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Electroless deposition of silver thin films on gold nanoparticles catalyst for micro and nanoelectronics applications
Microelectronic Engineering 98 (2012) 570–573
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Electroless deposition of silver thin films on gold nanoparticles catalyst for micro and nanoelectronics applications A. Inberg a, P. Livshits b,⇑, Z. Zalevsky b, Y. Shacham-Diamand a a b
School of Electrical Engineering, Tel Aviv University, Tel Aviv 66978, Israel School of Engineering, Bar Ilan University, Ramat Gan 52900, Israel
a r t i c l e
i n f o
Article history: Available online 14 July 2012 Keywords: Electroless deposition Silver ultra-thin films Gold nanoparticles catalyst
a b s t r a c t In this work, the electroless deposition (ELD) of silver (Ag) thin films onto SiO2/Si surface modified by self-assembled monolayer (SAM) and activated by gold nanoparticles (AuNPs) has been studied. Experimental results clearly indicate the successful deposition of the films. Three distinct stages, such as compact and isolated islands, coalesce equilibrium shapes, and continuous hole-free films, during the growth of the Ag films, were observed by HRSEM imaging. Optical and electrical properties of the films were investigated, and their dependence on the coating structure was discussed. The application of small AuNPs as catalyst for successful metallization is proposed. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The rapid developments of ULSI devices open up huge opportunities, but at the same time it creates new challenges for interconnect metallization. The success of deep-sub-micron or nano-scale interconnects depends on the deposition method and its capability to provide conformal uniform thin-films with desired properties and reliability [1]. In view of continuous downscaling of electronic circuits, the atomic layer deposition (ALD) [2] and different sputtering techniques [3] are widely applied in micro and nanoelectronics. However, alongside with numerous advantages, these methods also have different limitations. In particular, the ALD is relatively slow [4], whereas the sputtering method is incompatible with a lift-off process for the film structuring because of the diffusion transport that leads to contamination problems [5]. Moreover, both of them are expensive and require special equipment. Electroless deposition (ELD) offers high-quality ultra-thin films, which meet requirements for the sub-45 nm ULSI interconnects, contacts and via contacts as well as in high aspect ratio structures for M(N)EMS (micro and nano electronic mechanical systems) [6]. In addition, ELD is superior with regard to other deposition techniques since it is low cost, relatively simple, and highly selective method, which potentially may eliminate some lithography and chemical mechanical polishing steps. ELD can be produced on conductive or dielectric substrates, but in the second case it requires the surface activation. The latter has a strong influence on properties of the final metal films, and therefore it is under intensive research. In our previous studies the ultra-thin silver (Ag) films were ⇑ Corresponding author. Tel.: +972 52 8674734; fax: +972 3 7384050. E-mail address: [email protected] (P. Livshits). 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.06.020
successfully deposited on dielectrics using surface activation in an aqueous palladium (Pd) colloid [7] or Pd ionic solutions. In this work, we replaced the traditionally used in ELD expensive Pd activation [8] as a surface catalysis by monodisperse gold nanoparticles (AuNPs), which also offer less threat for contamination due to the deposition at low temperatures. The obtained electroless deposited Ag thin films as well as the influence of AuNPs on the structure and the properties of the films are presented and discussed in next sections. 2. Experimental details The silver thin films were deposited on 100 nm thick thermal SiO2 layer onto p-type Si h1 0 0i substrate of 1.5 cm 1.5 cm size. Prior to deposition, the samples were cleaned, first in hot (80 °C) NH4OH:H2O2:deionize water (DIW) (1:1:6) to remove metallic contamination and then, in hot (80 °C) HCl:H2O2:DIW (1:1:5) for organic residues removal. To improve metal adhesion silane based amino-terminated self-assembled monolayer (SAM) of 1–2 nm thickness was applied on the hydroxylate SiO2/Si samples by treatment in 1% aminopropyl-trimethoxysilan (APTMS) in ethanol solution at 65 °C for 3.5 h. After silanization, the samples were backed in an air oven at 100 °C for 30 min to achieve cross-linkage of the chains and densification of the SAM. Activation of the surface was performed by immersion of the modified samples in an aqueous colloid solution of 15 nm and 25 nm size AuNPs. ELD of Ag onto the activated surface was performed at room temperature from amino-acetate-complex based solution with hydrazine hydrate as reducing agent. A few additives were introduced in minute quantities as a surfactant and to level and brighten the deposit [7]. The thickness of the Ag films was measured
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A. Inberg et al. / Microelectronic Engineering 98 (2012) 570–573
using an Alpha-step 500. The resistance of the deposits was determined by In-line Four Point Probe Resistivity Tester, Model 301-6, Signatone, USA. To analyze and characterize the AuNPs precipitation on the SiO2/Si substrate as well as to observe the thin films’ morphology and microstructure, High-Resolution Scanning Electron Microscope (HRSEM), Helios 600 apparatus from FEI that enables a 0.9 nm resolution was applied. The topography and roughness (RMS) of the deposits were measured by Atomic Force Microscope (AFM), MultiMode Scanning Probe Microscope, DI 3100 model, Veeco. Commercially available NT-MDT gold coated silicon probe (NSG01/Au) was used for all measurements. The optical properties of the films were studied using UV/Vis/NIR spectrophotometer.
