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Thermal treatment effects on interfacial layer formation between ZrO2 thin films and Si substrates
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Vacuum 80 (2005) 310–316 www.elsevier.com/locate/vacuum
Thermal treatment effects on interfacial layer formation between ZrO2 thin films and Si substrates Hoon Sang Choia,, Kwang Soo Seolb, Do Young Kima, Jun Sang Kwaka, Chang-Sik Sonc,, In-Hoon Choia a
Department of Materials Science and Engineering, Korea University, 5-1 Anam-dong, Sungbuk-gu, Seoul 136-701, Republic of Korea b Nanomaterial Processing Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1Hirosawa, Wako, Saitama 351-0198, Japan c Department of Photonics, Silla University, San 1-1, Gwaebop-dong, Sasang-gu, Busan 617-736, Republic of Korea Received 7 March 2005; received in revised form 31 May 2005; accepted 31 May 2005
Abstract This paper describes the growth condition of stoichiometric ZrO2 thin films on Si substrates and the interfacial structure of ZrO2 and Si substrates. The ZrO2 thin films were prepared by rf-magnetron sputtering from Zr target with mixed gas of O2 and Ar at room temperature followed by post-annealing in O2 ambient. The stoichiometric ZrO2 thin films with smooth surface were grown at high oxygen partial pressure. The thick Zr-free SiO2 layer was formed with both Zr silicide and Zr silicate at the interface between ZrO2 and Si substrate during the post-annealing process due to rapid diffusion of oxygen atoms through the ZrO2 thin films. After post annealing at 650–750 1C, the multi-interfacial layer shows small leakage current of less than 108 A/cm2 that is corresponding to the high-temperature processed thermal oxidized SiO2. r 2005 Elsevier Ltd. All rights reserved. Keywords: ZrO2; Sputtering; Silicide; Silicate; Leakage current
1. Introduction
Corresponding author. Department of Materials Science and Engineering, Korea University, 5-1 Anam-dong, Sungbukgu, Seoul 136-701, Republic of Korea. Tel./fax: +82 292 884 66. Corresponding author. Fax: +82 51 309 5652. E-mail addresses: [email protected] (H.S. Choi), [email protected] (C.-S. Son).
Scaling rules of metal oxide–semiconductor field effect transistors (MOSFETs) impose further reduction of gate oxide thickness in order to achieve the required high-speed and low-power operation [1]. However, in the case of SiO2 gate oxide, the reduction of the gate oxide thickness below 2 nm implies too high tunneling currents for
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.05.004
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microelectronic applications [2]. As a result, a variety of high dielectric constant (high-k) materials, such as Ta2O5, ZrO2, Al2O3, Y2O3, (Ba,Sr)TiO3, and (ZrO2)1x(Y2O3)x, have been examined to replace the SiO2 [3–7]. Among oxides, ZrO2 films attract much attention because of a high dielectric constant (25) and a large band gap (7.8 eV). However, one of the major concerns in employing the ZrO2 gate dielectrics is a reaction between the ZrO2 and Si substrate resulting in formation of an interfacial layer with relatively lower dielectric constants. For most of high-k materials, the interfacial layers that form between the dielectrics and Si with relatively lower dielectric constants deteriorate the merit of using the high-k dielectrics, since the interfacial layers play the main role in determining the overall electrical properties of devices. According to the thermodynamic analyses of bulk ZrO2 in contact with Si, the ZrO2 layer is expected to be stable [8]. However, the thin ZrO2 materials may exhibit quite different thermal stability. Experimental verification is important for the stability of thin-film materials. Recently, there have been many studies focused on the interfacial reaction between ZrO2 and Si substrates [9,10] in relation to the improvement of the thermal stability [11–13]. For the fine control and the reproducibility of the device performance, the understanding of interfacial properties is very important. In this article, we report the results of an investigation of diffusion phenomena of Si and O through ZrO2/Si interface in relation to the electrical properties.
