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Ar plasma
Microelectronic Engineering 112 (2013) 74–79
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Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Surface reaction effects on dry etching of IGZO thin films in N2/BCl3/Ar plasma Young-Hee Joo, Jong-Chang Woo, Chang-Il Kim ⇑ Department of Electrical and Electronics Engineering, Chung-Ang University, 221, Heukseok-Dong, Dongjak-Gu, Seoul 156-759, Republic of Korea
a r t i c l e
i n f o
Article history: Received 23 January 2013 Received in revised form 2 May 2013 Accepted 1 June 2013 Available online 20 June 2013 Keywords: IGZO Etching ICP XPS FESEM
a b s t r a c t We were experimented the etching process of indium gallium zinc oxide (IGZO) thin films in an inductively coupled plasma. The dry etching characteristics of the IGZO thin films was studied by varying the N2/BCl3/Ar gas mixing ratio, RF power, DC-bias voltage, and process pressure. We determined the optimized process conditions that were as follows: a RF power of 700 W, a DC-bias voltage of 150 V, and a process pressure of 15 mTorr. The minimum etch rate of the IGZO thin film was 38.1 nm/min in N2/ BCl3/Ar = (3:10:10 sccm) plasma, and the selectivities of IGZO for Al and TiN were 0.26 and 0.38, respectively. From XPS analyses, the chemical reactions on the IGZO thin films were explained. From AFM and FESEM images, we found the etch conditions for the smooth surface and vertical sidewall conditions. Ó 2013 Published by Elsevier B.V.
1. Introduction Recently, transparent ZnO-based thin film transistors (TFTs) have attracted much attention in next generation display industry. Among them indium gallium zinc oxide (IGZO) has been demonstrated and widely studied for display panels because of its high mobility, low processing temperature, and good transparency to visible light [1–5]. The IGZO thin films are located between the dielectric layer and the source/drain electrodes in TFTs structure. For this reason, reduction in the contact resistance between the source/drain electrodes and IGZO thin films is important for achieving high-performance. Also, the IGZO thin films can be damaged during the etching process of source/drain electrode [6–9]. Clearly, the contact resistance and damage can significantly affect the device characteristics after the device is manufactured. Most previous reports on IGZO channel pattering have been wet chemical etching process. However, wet etching process cannot be suited for high resolution device application because wet chemical etching has the characteristics of isotropic. Therefore, in order to solve these problem, stable dry etching process with high etch rate, and high selectivity is required. In addition, many studies reported the effect of O2 plasma or Ar plasma on the contact resistance between source/drain electrodes and IGZO thin films, but there has been little study of the role of nitrogen plays when it was etched in BCl3/Ar plasma. In order to determine the role of nitrogen, we ⇑ Corresponding author. Tel.: +82 2 820 5334; fax: +82 2 812 9651. E-mail address: [email protected] (C.-I. Kim). 0167-9317/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.mee.2013.06.005
investigate the etch profiles and the surface reactions of IGZO thin films. In this work, we experimented the etching and treatment process using an inductively coupled plasma (ICP) system. We present the etching profile of IGZO thin films with various etching and treatment conditions such as gas mixing ratio and RF power, DCbias voltage, and process pressure using ICP. The chemical states on the surfaces of the etched IGZO thin films were investigated by X-ray photoelectron spectroscopy (XPS). The surface morphology of the IGZO thin films was observed by atomic force microscopy (AFM). The etch profiles were observed by field effect scanning electron microscopy (FE-SEM). 2. Experiment The IGZO thin films were deposited on SiO2 (100 nm)/Si substrates by plasma-enhanced atomic layer deposition (PEALD) system for the experiments at room temperature. The total thickness of the IGZO thin film was approximately 200 nm. The dry etching process was performed in an ICP system as shown schematically in Fig. 1. The system consisted of a cylindrical chamber. A 3.5 turn copper coil was located above horizontal quartz window lid in order to generate high density plasma. A 13.56 MHz power generator was connected to the copper coil. The bottom electrode, which was used as substrate holder, was connected to another 13.56 MHz RF power generator. The distance between the quartz window and substrate electrode was 9 cm. The chamber was evacuated to 10 6 Torr using both a mechanical pump (2M90,
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Fig. 1. Schematic diagram of the inductively coupled plasma system.