3. Results and discussion
Fig. 1. The comparison between the coverage (in percents) of the modified SiO2/Si surface by catalyst after immersion into the colloid solution of 25 nm and 15 nm AuNPs for different times.
Fig. 2. The HRSEM image showing a 1 lm 1 lm area of the SAM/SiO2/Si substrate after immersion into the colloid solution of 15 nm AuNPs for 90 min. Small white dots are NPs.
Ag film thickness [nm]
In case of ELD, activation of the insulating surface by the catalyst is extremely important for following metallization. Note, that the size of AuNPs and coverage of the SAM surface by AuNPs affects the morphology of the ELD film, and eventually the properties of the film. Thus, in the first step, we have studied the precipitation of 15 nm and 25 nm AuNPs (e.g. kinetics, distribution on surfaces, and surface coverage) [9]. It was found (Fig. 1) that 25 nm AuNPs have demonstrated lower surface coverage in comparison with the smallest ones. Therefore, 25 nm AuNPs left outside the scope of this work, and the modified SiO2/Si substrates were activated before Ag ELD by 15 nm AuNPs during 1.5 h. The obtained AuNPs distribution on the surface is presented in Fig. 2. The kinetics of the Ag film deposition (Fig. 3) was studied together with the coatings’ microstructure (Fig. 4) and the roughness (RMS) measurements. The Ag film deposition rate, as it can be obtained from Fig. 3, is 18 nm/s and close to the one obtained for the same process conditions using Pd activation. In Fig. 4 three distinct stages, such as compact and isolated islands, coalesce equilibrium shapes, and continuous hole-free deposits during Ag films growth are clearly observed. It is in accordance with the general acceptance that when metallic thin films grow on insulating surfaces they pass through a sequence of morphological changes [10]. At the early stages, where the film is extremely thin, it consists of isolated compact islands. As the deposition proceeds, the latter are growing and becoming larger, but still to be compact islands. At some certain film’s thickness, these islands coalesce into near equilibrium compact shapes forming percolating structures. Finally, the channels between the structures are filled in and a continuous, free of holes, deposit is created.
200 160 120 80 40 0
0
20
40
60
80
100
Ag deposition time [s] Fig. 3. The Ag film thickness as a function of the deposition time.
The roughness of Ag deposits was varied from 18.2 nm to 29.2 nm for island-like and full continues film, respectively. These values are higher than those obtained for the Ag film of the same thickness using Pd activation. Taking into account that for both ELD processes the film deposition rate was practically the same, such a difference in the film roughness could be attributed only to the catalytic layer quality resulted from the applied activator. Since in this study we used as catalyst relatively large AuNPs, the application of the smaller ones could be the way to resolve this drawback. The thin film roughness has a significant impact on its properties most likely due to surface electron scattering. It could be important for different microelectronic applications where surface roughness can allow or restrict subsequent surface treatment or lithography steps. The electrical conductivity is one of the key parameters for nanoscale metallization. The FPP resistance measurements of ELD Ag thin films as a function of their thickness are presented in Fig. 5. The resistivity decreases many orders of magnitude from almost entirely dielectric for very thin coating to conductive when the film achieves the thickness of 50 nm. At some thickness the resistivity reaches saturation (6 lX cm) and further thickness increase has no more influence. This value is higher than that obtained for Pd-activated process, and it can be explained by the porosity of the Ag film and its high roughness discussed above. The dark spots on the HRSEM images of 100–150 nm Ag deposits (Fig. 4(E) and (F)) and surface coverage of about 94% and 96%, respectively, substantiate this supposition.
