RU2371399C2 - Method of making antireflection coating based on mesoporous silica using sol-gel method in presence of certain polymers, static copolymers - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to thin-film interference coatings for coating optical elements. A method is proposed for making thin 50-200 nm single-layer antireflection coating based on mesoporous silica on objects made from silicate glass with transmission maximum of 97.0 to 98.5% in the visible spectrum, involving a) sol-gel process of silicon tetraalkoxide in the presence of an organic additive in concentration of 0.1 to 5.0 wt % to the weight of the sol, using nanostructure self-organising techniques, induced by evaporating solvent EISA, b) heating a sample with coating in an air atmosphere at 300 to 600°C for several hours for thermal decomposition of the organic additive. The organic additive, which determines spontaneous micro-separation of inorganic and organic phases during formation of solid coating on glass, is in form of carbon-chain polymers, which contain lateral ether or ester groups or carbon-chain static copolymers, containing ether groups and ester groups.

EFFECT: design of a method of obtaining cheap coating with low refraction index of 1,25-1,34.

1 cl, 7 dwg, 7 ex

Description

Thin-film interference coatings for the illumination of optical elements are widely used in the optical industry and in the commercial industry (enlightenment of display screens, photodetectors, fiber optical fibers, etc.). The main limitation of thin-film antireflection coatings is the impossibility of vacuum deposition on large optical parts (larger aperture optics based on optical elements from fused silica and optical glasses of various grades, nonlinear water-soluble crystals, etc.).

In general, light reflection occurs at the interface between two materials, such as silicate glass and air. With a glass refractive index of n≈1.51, about 8.6% of the light incident at an angle of 0 ° is reflected from two glass-air interfaces. Theoretically, to minimize the reflection of light with a wavelength λ to a minimum (0.0%), it is necessary to cover the glass with a transparent film with a refractive index n _p = √n≈1.23 and an optical thickness of λ / 4.

However, film-forming materials with such a low refractive index do not exist in nature.

For example, fluorinated compounds of magnesium difluoride (refractive index 1.38), Teflon (refractive index 1.33) have low refractive indices, but the refractive indices of these substances are much higher than the theoretical value of 1.23. Therefore, in the practice of enlightening optical parts, multilayer coatings are widely used: two-layer, three-layer, four-layer, etc. (N.A. Macleod, "Thin Film Optical Filters", Adam Hilger Ltd., Bristol, 1985). Two-layer film coatings have an M / L structure, where M is an internal (adjacent to the glass) coating of a certain thickness with a refractive index of 1.6 to 1.9, L is an external coating with a low refractive index of less than 1.6. Three-layer film coatings have an M / H / L structure, where M is an inner coating with a refractive index of 1.6 to 1.9, H is an intermediate coating with a refractive index of more than 1.9, and L is an outer coating with a refractive index of less than 1, 6. Four-layer film coatings have the structure H / L / H / L, six-layer coatings have H / L / H / L / H / L, etc.

Materials that have a high refractive index (H) are, for example, titanium dioxide (2.35 at 520 nm), tantalum pentoxide (2.25 at 520 nm), zirconia (2.05 ÷ 2.08 at 520 nm ), tin dioxide (2.0 at 520 nm), cerium dioxide (1.95 at 520 nm) and others.

Materials with a low refractive index (L) are, for example, silicon dioxide (1.46 at 520 nm), magnesium difluoride (1.38 at 520 nm) and others.

Materials with an average refractive index (M) of 1.6-1.9 are Al ₂ O ₃ (1.65), Sc ₂ O ₃ (1.85), LaF ₃ (1.54), CeF ₃ (1.57 ), Dy ₂ O ₃ (1.78) and others.

Various variants of multilayer antireflection coatings on silicate glass are well known and are discussed in detail, for example, in US patents: S. Katsube, et al., No. 3958042, 1976; O. Kamiya, et al., No. 3960441, 1976; F. Onoki, et al., No. 4,128,303, 1978; H. Tani, No. 4387960, 1983; J. Rijpers, et al., No. 4,798,994, 1989; Y. Iida, et al., No. 5073481, 1991; R. Austin, No. 5147125, 1992; R. Austin, No. 5332618; P. Boire, No. 5618579, 1997; S. Machol, No. 5719705, 1998; P. Macquart, et al., No. 5935702, 1999; C. Bright, et al., No. 5981059; 1991; C. Anderson, et al., No. 6,238,781, 2001; J. Sopko, et al., No. 6436541, 2002; C. Anderson, et al., No. 7005188, 2006.

