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Growth, structure and properties of sputtered niobium oxide thin films
Thin Solid Films 519 (2011) 3068–3073
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Growth, structure and properties of sputtered niobium oxide thin films Ali Foroughi-Abari ⁎, Kenneth C. Cadien Chemical and Materials Engineering Department, University of Alberta, Edmonton, Alberta, Canada, T6G 2V4
a r t i c l e
i n f o
Article history: Received 16 April 2010 Received in revised form 6 December 2010 Accepted 9 December 2010 Keywords: Reactive sputtering Niobium oxide Thin films Oxygen Optical properties Surface morphology X-ray photoelectron spectroscopy
a b s t r a c t Niobium oxide (NbOx) films were deposited by pulsed dc magnetron sputtering at different total gas pressures and oxygen flow rates. Various film properties were characterized by X-ray photoelectron spectroscopy, atomic force microscopy, variable angle spectroscopic ellipsometry and four point probe. It was found that oxygen flow rates required for preparing NbO, NbO2 and Nb2O5 at a constant total pressure of 0.93 Pa were approximately 2, 4 and N 6 sccm, respectively. The results showed that the film properties, specifically composition can be significantly changed by the total gas pressure and the oxygen flow rate. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Niobium and its oxides are important strategic high technology materials. Niobium monoxide is a metallic material which exhibits superconductivity at 1.38 K [1] and has been used as a resistor in superconducting circuits [2]. Niobium dioxide is a semiconductor [3] with unique field-switching properties [4], and has been shown to be a promising support for platinum in methanol oxidation and a reducing agent for fuel cell technology [5]. Niobium pentoxide is a transparent dielectric material which makes it ideal for capacitor technology as well as for use in optical systems [6]. The pentoxide has been extensively studied as a gate dielectric in complementary metaloxide-semiconductor (CMOS) devices [7] and also shows excellent catalytic properties. It has been used in polymerization and dehydration processes [8]. Thin films of niobium oxides have been prepared using reactive DC magnetron sputtering and RF diode sputtering [9–11]. The growth of oxides with multiple oxide states is complex. Niobium pentoxide is thermodynamically the most stable having the lowest free energy of formation [8,12]. Therefore, the pentoxide is always formed when enough oxygen is provided during reactive sputtering. However, when other oxide states are desired, the amount of provided oxygen must be limited to prevent the formation of Nb2O5. The challenge is to find the required oxygen flow rate to achieve the desired stoichiometry. Venkataraj et al. [9] have reported that an oxygen flow rate greater than 7 sccm is enough to form the pentoxide state, however
⁎ Corresponding author. E-mail address: [email protected] (A. Foroughi-Abari). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.12.036
they did not report on the sputter deposition of niobium monoxide and dioxide films. In this paper we report work on the pulsed DC magnetron sputter deposition of niobium oxides and present an investigation into the structure and properties of these oxides. We also investigate the effect of total gas pressure on the film properties and show that the Nb and O film composition is dependent on the total sputtering pressure as well as the oxygen flow rate. 2. Experimental procedure The reactive sputtering was done in a pulsed dc magnetron sputtering system. The bias was changed with a frequency of 150 kHz and a reverse duration of 0.5 μs to avoid charge accumulation and arcing. Argon and oxygen lines were connected to the chamber to provide working and reactive gases. The base pressure for all the runs was b2.6 × 10− 4 Pa. Sputtering was performed at room temperature with a constant power of 300 W using a 3-inch niobium metal target. During the first set of experiments, the oxygen flow rate was varied between 0 and 20 sccm to evaluate its effect on film structure and properties. Sputtering pressure was controlled by varying a throttle valve and was adjusted prior to each run to maintain a total chamber pressure of 0.93 Pa. During the second set of experiments, the oxygen flow rate was fixed at 6 sccm, while the total gas pressure was varied from 0.46 Pa to 0.93 Pa. For both set of experiments, an argon flow rate of 65 sccm was used. Films were deposited on Si(100) wafers and glass slides. To determine the stoichiometry of the NbOx films as well as their oxidation state, X-ray photoelectron spectroscopy (XPS) analysis was performed on an AXIS-165 spectrometer (Kratos Analytical) at the Alberta Centre
A. Foroughi-Abari, K.C. Cadien / Thin Solid Films 519 (2011) 3068–3073
