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Porous silica xerogel films as antireflective coatings – Fabrication and characterization
Optical Materials 33 (2011) 1989–1994
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Porous silica xerogel films as antireflective coatings – Fabrication and characterization Paweł Karasin´ski b, Janusz Jaglarz a,⇑, Manuela Reben c, Edyta Skoczek a, Jacek Mazur b a
Institute of Physics, Cracow University of Technology, ul. Podchora˛z_ ych 1, 30-084 Kraków, Poland Department of Optoelectronics, Silesian University of Technology, ul. Krzywoustego 2, Gliwice 44-100, Poland c Faculty of Materials Science and Ceramics, AGH – University of Science and Technology, al. Mickiewicza 30, 30-059 Cracow, Poland b
a r t i c l e
i n f o
Article history: Available online 11 May 2011 Keywords: Porous silica Sol gel technology Antireflective coatings
a b s t r a c t The paper presents a simple fabrication method of porous silica xerogel films. By adding a surface active agent Triton X-100™ to the starting solution, we can considerably reduce the surface tension, which, in turn, allows to fabricate silica films of high porosity. The paper presents the influence of surfactant content and the influence of heating temperature on the refractive index and thickness of the fabricated films. We fabricated silica films of the minimum refractive index below 1.3 and corresponding porosity 50%. Due to low refractive index, the elaborated porous silica xerogel films can be applied to reduce the light reflection coefficient in optical systems. In this work the spectral characteristics of the refractive index, extinction coefficients, the reflection and transmission coefficients and also depolarization factor are presented. The paper also provides results of surface morphology of produced layers, obtained using an atomic force microscope. Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction The sol–gel method is a chemical means of the production of glass and ceramics from liquid phase. The sol–gel method can be used to produce various materials where of structure and consequently physical properties can be transformed within a wide range [1]. This is a principal advantage of sol–gel method. The structure of the fabricated material, depending on the applied technological process, can be compact or porous, in which the pore size can vary from a few nanometers to dozens of nanometers. The sol–gel method can produce bulk materials as well as films on the different substrates. The porous silica films can offer various application in optoelectronics [2–4]. Due to low refractive index, the porous silica layers can be applied in optical systems to reduce the light reflection coefficient. The antireflective properties of porous SiO2 obtained by sol gel method are known and well described in the literature as for example in [5–8]. The paper is devoted to optical properties of the sol–gel derived porous silica films of refractive index below 1.3. The silica xerogel films on glass substrates were coated using dip-coating method. The tetraethyl ortosilane Si(OC2H5)4 (TEOS) hydrolysis processes catalyzed with hydrochloric acid were applied. High porosity was achieved by adding the non-polar surfactant Triton X-100 to the starting solution [9,10]. The surfactant lowering the surface tension between the solution and the glass surface results in better ⇑ Corresponding author. E-mail address: [email protected] (J. Jaglarz).
adhesion, increases the molecular weight of the polymerizing gel, gives higher homogeneity and decreases internal stresses during polymerization [11]. The porosity of the silica films fabricated in our method depends on the Triton X-100 quantity in the starting solution as well as the annealing temperature. In annealing process the final properties of the fabricated films are establishing. The main aim of the present paper is to report on the influence of the thermal conditions of the annealing process on the values of the refractive index and thickness of the obtained thin films of porous silica. Understanding these dependences is necessary to assessment and tailoring of the antireflective coatings. The basic processes in the sol–gel method are hydrolysis and condensation, which can be catalyzed with acid or base. At low pH levels, when the hydrolysis is fast, the silica tends to form a linear link that occasionally cross-links. At high pH levels, when the condensation is fast, highly branched clusters are formed, and the gelling is occurring as cluster linking. Hence, the silica produced in base-catalyzed processes is more porous than in acid-catalyzed. The final porosity of xerogel film depends on the nature of this gel network and its extent of collapse by capillary stresses developed during the final stage of drying [1]. In the traditional acid-catalyzed gel solutions, we obtain layers of the porosity lower than 10% [2,3], whereas in base-catalysis, we can obtain layers of the porosity lower than 25% [4]. To obtain higher porosities, supercritical drying is required or a surfactant must be added to the input solution. In both cases it results in lowering the surface tension of the solvents, and consequently, in lowering the capillary pressure which is the reason why silica structure collapse. Supercritical solvent extraction is applied
0925-3467/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.04.003
1990
´ ski et al. / Optical Materials 33 (2011) 1989–1994 P. Karasin
to fabricate aerogels of the porosity from 85% to 99% [11]. But this process is expensive and sometimes dangerous. The paper presents the fabrication method of silica films using sol–gel method as well as the influence of technological parameters on their refractive index and thickness. 2. Fabrication of films 2.1. Sol–gel process In the research presented here, the starting solutions were prepared with the application of the following solutions: TEOS, water H2O, ethyl alcohol C2H5OH (EtOH) and hydrochloric acid HCl as catalyst with the following molar ratios being applied: TEOS:EtOH:H2O:HCl = 1:4:4:0.02. A non-polar surfactant Triton X-100 was added to the output solutions. Over a dozen solutions were prepared in which the volumetric ratios Triton X-100: TEOS = C were from C = 0 to C = 1.2. After mixing the components, the sol formation was carried out for 3 h in a closed glass vessel at a temperature of 50 °C, using ultrasonic mixing. Then, after cooling up the solutions, they were filtered with the application of syringe filters of the pore size of 0.2 lm. In the research we applied microscopic substrate glass plates (Menzel-Glaser) of the dimensions 76 25 1 mm3 and with refractive index nsub = 1.518. The substrate glass plates were washed following the procedure which included: mechanical cleaning in water with detergent, rinsing in deionized water, soaking in 5% ammonia water solution, rinsing in deionized water, rinsing in acetone and drying. 2.2. Dip-coating method The films were coated on glass substrates using the dip-coating method. The sol in which the substrates were dipped was placed in a beaker and the opening was shielded by a glass cylinder. It ensured that there were no accidental air movements, and, in consequence, allowed to obtain films of uniform thickness. In the dipcoating method, it is substrate withdrawal speed, which is a basic parameter, which controls the thickness of the fabricated films. The equations that describe the dependence of film thickness of the coated sol on the substrate withdrawal speed v depend on the fact whether the substrate movement has any effect on the curvature of meniscus [1]. The sol films coated on the substrate are then dried and heated. In these processes the films undergo condensation, which brings about the decrease of their thickness [10]. For a general case, the dependence of final thickness d of the film on the substrate withdrawal speed v can be expressed in the following form [12]:
d ¼ A ðnv Þa
were applied in the research: a multiple angle monochromatic ellipsometer SE400 (Sentech, model 2003, Germany) and a spectroscopic ellipsometer Woollam M-2000 (J.A. Woollam Co., Inc., USA). For all produced waveguide films the measurements of thickness d and refractive index n were carried out with the monochromatic ellipsometer (k = 632.8 nm). The spectroscopic ellipsometer has been used to determine the dispersion dependences of refractive index n(k) and extinction coefficient j(k) of the silica film material within the range from ultraviolet to near infrared as well as spectral characteristics of depolarization. The error of refractive index is for both ellipsometers similar and less than 0.001. Accuracy of thickness finding is about 1 nm for SE400 and M-2000. Estimated relative error of j(k) of porous silica films in wavelength range 190–500 nm is less than 0.05 for M-2000 ellipsometer. The surface topography of the produced waveguide films was investigated with the application of an atomic force microscope XE-70 (Park System Corp., Korea). The AFM imaging was perform in the non contact AFM mode (NC-AFM). The Tip300Al cantilever from Budget Sensors was used. The total reflectance measurements were performed by the use of Perkin–Elmer Lambda 490 spectrophotometer equipped with an integrating double-beam sphere [15] of 120 mm diameter. This instrument in which the xenon lamp is used as a light source, allows one to measure the reflectivity spectra in the 190–2500 nm wavelength range with a mean accuracy of 1 nm. Sample is illuminated with light beam of 10 mm diameter.
4. Experimental results From each of the sols prepared using dip-coating method and applying different withdrawal speeds v, a series of 10 structures with films of different thickness was fabricated [16]. For each series, we determined the dependence of thickness on substrate withdrawal speed d(v) and the dependence of refractive index on substrate withdrawal speed n(v). The applied substrate withdrawal speeds v were within the range from 3.0 cm/min to 7.5 cm/min. The exemplary dependences d(v) and n(v) are presented in Fig. 1. The films were fabricated from the sol in which the Triton X-100 content was C = 0.7. After the fabrication of the films the structures were heated at the temperature of 500 °C for 30 min. On the charts, squares and triangles were used to mark. Experimental points and solid lines to draw up their approximations. Broken lines stand for uncertainty intervals. The experimental points d(v) were approximated with the dependence (1). For this dependence the power a = 0.6256 ± 0.0011. It means that the applied sol was a Newtonian liquid. In the presented speeds range v the
ð1Þ
The proportionality index A depends on sol viscosity, its density and on the surface tension on the surface sol-environment, whereas n is a unit scaling factor of the dimension of speed inverse. The exponent a is referred to as slope index, and its value for Newtonian liquid is within the range from 0.50 to 0.66 [1,12]. Detailed expressions on the dependence of film thickness on substrate withdrawal speed can be found in Ref. [13]. For a given technological process, the proportionality index A and power a can be determined empirically. 3. Experimental techniques The thickness d, refractive index n and extinction coefficient j (nfilm = n ij) of the sol–gel derived silica films have been determined by ellipsometric method [14]. Two types of ellipsometers
Fig. 1. Influence of substrate withdrawal speed from sol on film thickness and refractive index of porous SiO2.
´ ski et al. / Optical Materials 33 (2011) 1989–1994 P. Karasin
thickness of the fabricated films is changing from d = 445 nm for v = 3.0 cm/min to d = 790 nm for v = 7.5 cm/min. The experimental points n(v) were approximated with linear dependence. It can be observed from the characteristic n(v) that the refractive index n of the porous silica film has low value and is decreasing with the rise of substrate withdrawal speed v. We can see that the refractive index of the porous silica film depends on its thickness d. The dependence determined from the approximation in Fig. 1 of the refractive index n of porous silica film on its thickness d is presented in Fig. 2.
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lysis processes catalyzed with a hydrochloric acid has been discussed in the previous work [3]. The dependence of refractive index n on annealing temperature is presented in Fig. 4. The annealing time of the films was 30 min. The lowest value of porous silica layer n 1.228 for at the wavelength k = 633 nm was found in films annealed upon the temperature T = 425 °C. This dependence was determined for silica films obtained from the sol of the surfactant content C = 0.7, for substrate withdrawal speed v = 4.0 cm/min. The dependence of film thickness on annealing temperature is presented in Fig. 5.
4.1. Surfactant content 4.3. Spectral characteristics The influence of Triton X-100 content on the refractive index n is presented respectively in Fig. 3. Here, each of the marked points corresponds to a different technological process, and they were defined on the base of approximation of experimental characteristics, respectively with linear dependence for n(v). The heights of error bars correspond to the widths of uncertainty intervals determined from the respective approximations. The results presented here correspond to the structures heated at the temperature of 500 °C for 30 min and to the substrate withdrawal speed of v = 4.0 cm/min.
The spectral dependence of ellipsometric angles W and D for one sample from the sets of porous SiO2 films is shown in Fig. 6A and B respectively. As well in Fig. 6A and B generated values of D obtained from the fitting extended Cauchy model have been presented [17]. The dotted lines represent fitting obtained from a one layer model on soda-lime-silica glass substrate with n = 1.518. The extended Cauchy model describes dispersion relations for n and j indices namely:
4.2. Annealing
nðkÞ ¼ A þ
In the annealing process the removal of solvent remains is taking place as well as the condensation of the collapse of the structure effected by the action of capillary pressure. These effects lead to the changes of film thickness and of refractive index. In traditional processes heating leads to the rise of refractive index and decrease of layer thickness. The influence of heating and drying processes on the properties of silica films obtained in TEOS hydro-
B k2
þ
C k4
;
jðkÞ ¼ Kebð k Ebandedge Þ hc
ð2Þ
ð3Þ
where A, B, C and b are constant term. The K and EBandegde are the fit parameters whose describe Urbach absorption tail and allow to determine the shape of dispersion of extinction coefficient.
