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Self-catalyzed chemical vapor deposition method for the growth of device-quality metal thin films
Microelectronic Engineering 84 (2007) 2481–2485 www.elsevier.com/locate/mee
Self-catalyzed chemical vapor deposition method for the growth of device-quality metal thin films N. Bahlawane
a,*
, P. Antony Premkumar a, K. Onwuka a, G. Reiss b, K. Kohse-Ho¨inghaus a
b
Department of Chemistry, Bielefeld University, Universita¨tsstr. 25, 33615 Bielefeld, Germany Department of Physics, Bielefeld University, Universita¨tsstr. 25, 33615 Bielefeld, Germany
b
Received 8 May 2007; accepted 21 May 2007 Available online 26 May 2007
Abstract Deposition of metals and alloys was demonstrated using thermal chemical vapor deposition starting from commercially available precursors in the absence of molecular hydrogen. The adopted chemical strategy relies solely on the selective reactivity of alcohols with metal complexes at deposition temperature. In this report, particular interest was given to the growth of nickel and silver. This process allows the optimization of the growth of single hcp and fcc phases of nickel starting from Ni(acac)2, whereas several silver precursors allow the deposition of the fcc crystalline structure of silver. Steady growth kinetics, without incubation time, was noticed for all investigated precursors. The electrical conductivity of hcp-Ni, fcc-Ni and fcc-Ag shows the typical decay to the bulk value with increased film thickness, and the temperature resistivity coefficients are similar to the corresponding bulk material. Ó 2007 Elsevier B.V. All rights reserved. Keywords: CVD of metals; Direct liquid injection; Pulsed spray evaporation; Chemical reduction
1. Introduction Metal thin films play a prominent role in the manufacture of many electronic, magnetic and photonic devices. State-of-the-art devices are built in three-dimensional structures where filling deep and narrow trenches with metal coatings without leaving any voids is crucial. This makes chemical vapor deposition (CVD) the method of choice because of its superior step coverage. While CVD of semiconductors is well established, the widespread development of CVD of metals is hindered by the toxicity of the precursors used and the extent of carbon contamination which is associated with the use of hydrogen. This motivates the continuous development of high-performance CVD precursors which do not require hydrogen for the growth of metals [1–5]. This study aims to make heterogeneously catalyzed surface reactions as the primary deposi*
Corresponding author. Tel.: +49 0 521 106 2199; fax: +49 0 521 106 6027. E-mail address: [email protected] (N. Bahlawane). 0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.05.014
tion pathway, since most relevant metals are active redox catalysts for a multitude of selective reactions [6–9]. In particular Ni and Ag catalyze the reforming and dehydrogenation reactions of alcohols successively. The concept of self-catalyzed reactions is expected to render commercially available metalorganic precursors valuable to grow metals without using molecular hydrogen. The advantageous high yield and selectivity of the catalytic reactions are expected to improve the purity of metal films and to enhance their growth rate. 2. Experimental details A pulsed spray evaporation CVD reactor with cold wall geometry was used in this study. This process runs in a pulsed way by injecting a liquid feedstock in the form of a spray. The injection of the feedstock, organic solvent with a metal concentration of 5 mM, is ensured using a 4-opening spray nozzle. Injection proceeds with a frequency of 1 Hz and an opening time of 25 ms. In these conditions, the partial pressure of the precursor reaches 0.013 mbar
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N. Bahlawane et al. / Microelectronic Engineering 84 (2007) 2481–2485
during the pulse duration. The evaporation of the spray was performed at 180 °C and transport to the hot substrate was carried out at 200 °C. A preheated steady flow of nitrogen, 1 slm, at 220 °C was used to purge the reactor between pulses. The mean pressure during deposition was stabilized at 45 mbar. The above mentioned parameters were optimized as a result of a systematic study reported elsewhere [10]. The flow regime is highly unsteady and changes from the transient to the laminar regime during the pulse-purge periods. The reactor, which is schematically presented in Fig. 1, allows a dual, synchronized or alternated delivery of the precursor solution and the reactive solvent. Most results presented here correspond to a single delivery where the precursor is dissolved in the reactive solvent. The thickness of the obtained films was estimated gravimetrically, while their resistivity was measured in open atmosphere at controlled temperatures using a fourpoint-probe method. The identification of the crystalline phases was done using X-ray diffraction (XRD).
