CN111399103A

CN111399103A - Low-stress multilayer thin film optical filter and preparation method thereof

Info

Publication number: CN111399103A
Application number: CN202010323455.2A
Authority: CN
Inventors: 吴江波; 刘璐; 艾曼灵; 金波; 郑臻荣; 顾培夫
Original assignee: Hangzhou Koti Optical Technology Co ltd
Current assignee: Hangzhou Koti Optical Technology Co ltd
Priority date: 2020-04-22
Filing date: 2020-04-22
Publication date: 2020-07-10
Anticipated expiration: 2040-04-22
Also published as: CN111399103B

Abstract

The invention discloses a low-stress multilayer thin film optical filter and a preparation method thereof, wherein a substrate is 3.3 borosilicate glass, the multilayer thin film optical filter is a double-half-wave narrow-band interference optical filter, and the linear expansion coefficient of the 3.3 borosilicate glass is 3.3 × 10^‑6Degree/degree; in order to reduce the junction stress of the filter, two technologies of substrate heating and ion assistance are adopted. When the substrate temperature is 250 ℃, TiO₂And SiO₂The concentration density of the film is close to 1 and 0.92 respectively, and the junction accumulated tensile stress of the optical filter is 10.8 MPa; if the ion energy is chosen to be 350eV, the ratio J of the number of assist ions reaching the substrate to the number of deposited molecules is maintained_I/J_MTo TiO 2₂And SiO₂Film of 0.15 and 0.05, respectively, then TiO₂And SiO₂The concentration densities of (A) and (B) are close to 1 and 0.96 respectively, and the junction accumulated compressive stress of the optical filter is-7 MPa. The low stress thin film filter can be widely applied to various photoelectric instruments.

Description

Low-stress multilayer thin film optical filter and preparation method thereof

Technical Field

The invention relates to the field of optical thin films, in particular to a low-stress multilayer thin film optical filter and a preparation method thereof.

Background

Optical thin films formed from bulk solid materials by vacuum thermal evaporation or sputtering are typically very stressed. The film stress is composed of three parts of surface tension, thermal stress and internal stress. Since the surface tension is small and the surface tension of different materials does not vary much, it can be ignored. The thermal stress is caused by the difference of the thermal expansion coefficients of the substrate and the film due to the temperature difference between the film preparation and the film cooling to room temperature after the preparation, and if the thermal expansion coefficient of the substrate is not equal to that of the film, the thermal stress is generated certainly, and the thermal stress is mostly expressed as tensile stress. The internal stress is also called intrinsic stress, and mainly depends on a plurality of factors such as microstructure and defects of the film, so that the internal stress of the film is closely related to the film material and the preparation process, and although the relation between the film preparation process and the film structure is extremely complex, the invention provides that the film stress can be simply attributed to the relation with the film aggregation density. The film aggregation density is a physical quantity for representing the loose degree of the film, if the film is loose, the aggregation density is low, and the film presents tensile stress; on the other hand, if the film is dense, the aggregation density is high, and the film exhibits compressive stress. For convenience of representation, the tensile stress is customarily set to be positive and the compressive stress to be negative, and the habit is that most of the soft films are tensile stress at the early stage, but with the progress of the hard film preparation technology and the Ion Assisted (IAD) technology, most of the oxide films are compact, and the internal stress is changed into the compressive stress, so that the representation method of setting the compressive stress to be positive and the tensile stress to be negative is adopted at present. Although the above two methods of representing stress are easy to be confused, they are not significant to analyze whether the stress properties of the thin film are the same or opposite. Experienced film designers and manufacturers have always sought to ensure that the stresses of the two materials making up the multilayer film are kept low and opposite by selecting the film material or the manufacturing process, respectively, so that the ultimate multilayer film stresses cancel each other out, but this is a desirable desire that in practice the film material and manufacturing process are always subject to various constraints in order to ensure various properties of the multilayer film, so that film stresses are always difficult to control. If the materials are selected improperly or the process is not proper, the multilayer film is finally formed by two films with the same stress property, and the accumulated stress is not tolerable. Such large accumulated stress of the thin film junction tends to cause bending deformation of the substrate, thus, on one hand, the optical and mechanical properties of the multilayer film as the narrow-band interference filter are damaged, including central wavelength drift and non-uniformity along with the surface position, deterioration of the transmission curve of the filter, increase of optical loss and the like (as shown in fig. 1), and the mechanical properties of the multilayer film are reduced sharply, if the accumulated stress of the thin film junction exceeds the elastic limit of the thin film or the substrate, the film or the substrate is cracked, and the function of the interference filter is lost; on the other hand, the optical imaging system introduces large aberration, and the curved thin film optical filter causes large distortion of the wavefront of the incident light wave after being reflected and transmitted by the optical filter, so that the aberration of the whole optical system deviates from the design index, and even the optical system cannot work at all.

