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Oxidation of tungsten surface with reactive oxygen plasma
Nuclear Instruments and Methods in Physics Research B 232 (2005) 358–361 www.elsevier.com/locate/nimb
Oxidation of tungsten surface with reactive oxygen plasma Andriy Romanyuk a
a,*
, Viktor Melnik b, Peter Oelhafen
a
Institute of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland b IHP-Microelectronics, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany Available online 29 April 2005
Abstract We report on the oxidation of tungsten at different temperatures with an oxygen plasma studied with photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass-spectrometry (ToF-SIMS). The oxidation of the tungsten surface has been performed in a low-temperature oxygen plasma for different time intervals and various substrate temperatures. The stoichiometry of the formed oxide films was evaluated from O 1s and W 4f XPS peak ratio and was found to be independent of the plasma exposure time and the substrate temperature in the investigated range. The ToF-SIMS results show an increased thickness of the surface oxide layer upon increased substrate temperature during oxidation. The raise the substrate temperature in the process of oxide formation also causes the total amount of hydrogen and water bonded in the film structure to be reduced. 2005 Elsevier B.V. All rights reserved. PACS: 68.35.p; 68.47.Gh; 68.55.Nq; 73.20.r; 81.65.Mq Keywords: Tungsten oxides; Oxygen plasma; XPS; ToF-SIMS
1. Introduction Tungsten oxides are the subject of great interest from the viewpoint of the electrochromic phenomenon and its application in architecture, display devices etc. [1–3]. In all these fields the study of the oxidation mechanism is of paramount impor* Corresponding author. Tel.: +41 61 267 37 20; fax: +41 61 237 37 84. E-mail address: [email protected] (A. Romanyuk).
tance. The composition of the film, i.e. oxygen/ tungsten ratio, incorporated water and hydrogen, is considered to be important for the electrochromic response [4]. Interaction of oxygen with tungsten surfaces has been a subject of several investigations [5,6]. The works performed with electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), X-ray diffraction methods have focused on study of the local surface density-of-states (LSDOS), oxidation resistance and rates. The investigation of oxidation states of tungsten and dependence of the oxide thickness
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.03.073
A. Romanyuk et al. / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 358–361
on the temperature appears to be an important issue. An alternative oxidation procedure consists of the use of oxygen plasma since the oxidation can proceed to a large extent even at room temperature. This paper is concerned with the electronic properties, oxide composition and the dependence of the thickness of the oxide on the time and temperature used during the oxidation process.
2. Experimental procedures Tungsten films were prepared by magnetron sputtering of W target using standard PVD method with argon as a process gas. The films were deposited onto boron doped silicon (1 0 0) wafers with resistivity 10 X cm. The amount of metal deposited with this method was determined by a quartz microbalance and was equal to 200 nm for all depositions. The oxidation of the tungsten surface has been performed in low-temperature oxygen plasma at pressure of 0.5 Pa for time intervals from 10 s to 3600 s at ambient temperature in the same process chamber without breaking the vacuum. In the second set of experiments the substrate temperature during oxidation was increased to 390 C and 490 C while keeping the oxidation time constant at 3600 s. The samples were then transferred to XPS measurement chamber without breaking the vacuum and XPS spectra were recorded. It is imperative to prepare the samples in situ since the films transferred through the atmosphere are covered by a contamination overlayer containing stoichiometric WO3 and carbonaceous products. The atomic composition and the chemical states of the films were assessed with VG ESCALAB 210 spectrometer using Al Ka monochromatized radiation (1486.6 eV) with an energy resolution of better than 0.5 eV. The operation vacuum was less than 2 · 107 Pa. The energy position of the spectra were calibrated with reference to the 4f7/2 level of clean gold sample (positioned at 84.0 eV binding energy). Low-energy ToF-SIMS depth profiling was performed with a ION-TOF IV system in dual beam mode with 0.6 keV Cs+ sputtering and 10 keV
359
Ar+ primary ion beam for secondary ion generation.