Fig. 4. Refractive index of porous SiO2 versus annealing temperature. Withdrawal speed v = 4.0 cm/min. Fig. 2. Dependence of refractive index of porous SiO2 on film thickness.
Fig. 3. Influence of Triton X-100 content on refractive index of the silica xerogel film. v = 4.0 cm/min.
Fig. 5. Porous SiO2 Layer thickness versus annealing temperature. Withdrawal speed v = 4.0 cm/min.
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The determined refractive index was equal n = 1.258 for wavelength 632.8 nm and film thickness d = 692 nm at the room temperature (20 °C). During the measurements, the relative humidity in the room was about 45%. Assuming that the pores of these films are filled up solely with air, their porosity P can be calculated on the basis of the simplified effective medium approximation n = nairP + nd(1 P) with nair = 1 and nd = 1.46 – refractive index of dense silica at k = 632.8 nm. The obtained value of porosity (in%) for porous silica film was equal 48%. Fig. 8 shows total reflectance determined by the use Perkin Elmer 490 spectrometer on porous silica 724 nm thick film with refractive index n = 1.238. These values were determined from elipsometric study by the use M-2000 ellipsometer. As well in Fig. 8 the total reflectance on soda-lime-silica substrate is shown. Additionally elipsometric measurements allowed to determined depolarization of reflected from porous silica samples. In Fig. 9 the depolarization coefficient of porous SiO2 film measured by the using M2000 ellipsometer is presented. The sample was adjusted at an incident angle of 60°. Fig. 10 presents the transmittance of the same porous SiO2. Also in Fig. 10 the transmission curve calculated from Fresnel formula is shown. Fig. 11 shows generated specular reflectance from 95 nm thick porous SiO2 film with refractive index n = 1.258 – curve 2 on soda-lime-silica glass substrate with n = 1.518. As well as curves 1 and 3 show calculated for that optical model and maximum envelope respectively.
Fig. 6. Spectral dependences of ellipsometric angles for 692 nm thick porous silica thin film.
Fig. 8. Total reflection from 724 nm thick porous SiO2 film on soda-lime-silica glass substrate.
Fig. 7. Dispersion relation of n (A) and j (B) of porous 692 nm thick SiO2 film.
The dispersion of refractive index and extinction coefficient relation of porous SiO2 film determined from ellipsometric measurements is shown in Fig. 7A and B respectively.
Fig. 9. Spectral dependence of depolarization coefficient for 692 nm thick porous SiO2 film.
´ ski et al. / Optical Materials 33 (2011) 1989–1994 P. Karasin
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values of the RMS roughness are typical for the good quality sol–gel derived films [18].
5. Discussion
Fig. 10. Transmission from 724 nm thick porous SiO2 film on soda-lime-silica glass substrate.
Fig. 11. Calculated specular reflection from 95 nm thick porous SiO2 film on sodalime-silica glass substrate – curve 2, curves 1 and 3 are minimum and maximum envelopes determined from experimental data respectively.
Fig. 12. AFM profile of porous silica film.
4.4. Surface morphology In Fig. 12 the AFM image of the surface of porous silica film is presented. The surface of porous SiO2 film obtained by sol–gel method exhibit very flat nature. The root mean square (RMS) surface roughness is 0.18 nm over a 0.25 0.25 lm2 area, which reveals a very smooth surface of the silica film. Therefore lower
It may be observed in Fig. 1 that with the increase of the thickness d of the porous silica film, its refractive index n is decreasing. Such dependences are typical for a polymeric sol, where the polymers are weakly charged and the condensation rate is high [1,12]. In presented in Fig. 2 range of film thickness d, the changes of refractive index are 0.006. Here, the decrease of refractive index n with the rise of film thickness d is effected by stresses, and in consequence by the refractive index gradient. It can be seen from the characteristic presented in Fig. 3 that with the rise of Triton X-100 content in the input solution. Three dependence ranges n(C) can be observed on the characteristic. The first range, C < 0.2, where the refractive index of the final silica films is slightly decreasing with the rise of Triton X-100 in the input solution. The second range, 0.2 < C < 0.4, in which the refractive index of the final silica films is decreasing very fast with the rise of Triton X-100 content in the input solution, and the third range C > 0.4 in which the refractive index of final films reaches the constant level. As Results from Fig. 3 the refractive index of the surface tension of solvents is decreasing. In this way, the collapse effect of silica structure in the drying and heating process is limited, which leads to the rise of their porosity and to the decrease of refractive index. The increase of Triton X-100 content in the input solution has also an influence on the rise of sol viscosity. In the annealing process two effects compete with each other, which have different influences on the final value of refractive index. The first effect involves the collapse of silica structure, leading to the condensation of structure and consequently to the rise of refractive index. The second effect involves the removal of the remains of solvents and surfactant (Triton X-100) from the structure pores, which leads to the decrease of refractive index. We can observe from the characteristic presented in Fig. 4 that the dependence of the refractive index of the fabricated silica films on annealing temperature is non-monotonic. The said dependence reaches the minimum for the temperature of 425 °C. The decrease of refractive index with the rise of annealing temperature is here principally effected by the removal of Triton X-100 surfactant from the film material. The minimum of refractive index corresponds to the temperature in which all remains of organic solvents are removed. At the same time, the collapse of the structure is taking place, which results in the contraction of film thickness. It is presented in Fig. 5. The very good fit of model of Cauchy homogenous film on soda lime –silica glass to ellipsometric data (Fig. 6A and B) allowed to determine reliable values of refractive index, extinction coefficient and thickness of porous silica films whose are presented in Fig. 7A and B. As may see in Fig. 8 the upper envelope of reflection spectrum covers reflectance from soda-lime glass substrate alone in wavelength range 190–1200 nm. On the other hand for some spectral range the reflection from films is very low and even close to 0. It allows to easily designate porous SiO2 layers for antireflective application by matching appropriate film thickness for selected spectral range. Presence of depolarization in light reflected from porous SiO2 films (Fig. 9) may partly result from anisotropy arising from columnar-like spatial distribution of pores upon the normal to the film axis and due to nonuniformity of pore size in volume of films. The visible extrema in spectral dependence of depolarization coefficient are related with interference phenomena in a film. Depolarization is effect of appearing of incoherent light component in specular transmittance. Therefore one must be careful because
