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Effects of the thermal annealing processes on praseodymium oxide based films grown on silicon substrates
Materials Science and Engineering B 118 (2005) 192–196
Effects of the thermal annealing processes on praseodymium oxide based films grown on silicon substrates Raffaella Lo Nigroa,∗ , Roberta G. Torob , Graziella Malandrinob , Vito Raineria , Ignazio L. Fragal`ab,1 b
a IMM sezione di Catania, CNR, Stradale Primo Sole n 50, 95127 Catania, Italy Dipartimento di Scienze Chimiche, Universit`a di Catania, INSTM, U.d.R. Catania, Viale A. Doria 6, 95125 Catania, Italy
Abstract We have investigated the effects of thermal annealing processes on Pr2 O3 /Pr–O–Si system grown using the metal organic chemical vapor deposition (MOCVD) technique from the Pr(tmhd)3 [(H-tmhd = 2,2,6,6-tetramethylheptane-3,5-dione)] precursor. The influence of different atmospheres (Ar and O2 ) during the annealing process has been investigated using transmission electron microscopy (TEM). The annealing processes have been carried out at two different temperatures, 800 and 900 ◦ C, for 4 h. The praseodymium films have been found to be stable in argon atmosphere up to 800 ◦ C whilst at 900 ◦ C the film crystallization has been observed. On the other hand, in oxygen environment, evidence of crystallization processes has already been detected at 800 ◦ C. The electron diffraction patterns of the crystallized films have shown some of the most intense reflections of the stoichiometric Pr8 Si6 O24 phase. © 2004 Elsevier B.V. All rights reserved. Keywords: Praseodymium oxide; MOCVD; High k; Dielectric; Thermal stability
1. Introduction New dielectric materials with sufficiently high permittivity are needed as future insulators to replace SiO2 in complementary metal oxide semiconductor (CMOS) devices [1]. Any alternative dielectric candidate must fulfil stringent requirements, such as high breakdown strength, low leakage current at operating voltage and low oxide trap charge. Moreover, it is well known that one of the most critical issues for the implementation of new dielectrics in substitution of silicon oxide is their structural and chemical stability versus thermal processes occurring during the fabrication of MOS devices. Many high k gate dielectric candidates consist of rare earths and 4B oxide materials, such as Y2 O3 , La2 O3 , Pr2 O3 , HfO2 and ZrO2 [2–5]. These materials are not stable on silicon under the required processing conditions, and react ∗ 1
Corresponding author. Tel.: +39 095 5968218; fax: +39 095 5968312. E-mail address: [email protected] (R. Lo Nigro). Co-corresponding author.
0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.12.027
at high temperature forming interfacial layers with reduced high k value. In particular, the variability in the interfacial layer following ZrO2 and HfO2 depositions leads to the conclusion that the growth of such layers is difficult to control [6]. Moreover, another potential problem of ZrO2 and HfO2 is that they have been observed to crystallize at relatively low temperatures [7]. A better approach is the implementation of silicates and/or aluminates, which have shown a greater chemical and structural compatibility with silicon. In fact, hafnium or zirconium silicates have been tested as potential gate oxides and demonstrated to remain amorphous and chemically stable [8–10]. Generally, high k films have been grown by physical vapor deposition techniques, such as molecular beam epitaxy (MBE) and reactive sputtering. Nevertheless, there have been recently huge efforts to develop metal organic chemical vapor deposition (MOCVD) or atomic layer deposition (ALD) processes because of their major advantages such as high film uniformity, and superior conformal step coverage [11,12]. In particular, some interest has been devoted to the MOCVD and
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ALD growth of rare earth oxides and/or silicates and in this context praseodymium oxide based films can be interesting candidates. In our previous reports we have shown that Pr2 O3 /Pr–O–Si layers have been obtained by MOCVD process at 750 ◦ C using the Pr(tmhd)3 [(H-tmhd = 2,2,6,6-tetramethylheptane3,5-dione)] precursor [13–16]. Here we report the study of the thermal stability of the Pr2 O3 /Pr–O–Si system. We tested the structural and chemical stability of the deposited layers upon varying the annealing temperature in the 800–900 ◦ C range. Moreover, the influence of the process atmosphere has been evaluated as well.