BOC Edwards) and a turbo molecular pump (ATP 900, Adixen) [10]. The etch rates were measured using a depth profiler (a-step 500, KLA tencor). The XPS (K-alpha, Thermo VG) was employed to analyze the surface bonding structure. In all cases for XPS and AFM analysis, the sample size was approximately 1 1 cm2 without any photoresist patterns. The XPS was operated in fixed retarding ratio mode using Al Ka (1486.6 eV) excitation, and all the measured results were calibrated to C 1s (284.8 eV). The surface morphology of the IGZO thin films was examined using AFM (Park scientific instrument, Auto probe c pap-0100). FE-SEM (SIGMA, Carl Zeiss) images were used to monitor the profile of the IGZO thin films after etching.
3. Results and discussion For the etching characterization of IGZO thin films using ICP system, the dry etching conditions are given in Table 1. IGZO thin films were etched as a function of the N2/BCl3/Ar gas mixing ratio, RF power, DC-bias voltage, and process pressure. Fig. 2 shows the etch rate of the IGZO thin film and the selectivity of IGZO for Al and TiN as a function of the N2/BCl3/Ar gas mixing ratio. Other etching conditions are as follows: RF power
Table 1 Process conditions. Process parameter
Parameter range
Gas mixture (sccm) RF power (W) DC-bias voltage ( V) Process pressure (mTorr)
N2/BCl3/Ar = (0–9)/10/10 500–800 50–200 5–20
Fig. 2. The etch rate of the IGZO thin films and selectivity of IGZO for Al and TiN as a function of the N2/BCl3/Ar gas mixing ratio.
of 700 W, DC-bias voltage of 150 V, and process pressure of 15 mTorr. As the flow rate of added N2 gas in N2/BCl3/Ar plasma increased from 0 to 3 sccm, the etch rate of the IGZO thin films decreased from 52.25 to 38.1 nm/min. The etch rate began to increase with more than 3 sccm of N2 gas. The selectivity of IGZO for Al and TiN were 0.26 and 0.38, respectively. The etch rates of the IGZO thin film increased after the flow rate of N2 exceeded 3 sccm because the effect of physical bombardment was increased by increasing N concentration in the N2/BCl3/Ar plasma
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[11]. The comparisons of the etch rates of the IGZO thin film in BCl3/Ar = (10:10 sccm) and N2/BCl3/Ar = (3:10:10 sccm) show that the byproduct generated in N2/BCl3/Ar = (3:10:10 sccm) is more than generated in BCl3/Ar = (10:10 sccm) due to the generation of nonvolatile byproduct such as In–N, Ga–N and so on. Nevertheless, when the N2 concentration was increased, the etch rate of the IGZO thin film began to rises. This result indicates that the etch mechanism of the IGZO thin films in added N2 gas can be explained as follows. N radicals would be disturbed, which then reacted with B or Cl radicals on surface of IGZO thin films. Therefore, N radicals would help to increase the effect of physical sputtering and dissociation of radicals as increased flow rate of N2 gas [12]. Fig. 3 shows the etch rate of the IGZO thin film and the selectivity of IGZO for Al and TiN as a function of (a) RF power, (b) DC-Bias voltage, and (c) process pressure. In Fig. 3(a), the etch characteristics of IGZO were shown as the RF power applied to the ICP coil was increased from 500 to 800 W. The etching conditions in N2/BCl3/Ar (3:10:10 sccm) plasma were a DC-bias voltage of 150 V and a process pressure of 15 mTorr. As the RF power was increased, the etch rate was greatly increased and reached 41.72 nm/min. The selectivity of IGZO for Al and TiN was decreased slightly when the RF power increased. As a result, it can be inferred that increased densities of ion and radicals with increasing RF power
causes an increased etch rate. In addition, chemical and physical sputtering reactions would be more frequently generated due to the increases of both dissociation and ionization rates [13]. The effect of the DC-bias voltage applied to the bottom ICP coil was shown in Fig. 3(b), which increased from 50 to 200 V. When the DC-bias voltage of 50 V was applied, the etching did not proceed due to the weak physical sputtering effect. Therefore, the etch byproducts were deposited on IGZO thin films because they were hardly evaporated without physical sputtering. The etch rate increased when the DC-bias voltage was more than 50 V. This is due to the fact that increasing DC-bias voltage would increase the ion bombardment energy. In Fig. 3(c), the etch rate of the IGZO thin films and the selectivity of IGZO for Al and TiN as function of the process pressure were shown. The process pressure increased from 5 to 20 mTorr, then the etch rate of IGZO thin films greatly decreased from 93.5 to 19.3 nm/min, respectively. This results can be explained as follows: when the process pressure decreased, the more chemical and physical reaction has occurred on the IGZO thin films because the collision of ions and radicals can be reduced. For a more detailed analysis on the effect of the N2 content in N2/BCl3/Ar plasma, we investigated the surface composition and chemical binding states by using XPS narrow scan analysis. In order to analyze the chemical reaction, we compared the as-deposited surface with the etched surface samples. We prepared
Fig. 3. The etch rate of the IGZO thin film and selectivity of IGZO for Al and TiN as a function of (a) RF power, (b) DC-bias voltage, and (c) process pressure.
