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The structure and thermal stability of TiO2 grown by the plasma oxidation of sputtered metallic Ti thin films
Chemical Physics Letters 395 (2004) 259–263 www.elsevier.com/locate/cplett
The structure and thermal stability of TiO2 grown by the plasma oxidation of sputtered metallic Ti thin films Gang He *, Qi Fang, Liqiang Zhu, Mao Liu, Lide Zhang Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, PR China Received 18 June 2004; in final form 28 July 2004 Available online 20 August 2004
Abstract High-k TiO2 thin films have been fabricated by plasma oxidation of sputtered Ti films. The structure and thermal stability characteristics for TiO2 dielectrics were investigated. X-ray diffraction and Raman spectroscopy analysis show that the as-grown films are amorphous, and the O2-annealed TiO2 films undergo a transformation of anatase to rutile phase with increase of the annealing temperature. Thickness and optical constants of TiO2 films correlating annealing temperature have been determined by Spectroscopic Ellipsometer. By Fourier transform infrared spectroscopy (FTIR) characterization, the growth and properties of the interfacial SiO2 layer at the TiO2/Si interface were observed. The growth mechanism of the interfacial layer was discussed. 2004 Elsevier B.V. All rights reserved.
1. Introduction Aggressive scaling has led to silicon dioxide gate die˚ in state-of-the-art CMOS techlectrics as thin as 15 A nologies. As a consequence, static leakage power due to direct tunnelling through the gate oxide has been increasing at an exponential rate [1]. As an alternative to continuing to scale SiO2, recent efforts have focused on the development of alternative high-k gate oxides [2–5]. Unfortunately, many of the high-k materials are not thermally stable on silicon. The formation of SiO2 or metal silicates often occurs when these materials are grown on silicon or during subsequent annealing. Among many potential high-k candidates, TiO2 has attracted attention to replace conventional SiO2 for future complementary metal-oxide-semiconductor integrated circuit due to its relatively high dielectric constant [6] and superior interface thermal stability in contact with Si [7]. To date, many methods have been used to deposit ultrathin TiO2 films, such as reactive sputtering [8], plasma enhanced chemical vapor deposition (PECVD) [9], and *
Corresponding author. Fax: +860 551 5591434. E-mail address: [email protected] (G. He).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.090
atomic layer deposition (ALD) [10,11]. One fatal shortcoming of these deposition methods is the uncontrollable growth of low-k interfacial layer due to the oxidation of Si substrate surface. CVD processes are also suffering from contaminants that need to be removed by a high temperature deposition or a post-deposition annealing. Thus, it is difficult to obtain stoichiometric TiO2 films suppressing the interfacial layer growth. It is crucial to develop controllable low temperature processes for TiO2 preparation. In this letter, we prepared high-k TiO2thin films by means of plasma oxidation of sputtered metallic Ti films and systematically investigated the structure and thermal stability of TiO2 films in relation to annealing temperature.
2. Experimental 2.1. Preparation of TiO2 films Before deposition, n-type Si (100) substrates with a resistivity of 2–5 X cm were cleaned by a modified
G. He et al. / Chemical Physics Letters 395 (2004) 259–263
RCA process and dipped in 1% buffered HF solution to remove any native oxide and to hydrogen passivate the surface. After the chemical treatment process, the substrates were dried and put into the deposition chamber. Titanium films were prepared using a DC magnetron sputtering system with a cylindrical stainless-steel chamber connected to a turbomolecular pump. The sputtering target was a Ti disk of 99.99% purity with the diameter of 60 mm. The chamber was initially evacuated to 1.0 · 104 Pa, and ultra-high purity (99.999%) Ar was introduced to act as the sputtering enhancing gas. Prior to Ti deposition, the Ti metal target was pre-sputtered in an argon atmosphere for 10 min in order to remove surface oxide of the target, and then a Ti metal layer was pre-deposited on Si (100) substrates at room temperature with a constant dc power of 50 W. The sputtering time was 5 min under a working pressure of 0.2 Pa. After the Ti metal layer deposition, the wafers were transferred ex situ to plasma oxidation chamber and the Ti metal layer was oxidized by Ar/O2 plasma generated in the chamber. The plasma was inductively coupled plasma with an RF power of 2 kW, a frequency of 13.56 kHz and an Ar/O2 flow rate of 5. The plasma oxidation of sputtered Ti metal layer proceeded at 400 C for 2 min. In order to improve the qualities of the resultant TiO2 films, the as-grown TiO2 thin films were subjected to post-annealing treatment at temperatures ranging from 500 to 800 C for 5 min in O2. 2.2. Film characterization The film crystallinity was investigated by a Philips X-ray diffractometer (XRD) operating with Cu Ka radiation at 40 kV and 40 mA. The thickness and optical constants of the film were determined by using an ex situ spectroscopic phase modulated ellipsometer (Model UVISEL JOBIN-YVON) in the wavelength range of 200–800 nm. Analysis of the interfacial layer between the TiO2 films and the Si substrate was carried out on the as-grown and the as-annealed samples by Fourier transform infrared spectroscopy (FTIR) in a Nicolet Magna 750 operating in transmission mode. Raman scattering measurements excited with the 514.5 nm incident wavelength radiation were performed to detect and chemically identify the crystallization of the TiO2 films.