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optical constants found by the use of ellipsometry are determined from coherent part of reflected light only (Fresnel assumptions). So well for verification of these parameters we performed specular transmission measurements. As may notice values of specular transmission coefficient are larger than for one calculated from Fresnel formula represents by dashed curve in Fig. 11. If we take into account the well known expression for the antireflective coatings the lowest reflectance at selected wavelength k namely: n2 = nsub, should be fulfilled. Also thickness of film must be equal k/4. For many optical glasses nsub is in a range 1.45–1.7 [19], so our porous silica films with refractive indices range from 1.22 to 1.3 fulfill this condition. However the porous SiO2 sample for which obtained optical quantities are presented in Figs. 6–12 has n = 1.258. The best antireflective properties for that value of n may obtain for glass substrate with nsub = 1.582. As well in this figure extrema envelopes determined from porous SiO2 films with reflectance have been shown. For example if we assume that thickness of porous silica layer on soda-lime-silica glass substrate is equal 95 nm the value of reflection coefficient is less than 1% in most effective part of solar spectrum (i.e. for k in range 400–600 nm). 6. Conclusions It was shown that porous silica films obtained by sol–gel method have a low refractive index. It is caused by porosity and its possible gradient behavior with respect to the normal to the surface, which also implies the effective refractive index changes according to the depth of the sample since it depends on the SiO2/air ratio. Theoretical calculations show that better ARC effect for this method should be obtained for thinner porous SiO2 films. The refractive index values of the studied porous silica films were in range 1.2–1.3 These values allow one to regard SiO2 films as a very good material for the antireflective coatings (ARC) production for the glasses with their refractive indices in the range from 1.45 to 1.7 – met in many optical glasses. For deeper analysis of the refractive index the optical inhomogeneity should be assumed particularly for porous SiO2 layer with thickness less than 100 nm and for more than 1000 nm films. Although the porous SiO2 films have very useful properties as ARC there are still many problems to solve in order to apply them as efficient glass covers.
Acknowledgment This work was partly supported by N N503 137235 Grand of Polish Science Committee. References [1] C.J. Brinker, G.W. Scherer, in: Sol–gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, Inc., San Diego, 1990. [2] Y. Liu, W. Ren, L. Zhang, X. Yao, New method for making porous SiO2 thin films, Thin Solid Films 353 (1999) 124–128. [3] P. Karasin´ski, Influence of aging and annealing on the properties of silica films produced with sol–gel method, Optica Applicata 36 (2–3) (2006) 389–399. [4] L. Li, L. Zhang, X. Yao, Preparation and characterization of thick porous SiO2 film for multilayer pyroelectric thin film IR detector, Ceramic International 30 (2004) 1842–1846. [5] M.C. Bautista, A. Morales, Silica antireflective films on glass produced by the sol–gel method, Solar Energy Materials & Solar Cells 80 (2003) 217–225. [6] K. Cathro, D. Constable, T. Solaga, Silica low-reflection coatings for collector covers, by a dip-coating process, Solar Energy 32 (1984) 572–579. [7] L.C. Klein (Ed.), Sol–Gel Technology for Thin Films, Fibers, Preforms, Electronics, and Specialty Shapes, Noyes Publication, 1988, pp. 50–99. [8] X. Wang, J. Shen, Sol–gel derived durable antireflective coating for solar glass, Journal of Sol–Gel Science 53 (2) (2010) 322–327. [9] T. Murakata, S. Sato, T. Ohgawara, T. Watanabe, T. Suzuki, Control of pore size distribution of silica gel through sol–gel process using inorganic salts and surfactants as additives, Journal of Materials Science 27 (1992) 1567–1574. [10] D. Avnir, V.R. Kaufman, R. Reisfeld, Organic fluorescent dyes trapped in silica and silica–titania thin films by the sol–gel method. Photophysical, film and cage properties, Journal of Non-Crystalline Solids 74 (1985) 395–406. [11] H. Fan, H.R. Bentley, K.R. Kathan, P. Clem, Y. Lu, C.J. Brinker, Self-assembled aerogel-like low dielectric constant films, Journal of Non-Crystalline Solids 285 (2001) 79–83. [12] C.J. Brinker, G.C. Frye, A.J. Hurd, C.S. Ashley, Fundamentals of sol–gel dip coating, Thin Solid Films 201 (1991) 97–108. [13] I. Strawbridge, P.F. James, The factors affecting the thickness of sol–gel derived silica coatings prepared by dipping, Journal of Non-Crystalline Solids 86 (1986) 381–393. [14] R.M.A. Azzam, N.M. Bashara, in: Ellipsometry and Polarized Light, NorthHolland, Amsterdam, 1987. [15] A. Springsteen, in: J. Workmann (Ed.), Applied Spectroscopy, vol. 1, Academic Press, 1998, p. 205. [16] P. Karasin´ski, Influence of technological parameters on the properties of sol– gel silica films, Optica Applicata 35 (1) (2005) 117–128. [17] D.E. Aspnes, Optical properties of thin films, Thin Solids Films 89 (1982) 249– 262. [18] P. Karasin´ski, C. Tyszkiewicz, R. Rogozin´ski, J. Jaglarz, J. Mazur, Optical rib waveguides based on sol–gel derived silica–titania films, Thin Solid Films (2011). doi:10.1016/j.tsf.2011.02.064. [19] http://refractiveindex.info.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Porous silica xerogel films as antireflective coatings – Fabrication and characterization Paweł Karasin´ski b, Janusz Jaglarz a,⇑, Manuela Reben c, Edyta Skoczek a, Jacek Mazur b a
Institute of Physics, Cracow University of Technology, ul. Podchora˛z_ ych 1, 30-084 Kraków, Poland Department of Optoelectronics, Silesian University of Technology, ul. Krzywoustego 2, Gliwice 44-100, Poland c Faculty of Materials Science and Ceramics, AGH – University of Science and Technology, al. Mickiewicza 30, 30-059 Cracow, Poland b
a r t i c l e
i n f o
Article history: Available online 11 May 2011 Keywords: Porous silica Sol gel technology Antireflective coatings
