CN111856639A - An all-dielectric UV filter film - Google Patents

Abstract

The invention relates to an all-dielectric ultraviolet filter film, which takes glass as a base material and is formed by alternately laminating two section materials with different refractive indexes; the two different refractive index profile materials are a high refractive index profile material with broadband gap and high dielectric and a low refractive index material with good polarization. The all-dielectric ultraviolet filter film is a filter film which is based on the theoretical research result of a film, adopts a simplex optimization method, and is high in reflection at 210-260 nm and high in transmission at 280-700 nm through analysis and correction of the number of layers and the thickness of film layers in a film system structure.

Description

An all-dielectric UV filter film

technical field

The invention relates to the technical field of thin film manufacturing, and more particularly, to an all-dielectric ultraviolet filter film.

Background technique

With the rapid development of optical technology, ultraviolet communication has attracted much attention due to its unique advantages of small size and light weight. Various UV components and products are constantly entering people's lives and are widely used in life and production. In order to meet the technical demands of optical devices, in recent years, ultraviolet optical thin films have also been extensively studied by scientists.

The study found that in the ultraviolet communication system, the use of filters can achieve the purpose of reducing signal attenuation and eliminating other nuclear magnetic interference to enhance the ultraviolet light signal. The UV filter plays an important and critical role in the communication system, and its quality will directly affect whether the UV light communication system can work normally. To a certain extent, the development of UV filters directly determines the development level of UV communication systems. Therefore, the research on UV filters is particularly important.

In the past, UV mirrors were mainly realized by depositing a layer of protective film on the metal, but there were serious deficiencies in the absorption of UV light and the low reflectivity. Subsequent studies found that the absorption of the dielectric film was significantly lower than that of the metal, and the transparent area was also significantly wider. Ultraviolet communication requirements of high UV reflection and high pass in other bands cannot be achieved.

The current general UV reflective film structure is a metal-dielectric film, but the metal film is more reactive due to its chemical properties, and has strong absorbing and oxidizing ability outside, which will greatly affect the reflectivity, and a layer of coating is added on the metal film. Although the protective film can reduce the absorption to a certain extent, the absorption and scattering of the film itself are still very serious, and the reflectivity is not enough. It is also prone to scratches, rust and other conditions that affect the reflection. Considering that the dielectric film has a wide transparent area and absorbs Few good properties.

SUMMARY OF THE INVENTION

Aiming at the deficiencies of the prior art, an all-dielectric UV filter film is proposed.

The technical solution adopted by the present invention to solve the technical problem is as follows: constructing an all-dielectric UV filter film, using glass as the base material, and consisting of two different refractive index profiles alternately stacked; wherein, the two different refractive index profiles The material is a high-refractive-index profile material with broadband gap, high dielectric and a low-refractive-index material with good polarization.

Among them, hafnium dioxide is selected as the material of the high-refractive index profile, and magnesium fluoride is selected as the material of the low-refractive index profile.

Among them, the number of alternately stacked layers of high and low refractive index profile materials is set to 13 layers, of which 7 layers are hafnium dioxide materials, and a layer of magnesium fluoride material is arranged between every two layers of hafnium dioxide materials.

The thickness of the hafnium dioxide material film is set to 25 nm, and the thickness of the magnesium fluoride material film is set to 40 nm.

Different from the prior art, the present invention provides an all-dielectric UV filter film, which uses glass as a base material and is composed of two different refractive index profile materials alternately stacked; wherein, the two different refractive index profile materials have both broadband gaps. , high dielectric high refractive index profile material and low refractive index material with good polarization. The all-dielectric UV filter film of the present invention is based on the theoretical research results of thin films, adopts the method of simplex optimization, and analyzes and corrects the number of layers and thicknesses of the film layers in the film structure. 280 ~ 700nm high transmission filter film.

Description of drawings

The present invention will be further described below in conjunction with the accompanying drawings and embodiments, in which:

FIG. 1 is a schematic structural diagram of an all-dielectric UV filter provided by the present invention.

2 is a schematic diagram of an equivalent interface of a single-layer film in an all-dielectric UV filter provided by the present invention.

3 is a schematic diagram of an equivalent interface of a multilayer film in an all-dielectric UV filter provided by the present invention.