In the practice of applying thin-film antireflection coatings on optical parts, vacuum technologies are used (N.A. Macleod, "Thin Film Optical Filters", Adam Hilger Ltd., Bristol, 1985) and the sol-gel method (N.V. Suikovskaya, "Chemical methods for producing thin transparent films ", Khimiya Publishing House, 1971, 199 pages). In vacuum technologies, expensive equipment is used, the price of which increases with increasing size of optical parts. The dimensions of the optical parts are limited by the dimensions of the vacuum chamber of the spraying unit.

The sol-gel process is easier in experimental design and can be used for coating large optical parts.

In 1992, scientists at Mobil Oil Corp. (USA), an important discovery was made in the field of synthesis of new nanomaterials (JSBeck, US Pat. No. 5057296, 1991; JSBeck et al., US Pat. No. 5145816, 1992; S.T. Kresge et al., Nature, 1992, 359, 710). They developed a matrix synthesis of mesoporous silicates and aluminosilicates. For the first time, a group of M41S mesoporous materials was obtained (MCM-41 — hexagonal mesophase, MCM-48 — cubic mesophase, MCM-50 — lamellar mesophase) with a regular, well-defined system of nanoscale structures by conducting a sol-gel process in the presence of a cationic surfactant. In view of the relative ease of synthesis of new mesoporous materials and the wide possibilities of their practical use in catalysis, microelectronics, optics, as sensors, etc. this line of research subsequently developed very rapidly. In a review (Y. Wan, D. Zhao, Chem. Rev., 2007, 107, 2821), published in 2007 and devoted only to various aspects of the synthesis and properties of mesoporous silica, more than 350 works are presented. In all these works, the sol-gel process is carried out in the presence of various classes of surfactants, which play the role of a matrix that determines the self-organization of inorganic-organic nanoscale structures during gel formation.

It should be noted that in the pioneering work of Mobil Oil Corp. Mesoporous silicates and aluminosilicates were obtained only in the form of powders. In 1997 (CJBrinker et al., Nature, 1997, 389, 364; CJBrinker, et al. US Pat. No. 5,858,457, 1999; CJBrinker, et al., US Pat. No. 6,208,846, 2001), a technique was developed the self-organization of nanostructures caused by solvent evaporation [EISA (evaporation-induced self-assembly)], which is used to deposit nanoporous films of silicon dioxide on a substrate, and the way was opened to obtain single-layer transparent coatings with a low refractive index, with almost theoretical antireflection effect. In this work, cationic surfactants (cetyltrimethylammonium bromide, etc.) were used as a matrix — an organic compound that determines the self-organization of inorganic-organic nanoscale structures in the resulting film.

We found that the EISA technique can also produce antireflection coatings with a low refractive index (1.25-1.3) based on nanoporous silicon dioxide, if instead of expensive and hard-to-reach cationic surfactants in the sol-gel process, cheap and readily available carbochain polymers containing lateral ether, lateral ester groups, random copolymers containing lateral ether, lateral ester groups, in a concentration of 0.1-5.0 wt.%, preferably 0.5-3.0 wt.%, by weight of the sol composition. It has been established that there is an optimal concentration of polymer, a random copolymer in ash, at which nanoporous coatings with the lowest refractive index and, therefore, the highest antireflection effect are obtained. With a decrease or increase in the concentration of the polymer, the statistical copolymer in the ash from the optimum value, the refractive index of the coating increases and the antireflection effect decreases. It should be noted that the use of high molecular weight compounds as pore-forming substances in the preparation of films of mesoporous silicon dioxide on silicon is known: K.R. Carter et al., Pat. US No. 5773197, 1998; K.R. Carter et al., Pat. US No. 5895263, 1999; C.J. Hawker et al., Pat. US No. 6107357, 2000; M. Nishikawa et al, Pat. US No. 6376634, 2002; A. Shiota et al., Pat. U.S. No. 6,406,794, 2002; S. Jang et al., Pat. US No. 6495478, 2002; H. Wu et al., Pat. US No. 6495479, 2002; L.V. Interrante et al., Pat. US 6809041, 2004; M. Akiyama et al., Pat. US2005 / 0096415, 2005; H. Tsuchiya et al., Pat. US2006 / 0024980, 2006; H. Tsuchiya et al., Pat. US No. 7291567, 2007; B.K. Peterson et al., Pat. US No. 7294585, 2007. These US patents disclose methods for producing mesoporous silica films by the sol-gel method in the presence of polymers only on silicon substrates. It is proposed to use films as dielectric layers with a low dielectric constant (K <3.0) in microelectronic circuits; therefore, the dielectric constants of films were measured. In these US patents, substrates of silicate glass, quartz, and other optical materials were not studied, and the optical properties (transmission of light or reflection of light) of optical materials coated with mesoporous silica films were not studied either.