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O2:Ar flow ratios of 5:15, 10:15, and 15:15 during the sputtering. After deposition, the samples were post-annealed at an oxygen flow rate of 100 cm3/ min. The heat treatments were carried out from 450 to 750 1C in a step of 100 1C for 1 h. After each annealing step, the samples were cooled down to room temperature at a cooling rate of 20 1C/min and subsequently analyzed by different methods. The crystalline structures were analyzed with X-ray diffraction (XRD) (Rigaku, model No. D/Max-2A) for 2y values ranging from 201 to 601. For composition and chemical state analysis, we used Rutherford backscattering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS) (Physical Electronics, no. Phi 5800) with an Al Ka monochromated source and a pass energy of 15 keV. The surface roughness of ZrO2 films was evaluated by atomic force microscopy (AFM) (Park Scientific Instruments, Autoprobe CP) using a non-contact mode. We observed the interfacial morphology change as a function of annealing temperature using high-resolution transmission electron microscopy (HRTEM). The top Pt gate electrodes of 285 mm in diameter were patterned by dc-sputtering. The capacitance–voltage (C– V) and current–voltage (I– V) characteristics were measured by a HP 4284A precision LCR meter and a HP 4156 parameter analyzer, respectively. To measure the C– V characteristics, the voltage was applied from accumulation to inversion with sweep rate of 0.2 V/s at frequency 1 MHz and amplitude 30 mV.
3. Results and discussion 2. Experimental The p-type Si(1 0 0) substrates were etched in a diluted HF (10%) solution to remove native oxide and contaminations, and then ZrO2 films were grown at room temperature by an rf-magnetron sputtering system with a Zr metal target (purity ¼ 99.99%) in a reactive oxygen ambient. The sputter chamber was initially pumped down to achieve a base pressure of 6 104 Pa, and then the total pressure was maintained at about 0.2 Pa. The ZrO2 thin films were deposited at different
The effect of sputtering gas flow ratios of O2 to Ar on the chemical state of the sputtered ZrO2 films was analyzed by XPS. Fig. 1a and b show Zr 3d peak and O 1s peak, which were observed in 2.8-nm-thick ZrO2 films grown at various gas flow ratios of O2 to Ar. Both peaks of Zr 3d5/2 and Zr 3d3/2 together with O 1s peak shifted to higher binding energies with an increase in the flow ratio of O2 to Ar. This is attributed to the increasing oxygen content of the ZrO2. The spectrum of Zr 3d orbits in the sputtered films grown at O2:Ar flow ratio of 15:15 consists of a pair of peaks at
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Fig. 1. XPS spectra of the (a) Zr 3d peaks and (b) O 1s peaks of ZrO2 (2.8 nm) films grown with different O2:Ar flow ratios and annealed at 750 1C for 1 h.
Fig. 2. AFM images of the ZrO2 (2.8 nm) films grown with different O2:Ar flow ratios: (a) 5:15, (b) 10:15, and (c) 15:15.
182.2 and 184.1 eV, both of which corresponds to Zr4+ state of pure ZrO2 [14]. These ZrO2 films show nearly stoichiometric composition according to the RBS analyses. In order to investigate the surface morphologies of the ZrO2 films (2.8 nm in thickness) grown with different flow ratio (O2:Ar), AFM surface images were measured in an area of 2 mm 2 mm as shown in Fig. 2. As the flow ratio of O2 to Ar increases, the surface roughness in terms of the rms decreased from 1.5 to 0.7 nm. These results indicate that the oxygen flow ratio during the deposition of ZrO2 film is very sensitive to the chemical state and surface morphology of deposited film.
Fig. 3 shows XRD patterns of ZrO2 films (265 nm in thickness) deposited at O2:Ar flow ratio of 15:15 and annealed at different temperatures for 1 h in an oxygen ambient. The XRD patterns revealed that a monoclinic structure of ZrO2 coexists with a tetragonal structure in the films regardless of the annealing temperature in the range 450–750 1C. The monoclinic structure is a stable phase whereas the tetragonal structure is metastable at temperatures below 1170 1C for bulk ZrO2. In the present ZrO2 thin films, the tetragonal phase was conventionally formed after postannealing at temperature below 1100 1C [15,16]. The formation of the metastable phase leads to enhanced oxygen diffusion to the interface,
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Fig. 3. XRD patterns of ZrO2 (265 nm) films grown at O2:Ar flow ratio 15:15 and annealed at different temperatures for 1 h in O2; t and m refer to tetragonal and monoclinic structures, respectively.
