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etch systems
Microelectronic Engineering 83 (2006) 1499–1502 www.elsevier.com/locate/mee
Measurement and simulation of impinging precursor molecule distribution in focused particle beam deposition/etch systems Ivo Utke a
a,*
, Vinzenz Friedli
a,b
, Simone Amorosi b, Johann Michler a, Patrik Hoffmann
b
Nanomechanics and Nanopatterning Group, EMPA Materials Science and Technology Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland b Advanced Photonics Laboratory, Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), 1015 Lausanne, Switzerland Available online 20 February 2006
Abstract The distribution of metal-precursors supplied via a gas injection system to the substrate inside a focused electron beam (FEB) induced deposition system is investigated for the first time. The impinging precursor molecules are thermally decomposed using a heating stage. Resulting deposit thickness profiles obtained from [(PF3)2RhCl]2, Co2(CO)8, and (hfac)CuVTMS precursors are determined optically by interference colors or by profilometry. FEB access to the precursor flux peak and the flux peak value itself depend on tube tilt and vertical tube distance to the substrate. Monte Carlo simulation match best the experiments when assuming molecular flow conditions. 2006 Elsevier B.V. All rights reserved. Keywords: Gas injection system; Precursor distribution; Effusion; Focused electron beam; Focused ion beam; Deposition; Etching
1. Introduction Focused particle beam induced deposition/etch systems use mostly microtube based gas injection systems to supply the volatile precursor to the reaction region as shown schematically in Fig. 1. The overall area where precursor molecules impinge is often millimeter size and little is known about the precursor density at the nanometer sized deposition/etch spot. The precursor density determines whether the process is electron or precursor limited. These regimes, or more precisely the ratio between adsorbed precursor flux and charged particle flux (electrons or ions) at the deposition/etch area crucially determine the deposition and etch rate [1,2], minimum dot dimensions at nano-scale [3], the shape [4], the metal content and electrical resistivity [5], and mechanical properties [6]. Fundamental studies to above topics thus require the knowledge of the precursor density at the areas where the focused particle beam hits the substrate. The impinging flux strongly determines this value and is the subject of this article. *
Corresponding author. Tel.: +41 33 2282957; fax: +41 33 2284490. E-mail address: [email protected] (I. Utke).
0167-9317/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.01.136
The overall flow rate of tube-based supply systems can be experimentally determined by mass or volume loss measurements and allows average flux values at the tube exit to be deduced. The average precursor flux arriving on the substrate as a function of vertical microtube distance was measured using stagnation tubes [7] and as a function of temperature using quartz microbalances [1]. A micromechanical gas sensor together with a geometrical approach to estimate the average precursor flux on the substrate was reported in [8]. Recently, the distribution of impinging water molecules was measured on a cryo-cooled substrate inside a dual beam system and modeled solving numerically the continuum Navier–Stokes equation [9]. Our approach presented in this paper relies on the thermal decomposition of impinging precursor molecules being analogous to chemical vapor deposition (CVD) driven in the precursor flux limited regime. For the first time spatial distributions of FEB relevant precursors containing copper, rhodium, and cobalt could be accessed under typical injection conditions. A comparison with Monte Carlo simulations taking into account molecular and transient flow conditions is presented.
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I. Utke et al. / Microelectronic Engineering 83 (2006) 1499–1502
3. Monte Carlo Simulations
Fig. 1. Schematics of experiment: tube-based precursor supply. The substrate is mounted on a heating stage to thermally decompose all impinging molecules on the entire surface.
Monte Carlo (MC) simulations were performed assuming that molecules enter uniformly over the microtube entrance surface with an angular velocity distribution described by a cosine law. After collisions with the inner tube wall the molecule desorbs again with a cosine law distribution. Chemisorption of molecules to the inner tube wall can be included through a sticking factor [10]. Additionally, to allow for transient flow, our simulations consider molecule-molecule collisions with an isotropic angular velocity distribution. The distance between two subsequent molecule–molecule collisions follows an exponential distribution which averages to the mean free path k. It can be estimated from the vapor pressure Pvap according to [11]: p k ¼ kT =ð 2 p d2 P vap Þ; ð1Þ where k is the Boltzmann constant, T the temperature, and d the molecule diameter as listed in Table 1. We verified our MC simulations against published transmission factors and found agreement within five decimal digits [12].