3. Results and discussion Fig. 1 shows the XRD patterns of the as-grown and O2-annealed TiO2 thin films. All the peaks are indexed according to standard JCPDS patterns for the TiO2 lattice. In Fig. 1a, the as-grown TiO2 films show a featureless diffraction, characteristics of amorphous phase due to the scattering of X-rays by the short-range order in the amorphous phase. This is due to reduced atom
R(110)
R(211)
R(220)
R(110)
A(101) Intensity (a.u)
260
(e)800˚C (d)700˚C R(220)
(c)600˚C (b) 500˚C (a) as-grown
20
30
40
50
60
70
80
2 Theta (degree) Fig. 1. XRD pattern of as-grown and annealed TiO2 films at different temperatures for 5 min in O2-containing atmosphere. (A: anatase, R: rutile).
mobility during low temperature plasma oxidation, associated with decreased kinetic energy of arriving species because of the collisions with residual gas molecules. However, the film remains amorphous after annealing at 500 C in O2. These results appear to be attributed to the formation of a large quantity of oxygen vacancies in the as-grown films and a sufficient oxygen supply was needed to crystallize the TiO2 films by post-annealing. Spectrum(c) in Fig. 1 shows a strong diffraction peak attributed to the anatase phase of TiO2 film and a weak diffraction peak corresponding to the (2 2 0) plane of rutile phase. With increase of annealing temperature, a transformation from anatase to rutile phase occurs. For the sample annealed at 700 C, an X-ray peak corresponding to the (1 1 0) plane of rutile phase is observed in spectrum(d), and the previous anatase phase disappears. These observations are in agreement with those reported by Chu and San-Yuan [12] for thermal oxidation derived TiO2 thin films, where they found it to be rutile type up to 700 C annealing temperature and above that (700–1000 C) a predominant rutile phase. A further increase to 800 C results in an increase in diffraction intensity with an associated decrease in the amorphous background of XRD patterns, indicating an improved crystallinity of the films. By using a simple optical model consisting of a threelayer stack structure: c-Si/SiO2/TiO2, the thickness and the optical constants, i.e., refractive index and extinction coefficient, of the TiO2 films were calculated, which based on the best fit between the experimental and the simulated spectra. Fig. 2 shows the refractive index and extinction coefficient calculated from the extracted best-fit parameters of the model for the TiO2 films. It is obvious that there is a gradual increase in n with annealing temperature, which is attributed to the increase in packing density and the formation of the rutile
Extinction coefficient(k)
Refractive index(n)
G. He et al. / Chemical Physics Letters 395 (2004) 259–263 9 8 7 6 5 4 3 2 1 0 -1 -2 -3 -4
800˚C 700˚C 500˚C as-grown
300 350 400 450 500 550 600 650 700 750 800 Wavelengh (nm)
Fig. 2. Refractive indices and extinction coefficients calculated from the extracted best-fit parameters of a three-layer optical model for the TiO2 films.
