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Ar plasma
Vacuum 92 (2013) 85e89
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Dry etching properties of TiO2 thin films in O2/CF4/Ar plasma Kyung-Rok Choi, Jong-Chang Woo, Young-Hee Joo, Yoon-Soo Chun, Chang-Il Kim* Department of Electrical and Electronics Engineering, Chung-Ang University, Heukseok-dong, Dongjak-gu, Seoul 156-756, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 August 2012 Received in revised form 30 October 2012 Accepted 17 November 2012
In this work, the etching properties of titanium dioxide (TiO2) thin film in additions of O2 at CF4/Ar plasma were investigated. The maximum etch rate of 179.4 nm/min and selectivity of TiO2 of 0.6 were obtained at an O2/CF4/Ar (¼3:16:4 sccm) gas mixing ratio. In addition, the etch rate and selectivity were measured as a function of the etching parameters, such as the RF power, DC-bias voltage, and process pressure. The efficient destruction of the oxide bonds by ion bombardment, which was produced from the chemical reaction of the etched TiO2 thin film, was investigated by X-ray photoelectron spectroscopy. To determine the re-deposition of sputter products and reorganization of such residues on the surface, the surface roughness of TiO2 thin film were examined using atomic force microscopy. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: TiO2 XPS CF4/Ar Etching AFM
1. Introduction Recently, the dimension of memory cell has been shrunk to improve the speed of the conventional devices, and reduce the power consumption. However, due to tunneling effect, the current gate oxides used as the gate dielectric have the current leakage problem under 50 nm thicknesses. To overcome this problem, high dielectric constant materials, such as HfO2, Al2O3, ZrO2, Ta2O5, BST, PZT, and TiO2, have been studied to substitute for the conventional gate oxide [1]. These high-k materials have been used as insulators for metal-insulator-metal (MIM) capacitors to prevent the current leakage. However, when these high-k materials are substituted for SiO2, new insulator must meet the current metal semiconductor processing procedure. Therefore, to satisfy the current processing procedure, it is important to understand the etching mechanism of the materials. Among many high-k materials, TiO2 has a very high dielectric constant. Thus, despite the disadvantage of low band gap, TiO2 thin film is important candidate to use as the gate insulator. Using TiO2 thin film as the gate insulator can improve the density of integrated circuits by increasing the dielectric constant of the insulators, and it is necessary to attain a high etch rate and high selectivity for the harmony of stability and performance of MIM capacitor [2]. Until now, the dry etching properties of TiO2 thin film in fluorine-based plasma have been reported as ion enhanced
* Corresponding author. E-mail address: [email protected] (C.-I. Kim). 0042-207X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2012.11.009
chemical etching [3]. But, the effect of O2 gas in fluorine-based plasma has not been rarely reported, and the relationships between plasma parameters and chemical reactions were not fully explored in detail [4,5]. So, the dry etching properties of TiO2 thin film with addition of O2 should be investigated [6,7]. In this study, TiO2 thin film was etched using inductively coupled plasma system with O2 added CF4/Ar gas chemistries. We investigated the etching properties of TiO2 thin film and the selectivity of TiO2 to SiO2 in O2/CF4/Ar plasma. To understand chemical reaction, the etched surface of TiO2 thin film was investigated by X-ray photoelectron spectroscopy (XPS). To determine the re-deposition of sputter products and reorganization of such residues on the surface, atomic force microscopy (AFM) was used to examine the surface roughness of TiO2 thin film. 2. Experiment The TiO2 thin film was deposited by the E-beam evaporator system. The source material for deposition was TiO2 pellets. The overall thickness of the metal deposited was measured by a quartz crystal microbalance, and the final thickness of the TiO2 thin film is about 200 nm. The etching experiments are performed in a planar ICP system which is schematically shown in Fig. 1. The planar ICP etching equipment has a 3.5-turn copper coil which is located above the 24 mm thick horizontal quartz window and 13.56 MHz RF power was applied to the coil to induce inductively coupled plasma. Another 13.56 MHz RF power was applied to the substrate to induce DC-bias voltage to the wafer [8]. The etching properties of TiO2 thin film was investigated in addition of O2 into CF4/Ar gas
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Fig. 2. Schematic diagram of masking layer of TiO2 thin film and SiO2 thin film.
Fig. 1. Schematic diagram of an inductively coupled plasma system.
mixing ratio, and measured as a function of the etching parameters listed in Table 1. The etch rate of the TiO2 thin film were measured by a surface profiler (KLA Tencor, a-step 500). SiO2 thin film was used as masking layer of TiO2 thin film. As shown in Fig. 2(a), the thickness of TiO2 thin film is about 200 nm and SiO2 thin film is about 500 nm above the silicon. To measure the etch rate of the TiO2 thin film after etching process, SiO2 thin film was removed by Hydrofluoric acid (HF) through the wet etching. Also, SiO2 thin film was used to investigate the selectivity of TiO2 to SiO2. As shown in Fig. 2(b), the thickness of PR is about 2000 nm and SiO2 thin film is about 500 nm above the silicon. The chemical reaction on the surface of the etched TiO2 thin film was investigated using XPS (PHI 5000 VersaProbe. Ulvac-PHI). The Al Ka radiation (hv ¼ 1486.6 eV) was used as X-ray source and the minimum energy resolution is about 0.48 eV during the testing. The carbon C 1s peak at 284.6 eV is used as a reference for charging correction. The detailed information of the film was provided by the spectra angle of 45 . The surface roughness of TiO2 thin film was examined using AFM (park scientific instrument, Auto probe cpap-0100). All of the samples for the XPS and AFM analysis were bare TiO2 thin film without any photo-resist patterns and the size of the samples was 1 1 cm2.