2. Experimental In our experiments we used a modified internal syringe reservoir with the microtube length L = 6 mm, see Fig. 1. The other parameters were varied with the specific experiment. As volatile precursors, di-l-chloro-tetrakis-(trifluorophosphine)-dirhodium [(PF3)2RhCl]2 (CAS: 14876-98-3), dicobalt-octacarbonyl Co2(CO)8 (CAS: 10210-68-1), and hexafluoroacetylacetonato-copper(I)-vinyltri-methylsilane (hfac)CuVTMS (CAS: 139566-53-3) were used. The average precursor flux F with respect to the tube exit area was deduced by mass loss measurements, see Table 1. Control of substrate position with respect to the fixed microtube is achieved using an x–y–z table. Silicon substrates with a TiN layer as diffusion barrier were used. Heating the substrate above CVD temperature to 200–300 C was accomplished with a resistance heater. Using a copper block as fixation assured temperature homogeneity over the entire substrate size (about 1 cm2). The substrate temperature was measured with a thermocouple. In situ electron beam observation served for substrate positioning and showed negligible thermal drifts during typical deposition periods of less than 10 min. Ex situ observation of the deposits was performed with optical microscopy and profilometry.
4. Results and discussion Fig. 2 summarizes the deposition with Rh2Cl2(PF3)4, where the microtube was inclined to the substrate at 40 (D = 600 lm, H = 165 lm, d = 614 lm). The resulting deposit shows an interference color contrast in bright field optical microscopy and thus represents an optical thickness profile. Assuming a homogeneous refractive index, the colors reflect the thickness profile of the deposit. A steady color change from goldish (substrate) to yellow to red to green with increasing deposit thickness towards the microtube is observed. This specific color change is attributed to the reflected light spectrum of the underlying TiN/Si substrate. Superposition with the microtube geometry reveals that the position of precursor flux peak is covered by the microtube i.e., it is not accessible to the focused electron beam. The dimensions of the deposit are roughly three times D along the tube axis and about two times D perpendicular to the tube axis (D = inner tube diameter). MC simulations match best the interference image when molecular flux conditions are assumed, see Fig. 2a. Here the mean free path is set
Table 1 Summary of data from precursors used in experiments. Mean free path k calculated according to Eq. (1). The inner diameter of the tube is D = 600 lm (D = 424 lm for Co2(CO)8) and indicates transient flow (k/D < 1) with intramolecular collisions ˚ )b Precursor Vapor pressure (mbar)a Molecule diameter (A Mean free path (lm) Flux F (cm2 s1)c Reference [(PF3)2RhCl]2 (hfac)CuVTMS Co2(CO)8 a b c
0.075 0.1 0.4
5.7 8.6 7
380 124 47
At room temperature. Longest dimension of molecule. Value of (hfac)CuVTMS is estimated from density and molar mass. At tube exit (measured by mass loss).
3 · 1017 1 · 1018 2 · 1018
[13] [14] [15]
I. Utke et al. / Microelectronic Engineering 83 (2006) 1499–1502
Fig. 2. Top view interference image superposed with isodensity contour plots of precursor flux obtained from MC simulation. The projected view of the microtube with inner (dashed) and outer diameter is indicated. (a) Good match is obtained without molecule collisions (molecular flow). (b) Poor match is obtained including molecule collisions (transient flow).
much larger than tube dimensions such that only molecule-tube collisions take place. However, the mean free path of 380 lm for Rh2Cl2(PF3)4 reported in Table 1 is smaller than the tube dimensions. This suggests that intramolecular collisions should be considered, however, poor match with the experiment is obtained, see Fig. 2b. This contradiction can be explained by pressure decay inside the tube such that molecular flow conditions can be present near the tube exit into vacuum. From the MC simulations it can be calculated that the flux peak in Fig. 2a is 0.32F (F = average flux F reported in Table 1). The flux value accessible to the FEB/FIB is 60.15F. Molecular flow conditions were also found to fit best quantitative profile measurements on deposits obtained from Co2(CO)8 with normal incidence, see Fig. 3a. The absolute height scale was set by matching the area below the curves. Comparison with a singular cosine emitter at the tube exit center shows that within a distance d comparable to the inner tube diameter D the distribution cannot be adequately described. Mass measurements show
1501
Fig. 3. Deposit thickness scans along center line (x = 0). The origin is defined as in Fig. 1. (a) Deposit obtained from Co2(CO)8 with normal incidence, H = d = D = 424 lm. (b) Deposit obtained from (hfac)CuVTMS. The highest precursor flux value accessible by FEB/FIB is indicated.