phase along with the anatase at the elevated temperature, evidenced by the X-ray analysis. It is believed that the as-grown film has a lower packing density because of a loose arrangement. The high temperature annealing increases the mobility of the atoms or molecules of the film and favors the formation of more closely packed thin films, which leads to a higher refractive index. It is worth noting that the nonzero extinction coefficient for all the samples at lower photon energy indicates a scattering effect in the nanocrystalline TiO2 film which increases for high temperature annealing samples due to larger particle size. Fig. 3 shows the Raman spectra of the as-grown and the as-annealed TiO2 films at various temperatures. The most relevant features occur between 100 and 800 cm1. The intense emission bands at 520 cm1 are related to the longitudinal-optical (LO) phonons of the Si sub-
Intensity (a.u)
Si (100)
(d) 800˚C (c) 700˚C (b) 500˚C (a) as-grown
100
200
300
400
500
600
700
800
Wavenumber (cm-1) Fig. 3. Raman spectra of as-grown and annealed TiO2 thin films.
261
strate. Since orientation of the Si substrate was (100), the LO mode of Si is allowed according to the selection rules [13]. No peak due to crystalline TiO2 phase can be seen in the spectra of the as-grown TiO2 films, which indicates that the as-grown TiO2 films is in amorphous state, as confirmed by XRD. For the sample annealed at 700 C, emission bands (in addition to the 448 and 612 cm1 attributed to the rutile phase of TiO2) typical of the anatase form appear at 142,396 and 636 cm1,which are assignable to Eg, B1g and Eg modes, respectively [14]. With increase of annealing temperature, emission bands at 142,396 and 638 cm1 disappear, only 448 (Eg) and 614 cm1 (A1g) remained. Kin et al. [15] have demonstrated that the peaks occurring at 144, 236.5, 450 and 611.5 cm1 was considered to be due to the rutile TiO2 longitudinal optical phonons. It is noteworthy that the transformation of the TiO2 film structure from anatase to rutile phase occurs by increasing the annealing temperature. This result is in agreement with that confirmed by XRD above. The challenge facing high-k thin films is the formation of an interfacial SiO2-like layer with a relatively low dielectric constant due to the oxidation of the Si substrate surface. The amorphous SiO2 layer on silicon leaves dangling bonbs that may result in electronic defects disrupting translational symmetry at the interface. ItÕs desirable to minimize the thickness of any low-k SiO2 layer between the high-k film and the Si substrate. In our work, we investigated the interfacial layer formation information by using FTIR. The technique, based on the absorption of light in the infrared region of the spectrum, is sensitive to rotational, bending and stretching vibrational modes and provides additional information on the chemical nature of the interfacial region, in particular, the presence of SiO2 [16]. Fig. 4 illustrates examples of infrared absorption spectra in the 400–1200 cm1 range obtained for TiO2/Si samples. The samples are as-grown and postannealed at 500–800 C for 5 min in O2. In order to ensure that SiAO bonding is associated with the interfacial layer, the infrared absorption spectra were obtained with the backside of all wafers etched in diluted HF solution to remove any native oxide and subtract the absorbance of the sample and a reference from the same wafer with no TiO2 present. As a result, we detected the transverse optical (TO) vibrational modes from the interfacial SiO2 films. The weak absorption band at 1000–1200 cm1 observed in the O2-annealed samples is due to the SiAO bond stretching mode in the interfacial layer. For the as-grown TiO2 film, there is an absorption band centered at 1107 cm1. According to the report from [3], for the Si wafers as substrates, a strong absorption centered at 1105 cm1 attributed to the interstitial oxygen in the Si bulk. Therefore, we think that the characteristic absorbance is associated with interstitial oxygen in the
262
G. He et al. / Chemical Physics Letters 395 (2004) 259–263
Si-O
Absorbance (a.u)
(d) 800˚C
(c) 700˚C
(b) 500˚C (a) as-grown 400
500
600
700
800
900
1000
1100
1200
interface and directly reacts in the interfacial region. Annealing in oxygen ambient, partial oxygen radicals (ions and molecule ions) with high-energy might also diffuse into the Si substrate without reacting with the residual metallic Ti, and form the SiO2 interfacial layer. The FTIR data provide the evidence for the annealing-induced interaction of the Si substrate with oxygen from annealing ambient resulting in the production of increased numbers of SiAOASi bonds. Meanwhile, high temperature annealing-induced release of interstitial oxygen in the interfacial region brings about the decomposition of partial interfacial SiO2.This process can be described by the following chemical reaction:
Wavenumber (cm-1) Fig. 4. Annealing temperature dependence of absorption peak position of SiAOASi bonds by FTIR for TiO2 films in O2: (a) as-grown, (b) annealed TiO2 at 500 C, (c) annealed TiO2 at 700 C, (d) annealed TiO2 at 800 C.
