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Composition, structure and electrical properties of DC reactive magnetron sputtered Al2O3 thin films
Materials Science in Semiconductor Processing 16 (2013) 705–711
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Composition, structure and electrical properties of DC reactive magnetron sputtered Al2O3 thin films S. Prasanna a, G. Krishnendu a, S. Shalini a, P. Biji b, G. Mohan Rao c, S. Jayakumar a,n, R. Balasundaraprabhu a a b c
Thin Film Center, Department of Physics, PSG College of Technology, Coimbatore-641 004, India PSG Institute of Advanced Studies, Coimbatore-641 004, India Department of Instrumentation, Indian Institute of Science (IISc), Bangalore-560 012, India
a r t i c l e i n f o
abstract
Available online 16 January 2013
Thin films of alumina (Al2O3) were deposited over Si /1 0 0S substrates at room temperature at an oxygen gas pressure of 0.03 Pa and sputtering power of 60 W using DC reactive magnetron sputtering. The composition of the as-deposited film was analyzed by X-ray photoelectron spectroscopy and the O/Al atomic ratio was found to be 1.72. The films were then annealed in vacuum to 350, 550 and 750 1C and X-ray diffraction results revealed that both as-deposited and post deposition annealed films were amorphous. The surface morphology and topography of the films was studied using scanning electron microscopy and atomic force microscopy, respectively. A progressive decrease in the root mean square (RMS) roughness of the films from 1.53 nm to 0.7 nm was observed with increase in the annealing temperature. Al–Al2O3–Al thin film capacitors were then fabricated on p-type Si /1 0 0S substrate to study the effect of temperature and frequency on the dielectric property of the films and the results are discussed. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Thin films B. Reactive magnetron sputtering C. Electrical properties
1. Introduction Al2O3 is one of the widely used insulating materials for microelectronic applications with properties such as high dielectric constant (er ¼9), thermal stability, wide band gap (8.7 eV), high refractive index and excellent chemical stability [1–3]. Also, Al2O3 thin films are considered as one of the potential high-k dielectric materials to replace SiO2 layers in devices such as DRAMs and MOSFETs due to properties such low leakage current, high breakdown voltage (9 MV/cm), thermal stability on Si etc. [4]. The properties of the Al2O3 films are strongly influenced by its crystal structure, which can occur in several phases. Even
n Corresponding author. Tel.: þ91 422 4344453; fax: þ 91 0422 2573833. E-mail address: [email protected] (S. Jayakumar).
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.12.012
though, there are reports on more than 20 other phases of Al2O3 [6], the most important, and common, are a, g, y, and k [5]. Al2O3 films are amorphous up to 400 1C or more, and it is an important material for dielectric applications. For PVD, the phase transformation sequence is reported to be amorphous or a mix of amorphous and g-y þ d-a [7–9]. The amorphous films are found to differ in their electrical properties based on the preparation technique [10]. In particular, the deposition rate in evaporated films is found to have a profound effect on the dielectric constant. Recently, owing to the better control of the formation phases and quality, there has been a considerable amount of work pertaining to Al2O3 films prepared by physical methods, especially magnetron sputtering [11–14]. Reactive magnetron sputtering is one of the important methods for the deposition of wide range of compound thin films including oxides, nitrides, carbides, fluorides and arsenides [15]. DC reactive magnetron
706
S. Prasanna et al. / Materials Science in Semiconductor Processing 16 (2013) 705–711
sputtering is extensively used to prepare thin compound films of controllable stoichiometry and composition from an elemental target which can be easily purified hence resulting in the deposition of high-purity films [16–18]. In addition, by using DC reactive magnetron sputtering, the complexity of RF systems can be avoided since metallic targets are in general electrically and thermally conductive, which makes the cooling of these targets more efficient; thus the range of the applied power can be extended without the problem of target cracking [19,20]. Although Al2O3 thin films were studied extensively for many applications [15–20], not much work has been done on the effect of post-deposition annealing on the structure and morphology of Al2O3 films deposited by DC reactive magnetron sputtering and this is one of the objectives of the present work. In addition, Al–Al2O3–Al thin film capacitor was fabricated and dielectric properties and AC conduction mechanism were studied. 2. Experimental details Al2O3 thin films were prepared on well-cleaned Si /1 0 0S substrates held at room temperature by sputtering an aluminium target (99.99% pure) of thickness 3 mm and diameter 50 mm in an optimized oxygen partial pressure of 0.03 Pa and sputtering power of 60 W. The details of optimization are described elsewhere [21]. The target to substrate distance was fixed at 4 cm. High purity oxygen (4 N) and argon (4 N) was used as the reactive and sputtering gases, respectively. The sputtering chamber was evacuated to an ultimate vacuum of 9 10 4 Pa before admitting the reactive gas into the system. Both oxygen and argon were admitted into the chamber through needle valves and their flow rates were monitored individually using Aalborg mass flow controllers. Oxygen was introduced in the vicinity of the substrate holder to reduce target poisoning and to increase the partial pressure of oxygen close to the substrates. Prior to each deposition, the target was sputter cleaned in pure argon atmosphere to remove contaminants from the target surface. The sputtering gas was maintained at a pressure of 0.1 Pa and pre-sputtering was done for 10–20 min prior to deposition. The composition of the films was determined using a SPECS GmbH X-ray photoelectron spectrometer (Phoibos 100 MCD Energy Analyser) using Al Ka radiation (1486.6 eV). The residual pressure inside the analysis chamber was of the order of 2 10 8 Pa. Peaks were recorded with constant pass energy of 40 eV. The thickness of the films was measured using a Dektak 150 surface profilometer. The structure of the films was determined using a Shimadzu X-ray diffractometer operating in the 2y mode with Cu Ka radiation (l ¼ 0.15406 nm) and a fixed incidence angle of 21. Topography and surface roughness were determined with a root mean square (RMS) accuracy of 0.1 nm using an NTMDT atomic force microscope (AFM) in contact mode with a Si3N4 cantilever. Thickness of the films was measured using a Dektak 150 surface stylus profiler. For measuring the dielectric properties, Al/Al2O3/Al capacitor was fabricated on Si /1 0 0S substrate. Aluminium was used as the electrode material. The dielectric and AC electrical properties were studied
using a Chen Hwa LCZ meter in the frequency range of 40 Hz and 200 kHz at 50 mV. Dielectric parameters such as capacitance, relative permittivity, AC conductivity were measured. The dielectric measurements were carried out in vacuum by placing the sample in a small chamber maintained at a base pressure of 10 1 Pa.