Because of this, there has been no diligent research on film stress, which involves stress generation mechanisms, measurement methods, and removal methods. In the aspect of mechanism research, various theoretical models are established for the origin of stress, including quenching effect of film material thermal molecules on the surface of a substrate, phase transfer effect of tensile stress generated by film volume shrinkage or compressive stress generated by expansion, vacancy elimination or particle embedding effect of volume shrinkage or particle embedding caused by volume expansion due to elimination of vacancy defects in the film, lattice defects, microstructure change, recrystallization and phase change, impurity effect, surface tension model, bombardment model and the like. The mechanism of stress generation by the membrane is very complex and has not been fully understood so far. In the measurement technique, there are mainly a substrate deformation method and a diffraction method, wherein the former method includes a cantilever beam method, an optical interference method, a curvature method, and the like. In the aspect of stress relief, Ion Assisted Deposition (IAD), UV irradiation treatment, adjustment of thermal expansion coefficients of a substrate and a thin film material, doping, change of deposition parameters, annealing treatment and the like are included. Although significant progress has been made in thin film stress research, the problem is not solved well and still is a difficult obstacle in some important applications.

Disclosure of Invention

The invention aims to provide a low-stress multilayer thin film filter and a preparation method thereof, and the low-stress thin film narrow-band interference filter is used for avoiding the damage of optical and mechanical characteristics of the filter caused by too large accumulated stress, avoiding the function of the filter from being completely lost due to the rupture of a filter film layer or the rupture of the whole filter, and avoiding the abnormal work of an imaging system caused by too large aberration, so that the thin film filter is more effectively applied to various optical, photoelectric and laser systems and instruments.

The present invention is directed to the most commonly used titanium dioxide, TiO₂And silica SiO₂The stress of narrow-band interference filters composed of multilayer films has been subject to some research and study. Testing on TiO based on curvature method₂And SiO₂Experimental tests are carried out on the single-layer film and the narrow-band interference filter consisting of the two films, and the cognition that the substrate temperature and the IAD have important effects on reducing the film stress is obtained. It is proposed that the film packing density is an important factor for stress, low packing density is prone to tensile stress, and high packing density is prone to compressive stress. The purpose of adjusting the concentration density is achieved by properly adjusting the process parameters in the narrow-band interference filter, and the accumulated stress is expected to be greatly reduced.

In order to achieve the above object, the present invention proposes the following concepts and experiments:

first, understanding and analyzing various stress models, a stress variation model which is most suitable for the volume shrinkage or volume expansion of the substrate and the film of the invention is provided, and the model comprises double factors of the substrate temperature and the film structure, thereby simultaneously comprising thermal stress and internal stress.

On the basis of analyzing various stress models, the influence of the substrate temperature on the film stress is deduced, when the substrate is heated, the film expands, and conversely, when the substrate is cooled, the film contracts, and the film stress is generated under the two conditions. Since this is a thermally induced stress, it is simply referred to as thermal stress, in general, withThe substrate temperature is increased, and most of the generated thermal stress is tensile stress. This expansion or contraction of the film occurs simultaneously in the x, y and z directions (as shown in fig. 2), and the cross-sectional area is set to "d" according to Hooke' S law_fW film is subjected to a force F_fHere, d_fIs the film thickness and w is the film width, the film stress σ in the x-direction is taken into account_fAnd strain_fCan be respectively expressed as sigma_f＝F_fS and_f＝ΔL/L₀Δ L is the amount of expansion in the x-direction, and σ is satisfied between the film stress and strain_f＝E_f _fHere, E_fThe young index modulus of the film. Film stress sigma generated if the substrate temperature T is set_fExpressed as thermal stress σ_f(T), then σ_f(T) usable substrate thermal expansion coefficient α_sAnd film coefficient of thermal expansion α_fAnd the difference DeltaT between the film deposition temperature and the room temperature

V in the formula_fIs the poisson coefficient of the film.