As carbochain polymers containing side ether ether groups, we offer: a) polyalkyl vinyl ethers (polyvinyl methyl ether, polyvinyl ethyl ether, polyvinyl propyl ether, polyvinyl n-butyl ether, polyvinyl isobutyl ether and other similar compounds with a large alkyl radical); b) polyvinyl acetals (polyvinyl formal, polyvinyl ethyl, polyvinyl butyral, polyvinyl formal formal ethyl, polyvinyl butyral furfural).

As carbochain polymers containing side ester groups, we offer: a) polyester acrylates (polymethyl acrylate, polyethyl acrylate, polypropyl acrylate, polybutyl acrylate, poly-2-ethylhexyl acrylate, polyfluoroalkyl, polyalkyl, ); b) polyether methacrylates (polymethyl methacrylate, polyethyl methacrylate, polypropyl methacrylate, polybutyl methacrylate, polyoctyl methacrylate, poly-2-ethylhexyl methacrylate, polynonyl methacrylate, polydecyl methacrylate, polyhydrocylmethacrylate and other polyether ether) c) polyvinyl acetate, polyvinyl propionate, polyvinyl laurate, polyvinyl stearate.

As carbochain random copolymers containing side ether groups, ester groups, we offer alkyl acrylate copolymers, alkyl methacrylate copolymers, vinyl acetate copolymers, vinyl ester copolymers.

As the main component in the preparation of the sol can be used tetraalkoxysilanes: tetramethoxysilane, tetraethoxysilane.

As a solvent, methyl, ethyl, propyl, isopropyl, butyl alcohols, tetrahydrofuran can be used. Before use, solvents are freed from impurities by methods known in the literature. Water is bidistillate. The ratio of water to tetraalkoxysilane (in moles) is 4: 1.

Inorganic acids can be used as a catalyst for the hydrolysis of tetraalkoxysilanes: hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid in concentrations of 1.5-4.0 · 10 ^-3 mol per 1 mol of tetraalkoxysilane.

Silicate slides for microscopy were used as the substrate; the refractive index of the glass was 1.506. Glass thickness 2.0 mm, length 20 mm, width 10 mm.

The glass surface was cleaned of impurities by immersion in a solution of alkali with hydrogen peroxide, the glasses were washed with water, distilled water and dried in an thermostat at 150 ° C for 6-8 hours. The following examples illustrate the subject matter of the invention.

Example 1

In a 50 ml glass flask, 1.0 ml of tetraethoxysilane (TEOS), 5.0 ml of isopropyl alcohol were added, water (4: 1 in moles to TEOS) and hydrochloric acid were added at a concentration of 3.0 · 10 ^-3 mol per 1 mol TEOS. The glass flask was closed and the contents of the flask were stirred at room temperature using a magnetic stirrer for 1-4 hours.

A solution of polyvinyl butyral in isopropyl alcohol was introduced into the flask, the mixture was stirred for 1.0 hour at room temperature. The mixture was diluted with isopropyl alcohol to a concentration of 1.0 wt.% In terms of SiO ₂ .

The coatings were applied to glass by rotation at a speed of 500-2000 rpm for several minutes at room temperature. Glasses were left at room temperature for 12 hours. Next, coated glasses were placed in a thermostat and heated at a speed of 5 ° C / min from 150 to 500 ° C. At this temperature, the samples were kept for 5-6 hours. After slow cooling, the samples were removed from the thermostat and their light transmission was determined in the wavelength range of 200-1100 nm on a Perkin-Elmer Lambda 25 spectrometer with an accuracy of ± 0.1%.