enhances the F atom generation because of the oxidation of CFx, and that the increase in the etch rate of TiO2 thin film results from the increased concentration of F atoms in O2/CF4/Ar plasma. However, as the O2 gas was increased from 3 to 9 sccm, the etch rate of TiO2 thin film decreased rapidly from 179.4 nm/min to 137.5 nm/min. This excessive increase of O2 gas ratio induced an adsorption of O on the TiO2 surface and then partially blocked absorption of fluorine gas. As a result, the etch process was inhibited and the etch rate was decreased. On the other hand, the selectivity of TiO2 to SiO2 monotonically increased from 0.57 to 0.81. Fig. 4 shows the etch rate of TiO2 thin film and the selectivity of TiO2 to SiO2 as a function of the RF power in O2/CF4/Ar (¼3:16:4 sccm) plasma. The DC-bias voltage was maintained at 150 V, and the process pressure was 2 Pa. As the RF power was raised from 600 to 800 W, the etch rate of TiO2 thin film monotonically increased from 136 nm/min to 208.3 nm/min, and the selectivity of TiO2 to SiO2 increased from 0.66 to 0.83. It can be seen that the increase in the etch rate of TiO2 thin film with increasing RF power may be explained by the acceleration of both the physical and chemical etch pathway. The increase of RF power causes the increase of the densities and fluxes of positive ions and F atoms
3. Results and discussion Fig. 3 shows the etch rate of TiO2 thin film and selectivity of TiO2 to SiO2 with addition of O2 in CF4/Ar plasma. The RF power was maintained at 700 W, a DC-bias voltage was 150 V, and the process pressure was 2 Pa. It can be seen that, as the O2 gas was increased from 0 to 3 sccm in the CF4/Ar (¼16:4 sccm) plasma, the etch rate of TiO2 thin film increased from 154.1 nm/min to 179.4 nm/min. The maximum etch rate of TiO2 was 179.4 nm/min in O2/CF4/Ar (¼3:16:4 sccm) plasma. From these results, it may assume that the increasing gas ratio of O2 in CF4/Ar plasma Table 1 Process conditions. Gas mixture
O2/CF4/Ar
RF power (W) DC-bias voltage (V) Process pressure (Pa)
700 150 2
Fig. 3. Etch rate of TiO2 thin film and selectivity of TiO2 to SiO2 as a function of the O2/ CF4/Ar gas mixing ratio. The RF power was maintained at 700 W, the DC-bias voltage was 150 V, and the process pressure was 2 Pa.
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through the increase in both dissociation and ionization rates. Thus, the densities of reactive species and Ar ion flow rate were increased with increasing RF power, and the etch rate of TiO2 thin film enhanced. The acceleration of chemical reactions as well as physical ion bombardment occurred simultaneously. As a result, by increasing of physical and chemical etch pathway, it leads to an increased etch rate of TiO2 thin film. Fig. 5 shows the etch rate of TiO2 thin film and the selectivity of TiO2 to SiO2 as a function of the DC-bias voltage in O2/CF4/Ar (¼3:16:4 sccm) plasma. The RF power was maintained at 700 W, and the process pressure was 2 Pa. As the DC-bias voltage increases from 50 to 250 V, the etch rate of the TiO2 thin film monotonically increases from 130.9 nm/min to 197.2 nm/min, whereas the selectivity of TiO2 to SiO2 decreases from 0.65 to 0.56. The increase in the etch rate of TiO2 thin film is attributed to the
increase of ion bombarding energy, which increased the etch rate of both TiO2 thin film and reaction products. These results with increasing DC-bias voltage can be explained by following reasons. A high DC-bias voltage enhanced the mean ion energy, resulting in an increase in the sputtering yields for both TiO2 thin film and reaction products through the physical sputtering by ion bombardment. So, the increasing of DC-bias voltage enhanced bond-breaking and increased sputter desorption of etch products from the surface of TiO2 thin film, and then leads to an increase of the etch rate. Fig. 6 shows the etch rate of TiO2 thin film and the selectivity of TiO2 to SiO2 as a function of the process pressure in O2/CF4/Ar (¼3:16:4 sccm) plasma. The RF power was maintained at 700 W, and the DC-bias voltage was 150 V. As the process pressures increased from 1.2 to 2.8 Pa, the etch rate of TiO2 thin film and the selectivity of TiO2 to SiO2 non-monotonically decreased from 187.7 nm/min to 138.7 nm/min and 0.60 to 0.50, respectively. The increase of the process pressure enhanced the density of the chemically neutral active species, which accelerates the chemical reaction. As a result, the byproduct on the surface of TiO2 thin film was increased during the etching process through the acceleration of the chemical reaction. However, because the mean free path and the ion energy decrease with increasing process pressure, the probability of ion to reach the TiO2 thin film was decreased. Therefore, because the effect of ion bombardment was reduced, the etch rate of TiO2 thin film was decreased. Therefore, the etch rate of TiO2 thin film was decreased with increasing the process pressure [9]. According to the above results, the domination of the chemical reactions may be explained by the following reasons. First, since the melting point of TiF4 is about 284 C, the domination of the chemical pathway cannot be related hardly to the volatility. Next, because the strength of the TieF chemical bond (272 kJ/mol) is lower than that of the TieO bond (383 kJ/mol), the F atoms formed in the plasma can react with TiO2 spontaneously and the ion bombardment enhances the chemical reaction by breaking the oxide bonds. As a result, the density of the radicals and ions increased with increasing the RF power and DC-bias voltage. The bonds of molecular were broken by the ion bombardment on the surface of the TiO2 thin film. Due to the higher ion density and increased number of broken bonds of molecular, chemical reactions
Fig. 5. Etch rate of TiO2 thin film and selectivity of TiO2 to SiO2 as a function of the DCbias voltage. The O2/CF4/Ar gas mixture was maintained at 3:16:4 sccm, the RF power was 700 W, and the process pressure was 2 Pa.