2. Experimental The growth of praseodymium oxide films was carried out on HF treated p-type Si(0 0 1) substrate. A hot-wall, horizontal low-pressure MOCVD reactor was used and deposition experiments were performed at total pressure of about 13.3 Pa. The deposition temperature was fixed at 750 ◦ C and the Pr(tmhd)3 precursor was transported in the deposition zone by an Ar carrier gas flow. Details of deposition procedures were described elsewhere [13,15]. Thermal treatments were performed on a tube furnace at 800 and 900 ◦ C for 4 h. All the thermal processes were carried out at atmospheric pressure under both inert Ar atmosphere and reactive ambient using a N2 ÷ O2 (70 ÷ 30) mixture. During the tube furnace treatments the heating rate has been 40 ◦ C/min. Microstructures and morphologies were investigated by X-ray diffraction (XRD) [Bruker-AXS D5005 - diffractometer, using a G¨oebel mirror to parallel the Cu K␣ radiation operating at 40 kV and 30 mA] and cross-sectional transmission electron microscopy [(field emission guntransmission electron microscope) FEG-TEM JEOL 2010F equipped with the Gatan imagining filter (GIF), operating at 200 KeV].
3. Results The structural characterization of the praseodymium oxide layers obtained at 750 ◦ C through the MOCVD process from the Pr(tmhd)3 precursor has been performed by X-ray diffraction (XRD) as well as by transmission electron microscopy (TEM) analyses. The XRD and the selected area electron diffraction (SAED) patterns of praseodymium oxide films deposited at 750 ◦ C indicated that the random hexagonal Pr2 O3 phase has been formed [13–16]. The cross-sectional TEM micrographs of an as-deposited Pr2 O3 film on silicon substrate have been shown in Fig. 1. The low magnification image (Fig. 1a) consists of a Pr2 O3 film about 90 nm thick, which has been formed on silicon substrate for 45 min.
Fig. 1. (a) Low-magnification bright field TEM image of a 90 nm praseodymium oxide film and (b) high-resolution TEM image of a 9 nm praseodymium oxide film, both grown at 750 ◦ C.
Special attention has been devoted to the interface microstructure since the interface state density and the carrier transport in the channel region are strongly affected by the composition and electronic structure of the high k materials/silicon interface. In the present case, the interface microstructure has been investigated by high-resolution TEM. The HR-TEM image of an as-deposited Pr2 O3 film consists of a SiO2 layer, an amorphous praseodymium containing layer and crystalline Pr2 O3 grains. Previous energy filtered TEM studies on this deposited system have demonstrated that the amorphous intermediate praseodymium layer is an oxygen rich zone [13]. Moreover, by decreasing the deposition time to 10 min, only a ≈9 nm thick amorphous layer has been grown (Fig. 1b). Note that the film growth rate is much lower in the initial stages of deposition [16]. However, in order to assess the chemical composition of this layer X-ray photoelectron spectroscopy (XPS) analysis
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has been performed. In particular, significant compositional information has been deducted by the XPS results related to the Si 2p region. In fact, the Si 2p XPS spectrum (Fig. 2) revealed that the amorphous Pr-containing layer is consistent with the formation of a Pr silicate phase [17], probably caused by reaction between silicon substrate and Pr2 O3 film during the deposition process. However, there is no evidence for Pr Si bonds so concluding that no highly undesirable silicides [18] have been formed. Regarding the thermal stability of the as-deposited layers, post-deposition annealing processes have been carried out and particular attention has been devoted to the structural modification of the amorphous praseodymium silicate layer. Films consisting of the Pr2 O3 /Pr–O–Si system, about 30 nm thick, i.e. deposited for about 20 min at 750 ◦ C, have been prepared and their thermal stability has been tested in different atmospheres (N2 ÷ O2 and Ar) and at two different annealing temperatures (800 and 900 ◦ C) for 4 h. In Fig. 3a is shown the TEM cross-section of one of these samples subjected to a tube furnace annealing process in inert ambient (pure Ar) at 800 ◦ C. This treatment does not modify the structure of the entire system, namely, SiO2 layer, amorphous praseodymium silicate layer, and Pr2 O3 crystalline grains. In fact, neither the thickness nor the crystallinity of