consequently resulting in an excessively thick SiO2 interfacial layer [17,18]. Thin ZrO2 films (2.8 nm thickness) were used for investigating the effects of annealing temperature on interfacial layer growth. Fig. 4 shows the HRTEM cross-sectional images of ZrO2 films grown at O2:Ar flow ratio of 15:15 and annealed at different temperatures for 1 h in oxygen ambient. Interfacial layer thickness gradually increases with the increase of annealing temperature up to 650 1C accompanied by an abrupt increase at 750 1C, while the ZrO2 film thickness is maintained at the original value (2.8 nm), as shown in Fig. 4c. Some studies on the structure and stability of thin ZrO2 layers on Si(1 0 0) substrate showed that the interfacial layer is pure SiO2, and that the ZrO2 layer is stable against silicate formation up to 900 1C, i.e. there is no reaction between SiO2 and ZrO2 [10]. On the other hand, other studies showed that the interfacial layer is not pure SiO2, but includes Zr atoms as the result of the reaction between Si or SiO2 and ZrO2 [19]. Therefore, in order to confirm the silicate formation, it is important to investigate the depth profiles of Zr and Si chemical bonding states in the interfacial layers grown during the annealing. Fig. 5 shows XPS spectra of Zr 3d and Si 2p orbits measured in the ZrO2 films annealed at
Fig. 4. Cross-sectional HRTEM images of ZrO2 films annealed at (a) 450 1C and (b) 750 1C. (c) Thickness of ZrO2 and interfacial layer at different annealing temperatures.
750 1C and subsequently thinned by means of sputter using Ar+ ions for different times, i.e. 0.4, 0.8, and 2 min. The spectrum of Zr 3d orbits at the
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is also observed in the spectrum of Si 2p orbit (See Fig. 5). These results reveal that annealing at 750 1C in oxygen ambience leads to the multiinterfacial layers between a top ZrO2 film and a bottom Si substrate, which consists of silicide, silicate and pure SiO2 layers in depth sequence. After 2 min thinning, strong peaks of Si 2p spectrum remains corresponding to pure SiO2. Another notable result in the Si 2p spectrum is emergence of Si bonding states of pure silicon in the film thinned by sputtering for 0.8 min together with those of silicate. This shows that pure silicon inclusion forms in the silicate layer. At high temperature during annealing, Si diffuses into the structure with rapid diffusivity and ZrO2 shows composition change, resulting in Zr silicate with excess Si. This excess Si leads to film decomposition and possible reaction paths can be written as [3,22]:
pristine surface (not sputtered) consists of a pair of peaks at 182.2 and 184.1 eV, both of which corresponds to Zr4+ state of pure ZrO2. The peaks first shift to the lower binding energies, and then the peaks shift to the higher binding energies as the sputter time increases, finally the peaks become negligible. The peak shift to the lower binding energies is due to the formation of silicide (pure Zr–Si bonding states), while the shift to the higher energies is attributed to the formation of Zr silicate [20]. Guittet et al. [21] also reported similar result that the Zr 3d state binding energy in ZrSiO4 is larger than that in ZrO2. Corresponding change
(1)
2ZrSiO4 þ 7Si ! ZrO þ 7SiO þ ZrSi2 :
(2)
However, further studies of the detailed forming mechanism of Zr silicide are necessary for a better understanding of the thermal stability of thin Zr oxide. Fig. 6 shows the C– V characteristics of Pt/ ZrO2/Si MOS capacitors annealed at different temperatures. As the annealing temperature 8 Capacitance (fF/µm2)
Fig. 5. XPS data of Zr 3d and Si 2p before (as introduced) and after increasing time of 4 keV Ar+ sputtering of ZrO2 (2.8 nm) films annealed at 750 1C for 1 h.