for Surface Engineering and Science (ACSES). The base pressure in the analytical chamber was lower than 3 × 10− 8 Pa. Monochromated Al Kα (hν = 1486.6 eV) source was used at a power of 210 W. Survey spectra were collected for binding energy ranging from 1100 to 0 eV with pass energy (PE) of 160 eV and step of 0.35 eV. High-resolution spectra were measured with PE of 20 eV and step of 0.1 eV. The depth profile was measured using a 4 keV argon ion beam scanned with an amplitude of +/− 1.5 mm in two directions. The etch rate of silicon dioxide was measured to be 2.2 nm/min for the measurement conditions. Atomic Force Microscopy (Asylum MFP-3D AFM) was employed to measure the surface roughness of the films. Resistivity of the films was measured using a four point probe. Optical properties of the films deposited on glass substrates were measured using a J. A. Wollam Inc. Variable Angle Spectroscopic Ellipsometer (VASE). The incident angle for p-polarized light was 0° and the frequency range of 10,000 and 40,000 cm− 1 wavenumbers was chosen. 3. Results and discussion 3.1. Deposition characteristics Fig. 1 shows the variation of target cathode potential as a function of oxygen flow during the reactive sputtering of niobium oxide at a constant power of 300 W and total pressure of 0.93 Pa. The cathode potential shows small hysteresis for increasing and decreasing oxygen flow. The metallic mode was observed for oxygen flow rates below 5 sccm. At oxygen flow rates greater than 5 sccm the transition between metallic and oxide modes occurs. For oxygen flow rates higher than 10 sccm, the voltage was almost constant indicating that deposition was in the full oxide mode and the chamber was saturated with oxygen. Fig. 2 shows the variation of deposition rate as a function of oxygen flow inside the chamber while keeping the total pressure at 0.93 Pa. The trend is similar to previous reported observations [9]. It can be seen from Fig. 2 that the deposition rate increases when the oxygen flow rate changes from 0 to 4 sccm. Then it starts to decrease until the oxygen flow rate reaches 8 sccm. After this point, the deposition rate remains constant. 3.2. Film composition 3.2.1. Effect of oxygen flow rate To determine the stoichiometry of sputtered niobium oxide films, XPS measurements were performed on the first set of samples deposited at different oxygen flow rates and constant total gas pressure of 0.93 Pa. Initial XPS results indicated that a very thin layer of native niobium pentoxide was always formed on the surface of the films upon atmospheric exposure, regardless of the composition. This was confirmed by observing a very distinct Nb 3d doublet as seen in
Fig. 1. Variation of cathode potential of the niobium target as a function of oxygen flow rate for a constant power of 300 W and total pressure of 0.93 Pa.
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Fig. 2. Deposition rate as a function of oxygen flow rate at fixed total gas pressure of 0.93 Pa.
Fig. 3. This doublet is specific to Nb2O5 [13]. In order to determine the film composition, one solution was to sputter etch off the thin native oxide layer in the XPS prior to measurement. However by sputter etching, another problem arose; the preferential sputtering of different elements. Lighter elements tend to be sputtered faster than the heavier ones. This results in accumulation of heavier elements on the surface [14,15]. Since heavy elements have a lower etch rate, during sputter etching, their abundance on the surface gradually increases until a steady state regime is reached [16]. Therefore the concentration of the heavy elements calculated from the XPS data is higher than the actual values. To better understand the effect of oxygen flow on film stoichiometry, a multilayer film consisting of 7 layers with different oxygen flow rates up to 15 sccm was prepared. Each layer had a thickness of 17 nm. The initial deposition on the Si(100) substrate was started with pure argon and then the oxygen flow was slowly increased before beginning the growth of each layer. The total pressure was kept constant at 0.93 Pa. The thickness of the first and last layer was 34 nm. Fig. 4 shows the XPS depth profile of the multilayer film. The type of oxide corresponding to 15 sccm oxygen flow was Nb2O5 with O/Nb atomic ratio of 2.45. As the surface was gradually sputter etched, the ratio decreased to reach a steady value of 1.8 which did not change for a wide range of oxygen flow rates from 6 to 10 sccm. The initial decrease of the O/Nb ratio is caused by preferential sputtering which leads to gradual accumulation of Nb atoms on the surface until steady state is reached. It can be concluded that oxygen flow rates higher than 6 sccm are needed to deposit Nb2O5. Therefore NbO2 and NbO can be deposited with oxygen flow rates below 6 sccm. However, preferential sputtering makes it difficult to determine the exact oxygen flow required to produce each of these two oxidation states.
Fig. 3. Nb 3d doublet in sputtered niobium oxide film with 5 sccm oxygen flow; the distinct doublet on the higher energy side is a characteristic of Nb2O5. After 2 min of etching the thin native oxide layer is removed and the internal structure is revealed.
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A. Foroughi-Abari, K.C. Cadien / Thin Solid Films 519 (2011) 3068–3073 Table 1 Measured and normalized O/Nb atomic ratio extracted from XPS depth profile of a multilayer NbOx film for various oxygen flow rates. The film was prepared at a constant power of 300 W and constant total pressure of 0.93 Pa. Oxygen flow rate (sccm) Measured value O/Nb Normalized value O/Nb
20 2.0 2.5
5 1.7 2.1
4 1.6 2.0
3 1.1 1.4
2 0.7 0.9
1 0.4 0.5
0 0.2 0.3
The mean free path λ of gas atoms or molecules is longer at lower pressures, according to the following equation:
K T λ = pffiffiffi b 2 2πd p Fig. 4. Depth profile for a multilayer NbOx film with 7 layers with different oxygen flow rates from 0 to 15 sccm and fixed total pressure of 0.93 Pa.