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samples under the following conditions: an etch time of 1 min, RF power of 700 W, DC-bias voltage of 150 V, and the process pressure of 15 mTorr, while varying the flow of N2 gas ratio. Fig. 4 shows the XPS narrow scan spectra for (a) In 3d5/2, (b) Ga 2p3/2, (c) Zn 2p3/2, and (d) O 1s from the surface of the IGZO thin film. As shown in Fig. 4(a), the In 3d3/2 peak is observed at 445.18 eV with as-deposited sample. When the IGZO thin film was exposed to the N2/BCl3/Ar plasma, the In 3d3/2 peaks were shifted by approximately 0.5 eV toward a lower binding energy. These chemical shifts indicate that Inx–Oy bonds were broken, and In radicals reacted with B, Cl or N radicals resulting in the formation of In–Cl and In–N bonds on the surface. In Fig. 4(b), the Ga 2p3/2 peak is observed at 1118.29 eV with as-deposited sample. When the IGZO thin film was exposed to N2/BCl3/Ar plasma, the Ga 2p3/2 peaks were shifted by approximately 0.3 eV toward a higher binding energy. This indicates that Ga chemically reacted with N and Cl instead of oxygen, resulting in formation of Gax– Cly bonds on the surface. The resulting Gax–Cly byproducts can be removed without Ar+ sputtering because Ga and the Gax–Cly bond
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have a low melting point (78 °C) [14]. However, the Ga–N bonds are remained on the IGZO thin films because Ga–N bond have high melting point (>2500 °C). As shown Fig. 4(c), the Zn 2p3/2 peak is observed at 1022.09 eV with as-deposited sample. The binding energy of BCl3/Ar = (10:10 sccm), N2/BCl3/Ar = (3:10:10 sccm), and N2/BCl3/Ar = (9:10:10 sccm) plasma were 1022.09 eV, 1022.19 eV, and 1022.29 eV, respectively. When the flow of N2 gas ratio increased the Zn 2p3/2 peaks shifted to higher binding energy. This indicated that the Ar+ and N+ sputtering can provide more opportunities to react with B and Cl radicals. As shown Fig. 4(d), the narrow scan spectras for the O 1s peaks from the surface of the asdeposited IGZO thin film, after etching in BCl3/Ar plasma, and etching in N2/BCl3/Ar plasma were compared. After etching in N2/BCl3/ Ar plasma, the peak position negligibly shifted to lower binding energy. This indicated that the formation of B–O bonds would be remaining on the surface of the IGZO thin films but Zn–O bonds would be removed by the chemical reaction after etching process. In order to investigate the surface of the IGZO thin film, AFM measurement was carried out on the IGZO thin film as function
Fig. 4. The XPS narrow scan spectra on the surface of the etched IGZO thin film: (a) In 3d5/2, (b) Ga 2p3/2, (c) Zn 2p3/2, and (d) O 1s.
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Fig. 5. AFM images for the IGZO thin films: (a) as-deposited, (b) BCl3/Ar (10:10 sccm), (c) N2/BCl3/Ar (3:10:10 sccm), and (d) N2/BCl3/Ar (9:10:10 sccm).
