, the mechanisms of thin film deposition using powder targets were explored.

2. EXPERIMENTALA schematic of the film preparation setup and the

measurement characteristics of the RF plasma are shown in Fig. 1. The deposition chamber (ANELVA :PRF-065B) was made by stainless steel. First, the powder targets were set up on a stainless steel target holder. In this experiment, rutile Ti powder (99.95%) of 45 µm diameter was used for the powder targets. Thetarget holders were covered with the Ti powder such that the powder was level on target. The size of the powder target in the target holder was nearly the same as the bulk Ti target (99.9%). Next, the chamber was evacuated to 1 Pa using a rotary pump, and then to a base pressure of 4×10-4 Pa using a turbomolecular pump and rotary pump. The deposition total pressure (P) was 10 Pa and consisited of a mixture of oxygen (O2:99.99%) and argon (Ar:99.99%) at a flow rate of 10 sccm. An RF magnetron sputtering plasma was generated at a frequency of 13.56 MHz and 100 W in discharge RF power (Prf). The Si(100) substrates located 5.0 cm from the target. They were cleaned by repeated bathing in an ultrasonic agitator and then rinsed in high-purity deionized water prior to loading into the deposition chamber. The deposition time was fixed to 120 min, the substrate temperature (Ts) was increased from room temperature to 700°C, and the gas mixture was variedfrom Ar:O2=10:0 - 5:5.

The crystalline structure and crystallographic orientation of the thin films were characterized by XRD (RIGAKU;RINT2100V) using CuKα radiation. The composition of the films was measured by XPS (JEOL;JPS9010). The surface morphology of the films was observed using an atomic force microscope (AFM)(JOEL;JSPM4210).

Optical emission spectra were recorded through a spectrometer (Hamamatsu C5095) equipped with an ICCD camera (Hamamatsu C7164-03) after integrating 16 times. The deposition conditions are shown in Table 1.

Table 1. Deposition conditionsTarget Ti bulk(99.5%),

Ti Powder (45mmφ, 99.5%)Substrate Si (100)

Substrate Temp. Room Temp. , 400℃, 700℃Base Pressure 4×10

-4 Pa

Gas ArPressure 10 Pa

Gas flow rate 10 sccmRF Power 100 W

3. RESULTS AND DISCUSSION3.1 Surface morphology of the prepared film

Photographs of the films prepared using (a)a Ti bulk target and (b)a Ti powder target are shown in Fig. 2. Both the photographs looks nearly similar. The surface morphology of the film prepared using the Ti powder target at room temperature in pure Ar gas as measured by AFMis shown in Figure 3. The measurement was performed in non-contact AFM mode. The resultssuggest that the film is smooth and the mean roughness is several nm. In addition, the film wasfound to be composed of small particles of 30~50 nm diameter. These properties are also very

Fig. 1 Experimantal setup.

(a)Bulk target (b)Powder target

Fig. 2 Photograph of the film prepared on a Si substrate. Prf = 100 W, Ts = 700°C, P = 10 Pa and Ar:O2 = 9:1.

Fig. 3 Surface morphology of the film prepared using a Ti powder target at room temperature inpure Ar gas measured by AFM. Prf = 100 W, P =10 Pa and Ar:O2 =5:5.

149H. Kawasaki et al. Trans. Mat. Res. Soc. Japan 37[2] 147- 150 (2012)

similar to those observed for the film prepared using the Ti bulk target at room temperature, not shown here.

