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Air gap formation by UV-assisted decomposition of CVD material
Microelectronic Engineering 85 (2008) 2071–2074
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Air gap formation by UV-assisted decomposition of CVD material M. Pantouvaki a,*, A. Humbert b, E. VanBesien a, E. Camerotto a, Y. Travaly a, O. Richard a, M. Willegems a, H. Volders a, K. Kellens a, R. Daamen b, R.J.O.M. Hoofman b, G. Beyer a a b
IMEC, Kapeldreef 75, B-3001 Leuven, Belgium NXP/TSMC Research Center, Kapeldreef 75, B-3001 Leuven, Belgium
a r t i c l e
i n f o
Article history: Received 7 May 2008 Accepted 7 May 2008 Available online 27 May 2008 Keywords: Air gaps Low-k Sacrificial material Capacitance reduction CVD Decomposition
a b s t r a c t A sacrificial material deposited by CVD is used to demonstrate air gap formation in single damascene structures by UV-assisted decomposition. The material is removed through a porous low-k cap, after completion of the damascene scheme. The porosity of the low-k cap is shown to be critical for efficient air gap formation. Capacitance reduction of 50% is demonstrated using this technique compared to conventional SiOC(H) interconnects and an effective dielectric constant of 1.7 is extrapolated. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Interconnect scaling to smaller sizes increases the interline capacitance. The resulting RC delay and power consumption become critical drawbacks of the interconnects for the performance of integrated circuits. Although the integration of low-k dielectrics is now under development, air, with a k-value of 1, appears as the ultimate dielectric material for capacitance reduction at metal levels. The various approaches that have been proposed to create air gaps are generally classified into two categories [1,2]: air gap creation by material removal and non-conformal chemical vapor deposition (CVD) [3–5] or by selective sacrificial material removal through a cap using thermal decomposition or etching [6–8]. Nonconformal CVD initially created air gaps that were unintentional and undesirable, but they were eventually exploited to reduce interline capacitance. Although this is a technique that can be easily incorporated into fabs, good control of the air gap formation is needed to avoid the extension of the cavity into the via dielectric and corresponding problems of metal intrusion in the air gap in case of via misalignment. To demonstrate air gap formation via thermal decomposition of a sacrificial material through a porous cap, the sacrificial material has been typically a spin-on polymer. In [9], Chan et al. showed that a thermally decomposable sacrificial film can be deposited by CVD. In this paper we use a sacrificial material deposited by CVD and a porous SiOC low-k cap to inte-
* Corresponding author. E-mail address: [email protected] (M. Pantouvaki). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.05.020
grate air gap formation by UV-assisted decomposition into a standard damascene scheme on 300 mm-size wafers. The air gap formation is performed after chemical-mechanical polishing (CMP), increasing the integration scheme by one step. The UV-illumination during decomposition at high temperature is used to increase the decomposition rate.
2. CVD deposition The CVD deposition and decomposition processes were initially studied on blanket wafers. The CVD thermally decomposable film was developed such that deposition of the film was possible at the same temperature as other low-k dielectrics deposited in the same chamber. Evaluation of the sacrificial film decomposition was done by in-line ellipsometric measurements of the film thickness before and after decomposition. However, such measurements become complicated with a multilayer stack under study. Therefore, to test the efficiency of the decomposition of the sacrificial film through the porous low-k cap, a mass loss measurement technique was developed using a microbalance. The weight of the wafers was measured before and after deposition of the sacrificial film, after deposition of the low-k cap, and after decomposition. A correction was made for the mass loss of the porous cap due to UV-illumination, based on separate measurements on wafers with only the porous cap deposited, and the mass loss of the sacrificial film was extrapolated. A comparison of the thickness loss and the mass loss measurements on wafers with only the sacrificial material deposited is shown in Fig. 1. The two methods
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Table 1). In the case of the lowest porosity material A, the mass loss is reaching a plateau of 40% after 10 min treatment. The result is very similar when reducing the thickness of the cap by half. This suggests that the removal of the sacrificial film is blocked by the pore size rather than the pore network. The capping material B with a porosity of 25% results in a maximum mass loss of about 70%. The decomposition efficiency is further improved when using the capping material C of highest porosity, 40%. A mass loss of more than 80% is achieved, similar to the result without a low-k cap, indicating that the pore size and connectivity of this capping material are sufficiently high to let the decomposed film escape. 3. Integration and measurements
Fig. 1. Comparison of thickness loss and mass loss measurements of the sacrificial film as a function of UV-assisted decomposition time at 425 °C, for two experiments.