Table 2 Co2(CO)8 flux peaks (normalized to flux peak at normal incidence, see Fig. 3a) and maximum FEB/ FIB accessible flux (in units of total flux F, see Table 1) when pivoting the microtube around (0,0) at constant d (=424 lm), see Fig. 1 a ()
70
60
50
40
30
Peak (%) Access
80 0.16F
71 0.14F
59 0.12F
44 0.08F
28 0.05F
that chemisorption is less than 3% which did not result in observable changes in simulated distributions. In Table 2, MC simulations were extended to a range of typical supply angles showing that a maximum of 16% of the total flux can be accessed at the tail of the distribution obtained under a = 70. The peak decrease with lower incidence angles is due to an increase of the projected area of precursor impingement. For (hfac)CuVTMS in Fig. 3b, the precursor flux increases steeply starting at the lower end of the microtube, peaking at a position still covered from its upper end and decays smoothly with increasing distance from the tube. This confirms the observations from Fig. 2.
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I. Utke et al. / Microelectronic Engineering 83 (2006) 1499–1502
The MC-simulation yields comparable results at molecular and transient flow conditions and a good match for a = 60, D = 500 lm, and H = 70 lm. From the MC simulations it can be deduced that the flux peaks correspond to 0.26F (a = 90) for the Co2(CO)8 experiment and to 0.68F for (hfac)CuVTMS. The FEB/FEB accessible precursor flux value is 0.18F in Fig. 3b (for values of F refer to Table 1). Summarizing we found good agreement between impinging precursor distributions obtained by thermal decomposition and MC simulations assuming molecular flow conditions. FIB/FEB accessible flux values scale between 5% and 18% of the average exit flux in the geometries investigated. Further optimization of gas injection and mixed gas flows evolving from multi-nozzle systems are being investigated. Acknowledgements S.W. Rhee is acknowledged for supplying the TiN/Si substrate. The Swiss National Fund and CTI/TN21 are acknowledged for financial support.
References [1] V. Scheuer, H.W.P. Koops, T. Tschudi, Microelectron. Eng. 5 (1986) 423. [2] K. Edinger, T. Kraus, Microelectron. Eng. 57–58 (2001) 2631. [3] M. Tanaka, M. Shimojo, K. Mitsuishi, K. Furuya, Appl. Phys. A 78 (2004) 543. [4] P. Hoffmann, I. Utke, F. Cicoira, Proc. SPIE 5023 (2002) 4. [5] N. Antoniou, T.J. Gannon, Presented at EFUG, 2004. [6] J. Fujita, M. Ishida, T. Sakamoto, Y. Ochiai, T. Kaito, S. Matsui, J. Vac. Sci. Technol. B 19 (2001) 2834. [7] P.G. Blauner, J.S. Ro, Y. Butt, J. Melngailis, J. Vac. Sci. Technol. B 7 (1989) 609. [8] K.T. Kohlmann, M. Thiemann, W.H. Bru¨nger, Microelectron. Eng. 13 (1991) 279. [9] M.S. El-Morsi, A.C. Wei, G.F. Nellis, R.L. Engelstadd, S. Sijbrandij, D. Stewart, H. Mulders, Proc. SPIE 5567 (2004) 467. [10] C.G. Smith, G. Lewin, J. Vac. Sci. Technol. 3 (1966) 92. [11] A. Berman, Vacuum Engineering Calculations, Formulas, and Solved Exercises, Academic Press, San Diego, 1992. [12] B.C. Boude, J. Vac. Sci. Technol. B 9 (1972) 1090. [13] T. Ohta, F. Cicoira, P. Doppelt, L. Beitone, P. Hoffmann, Chem. Vap. Deposition 7 (2001) 33. [14] K.K. Choi, S.W. Rhee, Thin Solid Films 409 (2002) 147. [15] M. Rothschild, J.H.C. Sedlacek, D.C. Shaver, D.J. Ehrlich, Mater. Res. Soc. Symp. 158 (1990) 79.
