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Selective etching of (Ba,Sr)TiO3 thin films over silicon in an inductively coupled plasma
Microelectronic Engineering 84 (2007) 187–191 www.elsevier.com/locate/mee
Selective etching of (Ba,Sr)TiO3 thin films over silicon in an inductively coupled plasma Gwan-Ha Kim, Chang-Il Kim
*
School of Electrical and Electronics Engineering, Chung-Ang University, 221 Heukseok-Dong, Dongjak-Gu, Seoul 156-756, Republic of Korea Received 29 July 2006; received in revised form 10 October 2006; accepted 13 October 2006 Available online 9 November 2006
Abstract In this work, we investigated etching characteristics of BST thin films and higher selectivity of BST over Si using inductive coupled O2/Cl2/Ar plasma (ICP) system. The maximum etch rate of BST thin films and selectivity of BST over Si were 61.5 nm/min at a O2 addition of 1 sccm, 9.52 at a O2 addition of 4 sccm into the Cl2(30%)/Ar(70%) plasma, respectively. Plasma diagnostics was performed by Langmuir probe (LP), optical emission spectroscopy (OES) and quadrupole mass spectrometry (QMS). These results confirm that the increased etch rates at O2 addition of 1 sccm is the result of the enhanced chemical reaction between BST and Cl radicals and an ion bombardment effect. Ó 2006 Elsevier B.V. All rights reserved. Keywords: (Ba,Sr)TiO3; Selective etching; OES; QMS; Langmuir probe
1. Introduction The high-density dynamic random access memory (DRAM) requires capacitors with larger capacitance. The conventional capacitor using SiO2 film cannot be used for high density DRAM applications because of its low dielectric constant below 100 nm fabrication process [1,2]. To ensure sufficient accumulated electric charge with a smaller capacitor area, (Ba,Sr)TiO3 (BST) thin film is attractive for the capacitor dielectric for Gbit DRAM because of its large dielectric constant, low leakage current and low dielectric loss. But, there are several problems such as fine pattern transfer and no plasma induced-damage etc. In order to solve these problems, the etch behavior of BST with various gas mixture is performed with inductively coupled plasma. Until now, there are several works devoted to an investigation of etching characteristics of BST thin films using both chlorine-containing and fluorine-containing plasmas [3,4], including our earlier works [5,6]. As a result, etching *
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 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.10.081
products form a residue layer, which obstructs the access of active species and decreases the etch rate [3–6]. Also, any changes of process parameters causing higher intensity of ion bombardment increase the BST etch rate. But, the selectivity of BST over other material is not enough unfortunately. That is why the majority of researches were focused in additive gas mixtures because the addition of small amount of O2 enhances the generation of protective layer. In this work, we investigated etching characteristics of BST thin films and Si using inductively coupled plasma (ICP) system. Etching characteristics were investigated in the terms of BST and Si etch rate and selectivity of BST over Si as a function of O2 addition into the Cl2/Ar mixing ratio. Plasma diagnostics were performed by Langmuir probe (LP), optical emission spectroscopy (OES) and quadrupole mass spectrometry (QMS) measurements. 2. Experimental details The (Ba,Sr)TiO3 (BST) thin films were deposited by sol– gel method by using alcoxide precursor [7]. The BST films
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G.-H. Kim, C.-I. Kim / Microelectronic Engineering 84 (2007) 187–191
were spin coated at 4000 rpm for 30 s and then dried at 400 °C on a hot plate for 10 min to remove organic material. This procedure was performed several times to obtain the final thickness of 200 nm. The pre-baked films were annealed at 650 °C for 1 h under an oxygen atmosphere for crystallization. Etching experiments were performed in a planar inductively coupled plasma system with the chamber made from stainless steel which is schematically shown in Fig. 1. A 3.5turn copper coil, connected to 13.56 MHz power supply, is located above the 24 mm-thick horizontal quartz window. The height of working zone, i.e. the distance between quartz window and bottom electrode, was 14 cm. The substrate holder was connected to another asymmetric RF generator with 13.56 MHz to control DC bias voltage. The BST thin films were etched as a function O2 addition into the Cl2/Ar plasma. The standard conditions were a working pressure of 1.6 Pa, an RF power of 700 W and bias power of 300 W. Etch rates were measured by using a a-step surface profiler (a-step 500, KLA