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SiO2 nanocomposite films for optical coatings prepared by vacuum magnetron sputtering
Vacuum 86 (2012) 742e744
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Mo/SiO2 nanocomposite films for optical coatings prepared by vacuum magnetron sputtering I. Vávra a, *, Z. Kri zanová a, J. Dérer a, J. Humlí cek b a b
Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravská cesta 9, 841 04 Bratslava, Slovakia Department of Condensed Matter Physics, Faculty of Science MU, Kotlár ská 2, 611 37 Brno, Czech Republic
a b s t r a c t Keywords: Nanotechnology Nanocomposites Multilayers Coatings
The embedding of metallic nanoparticles in the traditional optical materials (e.g. SiO2) gives us the possibility to create new optical materials. Metallic particles of nanometric dimensions can be transparent in wide spectral ranges of light. The incorporation of nanocrystal inclusions in such nanocomposites provides the benefit of targeted manipulations of their macroscopic optical response. In this paper we present the possibility to create, using vacuum deposition methods, the nanocomposite coatings with fairly small refractivity. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
The vacuum preparation methods of metallic nanoparticles are generally well known. By these methods there is possible to prepare the pure metallic nanoparticles of good crystalline perfection. For this goal in thin film nanotechnology the early stage of thin film growth (nucleation) is used. Joining the technology of discontinuous metallic films together with the technology of periodic multilayers we can create thin film nanocomposite of nanometal/insulator type. The structural parameters of resulting composite can be adjusted by the multilayer periodicity and by the size and shape of metal nanoparticles. Microstructure of PVD deposited metallic layers is described by the well known Thornton diagram of film growth. A detailed study of Thornton’s model [1] for the deposition at low substrate temperature was preformed in 2003 by Petrov et al. [2]. In this work there was shown the possibility to deposit discontinuous films with various aspect ratios of individual metallic particles. In other words, there is possibility to prepare discontinuous metallic film consisting of separated metallic needle-shaped crystallites. We focus here on the structural and optical properties of the metal (Mo)/insulator (SiO2) composites. We aim at combining the specific features of the components in order to obtain interesting optical behavior of the composites. Namely, we obtain mixtures with a fairly small polarizability, having, at the same time, the penetration depth of light small enough to prevent reflections at the film/substrate interfaces.
In our study we used the magnetron deposition of Mo/SiO2 periodic multilayers with discontinuous Mo layers with various nanostructure parameters. The dc magnetron sputtering was used for Mo deposition; rf magnetron was used for SiO2 deposition. The discontinuity of Mo layers was achieved by the deposition at relatively high Ar pressure (more than 5 Pa). On the other hand the SiO2 layers were deposited at 0.1 Pa. The SiO2 layers prepared at this pressure are always continuous and conformal. By this procedure we obtain the nanocomposite of metallic particles of various shape, embedded in the amorphous SiO2 matrix. The “nanostructuring” of prepared composites was studied using in-plane and cross sectional transmission electron microscopy (TEM). The structural investigations were performed by conventional TEM (JEOL1200EX) microscope with the accelerating voltage of 120 kV. The periodic multilayers of Mo/SiO2 of various periods and also several single layers of Mo were prepared on oxidized Si wafers (the oxide thickness was 100 nm) with a surface roughness of 0.6 nm, at the substrate temperature of 50 C, by magnetron sputtering in a cryopumped apparatus with the base pressure of 10 5 Pa. For the Mo deposition we used dc magnetron and for SiO2 rf magnetron. The pressure during deposition was 0.1 Pa for Mo and 5 10 2 Pa for SiO2, respectively. The substrate e target distance was 70 mm. The sputtering rates were kept constant and the deposition rate was adjusted by the velocity of the substrate movement over the Mo and SiO2 targets. Standard optical reflectivity measurements were performed in the range of 0.5e5.5 eV on all prepared coatings, and also on polished Si single crystalline substrate and on the polished bulk Mo.
* Corresponding author. E-mail address: [email protected] (I. Vávra). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.07.034
I. Vávra et al. / Vacuum 86 (2012) 742e744
743
Fig. 1. Cross sectional TEM micrograph of Mo/SiO2 multilayer. a) bright field image. b) dark field image. Each Mo layer was deposited at different Ar pressure. SiO2 layers were deposited always at the pressure 0.1 Pa. The microstructure of Mo layers strongly depends on the Ar pressure e top Mo layers are polycrystalline whereas bottom Mo layers are non continuous and consist of Mo nanocrystals ordered in the columns.
3. Results First of all we observed a strong dependence of the Mo crystalline structure on the Ar pressure during the deposition. The
situation is depicted in Fig. 1. The deposition at higher Ar pressure involves the discontinuity of the Mo growth. The deposition at the Ar pressures lower than 2 Pa produces standard polycrystalline (microcrystalline) Mo layers.
Fig. 2. Cross sectional TEM micrograph of a periodic [Mo (85 nm)/SiO2 (2.5 nm)] 5 multilayer deposited at 10 Pa (Mo) and 0.1 Pa (SiO2). Columnar non continuous growth of Mo is clearly seen in the bright field image (a). The nanostructuring of Mo columns is revealed in the dark field image (b). The in-plane discontinuity and nanostructuring is shown in bright field micrograph (c) and in dark field micrograph (d).