2ZrO2 þ 5Si ! ZrO þ 3SiO þ ZrSi2
450°C 550°C 650°C 750°C
6
4
2
0 -2
-1
0 Voltage (V)
1
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Fig. 6. C–V characteristics of ZrO2 (2.8 nm) films annealed at different temperatures for 1 h in O2 ambient.
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4. Conclusions
10-2 Current density (A/cm2)
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450°C 550°C 650°C 750°C
10-3 10-4 10-5 10-6 10-7 10-8 10-9 0
2
4 6 Voltage (V)
8
10
Fig. 7. I–V curves of ZrO2 (2.8 nm) films annealed at different temperatures for 1 h in O2 ambient.
increases, the accumulation capacitance of the films decreases accompanied by the decrease of equivalent oxide thickness (EOT). The largest value of measured capacitance density in accumulation was obtained at 550 1C annealing temperature and the EOT value was 1.8 nm. Especially, above 650 1C annealing temperature, the abrupt capacitance decrease is observed due to the abrupt increase of thickness of the interfacial layer as shown in Fig. 4. Fig. 7 shows the I– V characteristics of Pt/ZrO2/ Si MOS capacitors annealed at different temperatures. We obtained leakage currents of 5.3 107 and 1.15 107 A/cm2 at an applied voltage of 1 V after annealing at 450 and 550 1C, respectively. However, after annealing at 650 and 750 1C, leakage currents were remarkably decreased respectively to 4.36 108 and 7.86 109 A/cm2 at an applied voltage of 1 V. These values were as low as high-quality SiO2 gate oxide [23]. This suggests the electrical properties of the interfacial layers are governed by the Zr-free high-quality SiO2 layer grown during the annealing process. The highquality SiO2 layer will be grown due to the rapid diffusion of active atomic oxygen through the ZrO2 layer. The EOT values of present ZrO2 MOS capacitor are governed by the high-quality SiO2 layer. The ZrO2 MOS capacitor with a small EOT value is provided at the process temperature lower than 550 1C [24].
The dependence of different flow ratio (O2:Ar) during deposition and the chemical state and surface morphology of ZrO2 films on Si was investigated. The stoichiometric ZrO2 films with a smooth surface could be obtained by controlling ratio of O2 to Ar. The effects of annealing temperature on interfacial layer growth were investigated. The Zr-free SiO2 interfacial layer abruptly increased with increasing annealing temperature, especially at 750 1C, due to rapid oxygen diffusion through the ZrO2. We observed an Zr 3d peak indicating Zr silicide between ZrO2 and Zr silicate. This suggests that excess Si due to rapid diffusivity of Si into the structure can form the silicide layer on Zr silicate.
Acknowledgment The study was supported by joint research project under the Korea–Japan basic scientific promotion program through KOSEF in Korea. References [1] Kurokawa A, Nakamura K, Ichimura S, Moon DW. Appl Phys Lett 2000;76:493–5. [2] Fukuda H, Yasuda M, Iwabuchi T. Appl Phys Lett 1992;61:693–5. [3] Chang JP, Lin YS, Berger S, Kepten A, Bloom R, Levy S. J Vac Sci Technol B 2001;19(6):2137–43. [4] Zhang H, Solanki R, Roberds B, Bai G, Banerjee I. J Appl Phys 2000;87:1921–4. [5] Kang JF, Yu HY, Ren C, Li MF, Chan DSH, Hu H, et al. Appl Phys Lett 2004;84:1588–90. [6] Jeon S, Matsuda T, Unno A, Wasa K, Ichikawa Y, Hwang H. Vacuum 2002;65:19–25. [7] Hata T, Nakano S, Masuda Y, Sasaki K, Haneda Y, Wasa K. Vacuum 1998;51:583–90. [8] Hubbard KJ, Schlom DG. J Mater Res 1996;11:2757–76. [9] Jeon TS, White JM, Kwong DL. Appl Phys Lett 2001;78:368–70. [10] Copel M, Gribelyuk M, Gusev E. Appl Phys Lett 2000;76:436–8. [11] Chang JP, Lin YS. Appl Phys Lett 2001;79:3824–6. [12] Cho MH, Moon DW, Park SA, Rho YS, Kim YK, Jeong K, et al. Appl Phys Lett 2004;84:678–80. [13] Ahn H, Chen HW, Landheer D, Wu X, Chou LJ, Chao TS. Thin Solid Films 2004;455:318–22.