In order to further investigate the effect of oxygen flow on film stoichiometry, another multilayer film was prepared with layers deposited with different oxygen flow rates separated by a 10 nm layer of tungsten to better distinguish the layers and to prohibit possible diffusion between the layers. The result of XPS depth profile for this sample is shown in Fig. 5. To allow for preferential sputtering the data extracted from the XPS data was normalized using an O/Nb ratio of 2.5 for 20 sccm of oxygen flow. The value calculated from the data is 2.1. If it is assumed that the preferential sputtering has the same effect on all the films with different oxygen contents, the approximate actual values of O/Nb ratio could be calculated as shown in Table 1. The oxygen flow rates required for depositing NbO2 and NbO are 4 and 2 sccm, respectively. 3.2.2. Effect of total gas pressure In order to evaluate the possible effect of total gas pressure on film composition, XPS analysis was performed on the second set of samples prepared under constant oxygen flow rate of 6 sccm and variable total pressure. The results presented in Fig. 6, confirm that the film composition was influenced by the total gas pressure. A relatively small change in total sputtering pressure, from 0.47 to 0.93 Pa, had a large difference in composition. Therefore, one can produce the desired niobium oxide composition by either changing the oxygen flow rate at constant total pressure or by varying the total sputtering pressure and keeping the oxygen flow rate constant. For example, NbO2 could be deposited at 4 sccm oxygen flow rate and 0.93 Pa of total gas pressure. It could also be deposited at 6 sccm oxygen flow and 0.66 Pa of total gas pressure. The data presented in Fig. 6 does not require any normalizing similar to the depth profile data, since the preferential sputtering effect is minimal for the case of surface composition.
Fig. 5. Depth profile for a multilayer NbOx film with 7 layers with different oxygen flow rates from 0 to 20 sccm separated by a thin tungsten layer.
ð1Þ
where Kb is the Boltzmann constant, T the room temperature, d the diameter of the gas atoms, and p the pressure. For the case of pure Ar at 0.93 Pa, the mean free path value would be 3 cm which is much smaller than the substrate/target separation of 17 cm. This means that there are several collisions between gas species (Nb, O) while traveling from the target to the substrate. According to the above equation, the mean free path at 0.93 Pa would be half that at 0.46 Pa. Therefore, more collisions occur between particles when the pressure is increased to 0.93 Pa. This would result in higher probability of Nb metal being oxidized and, therefore less unreacted metal is deposited at the substrate.
3.3. Film roughness Fig. 7 shows AFM images from three NbOx films deposited with different oxygen flow rates from 0 to 20 sccm at constant total pressure of 0.93 Pa. The films were relatively smooth with a root mean square (RMS) roughness value of 1 nm. The oxygen flow rate during deposition did not have any considerable effect on the roughness of the films. Fig. 8 shows the effect of total gas pressure on roughness of the NbOx samples deposited at constant oxygen flow rate of 6 sccm and total pressure of 0.47 to 0.93 Pa. Increasing the total pressure from 0.47 to 0.93 Pa, NbOx films became rougher and surface features increased in size. Fig. 9 also shows the gradual increase in roughness as the sputtering pressure is increased. As discussed earlier, the mean free path is longer at lower pressure which means there are fewer collisions between particles traveling from target to substrate. Since the scattering effect is lower, the target atoms lose less energy and arrive at the substrate with higher kinetic energy and form a denser and smoother film. The data presented in Fig. 10 showing the effect of total sputtering pressure on deposition rate confirms this explanation. As the pressure increased, the deposition rate also increased which means a rougher and more porous film was deposited.
Fig. 6. Variation of film oxygen/niobium ratio as a function of total sputtering pressure.
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Fig. 7. AFM images from NbOx films; a) 0 sccm; b) 4 sccm; c) 20 sccm oxygen flow. Films had a thickness of 300 nm and were deposited on Si(100) wafers.
3.4. Optical properties 3.4.1. Effect of oxygen flow rate Optical transmittance measurements were performed using a VASE with p-polarized light and an angle of incident of 0° and spectral range of 10,000 to 40,000 cm− 1. The measurements were performed on 400 nm thick NbOx films deposited on glass substrates with varying oxygen flow rate and constant total pressure of 0.93 Pa. Fig. 11 shows the transmittance spectra obtained for these films. Films deposited at oxygen flow rates higher than 6 sccm are transparent. This is lower than the 7.5 sccm threshold determined in prior published work [9]. This small discrepancy could be due to variation in target diameter and chamber volume. However, the film deposited at 5 sccm shows a different trend and confirms that oxygen flow rates below 6 sccm are not enough to produce Nb2O5 and thus the films are not transparent. To determine the band gap energy of the films, absorption coefficients were calculated for two samples deposited at 4 and 7 sccm (corresponding to NbO2 and Nb2O5) using the following equation [17] 1= 2 α = A h −Eg
ð2Þ
