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Electroless deposition of novel Ag–W thin films
Microelectronic Engineering 70 (2003) 489–494 www.elsevier.com / locate / mee
Electroless deposition of novel Ag–W thin films a, a a b a V. Bogush *, A. Inberg , N. Croitoru , V. Dubin , Y. Shacham-Diamand a
Department of Physical Electronics, Faculty of Engineering, Tel-Aviv University Ramat-Aviv, Tel Aviv 69978, Israel b Intel, Components Research, Hillsboro, OR 97124, USA
Abstract The seedless electroless deposition of silver–tungsten (Ag–W) thin films on silicon dioxide substrate was performed using wet palladium activation from ammonia–acetate and benzoate solutions. Introducing tungsten in the plating bath catalyzes the deposition for benzoic acid solution and decreases the deposition rate for ammonia–acetate solution. The tungsten content in the deposit was 0–1.0 at%, mainly in WO x form. It was found that the electrical, optical, and mechanical properties of the Ag–W films depends on the W content in the deposit. The optimal Ag–W thin films that were deposited from either the ammonia–acetate or benzoate bath demonstrate good adhesion to the substrate, high brightness, and do not corrode at temperatures up to 350 8C in air. Sub-100 nm thick Ag–W deposits have demonstrated resistivity of about 4 mV cm after vacuum annealing at 350 8C for 1 h. Finally, we present the film microstructure characterization and discuss the possibility of using Ag–W thin films for advanced microelectronics metallization. 2003 Elsevier B.V. All rights reserved. Keywords: Silver; Tungsten; Thin film; Electroless deposition
1. Introduction
The rapid down scaling of the integrated circuit creates resistivity and reliability problems for sub-0.1 mm interconnect technology [1]. Silver was recently promoted as a potential candidate for advanced ultralarge-scale-integration (ULSI) metallization due to its low room temperature bulk resistivity (1.59 mV cm), relatively high melting point and expected higher electromigration resistance in comparison to the common Al and Cu metallization [2,3]. Among the different technologies for thin film deposition, chemical methods are most suitable for *Corresponding author. Tel.: 1972-3-640-5326; fax: 1972-3642-3508. E-mail address: [email protected] (V. Bogush).
ULSI interconnects because of their compatibility with the Dual damascene scheme that requires filling of high aspect ratio features [4]. The superfilling of narrow submicron trenches by the electrochemical deposition of Cu and Ag has been demonstrated [5,6]. Nevertheless, the possibility to fill trenches and via with a high aspect ratio, as well as direct plating of the dielectric surface, makes the electroless deposition procedure a promising technology for future ULSI applications [7,8]. The common silver electroless technology was modified to improve the resistivity and corrosion stability of Ag thin films by using silver–tungsten (Ag–W). It was shown that Ag–W thin films with resistivity in the order of 2–10 mV cm and corrosion stability up to 350 8C in air can be deposited from ammonia–acetate [9] and benzoate [10] silver complexes. In this work we present the results of the influence of the electroless bath
0167-9317 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00414-3
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composition on the growth and properties of Ag–W films on a SiO 2 substrate.
2. Experimental Ag–W thin films were deposited from a AgNO 3 based solution. Hydrazine hydrate was used as a reducing agent. Ammonia together with acetic acid [9] and benzoic acid [10] were used to complex the Ag ions, providing high stability of the baths. It has been shown that both Na 2 WO 4 and WO 3 can be used as a source of tungsten ions, but in this study we investigated the effect of sodium tungstate only. A small amount of additives was introduced to improve the surface coverage, film brightness and uniformity. The plating bath composition is presented in Table 1. Wet palladium activation as described in Refs. [9,10] was applied to initiate film deposition on the SiO 2 surface. All depositions were performed at room temperature. After each step, samples were rinsed with 18 MV cm deionized water. After deposition the samples were washed in iso-propanol followed by drying in nitrogen. Vacuum annealing was applied under a residual pressure of less than 2 10 27 Torr at 350 8C for 1 h to improve the deposit electrical properties. The deposition kinetics, composition, microstructure and resistivity of the thin Ag–W films were studied. The layer thickness was measured using an Alpha-step 500 profilometer. The Ag–W thin film composition was analyzed qualitatively and quantita-
tively by X-ray photoelectron spectroscopy (XPS) using a Physical Electronics PHI model 590 unit. Optical microscopy (Nikon-Optiphot with magnifications of 100–1000), Digital Instruments atomic force microscopy (AFM) and field emission scanning electron microscopy (SEM) (JSM-6300 and JSM 6700 F units) were used to characterize the film microstructure, morphology and topography. The resistivity was measured using an in-line four point probe from Lucas / Signatone姠.