Thirdly, the invention also tries to deduce the expression of the internal stress caused by the film structure, but no matter the defect model, the grain boundary model or other existing models can not obtain a physical quantity capable of representing the film structure, so the invention proposes to use the aggregation density of the film to represent, the aggregation density of the film is equal to the solid part volume of the film divided by the total volume of the film (namely solid + gap), and obviously the aggregation density of the film is a physical quantity representing the loose degree of the film structure. Experiments of the present invention show that it is reasonable to characterize the film structure by the aggregation density: low packing density produces tensile stress and high packing density produces compressive stress, so that the structural characteristics of the film are closely related to the internal stress of the film. Further experiments have found that internal stresses in the film are dominant compared to thermal stresses in the film. Unfortunately, with the introduction of the concept of aggregate density, the expression for internal stress remains difficult to obtain, due to: 1) as the temperature of the substrate increases, as shown in fig. 3, on the one hand, the tensile stress of the film increases due to the temperature increase, and on the other hand, the temperature increase also causes the densification of the film structure, so that the concentration density increases, the tensile stress decreases, and even turns into compressive stress, and therefore the total stress of the film is the result of the common contribution of the tensile stress caused by thermal stress and the tensile stress generated by the continuous densification of the film decreases, and even turns into compressive stress. 2) Different thin film materials have completely different and even completely opposite internal stresses, also called intrinsic stresses, and the relationship between the internal stresses and manufacturing process factors is extremely sensitive, especially with respect to substrate temperature and ion-assisted deposition parameters. 3) The internal stress is related to the film thickness, and in the initial stage of film formation, as the film in the nucleation process has a liquid phase characteristic, the aggregation density is very low, the tensile stress is very large, generally speaking, the film thickness is about 10-20 nm, and when the film grows to be completely continuous, the film thickness is increased, and the internal stress tends to be stable. 4) The change of the internal stress is nonlinear, the stress of the same material can be tensile stress or compressive stress, and the specific stress can be generated only under specific preparation conditions. The above causes the resolution of internal stress to be very complex and not as straightforward as thermal stress.

Fourthly, although the invention can not independently derive a direct expression of the internal stress, a more important expression of the total stress of the film can be obtained, and the expression comprises two parts of thermal stress and internal stress. In general, although the substrate thickness of the filter is much greater than the film thickness, the film-substrate system still bends under the total stress of the film. As shown in FIG. 4, assuming that the curvature radius of the film-substrate bending is r, based on Hooke's law, considering the fact that the substrate thickness is much larger than the film thickness, the higher order term of the film thickness can be eliminated, thereby obtaining the total stress of the film as

In the formula, E_s、ν_sAnd d_sRespectively the young's modulus, poisson's coefficient and substrate thickness of the substrate. The formula (2) is a basic formula for measuring and calculating the total stress of the film by using a curvature method. In factIf the total stress of the film can be measured by the formula (2), the internal stress of the film can be obtained by subtracting the thermal stress of the film of the formula (1). It should be noted that, since the film is curved, there is a difference in the distribution of the total stress of the film in the thickness direction of the film, and the difference in stress has positive and negative maximum values at the upper and lower surfaces of the film, but the average value of the total stress in the thickness direction is actually obtained in the specific calculation.

And fifthly, the principle of testing the total stress by using the curvature method is that the total stress of the film is calculated by measuring the curvature change of the substrate before and after film coating. The curvature can be measured by means of a stage-finder or a laser interferometer, both of which can achieve satisfactory accuracy for thin substrates of circular shape. The invention adopts a step tester for testing for convenience. From the measured radius of curvature r, the total stress of the film can be determined by the equation (2). FIG. 5 is a schematic illustration of film-substrate curvature measurement. For the convenience of the test, let curve ABC denote the curved surface, AC the measurement range, BD the degree of curvature, and let AC ═ a, BD ═ h, the measured radius of curvature r can be found by geometric relationships as

In the experiment, a is 5 mm. Since a is much larger than h, equation (3) can be further simplified as r ═ a²And/8 h, substituting the simplified expression into the expression (2) to finally obtain the following expression of total stress of the film

Therefore, only the corresponding substrate bending h before and after coating is measured₀And h, calculating the total stress of the single-layer thin film or multi-layer thin film filter. For example, FIG. 6 shows ion-assisted TiO measured by a step profiler₂The curvature of the single-layer film interface, because the surface of the film contacted by the probe is convex, the corresponding total stress is compressive stress (see fig. 7 a); similarly, the tensile stress corresponds to a concave surface (see fig. 7 b). Because h is 43nm, a is 5mm, d is shown in FIG. 6_s＝1mm、E_s＝64GPa、ν_s0.2 and d_f220nm, the total compressive stress obtained from formula (4) is approximately-830 MPa, which is the result of high energy ion bombardment of TiO₂The monolayer film has high aggregation density, so that high total pressure stress is generated, which is a problem caused by strong ion assistance, and the practical work should avoid the negative effect as much as possible. The total stress of the film is closely related to the acting force among the crystal grains of the film, although the stress characteristic varies with materials, the film with high aggregation density on the whole generates microscopic repulsive force among the crystal grains because the gaps among the crystal grains are small, and the repulsive force necessarily generates a reaction force, so the macroscopic stress of the film presents a compressive stress (see fig. 8 a); on the contrary, if the film is loose and the aggregation density is low, microscopic attraction is generated among the crystal grains, and the reaction force is macroscopic tensile stress (see fig. 8 b).