where α is the absorption coefficient, A is a constant, hν is the photon energy, and Eg is the band gap. The band gap energy Eg can be determined by plotting α2 versus hν and extrapolating the linear part of the curve until it crosses the hν axis where α2 becomes zero. The value obtained in this manner is the band gap Eg. Fig. 12 shows the variation of α2 as a function of hν for NbO2 and Nb2O5 samples. The band gap value obtained for Nb2O5 (7 sccm) sample is 3.65 eV which is close to the 3.4–3.6 eV range reported previously [9,11]. The variations in the reported band gap may be due to thickness variations
which have a small effect on the band gap energy [17]. The NbO2 sample shows two linear parts, one with a high slope similar to that of the Nb2O5 sample and the other one has a much smaller slope and is related to NbO2. The existence of the higher slope suggests that a small part of the film is composed of Nb2O5, either on the surface or inside the film as mixture and the rest is NbO2. The band gap value calculated for NbO2 contribution is 1.1 eV. 3.4.2. Effect of total gas pressure Fig. 13 shows the transmittance spectra for NbOx films deposited at different total sputtering pressures. The oxygen flow rate was fixed at 6 sccm. By lowering pressure from 0.93 to 0.47 Pa, the transmittance was decreased which was due to more unreacted Nb metal in the films as discussed in Section 3.2.2. 3.5. Film resistivity Niobium oxide films present a wide range of electrical properties from conductor to insulator depending on their oxygen content. Film resistivity was calculated from the sheet resistance data measured by a four point probe and the film thickness. The sheet resistance data was corrected by a geometric factor as in the following formula V Rs = k I
ð3Þ
where V is the measured voltage, I is the measured current and k is the geometric factor which was 4.53 for these experiments [18]. Fig. 14 shows film resistivity for sputtered niobium oxide thin films deposited at different oxygen flow rates from 0 to 6 sccm and at two different total pressures. The four point probe used for measurements was not able to measure very high sheet resistance; thus the resistivity values for films deposited at 0 to 6 sccm of oxygen flow at a total pressure of 0.47 Pa are available as shown in Fig. 14. For the films deposited at 0.93 Pa, the data is available only to 4 sccm of oxygen flow. The film resistivity increases greatly as the oxygen content increases. The effect of total pressure on films resistivity is also shown in Fig. 14. The resistivity values are lower for films deposited at lower total pressures. This can be similarly explained by the change in the mean free path. The films deposited at higher pressures had relatively porous structures which reduces the ability of electrons to move through the film and thus results in higher resistivity values. 4. Summary
Fig. 8. RMS roughness as a function of total sputtering pressure (fixed oxygen flow rate).
Niobium oxide thin films were prepared by reactive pulsed dc magnetron sputtering on Si(100) and glass substrates at different oxygen flow rates and different total sputtering pressures to study the effect of oxygen flow rate and total sputtering pressure during
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Fig. 9. AFM images from NbOx films deposited at constant oxygen flow rate and varied total pressure; a) 0.47 Pa; b) 0.67 Pa; c) 0.80 Pa; d) 0.93 Pa.
deposition on various film properties. XPS depth profiles showed that oxygen flow rates required for preparing NbO, NbO2 and Nb2O5 at a constant total pressure of 0.93 Pa were approximately 2, 4 and N6 sccm, respectively. Further XPS analysis performed on films deposited at varied total pressures showed that more unreacted metal was found in the films when the total pressure was lowered. The significant implication drawn from this is that the film composition is dependent on the total sputtering pressure as well as the oxygen flow rate. Roughness measurements by AFM showed that oxygen flow rate had very little influence on the surface roughness. The total pressure was found to have effect on the roughness; at lower total pressures, the films
Fig. 10. Deposition rate as a function of total sputtering pressure.
Fig. 11. Transmittance spectra of NbOx films deposited at different oxygen flow rates as a function of wavenumber. Films were 400 nm-thick and were deposited on glass substrates.
Fig. 12. Variation of α2 as a function of photon energy for films sputtered at 4 and 7 sccm oxygen flow. The band gap is calculated to be 1.1 eV and 3.65 eV for NbO2 and Nb2O5 samples respectively.
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longer true if the total pressure was lowered to 0.47 Pa as the change in total pressure has a significant effect on optical properties. Film resistivity also increased with oxygen flow rate and total sputtering pressure.
Acknowledgments We would like to thank Alberta Innovates, Technology Futures, for support of this project through an Alberta Ingenuity Scholar Award. We would also like to thank the Alberta Centre for Surface Engineering and Science and the Nanofab in Electrical Engineering, University of Alberta for use of their facilities. Fig. 13. Transmittance spectra of NbOx films deposited at constant oxygen flow rate and varied total pressures.
Fig. 14. Variation of film resistivity as a function of oxygen flow rate during deposition; note that the resistivity axis is in logarithmic scale.