3. Results and discussion Fig. 1 shows the photoemission spectra of W 4f core-level doublet for different oxidation times as well as for a virgin sputtered film. To determine the contribution of oxide and metal components as well as to obtain peak positions, a fit procedure using Doniach–Sunjic functions [7] was applied. The spectra of the oxide layers consist of two doublets: one at binding energies 37.52 eV for the W 4f5/2 and 35.38 eV for the W 4f7/2, and a second doublet at binding energies 33.37 eV for the W 4f5/2 and 31.23 eV for the W 4f7/2 that is related to the metallic substrate. The intensity of the ‘‘metallic’’ doublet is decreasing with increasing of oxidation time indicating that oxide thickness is also increasing. The oxide thickness was estimated with formula: I s ¼ I S0 expd=k cos h ;
Fig. 1. XPS spectra of W 4f line for different oxidation times at 22 C. The positions of the oxidation states of W 4f7/2 are marked with arrows.
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A. Romanyuk et al. / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 358–361
where Is is flux emitted from the substrate with overlayer, IS0 – flux emitted from the substrate in absence of overlayer, d – thickness of the overlyer, h – the electron take-off angle measured from the sample normal, and k – electron inelastic mean free path (IMFP) in overlayer. The magnitude of IMFP was derived from predictive equation of Gries [8] giving us a value of 2.86 nm. The resulting thickness for oxidation at room temperature and times of 10 s and 3600 s is found to be 0.2 nm and 11.1 nm respectively. The latter corresponds well with SIMS depth profile measurements. The stoichiometry of the formed oxide films was evaluated from O 1s and W 4f XPS peak ratio and was found to be independent on the plasma exposure time and equals to 3.05 ± 0.05. It should be particularly emphasized that no lower oxidation states (W5+, W4+) have been detected. Increased oxidation temperature results in the ‘‘metallic’’ part of the spectra to disappear revealing that the oxide thickness is increased. The stoichiometry of the films as detected from XPS peak ratio has not been changed for all deposition runs.
The ToF-SIMS WO3 depth profiles for the corresponding samples are depicted on Fig. 2. The picture shows an increased thickness of the surface oxide layer upon increased substrate temperature during oxide formation. The effect is related to the enhanced oxygen diffusion towards the oxide/ metal interface upon the increased temperature. Many studies have confirmed that water in the film plays an important role in the coloration mechanism [4,9]. Structural water, i.e. the water that is incorporated in the oxide is important to obtain a high proton mobility in the WO3 layer. Zeller and Beyeler [10] concluded that the water not only provided a high ionic conductivity which is a condition for a fast electrochromic reaction, but also stabilized electrocatalytically active surface sites. Inspection of Fig. 3 with the H2O–W distribution in the films discloses that amount of the water in the oxide overlayer is decreasing with increase of oxidation temperature. Although in situ post-annealing of the samples in vacuum shows that oxide layers prepared at different temperatures exhibit pronounced change in color demonstrating good thermochromic properties, the 103
T = 22oC T = 390oC T = 490oC
6
10
o
T = 22 C T = 390oC T = 490oC
Intensity [a.u.]
102 5
Intensity [a.u.]
10
101 104
100
3
10
0
10 20 30 40 50 60 70 80 90 100 Depth [nm]
Fig. 2. ToF-SIMS depth profile of WO3 complex for different oxidation temperatures and constant oxidation time of 3600 s.
0
10 20 30 40 50 60 70 80 90 100 Depth [nm]
Fig. 3. ToF-SIMS depth profile of H2O–W complex for different oxidation temperatures and constant oxidation time of 3600 s.
A. Romanyuk et al. / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 358–361
105
T = 22oC T = 390oC T = 490oC
Intensity [a.u.]
104
band just below Fermi energy. The band could be induced as a stable state in the virgin layers making the films blue colored.