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Ag film resistivity [μΩcm]
Fig. 4. HRSEM images of ELD Ag films. With increasing thickness three distinct stages can be observed: compact and isolated islands, coalesce equilibrium shapes, and continuous hole-free film. Deposition time (s): A-10, B-15, C-20, D-45, E-60, and F-80.
the reflectance is very high, then it decreases in the visible region, and drops considerably into the ultra-violet one [11]. Therefore, the reflectivity measurements of Ag films of different thicknesses were performed in the range of 200–500 nm, and are selectively illustrated in Fig. 6 for several deposition times. In the figure only the range (300–400 nm) is shown, where a significant variation is expected to occur. The high optical absorption peak at approximately 320 nm, typical for pure Ag [12], was clearly observed only for relatively thick continuous films (100–150 nm). This phenomenon can be related to high porosity and discontinuity of very thin Ag films observed in this study and discussed above.
100
10
100
Ag film thickness [nm] Fig. 5. The Ag film electrical resistivity as a function of its thickness.
4. Summary and conclusion In this work ELD of Ag ultra-thin films onto SiO2/Si surface modified by SAM and activated by 15 nm AuNPs has been studied. Experimental results clearly indicate the successful deposition of the films. At the moment, in spite of the same process kinetics, the optical and electrical properties of the films are slightly worse than those of Ag films electroless deposited on top of Pd activated substrates. We attribute this effect to high roughness and porosity of obtained very thin films that resulted from the activation process. In this work we used relatively large AuNPs producing rough catalytic layer due to its own size and surface coverage limit. Therefore, based on our experience, replacement of large AuNPs with smaller ones (3–5 nm) is the key to improve Ag ultra-thin films properties, and is the subject of our future research. References
Fig. 6. Reflectance of Ag films of different thicknesses as a function of the wavelength.
The excellent optical properties of silver allow its wide application in optics and electro-optics. In the near infrared wavelengths
[1] International Technology Roadmap for Semiconductors, 2009 Edition. [2] M. Ritala, J. Niinisto, ECS Transactions 25 (2009) 641. [3] Y.K. Gautam, A.K. Chawla, R. Walia, R.D. Agrawal, R. Chandra, Appl. Surf. Sci. 257 (2011) 6291. [4] A.J.M. Mackus, S.B.S. Heil, E. Langereis, H.C.M. Knoops, M.C.M. van de Sanden, W.M.M. Kessels, J. Vac. Sci. Technol. A 28 (2010) 77. [5] O. Lühn, C. Van Hoof, W. Ruythooren, J.P. Celis, Microelectron. Eng. 85 (2008) 1947. [6] J.M. Warrender, M.J. Aziz, Appl. Phys. A: Mater. 79 (2004) 2573.
A. Inberg et al. / Microelectronic Engineering 98 (2012) 570–573 [7] V. Sabayev, N. Croitoru, A. Inberg, Y. Shacham-Diamand, Mater. Chem. Phys. 127 (2011) 214. [8] W. Grochala, P.P. Edwards, Chem. Rev. 104 (2004) 1283. [9] P. Livshits, A. Inberg, Y. Shacham-Diamand, D. Malka, Y. Fleger, Z. Zalevsky, submitted to Microelectronic Engineering. [10] E. Bauer, J.H. van der Merwe, Phys. Rev. B 33 (1986) 3657.