3. Results and discussion The solution composition and operating temperature were found to strongly affect the Ag–W layer morphology and conductivity. As-deposited shiny Ag–W films with good coverage and adhesion to SiO 2 were obtained from both solutions (ammonia– acetate or benzoate).
3.1. Deposition rate The almost linear dependence of the Ag–W deposition rate vs. Na 2 WO 4 concentration in the range 0–0.03 M indicates the catalytic effect of the sodium tungstate in the benzoate bath (Fig. 1). Moreover, the introduction of Na 2 WO 4 to the benzoate solution is necessary to perform plating because pure Ag cannot be deposited at all in the absence of Na 2 WO 4 . Excess sodium tungstate (.0.06 M) results in low solution stability and volume nucleation of silver after several minutes of
Table 1 Silver–tungsten plating solution composition Destination
Source Ag Source W Complexing agent C7H6O2 Reducing agent Additives
pH
Chemical
AgNO 3 Na 2 WO 4 25% NH 4 OH CH 3 COOH None Hydrazine hydrate Saccharine EDTA NH 4 CH 3 OO
Concentration, g / l (M) Ammonia–acetate bath
Benzoate bath
5.4 (0.032) 0–10 (0–0.034) 43 (1.22) 30 (0.5) 40 (0.328) 0.45 ml / l (0.09) 0–0.1 (0–0.0005) 5–15 (0.017–0.051) None 10.0–11.54
5 (0.029) 0–20 (0–0.068) None
0.45 ml / l (0.09) None None 20 (0.26) 9.07–9.25
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2). We assume, taking into account the effect of the deposition rate, that tungsten inclusion in the deposits has more of a mechanical than an electrochemical nature. Sub-100 nm Ag–W films with a tungsten content ranging from 0.6 to 1.0 at% demonstrated the lowest resistivity. It was found that sub-100 nm thick films are enriched by oxygen with the concentration decreasing with film thickness. The composition of films
Fig. 1. The Ag–W deposition rate as a function of the sodium tungstate concentration in solution.
deposition. In the case of ammonia–acetate solution, the film deposition rate decreases linearly with increasing Na 2 WO 4 content (Fig. 1), but the solution maintains stability at any sodium tungstate concentration. We assume that Na 2 WO 4 affects the Ag reduction reaction either directly or indirectly via one of its products after dissolution in water at high pH, but the exact mechanism of the reaction has not yet been fully studied. It was found that Ag–W films with the best uniformity, lowest roughness and resistivity were produced at a deposition rate from 0.3 to 0.7 nm / s.
3.2. Composition The composition of the Ag–W films was studied qualitatively by XPS analysis. It was shown that the deposits contain silver, tungsten and oxygen. It was previously shown that tungsten is introduced into the film mainly in the form of WO x in quantities of several atomic percent [9,10] and its content in the deposit varies with the Na 2 WO 4 concentration in the solution [11]. Ag–W films deposited at the same deposition rate from either benzoate or ammonia–acetate solution have similar composition vs. thickness curves (Fig.
Fig. 2. Ag–W layer composition as a function of film thickness for ammonia–acetate (a) and benzoate (b) solutions.
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thicker than 100 nm was almost constant. The [W] / [O] ratio was about 3, indicating the presence in the film of mainly WO 3 , tungsten oxide. Such a difference in composition may be explained by the elevated tungsten content of thinner films as well as island growth of electroless deposits on the Pdactivated dielectric surface. In the early stage of deposition, the film is discontinuous and contains pores and holes which allow the sorption of additional, unbounded oxygen from solution. The oxygen XPS signal is also increased due to sputtering of the
SiO 2 substrate through the pores. The following plating fills all free spaces, achieving continuity and uniformity in composition and resistivity.