these layers change after this treatment as confirmed by the TEM cross-section. On the other hand, the thermal process in Ar atmosphere at 900 ◦ C has caused the crystallization of the amorphous silicate layer and, as shown in Fig. 3b, instead of a two praseodymium containing layers only a single crystalline layer is visible. Moreover, the thickness of the SiO2 layer greatly increases from about 2 nm in the as-deposited sample to about 7 nm in the thermal treated sample. The thermal treatments carried out at 800 ◦ C in non-inert atmosphere consisting of a 70 ÷ 30 N2 ÷ O2 mixture, have transformed the Pr2 O3 /Pr–Si–O system in a single crystalline layer, as shown in Fig. 4, and the thickness of SiO2 bottom layer has increased up to 5 nm. The chemical composition of the crystallized samples has been determined by electron diffraction pattern. The observation of spots at 0.35, 0.28, 0.26, 0.23, 0.21, 0.19 nm distances, are related to the (0 0 2), (1 1 2), (2 0 2), (2 2 1), (1 1 3) and (3 1 2) reflections of the stoichiometric Pr6 Si8 O24 silicate phase.
Fig. 2. XPS Si 2p spectrum for a 6 nm thick praseodymium oxide based film deposited on Si(0 0 1) substrate at 750 ◦ C.
Fig. 3. High-resolution TEM image of a 30 nm praseodymium oxide film/Si substrate interface after annealing for 4 h in Ar ambient at 800 ◦ C (a) and at 900 ◦ C (b).
Fig. 4. High-resolution TEM image of the praseodymium oxide film/Si substrate interface after annealing at 800 ◦ C for 4 h in O2 ambient.
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The thermal process in reactive oxygen ambient has not been performed at 900 ◦ C, because it was intuitively expected to produce the same results. Finally, note that the film/substrate interface shows no mechanical damages after the annealings in both inert and oxidizing ambient.
4. Discussion The evaluation of thermal stability of candidate high k materials is a crucial issue in the perspective of their implementation as gate dielectric in CMOS devices. In some cases, prediction on the thermal stability can be deducted by considering the phase diagram. For instance, the ZrO2 –SiO2 phase diagram is very well characterized [19,20], while the HfO2 –SiO2 phase diagram, is only partially known [21]. Nevertheless, it has been assumed that differences between the two systems are too small to be relevant and thus the ZrSiO4 phase diagram characteristics have been extended to HfSiO4 system. From these data, it is expected that the Hf silicate films separates into crystalline HfO2 domains embedded into an amorphous SiO2 matrix upon cooling from high temperature annealing. This thermal behavior has been explained considering the presence of a liquid miscibility gap in the HfO2 –SiO2 system analogously to that observed in the ZrO2 –SiO2 system. The region of liquid immiscibility can be extended as a metastable miscibility gap that allows an amorphous film to lower its free energy by separating in two phases with compositions defined by the metastable liquid lines [22]. On the other hand, an understanding of the thermal stability of rare earth silicates is still limited and, in this work, thermal stability of the MOCVD fabricated Pr silicate layers has been experimentally evaluated. Changes in the praseodymium deposited film microstructure associated with phase segregation and crystallization as a function of both annealing temperature and process atmosphere have been observed. Experimental results clearly indicate that the oxygen atmosphere induces a crystallization process of the amorphous praseodymium silicate layer and an increase of SiO2 layer thickness at lower temperature (800 ◦ C) than in the case of argon treatments (900 ◦ C). The formation of a thick SiO2 layer beneath the silicate layer has been also observed in other rare earths M2 O3 /M–O–Si stacks, where M = La, Gd or Y, after thermal annealing. The growth of the interfacial SiO2 layer, during the annealing process in O2 ambient, is mainly attributed to the oxidation of the silicon substrate by the diffusion of oxygen species through M2 O3 films from the annealing ambient. The diffusivity of oxygen through Y2 O3 , for instance, has been estimated to be 8.8 × 10−12 cm2 /s at 800 ◦ C [23]. This value is high enough for the oxygen species to spread through 20 nm thick Y2 O3 films. Therefore, the diffusion of oxygen species through the overlaying Y2 O3 films induces the oxidation of silicon, and the subsequent growth of yttrium silicate.