Tencor). Plasma diagnostics was performed by Langmuir probe (LP), optical emission spectroscopy (OES) and quadrupole mass spectrometry (QMS). The installation of diagnostic tools was provided through the vertical view port on the chamber wall-side. LP measurements were carried out using single, cylindrical, and rf-compensated probe (ESPION, Hiden Analytical Ltd.). The probe was placed at 3 cm above the bottom electrode and centered in the radial position. For the treatment of ‘‘voltage–current’’ traces aimed to obtain electron energy distribution functions, we used the software supplied by the equipment manufacturer. OES measurements were carried out using a grating monochromator (NTSU101, Nanotek) with a wavelength range of 200–800 nm. The mass and the ion energy distributions (IEDs) were measured with a Hiden EQP plasma probe (EQP 510,
Hiden Analytical Ltd.). The EQP apparatus was mounted to the vertical view port on the chamber wall-side to allow sampling of positive ions through a 250 lm orifice. The IEDs are measured by setting the quadruple to a particular mass-to-charge ratio (m/z), then scanning the energy of the ions transmitted through the electrostatic ion-energy analyzer (ESA). 3. Results and discussion For any plasma etch process using a chemically active gas, there are two general factors affecting the behavior of the etch rate. The first is the flux of active species on the etched surface. The fluxes depend on stationary mass composition of the gas phase which results from the balance between formation and decay processes. The second factor is the reaction probability and sputtering yield for the main thin film to be searched as well as for low-volatility reaction products. According to Ref. [5,6], BST etch mechanism includes the simultaneous analysis of both volume and surface kinetics. However, selectivity of BST over Si was not high enough because of higher vapor pressure of etch by-products. First of all, at the complementary metal oxide semiconductor (CMOS) fabricates process, high selectivity was necessary for source/drain formation. Fig. 2 shows the etch rates of the BST thin films and Si and the selectivities BST to Si as a function of O2 addition into the Cl2(30%)/Ar(70%) plasma. The RF power/bias power was 700 W/300 W, the working pressure was 1.6 Pa, and substrate temperature was 20 °C. As the concentration of O2 into gas mixture increases, the etch rate of BST thin films reaches a maximum and then decreases. For O2 gas mixing ratio exceeding 1 sccm, the BST etch rate decreases due to the lower physical bombardment effect and chemical reaction by the decrease of Ar and Cl concentration in the etching gas. When the O2 content
Fig. 1. Schematic diagram of etch system with plasma diagnostics system.
G.-H. Kim, C.-I. Kim / Microelectronic Engineering 84 (2007) 187–191
189
ciation of Cl2 during the interaction with metastable oxygen molecules O2 ða3 RÞ. Cl2 þ O2 ða3 RÞ ! Cl þ Cl þ O2 ðx3 Rg Þ
ðR1Þ
Second, peculiarity is connected with possibility of formation of ClO radicals, which support chain reactions involving chlorine and oxygen atoms. To check a possibility of this mechanism, we added to basic kinetic scheme several reactions involving ClO radicals such as Cl þ O þ Cl2 ! ClO þ Cl2
Fig. 2. Etch rate of (Ba,Sr)TiO3 and Si and selectivity of (Ba,Sr)TiO3 over Si as a function of O2 addition into the Cl2/Ar plasma.
exceeds 1 sccm, the etch rate begin to fall down due to the ‘‘disappearance’’ of chemical channel and physical sputtering effect. In this case, chemical etching can give a noticeable contribution to the etch rate. To understand the roles of physical and chemical effects, the data about the influence of O2 addition on volume densities of active species intensity are needed. For these purposes we used OES. For the control of active species volume densities behavior in O2 addition plasmas, we selected such emission maximums as atomic lines of Cl (452.6, 725 nm), Ar (750.4 nm) and O (777 nm) [8]. These maximums are frequently used for analytical purposes including optical emission actinometry to determine absolute densities of active particles. Fig. 3 shows that the increasing of O2 content in Cl2/Ar mixture leads to a direct proportional increase of emission intensity of Ar, Cl and O atoms. The addition of more than 1 sccm O2 to the Cl2/Ar chemistry decreased the Cl radical and Ar ion intensity while O radical intensity was increased continuously. It is well known that Cl2/O2 plasma mixture is characterized by several peculiarities concerning neutral particle kinetics. First, peculiarity is a possibility of stepwise disso-
Fig. 3. Emission intensities of dominant radicals as a function of O2 addition into the Cl2/Ar plasma.