3.3. Microstructure As-deposited sub-100 nm Ag–W films have good surface coverage and adhesion to the substrate, high optical reflection and electrical uniformity from a thickness of about 30 nm. It was found that the film morphology is determined, in general, by the deposition conditions. The microstructure of highly conductive Ag–W films deposited at a rate from 0.3 to 0.7 nm / s depends on the type of complexing agent. Both deposition solutions (Fig. 3) produced polycrystalline films. The microstructure of the Ag–W films deposited from the benzoate solution was characterized by smaller grains (Fig. 3b) in comparison with the ammonia–acetate bath (Fig. 3a), probably due to the stronger silver complex. The grain size of the films was found to be a function of film thickness, attaining a value of about 140 nm for films thicker than 100 nm. The film topography was found to correspond to the grain structure. The surface roughness was in the range from 3 to 15 nm, that is larger than the ,1 nm for pure Ag films deposited by dc magnetron sputtering [3] and comparable to the roughness of electroplated copper and silver thin films [6,7].
3.4. Resistivity
Fig. 3. SEM images of 60 nm thick Ag–W films deposited from ammonia–acetate (a) and benzoate (b) solutions.
The electrical properties of the Ag–W films also depend on the W content in the deposit. Low-resistivity 100 nm thick Ag–W films were deposited at an optimal deposition rate from a solution containing from 0.015 to 0.03 M sodium tungstate at pH ¯9.17 and ¯11.56 for the benzoate and ammonia–acetic baths, respectively. Such layers, with a tungsten content of ,1 at%, exhibit resistivity values in the order of 10 25 V cm, which is several times higher than that of bulk silver, sputtered silver films [3] and electroless copper [7]. Fig. 4 presents resistivity vs. thickness for Ag–W films deposited from ammonia–acetate and benzoate baths in the as-deposited and annealed state. The resistivity reaches its lowest value for films thicker than 100 nm. We assume that such behavior is due to
V. Bogush et al. / Microelectronic Engineering 70 (2003) 489–494
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less solutions. Ag–W thin films deposited from the developed baths on activated silicon dioxide demonstrated good adhesion to the substrate, improved electrical resistivity, and good surface coverage. The deposition conditions strongly affect the Ag–W layer conductivity and morphology. The composition and resistivity of the silver–tungsten films was studied as a function of film thickness. It was shown that the electrical properties of Ag–W films deposited at a rate from 0.3 to 0.7 nm / s slightly depend on the nature of the complexing agent. The Ag–W films containing ,1 at% tungsten showed a minimum resistivity of |4 mV cm after vacuum annealing. The improved resistivity of electroless Ag–W films thinner than 100 nm and their high corrosion stability make them useful for microelectronic applications as a conductive or cap layer. Fig. 4. Ag–W film resistivity as a function of thickness in the as-deposited and annealed state.
Acknowledgements variation in the film composition and structure. Thinner films show a strong contribution of surface electron scattering and point defects, such as oxygencontaining impurities and voids, which may increase the resistivity in addition to grain boundary electron scattering, which, in general, determines the resistivity of relatively thick (.100 nm) films. Post-deposition vacuum annealing reduced the Ag–W film resistivity. This influence is less significant for thicker, .100 nm, films than for those of 50 nm thickness. According to the evolution of the microstructure after vacuum annealing, the corresponding doubling of grain growth cannot explain such a large conductivity improvement. Probably, the grain boundary effect decreases due to both grain growth and the removal of contamination from the grain boundaries. The achieved Ag–W resistivity of 4–6 mV cm after post-deposition vacuum annealing, together with the low Ag diffusion into the inter-level dielectric [12], makes Ag–W layers promising for ‘barrierless’ ULSI interconnect applications at sub-100 nm.
4. Conclusions Benzoic and ammonia–acetic complexes of silver are proposed for the development of stable electro-
The authors would like to thank the Semiconductor Research Corporation for support of this research (contract 2001-MJ-944). We also thank Dr. Larisa Burschstein and Dr. Zahava Barkay from the Wolfson Material Center at Tel-Aviv University.