Fig. 6. Etch rate of TiO2 thin film and selectivity of TiO2 to SiO2 as a function of the process pressure. The O2/CF4/Ar gas mixture was maintained at 3:16:4 sccm, the RF power was 700 W, and the DC-bias voltage was 150 V.
Fig. 4. Etch rate of TiO2 thin film and selectivity of TiO2 to SiO2 as a function of the RF power. The O2/CF4/Ar gas mixture was maintained at 3:16:4 sccm, the DC-bias voltage was 150 V, and the process pressure was 2 Pa.
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of the surface were generated more frequently in the plasma. The byproducts of etched surface were easily evaporated by physical and chemical pathway. This conclusion is in good agreement with the data shown in from Figs. 2e5 [10e15]. For more detailed investigations of the chemical reaction between the TiO2 thin film and the etched TiO2 thin film as a function of O2/CF4/Ar gas mixing ratio, XPS analysis was performed and the results are presented in Fig. 7. We compared the chemical reaction difference between the etched surface and the as-deposited surface of TiO2 thin film. Fig. 7 shows the XPS narrow scan spectra of (a) Ti, (b) O, (c) F and (d) C, which were obtained from the TiO2 thin film surfaces in as-deposition, CF4/Ar plasma and O2/CF4/Ar plasma. Fig. 7(a) shows the photoelectron peak of Ti 2p from the as-deposited and etched TiO2 thin film. It can be seen that the Ti 2p peak of the as-deposited film was observed at 458.8 eV. After the TiO2 thin film were exposed to the CF4/Ar (¼16:4 sccm) plasma, the peak of Ti 2p at 458.8 eV was shifted to a higher binding energy and the maximum deviation is about þ0.1 eV. The shift of peak indicates that Ti chemically reacted with F-component species, and it led to the formation of TieF bonds on the surface of TiO2 thin film by ion bombardment. In other words, the chemical reaction of F radical was increased on the surface of TiO2 thin film, the physical
sputtering of Ar ion influenced on the formation of Ti component such as TiFx. After the TiO2 thin film were exposed to the O2/CF4/Ar (¼3:16:4 sccm) plasma, the peak of Ti 2p at 458.8 eV was shifted to a higher binding energy and the maximum deviation is about þ0.2 eV. The shift of peak indicates that Ti chemically reacted with F-component as well as O-component, and it led to the formation of TieO and TieF bonds on the surface of TiO2 thin film by ion bombardment. In other words, the chemical reaction of O radical was increased on the surface of TiO2, and the physical sputtering of Ar ion influenced on the formation of Ti component such as TieF, and TieO. Fig. 7(b) shows the photoelectron peak of O 1s from the as-deposited and etched TiO2 thin film surface. It can be seen that, the O 1s peak of the as-deposited film was observed at 530.2 eV. After the TiO2 thin film were exposed to the CF4/Ar (¼16:4 sccm) plasma, the peak of O 1s at 530.2 eV was shifted to a higher binding energy and the maximum deviation is about þ0.1 eV. It is expected that the association of TieO bonds by ion bombardment become increase owing to the formation of FeO bonds. The CeO bonds were also formed in the same way. Fig. 7(c) shows the photoelectron peak of F 1s from the as-deposited and etched TiO2 thin film surface. It can be seen that the F 1s peak of the as-deposited film was not observed. After the TiO2 thin film were exposed to the CF4/Ar (¼16:4 sccm) plasma, the peak of F 1s was
Fig. 7. The XPS narrow scan spectra of the etched TiO2 thin film. The RF power was maintained at 700 W, the DC-bias voltage was 150 V, and the process pressure was 2 Pa. (a) Ti 2p, (b) O 1s, (c) F 1s, (d) C 1s.