3. Results and discussion 3.1. Composition Fig. 1 shows the X-ray photoelectron spectra of Al2O3 thin films sputtered at room temperature with an oxygen partial pressure of 0.03 Pa and sputtering power of 60 W. To a first approximation, the spectrum shows the presence of aluminium and oxygen with the presence of species such as Al 2s, Al 2p and O 1s. The X-ray photoelectron spectra were calibrated with respect to the C 1s spectral line. Figs. 2a, 2b and 3 show the high energy XPS scans for Al 2p, Al 2s and O 1s core levels, respectively. For determining the O/Al ratio high energy XPS scans for Al 2p and O 1s core levels were recorded for the sample shown as per standard procedure by using the sensitivity factors of the instrument [22]. The quantitative analysis of the oxygen and aluminium (O/Al atomic ratio) was carried out from a calculation of the peak area ratios of the respective states in the narrow band O 1s and Al 2p spectra, following Shirley background subtraction. The films were found to be oxygen rich with an O/Al atomic ratio of 1.72. The position of the Al 2p peak at 74.08 eV indicates that the aluminium present in the surface region is bound as Al2O3. Also, the Al 2p peak could be fitted only with a single peak, showing that the aluminium present in the film is completely oxidized in the form of Al2O3 [23–25]. The high resolution O 1s scan shows a single, broad peak at 531.03 eV indicating that the oxygen is in a bound state rather than in absorption state which can be expected at a higher binding energy of around 534.4 eV or more [26–28]. The peak pertaining to Al 2s is also present at 118.94 eV. The presence of carbon peak in the films at 284.5 eV could be attributed to the surface
Fig. 1. X-ray photoelectron spectra of Al2O3 film prepared at a power of 60 W.
S. Prasanna et al. / Materials Science in Semiconductor Processing 16 (2013) 705–711
707
Fig. 4. Gracing incidence X-ray diffraction pattern of as-deposited Al2O3 film.
Fig. 2. (a) and (b) Narrow range X-ray photoelectron spectra of Al 2p and Al 2 s peaks of as-deposited Al2O3 thin film. Fig. 5. SEM image of an as-deposited Al2O3 film.
Fig. 3. Narrow range X-ray photoelectron spectra of O 1s peak of as-deposited Al2O3 thin film.
contamination from the atmosphere before insertion into the chamber [29]. 3.2. Structure and surface morphology X-ray diffraction pattern of annealed Al2O3 thin films deposited at a sputtering pressure of 60 W is shown in Fig. 4. The as-deposited film was found to be amorphous
(figure not shown) as reported widely in the literature [12–14]. In order to understand the crystallization behavior of the magnetron sputtered Al2O3 films, post-deposition annealing was done at 350, 550 and 750 1C in vacuum for 60 min. The films were found to be completely amorphous even after annealing at a high temperature of 750 1C. Fig. 5 shows the scanning electron micrograph of an as-deposited Al2O3 thin film at a sputtering power of 60 W. It is evident from the SEM analysis that the surface morphology of the film is very smooth. AFM analysis was done to ascertain the changes in the surface morphology of the films with post-deposition annealing. AFM images of the films annealed at 350, 550 and 750 1C are shown in Fig. 6a,b and c, respectively. The root mean square roughness (RMS) values were determined from the AFM images using WSxM software (Nanotec Electronica). The RMS roughness of the film annealed at 350 1C was found to be 1.53 nm. The RMS roughness decreased to 0.77 and 0.73 nm upon post-deposition annealing at 550 and 750 1C, respectively. Zhang et al. [30] reported RMS roughness values between 0.2 nm and 0.5 nm for Al2O3 films synthesized using atomic layer deposition (ALD). Balakrishnan et al. [31] reported RMS roughness values between 1.4 and 3.5 nm for pulsed laser deposited Al2O3
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S. Prasanna et al. / Materials Science in Semiconductor Processing 16 (2013) 705–711
Fig. 6. AFM image of annealed Al2O3 films prepared at a sputtering power of 60 W (a) annealed at 350 1C, (b) annealed at 550 1C, (c) annealed at 750 1C.
Fig. 7. Variation of capacitance with frequency.
Fig. 8. Variation of dielectric constant with frequency.
films. The decrease in the RMS roughness shows that the films became progressively smoother and defects were reduced with post-deposition annealing particularly at higher temperatures above 500 1C in accordance with the reported results [30].
temperatures and found to be independent of temperature at high frequencies. The large increase in capacitance towards the lower frequency region can be explained on the basis of the charge carriers being blocked at the electrodes, which leads to a space charge layer resulting in an increase in capacitance [33]. The space charge region developed near the electrode leads to a substantial increase in the capacitance at lower frequency below 1 kHz. The decrease in the value of capacitance with increase in frequency is attributed to the screening of the electric field across the film by charge redistribution [34]. The dielectric constant (er) was calculated from the knowledge of capacitance (C), film thickness (d), permittivity of free space (eo) and the area of the capacitor using the relation,
3.3. Electrical properties Thin film capacitors of the type Al/Al2O3/Al were fabricated on p-type Si /1 0 0S substrates. Initially, aluminium electrode was sputtered onto Si /1 0 0S substrates followed by reactive deposition of Al2O3 thin films. The thickness of the dielectric film was measured using a stylus profiler and found to be 200 nm. Aluminium was sputtered once again as the top electrode onto the dielectric film and dot capacitors having an area of 0.0613 10 4 m2 were fabricated. The thickness of the aluminium film (electrode) was kept above 500 nm in order to reduce lead resistance [32]. The dielectric properties of Al2O3 films were measured as a function of AC frequency from 40 Hz to 2 105 Hz at 50 mV. The fabricated capacitor was stabilized by annealing at 250 1C for around 4 h in vacuum. Fig. 7 shows the variation of capacitance of an Al2O3 thin film capacitor with frequency. From Fig. 7, it is observed that the capacitance of Al–Al2O3–Al thin film capacitor decreases with increase in frequency at different
er ¼ Cd=eo A
ð1Þ
The dielectric constant at room temperature was found to be 15 at a frequency of 1 kHz. Similar to the capacitance, the dielectric constant also shows a decreasing trend with increasing frequency at all temperatures as shown in Fig. 8. The trend seems to resemble the Debye relaxation model for orientational polarization [35]. The dielectric loss tangent was found to decrease with frequency attaining a minimum value of around 0.17 as evident from the Fig. 9.