References [1] J.A. Davis, R. Venkatesan, A. Kaloyeros, M. Beylansky, S.J. Souri, K. Banerjee, K.C. Saraswat, A. Rahman, R. Reif, J.D. Meindl, Interconnect limits on gigascale integration (GSI) in 21st century, Proc. IEEE 89 (3) (2001) 305–324. [2] Y. Wang, T.L. Alford, Formation of aluminum oxynitride diffusion barriers for Ag metallization, Appl. Phys. Lett. 74 (1) (1999) 52–54. [3] M. Hauder, J. Gstottner, W. Hansch, D. Schmitt-Landsiedel, Scaling properties and electromigration resistance of sputtered Ag metallization lines, Appl. Phys. Lett. 78 (6) (2001) 838–840. [4] S.P. Murarka, Metallization: Theory and Practice for VLSI and ULSI, Butterworth–Heinemann, 1993. [5] T.P. Moffat, J.E. Bonevich, W.H. Huber, A. Stanishevsky, D.R. Kelly, G.R. Stafford, D. Josella, Superconformal electrodeposition of copper in 500–90 nm features, J. Electrochem. Soc. 147 (12) (2000) 4524–4535. [6] T.P. Moffat, B. Baker, D. Wheeler, J.E. Bonevich, M. Edelstein, D.R. Kelly, L. Gan, G.R. Stafford, P.J. Chen, W.F. Egelhoff, D. Josell, Superconformal electrodeposition of silver in submicrometer features, J. Electrochem. Soc. 149 (8) (2002) C423–C428.
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[7] Y. Shacham-Diamand, V. Dubin, Copper electroless deposition technology for ultra-large-scale-integration (ULSI) metallization, Microelectron. Eng. 33 (1997) 47–58. [8] Y. Shacham-Diamand, S. Lopatin, High aspect ratio quartermicron electroless copper integrated technology, Microelectron. Eng. 37 / 38 (1997) 77–88. [9] Y. Shacham-Diamand, A. Inberg, Y. Sverdlov, N. Croitoru, Electroless silver and silver with tungsten thin films for microelectronics and microelectromechanical system applications, J. Electrochem. Soc. 147 (9) (2000) 3345–3349. [10] A. Inberg, V. Bogush, N. Croitoru, V. Dubin, Y. Shacham-
Diamand, Novel highly conductive silver–tungsten thin films electroless deposited from benzoate solution for microelectronic applications, J. Electrochem. Soc. 150 (5) (2003) C285–C291. [11] V. Bogush, A. Inberg, N. Croitoru, V. Dubin, Y. ShachamDiamand, Material properties of very thin electroless silver– tungsten films, Thin Solid Films 426 (2003) 288–295. [12] A. Inberg, E. Ginsburg, Y. Shacham-Diamand, N. Croitoru, A. Seidman, Electroless and sputtered silver–tungsten thin films for microelectronics applications, Microelectron. Eng. 65 (2003) 197–207.
Electroless deposition of novel Ag–W thin films a, a a b a V. Bogush *, A. Inberg , N. Croitoru , V. Dubin , Y. Shacham-Diamand a
Department of Physical Electronics, Faculty of Engineering, Tel-Aviv University Ramat-Aviv, Tel Aviv 69978, Israel b Intel, Components Research, Hillsboro, OR 97124, USA
Abstract The seedless electroless deposition of silver–tungsten (Ag–W) thin films on silicon dioxide substrate was performed using wet palladium activation from ammonia–acetate and benzoate solutions. Introducing tungsten in the plating bath catalyzes the deposition for benzoic acid solution and decreases the deposition rate for ammonia–acetate solution. The tungsten content in the deposit was 0–1.0 at%, mainly in WO x form. It was found that the electrical, optical, and mechanical properties of the Ag–W films depends on the W content in the deposit. The optimal Ag–W thin films that were deposited from either the ammonia–acetate or benzoate bath demonstrate good adhesion to the substrate, high brightness, and do not corrode at temperatures up to 350 8C in air. Sub-100 nm thick Ag–W deposits have demonstrated resistivity of about 4 mV cm after vacuum annealing at 350 8C for 1 h. Finally, we present the film microstructure characterization and discuss the possibility of using Ag–W thin films for advanced microelectronics metallization. 2003 Elsevier B.V. All rights reserved. Keywords: Silver; Tungsten; Thin film; Electroless deposition
1. Introduction
The rapid down scaling of the integrated circuit creates resistivity and reliability problems for sub-0.1 mm interconnect technology [1]. Silver was recently promoted as a potential candidate for advanced ultralarge-scale-integration (ULSI) metallization due to its low room temperature bulk resistivity (1.59 mV cm), relatively high melting point and expected higher electromigration resistance in comparison to the common Al and Cu metallization [2,3]. Among the different technologies for thin film deposition, chemical methods are most suitable for *Corresponding author. Tel.: 1972-3-640-5326; fax: 1972-3642-3508. E-mail address: [email protected] (V. Bogush).