had a smoother surface. Optical measurements confirmed that the films deposited at total pressure of 0.93 Pa and oxygen flow rates higher than 6 sccm are transparent, as reported previously [9]. However, this was no
References [1] J.K. Hulm, C.K. Jones, R.A. Hein, J.W. Gibson, J. Low Temp. Phys. 7 (1972) 291. [2] C.I. Wilson, C.R. Ellis, L.S. Salinas, A.W. Lichtenberger, IEEE Trans. Appl. Supercond. 9 (1999) 1724. [3] M. Olszta, E. Dickey, Microsc. Microanal. 14 (2008) 451. [4] S.H. Shin, T. Halpern, P.M. Raccah, J. Appl. Phys. 48 (1977) 3150. [5] K. Sasaki, R.R. Adzik, J. Electrochem. Soc. 155 (2008) B180. [6] F. Lai, L. Lin, Z. Huang, R. Gai, Y. Qu, Appl. Surf. Sci. 253 (2006) 1801. [7] K. Kukli, M. Ritala, M. Leskelä, J. Electrochem. Soc. 148 (2001) F35. [8] K.T. Jacob, C. Shekhar, M. Vinay, J. Chem. Eng. Data (2010)8 DOI: 10.1021/ je1004609. [9] S. Venkataraj, R. Drese, O. Kappertz, R. Jayavel, M. Wuttig, Phys. Status Solidi (a) 188 (2001) 1047. [10] K. Yoshimura, T. Miki, S. Iwama, S. Tanemura, Thin Solid Films 235 (1996) 281. [11] M. Krishna, A. Bhattacharya, Mater. Sci. Eng. B 86 (2001) 41. [12] S. Venkataraj, R. Drese, O. Kappertz, R. Jayavel, M. Wuttig, J. Appl. Phys. 91 (2002) 4863. [13] M. Ziolek, I. Nowak, Catal. Today 78 (2003) 543. [14] S. Oswald, W. Bruckner, Surf. Interface Anal. 36 (2004) 17. [15] M. Bizjak, A. Zalar, P. Panjan, B. Zorko, B. Pracek, Appl. Surf. Sci. 253 (2007) 3977. [16] J.C. Vickerman, I.S. Gilmore, Surface Analysis: The Principal Techniques, 2nd, John Wiley and Sons Publishing, Chichester, U.K., 2009, p. 40. [17] M. Bouderbala, S. Hamzaoui, B. Amrani, A. Reshak, M. Adnane, T. Sahraoui, M. Zerdali, Phys. B 403 (2008) 3326. [18] K. Seshan, Handbook of Thin-film Deposition Processes and Techniques, 2nd, William Andrew Publishing/Noyes, New York, 2002, p. 262.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Growth, structure and properties of sputtered niobium oxide thin films Ali Foroughi-Abari ⁎, Kenneth C. Cadien Chemical and Materials Engineering Department, University of Alberta, Edmonton, Alberta, Canada, T6G 2V4
a r t i c l e
i n f o
Article history: Received 16 April 2010 Received in revised form 6 December 2010 Accepted 9 December 2010 Keywords: Reactive sputtering Niobium oxide Thin films Oxygen Optical properties Surface morphology X-ray photoelectron spectroscopy
a b s t r a c t Niobium oxide (NbOx) films were deposited by pulsed dc magnetron sputtering at different total gas pressures and oxygen flow rates. Various film properties were characterized by X-ray photoelectron spectroscopy, atomic force microscopy, variable angle spectroscopic ellipsometry and four point probe. It was found that oxygen flow rates required for preparing NbO, NbO2 and Nb2O5 at a constant total pressure of 0.93 Pa were approximately 2, 4 and N 6 sccm, respectively. The results showed that the film properties, specifically composition can be significantly changed by the total gas pressure and the oxygen flow rate. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Niobium and its oxides are important strategic high technology materials. Niobium monoxide is a metallic material which exhibits superconductivity at 1.38 K [1] and has been used as a resistor in superconducting circuits [2]. Niobium dioxide is a semiconductor [3] with unique field-switching properties [4], and has been shown to be a promising support for platinum in methanol oxidation and a reducing agent for fuel cell technology [5]. Niobium pentoxide is a transparent dielectric material which makes it ideal for capacitor technology as well as for use in optical systems [6]. The pentoxide has been extensively studied as a gate dielectric in complementary metaloxide-semiconductor (CMOS) devices [7] and also shows excellent catalytic properties. It has been used in polymerization and dehydration processes [8]. Thin films of niobium oxides have been prepared using reactive DC magnetron sputtering and RF diode sputtering [9–11]. The growth of oxides with multiple oxide states is complex. Niobium pentoxide is thermodynamically the most stable having the lowest free energy of formation [8,12]. Therefore, the pentoxide is always formed when enough oxygen is provided during reactive sputtering. However, when other oxide states are desired, the amount of provided oxygen must be limited to prevent the formation of Nb2O5. The challenge is to find the required oxygen flow rate to achieve the desired stoichiometry. Venkataraj et al. [9] have reported that an oxygen flow rate greater than 7 sccm is enough to form the pentoxide state, however
⁎ Corresponding author. E-mail address: [email protected] (A. Foroughi-Abari). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.12.036
they did not report on the sputter deposition of niobium monoxide and dioxide films. In this paper we report work on the pulsed DC magnetron sputter deposition of niobium oxides and present an investigation into the structure and properties of these oxides. We also investigate the effect of total gas pressure on the film properties and show that the Nb and O film composition is dependent on the total sputtering pressure as well as the oxygen flow rate. 2. Experimental procedure The reactive sputtering was done in a pulsed dc magnetron sputtering system. The bias was changed with a frequency of 150 kHz and a reverse duration of 0.5 μs to avoid charge accumulation and arcing. Argon and oxygen lines were connected to the chamber to provide working and reactive gases. The base pressure for all the runs was b2.6 × 10− 4 Pa. Sputtering was performed at room temperature with a constant power of 300 W using a 3-inch niobium metal target. During the first set of experiments, the oxygen flow rate was varied between 0 and 20 sccm to evaluate its effect on film structure and properties. Sputtering pressure was controlled by varying a throttle valve and was adjusted prior to each run to maintain a total chamber pressure of 0.93 Pa. During the second set of experiments, the oxygen flow rate was fixed at 6 sccm, while the total gas pressure was varied from 0.46 Pa to 0.93 Pa. For both set of experiments, an argon flow rate of 65 sccm was used. Films were deposited on Si(100) wafers and glass slides. To determine the stoichiometry of the NbOx films as well as their oxidation state, X-ray photoelectron spectroscopy (XPS) analysis was performed on an AXIS-165 spectrometer (Kratos Analytical) at the Alberta Centre