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We observed that the silicon oxide grew upon annealing in the Ar ambient as well as in the O2 ambient. In the former, for the formation of silicon oxide, the source of the oxygen species could be the ambient gas oxygen impurity or the Pr deposited film itself. The amount of oxygen in the Ar gas ambient is smaller than 1.33 × 10−2 Pa. On the other hand, the Pr2 O3 phase is a stable phase in the praseodymium–oxygen system. Therefore, we speculate that the growth of the silicon oxide during the annealing process in Ar gas ambient could be due to the existence of a great amount of oxygen in the amorphous silicate layer as already observed in the EF-TEM chemical maps [13]. In regard to the praseodymium silicate crystallization, it is likely that the Pr2 O3 /Pr–O–Si interface moves towards the surface thus giving rise to conversion of the Pr2 O3 layer into the silicate. However, more detailed studies are in progress to better understand the oxygen role in the silicate formation process.
5. Conclusions We investigated the interfacial thermal stability of praseodymium oxide based films fabricated by MOCVD upon annealing in Ar and O2 ambients using TEM characterizations. We observed that the thermal process at 800 ◦ C in O2 atmosphere induces the formation of the crystalline Pr6 Si8 O24 silicate phase. At the same annealing temperature and under Ar, the praseodymium films are stable, while at 900 ◦ C some compositional variations have been observed and the formation of the crystalline silicate layer is again evident. The crystallization process in oxygen atmosphere can be easily predicted on the basis of the results on other rare earth oxides, while only speculative observations can be proposed in the case of crystallization in inert ambient. In this latter case, the formation of the crystalline silicate could be a consequence of oxygen interdiffusion inside the deposited layers and towards the substrate surface resulting in the further growth of the SiO2 layer.
Acknowledgments The authors thank the MIUR for the financial support within the FISR thematic activities: Molecular Nanotechnologies for the storage and transmission of information (Nanotechnologies and Microsystems program).
References [1] D.A. Muller, T. Sorsch, S. Moccio, F.H. Baumann, G. Timp, Nature 399 (1999) 758. [2] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 89 (2001) 5243.
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[3] S. Guha, E. Cartier, M.A. Gribelyuk, N.A. Bojarczuk, M.C. Copel, Appl. Phys. Lett. 77 (2000) 2710. [4] J. Kwo, M. Hong, A.R. Koetan, K.T. Queeney, Y.J. Chabal, J.P. Mannaerts, T. Boone, J.J. Krajewski, A.M. Segent, J.M. Rosamilia, Appl. Phys. Lett. 77 (2000) 130. [5] H.J. Osten, J.P. Liu, H.J. Mussig, Appl. Phys. Lett. 80 (2002) 297. [6] J. Robertson, J. Vac. Sci. Technol. B 18 (2000) 1785. [7] P.S. Lysaght, P.J. Chen, R. Bergmann, T. Messina, R.W. Murto, H.R. Huff, J. Non-Cryst. Solids 304 (2002) 54. [8] G.D. Wilk, R.M. Wallace, Appl. Phys. Lett. 74 (1999) 2854. [9] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 87 (2000) 484. [10] B.C. Hendrix, A.S. Borovik, C. Xu, J.F. Roeder, T.H.J. Bevan, M.R. Visokay, J.J. Chambers, A.L.P. Rotondaro, H. Bu, L. Colombo, Appl. Phys. Lett. 80 (2002) 2362. [11] T. Kodas, M. Hampden-Smith, The Chemistry of Metal CVD, WileyVCH, Weinheim, Germany, 1994. [12] H.O. Pierson, Handbook of Chemical Vapor Deposition, Noesy, New York, 1992. [13] R. Lo Nigro, R. Toro, G. Malandrino, V. Raineri, I.L. Fragal`a, Adv. Mater. 15 (2003) 1071.