S. Prasanna et al. / Materials Science in Semiconductor Processing 16 (2013) 705–711
In the high frequency region the loss is proportional to frequency but at lower frequencies, depending upon temperature the loss is either decreasing or constant with frequency. Interestingly the dielectric loss tangent measured at room temperature was found to be almost constant with frequency and Argall and Jonscher [36] have reported similar results as a special case occurring only at room temperature. It is observed that the dielectric loss measured as the part of the polarization which is out of phase with the applied ac field, will generally increase with frequency after attaining a loss minimum.
3.3.1. Temperature coefficient of capacitance (TCC) The temperature coefficient of capacitance (TCC) was evaluated using the expression, TCC ¼ 1=C dC=dT
ð2Þ
The variation of capacitance with temperature is shown in Fig. 10 for different frequencies. It is seen that the TCC values increase with increase in frequency and found to be 227.16 104 ppm/K at a frequency of 10 kHz.
Fig. 9. Variation of dielectric loss tangent with frequency.
Fig. 10. Variation of capacitance of Al2O3 film with temperature.
709
3.3.2. AC conduction studies AC measurements are very important for any dielectric material as it gives a lot of information about dynamic properties such as capacitance, conductivity and loss factor. AC measurements are also helpful in identifying the nature of conduction mechanism exhibited by the dielectric film. In the present work, the AC conductivity (s) was calculated using the relation [37]
s ¼ 2pf tander e0
ð3Þ
Variation of AC conductivity of an as-deposited Al2O3 film with frequency and temperature is shown in Fig. 11. From Fig. 11 it is evident that AC conductivity increases linearly with frequency. Also, it is evident that the AC conductivity is not highly sensitive to temperature at high frequency, whereas at low frequencies, there is clear dependence on temperature. The variation of AC conductivity is related to a power law which can be described according the following relation:
s ¼ Aos
ð4Þ
where o is the angular frequency, s is the frequency exponent and A is a complex proportionality constant independent of frequency [38]. The frequency exponent s is derived from Fig. 11 by calculating the slopes for different frequency ranges. The variation of frequency exponent s with temperature is shown for four different frequency ranges in Fig. 12. It can be seen that s depends on temperature, decreasing with increasing temperature at all frequency ranges. Different theoretical models have been proposed to explain the frequency and temperature dependence of AC conduction in materials [39–42]. From the figure, it is clear that s gradually decreases with increase in temperature. The correlated barrier hopping mechanism (CBH) considers the hopping of two electrons from a D to a neighbouring D þ centre over the potential barrier between them [39]. It predicts that s should be less than 1, decreasing with increasing temperature. It is observed that only in the frequency range, 0.4–10 kHz the value of s is below 1, in the frequency range 10–20 kHz, s varies between 1.2 and 0.2. Anwar and Hogarth [43] have reported similar results with a
Fig. 11. Variation of AC conductivity of Al2O3 film with frequency and temperature.
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S. Prasanna et al. / Materials Science in Semiconductor Processing 16 (2013) 705–711
depend on the power law. The frequency component s was found decrease with increase in temperature suggesting a correlated barrier hopping (CBH) mechanism of conduction.
Acknowledgement The authors would like to thank the Management and Principal for the support extended towards this research work. References
Fig. 12. Variation of frequency exponent s with temperature at different frequency ranges.
decrease in the value of s with increasing temperature and an increase in the value of s at higher frequencies. Elliot [44] has given the following expression for s based on the CBH model s ¼ 1 6kB T=B ð5Þ where kB is the Boltzmann’s constant, T is the temperature in Kelvin and B is the optical band gap. Using the optical band gap value of bulk Al2O3 (9.1 eV) the value of s at room temperature was derived using the above expression and found to be 0.98 which is in close agreement with the experimental values of 0.8 and 1.2 measured at frequency ranges 0.4–10 kHz and 10–20 kHz, respectively. Jonscher [45] in his detailed review has postulated that this correlated barrier hopping mechanism (CBH) is relevant to the highly amorphous structure of dielectric films, which are expected to have a large number of localized levels in the forbidden gap. Hence, from the measured value of s and its variation with frequency and temperature it can be concluded that correlated barrier hopping mechanism (CBH) is the most suitable model for describing the carrier transport mechanism in Al2O3 thin films. [46,47] 4. Conclusions Al2O3 thin films were deposited onto Si /1 0 0S substrates by DC reactive magnetron sputtering. The composition was determined from XPS analysis and the asdeposited films were found to be oxygen rich with an O/ Al atomic ratio of 1.72. The effect of post-deposition annealing on the structure and surface morphology of the Al2O3 films were investigated. The films were found to be amorphous even after annealing at 750 1C. The RMS roughness of the films was found to decrease with postdeposition annealing attaining a minimum value of 0.73 nm at 750 1C. Al-–Al2O3–Al thin film capacitor was fabricated on Si /1 0 0S substrate and the dielectric constant was found to be 15 at a frequency of 1 kHz. The capacitance was found to decrease with increasing frequency and this is attributed to interfacial polarization. The AC conductivity was determined and was found to