Fig. 6. FESEM images for IGZO thin films etched at (a) BCl3/Ar (10:10 sccm) plasma, (b) N2/BCl3/Ar (3:10:10 sccm) plasma, and (c) N2/BCl3/Ar (9:10:10 sccm) plasma.
of flow rate of N2 gas ratio in BCl3/Ar = (10:10 sccm) plasma. The surface roughness of active layer is important because the characteristics of the channel-gate insulator are sensitive to the interface roughness [15]. Fig. 5 shows the AFM images of (a) as-deposited IGZO thin films, (b) the IGZO thin films etched at BCl3/ Ar = (10:10 sccm) plasma, (c) the IGZO thin films etched at N2/ BCl3/Ar = (3:10:10 sccm) plasma, and (d) the IGZO thin films etched at N2/BCl3/Ar = (9:10:10 sccm) plasma. The root mean square (RMS) values were 5.8 nm, 7.8 nm, 0.9 nm, and 2.6 nm, respectively. The surface of IGZO thin films etched at N2/BCl3/ Ar = (3:10:10 sccm) plasma was smoother than the other samples because the increased physical sputtering due to N ions made surface smooth. However the etched surface of IGZO films became rough because the N ions would be recombined to byproduct on surface of the IGZO thin films as the N2 gas was added more than
3 sccm. This result also proves the effect of roughness onto the surface of IGZO thin films, depending on the N2 concentration. FESEM images of the IGZO thin films etched at various concentration of N2 gas are shown in Fig. 6. When N2 gas flow rate increased the sidewall is smoother and more vertical because N2 helps to obtain a smooth sidewall with increased surface physical sputtering [16]. 4. Conclusion In this work, we reported the etch characteristics of the IGZO thin films using the ICP system. The etch rate of the IGZO thin film were varied in order to determine the effects of gas mixing ratio, RF power, DC-bias voltage, and process pressure, as well as the selectivity of IGZO for Al and TiN. The N2 gas is important to obtain vertical sidewall and smooth surface. From XPS analyses, the chemical
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reaction of B, Cl, and N radicals with IGZO thin films were explained. From AFM and FESEM images, we found the etch conditions for the smooth surface and vertical sidewall conditions. This process may be applied very usefully for etching process of IGZO channel layer in thin film transistors. Acknowledgement This research was supported by the Chung-Ang University Excellent Student Scholarship in 2012. References [1] C.T. Tsai, T.C. Chang, S.C. Chen, I. Lo, S.W. Tsao, M.C. Hung, J.J. Chang, C.Y. Wu, C.Y. Huang, Appl. Phys. Lett. 90 (2010) 242105. [2] J.S. Park, J.K. Jeong, Y.G. Mo, H.D. Kim, S.I. Kim, Appl. Phys. Lett. 90 (2007) 262106.
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[3] Y.M. Kim, K.S. Jeong, H.J. Yun, S.D. Yang, S.Y. Lee, H.D. Lee, G.W. Lee, Trans. Electr. Electron. Mater. 13 (2012) 47. [4] C. Lee, C. Chang, W. Shih, C. Dai, Thin Solid Films 518 (2010) 3992. [5] Y. Sun, J.A. Roger, Adv. Mater. 19 (2007) 1897. [6] M. Kim, J.H. Jeong, H.J. Lee, T.K. Ahn, H.S. Shin, J.S. Park, J.K. Jeong, Y.G. Mo, H.D. Kim, Appl. Phys. Lett. 90 (2007) 212114. [7] J.S. Park, J.K. Jeong, Y.G. Mo, H.D. Kim, Appl. Phys. Lett. 90 (2007) 262106. [8] J.H. Na, M. Kitamura, Y. Arakawa, Appl. Phys. Lett. 93 (2008) 133502. [9] D.H. Cho, S.H. Yang, J.H. Shin, C.W. Byun, M.K. Ryu, J.I. Lee, C.S. Hwang, H.Y. Chu, J. Korean Phys. Soc. 54 (2009) 531. [10] Y.H. Joo, J.C. Woo, C.I. Kim, Thin Solid Films 520 (2012) 2339. [11] X. Yang, D.S. Um, C.I. Kim, Thin Solid Films 518 (2010) 6441. [12] R. Chanson, A. Rhallabi, M.C. Fernandez, C. Cardinaud, J.P. Landesman, J. Vac. Sci. Technol. A31 (2013) 011301. [13] R.J. Shul, G.B. Mcclellan, R.D. Briggs, D.J. Rieger, S.J. Pearton, C.R. Abernathy, J.W. Lee, C. Constantine, C. Barratt, J. Vac. Sci. Technol. A 15 (1997) 633. [14] Y.B. Xiao, E.H. Kim, S.M. Kong, C.W. Chung, Jpn. J. Appl. Phys. 49 (2010) 08JB01. [15] K. Nomura, A. Takagi, H. Ohta, M. Hirano, H. Hosono, Jpn. J. Appl. Phys. 45 (2006) 4303. [16] J. Lin, A. Leven, N.G. Weimann, Y. Yang, R.F. Kopf, R. Reyes, Y.K. Chen, F.S. Choa, J. Vac. Sci. Technol. B 22 (2004) 510.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Surface reaction effects on dry etching of IGZO thin films in N2/BCl3/Ar plasma Young-Hee Joo, Jong-Chang Woo, Chang-Il Kim ⇑ Department of Electrical and Electronics Engineering, Chung-Ang University, 221, Heukseok-Dong, Dongjak-Gu, Seoul 156-759, Republic of Korea