744
I. Vávra et al. / Vacuum 86 (2012) 742e744 0,8 0,7
prevent the growth of larger islands in Mo layers. The structure of SiO2 was always amorphous. The observed islands are not single domain, but they are composed of Mo nanocrystallites, approx. 10 nm in size. The in-plane island size is approx. 50 nm (Fig. 2c,d). For the reflectance measurements we have prepared also microcrystalline Mo sample. The optical measurements were performed also on the Mo single layer. Fig. 3 provides an overview of the reflectivity measurements. The nanocrystalline discontinuous composites exhibit fairly low UV reflectivities, indicative of the real part of the refractive index close to unity, and a small imaginary (absorptive) part. However, the latter is large enough to prevent the reflections at the film/substrate interface.
Mo - µc
0,6
Reflectivity
0,5
Mo-bulk
0,4
1xMo-nc 85nm
0,3
Si bulk
0,2
5xMo-nc
0,1 0,0
4. Conclusions
1xMo-nc 330nm 1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
E (eV) Fig. 3. Reflectivity spectra of Mo/SiO2 optical coatings of various structuring. For comparison, the reflectivity of polished bulk molybdenum is shown. Its reflectivity is practically identical to that of the microcrystalline Mo. The reflectivity depends on the nanostructuring and is lowered significantly in the ultraviolet region. The reflectivity of the substrate Si is also shown for comparison.
We have developed the technology of optical coatings with low UV reflectivity. It was shown that this property depends on nanostructuring of the coatings. The nanostructuring of the coatings was achieved by the magnetron deposition at relatively high Ar pressure. It is possible to prepare also thicker coating by the multilayer technology. The described deposition method has a large application potential in the field of multicomponent coatings. Acknowledgment
Discontinuous Mo films consist of nanocrystals ordered in the “columns” which are perpendicular to the film plane. The nanocomposite structure was studied by transmission electron microscope (TEM) in cross sectional and plan view TEM specimens in the bright and dark field modes (Fig. 2). By this means we can completely structurally describe the prepared nanocomposite. The SiO2 layers in the multilayers serve as the thin separators, which
The research was supported by VEGA Agency project No.2/6100/ 27 and Center of Excellence “Nanosmart”. References [1] Thornton JA. Annu Rev Mat Sci 1977;7:239. [2] Petrov I, Barna PB, Hultman L, Greene JE. J Vac Sci Technol 2003;A 21:S117.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Mo/SiO2 nanocomposite films for optical coatings prepared by vacuum magnetron sputtering I. Vávra a, *, Z. Kri zanová a, J. Dérer a, J. Humlí cek b a b
Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravská cesta 9, 841 04 Bratslava, Slovakia Department of Condensed Matter Physics, Faculty of Science MU, Kotlár ská 2, 611 37 Brno, Czech Republic
a b s t r a c t Keywords: Nanotechnology Nanocomposites Multilayers Coatings
The embedding of metallic nanoparticles in the traditional optical materials (e.g. SiO2) gives us the possibility to create new optical materials. Metallic particles of nanometric dimensions can be transparent in wide spectral ranges of light. The incorporation of nanocrystal inclusions in such nanocomposites provides the benefit of targeted manipulations of their macroscopic optical response. In this paper we present the possibility to create, using vacuum deposition methods, the nanocomposite coatings with fairly small refractivity. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
The vacuum preparation methods of metallic nanoparticles are generally well known. By these methods there is possible to prepare the pure metallic nanoparticles of good crystalline perfection. For this goal in thin film nanotechnology the early stage of thin film growth (nucleation) is used. Joining the technology of discontinuous metallic films together with the technology of periodic multilayers we can create thin film nanocomposite of nanometal/insulator type. The structural parameters of resulting composite can be adjusted by the multilayer periodicity and by the size and shape of metal nanoparticles. Microstructure of PVD deposited metallic layers is described by the well known Thornton diagram of film growth. A detailed study of Thornton’s model [1] for the deposition at low substrate temperature was preformed in 2003 by Petrov et al. [2]. In this work there was shown the possibility to deposit discontinuous films with various aspect ratios of individual metallic particles. In other words, there is possibility to prepare discontinuous metallic film consisting of separated metallic needle-shaped crystallites. We focus here on the structural and optical properties of the metal (Mo)/insulator (SiO2) composites. We aim at combining the specific features of the components in order to obtain interesting optical behavior of the composites. Namely, we obtain mixtures with a fairly small polarizability, having, at the same time, the penetration depth of light small enough to prevent reflections at the film/substrate interfaces.