ðR2Þ
Cl þ O þ O2 ! ClO þ O2
ðR3Þ
O þ Cl2 ! ClO þ Cl Cl þ ClO ! Cl2 þ O
ðR4Þ ðR5Þ
O þ ClO ! O2 þ Cl
ðR6Þ
The correlation between the etch rate and dominant species emission intensity from Figs. 2 and 3 suggests that the etch reaction of BST thin films is affected by a physical sputtering and chemical reaction with Ar and Cl-containing species. Fig. 4 was confirmed our conclusion. Fig. 5 shows that the gas mixture dependence of the ion energy distributions (IEDs) for the Ar+, Cl+ and O+ sampled as a function of O2 addition into the Cl2/Ar gas mixing ratio for the same condition shown in Fig. 2. As the O2 addition increases, the Ar+ and O+ saddle structure becomes increasingly more constructed and the mean energy of the saddle shifts to higher values. On the other hand, for O2 gas mixing ratio exceeding 2 sccm, Cl ion energy intensity decrease due to recombination between Cl and O atoms. These results confirm that the increased etch rates at O2 addition of 1 sccm is the result of the enhanced chemical reaction between BST and Cl radicals and an ion bombardment effect. Fig. 6 illustrates variations of electron energy distribution function as a function of O2 addition into the Cl2/Ar plasma. This phenomenon may be caused by a well-known
Fig. 4. Mass scan results of ion species obtained by quadrupole mass spectrometer in various etching conditions.
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G.-H. Kim, C.-I. Kim / Microelectronic Engineering 84 (2007) 187–191
Fig. 6. Electron energy distribution functions as a function of O2 addition into the Cl2/Ar plasma.
the O2 addition leads to the increase of both number and efficiency of volume electron impact processes with high thresholds. This fact is a reason for increasing of fraction of ‘‘fast’’ electrons in electron energy distribution function and electron mean energy. 4. Conclusion
Fig. 5. Ion energy distributions of (a) Ar, (b) Cl and (c) O as a function of O2 addition into the Cl2/Ar plasma.
In this work, we investigated etching characteristics of BST thin films and higher selectivity of BST over Si using inductively coupled O2/Cl2/Ar plasma (ICP) system. The maximum etch rate of BST thin films and selectivity of BST over Si were 61.5 nm/min at a O2 addition of 1 sccm, 9.52 at a O2 addition of 4 sccm into the Cl2(30%)/Ar(70%) plasma, respectively. Other process conditions such as a RF power to the source, a bias power, a working pressure, and a substrate temperature were also maintained at 700 W, 300 W, 1.6 Pa, and 20 °C, respectively. To understand the roles of physical and chemical effects, the data about the influence of O2 addition on volume densities of active species intensity, dominant ion energy distributions and electron energy distribution function were measured. These results confirm that the increased etch rates at O2 addition of 1 sccm is the result of the enhanced chemical reaction between BST and Cl radicals and an ion bombardment effect. A chemically assisted physical etch of BST thin films was experimentally confirmed by ICP under various gas mixtures. References
‘‘transparency’’ effect, which is especially characteristic of chlorine mixtures with noble gases [9]. The presence of ‘‘transparency’’ effect in O2/Cl2/Ar mixture may be explained as follows. Electron impact processes for oxygen atoms and molecules are characterized by higher threshold energies in comparison with chlorine species. Accordingly,
[1] H.F. Okorn-Schmidt, Ibm J. Res. Dev. 43 (1999) 351. [2] P. Vitanov, A. Harizanova, T. Ivanova, D. Velkov, Zd. Raytcheva, Vacuum 69 (2003) 371. [3] H.M. Lee, D.C. Kim, W. Jo, K.Y. Kim, J. Vac. Sci. Technol. B 16 (1998) 1891. [4] T. Shibano, T. Takenage, K. Nakamura, T. Oomori, J. Vac. Sci. Technol. A 18 (2000) 2080.
G.-H. Kim, C.-I. Kim / Microelectronic Engineering 84 (2007) 187–191 [5] A.M. Efremov, D.P. Kim, K.T. Kim, C.I. Kim, Microelectron. Eng. 71 (2004) 54. [6] G.H. Kim, K.T. Kim, C.I. Kim, J. Vac. Sci. Technol. A 23 (2005) 894.
191
[7] K.T. Kim, C.I. Kim, S.G. Lee, H.M. Kim, Surf. Coat. Tech. 190 (2005) 190. [8] J.R. Roberts, J. Res. Natl. Inst. Stand. Technol. 100 (1995) 353. [9] S.V. Sazonov, N.V. Ustinov, Proc. SPIE 5402 (2004) 72.