[1] Sung Bo Lee, Chi Won Ahn, Sang-Hoon Lee, Eun Kyu Her, Kyu Hwan Oh, Doh-Yeon Kim, Dong Nyung, Thin Solid Films 518 (2010) 4304–4311. [2] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, second ed. John Wiley & Sons, New York, 1976. [3] J. Guzman-Mendoza, M. Garcia- Hipolito, M. Aguilar-Frutis, C. Falcony-Guajardo, Journal of Physics: Condensed Matter 13 (2001) L955–L959. [4] G.D. Wilk, R.M. Wallace, J.M. Anthony, Journal of Applied Physics 89 (10) (2001). [5] O. Zywitzki, G. Hoetzsch, Surface and Coatings Technology 86–87 (1996) 640–647. [6] W.H. Gitzen, Alumina as a Ceramic Material, The American Ceramic Society, Westerville, 1970 pp.3. [7] R.G. Frieser, Journal of the Electrochemical Society 113 (1966) 357–360. [8] A.L. Dragoo, J.J. Diamond, Journal of the American Ceramic Society 50 (1967) 568–574. [9] T.C. Chou, T.G. Nieh, Journal of the American Ceramic Society 74 (1991) 2270–2279. [10] A.K. Jonscher, Thin Solid Films 1 (1967) 213–234. [11] J.M. Andersson, E. Wallin, U. Helmersson, U. Kreissig, E.P. Munger, Thin Solid Films 513 (2006) 57–59. [12] C. Merckling, M. El-Kazzi, G. Delhaye, M. Gendry, G. Saint-Girons, G. Hollinger, L. Largeau, G. Patriarche, Applied Physics Letters 89 (2006) 232907. [13] O. Zywitzki, K. Goedicke, H. Morgner, Surface and Coatings Technology 14 (2002) 151–152. [14] K. Koski, J. Holsa, P. Juliet, Surface and Coatings Technology 115 (1999) 163–171. [15] I. Safi, Surface and Coatings Technology 127 (2000) 203–218. [16] J. Musil, V. Valvoda, S. Kadlee, J. Vyskocil, IPAT 87, Proceedings of the Sixth International Conference on Ion and Plasma Assisted Techniques, Brighton UK, 1987, p. 184. [17] T. Larsson, C. nender, S. Berg, Journal of Vacuum Science and Technology A6 (3) (1988) 1832. [18] A. Okamoto, T. Serikawa, Thin Solid Films 137 (1986) 143–151. [19] S. Maniv, W.D. Westwood, Journal of Applied Physics 51 (1) (1980) 718–725. [20] Yongtaek Hwang, Kyuyoung Heo, Chang Hwan Chang, Man Kil Joo, Moonhor Ree, Thin Solid Films 510 (2006) 159–163. [21] S. Prasanna, G. Mohan Rao, S. Jayakumar, M.D. Kannan, V. Ganesan, Thin Solid Films 520 (2012) 2689–2694. [22] /www.phy.cuhk.edu.hk/surface/XPSPEAK/S. [23] Jaebum Kim, Kuntal Chakrabarti, Jinho Lee, Ki-Young Oh, Chongmu Lee, Materials Chemistry and Physics 78 (2003) 733–738. [24] Chang-Soo Yang, Jung-Sik Kim, Jang-Woo Choi, Moo-Hyun Kwon, Young-Joo Kim, Jeong-Gil Choi, Geug-Tae Kim, Journal of Industrial and Engineering Chemistry 6 (3) (2000) 149–156. [25] N.R. Rajopadhye, S.B. Dake, S.V. Bhoraskar, Thin Solid Films (1986) 127–138. [26] L. Hodgkinson, W. Sheel, M. Yates, E. Pemble, Plasma Processes and Polymers 3 (2006) 597–605. [27] C. Tornow, P.-L.M. Noeske, S. Dieckhoff, R. Wilken, K. Gartner, Applied Surface Science 252 (2005) 1959–1965. [28] Niraj Joshi, A.K. Debanath, D.K. Aswal, K.P. Muthe, M. Senthilkumar, S.K. Gupta, J.V. Yakhmi, Vacuum 79 (2005) 178–185. [29] A. Anshu, S. Pradhan, Ismat Shah, M. Karl, Review of Scientific Instruments 73 (2002) 3841–3845. [30] L. Zhang, H.C. Jiang, C. Liu, J.W. Dong, P. Chow, Journal of Physics D: Applied Physics 40 (2007) 3707–3713.
S. Prasanna et al. / Materials Science in Semiconductor Processing 16 (2013) 705–711
[31] G. Balakrishnan, P. Kuppusami, S. Tripura Sundari, R. Thirumurugesan, V. Ganesan, E. Mohandas, D. Sastikumar, Thin Solid Films 518 (2010) 3898–3902. [32] B.G. Segda, M. Jacquet, C. Caperaa, G. Baud, J.P. Besse, Annales de l’Universite de Ouagadougou, serie B 4 (1996) 265. [33] R.D. Gould, A.K. Hassan, Thin Solid Films 223 (1993) 334–340. [34] B. Karunagaran, S.J. Chung, E.-K. Suh, D. Mangalaraj, Physica B 369 (2005) 129–134. [35] P. Matheswaran, R. Sathyamoorthy, R. Saravanakumar, S. Velumani, Materials Science and Engineering: B 174 (2010) 269–272. [36] F. Argall, A.K. Jonscher, Thin Solid Films 2 (1968) 185–210. [37] H. Birey, Journal of Applied Physics 49 (1978) 2898–2904. [38] M.A. Afifi, A.E. Bekheet, E. Abd Elwahhab, H.E. Atyia, Vacuum 61 (2001) 9–17.
[39] [40] [41] [42] [43] [44] [45] [46] [47]
711
Aswini Ghosh, Physical Review B: 41 (3) (1990) 1479–1488. A.R. Long, Advances in Physics 31 (1982) 553–637. M. Pollak, T.H. Geballe, Physical Review 122 (1961) 1742–1753. M. Pollak, G.E. Pike, Physical Review Letters 28 (1972) 1449–1451. M. Anwar, C.A. Hogarth, Journal of Materials Science 25 (9) (1990) 3906–3909. S.R. Elliot, Advances in Physics 36 (2) (1987) 135–218. A.K. Johnscher, Thin Solid Films 36 (1976) 1–20. A.I. Lakatos, M. Abkowitz, Physical Review B: 3 (1971) 1791–1800. J. Kocka, J. Kristofik, Journal of Physica Status Solidi (A) 45 (1978) 559–564.