In our study we used the magnetron deposition of Mo/SiO2 periodic multilayers with discontinuous Mo layers with various nanostructure parameters. The dc magnetron sputtering was used for Mo deposition; rf magnetron was used for SiO2 deposition. The discontinuity of Mo layers was achieved by the deposition at relatively high Ar pressure (more than 5 Pa). On the other hand the SiO2 layers were deposited at 0.1 Pa. The SiO2 layers prepared at this pressure are always continuous and conformal. By this procedure we obtain the nanocomposite of metallic particles of various shape, embedded in the amorphous SiO2 matrix. The “nanostructuring” of prepared composites was studied using in-plane and cross sectional transmission electron microscopy (TEM). The structural investigations were performed by conventional TEM (JEOL1200EX) microscope with the accelerating voltage of 120 kV. The periodic multilayers of Mo/SiO2 of various periods and also several single layers of Mo were prepared on oxidized Si wafers (the oxide thickness was 100 nm) with a surface roughness of 0.6 nm, at the substrate temperature of 50 C, by magnetron sputtering in a cryopumped apparatus with the base pressure of 10 5 Pa. For the Mo deposition we used dc magnetron and for SiO2 rf magnetron. The pressure during deposition was 0.1 Pa for Mo and 5 10 2 Pa for SiO2, respectively. The substrate e target distance was 70 mm. The sputtering rates were kept constant and the deposition rate was adjusted by the velocity of the substrate movement over the Mo and SiO2 targets. Standard optical reflectivity measurements were performed in the range of 0.5e5.5 eV on all prepared coatings, and also on polished Si single crystalline substrate and on the polished bulk Mo.
* Corresponding author. E-mail address: [email protected] (I. Vávra). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.07.034
I. Vávra et al. / Vacuum 86 (2012) 742e744
743
Fig. 1. Cross sectional TEM micrograph of Mo/SiO2 multilayer. a) bright field image. b) dark field image. Each Mo layer was deposited at different Ar pressure. SiO2 layers were deposited always at the pressure 0.1 Pa. The microstructure of Mo layers strongly depends on the Ar pressure e top Mo layers are polycrystalline whereas bottom Mo layers are non continuous and consist of Mo nanocrystals ordered in the columns.
3. Results First of all we observed a strong dependence of the Mo crystalline structure on the Ar pressure during the deposition. The
situation is depicted in Fig. 1. The deposition at higher Ar pressure involves the discontinuity of the Mo growth. The deposition at the Ar pressures lower than 2 Pa produces standard polycrystalline (microcrystalline) Mo layers.
Fig. 2. Cross sectional TEM micrograph of a periodic [Mo (85 nm)/SiO2 (2.5 nm)] 5 multilayer deposited at 10 Pa (Mo) and 0.1 Pa (SiO2). Columnar non continuous growth of Mo is clearly seen in the bright field image (a). The nanostructuring of Mo columns is revealed in the dark field image (b). The in-plane discontinuity and nanostructuring is shown in bright field micrograph (c) and in dark field micrograph (d).
744
I. Vávra et al. / Vacuum 86 (2012) 742e744 0,8 0,7
prevent the growth of larger islands in Mo layers. The structure of SiO2 was always amorphous. The observed islands are not single domain, but they are composed of Mo nanocrystallites, approx. 10 nm in size. The in-plane island size is approx. 50 nm (Fig. 2c,d). For the reflectance measurements we have prepared also microcrystalline Mo sample. The optical measurements were performed also on the Mo single layer. Fig. 3 provides an overview of the reflectivity measurements. The nanocrystalline discontinuous composites exhibit fairly low UV reflectivities, indicative of the real part of the refractive index close to unity, and a small imaginary (absorptive) part. However, the latter is large enough to prevent the reflections at the film/substrate interface.
Mo - µc
0,6
Reflectivity
0,5
Mo-bulk
0,4
1xMo-nc 85nm
0,3
Si bulk
0,2
5xMo-nc
0,1 0,0
4. Conclusions
1xMo-nc 330nm 1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
E (eV) Fig. 3. Reflectivity spectra of Mo/SiO2 optical coatings of various structuring. For comparison, the reflectivity of polished bulk molybdenum is shown. Its reflectivity is practically identical to that of the microcrystalline Mo. The reflectivity depends on the nanostructuring and is lowered significantly in the ultraviolet region. The reflectivity of the substrate Si is also shown for comparison.
We have developed the technology of optical coatings with low UV reflectivity. It was shown that this property depends on nanostructuring of the coatings. The nanostructuring of the coatings was achieved by the magnetron deposition at relatively high Ar pressure. It is possible to prepare also thicker coating by the multilayer technology. The described deposition method has a large application potential in the field of multicomponent coatings. Acknowledgment
Discontinuous Mo films consist of nanocrystals ordered in the “columns” which are perpendicular to the film plane. The nanocomposite structure was studied by transmission electron microscope (TEM) in cross sectional and plan view TEM specimens in the bright and dark field modes (Fig. 2). By this means we can completely structurally describe the prepared nanocomposite. The SiO2 layers in the multilayers serve as the thin separators, which
The research was supported by VEGA Agency project No.2/6100/ 27 and Center of Excellence “Nanosmart”. References [1] Thornton JA. Annu Rev Mat Sci 1977;7:239. [2] Petrov I, Barna PB, Hultman L, Greene JE. J Vac Sci Technol 2003;A 21:S117.