573
[11] N. Thomas, A. Erlandson, J. Farmer, H. Gregg, C. Marshall, W. Siekhaus, J. Wolfe, D. Fix, D. Ahre, H.E. Bennett, A.H. Guenther, M.R. Kozlowski, B.E. Newnam, M.J. Soileau, Laser-Induced Damage in Optical Materials-Proceeding of SPIE 3578 (1999) 730. [12] A. Rizzoa, M.A. Taglientea, M. Alvisia, S. Scaglioneb, Thin Solid Films 396 (2001) 29.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Electroless deposition of silver thin films on gold nanoparticles catalyst for micro and nanoelectronics applications A. Inberg a, P. Livshits b,⇑, Z. Zalevsky b, Y. Shacham-Diamand a a b
School of Electrical Engineering, Tel Aviv University, Tel Aviv 66978, Israel School of Engineering, Bar Ilan University, Ramat Gan 52900, Israel
a r t i c l e
i n f o
Article history: Available online 14 July 2012 Keywords: Electroless deposition Silver ultra-thin films Gold nanoparticles catalyst
a b s t r a c t In this work, the electroless deposition (ELD) of silver (Ag) thin films onto SiO2/Si surface modified by self-assembled monolayer (SAM) and activated by gold nanoparticles (AuNPs) has been studied. Experimental results clearly indicate the successful deposition of the films. Three distinct stages, such as compact and isolated islands, coalesce equilibrium shapes, and continuous hole-free films, during the growth of the Ag films, were observed by HRSEM imaging. Optical and electrical properties of the films were investigated, and their dependence on the coating structure was discussed. The application of small AuNPs as catalyst for successful metallization is proposed. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The rapid developments of ULSI devices open up huge opportunities, but at the same time it creates new challenges for interconnect metallization. The success of deep-sub-micron or nano-scale interconnects depends on the deposition method and its capability to provide conformal uniform thin-films with desired properties and reliability [1]. In view of continuous downscaling of electronic circuits, the atomic layer deposition (ALD) [2] and different sputtering techniques [3] are widely applied in micro and nanoelectronics. However, alongside with numerous advantages, these methods also have different limitations. In particular, the ALD is relatively slow [4], whereas the sputtering method is incompatible with a lift-off process for the film structuring because of the diffusion transport that leads to contamination problems [5]. Moreover, both of them are expensive and require special equipment. Electroless deposition (ELD) offers high-quality ultra-thin films, which meet requirements for the sub-45 nm ULSI interconnects, contacts and via contacts as well as in high aspect ratio structures for M(N)EMS (micro and nano electronic mechanical systems) [6]. In addition, ELD is superior with regard to other deposition techniques since it is low cost, relatively simple, and highly selective method, which potentially may eliminate some lithography and chemical mechanical polishing steps. ELD can be produced on conductive or dielectric substrates, but in the second case it requires the surface activation. The latter has a strong influence on properties of the final metal films, and therefore it is under intensive research. In our previous studies the ultra-thin silver (Ag) films were ⇑ Corresponding author. Tel.: +972 52 8674734; fax: +972 3 7384050. E-mail address: [email protected] (P. Livshits). 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.06.020
successfully deposited on dielectrics using surface activation in an aqueous palladium (Pd) colloid [7] or Pd ionic solutions. In this work, we replaced the traditionally used in ELD expensive Pd activation [8] as a surface catalysis by monodisperse gold nanoparticles (AuNPs), which also offer less threat for contamination due to the deposition at low temperatures. The obtained electroless deposited Ag thin films as well as the influence of AuNPs on the structure and the properties of the films are presented and discussed in next sections. 2. Experimental details The silver thin films were deposited on 100 nm thick thermal SiO2 layer onto p-type Si h1 0 0i substrate of 1.5 cm 1.5 cm size. Prior to deposition, the samples were cleaned, first in hot (80 °C) NH4OH:H2O2:deionize water (DIW) (1:1:6) to remove metallic contamination and then, in hot (80 °C) HCl:H2O2:DIW (1:1:5) for organic residues removal. To improve metal adhesion silane based amino-terminated self-assembled monolayer (SAM) of 1–2 nm thickness was applied on the hydroxylate SiO2/Si samples by treatment in 1% aminopropyl-trimethoxysilan (APTMS) in ethanol solution at 65 °C for 3.5 h. After silanization, the samples were backed in an air oven at 100 °C for 30 min to achieve cross-linkage of the chains and densification of the SAM. Activation of the surface was performed by immersion of the modified samples in an aqueous colloid solution of 15 nm and 25 nm size AuNPs. ELD of Ag onto the activated surface was performed at room temperature from amino-acetate-complex based solution with hydrazine hydrate as reducing agent. A few additives were introduced in minute quantities as a surfactant and to level and brighten the deposit [7]. The thickness of the Ag films was measured
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A. Inberg et al. / Microelectronic Engineering 98 (2012) 570–573
using an Alpha-step 500. The resistance of the deposits was determined by In-line Four Point Probe Resistivity Tester, Model 301-6, Signatone, USA. To analyze and characterize the AuNPs precipitation on the SiO2/Si substrate as well as to observe the thin films’ morphology and microstructure, High-Resolution Scanning Electron Microscope (HRSEM), Helios 600 apparatus from FEI that enables a 0.9 nm resolution was applied. The topography and roughness (RMS) of the deposits were measured by Atomic Force Microscope (AFM), MultiMode Scanning Probe Microscope, DI 3100 model, Veeco. Commercially available NT-MDT gold coated silicon probe (NSG01/Au) was used for all measurements. The optical properties of the films were studied using UV/Vis/NIR spectrophotometer.