ULSI interconnects because of their compatibility with the Dual damascene scheme that requires filling of high aspect ratio features [4]. The superfilling of narrow submicron trenches by the electrochemical deposition of Cu and Ag has been demonstrated [5,6]. Nevertheless, the possibility to fill trenches and via with a high aspect ratio, as well as direct plating of the dielectric surface, makes the electroless deposition procedure a promising technology for future ULSI applications [7,8]. The common silver electroless technology was modified to improve the resistivity and corrosion stability of Ag thin films by using silver–tungsten (Ag–W). It was shown that Ag–W thin films with resistivity in the order of 2–10 mV cm and corrosion stability up to 350 8C in air can be deposited from ammonia–acetate [9] and benzoate [10] silver complexes. In this work we present the results of the influence of the electroless bath
0167-9317 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00414-3
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V. Bogush et al. / Microelectronic Engineering 70 (2003) 489–494
composition on the growth and properties of Ag–W films on a SiO 2 substrate.
2. Experimental Ag–W thin films were deposited from a AgNO 3 based solution. Hydrazine hydrate was used as a reducing agent. Ammonia together with acetic acid [9] and benzoic acid [10] were used to complex the Ag ions, providing high stability of the baths. It has been shown that both Na 2 WO 4 and WO 3 can be used as a source of tungsten ions, but in this study we investigated the effect of sodium tungstate only. A small amount of additives was introduced to improve the surface coverage, film brightness and uniformity. The plating bath composition is presented in Table 1. Wet palladium activation as described in Refs. [9,10] was applied to initiate film deposition on the SiO 2 surface. All depositions were performed at room temperature. After each step, samples were rinsed with 18 MV cm deionized water. After deposition the samples were washed in iso-propanol followed by drying in nitrogen. Vacuum annealing was applied under a residual pressure of less than 2 10 27 Torr at 350 8C for 1 h to improve the deposit electrical properties. The deposition kinetics, composition, microstructure and resistivity of the thin Ag–W films were studied. The layer thickness was measured using an Alpha-step 500 profilometer. The Ag–W thin film composition was analyzed qualitatively and quantita-
tively by X-ray photoelectron spectroscopy (XPS) using a Physical Electronics PHI model 590 unit. Optical microscopy (Nikon-Optiphot with magnifications of 100–1000), Digital Instruments atomic force microscopy (AFM) and field emission scanning electron microscopy (SEM) (JSM-6300 and JSM 6700 F units) were used to characterize the film microstructure, morphology and topography. The resistivity was measured using an in-line four point probe from Lucas / Signatone姠.
3. Results and discussion The solution composition and operating temperature were found to strongly affect the Ag–W layer morphology and conductivity. As-deposited shiny Ag–W films with good coverage and adhesion to SiO 2 were obtained from both solutions (ammonia– acetate or benzoate).
3.1. Deposition rate The almost linear dependence of the Ag–W deposition rate vs. Na 2 WO 4 concentration in the range 0–0.03 M indicates the catalytic effect of the sodium tungstate in the benzoate bath (Fig. 1). Moreover, the introduction of Na 2 WO 4 to the benzoate solution is necessary to perform plating because pure Ag cannot be deposited at all in the absence of Na 2 WO 4 . Excess sodium tungstate (.0.06 M) results in low solution stability and volume nucleation of silver after several minutes of
Table 1 Silver–tungsten plating solution composition Destination
Source Ag Source W Complexing agent C7H6O2 Reducing agent Additives
pH
Chemical
AgNO 3 Na 2 WO 4 25% NH 4 OH CH 3 COOH None Hydrazine hydrate Saccharine EDTA NH 4 CH 3 OO
Concentration, g / l (M) Ammonia–acetate bath
Benzoate bath
5.4 (0.032) 0–10 (0–0.034) 43 (1.22) 30 (0.5) 40 (0.328) 0.45 ml / l (0.09) 0–0.1 (0–0.0005) 5–15 (0.017–0.051) None 10.0–11.54
5 (0.029) 0–20 (0–0.068) None
0.45 ml / l (0.09) None None 20 (0.26) 9.07–9.25
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2). We assume, taking into account the effect of the deposition rate, that tungsten inclusion in the deposits has more of a mechanical than an electrochemical nature. Sub-100 nm Ag–W films with a tungsten content ranging from 0.6 to 1.0 at% demonstrated the lowest resistivity. It was found that sub-100 nm thick films are enriched by oxygen with the concentration decreasing with film thickness. The composition of films
Fig. 1. The Ag–W deposition rate as a function of the sodium tungstate concentration in solution.
deposition. In the case of ammonia–acetate solution, the film deposition rate decreases linearly with increasing Na 2 WO 4 content (Fig. 1), but the solution maintains stability at any sodium tungstate concentration. We assume that Na 2 WO 4 affects the Ag reduction reaction either directly or indirectly via one of its products after dissolution in water at high pH, but the exact mechanism of the reaction has not yet been fully studied. It was found that Ag–W films with the best uniformity, lowest roughness and resistivity were produced at a deposition rate from 0.3 to 0.7 nm / s.