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Composition, structure and electrical properties of DC reactive magnetron sputtered Al2O3 thin films S. Prasanna a, G. Krishnendu a, S. Shalini a, P. Biji b, G. Mohan Rao c, S. Jayakumar a,n, R. Balasundaraprabhu a a b c
Thin Film Center, Department of Physics, PSG College of Technology, Coimbatore-641 004, India PSG Institute of Advanced Studies, Coimbatore-641 004, India Department of Instrumentation, Indian Institute of Science (IISc), Bangalore-560 012, India
a r t i c l e i n f o
abstract
Available online 16 January 2013
Thin films of alumina (Al2O3) were deposited over Si /1 0 0S substrates at room temperature at an oxygen gas pressure of 0.03 Pa and sputtering power of 60 W using DC reactive magnetron sputtering. The composition of the as-deposited film was analyzed by X-ray photoelectron spectroscopy and the O/Al atomic ratio was found to be 1.72. The films were then annealed in vacuum to 350, 550 and 750 1C and X-ray diffraction results revealed that both as-deposited and post deposition annealed films were amorphous. The surface morphology and topography of the films was studied using scanning electron microscopy and atomic force microscopy, respectively. A progressive decrease in the root mean square (RMS) roughness of the films from 1.53 nm to 0.7 nm was observed with increase in the annealing temperature. Al–Al2O3–Al thin film capacitors were then fabricated on p-type Si /1 0 0S substrate to study the effect of temperature and frequency on the dielectric property of the films and the results are discussed. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Thin films B. Reactive magnetron sputtering C. Electrical properties
1. Introduction Al2O3 is one of the widely used insulating materials for microelectronic applications with properties such as high dielectric constant (er ¼9), thermal stability, wide band gap (8.7 eV), high refractive index and excellent chemical stability [1–3]. Also, Al2O3 thin films are considered as one of the potential high-k dielectric materials to replace SiO2 layers in devices such as DRAMs and MOSFETs due to properties such low leakage current, high breakdown voltage (9 MV/cm), thermal stability on Si etc. [4]. The properties of the Al2O3 films are strongly influenced by its crystal structure, which can occur in several phases. Even
n Corresponding author. Tel.: þ91 422 4344453; fax: þ 91 0422 2573833. E-mail address: [email protected] (S. Jayakumar).
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.12.012
though, there are reports on more than 20 other phases of Al2O3 [6], the most important, and common, are a, g, y, and k [5]. Al2O3 films are amorphous up to 400 1C or more, and it is an important material for dielectric applications. For PVD, the phase transformation sequence is reported to be amorphous or a mix of amorphous and g-y þ d-a [7–9]. The amorphous films are found to differ in their electrical properties based on the preparation technique [10]. In particular, the deposition rate in evaporated films is found to have a profound effect on the dielectric constant. Recently, owing to the better control of the formation phases and quality, there has been a considerable amount of work pertaining to Al2O3 films prepared by physical methods, especially magnetron sputtering [11–14]. Reactive magnetron sputtering is one of the important methods for the deposition of wide range of compound thin films including oxides, nitrides, carbides, fluorides and arsenides [15]. DC reactive magnetron
706
S. Prasanna et al. / Materials Science in Semiconductor Processing 16 (2013) 705–711
sputtering is extensively used to prepare thin compound films of controllable stoichiometry and composition from an elemental target which can be easily purified hence resulting in the deposition of high-purity films [16–18]. In addition, by using DC reactive magnetron sputtering, the complexity of RF systems can be avoided since metallic targets are in general electrically and thermally conductive, which makes the cooling of these targets more efficient; thus the range of the applied power can be extended without the problem of target cracking [19,20]. Although Al2O3 thin films were studied extensively for many applications [15–20], not much work has been done on the effect of post-deposition annealing on the structure and morphology of Al2O3 films deposited by DC reactive magnetron sputtering and this is one of the objectives of the present work. In addition, Al–Al2O3–Al thin film capacitor was fabricated and dielectric properties and AC conduction mechanism were studied. 2. Experimental details Al2O3 thin films were prepared on well-cleaned Si /1 0 0S substrates held at room temperature by sputtering an aluminium target (99.99% pure) of thickness 3 mm and diameter 50 mm in an optimized oxygen partial pressure of 0.03 Pa and sputtering power of 60 W. The details of optimization are described elsewhere [21]. The target to substrate distance was fixed at 4 cm. High purity oxygen (4 N) and argon (4 N) was used as the reactive and sputtering gases, respectively. The sputtering chamber was evacuated to an ultimate vacuum of 9 10 4 Pa before admitting the reactive gas into the system. Both oxygen and argon were admitted into the chamber through needle valves and their flow rates were monitored individually using Aalborg mass flow controllers. Oxygen was introduced in the vicinity of the substrate holder to reduce target poisoning and to increase the partial pressure of oxygen close to the substrates. Prior to each deposition, the target was sputter cleaned in pure argon atmosphere to remove contaminants from the target surface. The sputtering gas was maintained at a pressure of 0.1 Pa and pre-sputtering was done for 10–20 min prior to deposition. The composition of the films was determined using a SPECS GmbH X-ray photoelectron spectrometer (Phoibos 100 MCD Energy Analyser) using Al Ka radiation (1486.6 eV). The residual pressure inside the analysis chamber was of the order of 2 10 8 Pa. Peaks were recorded with constant pass energy of 40 eV. The thickness of the films was measured using a Dektak 150 surface profilometer. The structure of the films was determined using a Shimadzu X-ray diffractometer operating in the 2y mode with Cu Ka radiation (l ¼ 0.15406 nm) and a fixed incidence angle of 21. Topography and surface roughness were determined with a root mean square (RMS) accuracy of 0.1 nm using an NTMDT atomic force microscope (AFM) in contact mode with a Si3N4 cantilever. Thickness of the films was measured using a Dektak 150 surface stylus profiler. For measuring the dielectric properties, Al/Al2O3/Al capacitor was fabricated on Si /1 0 0S substrate. Aluminium was used as the electrode material. The dielectric and AC electrical properties were studied
using a Chen Hwa LCZ meter in the frequency range of 40 Hz and 200 kHz at 50 mV. Dielectric parameters such as capacitance, relative permittivity, AC conductivity were measured. The dielectric measurements were carried out in vacuum by placing the sample in a small chamber maintained at a base pressure of 10 1 Pa.