K.-R. Choi et al. / Vacuum 92 (2013) 85e89
observed at 646.1 eV. After the TiO2 thin film were exposed to the O2/CF4/Ar (¼3:16:4 sccm) plasma, the core F 1s peak was decreased significantly and peak of F 1s was not shifted. The core F 1s peak of the TiO2 in CF4/Ar plasma was higher than that of the TiO2 thin film in as-deposition, as demonstrated by the increase in the number of TieF bonds on the surface of TiO2 thin film by ion bombardment. Fig. 7(d) shows the photoelectron peak of C 1s from the asdeposited and etched TiO2 thin film surface. It can be seen that, the C 1s peak of the as-deposited film was 284.8 eV. After the TiO2 thin film were exposed to the O2/CF4/Ar (¼3:16:4 sccm) plasma, the peak of C 1s at 284.6 eV was shifted to a higher binding energy and the maximum deviation is about þ0.2 eV. It is expected that the dissociation of CF4 gas become increase owing to the formation of CeO bonds. Based on the result of XPS, it was disclosed that the chemical reactions with the F radicals and the physical bombardment of the Ar ions removed Ti and O [16,17]. To understand the surface state of TiO2 thin film, atomic force microscopy (AFM) was used to examine the surface roughness of TiO2 thin film. We compared the root mean square (RMS) difference between the etched surface and the as-deposited surface of TiO2 thin film. A. The The RMS roughness of as-deposited TiO2 thin film was 36.5 RMS roughness of TiO2 thin film as a function of CF4/Ar (¼16:4 sccm) plasma was 59.8 A, and the RMS roughness of TiO2 thin film as A. When the a function of O2/CF4/Ar (¼3:16:4 sccm) plasma was 29.8 O2 gas was increased from 0 to 3 sccm, the RMS roughness of TiO2 thin film decreased from 59.8 A to 29.8 A. The changes in the RMS roughness as a function of O2/CF4/Ar gas ratio may be explained by the following reason. Because byproducts in the etching process were accumulated on the etched surface of TiO2 thin film, the RMS roughness in the CF4/Ar (¼16:4 sccm) plasma was increased. However, when the O2 gas was added, the RMS roughness in the O2/ CF4/Ar (¼3:16:4 sccm) plasma was decreased because the byproducts reacted with the O2 gas and were evaporated on the etched surface of TiO2 thin film. It is important to provide information that the surface of TiO2 thin film was reformed by O2 gas in O2/CF4/Ar (¼3:16:4 sccm) plasma. As a result, when compared with the asdeposited TiO2 thin film, the surface morphology of etched TiO2 thin film in O2/CF4/Ar plasma was become smoother [18,19]. 4. Conclusion In this work, we investigated the dry etching characteristics of the TiO2 thin film in O2/CF4/Ar plasma as functions of the etching parameters, such as the RF power, DC-bias voltage, and process pressure. The maximum etch rate and selectivity of TiO2 thin film were 179.4 nm/min and 0.6 in O2/CF4/Ar (¼3:16:4 sccm) plasma, respectively. The RF power was maintained at 700 W, DC-bias
89
voltage was 150 V, and the process pressure was 2 Pa. As increasing the gas ratio of O2 in CF4/Ar plasma, it enhances the concentration of F atom, and then it increased the etch rate of TiO2 thin film. However, an excessive increase of the gas ratio of O2 in CF4/Ar plasma results in an adsorption of O on the TiO2 surface which partially blocked absorption of fluorine gas. As a result, due to an adsorption of O on the TiO2 surface, the etch process was inhibited and the etch rate was decreased. To understand chemical reaction, the surface of the etched TiO2 thin film was analyzed by XPS. The Ti of TiO2 thin film interacted with the O radicals in the O2 containing plasmas, and TieO bond remained on the etched surface, due to the low volatility of TieO bond. These byproducts on the surface can be effectively removed with the help of ion bombardment. The surface roughness of TiO2 thin film was examined using AFM. The RMS roughness was decreased and the surface morphology of TiO2 thin film was smoothed after etched in O2/CF4/ Ar plasma. This tendency of TiO2 etch pathway was similar to the etch properties, which the chemical etching was enhanced by the ion energy. Acknowledgments This work was financially supported by the Department of Electrical and Electrics Engineering, Chung-Ang University, Korea. References [1] Wilk GD, Wallance RM, Anthony JM. J Appl Phys 2001;89:5243. [2] Campbell SA, Glimer DC, Wang X, Hsieh M, Kim HS, Gladfelter WL, et al. IEEE Trans Electron Devices 1997;44:104. [3] Woo JC, Joo YH, Kim CI. Jpn J Appl Phys 2011;50:08KC02. [4] Ding J, Jenq JS, Kim GH, Maynard HL, Hamers JS. J Vac Sci Technol A 1993;11: 1283. [5] Fracassi F, d’Agostino R, Lamendola R, Mangieri I. J Vac Sci Technol A 1995; 13:335. [6] Kim HS, Gilmer DC, Campbell SA, Polla DL. Appl Phys Lett 1996;69:25. [7] Pan TM, Lei TF, Chao TS. Appl Phys Lett 2001;78:1439. [8] Kim GH, Kim CI. J Vac Sci Technol A 2006;24:1514. [9] Yang X, Woo JC, Um DS, Kim CI. Trans Electr Electron Mater 2010;11:202. [10] Kim HK, Bae JW, Kim TK, Kim KK, Seong TY, Adesida I. J Vac Sci Technol B 2003;21:1273. [11] Kim GH, Kim CI, Efremov AM. Vacuum 2005;79:231. [12] Cheng B, Cao M, Rao R, Inani A, Noorde PV, Greene WM, et al. IEEE Trans Electron Devices 1999;46:1537. [13] Lee SJ, Cho IH, Kim HW, Hong SJ, Lee HY. Trans Electr Electron Mater 2009; 10:177. [14] Kim YS, Shimogaki Y. J Vac Sci Technol A 2001;19:2642. [15] d’Agostino R, Fracassi F, Pacifico C. J Appl Phys 1992;72:4351. [16] Bertoti I, Mohai M, Sullivan JL, Saied SO. Appl Surf Sci 1995;84:357. [17] Norasetthekul S, Park PY, Baik KH, Lee KP, Jeong BS, Shishodia V, et al. Appl Surf Sci 2001;185:27. [18] Chiarello G, Barberi R, Amoddeo A, Caputi LS, Colavita E. Appl Surf Sci 1996; 99:15. [19] Liu ZH, Brown NMD, Mckinley A. Appl Surf Sci 1997;108:319.