N2
Pulsed spray nozzle
Second reservoir for reactive solvent Evaporation zone
Liquid feedstock (precursor in an organic solvent)
Transport tube Hot plate and substrates
Exhaust
Fig. 1. Schematic presentation of the pulsed spray evaporation CVD reactor used for the deposition of metal films.
12
The results presented concern the growth of metals using the thermal CVD process which is achieved in hydrogen-free atmospheres on bare glass substrates. The performed investigations using different metals show that alcohols are essential for the growth of metal films. Inert solvents such as n-butyl acrylate and tetrahydrofuran (THF) do not result in the growth of metal films. This observation is illustrated with nickel in Fig. 2a where the growth rate was observed starting from alcoholic and non-alcoholic solutions of nickelII acetylacetonate (Ni(acac)2). The non-alcoholic precursor solution shows the first significant growth above 300 °C, corresponding to the thermolysis of Ni(acac)2 as reported by Maruyama et al. [11]; the growing films have no metallic appearance however. This is in agreement with the observation of Maruyama et al. [11], who did not notice any growth of metal films in the absence of hydrogen, while a metallic nickel film was observed with hydrogen starting at a substrate temperature of 250 °C [11]. Also presented in the Fig. 2a is the growth of nickel films at lower temperatures using alcoholic solutions of the same precursor. At low temperatures (200–240 °C), the growth rate increases exponentially with the temperature, indicating the limitation of the growth by the kinetics, while a mass-transfer limitation was observed above 240 °C. The film growth in the temperature range from 200 to 300 °C is conditioned by the presence of ethanol reduction. Therefore, a wide temperature range is available for the growth of nickel films by Atomic Layer Deposition (ALD) using the dual delivery experiment. In fact, the delivery of Ni(acac)2 in the n-butyl acrylate solvent leads to the adsorption of the precursor but no overgrowth, as shown in Fig. 2a. However, an alternated introduction of ethanol allows the reduction of the adsorbed precursor which liberates the surface from ligands. In this case the
b Solvent used with Ni(acac)2
60
Ethanol n-butyl acrylate
50
10
Thickness / nm
Growth rate / nm min-1
a
3. Results and discussions
8 6 4 2
40 30 20 10 0
0 220
240
260
280
300
Substrate temperature / °C
320
0
5
10
15
20
25
30
Time / min
Fig. 2. The growth rate of nickel thin films on glass substrates as a function of the deposition temperature, using dissolved Ni(acac)2 in ethanol and in n-butyl acrylate (a). While (b) represents the growth of nickel at 250 °C using the alternated delivery of the n-butyl acrylate precursor solution and of ethanol. Purging between both injections was ensured using nitrogen flow.
N. Bahlawane et al. / Microelectronic Engineering 84 (2007) 2481–2485
2483
The possibility of growing both phases individually is of particular interest for the deposition of alloys, since deposition conditions can be chosen that are favorable for the growth of comparable crystalline structures. The resistivity of the grown nickel thin films was investigated for both cubic and hexagonal crystalline structures obtained using ethanol precursor solutions. Fig. 4 shows a substantial decrease of the relative resistivity with increased thickness of the grown nickel films. The resistivity converges to the bulk value for thick films. The cubic structure, which is grown at high temperatures, shows better packed crystallites, and therefore better electrical conductivity is observed for thinner films, when compared to films grown at low temperature (hcp). The cubic structure reaches a relative resistivity of 1.12 at 63 nm, while a relative resistivity of 1.13 was only reached with a thickness of 270 nm with the hexagonal structure. Comparable performance to that observed with fcc-Ni was reported for 60–150 nm thick films obtained with
growth of nickel was observed, as shown in Fig. 2b. The trend observed in Fig. 2b shows that the initial stage of deposition presents slightly faster kinetics, which might be attributed to the change of the nature of the available adsorption sites. It is worth noting here that the alternated pulsing of Ni(acac)2 and hydrogen, using the conventional bubbler delivery method did not result in film growth on glass [12]. Unlike the process using hydrogen [11], the procedure with ethanol reduction allows the growth of nickel films in a wider temperature range. It thus enables the optimization of nickel single-phase either in a cubic or in a hexagonal lattice as shown in the XRD analysis in Fig. 3. Although organometallic precursors, such as cyclopentadienylallylnickel in the presence of hydrogen allow the deposition of nickel at temperatures below 190 °C, only the cubic crystalline structure is obtained [1]. Moreover, CVD metallization processes using hydrogen as a reducing agent are reported to induce carbon contamination [1–3].