a r t i c l e
i n f o
Article history: Received 23 January 2013 Received in revised form 2 May 2013 Accepted 1 June 2013 Available online 20 June 2013 Keywords: IGZO Etching ICP XPS FESEM
a b s t r a c t We were experimented the etching process of indium gallium zinc oxide (IGZO) thin films in an inductively coupled plasma. The dry etching characteristics of the IGZO thin films was studied by varying the N2/BCl3/Ar gas mixing ratio, RF power, DC-bias voltage, and process pressure. We determined the optimized process conditions that were as follows: a RF power of 700 W, a DC-bias voltage of 150 V, and a process pressure of 15 mTorr. The minimum etch rate of the IGZO thin film was 38.1 nm/min in N2/ BCl3/Ar = (3:10:10 sccm) plasma, and the selectivities of IGZO for Al and TiN were 0.26 and 0.38, respectively. From XPS analyses, the chemical reactions on the IGZO thin films were explained. From AFM and FESEM images, we found the etch conditions for the smooth surface and vertical sidewall conditions. Ó 2013 Published by Elsevier B.V.
1. Introduction Recently, transparent ZnO-based thin film transistors (TFTs) have attracted much attention in next generation display industry. Among them indium gallium zinc oxide (IGZO) has been demonstrated and widely studied for display panels because of its high mobility, low processing temperature, and good transparency to visible light [1–5]. The IGZO thin films are located between the dielectric layer and the source/drain electrodes in TFTs structure. For this reason, reduction in the contact resistance between the source/drain electrodes and IGZO thin films is important for achieving high-performance. Also, the IGZO thin films can be damaged during the etching process of source/drain electrode [6–9]. Clearly, the contact resistance and damage can significantly affect the device characteristics after the device is manufactured. Most previous reports on IGZO channel pattering have been wet chemical etching process. However, wet etching process cannot be suited for high resolution device application because wet chemical etching has the characteristics of isotropic. Therefore, in order to solve these problem, stable dry etching process with high etch rate, and high selectivity is required. In addition, many studies reported the effect of O2 plasma or Ar plasma on the contact resistance between source/drain electrodes and IGZO thin films, but there has been little study of the role of nitrogen plays when it was etched in BCl3/Ar plasma. In order to determine the role of nitrogen, we ⇑ Corresponding author. Tel.: +82 2 820 5334; fax: +82 2 812 9651. E-mail address: [email protected] (C.-I. Kim). 0167-9317/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.mee.2013.06.005
investigate the etch profiles and the surface reactions of IGZO thin films. In this work, we experimented the etching and treatment process using an inductively coupled plasma (ICP) system. We present the etching profile of IGZO thin films with various etching and treatment conditions such as gas mixing ratio and RF power, DCbias voltage, and process pressure using ICP. The chemical states on the surfaces of the etched IGZO thin films were investigated by X-ray photoelectron spectroscopy (XPS). The surface morphology of the IGZO thin films was observed by atomic force microscopy (AFM). The etch profiles were observed by field effect scanning electron microscopy (FE-SEM). 2. Experiment The IGZO thin films were deposited on SiO2 (100 nm)/Si substrates by plasma-enhanced atomic layer deposition (PEALD) system for the experiments at room temperature. The total thickness of the IGZO thin film was approximately 200 nm. The dry etching process was performed in an ICP system as shown schematically in Fig. 1. The system consisted of a cylindrical chamber. A 3.5 turn copper coil was located above horizontal quartz window lid in order to generate high density plasma. A 13.56 MHz power generator was connected to the copper coil. The bottom electrode, which was used as substrate holder, was connected to another 13.56 MHz RF power generator. The distance between the quartz window and substrate electrode was 9 cm. The chamber was evacuated to 10 6 Torr using both a mechanical pump (2M90,