A. Foroughi-Abari, K.C. Cadien / Thin Solid Films 519 (2011) 3068–3073
for Surface Engineering and Science (ACSES). The base pressure in the analytical chamber was lower than 3 × 10− 8 Pa. Monochromated Al Kα (hν = 1486.6 eV) source was used at a power of 210 W. Survey spectra were collected for binding energy ranging from 1100 to 0 eV with pass energy (PE) of 160 eV and step of 0.35 eV. High-resolution spectra were measured with PE of 20 eV and step of 0.1 eV. The depth profile was measured using a 4 keV argon ion beam scanned with an amplitude of +/− 1.5 mm in two directions. The etch rate of silicon dioxide was measured to be 2.2 nm/min for the measurement conditions. Atomic Force Microscopy (Asylum MFP-3D AFM) was employed to measure the surface roughness of the films. Resistivity of the films was measured using a four point probe. Optical properties of the films deposited on glass substrates were measured using a J. A. Wollam Inc. Variable Angle Spectroscopic Ellipsometer (VASE). The incident angle for p-polarized light was 0° and the frequency range of 10,000 and 40,000 cm− 1 wavenumbers was chosen. 3. Results and discussion 3.1. Deposition characteristics Fig. 1 shows the variation of target cathode potential as a function of oxygen flow during the reactive sputtering of niobium oxide at a constant power of 300 W and total pressure of 0.93 Pa. The cathode potential shows small hysteresis for increasing and decreasing oxygen flow. The metallic mode was observed for oxygen flow rates below 5 sccm. At oxygen flow rates greater than 5 sccm the transition between metallic and oxide modes occurs. For oxygen flow rates higher than 10 sccm, the voltage was almost constant indicating that deposition was in the full oxide mode and the chamber was saturated with oxygen. Fig. 2 shows the variation of deposition rate as a function of oxygen flow inside the chamber while keeping the total pressure at 0.93 Pa. The trend is similar to previous reported observations [9]. It can be seen from Fig. 2 that the deposition rate increases when the oxygen flow rate changes from 0 to 4 sccm. Then it starts to decrease until the oxygen flow rate reaches 8 sccm. After this point, the deposition rate remains constant. 3.2. Film composition 3.2.1. Effect of oxygen flow rate To determine the stoichiometry of sputtered niobium oxide films, XPS measurements were performed on the first set of samples deposited at different oxygen flow rates and constant total gas pressure of 0.93 Pa. Initial XPS results indicated that a very thin layer of native niobium pentoxide was always formed on the surface of the films upon atmospheric exposure, regardless of the composition. This was confirmed by observing a very distinct Nb 3d doublet as seen in
Fig. 1. Variation of cathode potential of the niobium target as a function of oxygen flow rate for a constant power of 300 W and total pressure of 0.93 Pa.
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Fig. 2. Deposition rate as a function of oxygen flow rate at fixed total gas pressure of 0.93 Pa.
Fig. 3. This doublet is specific to Nb2O5 [13]. In order to determine the film composition, one solution was to sputter etch off the thin native oxide layer in the XPS prior to measurement. However by sputter etching, another problem arose; the preferential sputtering of different elements. Lighter elements tend to be sputtered faster than the heavier ones. This results in accumulation of heavier elements on the surface [14,15]. Since heavy elements have a lower etch rate, during sputter etching, their abundance on the surface gradually increases until a steady state regime is reached [16]. Therefore the concentration of the heavy elements calculated from the XPS data is higher than the actual values. To better understand the effect of oxygen flow on film stoichiometry, a multilayer film consisting of 7 layers with different oxygen flow rates up to 15 sccm was prepared. Each layer had a thickness of 17 nm. The initial deposition on the Si(100) substrate was started with pure argon and then the oxygen flow was slowly increased before beginning the growth of each layer. The total pressure was kept constant at 0.93 Pa. The thickness of the first and last layer was 34 nm. Fig. 4 shows the XPS depth profile of the multilayer film. The type of oxide corresponding to 15 sccm oxygen flow was Nb2O5 with O/Nb atomic ratio of 2.45. As the surface was gradually sputter etched, the ratio decreased to reach a steady value of 1.8 which did not change for a wide range of oxygen flow rates from 6 to 10 sccm. The initial decrease of the O/Nb ratio is caused by preferential sputtering which leads to gradual accumulation of Nb atoms on the surface until steady state is reached. It can be concluded that oxygen flow rates higher than 6 sccm are needed to deposit Nb2O5. Therefore NbO2 and NbO can be deposited with oxygen flow rates below 6 sccm. However, preferential sputtering makes it difficult to determine the exact oxygen flow required to produce each of these two oxidation states.
Fig. 3. Nb 3d doublet in sputtered niobium oxide film with 5 sccm oxygen flow; the distinct doublet on the higher energy side is a characteristic of Nb2O5. After 2 min of etching the thin native oxide layer is removed and the internal structure is revealed.