3.2. Composition The composition of the Ag–W films was studied qualitatively by XPS analysis. It was shown that the deposits contain silver, tungsten and oxygen. It was previously shown that tungsten is introduced into the film mainly in the form of WO x in quantities of several atomic percent [9,10] and its content in the deposit varies with the Na 2 WO 4 concentration in the solution [11]. Ag–W films deposited at the same deposition rate from either benzoate or ammonia–acetate solution have similar composition vs. thickness curves (Fig.
Fig. 2. Ag–W layer composition as a function of film thickness for ammonia–acetate (a) and benzoate (b) solutions.
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V. Bogush et al. / Microelectronic Engineering 70 (2003) 489–494
thicker than 100 nm was almost constant. The [W] / [O] ratio was about 3, indicating the presence in the film of mainly WO 3 , tungsten oxide. Such a difference in composition may be explained by the elevated tungsten content of thinner films as well as island growth of electroless deposits on the Pdactivated dielectric surface. In the early stage of deposition, the film is discontinuous and contains pores and holes which allow the sorption of additional, unbounded oxygen from solution. The oxygen XPS signal is also increased due to sputtering of the
SiO 2 substrate through the pores. The following plating fills all free spaces, achieving continuity and uniformity in composition and resistivity.
3.3. Microstructure As-deposited sub-100 nm Ag–W films have good surface coverage and adhesion to the substrate, high optical reflection and electrical uniformity from a thickness of about 30 nm. It was found that the film morphology is determined, in general, by the deposition conditions. The microstructure of highly conductive Ag–W films deposited at a rate from 0.3 to 0.7 nm / s depends on the type of complexing agent. Both deposition solutions (Fig. 3) produced polycrystalline films. The microstructure of the Ag–W films deposited from the benzoate solution was characterized by smaller grains (Fig. 3b) in comparison with the ammonia–acetate bath (Fig. 3a), probably due to the stronger silver complex. The grain size of the films was found to be a function of film thickness, attaining a value of about 140 nm for films thicker than 100 nm. The film topography was found to correspond to the grain structure. The surface roughness was in the range from 3 to 15 nm, that is larger than the ,1 nm for pure Ag films deposited by dc magnetron sputtering [3] and comparable to the roughness of electroplated copper and silver thin films [6,7].
3.4. Resistivity
Fig. 3. SEM images of 60 nm thick Ag–W films deposited from ammonia–acetate (a) and benzoate (b) solutions.
The electrical properties of the Ag–W films also depend on the W content in the deposit. Low-resistivity 100 nm thick Ag–W films were deposited at an optimal deposition rate from a solution containing from 0.015 to 0.03 M sodium tungstate at pH ¯9.17 and ¯11.56 for the benzoate and ammonia–acetic baths, respectively. Such layers, with a tungsten content of ,1 at%, exhibit resistivity values in the order of 10 25 V cm, which is several times higher than that of bulk silver, sputtered silver films [3] and electroless copper [7]. Fig. 4 presents resistivity vs. thickness for Ag–W films deposited from ammonia–acetate and benzoate baths in the as-deposited and annealed state. The resistivity reaches its lowest value for films thicker than 100 nm. We assume that such behavior is due to
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less solutions. Ag–W thin films deposited from the developed baths on activated silicon dioxide demonstrated good adhesion to the substrate, improved electrical resistivity, and good surface coverage. The deposition conditions strongly affect the Ag–W layer conductivity and morphology. The composition and resistivity of the silver–tungsten films was studied as a function of film thickness. It was shown that the electrical properties of Ag–W films deposited at a rate from 0.3 to 0.7 nm / s slightly depend on the nature of the complexing agent. The Ag–W films containing ,1 at% tungsten showed a minimum resistivity of |4 mV cm after vacuum annealing. The improved resistivity of electroless Ag–W films thinner than 100 nm and their high corrosion stability make them useful for microelectronic applications as a conductive or cap layer. Fig. 4. Ag–W film resistivity as a function of thickness in the as-deposited and annealed state.