3. Results and discussion 3.1. Composition Fig. 1 shows the X-ray photoelectron spectra of Al2O3 thin films sputtered at room temperature with an oxygen partial pressure of 0.03 Pa and sputtering power of 60 W. To a first approximation, the spectrum shows the presence of aluminium and oxygen with the presence of species such as Al 2s, Al 2p and O 1s. The X-ray photoelectron spectra were calibrated with respect to the C 1s spectral line. Figs. 2a, 2b and 3 show the high energy XPS scans for Al 2p, Al 2s and O 1s core levels, respectively. For determining the O/Al ratio high energy XPS scans for Al 2p and O 1s core levels were recorded for the sample shown as per standard procedure by using the sensitivity factors of the instrument [22]. The quantitative analysis of the oxygen and aluminium (O/Al atomic ratio) was carried out from a calculation of the peak area ratios of the respective states in the narrow band O 1s and Al 2p spectra, following Shirley background subtraction. The films were found to be oxygen rich with an O/Al atomic ratio of 1.72. The position of the Al 2p peak at 74.08 eV indicates that the aluminium present in the surface region is bound as Al2O3. Also, the Al 2p peak could be fitted only with a single peak, showing that the aluminium present in the film is completely oxidized in the form of Al2O3 [23–25]. The high resolution O 1s scan shows a single, broad peak at 531.03 eV indicating that the oxygen is in a bound state rather than in absorption state which can be expected at a higher binding energy of around 534.4 eV or more [26–28]. The peak pertaining to Al 2s is also present at 118.94 eV. The presence of carbon peak in the films at 284.5 eV could be attributed to the surface
Fig. 1. X-ray photoelectron spectra of Al2O3 film prepared at a power of 60 W.
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Fig. 4. Gracing incidence X-ray diffraction pattern of as-deposited Al2O3 film.
Fig. 2. (a) and (b) Narrow range X-ray photoelectron spectra of Al 2p and Al 2 s peaks of as-deposited Al2O3 thin film. Fig. 5. SEM image of an as-deposited Al2O3 film.
Fig. 3. Narrow range X-ray photoelectron spectra of O 1s peak of as-deposited Al2O3 thin film.
contamination from the atmosphere before insertion into the chamber [29]. 3.2. Structure and surface morphology X-ray diffraction pattern of annealed Al2O3 thin films deposited at a sputtering pressure of 60 W is shown in Fig. 4. The as-deposited film was found to be amorphous
(figure not shown) as reported widely in the literature [12–14]. In order to understand the crystallization behavior of the magnetron sputtered Al2O3 films, post-deposition annealing was done at 350, 550 and 750 1C in vacuum for 60 min. The films were found to be completely amorphous even after annealing at a high temperature of 750 1C. Fig. 5 shows the scanning electron micrograph of an as-deposited Al2O3 thin film at a sputtering power of 60 W. It is evident from the SEM analysis that the surface morphology of the film is very smooth. AFM analysis was done to ascertain the changes in the surface morphology of the films with post-deposition annealing. AFM images of the films annealed at 350, 550 and 750 1C are shown in Fig. 6a,b and c, respectively. The root mean square roughness (RMS) values were determined from the AFM images using WSxM software (Nanotec Electronica). The RMS roughness of the film annealed at 350 1C was found to be 1.53 nm. The RMS roughness decreased to 0.77 and 0.73 nm upon post-deposition annealing at 550 and 750 1C, respectively. Zhang et al. [30] reported RMS roughness values between 0.2 nm and 0.5 nm for Al2O3 films synthesized using atomic layer deposition (ALD). Balakrishnan et al. [31] reported RMS roughness values between 1.4 and 3.5 nm for pulsed laser deposited Al2O3
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Fig. 6. AFM image of annealed Al2O3 films prepared at a sputtering power of 60 W (a) annealed at 350 1C, (b) annealed at 550 1C, (c) annealed at 750 1C.
Fig. 7. Variation of capacitance with frequency.
Fig. 8. Variation of dielectric constant with frequency.
films. The decrease in the RMS roughness shows that the films became progressively smoother and defects were reduced with post-deposition annealing particularly at higher temperatures above 500 1C in accordance with the reported results [30].
temperatures and found to be independent of temperature at high frequencies. The large increase in capacitance towards the lower frequency region can be explained on the basis of the charge carriers being blocked at the electrodes, which leads to a space charge layer resulting in an increase in capacitance [33]. The space charge region developed near the electrode leads to a substantial increase in the capacitance at lower frequency below 1 kHz. The decrease in the value of capacitance with increase in frequency is attributed to the screening of the electric field across the film by charge redistribution [34]. The dielectric constant (er) was calculated from the knowledge of capacitance (C), film thickness (d), permittivity of free space (eo) and the area of the capacitor using the relation,
3.3. Electrical properties Thin film capacitors of the type Al/Al2O3/Al were fabricated on p-type Si /1 0 0S substrates. Initially, aluminium electrode was sputtered onto Si /1 0 0S substrates followed by reactive deposition of Al2O3 thin films. The thickness of the dielectric film was measured using a stylus profiler and found to be 200 nm. Aluminium was sputtered once again as the top electrode onto the dielectric film and dot capacitors having an area of 0.0613 10 4 m2 were fabricated. The thickness of the aluminium film (electrode) was kept above 500 nm in order to reduce lead resistance [32]. The dielectric properties of Al2O3 films were measured as a function of AC frequency from 40 Hz to 2 105 Hz at 50 mV. The fabricated capacitor was stabilized by annealing at 250 1C for around 4 h in vacuum. Fig. 7 shows the variation of capacitance of an Al2O3 thin film capacitor with frequency. From Fig. 7, it is observed that the capacitance of Al–Al2O3–Al thin film capacitor decreases with increase in frequency at different
er ¼ Cd=eo A
ð1Þ
The dielectric constant at room temperature was found to be 15 at a frequency of 1 kHz. Similar to the capacitance, the dielectric constant also shows a decreasing trend with increasing frequency at all temperatures as shown in Fig. 8. The trend seems to resemble the Debye relaxation model for orientational polarization [35]. The dielectric loss tangent was found to decrease with frequency attaining a minimum value of around 0.17 as evident from the Fig. 9.
