CN111856639A

CN111856639A - All-dielectric ultraviolet filter film

Info

Publication number: CN111856639A
Application number: CN202010681397.0A
Authority: CN
Inventors: 赵亚丽; 石亚慧; 李旭峰; 仇瑞新; 白雪敏
Original assignee: Jinzhong University
Current assignee: Hunan Meihong Photoelectric Technology Co ltd
Priority date: 2020-07-15
Filing date: 2020-07-15
Publication date: 2020-10-30
Anticipated expiration: 2040-07-15
Also published as: AU2021100257A4; CN111856639B

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

All-dielectric ultraviolet filter film

Technical Field

The invention relates to the technical field of film manufacturing, in particular to an all-dielectric ultraviolet filter film.

Background

With the rapid development of optical technology, ultraviolet communication is attracting much attention due to its unique advantages of being small and light. Various ultraviolet components and products are continuously introduced into the life of people, and can be widely applied in the range of life, production and the like. In order to meet the technical requirements of optical devices, ultraviolet optical films have been studied by scientists in recent years.

Researches show that in an ultraviolet communication system, the filter can be used for achieving the purposes of reducing signal attenuation and eliminating other nuclear magnetic interference so as to enhance an ultraviolet light signal. The ultraviolet filter plays an important and key role in a communication system, and the quality of the ultraviolet filter directly influences whether the ultraviolet 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 the ultraviolet filter is particularly important.

The traditional ultraviolet reflecting mirror is mainly realized by depositing a protective film outside metal, but has the serious defects of large ultraviolet absorption and low reflectivity. Subsequent studies have found that the absorption of the dielectric film is significantly reduced and the transparent region is also significantly broadened compared to metal. The requirements of ultraviolet high reflection and high passing of other wave bands in ultraviolet communication cannot be realized.

The existing general ultraviolet reflecting film structure is a metal-dielectric film, but the metal film has active reaction due to chemical properties, and has strong absorption oxidation capacity outside, so that the reflectivity can be influenced to a great extent, while the metal film is additionally coated with a protective film, so that the absorption can be reduced to a certain extent, but the absorption and scattering of the film are serious, the reflectivity is not enough, and the reflection is influenced by the conditions of scratches, corrosion and the like.

Disclosure of Invention

Aiming at the defects of the prior art, an all-dielectric ultraviolet filter film is provided.

The technical scheme adopted by the invention for solving the technical problems is as follows: constructing an all-dielectric ultraviolet filter film, wherein glass is used as a base material and two section materials with different refractive indexes are alternately laminated to form the all-dielectric ultraviolet filter film; 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.

Wherein, the high refractive index section material is hafnium oxide, and the low refractive index section material is magnesium fluoride.

The number of the alternately laminated layers of the high-refractive-index profile materials is 13, wherein 7 layers of the profile materials are hafnium oxide materials, and a layer of magnesium fluoride material is arranged between every two layers of the hafnium oxide materials.

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

The invention provides an all-dielectric ultraviolet filter film which is characterized in that glass is used as a base material and two section materials with different refractive indexes are alternately laminated to form the all-dielectric ultraviolet filter film; 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.

Drawings

The invention will be further described with reference to the accompanying drawings and examples, in which:

fig. 1 is a schematic structural diagram of an all-dielectric ultraviolet filter film provided by the present invention.

FIG. 2 is a schematic view of an equivalent interface of a single layer film in an all-dielectric UV filter according to the present invention.

FIG. 3 is a schematic view of an equivalent interface of a multi-layer thin film in an all-dielectric UV filter according to the present invention.

Detailed Description

For a more clear understanding of the technical features, objects and effects of the present invention, embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

Referring to fig. 1, the invention provides an all-dielectric ultraviolet filter film, which uses glass as a substrate 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 existing general ultraviolet reflecting film structure is a metal-dielectric film, but the metal film is more active due to chemical property reaction, and has strong absorption oxidation capacity outside, so that the reflectivity can be influenced to a great extent, while the absorption can be reduced to a certain extent by additionally plating a protective film on the metal film, but the absorption and scattering of the film are still serious, the reflectivity is not enough, and the reflection is influenced by the conditions of scratches, corrosion and the like. In view of the excellent properties of a dielectric film, such as wide transparent area and low absorption, the present disclosure aims to design an all-dielectric uv filter film with high uv reflectivity and high visible light transmittance.

Firstly, deriving the reflectivity of the single-layer dielectric film according to Maxwell equation set, wherein the derivation process is as follows:

as shown in fig. 2, the film (single layer) can exhibit two interfaces with an equivalent interface on the mathematical locus plane, and the combined admittance coefficient of the film layer and the substrate is Y, so that:

H₀＝Y(k₀×E₀) (1)

in the formula

The amplitude reflection coefficient of the single-layer film is:

wherein eta is₀Theoretically, the transmittance and reflectance of the single-layer film can be deduced by calculating the combined admittance coefficient Y.