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20 2.0 2.5
5 1.7 2.1
4 1.6 2.0
3 1.1 1.4
2 0.7 0.9
1 0.4 0.5
0 0.2 0.3
The mean free path λ of gas atoms or molecules is longer at lower pressures, according to the following equation:
K T λ = pffiffiffi b 2 2πd p Fig. 4. Depth profile for a multilayer NbOx film with 7 layers with different oxygen flow rates from 0 to 15 sccm and fixed total pressure of 0.93 Pa.
In order to further investigate the effect of oxygen flow on film stoichiometry, another multilayer film was prepared with layers deposited with different oxygen flow rates separated by a 10 nm layer of tungsten to better distinguish the layers and to prohibit possible diffusion between the layers. The result of XPS depth profile for this sample is shown in Fig. 5. To allow for preferential sputtering the data extracted from the XPS data was normalized using an O/Nb ratio of 2.5 for 20 sccm of oxygen flow. The value calculated from the data is 2.1. If it is assumed that the preferential sputtering has the same effect on all the films with different oxygen contents, the approximate actual values of O/Nb ratio could be calculated as shown in Table 1. The oxygen flow rates required for depositing NbO2 and NbO are 4 and 2 sccm, respectively. 3.2.2. Effect of total gas pressure In order to evaluate the possible effect of total gas pressure on film composition, XPS analysis was performed on the second set of samples prepared under constant oxygen flow rate of 6 sccm and variable total pressure. The results presented in Fig. 6, confirm that the film composition was influenced by the total gas pressure. A relatively small change in total sputtering pressure, from 0.47 to 0.93 Pa, had a large difference in composition. Therefore, one can produce the desired niobium oxide composition by either changing the oxygen flow rate at constant total pressure or by varying the total sputtering pressure and keeping the oxygen flow rate constant. For example, NbO2 could be deposited at 4 sccm oxygen flow rate and 0.93 Pa of total gas pressure. It could also be deposited at 6 sccm oxygen flow and 0.66 Pa of total gas pressure. The data presented in Fig. 6 does not require any normalizing similar to the depth profile data, since the preferential sputtering effect is minimal for the case of surface composition.
Fig. 5. Depth profile for a multilayer NbOx film with 7 layers with different oxygen flow rates from 0 to 20 sccm separated by a thin tungsten layer.
ð1Þ
where Kb is the Boltzmann constant, T the room temperature, d the diameter of the gas atoms, and p the pressure. For the case of pure Ar at 0.93 Pa, the mean free path value would be 3 cm which is much smaller than the substrate/target separation of 17 cm. This means that there are several collisions between gas species (Nb, O) while traveling from the target to the substrate. According to the above equation, the mean free path at 0.93 Pa would be half that at 0.46 Pa. Therefore, more collisions occur between particles when the pressure is increased to 0.93 Pa. This would result in higher probability of Nb metal being oxidized and, therefore less unreacted metal is deposited at the substrate.
3.3. Film roughness Fig. 7 shows AFM images from three NbOx films deposited with different oxygen flow rates from 0 to 20 sccm at constant total pressure of 0.93 Pa. The films were relatively smooth with a root mean square (RMS) roughness value of 1 nm. The oxygen flow rate during deposition did not have any considerable effect on the roughness of the films. Fig. 8 shows the effect of total gas pressure on roughness of the NbOx samples deposited at constant oxygen flow rate of 6 sccm and total pressure of 0.47 to 0.93 Pa. Increasing the total pressure from 0.47 to 0.93 Pa, NbOx films became rougher and surface features increased in size. Fig. 9 also shows the gradual increase in roughness as the sputtering pressure is increased. As discussed earlier, the mean free path is longer at lower pressure which means there are fewer collisions between particles traveling from target to substrate. Since the scattering effect is lower, the target atoms lose less energy and arrive at the substrate with higher kinetic energy and form a denser and smoother film. The data presented in Fig. 10 showing the effect of total sputtering pressure on deposition rate confirms this explanation. As the pressure increased, the deposition rate also increased which means a rougher and more porous film was deposited.
Fig. 6. Variation of film oxygen/niobium ratio as a function of total sputtering pressure.
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Fig. 7. AFM images from NbOx films; a) 0 sccm; b) 4 sccm; c) 20 sccm oxygen flow. Films had a thickness of 300 nm and were deposited on Si(100) wafers.