Tsubstrate = 190 ˚C
Tsubstrate = 290 ˚C
(hcp) PDF: 45-1027
(fcc) PDF: 04-0850 20
30
40
50
60
70
80
2Θ/˚
Fig. 3. XRD patterns of obtained nickel films at different temperatures using ethanol precursor solution.
2.5
(fcc) Ni
3.0
Relative resistivity
Relative resistivity
(hcp) Ni 2.0
1.5
1.0
2.5
2.0
1.5
1.0 0
50
100
150
200
Thickness / nm
250
300
25
30
35
40
45
50
55
60
65
Thickness / nm
Fig. 4. Room temperature relative resistivity [film resistivity (lX cm)/bulk resistivity (lX cm)] of cubic (fcc) and hexagonal (hcp) nickel films as a function of their thickness.
N. Bahlawane et al. / Microelectronic Engineering 84 (2007) 2481–2485
Ni-(acac)2 in the presence of hydrogen, however, contamination levels were not reported [11]. In a different study the resistivity of CVD nickel films was reported to increase with the level of carbon contamination and a relative resistivity of 2.86 was observed for >300 nm thick films with less than 4% of contamination [13]. This investigation shows for the first time that using alcohols as reducing agents in the CVD process can lead to device-quality films. This is attributed to the more selective reactivity of alcohols with the precursor molecules leading to reduced contamination. The growth rate in this process was proven to increase linearly with the concentration of the precursor in the feedstock and a deposition rate of 30 nm/min was reached at a concentration of 25 mM in ethanol. This growth rate is competitive with the state-ofthe-art precursors using hydrogen reducing agents [1,3,11]. The reducing ability of alcohols was also demonstrated for the growth of copper, cobalt, iron, and silver [14]. The kinetic of the growth is characterized by zero incubation time on various substrates in contrast to H2-based CVD processes [13,15]. Particular interest was given to the growth of silver thin films using ethanol solution of a variety of precursors. Fig. 5 shows the growth kinetics of silver films starting from a variety of state-of-the-art precursors in an ethanol solution at a substrate temperature of 220 °C, as compared with silver nitrate. This has never been considered as a CVD precursor. In all these cases, silver films grow without incubation time, while an incubation time exceeding 7 min was observed for silverpentafluoropropionate [AgC3F5O2] in THF. The observed growth rates using ethanol solutions were 1.23, 1.4, 1.6, and 18 nm/min for AgC3F5O2, trimethylphosphine
(hexafluoroacetylacetonato)silver [Ag(hfac)(PMe3)], (1.5cyclooctadiene)(hexafluoroacetylacetonate)silver [Ag(hfac)(COD)], and silver nitrate [AgNO3], respectively. Presumably the ten-fold higher growth rate observed with silver nitrate may be a result of the sterically accessible metal center in contrast to the other precursors tested here. The XRD analysis of all silver films, not reported here, reveals a cubic crystalline structure. The electrical resistivity of films obtained from ethanol solutions of silver nitrate and Ag(hfac)(COD) were compared. Electrical conductivity is very sensitive to morphological defects and to contamination. Fig. 6 shows that films starting from AgNO3
4
Relative resistivity
2484
Ag(hfac)(COD) AgNO3
3
2
1
0
100
200
300
400
500
Thickness / nm
Fig. 6. Effect of the thickness of silver films on their resistivity. Films are grown on glass at 220 °C using different precursors.