Detailed ways

In order to have a clearer understanding of the technical features, objects and effects of the present invention, the specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

Referring to FIG. 1, the present invention provides an all-dielectric UV filter film, which uses glass as a base material and is composed of two different refractive index profile materials alternately stacked; wherein, the two different refractive index profile materials have both broadband gaps , high dielectric high refractive index profile material and low refractive index material with good polarization.

Among them, hafnium dioxide is selected as the material of the high-refractive index profile, and magnesium fluoride is selected as the material of the low-refractive index profile.

Among them, the number of alternately stacked layers of high and low refractive index profile materials is set to 13 layers, of which 7 layers are hafnium dioxide materials, and a layer of magnesium fluoride material is arranged between every two layers of hafnium dioxide materials.

The thickness of the hafnium dioxide material film is set to 25 nm, and the thickness of the magnesium fluoride material film is set to 40 nm.

The current general UV reflective film structure is a metal-dielectric film, but the metal film is more reactive due to its chemical properties, and has strong absorbing and oxidizing ability outside, which will greatly affect the reflectivity, and a layer of coating is added on the metal film. Although the protective film can reduce the absorption to a certain extent, the absorption and scattering of the film itself are still very serious, and the reflectivity is not enough, and it is also prone to scratches, rust and other conditions that affect the reflection. In view of the excellent properties of the dielectric film with wide transparent area and little absorption, this paper aims to design an all-dielectric UV filter film with high reflectivity to ultraviolet light and high transmittance of visible light.

First, the reflectivity of a single-layer dielectric film is deduced according to Maxwell's equations. The deduction process is as follows:

As shown in Figure 2, the film (single layer) can display the two interfaces as equivalent interfaces on the mathematical plane. The combined admittance coefficient of the film layer and the substrate is Y, so:

H ₀ =Y(k ₀ ×E ₀ ) (1)

in the formula

The amplitude reflection coefficient of the monolayer film is:

Among them, η ₀ is the admittance of the incident medium. Theoretically, the transmittance and reflectivity of the single-layer film can be deduced by calculating the combined admittance coefficient Y.

At interface 1, using the electromagnetic field boundary condition we get:

Take several points on the inner side of interface 1 and interface 2 to analyze, it is easy to find that there is a certain relationship between the complex amplitude strength of the electromagnetic field at two points with different ordinates and the phase difference introduced by the spatial distance, that is,

so,

Written as a matrix as:

In interface 2, there is

therefore:

Written in matrix form as:

Substitute equation (15) into equation (10) to get:

also because:

H ₀ =Y(k ₀ ×E ₀ ), H ₂ =η ₂ (k ₀ ×E ₂ ) (17)

Equation (16) can be written as:

make

but

Solutions have to

由膜层参数唯一确定，称为该膜层的特征矩阵，矩阵

是完全由膜系和基底参数决定的二阶矩阵。where the matrix

It is uniquely determined by the film parameters, called the characteristic matrix of the film, the matrix

is a second-order matrix completely determined by the film and substrate parameters.

得:Obviously, by

have to:

From this, the reflection coefficient r of a single-layer dielectric film can be calculated as:

Then, the reflectivity R of the single-layer dielectric film is calculated as:

With the basis of the calculation and research of the characteristics of the single-layer dielectric film, the recursive matrix calculation can be carried out for the multilayer dielectric film. The schematic diagram of the derivation is shown in Figure 3.

In interface 1, 2, interface 2, 3, by applying boundary conditions, we can get:

Repeat the whole process until the interface K and K+1, you can get:

Since the tangential components of each interface are continuous, there are:

After continuous linear transformation, we get:

Since Y=H ₀ /E ₀ and there is no reverse wave in the base, η _k+1 =H _k+1 /E _k+1 , so

So the characteristic matrix of the film system is:

After deliberation on the combined admittance of the single-layer film, the general formula of the combined admittance of the multi-layer film can be obtained: Y=C/B, then the R (reflectance) and T (transmittance) of the k-layer film are:

Interferometric cut-off filter means that there is a high transmittance beam in the wavelength range, and the deviated beam suddenly becomes highly reflective. Long-pass filters refer to short-wave suppression and long-wave transmission; similarly, short-pass filters refer to short-wave transmission and long-wave suppression.