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[14] XPS handbook of the elements and native oxides, XPS International Inc., 1999. [15] Meriani S, editor. In: Proceedings of the International Conference Zirconia ’88. Advances in zirconia science and technology. New York: Elsevier; 1989. [16] Venkataraj S, Kappertz O, Weis H, Drese R, Jayavel R, Wuttig M. J Appl Phys 2002;92(7):3599–607. [17] Brossmann U, Wurschum R, Sodervall U, Schaefer HE. J Appl Phys 1999;85:7646–54. [18] Busch BW, Schulte WH, Garfunkel E, Gustafsson T, Qi W, Nieh R, et al. Phys Rev B 2000;62:290–3.
[19] Wu X, Landheer D, Graham MJ, Chen HW, Huang TY, Chao TS. J Cryst Growth 2003;250:479–85. [20] Wilk GD, Wallace RM. Appl Phys Lett 2000;76:112–4. [21] Guittet M, Crocombette J, Gautier-Soyer M. Phys Rev B 2001;63:125117–24. [22] Fulton CC, Cook Jr TE, Lucovsky G, Nemanich RJ. J Appl Phys 2004;96:2665–73. [23] Depas M, Vermeire B, Mertens PW, Van Meirhaeghe RL, Heyns MM. Solid-State Electron 1995;38:1465–71. [24] Qi W-J, Nieh R, Lee BH, Kang L, Jeon Y, Onichi K, et al. IEDM technical digest 1999.p. 145–8.
Vacuum 80 (2005) 310–316 www.elsevier.com/locate/vacuum
Thermal treatment effects on interfacial layer formation between ZrO2 thin films and Si substrates Hoon Sang Choia,, Kwang Soo Seolb, Do Young Kima, Jun Sang Kwaka, Chang-Sik Sonc,, In-Hoon Choia a
Department of Materials Science and Engineering, Korea University, 5-1 Anam-dong, Sungbuk-gu, Seoul 136-701, Republic of Korea b Nanomaterial Processing Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1Hirosawa, Wako, Saitama 351-0198, Japan c Department of Photonics, Silla University, San 1-1, Gwaebop-dong, Sasang-gu, Busan 617-736, Republic of Korea Received 7 March 2005; received in revised form 31 May 2005; accepted 31 May 2005
Abstract This paper describes the growth condition of stoichiometric ZrO2 thin films on Si substrates and the interfacial structure of ZrO2 and Si substrates. The ZrO2 thin films were prepared by rf-magnetron sputtering from Zr target with mixed gas of O2 and Ar at room temperature followed by post-annealing in O2 ambient. The stoichiometric ZrO2 thin films with smooth surface were grown at high oxygen partial pressure. The thick Zr-free SiO2 layer was formed with both Zr silicide and Zr silicate at the interface between ZrO2 and Si substrate during the post-annealing process due to rapid diffusion of oxygen atoms through the ZrO2 thin films. After post annealing at 650–750 1C, the multi-interfacial layer shows small leakage current of less than 108 A/cm2 that is corresponding to the high-temperature processed thermal oxidized SiO2. r 2005 Elsevier Ltd. All rights reserved. Keywords: ZrO2; Sputtering; Silicide; Silicate; Leakage current
1. Introduction
Corresponding author. Department of Materials Science and Engineering, Korea University, 5-1 Anam-dong, Sungbukgu, Seoul 136-701, Republic of Korea. Tel./fax: +82 292 884 66. Corresponding author. Fax: +82 51 309 5652. E-mail addresses: [email protected] (H.S. Choi), [email protected] (C.-S. Son).