Si substrate. Plasma oxidation at low temperature leads to Ti ! TiO2, but there exists some residual Ti component in the TiO2 films due to not full oxidation. Meanwhile, in RF plasma oxidation, the self-bias voltage creates an electrical field across the oxide which, in turn, restrains the thermal diffusion of O1 throughout a TiO2 film much lower than normal thermal diffusion, the plasma oxygen can not diffuse to react with underlying substrate. So the interstitial oxygen instead of outside oxygen attributes to the absorption band centered at 1107 cm1. It can be seen that as the anneal conditions move from lower to higher temperature, the absorption band centered at 1107 cm1 disappears, a new broad absorption band centered near 1050 cm1 which can be associated with the SiAO stretching mode of the oxide located at the TiO2/Si interface appears. However, for the sample annealed at 500 C, where the absorption is very weak due to much lower thickness of the interfacial layer formed. It is noted that there is a constant increase in the intensity of the peak located around 1050 cm1 as well as a slight shift toward lower wavenumber with increase of annealing temperature. According to our result, we give our explanation as follow: Higher temperature annealing ensures the release of inset oxygen. It has been generally believed that the presence of a thin film SiO2 on the Si wafer may result from remnant oxide left after imperfect wafer cleaning prior to deposition or due to oxidation during the deposition process resulting from excited O neutrals in the plasma [17]. However, we think that the source of oxygen for the interfacial layer is a result of a combination of oxygen diffusing through the sample from the annealing atmosphere as well as oxygen that is inherently trapped in the Si substrate as an interstitial species that can migrate to the Si-film
Si þ O2 ðambientÞ þ Ox ðinsetÞ ! SiO2
ð1Þ
SiO2 ! Ox " þSiOx
ð2Þ
The decomposition of interfacial SiO2 leads to the production of the presence of Si suboxides near the Si/ SiO2 interface and attributes to the shifting of SiAO bonding vibrational peak position. So that we can see peak shift toward lower wavenumbers.
4. Conclusion In conclusion, by subjecting TiO2 films prepared by the plasma oxidation of sputtered Ti films on the Si substrate to oxygen post-deposition anneal, the structure and thermal stability in relation to annealing temperature were investigated. X-ray diffraction and Raman spectroscopy analysis reveal that the as-grown TiO2 films are amorphous, and O2-annealed samples suffer a transformation of anatase to rutile phase by increasing the annealing temperature. Spectroscopic Ellipsometer has demonstrates that there is gradual increase in n with annealing temperature and a nonzero extinction coefficient for the high temperature annealing samples. FTIR measurements have confirmed that the presence of the interfacial SiO2 layer for O2-annealed films. The growth of the interfacial layer is attributed to a combination of oxygen diffusion from the annealing ambient as well as oxygen trapped in the Si substrate as an interstitial species. High temperature annealing-induced decomposition of partial SiO2 layer brings about the slight shift of SiAO bonds vibration peak position towards lower wavenumbers.
Acknowledgements This work was supported by the National Key Project of Fundamental Research for Nanomaterials and Nanostructures and Hundred-talent Project of CAS.