Y.-H. Joo et al. / Microelectronic Engineering 112 (2013) 74–79
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Fig. 1. Schematic diagram of the inductively coupled plasma system.
BOC Edwards) and a turbo molecular pump (ATP 900, Adixen) [10]. The etch rates were measured using a depth profiler (a-step 500, KLA tencor). The XPS (K-alpha, Thermo VG) was employed to analyze the surface bonding structure. In all cases for XPS and AFM analysis, the sample size was approximately 1 1 cm2 without any photoresist patterns. The XPS was operated in fixed retarding ratio mode using Al Ka (1486.6 eV) excitation, and all the measured results were calibrated to C 1s (284.8 eV). The surface morphology of the IGZO thin films was examined using AFM (Park scientific instrument, Auto probe c pap-0100). FE-SEM (SIGMA, Carl Zeiss) images were used to monitor the profile of the IGZO thin films after etching.
3. Results and discussion For the etching characterization of IGZO thin films using ICP system, the dry etching conditions are given in Table 1. IGZO thin films were etched as a function of the N2/BCl3/Ar gas mixing ratio, RF power, DC-bias voltage, and process pressure. Fig. 2 shows the etch rate of the IGZO thin film and the selectivity of IGZO for Al and TiN as a function of the N2/BCl3/Ar gas mixing ratio. Other etching conditions are as follows: RF power
Table 1 Process conditions. Process parameter
Parameter range
Gas mixture (sccm) RF power (W) DC-bias voltage ( V) Process pressure (mTorr)
N2/BCl3/Ar = (0–9)/10/10 500–800 50–200 5–20
Fig. 2. The etch rate of the IGZO thin films and selectivity of IGZO for Al and TiN as a function of the N2/BCl3/Ar gas mixing ratio.
of 700 W, DC-bias voltage of 150 V, and process pressure of 15 mTorr. As the flow rate of added N2 gas in N2/BCl3/Ar plasma increased from 0 to 3 sccm, the etch rate of the IGZO thin films decreased from 52.25 to 38.1 nm/min. The etch rate began to increase with more than 3 sccm of N2 gas. The selectivity of IGZO for Al and TiN were 0.26 and 0.38, respectively. The etch rates of the IGZO thin film increased after the flow rate of N2 exceeded 3 sccm because the effect of physical bombardment was increased by increasing N concentration in the N2/BCl3/Ar plasma
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[11]. The comparisons of the etch rates of the IGZO thin film in BCl3/Ar = (10:10 sccm) and N2/BCl3/Ar = (3:10:10 sccm) show that the byproduct generated in N2/BCl3/Ar = (3:10:10 sccm) is more than generated in BCl3/Ar = (10:10 sccm) due to the generation of nonvolatile byproduct such as In–N, Ga–N and so on. Nevertheless, when the N2 concentration was increased, the etch rate of the IGZO thin film began to rises. This result indicates that the etch mechanism of the IGZO thin films in added N2 gas can be explained as follows. N radicals would be disturbed, which then reacted with B or Cl radicals on surface of IGZO thin films. Therefore, N radicals would help to increase the effect of physical sputtering and dissociation of radicals as increased flow rate of N2 gas [12]. Fig. 3 shows the etch rate of the IGZO thin film and the selectivity of IGZO for Al and TiN as a function of (a) RF power, (b) DC-Bias voltage, and (c) process pressure. In Fig. 3(a), the etch characteristics of IGZO were shown as the RF power applied to the ICP coil was increased from 500 to 800 W. The etching conditions in N2/BCl3/Ar (3:10:10 sccm) plasma were a DC-bias voltage of 150 V and a process pressure of 15 mTorr. As the RF power was increased, the etch rate was greatly increased and reached 41.72 nm/min. The selectivity of IGZO for Al and TiN was decreased slightly when the RF power increased. As a result, it can be inferred that increased densities of ion and radicals with increasing RF power