[14] R. Lo Nigro, V. Raineri, C. Bongiorno, R. Toro, G. Malandrino, I.L. Fragal`a, Appl. Phys. Lett. 83 (2003) 129. [15] R. Lo Nigro, R. Toro, G. Malandrino, I.L. Fragal`a, J. Electr. Soc. 151 (2004) F206. [16] R. Lo Nigro, R. Toro, G. Malandrino, G.G. Condorelli, V. Raineri, I.L. Fragal`a, Adv. Funct. Mater. 15 (2005), in press. [17] D. Schmeisser, H.J. Mussig, Solid State Electron. 46 (2003) 1607. [18] J.X. Wu, Z.M. Wang, F.Q. Li, M.S. Ma, Appl. Surf. Sci. 225 (2004) 229. [19] H. Kim, P.C. McIntyre, J. Appl. Phys. 92 (2002) 5094. [20] R.G.J. Ball, M.A. Mignanelli, T.I. Barry, J.A. Gibsby, J. Nucl. Mater. 201 (1993) 128. [21] V.N. Parfenenkov, R.B. Grebenshchikov, N.A. Toropov, Dokl. Akad. Nauk. SSSR 185 (1969) 840. [22] P. Lysaght, B. Foran, S. Stemmer, G. Bersuker, J. Bennett, R. Tichy, L. Larson, H.R. Huff, Microelectr. Eng. 69 (2003) 182. [23] S.-K. Kang, D.-H. Ko, E.-H. Kim, M.H. Cho, C.N. Wang, Thin Solid Films 353 (1999) 8.
Effects of the thermal annealing processes on praseodymium oxide based films grown on silicon substrates Raffaella Lo Nigroa,∗ , Roberta G. Torob , Graziella Malandrinob , Vito Raineria , Ignazio L. Fragal`ab,1 b
a IMM sezione di Catania, CNR, Stradale Primo Sole n 50, 95127 Catania, Italy Dipartimento di Scienze Chimiche, Universit`a di Catania, INSTM, U.d.R. Catania, Viale A. Doria 6, 95125 Catania, Italy
Abstract We have investigated the effects of thermal annealing processes on Pr2 O3 /Pr–O–Si system grown using the metal organic chemical vapor deposition (MOCVD) technique from the Pr(tmhd)3 [(H-tmhd = 2,2,6,6-tetramethylheptane-3,5-dione)] precursor. The influence of different atmospheres (Ar and O2 ) during the annealing process has been investigated using transmission electron microscopy (TEM). The annealing processes have been carried out at two different temperatures, 800 and 900 ◦ C, for 4 h. The praseodymium films have been found to be stable in argon atmosphere up to 800 ◦ C whilst at 900 ◦ C the film crystallization has been observed. On the other hand, in oxygen environment, evidence of crystallization processes has already been detected at 800 ◦ C. The electron diffraction patterns of the crystallized films have shown some of the most intense reflections of the stoichiometric Pr8 Si6 O24 phase. © 2004 Elsevier B.V. All rights reserved. Keywords: Praseodymium oxide; MOCVD; High k; Dielectric; Thermal stability
1. Introduction New dielectric materials with sufficiently high permittivity are needed as future insulators to replace SiO2 in complementary metal oxide semiconductor (CMOS) devices [1]. Any alternative dielectric candidate must fulfil stringent requirements, such as high breakdown strength, low leakage current at operating voltage and low oxide trap charge. Moreover, it is well known that one of the most critical issues for the implementation of new dielectrics in substitution of silicon oxide is their structural and chemical stability versus thermal processes occurring during the fabrication of MOS devices. Many high k gate dielectric candidates consist of rare earths and 4B oxide materials, such as Y2 O3 , La2 O3 , Pr2 O3 , HfO2 and ZrO2 [2–5]. These materials are not stable on silicon under the required processing conditions, and react ∗ 1
Corresponding author. Tel.: +39 095 5968218; fax: +39 095 5968312. E-mail address: [email protected] (R. Lo Nigro). Co-corresponding author.