G. He et al. / Chemical Physics Letters 395 (2004) 259–263
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Technical Digest, San Francisco, USA, December 6–9, 1998, p. 797. W.W. Xu, R. Kershaw, K. Dwigh, A. Wold, Mater. Res. Bull. 25 (1990) 1385. J. Aarik, A. Aidla, H. Mandar, V. Sammelselg, J. Cryst. Growth 220 (2000) 531. K. Kukli, A. Aidla, J. Aarik, M. Schuisky, A. Harsta, M. Ritala, M. Leskela, Langmuir 16 (2000) 8122. C.-C. Ting, S.-Y. Chen, D.-M. Liu, Thin Solid Films 102 (2002) 290. P.M. Airtharaj, F.H. Pollak, Appl. Phys. Lett. 45 (1984) 789. A. Turkovic, M. Ivanda, A. Drasner, V. Vranesa, M. Persin, Thin Solid Films 198 (1991) 199. T.W. Kim, M. Jung, H.J. Kim, T.H. Park, Y.S. Yoon, W.N. Kang, S.S. Yom, H.K. Na, Appl. Phys. Lett. 64 (1994) 1407. Y.J. Chabal, M.K. Weldon, V.E. Marsico, J. Phys. IV. 6 (1997). L. Koltunski, R.A.B. Devine, Appl. Phys. Lett. 79 (2001) 320.
The structure and thermal stability of TiO2 grown by the plasma oxidation of sputtered metallic Ti thin films Gang He *, Qi Fang, Liqiang Zhu, Mao Liu, Lide Zhang Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, PR China Received 18 June 2004; in final form 28 July 2004 Available online 20 August 2004
Abstract High-k TiO2 thin films have been fabricated by plasma oxidation of sputtered Ti films. The structure and thermal stability characteristics for TiO2 dielectrics were investigated. X-ray diffraction and Raman spectroscopy analysis show that the as-grown films are amorphous, and the O2-annealed TiO2 films undergo a transformation of anatase to rutile phase with increase of the annealing temperature. Thickness and optical constants of TiO2 films correlating annealing temperature have been determined by Spectroscopic Ellipsometer. By Fourier transform infrared spectroscopy (FTIR) characterization, the growth and properties of the interfacial SiO2 layer at the TiO2/Si interface were observed. The growth mechanism of the interfacial layer was discussed. 2004 Elsevier B.V. All rights reserved.
1. Introduction Aggressive scaling has led to silicon dioxide gate die˚ in state-of-the-art CMOS techlectrics as thin as 15 A nologies. As a consequence, static leakage power due to direct tunnelling through the gate oxide has been increasing at an exponential rate [1]. As an alternative to continuing to scale SiO2, recent efforts have focused on the development of alternative high-k gate oxides [2–5]. Unfortunately, many of the high-k materials are not thermally stable on silicon. The formation of SiO2 or metal silicates often occurs when these materials are grown on silicon or during subsequent annealing. Among many potential high-k candidates, TiO2 has attracted attention to replace conventional SiO2 for future complementary metal-oxide-semiconductor integrated circuit due to its relatively high dielectric constant [6] and superior interface thermal stability in contact with Si [7]. To date, many methods have been used to deposit ultrathin TiO2 films, such as reactive sputtering [8], plasma enhanced chemical vapor deposition (PECVD) [9], and *
Corresponding author. Fax: +860 551 5591434. E-mail address: [email protected] (G. He).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.090
atomic layer deposition (ALD) [10,11]. One fatal shortcoming of these deposition methods is the uncontrollable growth of low-k interfacial layer due to the oxidation of Si substrate surface. CVD processes are also suffering from contaminants that need to be removed by a high temperature deposition or a post-deposition annealing. Thus, it is difficult to obtain stoichiometric TiO2 films suppressing the interfacial layer growth. It is crucial to develop controllable low temperature processes for TiO2 preparation. In this letter, we prepared high-k TiO2thin films by means of plasma oxidation of sputtered metallic Ti films and systematically investigated the structure and thermal stability of TiO2 films in relation to annealing temperature.