causes an increased etch rate. In addition, chemical and physical sputtering reactions would be more frequently generated due to the increases of both dissociation and ionization rates [13]. The effect of the DC-bias voltage applied to the bottom ICP coil was shown in Fig. 3(b), which increased from 50 to 200 V. When the DC-bias voltage of 50 V was applied, the etching did not proceed due to the weak physical sputtering effect. Therefore, the etch byproducts were deposited on IGZO thin films because they were hardly evaporated without physical sputtering. The etch rate increased when the DC-bias voltage was more than 50 V. This is due to the fact that increasing DC-bias voltage would increase the ion bombardment energy. In Fig. 3(c), the etch rate of the IGZO thin films and the selectivity of IGZO for Al and TiN as function of the process pressure were shown. The process pressure increased from 5 to 20 mTorr, then the etch rate of IGZO thin films greatly decreased from 93.5 to 19.3 nm/min, respectively. This results can be explained as follows: when the process pressure decreased, the more chemical and physical reaction has occurred on the IGZO thin films because the collision of ions and radicals can be reduced. For a more detailed analysis on the effect of the N2 content in N2/BCl3/Ar plasma, we investigated the surface composition and chemical binding states by using XPS narrow scan analysis. In order to analyze the chemical reaction, we compared the as-deposited surface with the etched surface samples. We prepared
Fig. 3. The etch rate of the IGZO thin film and selectivity of IGZO for Al and TiN as a function of (a) RF power, (b) DC-bias voltage, and (c) process pressure.
Y.-H. Joo et al. / Microelectronic Engineering 112 (2013) 74–79
samples under the following conditions: an etch time of 1 min, RF power of 700 W, DC-bias voltage of 150 V, and the process pressure of 15 mTorr, while varying the flow of N2 gas ratio. Fig. 4 shows the XPS narrow scan spectra for (a) In 3d5/2, (b) Ga 2p3/2, (c) Zn 2p3/2, and (d) O 1s from the surface of the IGZO thin film. As shown in Fig. 4(a), the In 3d3/2 peak is observed at 445.18 eV with as-deposited sample. When the IGZO thin film was exposed to the N2/BCl3/Ar plasma, the In 3d3/2 peaks were shifted by approximately 0.5 eV toward a lower binding energy. These chemical shifts indicate that Inx–Oy bonds were broken, and In radicals reacted with B, Cl or N radicals resulting in the formation of In–Cl and In–N bonds on the surface. In Fig. 4(b), the Ga 2p3/2 peak is observed at 1118.29 eV with as-deposited sample. When the IGZO thin film was exposed to N2/BCl3/Ar plasma, the Ga 2p3/2 peaks were shifted by approximately 0.3 eV toward a higher binding energy. This indicates that Ga chemically reacted with N and Cl instead of oxygen, resulting in formation of Gax– Cly bonds on the surface. The resulting Gax–Cly byproducts can be removed without Ar+ sputtering because Ga and the Gax–Cly bond
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have a low melting point (78 °C) [14]. However, the Ga–N bonds are remained on the IGZO thin films because Ga–N bond have high melting point (>2500 °C). As shown Fig. 4(c), the Zn 2p3/2 peak is observed at 1022.09 eV with as-deposited sample. The binding energy of BCl3/Ar = (10:10 sccm), N2/BCl3/Ar = (3:10:10 sccm), and N2/BCl3/Ar = (9:10:10 sccm) plasma were 1022.09 eV, 1022.19 eV, and 1022.29 eV, respectively. When the flow of N2 gas ratio increased the Zn 2p3/2 peaks shifted to higher binding energy. This indicated that the Ar+ and N+ sputtering can provide more opportunities to react with B and Cl radicals. As shown Fig. 4(d), the narrow scan spectras for the O 1s peaks from the surface of the asdeposited IGZO thin film, after etching in BCl3/Ar plasma, and etching in N2/BCl3/Ar plasma were compared. After etching in N2/BCl3/ Ar plasma, the peak position negligibly shifted to lower binding energy. This indicated that the formation of B–O bonds would be remaining on the surface of the IGZO thin films but Zn–O bonds would be removed by the chemical reaction after etching process. In order to investigate the surface of the IGZO thin film, AFM measurement was carried out on the IGZO thin film as function
Fig. 4. The XPS narrow scan spectra on the surface of the etched IGZO thin film: (a) In 3d5/2, (b) Ga 2p3/2, (c) Zn 2p3/2, and (d) O 1s.