At the interface 1, using the electromagnetic field boundary conditions, one can obtain:

several points are taken on the inner side surfaces of the interface 1 and the interface 2 for analysis, so that the fact that the abscissa is fixed and the complex amplitude intensity of the electromagnetic field of the two points with different ordinates and the phase difference introduced by the space distance have a certain relation can be easily found, namely that

Therefore, the first and second electrodes are formed on the substrate,

written as a matrix:

at the interface 2, then

Thus:

written in matrix form as:

substituting formula (15) for formula (10) to obtain:

and because:

H₀＝Y(k₀×E₀)，H₂＝η₂(k₀×E₂) (17)

equation (16) can be written as:

order to

Then

Get it solved

Wherein the matrix

Uniquely determined by film parameters, called feature matrix, matrix of the film

Is a second order matrix determined entirely by the membrane system and substrate parameters.

Obviously, composed of

Obtaining:

therefore, the reflection coefficient r of the single-layer dielectric film can be calculated as follows:

and calculating the reflectivity R of the single-layer dielectric film as follows:

based on the calculation research of the characteristics of the single-layer dielectric film, recursive matrix calculation can be performed on the multilayer dielectric film, and the derivation schematic diagram is shown in fig. 3.

In

interfaces

1, 2,

interfaces

2, 3, by applying boundary conditions:

repeating the whole process until the interfaces K and K +1 obtain:

since the tangential component of each interface is continuous, there are:

after successive linear transformations, we obtain:

because Y is H₀/E₀And no backward wave, η, in the substrate _k+1＝H_k+1/E_k+1Therefore, it is

The characteristic matrix of the film system is then:

the research of the single-layer film combined admittance is carried out, so that the combined admittance formula of the multi-layer film can be obtained: y ═ C/B, then k-layer films R (reflectance) and T (transmittance) are:

the interference type cut-off filter means that a highly transparent light beam is present in a wavelength range, and the deviated light beam is abruptly and highly reflected. The long-wave pass filter refers to short-wave region inhibition and long-wave region transmission; similarly, the short-wave pass filter refers to transmission in the short-wave region and suppression in the long-wave region.

The film system underlying the cut-off filter (interference type) is a lambda/4 periodic film stack (LH)^sWherein L and H each represent low refractionThe film system adopting the structure can realize a series of high-transmission interval bands with high reflection band intervals. Can be represented by (LH)^sAnd the film layer structure design realizes the independent design of the reflection band and the transmission band.

When selecting materials, the physical and chemical properties of the material, such as refractive index, transparent region, purity of the material itself, density, melting point, etc., and the chemical reaction and matching degree with other materials, such as matching stress, thermal stability, firmness, etc., should be considered comprehensively, and of course, the absorption problem is also a very important issue for the material used as the uv film.

In practical production, the number of materials that can be used to prepare the uv film is limited, and commonly used materials include hafnium oxide, aluminum oxide, gadolinium fluoride, magnesium fluoride, lanthanum fluoride, neodymium fluoride, and aluminum fluoride. The material characteristics, namely the matching between materials, the preparation process conditions, the stability and the like are comprehensively considered, hafnium oxide is selected as a high-refractive-index material, and magnesium fluoride is selected as a low-refractive-index material.

Hafnium oxide (HfO)₂) Essentially one of a ceramic material, having a density of 9.68g/cm²White powder, with a melting point of 2812 ℃, a molecular weight of 210.49, wide band gap, high dielectric constant, corrosion resistance, stable physical and chemical properties and other excellent properties, is often widely used in the fields of radioactivity resistance, fire resistance and catalysis, and is a good high-refractive-index material because of high refractive index, low extinction coefficient and high laser damage threshold, so that the material is frequently used in the optical industry. However, in the thousands of natural substances, zirconium which is naturally grown with zirconium is likely to undergo chemical reaction due to its similar chemical properties, and affects the performance of zirconium to some extent, such as the purity, and the absorption of ultraviolet band is strongly interfered, especially when the wavelength is less than 250nm (the transparent band of zinc oxide starts from 250 nm). In addition, impurities in hafnium oxide have some effects indirectly or directly on the optical properties, film-forming quality, etc., and particularly in the ultraviolet range, the impurities are more strongly affected, so that hafnium oxide materials used in the ultraviolet range are oxidized The specific gravity of the zinc impurities must be less than 0.5%.