2. Experimental 2.1. Preparation of TiO2 films Before deposition, n-type Si (100) substrates with a resistivity of 2–5 X cm were cleaned by a modified
G. He et al. / Chemical Physics Letters 395 (2004) 259–263
RCA process and dipped in 1% buffered HF solution to remove any native oxide and to hydrogen passivate the surface. After the chemical treatment process, the substrates were dried and put into the deposition chamber. Titanium films were prepared using a DC magnetron sputtering system with a cylindrical stainless-steel chamber connected to a turbomolecular pump. The sputtering target was a Ti disk of 99.99% purity with the diameter of 60 mm. The chamber was initially evacuated to 1.0 · 104 Pa, and ultra-high purity (99.999%) Ar was introduced to act as the sputtering enhancing gas. Prior to Ti deposition, the Ti metal target was pre-sputtered in an argon atmosphere for 10 min in order to remove surface oxide of the target, and then a Ti metal layer was pre-deposited on Si (100) substrates at room temperature with a constant dc power of 50 W. The sputtering time was 5 min under a working pressure of 0.2 Pa. After the Ti metal layer deposition, the wafers were transferred ex situ to plasma oxidation chamber and the Ti metal layer was oxidized by Ar/O2 plasma generated in the chamber. The plasma was inductively coupled plasma with an RF power of 2 kW, a frequency of 13.56 kHz and an Ar/O2 flow rate of 5. The plasma oxidation of sputtered Ti metal layer proceeded at 400 C for 2 min. In order to improve the qualities of the resultant TiO2 films, the as-grown TiO2 thin films were subjected to post-annealing treatment at temperatures ranging from 500 to 800 C for 5 min in O2. 2.2. Film characterization The film crystallinity was investigated by a Philips X-ray diffractometer (XRD) operating with Cu Ka radiation at 40 kV and 40 mA. The thickness and optical constants of the film were determined by using an ex situ spectroscopic phase modulated ellipsometer (Model UVISEL JOBIN-YVON) in the wavelength range of 200–800 nm. Analysis of the interfacial layer between the TiO2 films and the Si substrate was carried out on the as-grown and the as-annealed samples by Fourier transform infrared spectroscopy (FTIR) in a Nicolet Magna 750 operating in transmission mode. Raman scattering measurements excited with the 514.5 nm incident wavelength radiation were performed to detect and chemically identify the crystallization of the TiO2 films.
3. Results and discussion Fig. 1 shows the XRD patterns of the as-grown and O2-annealed TiO2 thin films. All the peaks are indexed according to standard JCPDS patterns for the TiO2 lattice. In Fig. 1a, the as-grown TiO2 films show a featureless diffraction, characteristics of amorphous phase due to the scattering of X-rays by the short-range order in the amorphous phase. This is due to reduced atom
R(110)
R(211)
R(220)
R(110)
A(101) Intensity (a.u)
260
(e)800˚C (d)700˚C R(220)
(c)600˚C (b) 500˚C (a) as-grown
20
30
40
50
60
70
80
2 Theta (degree) Fig. 1. XRD pattern of as-grown and annealed TiO2 films at different temperatures for 5 min in O2-containing atmosphere. (A: anatase, R: rutile).
mobility during low temperature plasma oxidation, associated with decreased kinetic energy of arriving species because of the collisions with residual gas molecules. However, the film remains amorphous after annealing at 500 C in O2. These results appear to be attributed to the formation of a large quantity of oxygen vacancies in the as-grown films and a sufficient oxygen supply was needed to crystallize the TiO2 films by post-annealing. Spectrum(c) in Fig. 1 shows a strong diffraction peak attributed to the anatase phase of TiO2 film and a weak diffraction peak corresponding to the (2 2 0) plane of rutile phase. With increase of annealing temperature, a transformation from anatase to rutile phase occurs. For the sample annealed at 700 C, an X-ray peak corresponding to the (1 1 0) plane of rutile phase is observed in spectrum(d), and the previous anatase phase disappears. These observations are in agreement with those reported by Chu and San-Yuan [12] for thermal oxidation derived TiO2 thin films, where they found it to be rutile type up to 700 C annealing temperature and above that (700–1000 C) a predominant rutile phase. A further increase to 800 C results in an increase in diffraction intensity with an associated decrease in the amorphous background of XRD patterns, indicating an improved crystallinity of the films. By using a simple optical model consisting of a threelayer stack structure: c-Si/SiO2/TiO2, the thickness and the optical constants, i.e., refractive index and extinction coefficient, of the TiO2 films were calculated, which based on the best fit between the experimental and the simulated spectra. Fig. 2 shows the refractive index and extinction coefficient calculated from the extracted best-fit parameters of the model for the TiO2 films. It is obvious that there is a gradual increase in n with annealing temperature, which is attributed to the increase in packing density and the formation of the rutile
Extinction coefficient(k)
Refractive index(n)
G. He et al. / Chemical Physics Letters 395 (2004) 259–263 9 8 7 6 5 4 3 2 1 0 -1 -2 -3 -4
800˚C 700˚C 500˚C as-grown
300 350 400 450 500 550 600 650 700 750 800 Wavelengh (nm)
Fig. 2. Refractive indices and extinction coefficients calculated from the extracted best-fit parameters of a three-layer optical model for the TiO2 films.