Selective etching of (Ba,Sr)TiO3 thin films over silicon in an inductively coupled plasma Gwan-Ha Kim, Chang-Il Kim
*
School of Electrical and Electronics Engineering, Chung-Ang University, 221 Heukseok-Dong, Dongjak-Gu, Seoul 156-756, Republic of Korea Received 29 July 2006; received in revised form 10 October 2006; accepted 13 October 2006 Available online 9 November 2006
Abstract In this work, we investigated etching characteristics of BST thin films and higher selectivity of BST over Si using inductive coupled O2/Cl2/Ar plasma (ICP) system. The maximum etch rate of BST thin films and selectivity of BST over Si were 61.5 nm/min at a O2 addition of 1 sccm, 9.52 at a O2 addition of 4 sccm into the Cl2(30%)/Ar(70%) plasma, respectively. Plasma diagnostics was performed by Langmuir probe (LP), optical emission spectroscopy (OES) and quadrupole mass spectrometry (QMS). These results confirm that the increased etch rates at O2 addition of 1 sccm is the result of the enhanced chemical reaction between BST and Cl radicals and an ion bombardment effect. Ó 2006 Elsevier B.V. All rights reserved. Keywords: (Ba,Sr)TiO3; Selective etching; OES; QMS; Langmuir probe
1. Introduction The high-density dynamic random access memory (DRAM) requires capacitors with larger capacitance. The conventional capacitor using SiO2 film cannot be used for high density DRAM applications because of its low dielectric constant below 100 nm fabrication process [1,2]. To ensure sufficient accumulated electric charge with a smaller capacitor area, (Ba,Sr)TiO3 (BST) thin film is attractive for the capacitor dielectric for Gbit DRAM because of its large dielectric constant, low leakage current and low dielectric loss. But, there are several problems such as fine pattern transfer and no plasma induced-damage etc. In order to solve these problems, the etch behavior of BST with various gas mixture is performed with inductively coupled plasma. Until now, there are several works devoted to an investigation of etching characteristics of BST thin films using both chlorine-containing and fluorine-containing plasmas [3,4], including our earlier works [5,6]. As a result, etching *
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 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.10.081
products form a residue layer, which obstructs the access of active species and decreases the etch rate [3–6]. Also, any changes of process parameters causing higher intensity of ion bombardment increase the BST etch rate. But, the selectivity of BST over other material is not enough unfortunately. That is why the majority of researches were focused in additive gas mixtures because the addition of small amount of O2 enhances the generation of protective layer. In this work, we investigated etching characteristics of BST thin films and Si using inductively coupled plasma (ICP) system. Etching characteristics were investigated in the terms of BST and Si etch rate and selectivity of BST over Si as a function of O2 addition into the Cl2/Ar mixing ratio. Plasma diagnostics were performed by Langmuir probe (LP), optical emission spectroscopy (OES) and quadrupole mass spectrometry (QMS) measurements. 2. Experimental details The (Ba,Sr)TiO3 (BST) thin films were deposited by sol– gel method by using alcoxide precursor [7]. The BST films
188
G.-H. Kim, C.-I. Kim / Microelectronic Engineering 84 (2007) 187–191
were spin coated at 4000 rpm for 30 s and then dried at 400 °C on a hot plate for 10 min to remove organic material. This procedure was performed several times to obtain the final thickness of 200 nm. The pre-baked films were annealed at 650 °C for 1 h under an oxygen atmosphere for crystallization. Etching experiments were performed in a planar inductively coupled plasma system with the chamber made from stainless steel which is schematically shown in Fig. 1. A 3.5turn copper coil, connected to 13.56 MHz power supply, is located above the 24 mm-thick horizontal quartz window. The height of working zone, i.e. the distance between quartz window and bottom electrode, was 14 cm. The substrate holder was connected to another asymmetric RF generator with 13.56 MHz to control DC bias voltage. The BST thin films were etched as a function O2 addition into the Cl2/Ar plasma. The standard conditions were a working pressure of 1.6 Pa, an RF power of 700 W and bias power of 300 W. Etch rates were measured by using a a-step surface profiler (a-step 500, KLA Tencor). Plasma diagnostics was performed by Langmuir probe (LP), optical emission spectroscopy (OES) and quadrupole mass spectrometry (QMS). The installation of diagnostic tools was provided through the vertical view port on the chamber wall-side. LP measurements were carried out using single, cylindrical, and rf-compensated probe (ESPION, Hiden Analytical Ltd.). The probe was placed at 3 cm above the bottom electrode and centered in the radial position. For the treatment of ‘‘voltage–current’’ traces aimed to obtain electron energy distribution functions, we used the software supplied by the equipment manufacturer. OES measurements were carried out using a grating monochromator (NTSU101, Nanotek) with a wavelength range of 200–800 nm. The mass and the ion energy distributions (IEDs) were measured with a Hiden EQP plasma probe (EQP 510,
Hiden Analytical Ltd.). The EQP apparatus was mounted to the vertical view port on the chamber wall-side to allow sampling of positive ions through a 250 lm orifice. The IEDs are measured by setting the quadruple to a particular mass-to-charge ratio (m/z), then scanning the energy of the ions transmitted through the electrostatic ion-energy analyzer (ESA). 3. Results and discussion For any plasma etch process using a chemically active gas, there are two general factors affecting the behavior of the etch rate. The first is the flux of active species on the etched surface. The fluxes depend on stationary mass composition of the gas phase which results from the balance between formation and decay processes. The second factor is the reaction probability and sputtering yield for the main thin film to be searched as well as for low-volatility reaction products. According to Ref. [5,6], BST etch mechanism includes the simultaneous analysis of both volume and surface kinetics. However, selectivity of BST over Si was not high enough because of higher vapor pressure of etch by-products. First of all, at the complementary metal oxide semiconductor (CMOS) fabricates process, high selectivity was necessary for source/drain formation. Fig. 2 shows the etch rates of the BST thin films and Si and the selectivities BST to Si as a function of O2 addition into the Cl2(30%)/Ar(70%) plasma. The RF power/bias power was 700 W/300 W, the working pressure was 1.6 Pa, and substrate temperature was 20 °C. As the concentration of O2 into gas mixture increases, the etch rate of BST thin films reaches a maximum and then decreases. For O2 gas mixing ratio exceeding 1 sccm, the BST etch rate decreases due to the lower physical bombardment effect and chemical reaction by the decrease of Ar and Cl concentration in the etching gas. When the O2 content
Fig. 1. Schematic diagram of etch system with plasma diagnostics system.
G.-H. Kim, C.-I. Kim / Microelectronic Engineering 84 (2007) 187–191
189
ciation of Cl2 during the interaction with metastable oxygen molecules O2 ða3 RÞ. Cl2 þ O2 ða3 RÞ ! Cl þ Cl þ O2 ðx3 Rg Þ
ðR1Þ
Second, peculiarity is connected with possibility of formation of ClO radicals, which support chain reactions involving chlorine and oxygen atoms. To check a possibility of this mechanism, we added to basic kinetic scheme several reactions involving ClO radicals such as Cl þ O þ Cl2 ! ClO þ Cl2
Fig. 2. Etch rate of (Ba,Sr)TiO3 and Si and selectivity of (Ba,Sr)TiO3 over Si as a function of O2 addition into the Cl2/Ar plasma.
exceeds 1 sccm, the etch rate begin to fall down due to the ‘‘disappearance’’ of chemical channel and physical sputtering effect. In this case, chemical etching can give a noticeable contribution to the etch rate. To understand the roles of physical and chemical effects, the data about the influence of O2 addition on volume densities of active species intensity are needed. For these purposes we used OES. For the control of active species volume densities behavior in O2 addition plasmas, we selected such emission maximums as atomic lines of Cl (452.6, 725 nm), Ar (750.4 nm) and O (777 nm) [8]. These maximums are frequently used for analytical purposes including optical emission actinometry to determine absolute densities of active particles. Fig. 3 shows that the increasing of O2 content in Cl2/Ar mixture leads to a direct proportional increase of emission intensity of Ar, Cl and O atoms. The addition of more than 1 sccm O2 to the Cl2/Ar chemistry decreased the Cl radical and Ar ion intensity while O radical intensity was increased continuously. It is well known that Cl2/O2 plasma mixture is characterized by several peculiarities concerning neutral particle kinetics. First, peculiarity is a possibility of stepwise disso-
Fig. 3. Emission intensities of dominant radicals as a function of O2 addition into the Cl2/Ar plasma.