4. Conclusions Oxidation of the polycrystalline W surface can be performed using reactive species such as oxygen plasma. The chemical composition of the oxide formed involves W6+ states; no lower oxides states have been observed. Oxidation at higher temperatures leads to the increased oxide layer thickness with simultaneous decrease of water and hydrogen concentration in the film.
103
102
101
361
0
10 20 30 40 50 60 70 80 90 100 Depth [nm]
Fig. 4. ToF-SIMS depth profile of H2 for different oxidation temperatures and constant oxidation time of 3600 s.
observation of reduced water content implies that oxides prepared at higher temperatures may not be well suited for electrochromic applications. The hydrogen concentration in the films is also reduced with increase of oxidation temperatures as it is seen from Fig. 4. Hydrogen in WO3 layers is known [11] to exist at least under two states: ‘‘passive’’ and ‘‘active’’. In active state a part of the hydrogen atoms incorporated in the layer has the same function as the oxygen vacancies, that is to give electrons resulting in growth of a absorption
References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11]
K. Bange, Sol. Energy Mater. Sol. Cells 58 (1999) 1. C.M. Lampert, Sol. Energy Mater. Sol. Cells 52 (1998) 207. C.B. Greenberg, Thin Solid Films 251 (1994) 81. A. Georg, W. Graf, R. Neumann, V. Wittwer, Thin Solid Films 384 (2001) 269. J.J. Czyzewski, J. Krajniak, S. Klein, Vacuum 46 (1995) 445. Chich-Shang-Chang, Tai-Bor-Wu, Chun-Kai-Huang, Wong-Cheng-Shih, Lan-Lin-Chao, Jpn. J. Appl. Phys., Part 1 39 (2000) 6413. S. Doniach, M. Sunjic, J. Phys. C: Solid State. Phys. 3 (1970) 285. W.H. Gries, Surf. Interface. Anal. 24 (1996) 38. N. Yoshiike, S. Kondo, Electrochem. Sol. Cells 130 (1983) 2283. H.R. Zeller, H.V. Beyeler, Appl. Phys. 13 (1977) 231. A. Deneuville, P. Gerard, J. Electron. Mater. 7 (1978) 559.
Oxidation of tungsten surface with reactive oxygen plasma Andriy Romanyuk a
a,*
, Viktor Melnik b, Peter Oelhafen
a
Institute of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland b IHP-Microelectronics, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany Available online 29 April 2005
Abstract We report on the oxidation of tungsten at different temperatures with an oxygen plasma studied with photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass-spectrometry (ToF-SIMS). The oxidation of the tungsten surface has been performed in a low-temperature oxygen plasma for different time intervals and various substrate temperatures. The stoichiometry of the formed oxide films was evaluated from O 1s and W 4f XPS peak ratio and was found to be independent of the plasma exposure time and the substrate temperature in the investigated range. The ToF-SIMS results show an increased thickness of the surface oxide layer upon increased substrate temperature during oxidation. The raise the substrate temperature in the process of oxide formation also causes the total amount of hydrogen and water bonded in the film structure to be reduced. 2005 Elsevier B.V. All rights reserved. PACS: 68.35.p; 68.47.Gh; 68.55.Nq; 73.20.r; 81.65.Mq Keywords: Tungsten oxides; Oxygen plasma; XPS; ToF-SIMS
1. Introduction Tungsten oxides are the subject of great interest from the viewpoint of the electrochromic phenomenon and its application in architecture, display devices etc. [1–3]. In all these fields the study of the oxidation mechanism is of paramount impor* Corresponding author. Tel.: +41 61 267 37 20; fax: +41 61 237 37 84. E-mail address: [email protected] (A. Romanyuk).
tance. The composition of the film, i.e. oxygen/ tungsten ratio, incorporated water and hydrogen, is considered to be important for the electrochromic response [4]. Interaction of oxygen with tungsten surfaces has been a subject of several investigations [5,6]. The works performed with electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), X-ray diffraction methods have focused on study of the local surface density-of-states (LSDOS), oxidation resistance and rates. The investigation of oxidation states of tungsten and dependence of the oxide thickness
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.03.073
A. Romanyuk et al. / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 358–361
on the temperature appears to be an important issue. An alternative oxidation procedure consists of the use of oxygen plasma since the oxidation can proceed to a large extent even at room temperature. This paper is concerned with the electronic properties, oxide composition and the dependence of the thickness of the oxide on the time and temperature used during the oxidation process.