phase along with the anatase at the elevated temperature, evidenced by the X-ray analysis. It is believed that the as-grown film has a lower packing density because of a loose arrangement. The high temperature annealing increases the mobility of the atoms or molecules of the film and favors the formation of more closely packed thin films, which leads to a higher refractive index. It is worth noting that the nonzero extinction coefficient for all the samples at lower photon energy indicates a scattering effect in the nanocrystalline TiO2 film which increases for high temperature annealing samples due to larger particle size. Fig. 3 shows the Raman spectra of the as-grown and the as-annealed TiO2 films at various temperatures. The most relevant features occur between 100 and 800 cm1. The intense emission bands at 520 cm1 are related to the longitudinal-optical (LO) phonons of the Si sub-
Intensity (a.u)
Si (100)
(d) 800˚C (c) 700˚C (b) 500˚C (a) as-grown
100
200
300
400
500
600
700
800
Wavenumber (cm-1) Fig. 3. Raman spectra of as-grown and annealed TiO2 thin films.
261
strate. Since orientation of the Si substrate was (100), the LO mode of Si is allowed according to the selection rules [13]. No peak due to crystalline TiO2 phase can be seen in the spectra of the as-grown TiO2 films, which indicates that the as-grown TiO2 films is in amorphous state, as confirmed by XRD. For the sample annealed at 700 C, emission bands (in addition to the 448 and 612 cm1 attributed to the rutile phase of TiO2) typical of the anatase form appear at 142,396 and 636 cm1,which are assignable to Eg, B1g and Eg modes, respectively [14]. With increase of annealing temperature, emission bands at 142,396 and 638 cm1 disappear, only 448 (Eg) and 614 cm1 (A1g) remained. Kin et al. [15] have demonstrated that the peaks occurring at 144, 236.5, 450 and 611.5 cm1 was considered to be due to the rutile TiO2 longitudinal optical phonons. It is noteworthy that the transformation of the TiO2 film structure from anatase to rutile phase occurs by increasing the annealing temperature. This result is in agreement with that confirmed by XRD above. The challenge facing high-k thin films is the formation of an interfacial SiO2-like layer with a relatively low dielectric constant due to the oxidation of the Si substrate surface. The amorphous SiO2 layer on silicon leaves dangling bonbs that may result in electronic defects disrupting translational symmetry at the interface. ItÕs desirable to minimize the thickness of any low-k SiO2 layer between the high-k film and the Si substrate. In our work, we investigated the interfacial layer formation information by using FTIR. The technique, based on the absorption of light in the infrared region of the spectrum, is sensitive to rotational, bending and stretching vibrational modes and provides additional information on the chemical nature of the interfacial region, in particular, the presence of SiO2 [16]. Fig. 4 illustrates examples of infrared absorption spectra in the 400–1200 cm1 range obtained for TiO2/Si samples. The samples are as-grown and postannealed at 500–800 C for 5 min in O2. In order to ensure that SiAO bonding is associated with the interfacial layer, the infrared absorption spectra were obtained with the backside of all wafers etched in diluted HF solution to remove any native oxide and subtract the absorbance of the sample and a reference from the same wafer with no TiO2 present. As a result, we detected the transverse optical (TO) vibrational modes from the interfacial SiO2 films. The weak absorption band at 1000–1200 cm1 observed in the O2-annealed samples is due to the SiAO bond stretching mode in the interfacial layer. For the as-grown TiO2 film, there is an absorption band centered at 1107 cm1. According to the report from [3], for the Si wafers as substrates, a strong absorption centered at 1105 cm1 attributed to the interstitial oxygen in the Si bulk. Therefore, we think that the characteristic absorbance is associated with interstitial oxygen in the
262
G. He et al. / Chemical Physics Letters 395 (2004) 259–263
Si-O
Absorbance (a.u)
(d) 800˚C
(c) 700˚C
(b) 500˚C (a) as-grown 400
500
600
700
800
900
1000
1100
1200
interface and directly reacts in the interfacial region. Annealing in oxygen ambient, partial oxygen radicals (ions and molecule ions) with high-energy might also diffuse into the Si substrate without reacting with the residual metallic Ti, and form the SiO2 interfacial layer. The FTIR data provide the evidence for the annealing-induced interaction of the Si substrate with oxygen from annealing ambient resulting in the production of increased numbers of SiAOASi bonds. Meanwhile, high temperature annealing-induced release of interstitial oxygen in the interfacial region brings about the decomposition of partial interfacial SiO2.This process can be described by the following chemical reaction:
Wavenumber (cm-1) Fig. 4. Annealing temperature dependence of absorption peak position of SiAOASi bonds by FTIR for TiO2 films in O2: (a) as-grown, (b) annealed TiO2 at 500 C, (c) annealed TiO2 at 700 C, (d) annealed TiO2 at 800 C.