The basic film system of the cut-off filter (interference type) is λ/4 periodic film stack (LH) ^s , where L and H represent low refractive index and high refractive index materials respectively. The film system with this structure can be used. The realized series has high transmission interval bands spaced apart by high reflection bands. The independent design of the reflection band and the transmission band can be achieved through the design of the (LH) ^s film layer structure.

When selecting materials, it is necessary to comprehensively consider the refractive index of the material, the transparent area, the material's own purity, density, melting point and other physical and chemical properties, as well as the chemical reaction and matching degree with other materials, such as matching stress, thermal stability. , firmness and other issues, of course, for the materials used as ultraviolet films, the absorption problem also needs special attention, and it is an extremely important part.

In actual production, the number of materials that can be used to prepare UV thin films is limited. Commonly used materials are hafnium dioxide, aluminum oxide, gadolinium fluoride, magnesium fluoride, lanthanum fluoride, neodymium fluoride, and aluminum fluoride. Taking into account the material characteristics, that is, the matching between materials, the process conditions and stability of the preparation, etc., hafnium dioxide is selected as the high refractive index material, and magnesium fluoride is used as the low refractive index material.

Hafnium dioxide (HfO ₂ ) is essentially a kind of ceramic material, with a density of 9.68g/cm ² , white powder, melting point of 2812°C, molecular weight of 210.49, with wide band gap and high dielectric constant, corrosion resistance , stable physical and chemical properties and other excellent properties, are often widely used in anti-radiation, fire resistance, catalysis, because of its high refractive index, low extinction coefficient, high laser damage threshold, so it is widely used in the optical industry, it is very Good high refractive index material. But among the thousands of things in nature, there is indeed a zirconium that is related to it. Because of its similar chemical properties, it is prone to chemical reactions, which affects its performance to a certain extent, such as its purity and strong absorption in the ultraviolet band. The interference is especially obvious when the wavelength is less than 250nm (the transparent band of zinc oxide starts from 250nm). In addition, as an impurity, it also indirectly or directly affects the optical properties of hafnium dioxide, the quality of film formation, etc., especially in the ultraviolet band, the impact is greater, so the hafnium dioxide used in the ultraviolet band The proportion of zinc oxide impurities in the material must be less than 0.5%.

Magnesium fluoride (MgF ₂ ) has a boiling point of 2239°C, insoluble in water, a melting point of 1395°C, and a transparent region of 0.21 to 10 μm. It is especially suitable for ultraviolet and infrared spectroscopy due to its good performance of crystal polarization. Coating magnesium fluoride film on the outside of optical components can reduce the reflection of the incident light (thin film interference) at the lens interface. It has high transmittance in ultraviolet and infrared, and the refractive index and extinction coefficient are relatively low. These excellent characteristics make it a commonly used optical material in the ultraviolet region.

According to the difficulty of preparation, when designing the film system, we should first consider simplifying the film system structure, and reduce the film layer as much as possible to meet the performance requirements of the optical filter. When reducing, we should also pay attention to keeping the thickness uniform. The primary purpose is to In order to avoid the thickness of the single-layer film being too thick or too thin. If the thickness is too thin, the thickness will be difficult to precisely control, which increases the difficulty of the manufacturing process; if the thickness of the single-layer film is too thick, the stress will be too large and easy to cause demoulding.

The invention adopts the basic film system G|(LH)^s|AIR of the interference filter as the basic film system structure, and H hafnium dioxide (HfO ₂ ) is a high refractive index profile with both wide band gap and high dielectric. L magnesium fluoride (MgF ₂ ), a low refractive index material with good polarization, G (K9 glass) as the base material, s period number, AIR air. The basic film system based on the comprehensive design of the above coefficients cannot fully meet the application requirements in terms of transmittance and reflectivity. Based on this, the TFCalc film system design software is introduced to optimize it by the simplex method, so as to obtain an ideal spectral curve. the design of.

垂直入射的反射率为：By analyzing the reflectivity formula of a single-layer dielectric film, it can be seen that after a high refractive index (n ₁ ) film with an optical thickness of λ ₀ /4 is coated on a substrate with a refractive index of n _G , the reflectivity increases. For the central wavelength λ ₀ , the admittance of the combination of monolayer film and substrate is

The reflectivity at normal incidence is:

越大，反射率越高.但是实际中的膜层折射率(n₁)是有限的，理论上可达到的单层膜反射率最大值低于50％。obviously

The larger the reflectivity, the higher the reflectivity. However, the refractive index (n ₁ ) of the film layer in practice is limited, and the theoretically attainable maximum reflectivity of a single-layer film is lower than 50%.