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Fig. 5. AFM images for the IGZO thin films: (a) as-deposited, (b) BCl3/Ar (10:10 sccm), (c) N2/BCl3/Ar (3:10:10 sccm), and (d) N2/BCl3/Ar (9:10:10 sccm).
Fig. 6. FESEM images for IGZO thin films etched at (a) BCl3/Ar (10:10 sccm) plasma, (b) N2/BCl3/Ar (3:10:10 sccm) plasma, and (c) N2/BCl3/Ar (9:10:10 sccm) plasma.
of flow rate of N2 gas ratio in BCl3/Ar = (10:10 sccm) plasma. The surface roughness of active layer is important because the characteristics of the channel-gate insulator are sensitive to the interface roughness [15]. Fig. 5 shows the AFM images of (a) as-deposited IGZO thin films, (b) the IGZO thin films etched at BCl3/ Ar = (10:10 sccm) plasma, (c) the IGZO thin films etched at N2/ BCl3/Ar = (3:10:10 sccm) plasma, and (d) the IGZO thin films etched at N2/BCl3/Ar = (9:10:10 sccm) plasma. The root mean square (RMS) values were 5.8 nm, 7.8 nm, 0.9 nm, and 2.6 nm, respectively. The surface of IGZO thin films etched at N2/BCl3/ Ar = (3:10:10 sccm) plasma was smoother than the other samples because the increased physical sputtering due to N ions made surface smooth. However the etched surface of IGZO films became rough because the N ions would be recombined to byproduct on surface of the IGZO thin films as the N2 gas was added more than
3 sccm. This result also proves the effect of roughness onto the surface of IGZO thin films, depending on the N2 concentration. FESEM images of the IGZO thin films etched at various concentration of N2 gas are shown in Fig. 6. When N2 gas flow rate increased the sidewall is smoother and more vertical because N2 helps to obtain a smooth sidewall with increased surface physical sputtering [16]. 4. Conclusion In this work, we reported the etch characteristics of the IGZO thin films using the ICP system. The etch rate of the IGZO thin film were varied in order to determine the effects of gas mixing ratio, RF power, DC-bias voltage, and process pressure, as well as the selectivity of IGZO for Al and TiN. The N2 gas is important to obtain vertical sidewall and smooth surface. From XPS analyses, the chemical
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reaction of B, Cl, and N radicals with IGZO thin films were explained. From AFM and FESEM images, we found the etch conditions for the smooth surface and vertical sidewall conditions. This process may be applied very usefully for etching process of IGZO channel layer in thin film transistors. Acknowledgement This research was supported by the Chung-Ang University Excellent Student Scholarship in 2012. References [1] C.T. Tsai, T.C. Chang, S.C. Chen, I. Lo, S.W. Tsao, M.C. Hung, J.J. Chang, C.Y. Wu, C.Y. Huang, Appl. Phys. Lett. 90 (2010) 242105. [2] J.S. Park, J.K. Jeong, Y.G. Mo, H.D. Kim, S.I. Kim, Appl. Phys. Lett. 90 (2007) 262106.
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