give comparable results. For characterization of the decomposition of the sacrificial film through a porous cap, a film of 140 nm thickness was deposited on blanket wafers with a 150 nm thick porous low-k cap on top. The porosity and the pore size of the low-k material were varied as shown in Table 1. Decomposition tests through the low-k cap were performed at 425 °C with UV-light on. The sacrificial material would be decomposed and escape through the pores of the capping layer. The pore size and interconnectivity of this low-k cap should be adequately high to let the decomposed material escape during UV-treatment. Fig. 2 shows the mass loss of the sacrificial material due to UV-assisted decomposition as a function of time for capping material of varying porosity (as in
Table 1 Characteristics of different low-k capping materials Capping material
Porosity (%)
Pore radius (nm)
A B C
15 25 39
0.8 0.9 1.2
Fig. 2. Mass loss of the sacrificial film versus decomposition time for capping material of varying porosity.
The integration scheme used for air gap formation in a single damascene flow is shown in Fig. 3. The sacrificial film and a porous capping layer that formed the dielectric stack of the damascene were deposited by CVD. The stack was then patterned using conventional lithography, dry etch, metallization and CMP steps. A non-selective slurry was used for CMP, which allowed to vary the low-k cap thickness without the need for extra etch development. Although the cap thickness was not found to affect the decomposition of the sacrificial material, a thinner cap would increase the percentage of air between the metal lines, reducing further the capacitance. After CMP, the sacrificial CVD material was removed by UV-assisted thermal decomposition and the wafers were then fully passivated and electrically measured. Fig. 4 shows SEM images from samples after air gap formation. Air gaps were formed simultaneously in wide and narrow spacings across the wafer (Figs. 4(a), (b)). Mechanical integrity of the low-k cap bridges was observed for spacings up to 1.3 lm, depending on the material and the thickness of the porous cap used (Fig. 4(c)). When the low-k cap thickness was reduced to 30 nm, the material collapsed for intermetallic widths greater than 550 nm (Fig. 4(d)). Electrical measurements were performed on meander-fork structures of 120 nm / 120 nm linewidth / spacing. Two cases were eventually examined, of air gaps with low-k cap of material A and an air-to-metal height ratio of 60% and of air gaps with low-k cap of material C and air-to-metal-height ratio of 85%. Fig. 5 shows the capacitance of the two air gap versions versus a SiOC(H) reference. A capacitance reduction of 35% was measured on the wafer with low-k material A compared to the SiOC(H) low-k reference. This was further reduced to 50% for the low-k capping material C of higher porosity. No significant leakage current increase was observed in the case of air gaps compared to the reference wafer at
Fig. 3. Air gap formation integration scheme: (1) Dielectric stack deposition, (2) patterning, (3) metallization and CMP, and (4) UV-assisted decomposition of sacrificial film.
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0.1 MV/cm, as shown in Fig. 6. A breakdown voltage of 3.5 MV/ cm was measured on wafers with air gaps and low-k material A (Fig. 7). Further analysis of the integrated structures with air gaps was done by TEM and EFTEM. Fig. 8 shows TEM images from the two cases. Incomplete air gap formation was observed in some areas in the case of lower porosity capping material A. EFTEM analysis indicated a carbon rich ring around the air gap due to insufficient removal of the sacrificial film. Using a higher porosity capping material improved the removal efficiency of the sacrificial film. The integrated k-value of the intermetal stack was extrapolated by RaphaelTM simulations. In this approach, the real geometry and spacing of the metallic lines as observed by TEM is reproduced by simulation and the k-value is optimized to fit the measured capacitance values. The extracted k-value using this technique for air
Fig. 4. SEM images of UV-treated structures for air gap formation: (a) 100 nm/ 100 nm width/spacing, low-k material A, air gap 60%, (b) 500 nm/500 nm width/ spacing, low-k material C, air gap 85%, (c) 1.3 lm low-k bridges of material A, air gap 60%, remain intact, and (d) low-k bridges of material C, air gap 85%, are collapsing at spacings >550 nm.
Fig. 6. Open resistance of air gap wafer with capping material A compared to a reference wafer with PECVD SiOC(H) low-k dielectric measured at 0.1 MV/cm.