Magnesium fluoride (MgF)₂) The material has a boiling point of 2239 ℃, is insoluble in water, has a melting point of 1395 ℃, and has a transparent region of 0.21-10 mu m, and is particularly suitable for ultraviolet and infrared spectrums due to good performance of crystal polarization. The magnesium fluoride film layer is plated outside the optical component, so that the reflection (thin film interference) of incident light on a lens interface can be reduced, the high transmittance is realized in ultraviolet and infrared, the refractive index and the extinction coefficient are relatively low, and the excellent characteristics make the optical component become an optical material commonly used in an ultraviolet region.

According to the difficulty of preparation, when designing a film system, firstly, the simplification of the structure of the film system is considered, and the performance requirement of the optical filter is met by reducing the film layers as much as possible; if the thickness of the single layer film is too thick, the stress is large and the mold release is likely to occur.

The invention adopts the basic film system G | (LH) ^ s | AIR of the interference filter as the basic film system structure, and the hafnium H oxide (HfO)₂) Is a high refractive index profile with wide band gap and high dielectric constant, and is L-magnesium fluoride (MgF) ₂) Low refractive index material with good polarization, G (K9 glass) as substrate, s cycles, AIR. The basic membrane system based on the comprehensive design of the coefficients cannot completely meet the use requirements on transmittance and reflectivity, and based on the use requirements, TFCalc membrane system design software is introduced to optimize the basic membrane system by a simplex method, so that an ideal spectral curve is designed.

By analyzing the single-layer dielectric film reflectivity formula, the refractive index is n_GIs plated with an optical thickness of lambda₀High refractive index (n) of/4₁) After the film layer, the reflectivity is increased for the central wavelength lambda₀Admittance of the monolayer film and substrate combination

The reflectivity at normal incidence is:

it is obvious that

The larger the reflectivity, but the actual film refractive index (n)₁) Is limited and theoretically the maximum achievable single layer film reflectivity is below 50%.

If each layer thickness d is lambda₀The higher reflectivity is achieved with multilayer dielectric films that alternate between high and low refractive index, primarily because the reflection of the light back to the same phase at the front surface at the film interface creates relatively long interference, and theoretically it is possible to achieve reflectivity approaching 100% indefinitely for such dielectric film systems.

If n is_HRepresents a high refractive index, n_LRepresenting a low refractive index, the two outermost layers of the dielectric film system are high refractive index layers, and the thickness d of each layer is

I.e., G | H (LH) s | A, for a central wavelength λ₀Comprises the following steps:

thereby centering the center wavelength lambda at normal incidence₀The reflectance and transmittance of (a) are:

obviously, n_H/n_LThe larger the ratio or the larger the number of layers (2S +1), the larger the value of R and the smaller the value of T. Through analysis, in a 210-260 nm wave band, when the number of layers is more than 12, the reflectivity of the film reaches 95%, and after the reflectivity reaches 16 layers, the film is addedThe layer does not provide a significant improvement in reflectivity. Theoretically, the reflectivity approaches to a value of 1 as the number of layers of the film system increases, and the number of oscillation ripples in the 280-700 nm band increases with the increase of the number of layers, so that the transmittance of the band decreases, and finally, a 13-layer film structure is selected as the dielectric film, wherein the film structure is G | h (lh) 6| AIR.

In actual production, in addition to the influence of the number of layers on the film forming properties, the influence of the film thickness on the film forming properties must be considered. When the total thickness of the film layer is too thick, the sensitivity of a part of the film layer is increased, the overall fluctuation is large, and errors generated in the preparation process are multiplied along with the increase of the number of the layers. In order to minimize such effects, the physical thickness satisfying the conditions was found by studying the effect of changing the physical thickness of the high and low refractive index materials on the UV reflection and visible transmission based on the film structure G | H (LH) 6| AIR under the condition of normal incidence of light.

Based on the film-system structure G | H (LH) 6| AIR, without changing the physical thickness (d) of the low-refractive index material magnesium fluoride_L47nm), the reflectance and transmittance of the high refractive index material hafnium dioxide over the wavelength band of 200-800 nm were calculated by the TFCalc film system design software with 20nm as the minimum physical thickness and 5nm as the interval, and the results are shown in table 1:

film thickness (nm)	20	25	30	35	40
						Average reflectivity (210 to 260nm)	56.90％	93.82％	91.30％	64.18％	32.57％
Average transmittance (280 to 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 for different hafnium oxide film thicknesses

The central wavelength of the reflection band gradually shifts to the long-wave direction on the wave band of 210-260 nm along with the increase of the film thickness, and as can be seen from table 1, the average reflectivity firstly rises and then falls, and the average reflectivity of 25nm and 30nm is higher than that of other thicknesses and reaches 90 percent; the number of oscillation ripples with the thickness of 30nm between 280 nm and 700nm is obviously more than that of oscillation ripples with the thickness of 25nm, and the average transmittance is obviously reduced by combining the table 1; thus 25nm was chosen as the physical thickness of the high index material hafnium oxide.