Sixthly, the TiO manufactured by the conventional process manufacturing and ion-assisted technology is prepared by the method₂And SiO₂The two films are respectively subjected to stress measurement and analysis, and the aim is to search the preparation process conditions of the low-stress optical filter. 1) TiO manufactured by conventional process₂Film and SiO₂Film, TiO when substrate temperature is 250 DEG C₂Film and SiO₂The absolute values of the total stress of the film can be relatively close and opposite, wherein, the total stress of the TiO film is opposite₂The film is of compressive stress of about-60 MPa, SiO₂The tensile stress of the film is about 78MPa, when the two films are used for alternately plating the multilayer thin film filter, the compressive stress and the tensile stress can be mutually counteracted to greatly reduce the accumulative stress of the whole filter, finally, the deformation of the whole narrow-band interference filter is measured to be about 12.5nm (see figure 9), and the deformation is also substituted into the formula (4) (note that the total physical film thickness of the filter is 4800nm) to obtain the corresponding accumulative tensile stress sigma_{Accumulation in the body}10.8 MPa. 2) TiO manufactured by counter ion auxiliary technology₂Film and SiO₂Film, substrate without heating, selected ion energy of 350eV, and maintaining the ratio J of the number of assist ions reaching the substrate per unit time and unit area to the number of deposited molecules_I/J_MTo TiO 2₂Film and SiO₂The membranes are about 0.15 and 0.05 respectively, here weakly ion-assistedUnder the conditions of preparation, TiO₂Film and SiO₂The total stress of the film is small and opposite, wherein, TiO₂The film is a film with a compressive stress of about-26 MPa, SiO₂The tensile stress of the film is about 18MPa, when the two films are used for alternately plating the multilayer thin film filter, the stress is small, the two films can be mutually counteracted, finally, the deformation amount is measured to be about 8nm (see figure 10), and the obtained deformation amount is substituted into the formula (4), so that the accumulated compressive stress sigma corresponding to the narrow-band interference filter is obtained_{Accumulation in the body}＝-7MPa。

Seventhly, analyzing the TiO manufactured by the conventional process and the ion assistance₂And SiO₂The stress conditions of both films can be inspired by 1) the temperature of the substrate and the ion assist have a significant effect on the film stress, which is an issue in the design of low stress film fabrication processes.2. heating of the substrate increases the thermal stress, which is often expressed in the form of tensile stress.A difference in the thermal expansion coefficients of the substrate and the film is reduced by equation (1) (α)_s－α_f) Is the most effective method for reducing thermal stress, which is the reason for selecting 3.3 borosilicate glass in the present invention, since substrate heating is an important measure for improving film properties in conventional processes, the Δ T adjustment margin ratio (α) in formula (1)_s－α_f) Is small. On the other hand, the increase of the substrate temperature also increases the film aggregation density, and the internal stress changes, and the expression form of the internal stress can be tensile stress or compressive stress according to the film aggregation density and the film sensitivity to the temperature. In general, as the packing density gradually increases, the tensile stress gradually decreases, even converting into compressive stress. For example, TiO when the substrate temperature is 250 deg.C₂The film has an aggregate density of approximately 1, a compressive stress of about-60 MPa, SiO₂The film had an aggregate density of about 0.92 and a tensile stress of about 78 MPa. 3) When the ion-assisted thin film is adopted, in order to improve the preparation efficiency, the substrate is not heated generally, or the ion-assisted thin film can completely replace the aggregation density increase caused by substrate heating, so that the thermal stress can be ignored. The ion-assisted function is to obviously improve the film aggregation density, and the ion energy is transferred to the molecules of the deposited film by bombarding the deposited film by charged ions to densify the film and improve the aggregation density, so the internal stress of the filmThe manifestation is mostly compressive stress. Appropriate control of ion energy (eV) and ion current density (J)_I) The stress of the film can be greatly reduced. For TiO of the present invention₂Film and SiO₂Film, at the same ion energy (350eV), due to SiO₂Deposition rate of film (J)_M) To compare with TiO₂The film is much faster, so the ratio J of the number of assist ions reaching the substrate to the number of deposited molecules_I/J_MThe titanium dioxide and silicon dioxide films are about 0.15 and 0.05 respectively. This makes the TiO compound₂The film also has an aggregate density close to 1, exhibits a smaller compressive stress of-26 MPa, and SiO₂The film had an aggregate density of 0.96 and exhibited a lower tensile stress of about 18 MPa. 4) The narrow-band interference filter of the invention can be prepared by conventional process or ion-assisted technique, so that TiO can be used as the filter₂Film and SiO₂The two alternating films respectively present compressive stress and tensile stress, so that the two alternating films can mutually offset to greatly reduce the junction stress of the whole interference filter. Theoretically, the accumulated stress σ 'of the multilayer interference filter'_{Accumulation in the body}Can be simply expressed as