Fig. 5. Cumulative probability plot of capacitance of air gap structures versus a reference with PECVD SiOC(H) low-k dielectric. Fig. 7. Breakdown voltage of air gap wafer with capping material A.
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Fig. 8. (a) TEM, (b) EFTEM map of carbon for a sample with low-k material A as a cap, and (c) TEM image of sample with low-k material C as cap.
ity of which is critical for effective air gap formation. Significant capacitance reduction was measured on wafers with air gaps compared to a SiOC(H) reference for both cases of capping material of 15% and 39% porosity, reaching a 50% capacitance reduction in the latter case. The integrated k-value of this interconnect was calculated to be 1.7. This value is significantly lower than that of the low-k dielectrics typically used today, k = 2.5 as-deposited. Further optimization of the integration scheme is required to improve the reliability performance of these interconnects. Acknowledgement This work was partially funded by the European Commission’s Information Society Technologies Program (IST) under PULLNANO project contract No: IST 026828. References Fig. 9. Integrated k-value versus air-to-metal line height ratio. The lines indicate calculations of the ideal case of capping material with an as-deposited k-value of 2.8 (solid line), 2.5 (dashed line) and 2.2 (dashed-dotted line). Symbols indicate the experimental values for 60% ratio and capping material A (circle) and for 85% ratio and capping material C (triangle).
gaps with the low-k capping material C was k 1.7 for the case of 85% air-to-metal line height ratio. Fig. 9 shows a comparison of the extrapolated k-values for the experimental structures with the ideal case where the capping material would preserve the asdeposited k-value after integration, versus air-to-metal height ratio. Although the experimental values are not optimal, they are significantly lower than what can be achieved with porous low-k dielectrics in both cases. 4. Conclusions UV-assisted decomposition of a sacrificial film deposited by CVD was demonstrated on blanket and integrated structures. The decomposed material is removed through a porous cap, the poros-
[1] R. Daamen, P.H.L. Bancken, V.H. Nguyen, A. Humbert, G.J.A.M. Verheijden, R.J.O.M. Hoofman, Microelectron. Eng. 84 (2007) 2177. [2] R.J.O.M. Hoofman, R. Daamen, V.H. Nguyen, J. Michelon, L.G. Gosset, V. Arnal, J.D. Pontcharra, F. Gaillard, R. Caluwaerts, C. Bruynseraede, G. Beyer, Mater. Res. Soc. Symp. Proc. 914 (2006). 0914-F10-01. [3] J. Noguchi, K. Sato, N. Konishi, S. Uno, T. Oshima, K. Ishikawa, H. Ashihara, T. Saito, M. Kubo, T. Tamaru, Y. Yamada, H. Aoki, T. Fujiwara, IEEE Trans. Electron Dev. 52 (3) (2005) 352. [4] V. Arnal, J. Torres, P. Gayet, R. Gonella, P. Spinelli, M. Guillernet, J.-P. Reynard, C. Verove, in: IEEE International Interconnect Technology Conference, 2001, pp. 298–300. [5] R.J.O.M. Hoofman, R. Caluwaerts, J. Michelon, P. Herrero Bernabe, J.P. Gueneau de Mussy, C. Bruynseraede, J.M. Lee, S. List, P.H.L. Bancken, G. Beyer, Microelectronic Eng. 83 (2006) 2150–2154. [6] F. Gaillard, J. de Pontcharra, L.G. Gosset, Ph. Lyan, D. Bouchu, R. Daamen, O. Louveau, P. Besson, G. Passemard, J. Torres, Microelectronic Eng. 83 (2007) 2309. [7] R. Daamen, G.J.A.M. Verheijden, P.H.L. Bancken, T. Vandeweyer, J. Michelon, V. Nguyen, Hoang, R.O.J.M. Hoofman, M.K. Gallagher, in: IEEE International Interconnect Technology Conference, 2005, pp. 240–242. [8] R. Daamen, P.H.L. Bancken, D.E. Badaroglu, J. Michelon, V.H. Nguyen, G.J.A.M. Verheijden, A. Humbert, J. Waeterloos, A. Yang, J.K. Cheng, L. Chen, T. Martens, R.J.O.M. Hoofman, in: IEEE International Interconnect Technology Conference, 2007, pp. 61–63. [9] K. Chan, K.K. Gleason, J. Electrochem. Soc. 153 (4) (2006) C223.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Air gap formation by UV-assisted decomposition of CVD material M. Pantouvaki a,*, A. Humbert b, E. VanBesien a, E. Camerotto a, Y. Travaly a, O. Richard a, M. Willegems a, H. Volders a, K. Kellens a, R. Daamen b, R.J.O.M. Hoofman b, G. Beyer a a b
IMEC, Kapeldreef 75, B-3001 Leuven, Belgium NXP/TSMC Research Center, Kapeldreef 75, B-3001 Leuven, Belgium
a r t i c l e
i n f o
Article history: Received 7 May 2008 Accepted 7 May 2008 Available online 27 May 2008 Keywords: Air gaps Low-k Sacrificial material Capacitance reduction CVD Decomposition