Measurement and simulation of impinging precursor molecule distribution in focused particle beam deposition/etch systems Ivo Utke a
a,*
, Vinzenz Friedli
a,b
, Simone Amorosi b, Johann Michler a, Patrik Hoffmann
b
Nanomechanics and Nanopatterning Group, EMPA Materials Science and Technology Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland b Advanced Photonics Laboratory, Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), 1015 Lausanne, Switzerland Available online 20 February 2006
Abstract The distribution of metal-precursors supplied via a gas injection system to the substrate inside a focused electron beam (FEB) induced deposition system is investigated for the first time. The impinging precursor molecules are thermally decomposed using a heating stage. Resulting deposit thickness profiles obtained from [(PF3)2RhCl]2, Co2(CO)8, and (hfac)CuVTMS precursors are determined optically by interference colors or by profilometry. FEB access to the precursor flux peak and the flux peak value itself depend on tube tilt and vertical tube distance to the substrate. Monte Carlo simulation match best the experiments when assuming molecular flow conditions. 2006 Elsevier B.V. All rights reserved. Keywords: Gas injection system; Precursor distribution; Effusion; Focused electron beam; Focused ion beam; Deposition; Etching
1. Introduction Focused particle beam induced deposition/etch systems use mostly microtube based gas injection systems to supply the volatile precursor to the reaction region as shown schematically in Fig. 1. The overall area where precursor molecules impinge is often millimeter size and little is known about the precursor density at the nanometer sized deposition/etch spot. The precursor density determines whether the process is electron or precursor limited. These regimes, or more precisely the ratio between adsorbed precursor flux and charged particle flux (electrons or ions) at the deposition/etch area crucially determine the deposition and etch rate [1,2], minimum dot dimensions at nano-scale [3], the shape [4], the metal content and electrical resistivity [5], and mechanical properties [6]. Fundamental studies to above topics thus require the knowledge of the precursor density at the areas where the focused particle beam hits the substrate. The impinging flux strongly determines this value and is the subject of this article. *
Corresponding author. Tel.: +41 33 2282957; fax: +41 33 2284490. E-mail address: [email protected] (I. Utke).
0167-9317/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.01.136
The overall flow rate of tube-based supply systems can be experimentally determined by mass or volume loss measurements and allows average flux values at the tube exit to be deduced. The average precursor flux arriving on the substrate as a function of vertical microtube distance was measured using stagnation tubes [7] and as a function of temperature using quartz microbalances [1]. A micromechanical gas sensor together with a geometrical approach to estimate the average precursor flux on the substrate was reported in [8]. Recently, the distribution of impinging water molecules was measured on a cryo-cooled substrate inside a dual beam system and modeled solving numerically the continuum Navier–Stokes equation [9]. Our approach presented in this paper relies on the thermal decomposition of impinging precursor molecules being analogous to chemical vapor deposition (CVD) driven in the precursor flux limited regime. For the first time spatial distributions of FEB relevant precursors containing copper, rhodium, and cobalt could be accessed under typical injection conditions. A comparison with Monte Carlo simulations taking into account molecular and transient flow conditions is presented.
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I. Utke et al. / Microelectronic Engineering 83 (2006) 1499–1502
3. Monte Carlo Simulations
Fig. 1. Schematics of experiment: tube-based precursor supply. The substrate is mounted on a heating stage to thermally decompose all impinging molecules on the entire surface.
Monte Carlo (MC) simulations were performed assuming that molecules enter uniformly over the microtube entrance surface with an angular velocity distribution described by a cosine law. After collisions with the inner tube wall the molecule desorbs again with a cosine law distribution. Chemisorption of molecules to the inner tube wall can be included through a sticking factor [10]. Additionally, to allow for transient flow, our simulations consider molecule-molecule collisions with an isotropic angular velocity distribution. The distance between two subsequent molecule–molecule collisions follows an exponential distribution which averages to the mean free path k. It can be estimated from the vapor pressure Pvap according to [11]: p k ¼ kT =ð 2 p d2 P vap Þ; ð1Þ where k is the Boltzmann constant, T the temperature, and d the molecule diameter as listed in Table 1. We verified our MC simulations against published transmission factors and found agreement within five decimal digits [12].