400 80
AgC3F5O2 in THF 70
Ethanol solutions of:
AgNO3
60
Ag(hfac)(COD) Ag(hfac)(PMe3)
50
AgC3F5O2
200
100
40
30
Incubation time
Thickness / nm
Thickness / nm
300
20
10
0
0 0
20
40
60
Deposition time / min
80
100
0
10
20
30
40
Deposition time / min
Fig. 5. Growth kinetics of silver starting from some selected precursors using ethanol and THF precursor solutions with a metal concentration of 5 mM.
N. Bahlawane et al. / Microelectronic Engineering 84 (2007) 2481–2485
4. Conclusions
3.5 Nickel Silver
3.0
Relative resistivity
2485
A novel concept was developed for the deposition of metals by CVD relying on the intrinsic reactivity of transition metals with specific organic functional groups. The adopted approach permits the enhancement of the growth without incubation time. Moreover, this process allows the attainment of metal coatings in cases where no films grow in the absence of molecular hydrogen. Film growth was observed for the deposition of silver films using inorganic silver precursor, and nickel using Ni(acac)2. Device-quality metal films were grown with this process without any seed layer on bare glass.
2.5
2.0 1.5
1.0 0
50
100
150
200
250
300
350
400
Temperature / K
Fig. 7. The resistivity of nickel and silver films, in air atmosphere, as a function of temperature.
present electrical conductivity which can not be discerned from that obtained using Ag(hfac)(COD) precursor. The relative resistivity of 1.29 starting at a thickness of 160 nm is very competitive to the reported values of silver films starting from tailor-made precursors [4,16]. The measurement of the electrical resistivity as a function of temperature, in open air, shows a linear increase. The calculated slope from this linear dependence gives a temperature coefficient of 7.6 10 3 °C 1 for nickel and 4.29 10 3 °C 1 for silver. The obtained value for nickel lies between that of bulk material and PVD (Physical Vapor Deposition) nickel films [17], while the obtained value for silver lies within the measured range for PVD silver films [18]. These results confirm that obtained films possess electrical properties that are similar to metal films obtained by PVD. The films obtained here show neither oxidation nor carbon segregation in the investigated range of temperature Fig. 7. Chemical reduction was introduced as strategy to form metal films by CVD. This process allows the modification of the surface chemistry without the cumbersome and costly investigations related to the synthesis of new families of CVD precursors. The approach exploited here, relies on the selective reactivity of metals with ethanol. Further investigations indicate the advantageous simplicity encountered for the growth of numerous binary alloys involving iron, nickel, copper, cobalt, and silver. The adjustment of the liquid feedstock composition is a straightforward means of growing controlled compositions of these alloys.
Acknowledgments One of the authors (PAPK) wishes to acknowledge a Fellowship of the Alexander von Humboldt (AvH) Foundation for his postdoctoral stay in Germany. References [1] T. Kada, M. Ishikawa, H. Machida, A. Ogura, Y. Ohshita, K. Soai, J. Cryst. Growth 275 (2005) e1115–e1119. [2] H. Choi, S. Park, T.M. Kim, Chem. Mater. 15 (2003) 3735–3738. [3] J.D. Martin, P. Hogan, K.A. Abdoud, K.-H. Dahmen, Chem. Mater. 10 (1998) 2525–2532. [4] L. Gao, P. Harter, C. Linsmeier, A. Wiltner, R. Emling, D. SchmittLandsiedel, Micrelectron. Eng. 82 (2005) 296–300. [5] D.A. Edwards, M.F. Mahon, K.C. Molloy, V. Orgrodnik, J. Mater. Chem. 13 (2003) 563–570. [6] S.S. Bhoware, S. Shylesh, K.R. Kamble, A.P. Singh, J. Mol. Catal. A 255 (2006) 123–130. [7] W. Shan, Z. Feng, Z. Li, J. Zhang, W. Shen, C. Li, J. Catal. 228 (2004) 206–217. [8] J. Shen, W. Shan, Y. Zhang, J. Du, H. Xu, K. Fan, W. Shen, Y. Tang, J. Catal. 237 (2006) 94–101. [9] R. Zhang, Y. Wang, R.C. Brown, Energ. Convers. Manage. 48 (2007) 68–77. [10] P. Antony Premkumar, N. Bahlawane, K. Hohse-Ho¨inghaus, Chem. Vap. Deposition 13 (2007) 219–226. [11] T. Maruyama, T. Tago, J. Mater. Sci. 28 (1993) 5345–5348. [12] M. Utriainen, M. Kro¨ger-Laukkanen, L.-S. Johansson, L. Niinisto¨, Appl. Surf. Sci. 157 (2000) 151–158. [13] L. Brissonneau, C. Vahlas, Chem. Vap. Deposition 5 (1999) 135–142. [14] P. Antony Premkumar, N. Bahlawane, G. Reiss, K. Hohse-Ho¨inghaus, Chem. Vap. Deposition 13 (2007) 227–231. [15] S. Kim, J.M. Park, D.J. Choi, Thin Solid Films 320 (1998) 95–102. [16] C. Xu, M.J. Hampden-Smith, T.T. Kodas, Adv. Mater. 6 (1994) 746– 748. [17] B.C. Johnson, J. Appl. Phys. 67 (1990) 3018–3024. [18] K.C. Hewitt, P.A. Casey, R.J. Sanderson, M.A. White, R. Sun, Rev. Sci. Instr. 76 (2005) 093906.