a b s t r a c t A sacrificial material deposited by CVD is used to demonstrate air gap formation in single damascene structures by UV-assisted decomposition. The material is removed through a porous low-k cap, after completion of the damascene scheme. The porosity of the low-k cap is shown to be critical for efficient air gap formation. Capacitance reduction of 50% is demonstrated using this technique compared to conventional SiOC(H) interconnects and an effective dielectric constant of 1.7 is extrapolated. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Interconnect scaling to smaller sizes increases the interline capacitance. The resulting RC delay and power consumption become critical drawbacks of the interconnects for the performance of integrated circuits. Although the integration of low-k dielectrics is now under development, air, with a k-value of 1, appears as the ultimate dielectric material for capacitance reduction at metal levels. The various approaches that have been proposed to create air gaps are generally classified into two categories [1,2]: air gap creation by material removal and non-conformal chemical vapor deposition (CVD) [3–5] or by selective sacrificial material removal through a cap using thermal decomposition or etching [6–8]. Nonconformal CVD initially created air gaps that were unintentional and undesirable, but they were eventually exploited to reduce interline capacitance. Although this is a technique that can be easily incorporated into fabs, good control of the air gap formation is needed to avoid the extension of the cavity into the via dielectric and corresponding problems of metal intrusion in the air gap in case of via misalignment. To demonstrate air gap formation via thermal decomposition of a sacrificial material through a porous cap, the sacrificial material has been typically a spin-on polymer. In [9], Chan et al. showed that a thermally decomposable sacrificial film can be deposited by CVD. In this paper we use a sacrificial material deposited by CVD and a porous SiOC low-k cap to inte-
* Corresponding author. E-mail address: [email protected] (M. Pantouvaki). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.05.020
grate air gap formation by UV-assisted decomposition into a standard damascene scheme on 300 mm-size wafers. The air gap formation is performed after chemical-mechanical polishing (CMP), increasing the integration scheme by one step. The UV-illumination during decomposition at high temperature is used to increase the decomposition rate.
2. CVD deposition The CVD deposition and decomposition processes were initially studied on blanket wafers. The CVD thermally decomposable film was developed such that deposition of the film was possible at the same temperature as other low-k dielectrics deposited in the same chamber. Evaluation of the sacrificial film decomposition was done by in-line ellipsometric measurements of the film thickness before and after decomposition. However, such measurements become complicated with a multilayer stack under study. Therefore, to test the efficiency of the decomposition of the sacrificial film through the porous low-k cap, a mass loss measurement technique was developed using a microbalance. The weight of the wafers was measured before and after deposition of the sacrificial film, after deposition of the low-k cap, and after decomposition. A correction was made for the mass loss of the porous cap due to UV-illumination, based on separate measurements on wafers with only the porous cap deposited, and the mass loss of the sacrificial film was extrapolated. A comparison of the thickness loss and the mass loss measurements on wafers with only the sacrificial material deposited is shown in Fig. 1. The two methods
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Table 1). In the case of the lowest porosity material A, the mass loss is reaching a plateau of 40% after 10 min treatment. The result is very similar when reducing the thickness of the cap by half. This suggests that the removal of the sacrificial film is blocked by the pore size rather than the pore network. The capping material B with a porosity of 25% results in a maximum mass loss of about 70%. The decomposition efficiency is further improved when using the capping material C of highest porosity, 40%. A mass loss of more than 80% is achieved, similar to the result without a low-k cap, indicating that the pore size and connectivity of this capping material are sufficiently high to let the decomposed film escape. 3. Integration and measurements
Fig. 1. Comparison of thickness loss and mass loss measurements of the sacrificial film as a function of UV-assisted decomposition time at 425 °C, for two experiments.