Si substrate. Plasma oxidation at low temperature leads to Ti ! TiO2, but there exists some residual Ti component in the TiO2 films due to not full oxidation. Meanwhile, in RF plasma oxidation, the self-bias voltage creates an electrical field across the oxide which, in turn, restrains the thermal diffusion of O1 throughout a TiO2 film much lower than normal thermal diffusion, the plasma oxygen can not diffuse to react with underlying substrate. So the interstitial oxygen instead of outside oxygen attributes to the absorption band centered at 1107 cm1. It can be seen that as the anneal conditions move from lower to higher temperature, the absorption band centered at 1107 cm1 disappears, a new broad absorption band centered near 1050 cm1 which can be associated with the SiAO stretching mode of the oxide located at the TiO2/Si interface appears. However, for the sample annealed at 500 C, where the absorption is very weak due to much lower thickness of the interfacial layer formed. It is noted that there is a constant increase in the intensity of the peak located around 1050 cm1 as well as a slight shift toward lower wavenumber with increase of annealing temperature. According to our result, we give our explanation as follow: Higher temperature annealing ensures the release of inset oxygen. It has been generally believed that the presence of a thin film SiO2 on the Si wafer may result from remnant oxide left after imperfect wafer cleaning prior to deposition or due to oxidation during the deposition process resulting from excited O neutrals in the plasma [17]. However, we think that the source of oxygen for the interfacial layer is a result of a combination of oxygen diffusing through the sample from the annealing atmosphere as well as oxygen that is inherently trapped in the Si substrate as an interstitial species that can migrate to the Si-film
Si þ O2 ðambientÞ þ Ox ðinsetÞ ! SiO2
ð1Þ
SiO2 ! Ox " þSiOx
ð2Þ
The decomposition of interfacial SiO2 leads to the production of the presence of Si suboxides near the Si/ SiO2 interface and attributes to the shifting of SiAO bonding vibrational peak position. So that we can see peak shift toward lower wavenumbers.
4. Conclusion In conclusion, by subjecting TiO2 films prepared by the plasma oxidation of sputtered Ti films on the Si substrate to oxygen post-deposition anneal, the structure and thermal stability in relation to annealing temperature were investigated. X-ray diffraction and Raman spectroscopy analysis reveal that the as-grown TiO2 films are amorphous, and O2-annealed samples suffer a transformation of anatase to rutile phase by increasing the annealing temperature. Spectroscopic Ellipsometer has demonstrates that there is gradual increase in n with annealing temperature and a nonzero extinction coefficient for the high temperature annealing samples. FTIR measurements have confirmed that the presence of the interfacial SiO2 layer for O2-annealed films. The growth of the interfacial layer is attributed to a combination of oxygen diffusion from the annealing ambient as well as oxygen trapped in the Si substrate as an interstitial species. High temperature annealing-induced decomposition of partial SiO2 layer brings about the slight shift of SiAO bonds vibration peak position towards lower wavenumbers.
Acknowledgements This work was supported by the National Key Project of Fundamental Research for Nanomaterials and Nanostructures and Hundred-talent Project of CAS.
G. He et al. / Chemical Physics Letters 395 (2004) 259–263
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