Scaling rules of metal oxide–semiconductor field effect transistors (MOSFETs) impose further reduction of gate oxide thickness in order to achieve the required high-speed and low-power operation [1]. However, in the case of SiO2 gate oxide, the reduction of the gate oxide thickness below 2 nm implies too high tunneling currents for
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.05.004
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microelectronic applications [2]. As a result, a variety of high dielectric constant (high-k) materials, such as Ta2O5, ZrO2, Al2O3, Y2O3, (Ba,Sr)TiO3, and (ZrO2)1x(Y2O3)x, have been examined to replace the SiO2 [3–7]. Among oxides, ZrO2 films attract much attention because of a high dielectric constant (25) and a large band gap (7.8 eV). However, one of the major concerns in employing the ZrO2 gate dielectrics is a reaction between the ZrO2 and Si substrate resulting in formation of an interfacial layer with relatively lower dielectric constants. For most of high-k materials, the interfacial layers that form between the dielectrics and Si with relatively lower dielectric constants deteriorate the merit of using the high-k dielectrics, since the interfacial layers play the main role in determining the overall electrical properties of devices. According to the thermodynamic analyses of bulk ZrO2 in contact with Si, the ZrO2 layer is expected to be stable [8]. However, the thin ZrO2 materials may exhibit quite different thermal stability. Experimental verification is important for the stability of thin-film materials. Recently, there have been many studies focused on the interfacial reaction between ZrO2 and Si substrates [9,10] in relation to the improvement of the thermal stability [11–13]. For the fine control and the reproducibility of the device performance, the understanding of interfacial properties is very important. In this article, we report the results of an investigation of diffusion phenomena of Si and O through ZrO2/Si interface in relation to the electrical properties.
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O2:Ar flow ratios of 5:15, 10:15, and 15:15 during the sputtering. After deposition, the samples were post-annealed at an oxygen flow rate of 100 cm3/ min. The heat treatments were carried out from 450 to 750 1C in a step of 100 1C for 1 h. After each annealing step, the samples were cooled down to room temperature at a cooling rate of 20 1C/min and subsequently analyzed by different methods. The crystalline structures were analyzed with X-ray diffraction (XRD) (Rigaku, model No. D/Max-2A) for 2y values ranging from 201 to 601. For composition and chemical state analysis, we used Rutherford backscattering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS) (Physical Electronics, no. Phi 5800) with an Al Ka monochromated source and a pass energy of 15 keV. The surface roughness of ZrO2 films was evaluated by atomic force microscopy (AFM) (Park Scientific Instruments, Autoprobe CP) using a non-contact mode. We observed the interfacial morphology change as a function of annealing temperature using high-resolution transmission electron microscopy (HRTEM). The top Pt gate electrodes of 285 mm in diameter were patterned by dc-sputtering. The capacitance–voltage (C– V) and current–voltage (I– V) characteristics were measured by a HP 4284A precision LCR meter and a HP 4156 parameter analyzer, respectively. To measure the C– V characteristics, the voltage was applied from accumulation to inversion with sweep rate of 0.2 V/s at frequency 1 MHz and amplitude 30 mV.
3. Results and discussion 2. Experimental The p-type Si(1 0 0) substrates were etched in a diluted HF (10%) solution to remove native oxide and contaminations, and then ZrO2 films were grown at room temperature by an rf-magnetron sputtering system with a Zr metal target (purity ¼ 99.99%) in a reactive oxygen ambient. The sputter chamber was initially pumped down to achieve a base pressure of 6 104 Pa, and then the total pressure was maintained at about 0.2 Pa. The ZrO2 thin films were deposited at different
The effect of sputtering gas flow ratios of O2 to Ar on the chemical state of the sputtered ZrO2 films was analyzed by XPS. Fig. 1a and b show Zr 3d peak and O 1s peak, which were observed in 2.8-nm-thick ZrO2 films grown at various gas flow ratios of O2 to Ar. Both peaks of Zr 3d5/2 and Zr 3d3/2 together with O 1s peak shifted to higher binding energies with an increase in the flow ratio of O2 to Ar. This is attributed to the increasing oxygen content of the ZrO2. The spectrum of Zr 3d orbits in the sputtered films grown at O2:Ar flow ratio of 15:15 consists of a pair of peaks at
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Å
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Fig. 1. XPS spectra of the (a) Zr 3d peaks and (b) O 1s peaks of ZrO2 (2.8 nm) films grown with different O2:Ar flow ratios and annealed at 750 1C for 1 h.