The optical thickness of the films and their refractive index were determined on an LEF-ZM1 ellipsometer.

Figure 1 shows the light transmission curves of uncoated glasses and with double-sided single-layer coatings based on nanoporous silicon dioxide.

Uncoated glass has a maximum transmittance of 91.1% at wavelengths of 515-520 nm (curve 1), the transmittance monotonically decreases to 83.3% when the wavelength is shifted to the near infrared region. Glass does not transmit UV radiation with a wavelength of less than 325 nm. Transparent coatings of nanoporous silicon dioxide increase the light transmission of glass (there is a bleaching effect) in the wavelength range of 350-1100 nm: curve 2, additive - 0.35 wt.%, Curve 3 - 1.0 wt.%, Curve 4 - 1, 5 wt.%, Curve 5 - 2.0 wt.%. The effect of enlightenment depends on the concentration of polyvinyl butyral in the ash, first increases, passes through a maximum and then decreases with increasing concentration of the additive. The maximum transmittance of 97.0% (clarification 5.9%) is observed at wavelengths of 530-540 nm (curve 3) with an optimal concentration of polyvinyl butyral equal to 1.0 wt.%. The optical film thickness is 120 nm, the refractive index is 1.34.

Example 2

The experimental conditions are the same as in example 1, however, tetrahydrofuran was used as a solvent, and polymethyl methacrylate was used as an additive. Figure 2 shows the light transmission curves of uncoated glass (curve 1) and glasses with double-sided single-layer coatings based on mesoporous silica, obtained by the sol-gel method in the presence of different concentrations of polymethyl methacrylate in ash: curve 6, additive - 1.0 wt.% curve 7 - 2.0 wt.%. The effect of enlightenment depends on the concentration of polymethyl methacrylate in the ash, first increases, passes through a maximum and then decreases with increasing polymer concentration (figure 2, curves 6, 7). A transmission maximum of 98.4% (7.3% bleaching) is observed at wavelengths of 540-550 nm (curve 6) with an optimal polymer concentration of 1.0% by weight. The optical film thickness of 137 nm, the refractive index of 1.26.

Example 3

The experimental conditions are the same as in example 2, however, a random copolymer of methyl methacrylate with hydroxyethyl methacrylate (5 mol%) was used as an additive. Figure 3 shows the light transmission curves of uncoated glass (curve 1) and glasses with double-sided single-layer coatings based on nanoporous silicon dioxide obtained by the sol-gel method in the presence of different concentrations of copolymer in ash: curve 8, additive 0.5 wt.%, curve 9 - 0.7 wt.%, curve 10 - 1.0 wt.%.

The enlightenment effect depends on the concentration of the copolymer in the ash, first increases, passes through a maximum and then decreases with increasing concentration of the additive (Fig. 3, curves 8-10). The maximum transmittance of 98.0% (clarification 6.9%) is observed at wavelengths of 580-590 nm (curve 9) with an optimal copolymer concentration of 0.7 wt.%. The optical film thickness of 150 nm, the refractive index of 1.285.

Example 4

The experimental conditions are the same as in example 2, however, a random copolymer of methyl methacrylate with methacrylic acid (20.0 mol%) was used as an additive. Figure 4 shows the light transmission curves of uncoated glass (curve 1) and glasses with double-sided single-layer coatings based on nanoporous silicon dioxide obtained by the sol-gel method in the presence of different concentrations of copolymer in ash: curve 11, additive 0.5 wt.%, curve 12 - 1.0 wt.%, curve 13 - 1.2 wt.%, curve 14 - 1.5 wt.%.

The effect of enlightenment depends on the concentration of the copolymer in the ash, first increases, passes through a maximum and then decreases with increasing concentration of the additive (figure 4, curves 11-14). The maximum transmittance of 98.1% (clarification 7.0%) is observed at wavelengths of 580-590 nm (curve 12) at an optimal concentration of 1.0 wt.%. The optical film thickness of 140 nm, the refractive index of 1.28.