0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.12.027
at high temperature forming interfacial layers with reduced high k value. In particular, the variability in the interfacial layer following ZrO2 and HfO2 depositions leads to the conclusion that the growth of such layers is difficult to control [6]. Moreover, another potential problem of ZrO2 and HfO2 is that they have been observed to crystallize at relatively low temperatures [7]. A better approach is the implementation of silicates and/or aluminates, which have shown a greater chemical and structural compatibility with silicon. In fact, hafnium or zirconium silicates have been tested as potential gate oxides and demonstrated to remain amorphous and chemically stable [8–10]. Generally, high k films have been grown by physical vapor deposition techniques, such as molecular beam epitaxy (MBE) and reactive sputtering. Nevertheless, there have been recently huge efforts to develop metal organic chemical vapor deposition (MOCVD) or atomic layer deposition (ALD) processes because of their major advantages such as high film uniformity, and superior conformal step coverage [11,12]. In particular, some interest has been devoted to the MOCVD and
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ALD growth of rare earth oxides and/or silicates and in this context praseodymium oxide based films can be interesting candidates. In our previous reports we have shown that Pr2 O3 /Pr–O–Si layers have been obtained by MOCVD process at 750 ◦ C using the Pr(tmhd)3 [(H-tmhd = 2,2,6,6-tetramethylheptane3,5-dione)] precursor [13–16]. Here we report the study of the thermal stability of the Pr2 O3 /Pr–O–Si system. We tested the structural and chemical stability of the deposited layers upon varying the annealing temperature in the 800–900 ◦ C range. Moreover, the influence of the process atmosphere has been evaluated as well.
2. Experimental The growth of praseodymium oxide films was carried out on HF treated p-type Si(0 0 1) substrate. A hot-wall, horizontal low-pressure MOCVD reactor was used and deposition experiments were performed at total pressure of about 13.3 Pa. The deposition temperature was fixed at 750 ◦ C and the Pr(tmhd)3 precursor was transported in the deposition zone by an Ar carrier gas flow. Details of deposition procedures were described elsewhere [13,15]. Thermal treatments were performed on a tube furnace at 800 and 900 ◦ C for 4 h. All the thermal processes were carried out at atmospheric pressure under both inert Ar atmosphere and reactive ambient using a N2 ÷ O2 (70 ÷ 30) mixture. During the tube furnace treatments the heating rate has been 40 ◦ C/min. Microstructures and morphologies were investigated by X-ray diffraction (XRD) [Bruker-AXS D5005 - diffractometer, using a G¨oebel mirror to parallel the Cu K␣ radiation operating at 40 kV and 30 mA] and cross-sectional transmission electron microscopy [(field emission guntransmission electron microscope) FEG-TEM JEOL 2010F equipped with the Gatan imagining filter (GIF), operating at 200 KeV].
3. Results The structural characterization of the praseodymium oxide layers obtained at 750 ◦ C through the MOCVD process from the Pr(tmhd)3 precursor has been performed by X-ray diffraction (XRD) as well as by transmission electron microscopy (TEM) analyses. The XRD and the selected area electron diffraction (SAED) patterns of praseodymium oxide films deposited at 750 ◦ C indicated that the random hexagonal Pr2 O3 phase has been formed [13–16]. The cross-sectional TEM micrographs of an as-deposited Pr2 O3 film on silicon substrate have been shown in Fig. 1. The low magnification image (Fig. 1a) consists of a Pr2 O3 film about 90 nm thick, which has been formed on silicon substrate for 45 min.
Fig. 1. (a) Low-magnification bright field TEM image of a 90 nm praseodymium oxide film and (b) high-resolution TEM image of a 9 nm praseodymium oxide film, both grown at 750 ◦ C.