give comparable results. For characterization of the decomposition of the sacrificial film through a porous cap, a film of 140 nm thickness was deposited on blanket wafers with a 150 nm thick porous low-k cap on top. The porosity and the pore size of the low-k material were varied as shown in Table 1. Decomposition tests through the low-k cap were performed at 425 °C with UV-light on. The sacrificial material would be decomposed and escape through the pores of the capping layer. The pore size and interconnectivity of this low-k cap should be adequately high to let the decomposed material escape during UV-treatment. Fig. 2 shows the mass loss of the sacrificial material due to UV-assisted decomposition as a function of time for capping material of varying porosity (as in
Table 1 Characteristics of different low-k capping materials Capping material
Porosity (%)
Pore radius (nm)
A B C
15 25 39
0.8 0.9 1.2
Fig. 2. Mass loss of the sacrificial film versus decomposition time for capping material of varying porosity.
The integration scheme used for air gap formation in a single damascene flow is shown in Fig. 3. The sacrificial film and a porous capping layer that formed the dielectric stack of the damascene were deposited by CVD. The stack was then patterned using conventional lithography, dry etch, metallization and CMP steps. A non-selective slurry was used for CMP, which allowed to vary the low-k cap thickness without the need for extra etch development. Although the cap thickness was not found to affect the decomposition of the sacrificial material, a thinner cap would increase the percentage of air between the metal lines, reducing further the capacitance. After CMP, the sacrificial CVD material was removed by UV-assisted thermal decomposition and the wafers were then fully passivated and electrically measured. Fig. 4 shows SEM images from samples after air gap formation. Air gaps were formed simultaneously in wide and narrow spacings across the wafer (Figs. 4(a), (b)). Mechanical integrity of the low-k cap bridges was observed for spacings up to 1.3 lm, depending on the material and the thickness of the porous cap used (Fig. 4(c)). When the low-k cap thickness was reduced to 30 nm, the material collapsed for intermetallic widths greater than 550 nm (Fig. 4(d)). Electrical measurements were performed on meander-fork structures of 120 nm / 120 nm linewidth / spacing. Two cases were eventually examined, of air gaps with low-k cap of material A and an air-to-metal height ratio of 60% and of air gaps with low-k cap of material C and air-to-metal-height ratio of 85%. Fig. 5 shows the capacitance of the two air gap versions versus a SiOC(H) reference. A capacitance reduction of 35% was measured on the wafer with low-k material A compared to the SiOC(H) low-k reference. This was further reduced to 50% for the low-k capping material C of higher porosity. No significant leakage current increase was observed in the case of air gaps compared to the reference wafer at
Fig. 3. Air gap formation integration scheme: (1) Dielectric stack deposition, (2) patterning, (3) metallization and CMP, and (4) UV-assisted decomposition of sacrificial film.
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0.1 MV/cm, as shown in Fig. 6. A breakdown voltage of 3.5 MV/ cm was measured on wafers with air gaps and low-k material A (Fig. 7). Further analysis of the integrated structures with air gaps was done by TEM and EFTEM. Fig. 8 shows TEM images from the two cases. Incomplete air gap formation was observed in some areas in the case of lower porosity capping material A. EFTEM analysis indicated a carbon rich ring around the air gap due to insufficient removal of the sacrificial film. Using a higher porosity capping material improved the removal efficiency of the sacrificial film. The integrated k-value of the intermetal stack was extrapolated by RaphaelTM simulations. In this approach, the real geometry and spacing of the metallic lines as observed by TEM is reproduced by simulation and the k-value is optimized to fit the measured capacitance values. The extracted k-value using this technique for air
Fig. 4. SEM images of UV-treated structures for air gap formation: (a) 100 nm/ 100 nm width/spacing, low-k material A, air gap 60%, (b) 500 nm/500 nm width/ spacing, low-k material C, air gap 85%, (c) 1.3 lm low-k bridges of material A, air gap 60%, remain intact, and (d) low-k bridges of material C, air gap 85%, are collapsing at spacings >550 nm.
Fig. 6. Open resistance of air gap wafer with capping material A compared to a reference wafer with PECVD SiOC(H) low-k dielectric measured at 0.1 MV/cm.