σ’_{Accumulation in the body}＝(σ_HN_H+σ_LN_L)/(N_H+N_L) (5)

In the formula, σ_HIndicating high refractive index TiO₂Total stress of the membrane, σ_LSiO with a low refractive index₂The total stress of the film; n is a radical of_H、N_LThe number of layers of high and low refractive index films, i.e. 16 and 15, respectively, is indicated. For conventional processes, use σ_H＝-60MPa，σ_LSubstituting 78MPa into formula (5) to obtain sigma'_{Accumulation in the body}6.8 Mpa; for weak ion assisted techniques, using sigma_H＝-26MPa，σ_LSubstituting 18MPa into formula (5) to obtain sigma'_{Accumulation in the body}-4.7 Mpa. This is in contrast to the value σ actually measured in the present invention above_{Accumulation in the body}10.8MPa and σ_{Accumulation in the body}The comparison was reasonable at-7 Mpa, but the measured value was slightly higher than the theoretical value.

And eighthly, considering that the thickness of each film of most dielectric multilayer thin film filters is generally larger than 20nm, the stress tends to be stable along with the change of the film thickness, and based on the change, the application of the invention is not limited to narrow-band interference filters and can be popularized to various dielectric high-reflection films, cut-off filters, band-pass filters, polarization and depolarization filters and the like.

Specifically, the technical scheme adopted by the invention is as follows:

a low-stress multilayer thin film filter comprises a substrate and a multilayer thin film arranged on the substrate, wherein the low-stress multilayer thin film filter is a narrow-band interference filter adopting G | (H L)³H2LH(LH)³L(HL)³H2LH(LH)³The film system structure of | Air, in which G denotes a substrate, H denotes a high refractive index film of a quarter center wavelength thickness, L denotes a low refractive index film of a quarter center wavelength thickness, Air denotes Air, and the high refractive index film is titanium dioxide (TiO)₂) A film, the low refractive index film being silicon dioxide (SiO)₂) And (3) a membrane.

The total number of the layers of the multilayer film is 31, wherein the number of the layers of the high-refractive-index film is 16, the number of the layers of the low-refractive-index film is 15, the layers from the substrate to the

air direction

1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29 and 31 are the high-refractive-index films, and the layers from the substrate to the

air direction

2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28 and 30 are the low-refractive-index films, the total thickness of the multilayer film is about 4800nm, the thickness of each L layer is 182.76nm, and the thickness of each H layer is 117.25 nm.

Further, the substrate is optical glass, and further preferably, the substrate is 3.3 borosilicate glass with low thermal expansion coefficient, and the thermal linear expansion coefficient of the substrate is about 3.3 × 10^-6Degree, its refractive index at a wavelength of 1060nm is 1.46. In order to reduce the thermal stress caused by the thermal expansion coefficient, 3.3 borosilicate glass with the thermal expansion coefficient as close as possible to that of the film is selected as the filter substrate.

Further, the central wavelength is 1060 nm.

Further, said titanium dioxide (TiO)₂) Film refractive index at 1060nm wavelength of 2.26, said titanium dioxide (TiO)₂) The film has a coefficient of thermal linear expansion of 2 to2.5×10^-6Degree/deg.

Further, the silicon dioxide (SiO)₂) Film refractive index at 1060nm wavelength of 1.45, said silicon dioxide (SiO)₂) The film had a coefficient of thermal linear expansion of about 0.7 × 10^-6Degree/deg.

A method for preparing a low-stress multilayer thin film filter comprises the following steps:

controlling the temperature of the substrate at 245-255 ℃, and alternately preparing a high-refractive-index film and a low-refractive-index film on the substrate by adopting coating equipment.

The coating equipment adopts titanium dioxide (Ti)₂O₅) As the initial evaporation material of the high-refractive-index film, the coating equipment adopts silicon dioxide (SiO)₂) As the initial evaporation material for the low refractive index film.

Further, the temperature of the substrate is controlled to be 250 ℃, a conventional process can be selected for preparing the multilayer thin film optical filter, and the temperature of the substrate is 250 ℃ for reducing the junction stress of the optical filter.

Further, conventional process for preparing TiO₂Film bulk density close to 1.0, SiO₂The film had an aggregate density of about 0.92.

The preparation method of the low-stress multilayer thin film optical filter comprises the following steps:

a coating device with an ion source is adopted, and a high-refractive-index film and a low-refractive-index film are alternately prepared on a substrate by weak ion assistance.