Example 5

The experimental conditions are the same as in example 2, however, a random copolymer of methyl methacrylate with vinyl pyrrolidone (10 mol%) was used as an additive. Figure 5 shows the light transmission curves of uncoated glass (curve 1) and glasses with double-sided single-layer coatings based on mesoporous silicon dioxide obtained by the sol-gel method in the presence of different concentrations of copolymer in ash: curve 15, additive 0.5 wt.%, curve 16 - 0.7 wt.%, curve 17 - 1.0 wt.%, curve 18 - 1.5 wt.%. The effect of enlightenment depends on the concentration of the copolymer in the ash, first increases, passes through a maximum and then decreases with increasing concentration of the copolymer (Fig. 5, curves 15-18). The maximum transmittance of 98.4% (7.3% bleaching) is observed at wavelengths of 530-540 nm (curve 17) with an optimal copolymer concentration of 1.0 wt.%. The optical film thickness of 130 nm, the refractive index of 1.26.

Example 6

The experimental conditions are the same as in example 2, however, as additives, a statistical copolymer of methyl methacrylate with acrylamide (5 mol%) (Fig. 6, curve 19) and a statistical copolymer of methyl methacrylate with maleic anhydride (Fig. 6, curve 20 were used) ) The transmission maximums of 98.4% (7.3% bleaching) and 98.5% (7.4% bleaching) are observed at wavelengths of 540-550 nm with a copolymer concentration of 1.0% by weight. The optical film thickness of 132 nm, the refractive index of 1.26.

Example 7

The experimental conditions are the same as in example 1, however, polyvinyl acetate was used as an additive. Figure 7 shows the light transmission curves of uncoated glass (curve 1) and glasses with double-sided single-layer coatings based on mesoporous silica obtained by the sol-gel method in the presence of different concentrations of polyvinyl acetate in ash: curve 21, additive 1.0 wt.%, curve 22 - 1.5 wt.%. A transmission maximum of 98.6% (7.5% bleaching) is observed at a wavelength of 530 nm (curve 22) at a polymer concentration of 1.5% by weight. The optical film thickness is 136 nm, the refractive index is 1.25.

Thus, from the above examples it follows that the additives of carbochain polymers, random copolymers in the sol-gel process using the EISA technology at optimal concentration of compounds, which are proposed as organic compounds that determine the self-organization of inorganic-organic nanoscale structures in the resulting film, lead to the formation of nanoporous coatings based on silicon dioxide with a low refractive index of 1.25-1.34. These single-layer film coatings on silicate glass give a high antireflection effect.

Previously (CJBrinker et al., Nature, 1997, 389, 364; CJBrinker et al., US Pat. No. 5,858,457, 1999; CJBrinker et al., US Pat. No. 6,208,846, 2001; Y. Wan, D. Zhao, Chem. Rev., 2007, 107, 2821) similar low refractive index nanoporous film coatings based on mesoporous silica were obtained by the sol-gel process using the EISA technology only in the presence of hard-to-reach and expensive surfactants, for example, cationic surfactants, amphiphilic block copolymers.

Claims (1)

A method for producing thin 50-200 nm single-word antireflection coatings with a low refractive index of 1.25-1.34 based on mesoporous silica on silicate glass products with a transmission maximum of 97-98.6% in the visible spectral region, which includes a) the sol-gel process of silicon tetraalkoxide in the presence of an organic additive in a concentration of 0.1-5.0 wt.%, preferably 0.5-3.0 wt.% to the weight of the sol, using the technique of self-organization of nanostructures caused by evaporation of the solvent (EISA) b) heating the coated sample in an air atmosphere 300-600 ° C for several hours to thermal degradation of the organic additive, characterized in that
1) as an organic additive, which determines the spontaneous micro-separation of inorganic and organic phases during the formation of a solid coating on glass, polymers are used selected from the group:
carbochain polymers containing side ether groups (polyalkyl vinyl ethers, polyvinyl acetals), carbochain polymers containing side ester groups (polyether acrylates, polyester methacrylates, polyvinyl acetate), carbochain random copolymers containing ether groups, ester groups, alkyl copolymers, copolymers, alkyl copolymers copolymers of vinyl esters);
2) the organic additive has an optimal concentration in the ash, in which a transparent film of mesoporous silica with the maximum antireflection effect is formed in the sol-gel process, followed by heating the coated sample.

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