Fig. 2. AFM images of the ZrO2 (2.8 nm) films grown with different O2:Ar flow ratios: (a) 5:15, (b) 10:15, and (c) 15:15.
182.2 and 184.1 eV, both of which corresponds to Zr4+ state of pure ZrO2 [14]. These ZrO2 films show nearly stoichiometric composition according to the RBS analyses. In order to investigate the surface morphologies of the ZrO2 films (2.8 nm in thickness) grown with different flow ratio (O2:Ar), AFM surface images were measured in an area of 2 mm 2 mm as shown in Fig. 2. As the flow ratio of O2 to Ar increases, the surface roughness in terms of the rms decreased from 1.5 to 0.7 nm. These results indicate that the oxygen flow ratio during the deposition of ZrO2 film is very sensitive to the chemical state and surface morphology of deposited film.
Fig. 3 shows XRD patterns of ZrO2 films (265 nm in thickness) deposited at O2:Ar flow ratio of 15:15 and annealed at different temperatures for 1 h in an oxygen ambient. The XRD patterns revealed that a monoclinic structure of ZrO2 coexists with a tetragonal structure in the films regardless of the annealing temperature in the range 450–750 1C. The monoclinic structure is a stable phase whereas the tetragonal structure is metastable at temperatures below 1170 1C for bulk ZrO2. In the present ZrO2 thin films, the tetragonal phase was conventionally formed after postannealing at temperature below 1100 1C [15,16]. The formation of the metastable phase leads to enhanced oxygen diffusion to the interface,
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Fig. 3. XRD patterns of ZrO2 (265 nm) films grown at O2:Ar flow ratio 15:15 and annealed at different temperatures for 1 h in O2; t and m refer to tetragonal and monoclinic structures, respectively.
consequently resulting in an excessively thick SiO2 interfacial layer [17,18]. Thin ZrO2 films (2.8 nm thickness) were used for investigating the effects of annealing temperature on interfacial layer growth. Fig. 4 shows the HRTEM cross-sectional images of ZrO2 films grown at O2:Ar flow ratio of 15:15 and annealed at different temperatures for 1 h in oxygen ambient. Interfacial layer thickness gradually increases with the increase of annealing temperature up to 650 1C accompanied by an abrupt increase at 750 1C, while the ZrO2 film thickness is maintained at the original value (2.8 nm), as shown in Fig. 4c. Some studies on the structure and stability of thin ZrO2 layers on Si(1 0 0) substrate showed that the interfacial layer is pure SiO2, and that the ZrO2 layer is stable against silicate formation up to 900 1C, i.e. there is no reaction between SiO2 and ZrO2 [10]. On the other hand, other studies showed that the interfacial layer is not pure SiO2, but includes Zr atoms as the result of the reaction between Si or SiO2 and ZrO2 [19]. Therefore, in order to confirm the silicate formation, it is important to investigate the depth profiles of Zr and Si chemical bonding states in the interfacial layers grown during the annealing. Fig. 5 shows XPS spectra of Zr 3d and Si 2p orbits measured in the ZrO2 films annealed at
Fig. 4. Cross-sectional HRTEM images of ZrO2 films annealed at (a) 450 1C and (b) 750 1C. (c) Thickness of ZrO2 and interfacial layer at different annealing temperatures.