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Dry etching properties of TiO2 thin films in O2/CF4/Ar plasma Kyung-Rok Choi, Jong-Chang Woo, Young-Hee Joo, Yoon-Soo Chun, Chang-Il Kim* Department of Electrical and Electronics Engineering, Chung-Ang University, Heukseok-dong, Dongjak-gu, Seoul 156-756, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 August 2012 Received in revised form 30 October 2012 Accepted 17 November 2012
In this work, the etching properties of titanium dioxide (TiO2) thin film in additions of O2 at CF4/Ar plasma were investigated. The maximum etch rate of 179.4 nm/min and selectivity of TiO2 of 0.6 were obtained at an O2/CF4/Ar (¼3:16:4 sccm) gas mixing ratio. In addition, the etch rate and selectivity were measured as a function of the etching parameters, such as the RF power, DC-bias voltage, and process pressure. The efficient destruction of the oxide bonds by ion bombardment, which was produced from the chemical reaction of the etched TiO2 thin film, was investigated by X-ray photoelectron spectroscopy. To determine the re-deposition of sputter products and reorganization of such residues on the surface, the surface roughness of TiO2 thin film were examined using atomic force microscopy. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: TiO2 XPS CF4/Ar Etching AFM
1. Introduction Recently, the dimension of memory cell has been shrunk to improve the speed of the conventional devices, and reduce the power consumption. However, due to tunneling effect, the current gate oxides used as the gate dielectric have the current leakage problem under 50 nm thicknesses. To overcome this problem, high dielectric constant materials, such as HfO2, Al2O3, ZrO2, Ta2O5, BST, PZT, and TiO2, have been studied to substitute for the conventional gate oxide [1]. These high-k materials have been used as insulators for metal-insulator-metal (MIM) capacitors to prevent the current leakage. However, when these high-k materials are substituted for SiO2, new insulator must meet the current metal semiconductor processing procedure. Therefore, to satisfy the current processing procedure, it is important to understand the etching mechanism of the materials. Among many high-k materials, TiO2 has a very high dielectric constant. Thus, despite the disadvantage of low band gap, TiO2 thin film is important candidate to use as the gate insulator. Using TiO2 thin film as the gate insulator can improve the density of integrated circuits by increasing the dielectric constant of the insulators, and it is necessary to attain a high etch rate and high selectivity for the harmony of stability and performance of MIM capacitor [2]. Until now, the dry etching properties of TiO2 thin film in fluorine-based plasma have been reported as ion enhanced
* Corresponding author. E-mail address: [email protected] (C.-I. Kim). 0042-207X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2012.11.009
chemical etching [3]. But, the effect of O2 gas in fluorine-based plasma has not been rarely reported, and the relationships between plasma parameters and chemical reactions were not fully explored in detail [4,5]. So, the dry etching properties of TiO2 thin film with addition of O2 should be investigated [6,7]. In this study, TiO2 thin film was etched using inductively coupled plasma system with O2 added CF4/Ar gas chemistries. We investigated the etching properties of TiO2 thin film and the selectivity of TiO2 to SiO2 in O2/CF4/Ar plasma. To understand chemical reaction, the etched surface of TiO2 thin film was investigated by X-ray photoelectron spectroscopy (XPS). To determine the re-deposition of sputter products and reorganization of such residues on the surface, atomic force microscopy (AFM) was used to examine the surface roughness of TiO2 thin film. 2. Experiment The TiO2 thin film was deposited by the E-beam evaporator system. The source material for deposition was TiO2 pellets. The overall thickness of the metal deposited was measured by a quartz crystal microbalance, and the final thickness of the TiO2 thin film is about 200 nm. The etching experiments are performed in a planar ICP system which is schematically shown in Fig. 1. The planar ICP etching equipment has a 3.5-turn copper coil which is located above the 24 mm thick horizontal quartz window and 13.56 MHz RF power was applied to the coil to induce inductively coupled plasma. Another 13.56 MHz RF power was applied to the substrate to induce DC-bias voltage to the wafer [8]. The etching properties of TiO2 thin film was investigated in addition of O2 into CF4/Ar gas
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Fig. 2. Schematic diagram of masking layer of TiO2 thin film and SiO2 thin film.
Fig. 1. Schematic diagram of an inductively coupled plasma system.
mixing ratio, and measured as a function of the etching parameters listed in Table 1. The etch rate of the TiO2 thin film were measured by a surface profiler (KLA Tencor, a-step 500). SiO2 thin film was used as masking layer of TiO2 thin film. As shown in Fig. 2(a), the thickness of TiO2 thin film is about 200 nm and SiO2 thin film is about 500 nm above the silicon. To measure the etch rate of the TiO2 thin film after etching process, SiO2 thin film was removed by Hydrofluoric acid (HF) through the wet etching. Also, SiO2 thin film was used to investigate the selectivity of TiO2 to SiO2. As shown in Fig. 2(b), the thickness of PR is about 2000 nm and SiO2 thin film is about 500 nm above the silicon. The chemical reaction on the surface of the etched TiO2 thin film was investigated using XPS (PHI 5000 VersaProbe. Ulvac-PHI). The Al Ka radiation (hv ¼ 1486.6 eV) was used as X-ray source and the minimum energy resolution is about 0.48 eV during the testing. The carbon C 1s peak at 284.6 eV is used as a reference for charging correction. The detailed information of the film was provided by the spectra angle of 45 . The surface roughness of TiO2 thin film was examined using AFM (park scientific instrument, Auto probe cpap-0100). All of the samples for the XPS and AFM analysis were bare TiO2 thin film without any photo-resist patterns and the size of the samples was 1 1 cm2.