Fig. 5. Cumulative probability plot of capacitance of air gap structures versus a reference with PECVD SiOC(H) low-k dielectric. Fig. 7. Breakdown voltage of air gap wafer with capping material A.
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Fig. 8. (a) TEM, (b) EFTEM map of carbon for a sample with low-k material A as a cap, and (c) TEM image of sample with low-k material C as cap.
ity of which is critical for effective air gap formation. Significant capacitance reduction was measured on wafers with air gaps compared to a SiOC(H) reference for both cases of capping material of 15% and 39% porosity, reaching a 50% capacitance reduction in the latter case. The integrated k-value of this interconnect was calculated to be 1.7. This value is significantly lower than that of the low-k dielectrics typically used today, k = 2.5 as-deposited. Further optimization of the integration scheme is required to improve the reliability performance of these interconnects. Acknowledgement This work was partially funded by the European Commission’s Information Society Technologies Program (IST) under PULLNANO project contract No: IST 026828. References Fig. 9. Integrated k-value versus air-to-metal line height ratio. The lines indicate calculations of the ideal case of capping material with an as-deposited k-value of 2.8 (solid line), 2.5 (dashed line) and 2.2 (dashed-dotted line). Symbols indicate the experimental values for 60% ratio and capping material A (circle) and for 85% ratio and capping material C (triangle).
gaps with the low-k capping material C was k 1.7 for the case of 85% air-to-metal line height ratio. Fig. 9 shows a comparison of the extrapolated k-values for the experimental structures with the ideal case where the capping material would preserve the asdeposited k-value after integration, versus air-to-metal height ratio. Although the experimental values are not optimal, they are significantly lower than what can be achieved with porous low-k dielectrics in both cases. 4. Conclusions UV-assisted decomposition of a sacrificial film deposited by CVD was demonstrated on blanket and integrated structures. The decomposed material is removed through a porous cap, the poros-
[1] R. Daamen, P.H.L. Bancken, V.H. Nguyen, A. Humbert, G.J.A.M. Verheijden, R.J.O.M. Hoofman, Microelectron. Eng. 84 (2007) 2177. [2] R.J.O.M. Hoofman, R. Daamen, V.H. Nguyen, J. Michelon, L.G. Gosset, V. Arnal, J.D. Pontcharra, F. Gaillard, R. Caluwaerts, C. Bruynseraede, G. Beyer, Mater. Res. Soc. Symp. Proc. 914 (2006). 0914-F10-01. [3] J. Noguchi, K. Sato, N. Konishi, S. Uno, T. Oshima, K. Ishikawa, H. Ashihara, T. Saito, M. Kubo, T. Tamaru, Y. Yamada, H. Aoki, T. Fujiwara, IEEE Trans. Electron Dev. 52 (3) (2005) 352. [4] V. Arnal, J. Torres, P. Gayet, R. Gonella, P. Spinelli, M. Guillernet, J.-P. Reynard, C. Verove, in: IEEE International Interconnect Technology Conference, 2001, pp. 298–300. [5] R.J.O.M. Hoofman, R. Caluwaerts, J. Michelon, P. Herrero Bernabe, J.P. Gueneau de Mussy, C. Bruynseraede, J.M. Lee, S. List, P.H.L. Bancken, G. Beyer, Microelectronic Eng. 83 (2006) 2150–2154. [6] F. Gaillard, J. de Pontcharra, L.G. Gosset, Ph. Lyan, D. Bouchu, R. Daamen, O. Louveau, P. Besson, G. Passemard, J. Torres, Microelectronic Eng. 83 (2007) 2309. [7] R. Daamen, G.J.A.M. Verheijden, P.H.L. Bancken, T. Vandeweyer, J. Michelon, V. Nguyen, Hoang, R.O.J.M. Hoofman, M.K. Gallagher, in: IEEE International Interconnect Technology Conference, 2005, pp. 240–242. [8] R. Daamen, P.H.L. Bancken, D.E. Badaroglu, J. Michelon, V.H. Nguyen, G.J.A.M. Verheijden, A. Humbert, J. Waeterloos, A. Yang, J.K. Cheng, L. Chen, T. Martens, R.J.O.M. Hoofman, in: IEEE International Interconnect Technology Conference, 2007, pp. 61–63. [9] K. Chan, K.K. Gleason, J. Electrochem. Soc. 153 (4) (2006) C223.