2. Experimental procedures Tungsten films were prepared by magnetron sputtering of W target using standard PVD method with argon as a process gas. The films were deposited onto boron doped silicon (1 0 0) wafers with resistivity 10 X cm. The amount of metal deposited with this method was determined by a quartz microbalance and was equal to 200 nm for all depositions. The oxidation of the tungsten surface has been performed in low-temperature oxygen plasma at pressure of 0.5 Pa for time intervals from 10 s to 3600 s at ambient temperature in the same process chamber without breaking the vacuum. In the second set of experiments the substrate temperature during oxidation was increased to 390 C and 490 C while keeping the oxidation time constant at 3600 s. The samples were then transferred to XPS measurement chamber without breaking the vacuum and XPS spectra were recorded. It is imperative to prepare the samples in situ since the films transferred through the atmosphere are covered by a contamination overlayer containing stoichiometric WO3 and carbonaceous products. The atomic composition and the chemical states of the films were assessed with VG ESCALAB 210 spectrometer using Al Ka monochromatized radiation (1486.6 eV) with an energy resolution of better than 0.5 eV. The operation vacuum was less than 2 · 107 Pa. The energy position of the spectra were calibrated with reference to the 4f7/2 level of clean gold sample (positioned at 84.0 eV binding energy). Low-energy ToF-SIMS depth profiling was performed with a ION-TOF IV system in dual beam mode with 0.6 keV Cs+ sputtering and 10 keV
359
Ar+ primary ion beam for secondary ion generation.
3. Results and discussion Fig. 1 shows the photoemission spectra of W 4f core-level doublet for different oxidation times as well as for a virgin sputtered film. To determine the contribution of oxide and metal components as well as to obtain peak positions, a fit procedure using Doniach–Sunjic functions [7] was applied. The spectra of the oxide layers consist of two doublets: one at binding energies 37.52 eV for the W 4f5/2 and 35.38 eV for the W 4f7/2, and a second doublet at binding energies 33.37 eV for the W 4f5/2 and 31.23 eV for the W 4f7/2 that is related to the metallic substrate. The intensity of the ‘‘metallic’’ doublet is decreasing with increasing of oxidation time indicating that oxide thickness is also increasing. The oxide thickness was estimated with formula: I s ¼ I S0 expd=k cos h ;
Fig. 1. XPS spectra of W 4f line for different oxidation times at 22 C. The positions of the oxidation states of W 4f7/2 are marked with arrows.
360
A. Romanyuk et al. / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 358–361
where Is is flux emitted from the substrate with overlayer, IS0 – flux emitted from the substrate in absence of overlayer, d – thickness of the overlyer, h – the electron take-off angle measured from the sample normal, and k – electron inelastic mean free path (IMFP) in overlayer. The magnitude of IMFP was derived from predictive equation of Gries [8] giving us a value of 2.86 nm. The resulting thickness for oxidation at room temperature and times of 10 s and 3600 s is found to be 0.2 nm and 11.1 nm respectively. The latter corresponds well with SIMS depth profile measurements. The stoichiometry of the formed oxide films was evaluated from O 1s and W 4f XPS peak ratio and was found to be independent on the plasma exposure time and equals to 3.05 ± 0.05. It should be particularly emphasized that no lower oxidation states (W5+, W4+) have been detected. Increased oxidation temperature results in the ‘‘metallic’’ part of the spectra to disappear revealing that the oxide thickness is increased. The stoichiometry of the films as detected from XPS peak ratio has not been changed for all deposition runs.