enhances the F atom generation because of the oxidation of CFx, and that the increase in the etch rate of TiO2 thin film results from the increased concentration of F atoms in O2/CF4/Ar plasma. However, as the O2 gas was increased from 3 to 9 sccm, the etch rate of TiO2 thin film decreased rapidly from 179.4 nm/min to 137.5 nm/min. This excessive increase of O2 gas ratio induced an adsorption of O on the TiO2 surface and then partially blocked absorption of fluorine gas. As a result, the etch process was inhibited and the etch rate was decreased. On the other hand, the selectivity of TiO2 to SiO2 monotonically increased from 0.57 to 0.81. Fig. 4 shows the etch rate of TiO2 thin film and the selectivity of TiO2 to SiO2 as a function of the RF power in O2/CF4/Ar (¼3:16:4 sccm) plasma. The DC-bias voltage was maintained at 150 V, and the process pressure was 2 Pa. As the RF power was raised from 600 to 800 W, the etch rate of TiO2 thin film monotonically increased from 136 nm/min to 208.3 nm/min, and the selectivity of TiO2 to SiO2 increased from 0.66 to 0.83. It can be seen that the increase in the etch rate of TiO2 thin film with increasing RF power may be explained by the acceleration of both the physical and chemical etch pathway. The increase of RF power causes the increase of the densities and fluxes of positive ions and F atoms
3. Results and discussion Fig. 3 shows the etch rate of TiO2 thin film and selectivity of TiO2 to SiO2 with addition of O2 in CF4/Ar plasma. The RF power was maintained at 700 W, a DC-bias voltage was 150 V, and the process pressure was 2 Pa. It can be seen that, as the O2 gas was increased from 0 to 3 sccm in the CF4/Ar (¼16:4 sccm) plasma, the etch rate of TiO2 thin film increased from 154.1 nm/min to 179.4 nm/min. The maximum etch rate of TiO2 was 179.4 nm/min in O2/CF4/Ar (¼3:16:4 sccm) plasma. From these results, it may assume that the increasing gas ratio of O2 in CF4/Ar plasma Table 1 Process conditions. Gas mixture
O2/CF4/Ar
RF power (W) DC-bias voltage (V) Process pressure (Pa)
700 150 2
Fig. 3. Etch rate of TiO2 thin film and selectivity of TiO2 to SiO2 as a function of the O2/ CF4/Ar gas mixing ratio. The RF power was maintained at 700 W, the DC-bias voltage was 150 V, and the process pressure was 2 Pa.
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through the increase in both dissociation and ionization rates. Thus, the densities of reactive species and Ar ion flow rate were increased with increasing RF power, and the etch rate of TiO2 thin film enhanced. The acceleration of chemical reactions as well as physical ion bombardment occurred simultaneously. As a result, by increasing of physical and chemical etch pathway, it leads to an increased etch rate of TiO2 thin film. Fig. 5 shows the etch rate of TiO2 thin film and the selectivity of TiO2 to SiO2 as a function of the DC-bias voltage in O2/CF4/Ar (¼3:16:4 sccm) plasma. The RF power was maintained at 700 W, and the process pressure was 2 Pa. As the DC-bias voltage increases from 50 to 250 V, the etch rate of the TiO2 thin film monotonically increases from 130.9 nm/min to 197.2 nm/min, whereas the selectivity of TiO2 to SiO2 decreases from 0.65 to 0.56. The increase in the etch rate of TiO2 thin film is attributed to the
increase of ion bombarding energy, which increased the etch rate of both TiO2 thin film and reaction products. These results with increasing DC-bias voltage can be explained by following reasons. A high DC-bias voltage enhanced the mean ion energy, resulting in an increase in the sputtering yields for both TiO2 thin film and reaction products through the physical sputtering by ion bombardment. So, the increasing of DC-bias voltage enhanced bond-breaking and increased sputter desorption of etch products from the surface of TiO2 thin film, and then leads to an increase of the etch rate. Fig. 6 shows the etch rate of TiO2 thin film and the selectivity of TiO2 to SiO2 as a function of the process pressure in O2/CF4/Ar (¼3:16:4 sccm) plasma. The RF power was maintained at 700 W, and the DC-bias voltage was 150 V. As the process pressures increased from 1.2 to 2.8 Pa, the etch rate of TiO2 thin film and the selectivity of TiO2 to SiO2 non-monotonically decreased from 187.7 nm/min to 138.7 nm/min and 0.60 to 0.50, respectively. The increase of the process pressure enhanced the density of the chemically neutral active species, which accelerates the chemical reaction. As a result, the byproduct on the surface of TiO2 thin film was increased during the etching process through the acceleration of the chemical reaction. However, because the mean free path and the ion energy decrease with increasing process pressure, the probability of ion to reach the TiO2 thin film was decreased. Therefore, because the effect of ion bombardment was reduced, the etch rate of TiO2 thin film was decreased. Therefore, the etch rate of TiO2 thin film was decreased with increasing the process pressure [9]. According to the above results, the domination of the chemical reactions may be explained by the following reasons. First, since the melting point of TiF4 is about 284 C, the domination of the chemical pathway cannot be related hardly to the volatility. Next, because the strength of the TieF chemical bond (272 kJ/mol) is lower than that of the TieO bond (383 kJ/mol), the F atoms formed in the plasma can react with TiO2 spontaneously and the ion bombardment enhances the chemical reaction by breaking the oxide bonds. As a result, the density of the radicals and ions increased with increasing the RF power and DC-bias voltage. The bonds of molecular were broken by the ion bombardment on the surface of the TiO2 thin film. Due to the higher ion density and increased number of broken bonds of molecular, chemical reactions
Fig. 5. Etch rate of TiO2 thin film and selectivity of TiO2 to SiO2 as a function of the DCbias voltage. The O2/CF4/Ar gas mixture was maintained at 3:16:4 sccm, the RF power was 700 W, and the process pressure was 2 Pa.