The coating equipment with the ion source adopts titanium pentoxide (Ti)₂O₅) As the initial evaporation material of the high refractive index film, the coating equipment with the ion source adopts silicon dioxide (SiO)₂) As the initial evaporation material for the low refractive index film.

Further, the ion energy of the ion source is 350 + -50 eV, and the ratio J of the number of assist ions reaching the substrate per unit time and unit area to the number of deposited molecules is maintained_I/J_M: 0.15 + -0.03 for titanium dioxide film and 0.05 + -0.01 for silicon dioxide film.

Furthermore, the ion-assisted technique can be selected to produce a multilayer thin film filter, in order to reduce the junction stress of the filter as much as possible, the ion energy is selected to be 350 + -20 eV, and the ratio J of the number of assisting ions reaching the substrate per unit time and unit area to the number of deposited molecules is maintained_I/J_M: 0.15. + -. 0.015 for titanium dioxide films and 0.05. + -. 0.005 for silicon dioxide films.

Further, TiO prepared by ion-assisted technique₂The film also has an aggregate density of approximately 1.0, SiO₂The membrane had an aggregate density of about 0.96.

Furthermore, the multilayer thin film filter comprises various dielectric high-reflection films, cut-off filters, band-pass filters, polarization and depolarization filters and the like. The filter can be widely applied to various systems and instruments such as optics, photoelectricity, laser and the like as a filter.

Compared with the prior art, the invention has the beneficial effects that:

1. the research of the prior art on the film stress mainly focuses on the aspects of theoretical models and detection technologies. Many models for generating internal stress of a thin film have defects, grain boundary models and the like, but the models are isolated actually, and none of the models can fully describe the real situation of the internal stress generation, wherein the main reason is that the mechanism of the stress generation is too complicated. In contrast, several practical techniques have been achieved in stress detection, including cantilever methods, x-ray diffraction methods, curvature methods (including optical interferometer and step-meter testing), and the like. However, there are few reports on the research on how to reduce the film stress, and there are few researches on the characteristics and evolution of the film stress and the relevance of the film stress to the preparation process, and the invention provides two points of view for reducing the film stress: substrate temperature and ion assisted techniques. Heating of the substrate produces thermal stress, which is expressed in the form of tensile stress. Furthermore, it is proposed that the most effective way to reduce the thermal stress is to reduce the difference between the thermal expansion coefficients of the substrate and the thin film, which is not the choice of K in the present invention₉3.3 borosilicate glass is selected for optical glass because of K₉The coefficient of thermal expansion of the optical glass is 7 to 8 × 10^-6Boron in a/degree/ratio of 3.3Coefficient of thermal linear expansion of silica glass 3.3 × 10^-6The/degree is more than doubled. On the other hand, the increase of the substrate temperature also increases the film aggregation density, the invention provides that the aggregation density is the most important factor influencing the internal stress, the expression form of the internal stress can be tensile stress or compressive stress according to the aggregation density of the film and the sensitivity of the film to the temperature, and the evolution process is that the tensile stress is gradually reduced along with the gradual increase of the aggregation density, even the tensile stress approaches to zero, and then the tensile stress is converted into the compressive stress, and the compressive stress is gradually increased along with the continuous increase of the aggregation density. It follows that controlling the packing density is the most important method for controlling internal stresses. Based on this recognition, the present invention proposes to adjust the concentration density of the thin film by ion assist, because the ion assist can effectively increase the concentration density of the thin film, and the stress of the thin film approaches zero from the tensile stress by adjusting the ion energy and ion current density. If the packing density of the film is too high, the stress in the film will be expressed as compressive stress, which, of course, is to be avoided in the present invention.