a b s t r a c t The paper presents a simple fabrication method of porous silica xerogel films. By adding a surface active agent Triton X-100™ to the starting solution, we can considerably reduce the surface tension, which, in turn, allows to fabricate silica films of high porosity. The paper presents the influence of surfactant content and the influence of heating temperature on the refractive index and thickness of the fabricated films. We fabricated silica films of the minimum refractive index below 1.3 and corresponding porosity 50%. Due to low refractive index, the elaborated porous silica xerogel films can be applied to reduce the light reflection coefficient in optical systems. In this work the spectral characteristics of the refractive index, extinction coefficients, the reflection and transmission coefficients and also depolarization factor are presented. The paper also provides results of surface morphology of produced layers, obtained using an atomic force microscope. Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction The sol–gel method is a chemical means of the production of glass and ceramics from liquid phase. The sol–gel method can be used to produce various materials where of structure and consequently physical properties can be transformed within a wide range [1]. This is a principal advantage of sol–gel method. The structure of the fabricated material, depending on the applied technological process, can be compact or porous, in which the pore size can vary from a few nanometers to dozens of nanometers. The sol–gel method can produce bulk materials as well as films on the different substrates. The porous silica films can offer various application in optoelectronics [2–4]. Due to low refractive index, the porous silica layers can be applied in optical systems to reduce the light reflection coefficient. The antireflective properties of porous SiO2 obtained by sol gel method are known and well described in the literature as for example in [5–8]. The paper is devoted to optical properties of the sol–gel derived porous silica films of refractive index below 1.3. The silica xerogel films on glass substrates were coated using dip-coating method. The tetraethyl ortosilane Si(OC2H5)4 (TEOS) hydrolysis processes catalyzed with hydrochloric acid were applied. High porosity was achieved by adding the non-polar surfactant Triton X-100 to the starting solution [9,10]. The surfactant lowering the surface tension between the solution and the glass surface results in better ⇑ Corresponding author. E-mail address: [email protected] (J. Jaglarz).
adhesion, increases the molecular weight of the polymerizing gel, gives higher homogeneity and decreases internal stresses during polymerization [11]. The porosity of the silica films fabricated in our method depends on the Triton X-100 quantity in the starting solution as well as the annealing temperature. In annealing process the final properties of the fabricated films are establishing. The main aim of the present paper is to report on the influence of the thermal conditions of the annealing process on the values of the refractive index and thickness of the obtained thin films of porous silica. Understanding these dependences is necessary to assessment and tailoring of the antireflective coatings. The basic processes in the sol–gel method are hydrolysis and condensation, which can be catalyzed with acid or base. At low pH levels, when the hydrolysis is fast, the silica tends to form a linear link that occasionally cross-links. At high pH levels, when the condensation is fast, highly branched clusters are formed, and the gelling is occurring as cluster linking. Hence, the silica produced in base-catalyzed processes is more porous than in acid-catalyzed. The final porosity of xerogel film depends on the nature of this gel network and its extent of collapse by capillary stresses developed during the final stage of drying [1]. In the traditional acid-catalyzed gel solutions, we obtain layers of the porosity lower than 10% [2,3], whereas in base-catalysis, we can obtain layers of the porosity lower than 25% [4]. To obtain higher porosities, supercritical drying is required or a surfactant must be added to the input solution. In both cases it results in lowering the surface tension of the solvents, and consequently, in lowering the capillary pressure which is the reason why silica structure collapse. Supercritical solvent extraction is applied
0925-3467/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.04.003
1990
´ ski et al. / Optical Materials 33 (2011) 1989–1994 P. Karasin
to fabricate aerogels of the porosity from 85% to 99% [11]. But this process is expensive and sometimes dangerous. The paper presents the fabrication method of silica films using sol–gel method as well as the influence of technological parameters on their refractive index and thickness. 2. Fabrication of films 2.1. Sol–gel process In the research presented here, the starting solutions were prepared with the application of the following solutions: TEOS, water H2O, ethyl alcohol C2H5OH (EtOH) and hydrochloric acid HCl as catalyst with the following molar ratios being applied: TEOS:EtOH:H2O:HCl = 1:4:4:0.02. A non-polar surfactant Triton X-100 was added to the output solutions. Over a dozen solutions were prepared in which the volumetric ratios Triton X-100: TEOS = C were from C = 0 to C = 1.2. After mixing the components, the sol formation was carried out for 3 h in a closed glass vessel at a temperature of 50 °C, using ultrasonic mixing. Then, after cooling up the solutions, they were filtered with the application of syringe filters of the pore size of 0.2 lm. In the research we applied microscopic substrate glass plates (Menzel-Glaser) of the dimensions 76 25 1 mm3 and with refractive index nsub = 1.518. The substrate glass plates were washed following the procedure which included: mechanical cleaning in water with detergent, rinsing in deionized water, soaking in 5% ammonia water solution, rinsing in deionized water, rinsing in acetone and drying. 2.2. Dip-coating method The films were coated on glass substrates using the dip-coating method. The sol in which the substrates were dipped was placed in a beaker and the opening was shielded by a glass cylinder. It ensured that there were no accidental air movements, and, in consequence, allowed to obtain films of uniform thickness. In the dipcoating method, it is substrate withdrawal speed, which is a basic parameter, which controls the thickness of the fabricated films. The equations that describe the dependence of film thickness of the coated sol on the substrate withdrawal speed v depend on the fact whether the substrate movement has any effect on the curvature of meniscus [1]. The sol films coated on the substrate are then dried and heated. In these processes the films undergo condensation, which brings about the decrease of their thickness [10]. For a general case, the dependence of final thickness d of the film on the substrate withdrawal speed v can be expressed in the following form [12]:
d ¼ A ðnv Þa
were applied in the research: a multiple angle monochromatic ellipsometer SE400 (Sentech, model 2003, Germany) and a spectroscopic ellipsometer Woollam M-2000 (J.A. Woollam Co., Inc., USA). For all produced waveguide films the measurements of thickness d and refractive index n were carried out with the monochromatic ellipsometer (k = 632.8 nm). The spectroscopic ellipsometer has been used to determine the dispersion dependences of refractive index n(k) and extinction coefficient j(k) of the silica film material within the range from ultraviolet to near infrared as well as spectral characteristics of depolarization. The error of refractive index is for both ellipsometers similar and less than 0.001. Accuracy of thickness finding is about 1 nm for SE400 and M-2000. Estimated relative error of j(k) of porous silica films in wavelength range 190–500 nm is less than 0.05 for M-2000 ellipsometer. The surface topography of the produced waveguide films was investigated with the application of an atomic force microscope XE-70 (Park System Corp., Korea). The AFM imaging was perform in the non contact AFM mode (NC-AFM). The Tip300Al cantilever from Budget Sensors was used. The total reflectance measurements were performed by the use of Perkin–Elmer Lambda 490 spectrophotometer equipped with an integrating double-beam sphere [15] of 120 mm diameter. This instrument in which the xenon lamp is used as a light source, allows one to measure the reflectivity spectra in the 190–2500 nm wavelength range with a mean accuracy of 1 nm. Sample is illuminated with light beam of 10 mm diameter.
4. Experimental results From each of the sols prepared using dip-coating method and applying different withdrawal speeds v, a series of 10 structures with films of different thickness was fabricated [16]. For each series, we determined the dependence of thickness on substrate withdrawal speed d(v) and the dependence of refractive index on substrate withdrawal speed n(v). The applied substrate withdrawal speeds v were within the range from 3.0 cm/min to 7.5 cm/min. The exemplary dependences d(v) and n(v) are presented in Fig. 1. The films were fabricated from the sol in which the Triton X-100 content was C = 0.7. After the fabrication of the films the structures were heated at the temperature of 500 °C for 30 min. On the charts, squares and triangles were used to mark. Experimental points and solid lines to draw up their approximations. Broken lines stand for uncertainty intervals. The experimental points d(v) were approximated with the dependence (1). For this dependence the power a = 0.6256 ± 0.0011. It means that the applied sol was a Newtonian liquid. In the presented speeds range v the
ð1Þ
The proportionality index A depends on sol viscosity, its density and on the surface tension on the surface sol-environment, whereas n is a unit scaling factor of the dimension of speed inverse. The exponent a is referred to as slope index, and its value for Newtonian liquid is within the range from 0.50 to 0.66 [1,12]. Detailed expressions on the dependence of film thickness on substrate withdrawal speed can be found in Ref. [13]. For a given technological process, the proportionality index A and power a can be determined empirically. 3. Experimental techniques The thickness d, refractive index n and extinction coefficient j (nfilm = n ij) of the sol–gel derived silica films have been determined by ellipsometric method [14]. Two types of ellipsometers
Fig. 1. Influence of substrate withdrawal speed from sol on film thickness and refractive index of porous SiO2.
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thickness of the fabricated films is changing from d = 445 nm for v = 3.0 cm/min to d = 790 nm for v = 7.5 cm/min. The experimental points n(v) were approximated with linear dependence. It can be observed from the characteristic n(v) that the refractive index n of the porous silica film has low value and is decreasing with the rise of substrate withdrawal speed v. We can see that the refractive index of the porous silica film depends on its thickness d. The dependence determined from the approximation in Fig. 1 of the refractive index n of porous silica film on its thickness d is presented in Fig. 2.
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lysis processes catalyzed with a hydrochloric acid has been discussed in the previous work [3]. The dependence of refractive index n on annealing temperature is presented in Fig. 4. The annealing time of the films was 30 min. The lowest value of porous silica layer n 1.228 for at the wavelength k = 633 nm was found in films annealed upon the temperature T = 425 °C. This dependence was determined for silica films obtained from the sol of the surfactant content C = 0.7, for substrate withdrawal speed v = 4.0 cm/min. The dependence of film thickness on annealing temperature is presented in Fig. 5.
4.1. Surfactant content 4.3. Spectral characteristics The influence of Triton X-100 content on the refractive index n is presented respectively in Fig. 3. Here, each of the marked points corresponds to a different technological process, and they were defined on the base of approximation of experimental characteristics, respectively with linear dependence for n(v). The heights of error bars correspond to the widths of uncertainty intervals determined from the respective approximations. The results presented here correspond to the structures heated at the temperature of 500 °C for 30 min and to the substrate withdrawal speed of v = 4.0 cm/min.
The spectral dependence of ellipsometric angles W and D for one sample from the sets of porous SiO2 films is shown in Fig. 6A and B respectively. As well in Fig. 6A and B generated values of D obtained from the fitting extended Cauchy model have been presented [17]. The dotted lines represent fitting obtained from a one layer model on soda-lime-silica glass substrate with n = 1.518. The extended Cauchy model describes dispersion relations for n and j indices namely:
4.2. Annealing
nðkÞ ¼ A þ
In the annealing process the removal of solvent remains is taking place as well as the condensation of the collapse of the structure effected by the action of capillary pressure. These effects lead to the changes of film thickness and of refractive index. In traditional processes heating leads to the rise of refractive index and decrease of layer thickness. The influence of heating and drying processes on the properties of silica films obtained in TEOS hydro-
B k2
þ
C k4
;
jðkÞ ¼ Kebð k Ebandedge Þ hc
ð2Þ
ð3Þ
where A, B, C and b are constant term. The K and EBandegde are the fit parameters whose describe Urbach absorption tail and allow to determine the shape of dispersion of extinction coefficient.