Fig. 6. Etch rate of TiO2 thin film and selectivity of TiO2 to SiO2 as a function of the process pressure. The O2/CF4/Ar gas mixture was maintained at 3:16:4 sccm, the RF power was 700 W, and the DC-bias voltage was 150 V.
Fig. 4. Etch rate of TiO2 thin film and selectivity of TiO2 to SiO2 as a function of the RF power. The O2/CF4/Ar gas mixture was maintained at 3:16:4 sccm, the DC-bias voltage was 150 V, and the process pressure was 2 Pa.
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of the surface were generated more frequently in the plasma. The byproducts of etched surface were easily evaporated by physical and chemical pathway. This conclusion is in good agreement with the data shown in from Figs. 2e5 [10e15]. For more detailed investigations of the chemical reaction between the TiO2 thin film and the etched TiO2 thin film as a function of O2/CF4/Ar gas mixing ratio, XPS analysis was performed and the results are presented in Fig. 7. We compared the chemical reaction difference between the etched surface and the as-deposited surface of TiO2 thin film. Fig. 7 shows the XPS narrow scan spectra of (a) Ti, (b) O, (c) F and (d) C, which were obtained from the TiO2 thin film surfaces in as-deposition, CF4/Ar plasma and O2/CF4/Ar plasma. Fig. 7(a) shows the photoelectron peak of Ti 2p from the as-deposited and etched TiO2 thin film. It can be seen that the Ti 2p peak of the as-deposited film was observed at 458.8 eV. After the TiO2 thin film were exposed to the CF4/Ar (¼16:4 sccm) plasma, the peak of Ti 2p at 458.8 eV was shifted to a higher binding energy and the maximum deviation is about þ0.1 eV. The shift of peak indicates that Ti chemically reacted with F-component species, and it led to the formation of TieF bonds on the surface of TiO2 thin film by ion bombardment. In other words, the chemical reaction of F radical was increased on the surface of TiO2 thin film, the physical
sputtering of Ar ion influenced on the formation of Ti component such as TiFx. After the TiO2 thin film were exposed to the O2/CF4/Ar (¼3:16:4 sccm) plasma, the peak of Ti 2p at 458.8 eV was shifted to a higher binding energy and the maximum deviation is about þ0.2 eV. The shift of peak indicates that Ti chemically reacted with F-component as well as O-component, and it led to the formation of TieO and TieF bonds on the surface of TiO2 thin film by ion bombardment. In other words, the chemical reaction of O radical was increased on the surface of TiO2, and the physical sputtering of Ar ion influenced on the formation of Ti component such as TieF, and TieO. Fig. 7(b) shows the photoelectron peak of O 1s from the as-deposited and etched TiO2 thin film surface. It can be seen that, the O 1s peak of the as-deposited film was observed at 530.2 eV. After the TiO2 thin film were exposed to the CF4/Ar (¼16:4 sccm) plasma, the peak of O 1s at 530.2 eV was shifted to a higher binding energy and the maximum deviation is about þ0.1 eV. It is expected that the association of TieO bonds by ion bombardment become increase owing to the formation of FeO bonds. The CeO bonds were also formed in the same way. Fig. 7(c) shows the photoelectron peak of F 1s from the as-deposited and etched TiO2 thin film surface. It can be seen that the F 1s peak of the as-deposited film was not observed. After the TiO2 thin film were exposed to the CF4/Ar (¼16:4 sccm) plasma, the peak of F 1s was
Fig. 7. The XPS narrow scan spectra of the etched TiO2 thin film. The RF power was maintained at 700 W, the DC-bias voltage was 150 V, and the process pressure was 2 Pa. (a) Ti 2p, (b) O 1s, (c) F 1s, (d) C 1s.