Based on the film system structure G | H (LH) 6| AIR, without changing the physical thickness (d) of the high refractive index material hafnium oxide_H33nm), the reflectance and transmittance of the low refractive index material magnesium fluoride over the wavelength band of 200 to 800nm were investigated by TFCalc film system design software with 35nm as the minimum physical thickness and 5nm as the interval, and the results are shown in table 2:

Film thickness (nm)	35	40	45	50	55
						Average reflectivity (210 to 260nm)	92.20％	96.18％	84.05％	66.97％	47.32％
Average transmittance (280 to 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 for different magnesium fluoride film thicknesses

As can be seen from Table 2, the increase of the film thickness causes the central wavelength of the reflection band to gradually shift towards the long-wave direction, and the average reflectivity of two curves with the thicknesses of 35nm and 40nm reaches 92% within the range of 210-260 nm; the number of moire oscillations with a thickness of 35nm between 280 and 700nm is significantly greater than 40nm, and it can be seen from table 2 that the average transmission with a thickness of 40nm is higher than 35nm and 90.45%, so 40nm is selected as the physical thickness of the low refractive index material magnesium fluoride.

The film system structure was finally determined to be G | H (LH) 6| AIR by the above studies, wherein H has a thickness of 25nm and L has a thickness of 40 nm. The film system is input into the TFCalc film system design software to be optimized by a simplex method, and the reflectance, the transmittance and the thickness of each film before optimization are compared to finally obtain the table 3.

TABLE 3 film thickness (nm) comparison before and after optimization

The optimized film system is (G |0.69H 0.83L 0.96H 0.86L 0.93H 0.96L 0.89H 0.95L 0.97H0.86L 0.94.94H 1.07L 0.52H | AIR), is an aperiodic film system, has the total physical thickness of 450nm, and has 13 layers, the finally obtained theoretical reflectivity and transmissivity, the average reflection value in the waveband range of 210-260 nm is 95.23%, and the average transmissivity in the 280-700 nm range is 96.68%.

According to the all-dielectric ultraviolet filter film, the reflectivity of the film system is gradually improved and the transmissivity is gradually reduced along with the increase of the number of the film layers, but the reflectivity is always larger than an even number when the number of the film layers is an odd number; when the number of the film layers is more than 12, the reflectivity of the film layers reaches 95% in the wave band of 210-260 nm; in summary, a 13-layer film structure is finally selected as the dielectric film, and the film system structure is G | H (LH) 6| AIR.

In a wave band of 210-260 nm, the central wavelength of the reflection band gradually shifts to the long wave direction along with the increase of the film thickness; in the range of 280-700 nm, the transmissivity of the high refractive index material (hafnium oxide) is gradually reduced and the transmissivity of the low refractive index material (magnesium fluoride) is gradually improved along with the increase of the film thickness. Through research and analysis, 25nm and 40nm are finally selected as the physical thicknesses of the high-refractive-index material hafnium oxide and the low-refractive-index material magnesium fluoride respectively.

Finally, a simplex method in TFCalc film system design software is used for optimization, and the full-medium ultraviolet filter film with the average reflectivity of 210-260 nm wave bands of 95.23% and the average transmittance of 280-700 nm wave bands of 96.68% is designed, wherein the film structure of the full-medium ultraviolet filter film is (G |0.68H 0.82L 0.97H 0.85L 0.93H 0.97L 0.88H 0.95L 0.98H 0.84L 0.94 35 0.94H1.11L 0.47.47H | AIR), the total physical thickness is 452.12nm, and the full-medium ultraviolet filter film is an aperiodic film system.

While the present invention has been described with reference to the embodiments shown in the drawings, the present invention is not limited to the embodiments, which are illustrative and not restrictive, and it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. The all-dielectric ultraviolet filter film is characterized in that glass is used as a base material, and two profile materials with different refractive indexes are alternately laminated to form the all-dielectric ultraviolet filter film; 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.

2. The all-dielectric ultraviolet filter film of claim 1, wherein the high refractive index profile material is hafnium oxide and the low refractive index profile material is magnesium fluoride.

3. The all-dielectric ultraviolet filter film of claim 2, wherein the number of the alternately stacked high-refractive-index profile materials is 13, 7 of the layers are hafnium oxide materials, and a layer of magnesium fluoride material is arranged between each two layers of hafnium oxide materials.

4. The all-dielectric ultraviolet filter film of claim 2, wherein the thickness of the hafnium oxide material thin film is set to 25 nm, and the thickness of the magnesium fluoride material thin film is set to 40 nm.