Acknowledgements variation in the film composition and structure. Thinner films show a strong contribution of surface electron scattering and point defects, such as oxygencontaining impurities and voids, which may increase the resistivity in addition to grain boundary electron scattering, which, in general, determines the resistivity of relatively thick (.100 nm) films. Post-deposition vacuum annealing reduced the Ag–W film resistivity. This influence is less significant for thicker, .100 nm, films than for those of 50 nm thickness. According to the evolution of the microstructure after vacuum annealing, the corresponding doubling of grain growth cannot explain such a large conductivity improvement. Probably, the grain boundary effect decreases due to both grain growth and the removal of contamination from the grain boundaries. The achieved Ag–W resistivity of 4–6 mV cm after post-deposition vacuum annealing, together with the low Ag diffusion into the inter-level dielectric [12], makes Ag–W layers promising for ‘barrierless’ ULSI interconnect applications at sub-100 nm.
4. Conclusions Benzoic and ammonia–acetic complexes of silver are proposed for the development of stable electro-
The authors would like to thank the Semiconductor Research Corporation for support of this research (contract 2001-MJ-944). We also thank Dr. Larisa Burschstein and Dr. Zahava Barkay from the Wolfson Material Center at Tel-Aviv University.
References [1] J.A. Davis, R. Venkatesan, A. Kaloyeros, M. Beylansky, S.J. Souri, K. Banerjee, K.C. Saraswat, A. Rahman, R. Reif, J.D. Meindl, Interconnect limits on gigascale integration (GSI) in 21st century, Proc. IEEE 89 (3) (2001) 305–324. [2] Y. Wang, T.L. Alford, Formation of aluminum oxynitride diffusion barriers for Ag metallization, Appl. Phys. Lett. 74 (1) (1999) 52–54. [3] M. Hauder, J. Gstottner, W. Hansch, D. Schmitt-Landsiedel, Scaling properties and electromigration resistance of sputtered Ag metallization lines, Appl. Phys. Lett. 78 (6) (2001) 838–840. [4] S.P. Murarka, Metallization: Theory and Practice for VLSI and ULSI, Butterworth–Heinemann, 1993. [5] T.P. Moffat, J.E. Bonevich, W.H. Huber, A. Stanishevsky, D.R. Kelly, G.R. Stafford, D. Josella, Superconformal electrodeposition of copper in 500–90 nm features, J. Electrochem. Soc. 147 (12) (2000) 4524–4535. [6] T.P. Moffat, B. Baker, D. Wheeler, J.E. Bonevich, M. Edelstein, D.R. Kelly, L. Gan, G.R. Stafford, P.J. Chen, W.F. Egelhoff, D. Josell, Superconformal electrodeposition of silver in submicrometer features, J. Electrochem. Soc. 149 (8) (2002) C423–C428.
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V. Bogush et al. / Microelectronic Engineering 70 (2003) 489–494
[7] Y. Shacham-Diamand, V. Dubin, Copper electroless deposition technology for ultra-large-scale-integration (ULSI) metallization, Microelectron. Eng. 33 (1997) 47–58. [8] Y. Shacham-Diamand, S. Lopatin, High aspect ratio quartermicron electroless copper integrated technology, Microelectron. Eng. 37 / 38 (1997) 77–88. [9] Y. Shacham-Diamand, A. Inberg, Y. Sverdlov, N. Croitoru, Electroless silver and silver with tungsten thin films for microelectronics and microelectromechanical system applications, J. Electrochem. Soc. 147 (9) (2000) 3345–3349. [10] A. Inberg, V. Bogush, N. Croitoru, V. Dubin, Y. Shacham-
Diamand, Novel highly conductive silver–tungsten thin films electroless deposited from benzoate solution for microelectronic applications, J. Electrochem. Soc. 150 (5) (2003) C285–C291. [11] V. Bogush, A. Inberg, N. Croitoru, V. Dubin, Y. ShachamDiamand, Material properties of very thin electroless silver– tungsten films, Thin Solid Films 426 (2003) 288–295. [12] A. Inberg, E. Ginsburg, Y. Shacham-Diamand, N. Croitoru, A. Seidman, Electroless and sputtered silver–tungsten thin films for microelectronics applications, Microelectron. Eng. 65 (2003) 197–207.