2. Experimental In our experiments we used a modified internal syringe reservoir with the microtube length L = 6 mm, see Fig. 1. The other parameters were varied with the specific experiment. As volatile precursors, di-l-chloro-tetrakis-(trifluorophosphine)-dirhodium [(PF3)2RhCl]2 (CAS: 14876-98-3), dicobalt-octacarbonyl Co2(CO)8 (CAS: 10210-68-1), and hexafluoroacetylacetonato-copper(I)-vinyltri-methylsilane (hfac)CuVTMS (CAS: 139566-53-3) were used. The average precursor flux F with respect to the tube exit area was deduced by mass loss measurements, see Table 1. Control of substrate position with respect to the fixed microtube is achieved using an x–y–z table. Silicon substrates with a TiN layer as diffusion barrier were used. Heating the substrate above CVD temperature to 200–300 C was accomplished with a resistance heater. Using a copper block as fixation assured temperature homogeneity over the entire substrate size (about 1 cm2). The substrate temperature was measured with a thermocouple. In situ electron beam observation served for substrate positioning and showed negligible thermal drifts during typical deposition periods of less than 10 min. Ex situ observation of the deposits was performed with optical microscopy and profilometry.
4. Results and discussion Fig. 2 summarizes the deposition with Rh2Cl2(PF3)4, where the microtube was inclined to the substrate at 40 (D = 600 lm, H = 165 lm, d = 614 lm). The resulting deposit shows an interference color contrast in bright field optical microscopy and thus represents an optical thickness profile. Assuming a homogeneous refractive index, the colors reflect the thickness profile of the deposit. A steady color change from goldish (substrate) to yellow to red to green with increasing deposit thickness towards the microtube is observed. This specific color change is attributed to the reflected light spectrum of the underlying TiN/Si substrate. Superposition with the microtube geometry reveals that the position of precursor flux peak is covered by the microtube i.e., it is not accessible to the focused electron beam. The dimensions of the deposit are roughly three times D along the tube axis and about two times D perpendicular to the tube axis (D = inner tube diameter). MC simulations match best the interference image when molecular flux conditions are assumed, see Fig. 2a. Here the mean free path is set
Table 1 Summary of data from precursors used in experiments. Mean free path k calculated according to Eq. (1). The inner diameter of the tube is D = 600 lm (D = 424 lm for Co2(CO)8) and indicates transient flow (k/D < 1) with intramolecular collisions ˚ )b Precursor Vapor pressure (mbar)a Molecule diameter (A Mean free path (lm) Flux F (cm2 s1)c Reference [(PF3)2RhCl]2 (hfac)CuVTMS Co2(CO)8 a b c
0.075 0.1 0.4
5.7 8.6 7
380 124 47
At room temperature. Longest dimension of molecule. Value of (hfac)CuVTMS is estimated from density and molar mass. At tube exit (measured by mass loss).
3 · 1017 1 · 1018 2 · 1018
[13] [14] [15]
I. Utke et al. / Microelectronic Engineering 83 (2006) 1499–1502
Fig. 2. Top view interference image superposed with isodensity contour plots of precursor flux obtained from MC simulation. The projected view of the microtube with inner (dashed) and outer diameter is indicated. (a) Good match is obtained without molecule collisions (molecular flow). (b) Poor match is obtained including molecule collisions (transient flow).
much larger than tube dimensions such that only molecule-tube collisions take place. However, the mean free path of 380 lm for Rh2Cl2(PF3)4 reported in Table 1 is smaller than the tube dimensions. This suggests that intramolecular collisions should be considered, however, poor match with the experiment is obtained, see Fig. 2b. This contradiction can be explained by pressure decay inside the tube such that molecular flow conditions can be present near the tube exit into vacuum. From the MC simulations it can be calculated that the flux peak in Fig. 2a is 0.32F (F = average flux F reported in Table 1). The flux value accessible to the FEB/FIB is 60.15F. Molecular flow conditions were also found to fit best quantitative profile measurements on deposits obtained from Co2(CO)8 with normal incidence, see Fig. 3a. The absolute height scale was set by matching the area below the curves. Comparison with a singular cosine emitter at the tube exit center shows that within a distance d comparable to the inner tube diameter D the distribution cannot be adequately described. Mass measurements show
1501
Fig. 3. Deposit thickness scans along center line (x = 0). The origin is defined as in Fig. 1. (a) Deposit obtained from Co2(CO)8 with normal incidence, H = d = D = 424 lm. (b) Deposit obtained from (hfac)CuVTMS. The highest precursor flux value accessible by FEB/FIB is indicated.