Special attention has been devoted to the interface microstructure since the interface state density and the carrier transport in the channel region are strongly affected by the composition and electronic structure of the high k materials/silicon interface. In the present case, the interface microstructure has been investigated by high-resolution TEM. The HR-TEM image of an as-deposited Pr2 O3 film consists of a SiO2 layer, an amorphous praseodymium containing layer and crystalline Pr2 O3 grains. Previous energy filtered TEM studies on this deposited system have demonstrated that the amorphous intermediate praseodymium layer is an oxygen rich zone [13]. Moreover, by decreasing the deposition time to 10 min, only a ≈9 nm thick amorphous layer has been grown (Fig. 1b). Note that the film growth rate is much lower in the initial stages of deposition [16]. However, in order to assess the chemical composition of this layer X-ray photoelectron spectroscopy (XPS) analysis
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has been performed. In particular, significant compositional information has been deducted by the XPS results related to the Si 2p region. In fact, the Si 2p XPS spectrum (Fig. 2) revealed that the amorphous Pr-containing layer is consistent with the formation of a Pr silicate phase [17], probably caused by reaction between silicon substrate and Pr2 O3 film during the deposition process. However, there is no evidence for Pr Si bonds so concluding that no highly undesirable silicides [18] have been formed. Regarding the thermal stability of the as-deposited layers, post-deposition annealing processes have been carried out and particular attention has been devoted to the structural modification of the amorphous praseodymium silicate layer. Films consisting of the Pr2 O3 /Pr–O–Si system, about 30 nm thick, i.e. deposited for about 20 min at 750 ◦ C, have been prepared and their thermal stability has been tested in different atmospheres (N2 ÷ O2 and Ar) and at two different annealing temperatures (800 and 900 ◦ C) for 4 h. In Fig. 3a is shown the TEM cross-section of one of these samples subjected to a tube furnace annealing process in inert ambient (pure Ar) at 800 ◦ C. This treatment does not modify the structure of the entire system, namely, SiO2 layer, amorphous praseodymium silicate layer, and Pr2 O3 crystalline grains. In fact, neither the thickness nor the crystallinity of these layers change after this treatment as confirmed by the TEM cross-section. On the other hand, the thermal process in Ar atmosphere at 900 ◦ C has caused the crystallization of the amorphous silicate layer and, as shown in Fig. 3b, instead of a two praseodymium containing layers only a single crystalline layer is visible. Moreover, the thickness of the SiO2 layer greatly increases from about 2 nm in the as-deposited sample to about 7 nm in the thermal treated sample. The thermal treatments carried out at 800 ◦ C in non-inert atmosphere consisting of a 70 ÷ 30 N2 ÷ O2 mixture, have transformed the Pr2 O3 /Pr–Si–O system in a single crystalline layer, as shown in Fig. 4, and the thickness of SiO2 bottom layer has increased up to 5 nm. The chemical composition of the crystallized samples has been determined by electron diffraction pattern. The observation of spots at 0.35, 0.28, 0.26, 0.23, 0.21, 0.19 nm distances, are related to the (0 0 2), (1 1 2), (2 0 2), (2 2 1), (1 1 3) and (3 1 2) reflections of the stoichiometric Pr6 Si8 O24 silicate phase.
Fig. 2. XPS Si 2p spectrum for a 6 nm thick praseodymium oxide based film deposited on Si(0 0 1) substrate at 750 ◦ C.
Fig. 3. High-resolution TEM image of a 30 nm praseodymium oxide film/Si substrate interface after annealing for 4 h in Ar ambient at 800 ◦ C (a) and at 900 ◦ C (b).
Fig. 4. High-resolution TEM image of the praseodymium oxide film/Si substrate interface after annealing at 800 ◦ C for 4 h in O2 ambient.
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The thermal process in reactive oxygen ambient has not been performed at 900 ◦ C, because it was intuitively expected to produce the same results. Finally, note that the film/substrate interface shows no mechanical damages after the annealings in both inert and oxidizing ambient.