The ToF-SIMS WO3 depth profiles for the corresponding samples are depicted on Fig. 2. The picture shows an increased thickness of the surface oxide layer upon increased substrate temperature during oxide formation. The effect is related to the enhanced oxygen diffusion towards the oxide/ metal interface upon the increased temperature. Many studies have confirmed that water in the film plays an important role in the coloration mechanism [4,9]. Structural water, i.e. the water that is incorporated in the oxide is important to obtain a high proton mobility in the WO3 layer. Zeller and Beyeler [10] concluded that the water not only provided a high ionic conductivity which is a condition for a fast electrochromic reaction, but also stabilized electrocatalytically active surface sites. Inspection of Fig. 3 with the H2O–W distribution in the films discloses that amount of the water in the oxide overlayer is decreasing with increase of oxidation temperature. Although in situ post-annealing of the samples in vacuum shows that oxide layers prepared at different temperatures exhibit pronounced change in color demonstrating good thermochromic properties, the 103
T = 22oC T = 390oC T = 490oC
6
10
o
T = 22 C T = 390oC T = 490oC
Intensity [a.u.]
102 5
Intensity [a.u.]
10
101 104
100
3
10
0
10 20 30 40 50 60 70 80 90 100 Depth [nm]
Fig. 2. ToF-SIMS depth profile of WO3 complex for different oxidation temperatures and constant oxidation time of 3600 s.
0
10 20 30 40 50 60 70 80 90 100 Depth [nm]
Fig. 3. ToF-SIMS depth profile of H2O–W complex for different oxidation temperatures and constant oxidation time of 3600 s.
A. Romanyuk et al. / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 358–361
105
T = 22oC T = 390oC T = 490oC
Intensity [a.u.]
104
band just below Fermi energy. The band could be induced as a stable state in the virgin layers making the films blue colored.
4. Conclusions Oxidation of the polycrystalline W surface can be performed using reactive species such as oxygen plasma. The chemical composition of the oxide formed involves W6+ states; no lower oxides states have been observed. Oxidation at higher temperatures leads to the increased oxide layer thickness with simultaneous decrease of water and hydrogen concentration in the film.
103
102
101
361
0
10 20 30 40 50 60 70 80 90 100 Depth [nm]
Fig. 4. ToF-SIMS depth profile of H2 for different oxidation temperatures and constant oxidation time of 3600 s.
observation of reduced water content implies that oxides prepared at higher temperatures may not be well suited for electrochromic applications. The hydrogen concentration in the films is also reduced with increase of oxidation temperatures as it is seen from Fig. 4. Hydrogen in WO3 layers is known [11] to exist at least under two states: ‘‘passive’’ and ‘‘active’’. In active state a part of the hydrogen atoms incorporated in the layer has the same function as the oxygen vacancies, that is to give electrons resulting in growth of a absorption
References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11]
K. Bange, Sol. Energy Mater. Sol. Cells 58 (1999) 1. C.M. Lampert, Sol. Energy Mater. Sol. Cells 52 (1998) 207. C.B. Greenberg, Thin Solid Films 251 (1994) 81. A. Georg, W. Graf, R. Neumann, V. Wittwer, Thin Solid Films 384 (2001) 269. J.J. Czyzewski, J. Krajniak, S. Klein, Vacuum 46 (1995) 445. Chich-Shang-Chang, Tai-Bor-Wu, Chun-Kai-Huang, Wong-Cheng-Shih, Lan-Lin-Chao, Jpn. J. Appl. Phys., Part 1 39 (2000) 6413. S. Doniach, M. Sunjic, J. Phys. C: Solid State. Phys. 3 (1970) 285. W.H. Gries, Surf. Interface. Anal. 24 (1996) 38. N. Yoshiike, S. Kondo, Electrochem. Sol. Cells 130 (1983) 2283. H.R. Zeller, H.V. Beyeler, Appl. Phys. 13 (1977) 231. A. Deneuville, P. Gerard, J. Electron. Mater. 7 (1978) 559.