Table 2 Co2(CO)8 flux peaks (normalized to flux peak at normal incidence, see Fig. 3a) and maximum FEB/ FIB accessible flux (in units of total flux F, see Table 1) when pivoting the microtube around (0,0) at constant d (=424 lm), see Fig. 1 a ()
70
60
50
40
30
Peak (%) Access
80 0.16F
71 0.14F
59 0.12F
44 0.08F
28 0.05F
that chemisorption is less than 3% which did not result in observable changes in simulated distributions. In Table 2, MC simulations were extended to a range of typical supply angles showing that a maximum of 16% of the total flux can be accessed at the tail of the distribution obtained under a = 70. The peak decrease with lower incidence angles is due to an increase of the projected area of precursor impingement. For (hfac)CuVTMS in Fig. 3b, the precursor flux increases steeply starting at the lower end of the microtube, peaking at a position still covered from its upper end and decays smoothly with increasing distance from the tube. This confirms the observations from Fig. 2.
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I. Utke et al. / Microelectronic Engineering 83 (2006) 1499–1502
The MC-simulation yields comparable results at molecular and transient flow conditions and a good match for a = 60, D = 500 lm, and H = 70 lm. From the MC simulations it can be deduced that the flux peaks correspond to 0.26F (a = 90) for the Co2(CO)8 experiment and to 0.68F for (hfac)CuVTMS. The FEB/FEB accessible precursor flux value is 0.18F in Fig. 3b (for values of F refer to Table 1). Summarizing we found good agreement between impinging precursor distributions obtained by thermal decomposition and MC simulations assuming molecular flow conditions. FIB/FEB accessible flux values scale between 5% and 18% of the average exit flux in the geometries investigated. Further optimization of gas injection and mixed gas flows evolving from multi-nozzle systems are being investigated. Acknowledgements S.W. Rhee is acknowledged for supplying the TiN/Si substrate. The Swiss National Fund and CTI/TN21 are acknowledged for financial support.
References [1] V. Scheuer, H.W.P. Koops, T. Tschudi, Microelectron. Eng. 5 (1986) 423. [2] K. Edinger, T. Kraus, Microelectron. Eng. 57–58 (2001) 2631. [3] M. Tanaka, M. Shimojo, K. Mitsuishi, K. Furuya, Appl. Phys. A 78 (2004) 543. [4] P. Hoffmann, I. Utke, F. Cicoira, Proc. SPIE 5023 (2002) 4. [5] N. Antoniou, T.J. Gannon, Presented at EFUG, 2004. [6] J. Fujita, M. Ishida, T. Sakamoto, Y. Ochiai, T. Kaito, S. Matsui, J. Vac. Sci. Technol. B 19 (2001) 2834. [7] P.G. Blauner, J.S. Ro, Y. Butt, J. Melngailis, J. Vac. Sci. Technol. B 7 (1989) 609. [8] K.T. Kohlmann, M. Thiemann, W.H. Bru¨nger, Microelectron. Eng. 13 (1991) 279. [9] M.S. El-Morsi, A.C. Wei, G.F. Nellis, R.L. Engelstadd, S. Sijbrandij, D. Stewart, H. Mulders, Proc. SPIE 5567 (2004) 467. [10] C.G. Smith, G. Lewin, J. Vac. Sci. Technol. 3 (1966) 92. [11] A. Berman, Vacuum Engineering Calculations, Formulas, and Solved Exercises, Academic Press, San Diego, 1992. [12] B.C. Boude, J. Vac. Sci. Technol. B 9 (1972) 1090. [13] T. Ohta, F. Cicoira, P. Doppelt, L. Beitone, P. Hoffmann, Chem. Vap. Deposition 7 (2001) 33. [14] K.K. Choi, S.W. Rhee, Thin Solid Films 409 (2002) 147. [15] M. Rothschild, J.H.C. Sedlacek, D.C. Shaver, D.J. Ehrlich, Mater. Res. Soc. Symp. 158 (1990) 79.