2. The prior art generally does not provide technical indexes for the film stress, and under the condition of stress requirement, the main measure is to increase the thickness of the substrate or reduce the area of a plating film so as to reduce various adverse effects caused by deformation of the substrate as much as possible. However, this approach is clearly limited and many applications do not allow the use of thick substrates, which can lead to increased aberrations and increased system size and weight, while reducing the coating area reduces the light flux and decreases the brightness and signal-to-noise ratio. The invention directly realizes the low stress requirement of the multilayer thin film optical filter by two preparation technologies: the first is to adopt the conventional process to manufacture TiO₂Film and SiO₂Film, TiO when substrate temperature is 250 DEG C₂Film and SiO₂The absolute values of the total stress of the films are relatively close and opposite, wherein, TiO₂The film has an aggregate density of approximately 1, a compressive stress of about-60 MPa, SiO₂The film concentration density is about 0.92, the tensile stress is about 78MPa, and the compressive stress and the tensile stress can be mutually counteracted when the two films are used for alternately plating the multilayer thin film filter, thereby greatly reducing the whole narrow-band interference filterThe tensile stress at the junction of (A) is 10.8 MPa. The second method is to adopt weak ion auxiliary technology to manufacture TiO₂Film and SiO₂The membrane, and the appropriate control of ion energy and ion current density can significantly reduce the stress of the membrane. The invention selects the ion energy to be 350eV and keeps TiO₂Film and SiO₂Ratio J of number of auxiliary ions from film to number of deposited molecules reaching substrate_I/J_MRespectively about 0.15 and 0.05, under the weak ion auxiliary preparation condition, TiO₂The film still has an aggregate density close to 1, but exhibits a lower compressive stress of about-26 MPa, SiO₂The concentration density of the film is about 0.96, the lower tensile stress is about 18MPa, when the two films are used for alternately plating the multilayer thin film filter, the compressive stress and the tensile stress are small, and the compressive stress and the tensile stress can be mutually counteracted, so that the accumulated compressive stress of the narrow-band interference filter can reach-7 MPa finally. As can be seen from the above, the present invention has made a significant advance in realizing a low stress multilayer thin film filter.

Drawings

FIG. 1 is a graph of the spectral profile before and after strain of a narrow-band interference filter, a) before strain, b) after strain.

Fig. 2 is a schematic of stress and strain for a substrate-film system.

FIG. 3 is a graph of film stress for different substrate temperatures.

FIG. 4 is a diagram illustrating substrate-film bending and film stress distribution.

FIG. 5 is a schematic view of a substrate-film system curvature measurement.

FIG. 6 ion assisted TiO measurement by step profiler₂Curvature test curve of film.

Fig. 7 shows two deformations resulting from film stress, a) compressive stress, b) tensile stress.

FIG. 8 is a diagram illustrating the intergranular forces of the microstructure of the film, a) compressive stress, b) tensile stress.

FIG. 9 is a plot of a step-profiler measured curvature test of a 250 ℃ narrow-band interference filter.

FIG. 10 is a curvature test curve of a weak ion-assisted narrow-band interference filter measured by a step profiler.

Detailed Description

FIG. 1 is a graph of the spectral profile before and after strain of a narrow-band interference filter, where a) is before strain and b) is after strain. In fig. 1, curve 1 shows the spectral transmission curve of the filter without stress-induced strain, while curve 2 shows the stress-induced strain of the filter, and the circular planar filter is curved into a spherical surface due to the strain, so that the light rays at the edge of the field of view will be deflected, and curve 2 is the spectral transmission curve when the deflection angle is 12 °. The center wavelength of curve 1 is 1060nm, the maximum transmittance is 96.7%, and the half-width is 8 nm; curve 2 becomes: the center wavelength was 1056nm, the maximum transmittance was 87%, and the half width was 10 nm. It is apparent that the filter characteristic curve is remarkably deteriorated, and the transmittance at 1060nm is decreased from 96.7% to 53.5%, and further, where the strain is small, it is very important to reduce the stress of the narrow-band interference filter to secure the filter characteristic of the filter, and moreover, the stress causes deterioration of aberration, even breakage of the filter, and thus the necessity of reducing the stress of the thin film is observed.

Fig. 2 is a schematic of stress and strain for a substrate-film system. In order to obtain good filter performance, substrate heating is usually required when conventional processes are used. The heating temperature of the substrate depends on the specific requirements of the optical-mechanical characteristics of the film, and is generally 100-300 ℃. The present invention selects 250 ℃, that is, the forces of the deposited film 3 (i.e. multilayer film) and the substrate 4 at the substrate temperature of 250 ℃ are balanced, but after the preparation, the substrate-film system is cooled to room temperature gradually, the substrate-film cooling shrinks, and the force of the film shrinking to the-x direction varies with the difference between the thermal expansion coefficients of the substrate and the film and the ratio between the thicknesses of the substrate and the film because the substrate-film system has different thermal expansion coefficients and the substrate has a much larger thickness than the film. The force of the film contracting in the-x direction produces a reaction force in the x direction, the resultant force F of the film reaction forces_fA stress is generated and this stress is a tensile stress. This film shrinkage occurs in the x, y and z directions simultaneously, and for convenience, FIG. 2 shows only the force F in the x direction_f. According to Hooke' S law, the cross-sectional area is S ═ d_f·w is subjected to a force F_fHere, d_fIs the film thickness and w is the film width, the film stress σ in the x-direction is taken into account_fAnd strain_fCan be respectively expressed as sigma_f＝F_fS and_f＝ΔL/L₀Δ L is the amount of shrinkage in the x-direction, and σ is satisfied between the film stress and strain_f＝E_f _fWherein E is_fThe young index modulus of the film. Thermal stress sigma generated by the film_f(T) usable substrate thermal expansion coefficient α_sAnd film coefficient of thermal expansion α_fThe difference and the difference Δ T between the film deposition temperature and the room temperature are expressed as shown in the above equation (1).