Self-catalyzed chemical vapor deposition method for the growth of device-quality metal thin films N. Bahlawane
a,*
, P. Antony Premkumar a, K. Onwuka a, G. Reiss b, K. Kohse-Ho¨inghaus a
b
Department of Chemistry, Bielefeld University, Universita¨tsstr. 25, 33615 Bielefeld, Germany Department of Physics, Bielefeld University, Universita¨tsstr. 25, 33615 Bielefeld, Germany
b
Received 8 May 2007; accepted 21 May 2007 Available online 26 May 2007
Abstract Deposition of metals and alloys was demonstrated using thermal chemical vapor deposition starting from commercially available precursors in the absence of molecular hydrogen. The adopted chemical strategy relies solely on the selective reactivity of alcohols with metal complexes at deposition temperature. In this report, particular interest was given to the growth of nickel and silver. This process allows the optimization of the growth of single hcp and fcc phases of nickel starting from Ni(acac)2, whereas several silver precursors allow the deposition of the fcc crystalline structure of silver. Steady growth kinetics, without incubation time, was noticed for all investigated precursors. The electrical conductivity of hcp-Ni, fcc-Ni and fcc-Ag shows the typical decay to the bulk value with increased film thickness, and the temperature resistivity coefficients are similar to the corresponding bulk material. Ó 2007 Elsevier B.V. All rights reserved. Keywords: CVD of metals; Direct liquid injection; Pulsed spray evaporation; Chemical reduction
1. Introduction Metal thin films play a prominent role in the manufacture of many electronic, magnetic and photonic devices. State-of-the-art devices are built in three-dimensional structures where filling deep and narrow trenches with metal coatings without leaving any voids is crucial. This makes chemical vapor deposition (CVD) the method of choice because of its superior step coverage. While CVD of semiconductors is well established, the widespread development of CVD of metals is hindered by the toxicity of the precursors used and the extent of carbon contamination which is associated with the use of hydrogen. This motivates the continuous development of high-performance CVD precursors which do not require hydrogen for the growth of metals [1–5]. This study aims to make heterogeneously catalyzed surface reactions as the primary deposi*
Corresponding author. Tel.: +49 0 521 106 2199; fax: +49 0 521 106 6027. E-mail address: [email protected] (N. Bahlawane). 0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.05.014
tion pathway, since most relevant metals are active redox catalysts for a multitude of selective reactions [6–9]. In particular Ni and Ag catalyze the reforming and dehydrogenation reactions of alcohols successively. The concept of self-catalyzed reactions is expected to render commercially available metalorganic precursors valuable to grow metals without using molecular hydrogen. The advantageous high yield and selectivity of the catalytic reactions are expected to improve the purity of metal films and to enhance their growth rate. 2. Experimental details A pulsed spray evaporation CVD reactor with cold wall geometry was used in this study. This process runs in a pulsed way by injecting a liquid feedstock in the form of a spray. The injection of the feedstock, organic solvent with a metal concentration of 5 mM, is ensured using a 4-opening spray nozzle. Injection proceeds with a frequency of 1 Hz and an opening time of 25 ms. In these conditions, the partial pressure of the precursor reaches 0.013 mbar