If the thickness d of each layer is λ ₀ /4, and the multilayer dielectric film alternates between high and low refractive index, higher reflectivity can be obtained. This is mainly because the reflected light beam from the film interface returns to the same phase on the front surface. Where there is a relatively long interference, for such a dielectric film system, it is theoretically possible to obtain a reflectivity that is close to 100% infinity.

即G|H(LH)^s|A，对于中心波长λ₀有:

因而垂直入射时对中心波长λ₀的反射率和透射率为：If n _H represents high refractive index and n _L represents low refractive index, the outermost layers on both sides of the dielectric film system are high refractive index layers, and the thickness d of each layer is

That is, G|H(LH)^s|A, for the central wavelength λ ₀ :

Therefore, the reflectivity and transmittance for the central wavelength λ ₀ at normal incidence are:

Obviously, the larger the ratio of n _H /n _L or the more layers (2S+1), the larger the value of R will be, and the smaller the value of T will be. Through analysis, in the 210-260nm band, when the number of layers is greater than 12 layers, the reflectivity reaches 95%, and after reaching 16 layers, adding additional layers cannot significantly improve the reflectivity. Theoretically speaking, the more the number of layers in the film system, the closer the reflectivity will be to the value of 1. With the increase of the number of layers, the number of oscillating ripples in the 280-700nm band will increase, resulting in the transmittance of this band. It is getting lower and lower, and after comprehensive consideration, the 13-layer film structure is finally selected as the dielectric film, and its film system structure is G|H(LH)^6|AIR.

In the actual preparation, in addition to the influence of the number of layers on the film-forming properties, the influence of the film thickness on it must also be considered. When the total thickness of the film layer is too thick, the sensitivity of some film layers will also increase, and the overall fluctuation will be large, and the error generated during the preparation process will multiply with the increase of the number of layers. In order to minimize such effects, the effects of changing the physical thickness of high and low refractive index materials on UV reflection and visible transmission were studied based on the film structure G|H(LH)^6|AIR under the condition of normal incidence of light. , find the physical thickness that satisfies the condition.

Based on the film structure G|H(LH)^6|AIR, without changing the physical thickness (d _L = 47nm) of the low-refractive-index material magnesium fluoride, the high-refractive-index material was oxidized by TFCalc film system design software. Hafnium takes 20nm as the minimum physical thickness and calculates its reflectivity and transmittance in the 200-800nm band at intervals of 5nm. The results are shown in Table 1:

Film thickness (nm) 20 25 30 35 40 Average reflectance (210～260nm) 56.90% 93.82% 91.30% 64.18% 32.57% Average transmittance (280～700nm) 94.27% 92.30% 90.10% 84.87% 79.24%

Table 1 Comparison of average reflectance at 210-260 nm and average transmittance at 280-700 nm under different hafnium dioxide film thicknesses

It is found that with the increase of film thickness, in the 210-260nm band, the center wavelength of the reflection band gradually shifts to the long-wave direction. And it can be seen from Table 1 that the average reflection first rises and then falls, and the average reflection with thicknesses of 25nm and 30nm Compared with other thicknesses, the transmittance rate is higher than that of other thicknesses, reaching 90%; the number of oscillating ripples with a thickness of 30nm between 280 and 700nm is significantly more than that of 25nm. Combined with Table 1, it is found that the average transmittance is significantly reduced; therefore, 25nm is selected as the high The physical thickness of the refractive index material hafnium dioxide.