Fig. 4. Refractive index of porous SiO2 versus annealing temperature. Withdrawal speed v = 4.0 cm/min. Fig. 2. Dependence of refractive index of porous SiO2 on film thickness.
Fig. 3. Influence of Triton X-100 content on refractive index of the silica xerogel film. v = 4.0 cm/min.
Fig. 5. Porous SiO2 Layer thickness versus annealing temperature. Withdrawal speed v = 4.0 cm/min.
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The determined refractive index was equal n = 1.258 for wavelength 632.8 nm and film thickness d = 692 nm at the room temperature (20 °C). During the measurements, the relative humidity in the room was about 45%. Assuming that the pores of these films are filled up solely with air, their porosity P can be calculated on the basis of the simplified effective medium approximation n = nairP + nd(1 P) with nair = 1 and nd = 1.46 – refractive index of dense silica at k = 632.8 nm. The obtained value of porosity (in%) for porous silica film was equal 48%. Fig. 8 shows total reflectance determined by the use Perkin Elmer 490 spectrometer on porous silica 724 nm thick film with refractive index n = 1.238. These values were determined from elipsometric study by the use M-2000 ellipsometer. As well in Fig. 8 the total reflectance on soda-lime-silica substrate is shown. Additionally elipsometric measurements allowed to determined depolarization of reflected from porous silica samples. In Fig. 9 the depolarization coefficient of porous SiO2 film measured by the using M2000 ellipsometer is presented. The sample was adjusted at an incident angle of 60°. Fig. 10 presents the transmittance of the same porous SiO2. Also in Fig. 10 the transmission curve calculated from Fresnel formula is shown. Fig. 11 shows generated specular reflectance from 95 nm thick porous SiO2 film with refractive index n = 1.258 – curve 2 on soda-lime-silica glass substrate with n = 1.518. As well as curves 1 and 3 show calculated for that optical model and maximum envelope respectively.
Fig. 6. Spectral dependences of ellipsometric angles for 692 nm thick porous silica thin film.
Fig. 8. Total reflection from 724 nm thick porous SiO2 film on soda-lime-silica glass substrate.
Fig. 7. Dispersion relation of n (A) and j (B) of porous 692 nm thick SiO2 film.
The dispersion of refractive index and extinction coefficient relation of porous SiO2 film determined from ellipsometric measurements is shown in Fig. 7A and B respectively.
Fig. 9. Spectral dependence of depolarization coefficient for 692 nm thick porous SiO2 film.
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values of the RMS roughness are typical for the good quality sol–gel derived films [18].
5. Discussion
Fig. 10. Transmission from 724 nm thick porous SiO2 film on soda-lime-silica glass substrate.
Fig. 11. Calculated specular reflection from 95 nm thick porous SiO2 film on sodalime-silica glass substrate – curve 2, curves 1 and 3 are minimum and maximum envelopes determined from experimental data respectively.
Fig. 12. AFM profile of porous silica film.
4.4. Surface morphology In Fig. 12 the AFM image of the surface of porous silica film is presented. The surface of porous SiO2 film obtained by sol–gel method exhibit very flat nature. The root mean square (RMS) surface roughness is 0.18 nm over a 0.25 0.25 lm2 area, which reveals a very smooth surface of the silica film. Therefore lower
It may be observed in Fig. 1 that with the increase of the thickness d of the porous silica film, its refractive index n is decreasing. Such dependences are typical for a polymeric sol, where the polymers are weakly charged and the condensation rate is high [1,12]. In presented in Fig. 2 range of film thickness d, the changes of refractive index are 0.006. Here, the decrease of refractive index n with the rise of film thickness d is effected by stresses, and in consequence by the refractive index gradient. It can be seen from the characteristic presented in Fig. 3 that with the rise of Triton X-100 content in the input solution. Three dependence ranges n(C) can be observed on the characteristic. The first range, C < 0.2, where the refractive index of the final silica films is slightly decreasing with the rise of Triton X-100 in the input solution. The second range, 0.2 < C < 0.4, in which the refractive index of the final silica films is decreasing very fast with the rise of Triton X-100 content in the input solution, and the third range C > 0.4 in which the refractive index of final films reaches the constant level. As Results from Fig. 3 the refractive index of the surface tension of solvents is decreasing. In this way, the collapse effect of silica structure in the drying and heating process is limited, which leads to the rise of their porosity and to the decrease of refractive index. The increase of Triton X-100 content in the input solution has also an influence on the rise of sol viscosity. In the annealing process two effects compete with each other, which have different influences on the final value of refractive index. The first effect involves the collapse of silica structure, leading to the condensation of structure and consequently to the rise of refractive index. The second effect involves the removal of the remains of solvents and surfactant (Triton X-100) from the structure pores, which leads to the decrease of refractive index. We can observe from the characteristic presented in Fig. 4 that the dependence of the refractive index of the fabricated silica films on annealing temperature is non-monotonic. The said dependence reaches the minimum for the temperature of 425 °C. The decrease of refractive index with the rise of annealing temperature is here principally effected by the removal of Triton X-100 surfactant from the film material. The minimum of refractive index corresponds to the temperature in which all remains of organic solvents are removed. At the same time, the collapse of the structure is taking place, which results in the contraction of film thickness. It is presented in Fig. 5. The very good fit of model of Cauchy homogenous film on soda lime –silica glass to ellipsometric data (Fig. 6A and B) allowed to determine reliable values of refractive index, extinction coefficient and thickness of porous silica films whose are presented in Fig. 7A and B. As may see in Fig. 8 the upper envelope of reflection spectrum covers reflectance from soda-lime glass substrate alone in wavelength range 190–1200 nm. On the other hand for some spectral range the reflection from films is very low and even close to 0. It allows to easily designate porous SiO2 layers for antireflective application by matching appropriate film thickness for selected spectral range. Presence of depolarization in light reflected from porous SiO2 films (Fig. 9) may partly result from anisotropy arising from columnar-like spatial distribution of pores upon the normal to the film axis and due to nonuniformity of pore size in volume of films. The visible extrema in spectral dependence of depolarization coefficient are related with interference phenomena in a film. Depolarization is effect of appearing of incoherent light component in specular transmittance. Therefore one must be careful because