750 1C and subsequently thinned by means of sputter using Ar+ ions for different times, i.e. 0.4, 0.8, and 2 min. The spectrum of Zr 3d orbits at the
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is also observed in the spectrum of Si 2p orbit (See Fig. 5). These results reveal that annealing at 750 1C in oxygen ambience leads to the multiinterfacial layers between a top ZrO2 film and a bottom Si substrate, which consists of silicide, silicate and pure SiO2 layers in depth sequence. After 2 min thinning, strong peaks of Si 2p spectrum remains corresponding to pure SiO2. Another notable result in the Si 2p spectrum is emergence of Si bonding states of pure silicon in the film thinned by sputtering for 0.8 min together with those of silicate. This shows that pure silicon inclusion forms in the silicate layer. At high temperature during annealing, Si diffuses into the structure with rapid diffusivity and ZrO2 shows composition change, resulting in Zr silicate with excess Si. This excess Si leads to film decomposition and possible reaction paths can be written as [3,22]:
pristine surface (not sputtered) consists of a pair of peaks at 182.2 and 184.1 eV, both of which corresponds to Zr4+ state of pure ZrO2. The peaks first shift to the lower binding energies, and then the peaks shift to the higher binding energies as the sputter time increases, finally the peaks become negligible. The peak shift to the lower binding energies is due to the formation of silicide (pure Zr–Si bonding states), while the shift to the higher energies is attributed to the formation of Zr silicate [20]. Guittet et al. [21] also reported similar result that the Zr 3d state binding energy in ZrSiO4 is larger than that in ZrO2. Corresponding change
(1)
2ZrSiO4 þ 7Si ! ZrO þ 7SiO þ ZrSi2 :
(2)
However, further studies of the detailed forming mechanism of Zr silicide are necessary for a better understanding of the thermal stability of thin Zr oxide. Fig. 6 shows the C– V characteristics of Pt/ ZrO2/Si MOS capacitors annealed at different temperatures. As the annealing temperature 8 Capacitance (fF/µm2)
Fig. 5. XPS data of Zr 3d and Si 2p before (as introduced) and after increasing time of 4 keV Ar+ sputtering of ZrO2 (2.8 nm) films annealed at 750 1C for 1 h.
2ZrO2 þ 5Si ! ZrO þ 3SiO þ ZrSi2
450°C 550°C 650°C 750°C
6
4
2
0 -2
-1
0 Voltage (V)
1
2
Fig. 6. C–V characteristics of ZrO2 (2.8 nm) films annealed at different temperatures for 1 h in O2 ambient.
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4. Conclusions
10-2 Current density (A/cm2)
315
450°C 550°C 650°C 750°C
10-3 10-4 10-5 10-6 10-7 10-8 10-9 0
2
4 6 Voltage (V)
8
10
Fig. 7. I–V curves of ZrO2 (2.8 nm) films annealed at different temperatures for 1 h in O2 ambient.
increases, the accumulation capacitance of the films decreases accompanied by the decrease of equivalent oxide thickness (EOT). The largest value of measured capacitance density in accumulation was obtained at 550 1C annealing temperature and the EOT value was 1.8 nm. Especially, above 650 1C annealing temperature, the abrupt capacitance decrease is observed due to the abrupt increase of thickness of the interfacial layer as shown in Fig. 4. Fig. 7 shows the I– V characteristics of Pt/ZrO2/ Si MOS capacitors annealed at different temperatures. We obtained leakage currents of 5.3 107 and 1.15 107 A/cm2 at an applied voltage of 1 V after annealing at 450 and 550 1C, respectively. However, after annealing at 650 and 750 1C, leakage currents were remarkably decreased respectively to 4.36 108 and 7.86 109 A/cm2 at an applied voltage of 1 V. These values were as low as high-quality SiO2 gate oxide [23]. This suggests the electrical properties of the interfacial layers are governed by the Zr-free high-quality SiO2 layer grown during the annealing process. The highquality SiO2 layer will be grown due to the rapid diffusion of active atomic oxygen through the ZrO2 layer. The EOT values of present ZrO2 MOS capacitor are governed by the high-quality SiO2 layer. The ZrO2 MOS capacitor with a small EOT value is provided at the process temperature lower than 550 1C [24].
The dependence of different flow ratio (O2:Ar) during deposition and the chemical state and surface morphology of ZrO2 films on Si was investigated. The stoichiometric ZrO2 films with a smooth surface could be obtained by controlling ratio of O2 to Ar. The effects of annealing temperature on interfacial layer growth were investigated. The Zr-free SiO2 interfacial layer abruptly increased with increasing annealing temperature, especially at 750 1C, due to rapid oxygen diffusion through the ZrO2. We observed an Zr 3d peak indicating Zr silicide between ZrO2 and Zr silicate. This suggests that excess Si due to rapid diffusivity of Si into the structure can form the silicide layer on Zr silicate.
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