Based on the film structure G|H(LH)^6|AIR, the low refractive index material was fluorinated by TFCalc film system design software without changing the physical thickness of the high refractive index material hafnium dioxide (d _H = 33 nm). The minimum physical thickness of magnesium is 35nm, and its reflectivity and transmittance in the 200-800nm band are studied at intervals of 5nm. The results are shown in Table 2:

Film thickness (nm) 35 40 45 50 55 Average reflectance (210～260nm) 92.20% 96.18% 84.05% 66.97% 47.32% Average transmittance (280～700nm) 90.15% 90.45% 88.58% 85.80% 82.73%

Table 2 Comparison of average reflectance at 210-260 nm and average transmittance at 280-700 nm under different magnesium fluoride film thicknesses

It can be seen from Table 2 that the increase of the film thickness makes the central wavelength of the reflection band gradually shift to the long wave direction. In the wavelength range of 210-260 nm, the average reflectivity of the two curves with thicknesses of 35 nm and 40 nm has reached 92%. ; The number of ripple oscillations with a thickness of 35nm between 280 and 700nm is significantly more than that of 40nm, and it can also be seen from Table 2 that the average transmittance with a thickness of 40nm is higher than 35nm, which is 90.45%. Therefore, 40nm is selected as the low refractive index material Physical thickness of magnesium fluoride.

Through the above research, the film structure was finally determined as G|H(LH)^6|AIR, where the thickness of H was 25 nm and the thickness of L was 40 nm. The film system was input into the TFCalc film system design software for optimization by the simplex method, and compared with the reflectivity, transmittance and thickness of each film before optimization, and Table 3 was finally obtained.

Table 3 Comparison of film thickness (nm) before and after optimization

The optimized film is (G|0.69H 0.83L 0.96H 0.86L 0.93H 0.96L 0.89H 0.95L 0.97H0.86L 0.94H 1.07L 0.52H|AIR), which is a non-periodic film with a total physical thickness of 450nm. 13 layers, the theoretical reflectivity and transmittance finally obtained, the average reflectance in the wavelength range of 210-260nm is 95.23%, and the average transmittance at 280-700nm is 96.68%.

For the all-dielectric UV filter film of the present invention, with the increase of the number of film layers, the reflectivity of the film system gradually increases, and the transmittance gradually decreases, but the reflectivity is always greater than the even number when the number of film layers is an odd number; When it is more than 12 layers, its reflectivity reaches 95% in the wavelength range of 210-260 nm; after comprehensive consideration, a 13-layer film structure is finally selected as the dielectric film, and its film structure is G|H(LH)^6|AIR.

In the 210-260 nm band, the central wavelength of the reflection band gradually shifts to the long-wave direction with the increase of the film thickness; in the range of 280-700 nm, with the increase of the film thickness, the transmittance of the high refractive index material (hafnium dioxide) gradually increases. decrease, the transmittance of the low refractive index material (magnesium fluoride) gradually increases. After research and analysis, 25nm and 40nm were selected as the physical thickness of high refractive index material hafnium dioxide and low refractive index material magnesium fluoride respectively.

Finally, the simplex method in the TFCalc film system design software was used for optimization, and an all-dielectric UV filter film with an average reflectivity of 95.23% in the 210-260nm band and an average transmittance of 96.68% in the 280-700nm band was designed. The structure is (G|0.68H 0.82L 0.97H 0.85L 0.93H 0.97L 0.88H 0.95L 0.98H 0.84L 0.94H1.11L 0.47H|AIR), the total physical thickness is 452.12nm, and it is a non-periodic film system.

The embodiments of the present invention have been described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific embodiments, which are merely illustrative rather than restrictive. Under the inspiration of the present invention, without departing from the scope of protection of the present invention and the claims, many forms can be made, which all belong to the protection of the present invention.

Claims (4)

1. an all-medium ultraviolet filter film, it is characterized in that, described all-medium ultraviolet filter film takes glass as base material, is formed by alternately stacking the profile material of two kinds of different refractive indices; wherein, two kinds of different refractive index profiles The material is a high-refractive-index profile material with broadband gap, high dielectric and a low-refractive-index material with good polarization. 2 . The all-dielectric UV filter film according to claim 1 , wherein the high-refractive-index profile material is selected from hafnium dioxide, and the low-refractive-index profile material is selected from magnesium fluoride. 3 . 3. The all-dielectric UV filter film according to claim 2, wherein the number of layers of alternately stacked profile materials of high and low refractive index is set to 13 layers, of which 7 layers are hafnium dioxide materials, and every two layers of carbon dioxide A layer of magnesium fluoride material is arranged between the hafnium materials. 4 . The all-dielectric UV filter film according to claim 2 , wherein the thickness of the hafnium dioxide material film is set to 25 nm, and the thickness of the magnesium fluoride material film is set to 40 nm. 5 .

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