4. Discussion The evaluation of thermal stability of candidate high k materials is a crucial issue in the perspective of their implementation as gate dielectric in CMOS devices. In some cases, prediction on the thermal stability can be deducted by considering the phase diagram. For instance, the ZrO2 –SiO2 phase diagram is very well characterized [19,20], while the HfO2 –SiO2 phase diagram, is only partially known [21]. Nevertheless, it has been assumed that differences between the two systems are too small to be relevant and thus the ZrSiO4 phase diagram characteristics have been extended to HfSiO4 system. From these data, it is expected that the Hf silicate films separates into crystalline HfO2 domains embedded into an amorphous SiO2 matrix upon cooling from high temperature annealing. This thermal behavior has been explained considering the presence of a liquid miscibility gap in the HfO2 –SiO2 system analogously to that observed in the ZrO2 –SiO2 system. The region of liquid immiscibility can be extended as a metastable miscibility gap that allows an amorphous film to lower its free energy by separating in two phases with compositions defined by the metastable liquid lines [22]. On the other hand, an understanding of the thermal stability of rare earth silicates is still limited and, in this work, thermal stability of the MOCVD fabricated Pr silicate layers has been experimentally evaluated. Changes in the praseodymium deposited film microstructure associated with phase segregation and crystallization as a function of both annealing temperature and process atmosphere have been observed. Experimental results clearly indicate that the oxygen atmosphere induces a crystallization process of the amorphous praseodymium silicate layer and an increase of SiO2 layer thickness at lower temperature (800 ◦ C) than in the case of argon treatments (900 ◦ C). The formation of a thick SiO2 layer beneath the silicate layer has been also observed in other rare earths M2 O3 /M–O–Si stacks, where M = La, Gd or Y, after thermal annealing. The growth of the interfacial SiO2 layer, during the annealing process in O2 ambient, is mainly attributed to the oxidation of the silicon substrate by the diffusion of oxygen species through M2 O3 films from the annealing ambient. The diffusivity of oxygen through Y2 O3 , for instance, has been estimated to be 8.8 × 10−12 cm2 /s at 800 ◦ C [23]. This value is high enough for the oxygen species to spread through 20 nm thick Y2 O3 films. Therefore, the diffusion of oxygen species through the overlaying Y2 O3 films induces the oxidation of silicon, and the subsequent growth of yttrium silicate.
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We observed that the silicon oxide grew upon annealing in the Ar ambient as well as in the O2 ambient. In the former, for the formation of silicon oxide, the source of the oxygen species could be the ambient gas oxygen impurity or the Pr deposited film itself. The amount of oxygen in the Ar gas ambient is smaller than 1.33 × 10−2 Pa. On the other hand, the Pr2 O3 phase is a stable phase in the praseodymium–oxygen system. Therefore, we speculate that the growth of the silicon oxide during the annealing process in Ar gas ambient could be due to the existence of a great amount of oxygen in the amorphous silicate layer as already observed in the EF-TEM chemical maps [13]. In regard to the praseodymium silicate crystallization, it is likely that the Pr2 O3 /Pr–O–Si interface moves towards the surface thus giving rise to conversion of the Pr2 O3 layer into the silicate. However, more detailed studies are in progress to better understand the oxygen role in the silicate formation process.
5. Conclusions We investigated the interfacial thermal stability of praseodymium oxide based films fabricated by MOCVD upon annealing in Ar and O2 ambients using TEM characterizations. We observed that the thermal process at 800 ◦ C in O2 atmosphere induces the formation of the crystalline Pr6 Si8 O24 silicate phase. At the same annealing temperature and under Ar, the praseodymium films are stable, while at 900 ◦ C some compositional variations have been observed and the formation of the crystalline silicate layer is again evident. The crystallization process in oxygen atmosphere can be easily predicted on the basis of the results on other rare earth oxides, while only speculative observations can be proposed in the case of crystallization in inert ambient. In this latter case, the formation of the crystalline silicate could be a consequence of oxygen interdiffusion inside the deposited layers and towards the substrate surface resulting in the further growth of the SiO2 layer.
Acknowledgments The authors thank the MIUR for the financial support within the FISR thematic activities: Molecular Nanotechnologies for the storage and transmission of information (Nanotechnologies and Microsystems program).
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