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observed at 646.1 eV. After the TiO2 thin film were exposed to the O2/CF4/Ar (¼3:16:4 sccm) plasma, the core F 1s peak was decreased significantly and peak of F 1s was not shifted. The core F 1s peak of the TiO2 in CF4/Ar plasma was higher than that of the TiO2 thin film in as-deposition, as demonstrated by the increase in the number of TieF bonds on the surface of TiO2 thin film by ion bombardment. Fig. 7(d) shows the photoelectron peak of C 1s from the asdeposited and etched TiO2 thin film surface. It can be seen that, the C 1s peak of the as-deposited film was 284.8 eV. After the TiO2 thin film were exposed to the O2/CF4/Ar (¼3:16:4 sccm) plasma, the peak of C 1s at 284.6 eV was shifted to a higher binding energy and the maximum deviation is about þ0.2 eV. It is expected that the dissociation of CF4 gas become increase owing to the formation of CeO bonds. Based on the result of XPS, it was disclosed that the chemical reactions with the F radicals and the physical bombardment of the Ar ions removed Ti and O [16,17]. To understand the surface state of TiO2 thin film, atomic force microscopy (AFM) was used to examine the surface roughness of TiO2 thin film. We compared the root mean square (RMS) difference between the etched surface and the as-deposited surface of TiO2 thin film. A. The The RMS roughness of as-deposited TiO2 thin film was 36.5 RMS roughness of TiO2 thin film as a function of CF4/Ar (¼16:4 sccm) plasma was 59.8 A, and the RMS roughness of TiO2 thin film as A. When the a function of O2/CF4/Ar (¼3:16:4 sccm) plasma was 29.8 O2 gas was increased from 0 to 3 sccm, the RMS roughness of TiO2 thin film decreased from 59.8 A to 29.8 A. The changes in the RMS roughness as a function of O2/CF4/Ar gas ratio may be explained by the following reason. Because byproducts in the etching process were accumulated on the etched surface of TiO2 thin film, the RMS roughness in the CF4/Ar (¼16:4 sccm) plasma was increased. However, when the O2 gas was added, the RMS roughness in the O2/ CF4/Ar (¼3:16:4 sccm) plasma was decreased because the byproducts reacted with the O2 gas and were evaporated on the etched surface of TiO2 thin film. It is important to provide information that the surface of TiO2 thin film was reformed by O2 gas in O2/CF4/Ar (¼3:16:4 sccm) plasma. As a result, when compared with the asdeposited TiO2 thin film, the surface morphology of etched TiO2 thin film in O2/CF4/Ar plasma was become smoother [18,19]. 4. Conclusion In this work, we investigated the dry etching characteristics of the TiO2 thin film in O2/CF4/Ar plasma as functions of the etching parameters, such as the RF power, DC-bias voltage, and process pressure. The maximum etch rate and selectivity of TiO2 thin film were 179.4 nm/min and 0.6 in O2/CF4/Ar (¼3:16:4 sccm) plasma, respectively. The RF power was maintained at 700 W, DC-bias
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voltage was 150 V, and the process pressure was 2 Pa. As increasing the gas ratio of O2 in CF4/Ar plasma, it enhances the concentration of F atom, and then it increased the etch rate of TiO2 thin film. However, an excessive increase of the gas ratio of O2 in CF4/Ar plasma results in an adsorption of O on the TiO2 surface which partially blocked absorption of fluorine gas. As a result, due to an adsorption of O on the TiO2 surface, the etch process was inhibited and the etch rate was decreased. To understand chemical reaction, the surface of the etched TiO2 thin film was analyzed by XPS. The Ti of TiO2 thin film interacted with the O radicals in the O2 containing plasmas, and TieO bond remained on the etched surface, due to the low volatility of TieO bond. These byproducts on the surface can be effectively removed with the help of ion bombardment. The surface roughness of TiO2 thin film was examined using AFM. The RMS roughness was decreased and the surface morphology of TiO2 thin film was smoothed after etched in O2/CF4/ Ar plasma. This tendency of TiO2 etch pathway was similar to the etch properties, which the chemical etching was enhanced by the ion energy. Acknowledgments This work was financially supported by the Department of Electrical and Electrics Engineering, Chung-Ang University, Korea. References [1] Wilk GD, Wallance RM, Anthony JM. J Appl Phys 2001;89:5243. [2] Campbell SA, Glimer DC, Wang X, Hsieh M, Kim HS, Gladfelter WL, et al. IEEE Trans Electron Devices 1997;44:104. [3] Woo JC, Joo YH, Kim CI. Jpn J Appl Phys 2011;50:08KC02. [4] Ding J, Jenq JS, Kim GH, Maynard HL, Hamers JS. J Vac Sci Technol A 1993;11: 1283. [5] Fracassi F, d’Agostino R, Lamendola R, Mangieri I. J Vac Sci Technol A 1995; 13:335. [6] Kim HS, Gilmer DC, Campbell SA, Polla DL. Appl Phys Lett 1996;69:25. [7] Pan TM, Lei TF, Chao TS. Appl Phys Lett 2001;78:1439. [8] Kim GH, Kim CI. J Vac Sci Technol A 2006;24:1514. [9] Yang X, Woo JC, Um DS, Kim CI. Trans Electr Electron Mater 2010;11:202. [10] Kim HK, Bae JW, Kim TK, Kim KK, Seong TY, Adesida I. J Vac Sci Technol B 2003;21:1273. [11] Kim GH, Kim CI, Efremov AM. Vacuum 2005;79:231. [12] Cheng B, Cao M, Rao R, Inani A, Noorde PV, Greene WM, et al. IEEE Trans Electron Devices 1999;46:1537. [13] Lee SJ, Cho IH, Kim HW, Hong SJ, Lee HY. Trans Electr Electron Mater 2009; 10:177. [14] Kim YS, Shimogaki Y. J Vac Sci Technol A 2001;19:2642. [15] d’Agostino R, Fracassi F, Pacifico C. J Appl Phys 1992;72:4351. [16] Bertoti I, Mohai M, Sullivan JL, Saied SO. Appl Surf Sci 1995;84:357. [17] Norasetthekul S, Park PY, Baik KH, Lee KP, Jeong BS, Shishodia V, et al. Appl Surf Sci 2001;185:27. [18] Chiarello G, Barberi R, Amoddeo A, Caputi LS, Colavita E. Appl Surf Sci 1996; 99:15. [19] Liu ZH, Brown NMD, Mckinley A. Appl Surf Sci 1997;108:319.