3.2 XRD measurement resultFigure 4 shows the dependence of the XRD

patterns of the films prepared using the Ti powdertargets on the substrate temperature. The Ar:O2

ratio of the gas mixture was 9:1, and total gas pressure was 10 Pa. As can be observed in the figure, the crystallinity of the films strongly depended on the substrate temperature (Ts). Atroom temperature, there is no crystalline peakexcept for the Si substrate. These suggest that the film prepared under these conditions is amorphous. At a substrate temperature of 400°C,several anatase peaks for TiO2 can be observed,while at a substrate temperature of 700°C, several rutile peaks for TiO2 can be observed. Thedependence of the XRD patterns of the films using the Ti powder target on the gas mixture is shown in Fig. 5. The substrate temperature was 700°C, and the total gas pressure was 10 Pa. In

pure Ar gas, TiO(200) peaks can be observedalong with the Si substrate peaks, however, rutile TiO2 peaks increased with increasing O2 content in the gas mixture. These results suggest that the oxygen atomic concentration of the films depends on the O2 gas mixture.

3.2 XPS measurement resultsTo investigate the oxygen concentration

ratio of the films, XPS analyses were carried out to determine the composition of the films and identify the valence states of the various species present therein. The XPS spectra for the Ti 2p peaks of the films after sputtering are shown in Fig. 6. The films were etched by sputtering with the Ar ion bombardment with an acceleration of600 V, and etching number shown in Fig. 6 represents 60 s per 1 time. The films were prepared in an argon and oxygen gas mixture of Ar:O2=9:1.

The Ti 2p spectra show a doublet structure with slight asymmetry. The binding energies of the Ti 2p transition correspond to the valence states obtained from the deconvoluted spectra.The spectrum at the surface of the prepared filmsshows the presence of one large peak for the Ti 2p incorporated with TiO2(460 eV) ; however, the binding energy of approximately 460 eV is 1.7 eV higher than that of the Ti state in TiO2(458.3 eV)with increasing etching times. This result may be because of the incorporation of other gases into the TiO2 films.

3.3 Optical emission spectrumThe optical emission spectrum of the RF

plasma at 10 Pa under pure O2 gas using the Ti powder target is shown in Fig. 7. Tungsten atoms(Ti I) and Ti ions (Ti II) can mainly be detected [12]. The OES results showed the presence of

Fig. 6 XPS spectra of the Ti 2p peaks of the filmsafter sputtering. Prf = 100 W, Ts = 700°C, P = 10Pa and Ar:O2 = 9:1.

Fig. 4 Dependence of the patterns of the films prepared using the Ti powder targets on the substrate temperature. Prf=100W, P=10Pa and Ar:O2=9:1.

Fig. 5 Dependence of the XRD patterns of the films prepared using the Ti powder target on the gas mixture. Prf = 100 W, Ts = 700°C and P = 10Pa.

mono-atomic neutral tungsten atoms and ions, but neutral O atoms and molecular O species are not detected under these conditions.

4. CONCLUSIONTitanium thin films were produced using an

RF magnetron sputtering method with a Tipowder material target and the processing plasma was analyzed using the optical emission method.AFM images of the films prepared using Ti powder targets show nearly the same images as those of the film prepared using a Ti bulk target. XRD and XPS measurements suggest that titanium oxide thin films can be prepared using Ti powder targets, and their properties depend on the substrate temperature and argon and oxygen gas mixture. Mono-atomic neutral tungsten atoms and ions were identified by optical emission spectroscopy

5. AcknowledgementsThis work was supported in part by a

Grant-in-Aid for Scientific Research in Priority Areas (B) (No.23340181), a Grant-in-Aid for Scientific Research on Innovative Areas (No.22110519), Nippon Sheet Glass Foundation for Materials Science and Engineering, and the Circle for the Promotion of Science and Engineering. The authors wish to thank Prof. H. Fujiyama of Nagasaki University, Prof Shiratani of Kyusyu University and Prof. Inoue of Toyohasi University of Technology for their helpful discussions.

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Fig. 7 Optical emission spectrum of the RF plasma at 10 Pa and pure O2 gas using the Ti powder target.Prf = 100 W, Ts = 700°C, P = 10 Pa and Ar:O2 =9:1.

150 Preparation of a titanium thin film using a sputtering deposition process with a powder material target

(Received January 3, 2012; Accepted March 7, 2012)