2482
N. Bahlawane et al. / Microelectronic Engineering 84 (2007) 2481–2485
during the pulse duration. The evaporation of the spray was performed at 180 °C and transport to the hot substrate was carried out at 200 °C. A preheated steady flow of nitrogen, 1 slm, at 220 °C was used to purge the reactor between pulses. The mean pressure during deposition was stabilized at 45 mbar. The above mentioned parameters were optimized as a result of a systematic study reported elsewhere [10]. The flow regime is highly unsteady and changes from the transient to the laminar regime during the pulse-purge periods. The reactor, which is schematically presented in Fig. 1, allows a dual, synchronized or alternated delivery of the precursor solution and the reactive solvent. Most results presented here correspond to a single delivery where the precursor is dissolved in the reactive solvent. The thickness of the obtained films was estimated gravimetrically, while their resistivity was measured in open atmosphere at controlled temperatures using a fourpoint-probe method. The identification of the crystalline phases was done using X-ray diffraction (XRD).
N2
Pulsed spray nozzle
Second reservoir for reactive solvent Evaporation zone
Liquid feedstock (precursor in an organic solvent)
Transport tube Hot plate and substrates
Exhaust
Fig. 1. Schematic presentation of the pulsed spray evaporation CVD reactor used for the deposition of metal films.
12
The results presented concern the growth of metals using the thermal CVD process which is achieved in hydrogen-free atmospheres on bare glass substrates. The performed investigations using different metals show that alcohols are essential for the growth of metal films. Inert solvents such as n-butyl acrylate and tetrahydrofuran (THF) do not result in the growth of metal films. This observation is illustrated with nickel in Fig. 2a where the growth rate was observed starting from alcoholic and non-alcoholic solutions of nickelII acetylacetonate (Ni(acac)2). The non-alcoholic precursor solution shows the first significant growth above 300 °C, corresponding to the thermolysis of Ni(acac)2 as reported by Maruyama et al. [11]; the growing films have no metallic appearance however. This is in agreement with the observation of Maruyama et al. [11], who did not notice any growth of metal films in the absence of hydrogen, while a metallic nickel film was observed with hydrogen starting at a substrate temperature of 250 °C [11]. Also presented in the Fig. 2a is the growth of nickel films at lower temperatures using alcoholic solutions of the same precursor. At low temperatures (200–240 °C), the growth rate increases exponentially with the temperature, indicating the limitation of the growth by the kinetics, while a mass-transfer limitation was observed above 240 °C. The film growth in the temperature range from 200 to 300 °C is conditioned by the presence of ethanol reduction. Therefore, a wide temperature range is available for the growth of nickel films by Atomic Layer Deposition (ALD) using the dual delivery experiment. In fact, the delivery of Ni(acac)2 in the n-butyl acrylate solvent leads to the adsorption of the precursor but no overgrowth, as shown in Fig. 2a. However, an alternated introduction of ethanol allows the reduction of the adsorbed precursor which liberates the surface from ligands. In this case the
b Solvent used with Ni(acac)2
60
Ethanol n-butyl acrylate
50
10
Thickness / nm
Growth rate / nm min-1
a
3. Results and discussions
8 6 4 2
40 30 20 10 0
0 220
240
260
280
300
Substrate temperature / °C
320
0
5
10
15
20
25
30
Time / min
Fig. 2. The growth rate of nickel thin films on glass substrates as a function of the deposition temperature, using dissolved Ni(acac)2 in ethanol and in n-butyl acrylate (a). While (b) represents the growth of nickel at 250 °C using the alternated delivery of the n-butyl acrylate precursor solution and of ethanol. Purging between both injections was ensured using nitrogen flow.