3. Results and discussion
Fig. 1. The comparison between the coverage (in percents) of the modified SiO2/Si surface by catalyst after immersion into the colloid solution of 25 nm and 15 nm AuNPs for different times.
Fig. 2. The HRSEM image showing a 1 lm 1 lm area of the SAM/SiO2/Si substrate after immersion into the colloid solution of 15 nm AuNPs for 90 min. Small white dots are NPs.
Ag film thickness [nm]
In case of ELD, activation of the insulating surface by the catalyst is extremely important for following metallization. Note, that the size of AuNPs and coverage of the SAM surface by AuNPs affects the morphology of the ELD film, and eventually the properties of the film. Thus, in the first step, we have studied the precipitation of 15 nm and 25 nm AuNPs (e.g. kinetics, distribution on surfaces, and surface coverage) [9]. It was found (Fig. 1) that 25 nm AuNPs have demonstrated lower surface coverage in comparison with the smallest ones. Therefore, 25 nm AuNPs left outside the scope of this work, and the modified SiO2/Si substrates were activated before Ag ELD by 15 nm AuNPs during 1.5 h. The obtained AuNPs distribution on the surface is presented in Fig. 2. The kinetics of the Ag film deposition (Fig. 3) was studied together with the coatings’ microstructure (Fig. 4) and the roughness (RMS) measurements. The Ag film deposition rate, as it can be obtained from Fig. 3, is 18 nm/s and close to the one obtained for the same process conditions using Pd activation. In Fig. 4 three distinct stages, such as compact and isolated islands, coalesce equilibrium shapes, and continuous hole-free deposits during Ag films growth are clearly observed. It is in accordance with the general acceptance that when metallic thin films grow on insulating surfaces they pass through a sequence of morphological changes [10]. At the early stages, where the film is extremely thin, it consists of isolated compact islands. As the deposition proceeds, the latter are growing and becoming larger, but still to be compact islands. At some certain film’s thickness, these islands coalesce into near equilibrium compact shapes forming percolating structures. Finally, the channels between the structures are filled in and a continuous, free of holes, deposit is created.
200 160 120 80 40 0
0
20
40
60
80
100
Ag deposition time [s] Fig. 3. The Ag film thickness as a function of the deposition time.
The roughness of Ag deposits was varied from 18.2 nm to 29.2 nm for island-like and full continues film, respectively. These values are higher than those obtained for the Ag film of the same thickness using Pd activation. Taking into account that for both ELD processes the film deposition rate was practically the same, such a difference in the film roughness could be attributed only to the catalytic layer quality resulted from the applied activator. Since in this study we used as catalyst relatively large AuNPs, the application of the smaller ones could be the way to resolve this drawback. The thin film roughness has a significant impact on its properties most likely due to surface electron scattering. It could be important for different microelectronic applications where surface roughness can allow or restrict subsequent surface treatment or lithography steps. The electrical conductivity is one of the key parameters for nanoscale metallization. The FPP resistance measurements of ELD Ag thin films as a function of their thickness are presented in Fig. 5. The resistivity decreases many orders of magnitude from almost entirely dielectric for very thin coating to conductive when the film achieves the thickness of 50 nm. At some thickness the resistivity reaches saturation (6 lX cm) and further thickness increase has no more influence. This value is higher than that obtained for Pd-activated process, and it can be explained by the porosity of the Ag film and its high roughness discussed above. The dark spots on the HRSEM images of 100–150 nm Ag deposits (Fig. 4(E) and (F)) and surface coverage of about 94% and 96%, respectively, substantiate this supposition.