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In the high frequency region the loss is proportional to frequency but at lower frequencies, depending upon temperature the loss is either decreasing or constant with frequency. Interestingly the dielectric loss tangent measured at room temperature was found to be almost constant with frequency and Argall and Jonscher [36] have reported similar results as a special case occurring only at room temperature. It is observed that the dielectric loss measured as the part of the polarization which is out of phase with the applied ac field, will generally increase with frequency after attaining a loss minimum.
3.3.1. Temperature coefficient of capacitance (TCC) The temperature coefficient of capacitance (TCC) was evaluated using the expression, TCC ¼ 1=C dC=dT
ð2Þ
The variation of capacitance with temperature is shown in Fig. 10 for different frequencies. It is seen that the TCC values increase with increase in frequency and found to be 227.16 104 ppm/K at a frequency of 10 kHz.
Fig. 9. Variation of dielectric loss tangent with frequency.
Fig. 10. Variation of capacitance of Al2O3 film with temperature.
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3.3.2. AC conduction studies AC measurements are very important for any dielectric material as it gives a lot of information about dynamic properties such as capacitance, conductivity and loss factor. AC measurements are also helpful in identifying the nature of conduction mechanism exhibited by the dielectric film. In the present work, the AC conductivity (s) was calculated using the relation [37]
s ¼ 2pf tander e0
ð3Þ
Variation of AC conductivity of an as-deposited Al2O3 film with frequency and temperature is shown in Fig. 11. From Fig. 11 it is evident that AC conductivity increases linearly with frequency. Also, it is evident that the AC conductivity is not highly sensitive to temperature at high frequency, whereas at low frequencies, there is clear dependence on temperature. The variation of AC conductivity is related to a power law which can be described according the following relation:
s ¼ Aos
ð4Þ
where o is the angular frequency, s is the frequency exponent and A is a complex proportionality constant independent of frequency [38]. The frequency exponent s is derived from Fig. 11 by calculating the slopes for different frequency ranges. The variation of frequency exponent s with temperature is shown for four different frequency ranges in Fig. 12. It can be seen that s depends on temperature, decreasing with increasing temperature at all frequency ranges. Different theoretical models have been proposed to explain the frequency and temperature dependence of AC conduction in materials [39–42]. From the figure, it is clear that s gradually decreases with increase in temperature. The correlated barrier hopping mechanism (CBH) considers the hopping of two electrons from a D to a neighbouring D þ centre over the potential barrier between them [39]. It predicts that s should be less than 1, decreasing with increasing temperature. It is observed that only in the frequency range, 0.4–10 kHz the value of s is below 1, in the frequency range 10–20 kHz, s varies between 1.2 and 0.2. Anwar and Hogarth [43] have reported similar results with a
Fig. 11. Variation of AC conductivity of Al2O3 film with frequency and temperature.
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depend on the power law. The frequency component s was found decrease with increase in temperature suggesting a correlated barrier hopping (CBH) mechanism of conduction.
Acknowledgement The authors would like to thank the Management and Principal for the support extended towards this research work. References
Fig. 12. Variation of frequency exponent s with temperature at different frequency ranges.
decrease in the value of s with increasing temperature and an increase in the value of s at higher frequencies. Elliot [44] has given the following expression for s based on the CBH model s ¼ 1 6kB T=B ð5Þ where kB is the Boltzmann’s constant, T is the temperature in Kelvin and B is the optical band gap. Using the optical band gap value of bulk Al2O3 (9.1 eV) the value of s at room temperature was derived using the above expression and found to be 0.98 which is in close agreement with the experimental values of 0.8 and 1.2 measured at frequency ranges 0.4–10 kHz and 10–20 kHz, respectively. Jonscher [45] in his detailed review has postulated that this correlated barrier hopping mechanism (CBH) is relevant to the highly amorphous structure of dielectric films, which are expected to have a large number of localized levels in the forbidden gap. Hence, from the measured value of s and its variation with frequency and temperature it can be concluded that correlated barrier hopping mechanism (CBH) is the most suitable model for describing the carrier transport mechanism in Al2O3 thin films. [46,47] 4. Conclusions Al2O3 thin films were deposited onto Si /1 0 0S substrates by DC reactive magnetron sputtering. The composition was determined from XPS analysis and the asdeposited films were found to be oxygen rich with an O/ Al atomic ratio of 1.72. The effect of post-deposition annealing on the structure and surface morphology of the Al2O3 films were investigated. The films were found to be amorphous even after annealing at 750 1C. The RMS roughness of the films was found to decrease with postdeposition annealing attaining a minimum value of 0.73 nm at 750 1C. Al-–Al2O3–Al thin film capacitor was fabricated on Si /1 0 0S substrate and the dielectric constant was found to be 15 at a frequency of 1 kHz. The capacitance was found to decrease with increasing frequency and this is attributed to interfacial polarization. The AC conductivity was determined and was found to