N. Bahlawane et al. / Microelectronic Engineering 84 (2007) 2481–2485
2483
The possibility of growing both phases individually is of particular interest for the deposition of alloys, since deposition conditions can be chosen that are favorable for the growth of comparable crystalline structures. The resistivity of the grown nickel thin films was investigated for both cubic and hexagonal crystalline structures obtained using ethanol precursor solutions. Fig. 4 shows a substantial decrease of the relative resistivity with increased thickness of the grown nickel films. The resistivity converges to the bulk value for thick films. The cubic structure, which is grown at high temperatures, shows better packed crystallites, and therefore better electrical conductivity is observed for thinner films, when compared to films grown at low temperature (hcp). The cubic structure reaches a relative resistivity of 1.12 at 63 nm, while a relative resistivity of 1.13 was only reached with a thickness of 270 nm with the hexagonal structure. Comparable performance to that observed with fcc-Ni was reported for 60–150 nm thick films obtained with
growth of nickel was observed, as shown in Fig. 2b. The trend observed in Fig. 2b shows that the initial stage of deposition presents slightly faster kinetics, which might be attributed to the change of the nature of the available adsorption sites. It is worth noting here that the alternated pulsing of Ni(acac)2 and hydrogen, using the conventional bubbler delivery method did not result in film growth on glass [12]. Unlike the process using hydrogen [11], the procedure with ethanol reduction allows the growth of nickel films in a wider temperature range. It thus enables the optimization of nickel single-phase either in a cubic or in a hexagonal lattice as shown in the XRD analysis in Fig. 3. Although organometallic precursors, such as cyclopentadienylallylnickel in the presence of hydrogen allow the deposition of nickel at temperatures below 190 °C, only the cubic crystalline structure is obtained [1]. Moreover, CVD metallization processes using hydrogen as a reducing agent are reported to induce carbon contamination [1–3].
Tsubstrate = 190 ˚C
Tsubstrate = 290 ˚C
(hcp) PDF: 45-1027
(fcc) PDF: 04-0850 20
30
40
50
60
70
80
2Θ/˚
Fig. 3. XRD patterns of obtained nickel films at different temperatures using ethanol precursor solution.
2.5
(fcc) Ni
3.0
Relative resistivity
Relative resistivity
(hcp) Ni 2.0
1.5
1.0
2.5
2.0
1.5
1.0 0
50
100
150
200
Thickness / nm
250
300
25
30
35
40
45
50
55
60
65
Thickness / nm
Fig. 4. Room temperature relative resistivity [film resistivity (lX cm)/bulk resistivity (lX cm)] of cubic (fcc) and hexagonal (hcp) nickel films as a function of their thickness.