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A. Inberg et al. / Microelectronic Engineering 98 (2012) 570–573
Ag film resistivity [μΩcm]
Fig. 4. HRSEM images of ELD Ag films. With increasing thickness three distinct stages can be observed: compact and isolated islands, coalesce equilibrium shapes, and continuous hole-free film. Deposition time (s): A-10, B-15, C-20, D-45, E-60, and F-80.
the reflectance is very high, then it decreases in the visible region, and drops considerably into the ultra-violet one [11]. Therefore, the reflectivity measurements of Ag films of different thicknesses were performed in the range of 200–500 nm, and are selectively illustrated in Fig. 6 for several deposition times. In the figure only the range (300–400 nm) is shown, where a significant variation is expected to occur. The high optical absorption peak at approximately 320 nm, typical for pure Ag [12], was clearly observed only for relatively thick continuous films (100–150 nm). This phenomenon can be related to high porosity and discontinuity of very thin Ag films observed in this study and discussed above.
100
10
100
Ag film thickness [nm] Fig. 5. The Ag film electrical resistivity as a function of its thickness.
4. Summary and conclusion In this work ELD of Ag ultra-thin films onto SiO2/Si surface modified by SAM and activated by 15 nm AuNPs has been studied. Experimental results clearly indicate the successful deposition of the films. At the moment, in spite of the same process kinetics, the optical and electrical properties of the films are slightly worse than those of Ag films electroless deposited on top of Pd activated substrates. We attribute this effect to high roughness and porosity of obtained very thin films that resulted from the activation process. In this work we used relatively large AuNPs producing rough catalytic layer due to its own size and surface coverage limit. Therefore, based on our experience, replacement of large AuNPs with smaller ones (3–5 nm) is the key to improve Ag ultra-thin films properties, and is the subject of our future research. References
Fig. 6. Reflectance of Ag films of different thicknesses as a function of the wavelength.
The excellent optical properties of silver allow its wide application in optics and electro-optics. In the near infrared wavelengths
[1] International Technology Roadmap for Semiconductors, 2009 Edition. [2] M. Ritala, J. Niinisto, ECS Transactions 25 (2009) 641. [3] Y.K. Gautam, A.K. Chawla, R. Walia, R.D. Agrawal, R. Chandra, Appl. Surf. Sci. 257 (2011) 6291. [4] A.J.M. Mackus, S.B.S. Heil, E. Langereis, H.C.M. Knoops, M.C.M. van de Sanden, W.M.M. Kessels, J. Vac. Sci. Technol. A 28 (2010) 77. [5] O. Lühn, C. Van Hoof, W. Ruythooren, J.P. Celis, Microelectron. Eng. 85 (2008) 1947. [6] J.M. Warrender, M.J. Aziz, Appl. Phys. A: Mater. 79 (2004) 2573.
A. Inberg et al. / Microelectronic Engineering 98 (2012) 570–573 [7] V. Sabayev, N. Croitoru, A. Inberg, Y. Shacham-Diamand, Mater. Chem. Phys. 127 (2011) 214. [8] W. Grochala, P.P. Edwards, Chem. Rev. 104 (2004) 1283. [9] P. Livshits, A. Inberg, Y. Shacham-Diamand, D. Malka, Y. Fleger, Z. Zalevsky, submitted to Microelectronic Engineering. [10] E. Bauer, J.H. van der Merwe, Phys. Rev. B 33 (1986) 3657.
573
[11] N. Thomas, A. Erlandson, J. Farmer, H. Gregg, C. Marshall, W. Siekhaus, J. Wolfe, D. Fix, D. Ahre, H.E. Bennett, A.H. Guenther, M.R. Kozlowski, B.E. Newnam, M.J. Soileau, Laser-Induced Damage in Optical Materials-Proceeding of SPIE 3578 (1999) 730. [12] A. Rizzoa, M.A. Taglientea, M. Alvisia, S. Scaglioneb, Thin Solid Films 396 (2001) 29.