ðR2Þ
Cl þ O þ O2 ! ClO þ O2
ðR3Þ
O þ Cl2 ! ClO þ Cl Cl þ ClO ! Cl2 þ O
ðR4Þ ðR5Þ
O þ ClO ! O2 þ Cl
ðR6Þ
The correlation between the etch rate and dominant species emission intensity from Figs. 2 and 3 suggests that the etch reaction of BST thin films is affected by a physical sputtering and chemical reaction with Ar and Cl-containing species. Fig. 4 was confirmed our conclusion. Fig. 5 shows that the gas mixture dependence of the ion energy distributions (IEDs) for the Ar+, Cl+ and O+ sampled as a function of O2 addition into the Cl2/Ar gas mixing ratio for the same condition shown in Fig. 2. As the O2 addition increases, the Ar+ and O+ saddle structure becomes increasingly more constructed and the mean energy of the saddle shifts to higher values. On the other hand, for O2 gas mixing ratio exceeding 2 sccm, Cl ion energy intensity decrease due to recombination between Cl and O atoms. These results confirm that the increased etch rates at O2 addition of 1 sccm is the result of the enhanced chemical reaction between BST and Cl radicals and an ion bombardment effect. Fig. 6 illustrates variations of electron energy distribution function as a function of O2 addition into the Cl2/Ar plasma. This phenomenon may be caused by a well-known
Fig. 4. Mass scan results of ion species obtained by quadrupole mass spectrometer in various etching conditions.
190
G.-H. Kim, C.-I. Kim / Microelectronic Engineering 84 (2007) 187–191
Fig. 6. Electron energy distribution functions as a function of O2 addition into the Cl2/Ar plasma.
the O2 addition leads to the increase of both number and efficiency of volume electron impact processes with high thresholds. This fact is a reason for increasing of fraction of ‘‘fast’’ electrons in electron energy distribution function and electron mean energy. 4. Conclusion
Fig. 5. Ion energy distributions of (a) Ar, (b) Cl and (c) O as a function of O2 addition into the Cl2/Ar plasma.
In this work, we investigated etching characteristics of BST thin films and higher selectivity of BST over Si using inductively coupled O2/Cl2/Ar plasma (ICP) system. The maximum etch rate of BST thin films and selectivity of BST over Si were 61.5 nm/min at a O2 addition of 1 sccm, 9.52 at a O2 addition of 4 sccm into the Cl2(30%)/Ar(70%) plasma, respectively. Other process conditions such as a RF power to the source, a bias power, a working pressure, and a substrate temperature were also maintained at 700 W, 300 W, 1.6 Pa, and 20 °C, respectively. To understand the roles of physical and chemical effects, the data about the influence of O2 addition on volume densities of active species intensity, dominant ion energy distributions and electron energy distribution function were measured. These results confirm that the increased etch rates at O2 addition of 1 sccm is the result of the enhanced chemical reaction between BST and Cl radicals and an ion bombardment effect. A chemically assisted physical etch of BST thin films was experimentally confirmed by ICP under various gas mixtures. References
‘‘transparency’’ effect, which is especially characteristic of chlorine mixtures with noble gases [9]. The presence of ‘‘transparency’’ effect in O2/Cl2/Ar mixture may be explained as follows. Electron impact processes for oxygen atoms and molecules are characterized by higher threshold energies in comparison with chlorine species. Accordingly,
[1] H.F. Okorn-Schmidt, Ibm J. Res. Dev. 43 (1999) 351. [2] P. Vitanov, A. Harizanova, T. Ivanova, D. Velkov, Zd. Raytcheva, Vacuum 69 (2003) 371. [3] H.M. Lee, D.C. Kim, W. Jo, K.Y. Kim, J. Vac. Sci. Technol. B 16 (1998) 1891. [4] T. Shibano, T. Takenage, K. Nakamura, T. Oomori, J. Vac. Sci. Technol. A 18 (2000) 2080.
G.-H. Kim, C.-I. Kim / Microelectronic Engineering 84 (2007) 187–191 [5] A.M. Efremov, D.P. Kim, K.T. Kim, C.I. Kim, Microelectron. Eng. 71 (2004) 54. [6] G.H. Kim, K.T. Kim, C.I. Kim, J. Vac. Sci. Technol. A 23 (2005) 894.
191
[7] K.T. Kim, C.I. Kim, S.G. Lee, H.M. Kim, Surf. Coat. Tech. 190 (2005) 190. [8] J.R. Roberts, J. Res. Natl. Inst. Stand. Technol. 100 (1995) 353. [9] S.V. Sazonov, N.V. Ustinov, Proc. SPIE 5402 (2004) 72.