[1] Sung Bo Lee, Chi Won Ahn, Sang-Hoon Lee, Eun Kyu Her, Kyu Hwan Oh, Doh-Yeon Kim, Dong Nyung, Thin Solid Films 518 (2010) 4304–4311. [2] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, second ed. John Wiley & Sons, New York, 1976. [3] J. Guzman-Mendoza, M. Garcia- Hipolito, M. Aguilar-Frutis, C. Falcony-Guajardo, Journal of Physics: Condensed Matter 13 (2001) L955–L959. [4] G.D. Wilk, R.M. Wallace, J.M. Anthony, Journal of Applied Physics 89 (10) (2001). [5] O. Zywitzki, G. Hoetzsch, Surface and Coatings Technology 86–87 (1996) 640–647. [6] W.H. Gitzen, Alumina as a Ceramic Material, The American Ceramic Society, Westerville, 1970 pp.3. [7] R.G. Frieser, Journal of the Electrochemical Society 113 (1966) 357–360. [8] A.L. Dragoo, J.J. Diamond, Journal of the American Ceramic Society 50 (1967) 568–574. [9] T.C. Chou, T.G. Nieh, Journal of the American Ceramic Society 74 (1991) 2270–2279. [10] A.K. Jonscher, Thin Solid Films 1 (1967) 213–234. [11] J.M. Andersson, E. Wallin, U. Helmersson, U. Kreissig, E.P. Munger, Thin Solid Films 513 (2006) 57–59. [12] C. Merckling, M. El-Kazzi, G. Delhaye, M. Gendry, G. Saint-Girons, G. Hollinger, L. Largeau, G. Patriarche, Applied Physics Letters 89 (2006) 232907. [13] O. Zywitzki, K. Goedicke, H. Morgner, Surface and Coatings Technology 14 (2002) 151–152. [14] K. Koski, J. Holsa, P. Juliet, Surface and Coatings Technology 115 (1999) 163–171. [15] I. Safi, Surface and Coatings Technology 127 (2000) 203–218. [16] J. Musil, V. Valvoda, S. Kadlee, J. Vyskocil, IPAT 87, Proceedings of the Sixth International Conference on Ion and Plasma Assisted Techniques, Brighton UK, 1987, p. 184. [17] T. Larsson, C. nender, S. Berg, Journal of Vacuum Science and Technology A6 (3) (1988) 1832. [18] A. Okamoto, T. Serikawa, Thin Solid Films 137 (1986) 143–151. [19] S. Maniv, W.D. Westwood, Journal of Applied Physics 51 (1) (1980) 718–725. [20] Yongtaek Hwang, Kyuyoung Heo, Chang Hwan Chang, Man Kil Joo, Moonhor Ree, Thin Solid Films 510 (2006) 159–163. [21] S. Prasanna, G. Mohan Rao, S. Jayakumar, M.D. Kannan, V. Ganesan, Thin Solid Films 520 (2012) 2689–2694. [22] /www.phy.cuhk.edu.hk/surface/XPSPEAK/S. [23] Jaebum Kim, Kuntal Chakrabarti, Jinho Lee, Ki-Young Oh, Chongmu Lee, Materials Chemistry and Physics 78 (2003) 733–738. [24] Chang-Soo Yang, Jung-Sik Kim, Jang-Woo Choi, Moo-Hyun Kwon, Young-Joo Kim, Jeong-Gil Choi, Geug-Tae Kim, Journal of Industrial and Engineering Chemistry 6 (3) (2000) 149–156. [25] N.R. Rajopadhye, S.B. Dake, S.V. Bhoraskar, Thin Solid Films (1986) 127–138. [26] L. Hodgkinson, W. Sheel, M. Yates, E. Pemble, Plasma Processes and Polymers 3 (2006) 597–605. [27] C. Tornow, P.-L.M. Noeske, S. Dieckhoff, R. Wilken, K. Gartner, Applied Surface Science 252 (2005) 1959–1965. [28] Niraj Joshi, A.K. Debanath, D.K. Aswal, K.P. Muthe, M. Senthilkumar, S.K. Gupta, J.V. Yakhmi, Vacuum 79 (2005) 178–185. [29] A. Anshu, S. Pradhan, Ismat Shah, M. Karl, Review of Scientific Instruments 73 (2002) 3841–3845. [30] L. Zhang, H.C. Jiang, C. Liu, J.W. Dong, P. Chow, Journal of Physics D: Applied Physics 40 (2007) 3707–3713.
S. Prasanna et al. / Materials Science in Semiconductor Processing 16 (2013) 705–711
[31] G. Balakrishnan, P. Kuppusami, S. Tripura Sundari, R. Thirumurugesan, V. Ganesan, E. Mohandas, D. Sastikumar, Thin Solid Films 518 (2010) 3898–3902. [32] B.G. Segda, M. Jacquet, C. Caperaa, G. Baud, J.P. Besse, Annales de l’Universite de Ouagadougou, serie B 4 (1996) 265. [33] R.D. Gould, A.K. Hassan, Thin Solid Films 223 (1993) 334–340. [34] B. Karunagaran, S.J. Chung, E.-K. Suh, D. Mangalaraj, Physica B 369 (2005) 129–134. [35] P. Matheswaran, R. Sathyamoorthy, R. Saravanakumar, S. Velumani, Materials Science and Engineering: B 174 (2010) 269–272. [36] F. Argall, A.K. Jonscher, Thin Solid Films 2 (1968) 185–210. [37] H. Birey, Journal of Applied Physics 49 (1978) 2898–2904. [38] M.A. Afifi, A.E. Bekheet, E. Abd Elwahhab, H.E. Atyia, Vacuum 61 (2001) 9–17.
[39] [40] [41] [42] [43] [44] [45] [46] [47]
711
Aswini Ghosh, Physical Review B: 41 (3) (1990) 1479–1488. A.R. Long, Advances in Physics 31 (1982) 553–637. M. Pollak, T.H. Geballe, Physical Review 122 (1961) 1742–1753. M. Pollak, G.E. Pike, Physical Review Letters 28 (1972) 1449–1451. M. Anwar, C.A. Hogarth, Journal of Materials Science 25 (9) (1990) 3906–3909. S.R. Elliot, Advances in Physics 36 (2) (1987) 135–218. A.K. Johnscher, Thin Solid Films 36 (1976) 1–20. A.I. Lakatos, M. Abkowitz, Physical Review B: 3 (1971) 1791–1800. J. Kocka, J. Kristofik, Journal of Physica Status Solidi (A) 45 (1978) 559–564.