N. Bahlawane et al. / Microelectronic Engineering 84 (2007) 2481–2485
Ni-(acac)2 in the presence of hydrogen, however, contamination levels were not reported [11]. In a different study the resistivity of CVD nickel films was reported to increase with the level of carbon contamination and a relative resistivity of 2.86 was observed for >300 nm thick films with less than 4% of contamination [13]. This investigation shows for the first time that using alcohols as reducing agents in the CVD process can lead to device-quality films. This is attributed to the more selective reactivity of alcohols with the precursor molecules leading to reduced contamination. The growth rate in this process was proven to increase linearly with the concentration of the precursor in the feedstock and a deposition rate of 30 nm/min was reached at a concentration of 25 mM in ethanol. This growth rate is competitive with the state-ofthe-art precursors using hydrogen reducing agents [1,3,11]. The reducing ability of alcohols was also demonstrated for the growth of copper, cobalt, iron, and silver [14]. The kinetic of the growth is characterized by zero incubation time on various substrates in contrast to H2-based CVD processes [13,15]. Particular interest was given to the growth of silver thin films using ethanol solution of a variety of precursors. Fig. 5 shows the growth kinetics of silver films starting from a variety of state-of-the-art precursors in an ethanol solution at a substrate temperature of 220 °C, as compared with silver nitrate. This has never been considered as a CVD precursor. In all these cases, silver films grow without incubation time, while an incubation time exceeding 7 min was observed for silverpentafluoropropionate [AgC3F5O2] in THF. The observed growth rates using ethanol solutions were 1.23, 1.4, 1.6, and 18 nm/min for AgC3F5O2, trimethylphosphine
(hexafluoroacetylacetonato)silver [Ag(hfac)(PMe3)], (1.5cyclooctadiene)(hexafluoroacetylacetonate)silver [Ag(hfac)(COD)], and silver nitrate [AgNO3], respectively. Presumably the ten-fold higher growth rate observed with silver nitrate may be a result of the sterically accessible metal center in contrast to the other precursors tested here. The XRD analysis of all silver films, not reported here, reveals a cubic crystalline structure. The electrical resistivity of films obtained from ethanol solutions of silver nitrate and Ag(hfac)(COD) were compared. Electrical conductivity is very sensitive to morphological defects and to contamination. Fig. 6 shows that films starting from AgNO3
4
Relative resistivity
2484
Ag(hfac)(COD) AgNO3
3
2
1
0
100
200
300
400
500
Thickness / nm
Fig. 6. Effect of the thickness of silver films on their resistivity. Films are grown on glass at 220 °C using different precursors.
400 80
AgC3F5O2 in THF 70
Ethanol solutions of:
AgNO3
60
Ag(hfac)(COD) Ag(hfac)(PMe3)
50
AgC3F5O2
200
100
40
30
Incubation time
Thickness / nm
Thickness / nm
300
20
10
0
0 0
20
40
60
Deposition time / min
80
100
0
10
20
30
40
Deposition time / min
Fig. 5. Growth kinetics of silver starting from some selected precursors using ethanol and THF precursor solutions with a metal concentration of 5 mM.
N. Bahlawane et al. / Microelectronic Engineering 84 (2007) 2481–2485
4. Conclusions
3.5 Nickel Silver
3.0
Relative resistivity
2485
A novel concept was developed for the deposition of metals by CVD relying on the intrinsic reactivity of transition metals with specific organic functional groups. The adopted approach permits the enhancement of the growth without incubation time. Moreover, this process allows the attainment of metal coatings in cases where no films grow in the absence of molecular hydrogen. Film growth was observed for the deposition of silver films using inorganic silver precursor, and nickel using Ni(acac)2. Device-quality metal films were grown with this process without any seed layer on bare glass.
2.5
2.0 1.5
1.0 0
50
100
150
200
250
300
350
400
Temperature / K
Fig. 7. The resistivity of nickel and silver films, in air atmosphere, as a function of temperature.
present electrical conductivity which can not be discerned from that obtained using Ag(hfac)(COD) precursor. The relative resistivity of 1.29 starting at a thickness of 160 nm is very competitive to the reported values of silver films starting from tailor-made precursors [4,16]. The measurement of the electrical resistivity as a function of temperature, in open air, shows a linear increase. The calculated slope from this linear dependence gives a temperature coefficient of 7.6 10 3 °C 1 for nickel and 4.29 10 3 °C 1 for silver. The obtained value for nickel lies between that of bulk material and PVD (Physical Vapor Deposition) nickel films [17], while the obtained value for silver lies within the measured range for PVD silver films [18]. These results confirm that obtained films possess electrical properties that are similar to metal films obtained by PVD. The films obtained here show neither oxidation nor carbon segregation in the investigated range of temperature Fig. 7. Chemical reduction was introduced as strategy to form metal films by CVD. This process allows the modification of the surface chemistry without the cumbersome and costly investigations related to the synthesis of new families of CVD precursors. The approach exploited here, relies on the selective reactivity of metals with ethanol. Further investigations indicate the advantageous simplicity encountered for the growth of numerous binary alloys involving iron, nickel, copper, cobalt, and silver. The adjustment of the liquid feedstock composition is a straightforward means of growing controlled compositions of these alloys.
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