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optical constants found by the use of ellipsometry are determined from coherent part of reflected light only (Fresnel assumptions). So well for verification of these parameters we performed specular transmission measurements. As may notice values of specular transmission coefficient are larger than for one calculated from Fresnel formula represents by dashed curve in Fig. 11. If we take into account the well known expression for the antireflective coatings the lowest reflectance at selected wavelength k namely: n2 = nsub, should be fulfilled. Also thickness of film must be equal k/4. For many optical glasses nsub is in a range 1.45–1.7 [19], so our porous silica films with refractive indices range from 1.22 to 1.3 fulfill this condition. However the porous SiO2 sample for which obtained optical quantities are presented in Figs. 6–12 has n = 1.258. The best antireflective properties for that value of n may obtain for glass substrate with nsub = 1.582. As well in this figure extrema envelopes determined from porous SiO2 films with reflectance have been shown. For example if we assume that thickness of porous silica layer on soda-lime-silica glass substrate is equal 95 nm the value of reflection coefficient is less than 1% in most effective part of solar spectrum (i.e. for k in range 400–600 nm). 6. Conclusions It was shown that porous silica films obtained by sol–gel method have a low refractive index. It is caused by porosity and its possible gradient behavior with respect to the normal to the surface, which also implies the effective refractive index changes according to the depth of the sample since it depends on the SiO2/air ratio. Theoretical calculations show that better ARC effect for this method should be obtained for thinner porous SiO2 films. The refractive index values of the studied porous silica films were in range 1.2–1.3 These values allow one to regard SiO2 films as a very good material for the antireflective coatings (ARC) production for the glasses with their refractive indices in the range from 1.45 to 1.7 – met in many optical glasses. For deeper analysis of the refractive index the optical inhomogeneity should be assumed particularly for porous SiO2 layer with thickness less than 100 nm and for more than 1000 nm films. Although the porous SiO2 films have very useful properties as ARC there are still many problems to solve in order to apply them as efficient glass covers.
Acknowledgment This work was partly supported by N N503 137235 Grand of Polish Science Committee. References [1] C.J. Brinker, G.W. Scherer, in: Sol–gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, Inc., San Diego, 1990. [2] Y. Liu, W. Ren, L. Zhang, X. Yao, New method for making porous SiO2 thin films, Thin Solid Films 353 (1999) 124–128. [3] P. Karasin´ski, Influence of aging and annealing on the properties of silica films produced with sol–gel method, Optica Applicata 36 (2–3) (2006) 389–399. [4] L. Li, L. Zhang, X. Yao, Preparation and characterization of thick porous SiO2 film for multilayer pyroelectric thin film IR detector, Ceramic International 30 (2004) 1842–1846. [5] M.C. Bautista, A. Morales, Silica antireflective films on glass produced by the sol–gel method, Solar Energy Materials & Solar Cells 80 (2003) 217–225. [6] K. Cathro, D. Constable, T. Solaga, Silica low-reflection coatings for collector covers, by a dip-coating process, Solar Energy 32 (1984) 572–579. [7] L.C. Klein (Ed.), Sol–Gel Technology for Thin Films, Fibers, Preforms, Electronics, and Specialty Shapes, Noyes Publication, 1988, pp. 50–99. [8] X. Wang, J. Shen, Sol–gel derived durable antireflective coating for solar glass, Journal of Sol–Gel Science 53 (2) (2010) 322–327. [9] T. Murakata, S. Sato, T. Ohgawara, T. Watanabe, T. Suzuki, Control of pore size distribution of silica gel through sol–gel process using inorganic salts and surfactants as additives, Journal of Materials Science 27 (1992) 1567–1574. [10] D. Avnir, V.R. Kaufman, R. Reisfeld, Organic fluorescent dyes trapped in silica and silica–titania thin films by the sol–gel method. Photophysical, film and cage properties, Journal of Non-Crystalline Solids 74 (1985) 395–406. [11] H. Fan, H.R. Bentley, K.R. Kathan, P. Clem, Y. Lu, C.J. Brinker, Self-assembled aerogel-like low dielectric constant films, Journal of Non-Crystalline Solids 285 (2001) 79–83. [12] C.J. Brinker, G.C. Frye, A.J. Hurd, C.S. Ashley, Fundamentals of sol–gel dip coating, Thin Solid Films 201 (1991) 97–108. [13] I. Strawbridge, P.F. James, The factors affecting the thickness of sol–gel derived silica coatings prepared by dipping, Journal of Non-Crystalline Solids 86 (1986) 381–393. [14] R.M.A. Azzam, N.M. Bashara, in: Ellipsometry and Polarized Light, NorthHolland, Amsterdam, 1987. [15] A. Springsteen, in: J. Workmann (Ed.), Applied Spectroscopy, vol. 1, Academic Press, 1998, p. 205. [16] P. Karasin´ski, Influence of technological parameters on the properties of sol– gel silica films, Optica Applicata 35 (1) (2005) 117–128. [17] D.E. Aspnes, Optical properties of thin films, Thin Solids Films 89 (1982) 249– 262. [18] P. Karasin´ski, C. Tyszkiewicz, R. Rogozin´ski, J. Jaglarz, J. Mazur, Optical rib waveguides based on sol–gel derived silica–titania films, Thin Solid Films (2011). doi:10.1016/j.tsf.2011.02.064. [19] http://refractiveindex.info.