3.4. Optical properties 3.4.1. Effect of oxygen flow rate Optical transmittance measurements were performed using a VASE with p-polarized light and an angle of incident of 0° and spectral range of 10,000 to 40,000 cm− 1. The measurements were performed on 400 nm thick NbOx films deposited on glass substrates with varying oxygen flow rate and constant total pressure of 0.93 Pa. Fig. 11 shows the transmittance spectra obtained for these films. Films deposited at oxygen flow rates higher than 6 sccm are transparent. This is lower than the 7.5 sccm threshold determined in prior published work [9]. This small discrepancy could be due to variation in target diameter and chamber volume. However, the film deposited at 5 sccm shows a different trend and confirms that oxygen flow rates below 6 sccm are not enough to produce Nb2O5 and thus the films are not transparent. To determine the band gap energy of the films, absorption coefficients were calculated for two samples deposited at 4 and 7 sccm (corresponding to NbO2 and Nb2O5) using the following equation [17] 1= 2 α = A h −Eg
ð2Þ
where α is the absorption coefficient, A is a constant, hν is the photon energy, and Eg is the band gap. The band gap energy Eg can be determined by plotting α2 versus hν and extrapolating the linear part of the curve until it crosses the hν axis where α2 becomes zero. The value obtained in this manner is the band gap Eg. Fig. 12 shows the variation of α2 as a function of hν for NbO2 and Nb2O5 samples. The band gap value obtained for Nb2O5 (7 sccm) sample is 3.65 eV which is close to the 3.4–3.6 eV range reported previously [9,11]. The variations in the reported band gap may be due to thickness variations
which have a small effect on the band gap energy [17]. The NbO2 sample shows two linear parts, one with a high slope similar to that of the Nb2O5 sample and the other one has a much smaller slope and is related to NbO2. The existence of the higher slope suggests that a small part of the film is composed of Nb2O5, either on the surface or inside the film as mixture and the rest is NbO2. The band gap value calculated for NbO2 contribution is 1.1 eV. 3.4.2. Effect of total gas pressure Fig. 13 shows the transmittance spectra for NbOx films deposited at different total sputtering pressures. The oxygen flow rate was fixed at 6 sccm. By lowering pressure from 0.93 to 0.47 Pa, the transmittance was decreased which was due to more unreacted Nb metal in the films as discussed in Section 3.2.2. 3.5. Film resistivity Niobium oxide films present a wide range of electrical properties from conductor to insulator depending on their oxygen content. Film resistivity was calculated from the sheet resistance data measured by a four point probe and the film thickness. The sheet resistance data was corrected by a geometric factor as in the following formula V Rs = k I
ð3Þ
where V is the measured voltage, I is the measured current and k is the geometric factor which was 4.53 for these experiments [18]. Fig. 14 shows film resistivity for sputtered niobium oxide thin films deposited at different oxygen flow rates from 0 to 6 sccm and at two different total pressures. The four point probe used for measurements was not able to measure very high sheet resistance; thus the resistivity values for films deposited at 0 to 6 sccm of oxygen flow at a total pressure of 0.47 Pa are available as shown in Fig. 14. For the films deposited at 0.93 Pa, the data is available only to 4 sccm of oxygen flow. The film resistivity increases greatly as the oxygen content increases. The effect of total pressure on films resistivity is also shown in Fig. 14. The resistivity values are lower for films deposited at lower total pressures. This can be similarly explained by the change in the mean free path. The films deposited at higher pressures had relatively porous structures which reduces the ability of electrons to move through the film and thus results in higher resistivity values. 4. Summary
Fig. 8. RMS roughness as a function of total sputtering pressure (fixed oxygen flow rate).
Niobium oxide thin films were prepared by reactive pulsed dc magnetron sputtering on Si(100) and glass substrates at different oxygen flow rates and different total sputtering pressures to study the effect of oxygen flow rate and total sputtering pressure during
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Fig. 9. AFM images from NbOx films deposited at constant oxygen flow rate and varied total pressure; a) 0.47 Pa; b) 0.67 Pa; c) 0.80 Pa; d) 0.93 Pa.
deposition on various film properties. XPS depth profiles showed that oxygen flow rates required for preparing NbO, NbO2 and Nb2O5 at a constant total pressure of 0.93 Pa were approximately 2, 4 and N6 sccm, respectively. Further XPS analysis performed on films deposited at varied total pressures showed that more unreacted metal was found in the films when the total pressure was lowered. The significant implication drawn from this is that the film composition is dependent on the total sputtering pressure as well as the oxygen flow rate. Roughness measurements by AFM showed that oxygen flow rate had very little influence on the surface roughness. The total pressure was found to have effect on the roughness; at lower total pressures, the films
Fig. 10. Deposition rate as a function of total sputtering pressure.
Fig. 11. Transmittance spectra of NbOx films deposited at different oxygen flow rates as a function of wavenumber. Films were 400 nm-thick and were deposited on glass substrates.
Fig. 12. Variation of α2 as a function of photon energy for films sputtered at 4 and 7 sccm oxygen flow. The band gap is calculated to be 1.1 eV and 3.65 eV for NbO2 and Nb2O5 samples respectively.
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longer true if the total pressure was lowered to 0.47 Pa as the change in total pressure has a significant effect on optical properties. Film resistivity also increased with oxygen flow rate and total sputtering pressure.
Acknowledgments We would like to thank Alberta Innovates, Technology Futures, for support of this project through an Alberta Ingenuity Scholar Award. We would also like to thank the Alberta Centre for Surface Engineering and Science and the Nanofab in Electrical Engineering, University of Alberta for use of their facilities. Fig. 13. Transmittance spectra of NbOx films deposited at constant oxygen flow rate and varied total pressures.
Fig. 14. Variation of film resistivity as a function of oxygen flow rate during deposition; note that the resistivity axis is in logarithmic scale.
had a smoother surface. Optical measurements confirmed that the films deposited at total pressure of 0.93 Pa and oxygen flow rates higher than 6 sccm are transparent, as reported previously [9]. However, this was no
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