FIG. 3 shows a graph of film stress for different substrate temperatures. As the temperature of the substrate increases, on the one hand, the film increases in thermal stress 5 due to the temperature increase, which often appears as tensile stress (dotted line), and on the other hand, the temperature increase also causes the film structure to densify, the concentration density increases, and the internal stress 6 changes: the tensile stress is reduced and even converted into compressive stress (thin solid line). It can be seen that the total stress 7 of the film is a result of the co-contribution of the tensile stress caused by the thermal stress 5 and the internal stress variation (reduced tensile stress or even compressive stress) generated by the continuous densification of the film (bold solid line). The film stress is also related to the film thickness, the film in the nucleation process has liquid phase characteristics in the initial stage of film formation, the aggregation density is very low, the tensile stress is very large, the film thickness is generally 10-20 nm at the moment, and when the film grows to be completely continuous, the film thickness is increased, and the stress tends to be stable. The stress variation with the film thickness is generally nonlinear, the stress of the same material can be tensile stress or compressive stress, and specific stress can be generated only under specific preparation conditions.

FIG. 4 is a diagram illustrating substrate-film bending and film stress distribution. Although the thickness of the substrate 4 of the substrate-film system will be much greater than the thickness of the film 3, the substrate-film system will still bend under the film stress. When the bending strain exceeds the elastic limit borne by the membrane 3, the membrane 3 will break; if the bending strain exceeds the elastic limit to which the substrate 4 is subjected, the substrate 4 and the membrane 3 will break together, which is of interest for monitoring the total stress of the membrane. Assuming that the curvature radius of the substrate-film bending is r, based on Hooke's law, considering the fact that the thickness of the substrate is much larger than that of the film, the high-order term of the film thickness can be eliminated, so as to obtain the total stress of the film as shown in the above formula (2), which is the basic formula for calculating the total stress of the film by using curvature measurement, and the formula is suitable for single-layer films and multi-layer thin-film filters. Because the substrate-film system is curved, the distribution of the total stress of the film in the thickness direction of the film has a certain difference, the stress difference has positive and negative maximum values on the upper and lower surfaces of the film, and the average value of the total stress in the thickness direction is obtained in the actual calculation.

FIG. 5 is a schematic illustration of curvature measurement of a substrate-film system. Let curve ABC denote curved surface, AC denote measurement range, BD denote degree of curvature, and let AC ═ a ═ 5mm, BD ═ h, since a is much larger than h, the measured radius of curvature r ═ a can be obtained from geometric relations²The total stress of the finally obtained film is shown as the formula (4), so that the corresponding substrate bending degree h before and after coating is measured₀And h, the total stress of the film can be calculated. FIG. 6 is an ion assisted TiO measurement using a step profiler₂Curvature test curve of film. From FIG. 6, h is 43nm, and a is 5mm, d_s＝1mm、E_s＝64GPa、ν_s0.2 and d_fSince 220nm, the total compressive stress obtained by the above formula (4) is about-830 MPa. Obviously, the method can be used for conveniently measuring the stress condition of the single-layer film or the multi-layer thin film filter.

The strain generated by the film stress has two forms, fig. 7 shows a schematic diagram of two deformation generated by the film stress, wherein a) is compressive stress, the surface of the film 3 corresponding to the measurement is convex, the film 3 has an expansion tendency relative to the surface of the substrate 4, the thickness of the film 3 is thin, and the peeled film 3 shows an inward rolling state when the film 3 is cracked; b) the tensile stress corresponds to the concave surface of the film 3 to be measured, the film 3 tends to shrink relative to the surface of the substrate 4, the thickness of the film 3 becomes thicker, and the peeled film 3 shows a state of outward curling when the film 3 is broken. For an experienced membrane engineer, it is possible to accurately determine whether the film is a tensile stress or a compressive stress, or even roughly estimate the range of the stress, based on the degree of unevenness of the membrane surface or the state of crimp at the time of membrane breakage.

The film stress is closely related to the inter-grain forces of the film, and fig. 8 shows a schematic diagram of the inter-grain forces of the microstructure of the film, in which a) compressive stress is generated, and b) tensile stress is generated. Although the stress characteristics vary depending on the material, it is concluded that the high aggregation density thin film generates a microscopic repulsive force between the crystal grains (AA ') because the crystal grains (AA') are very close to each other, and the repulsive force necessarily generates a reaction force, which is a macroscopic compressive stress (see fig. 8 a); on the contrary, if the film is loose and the aggregation density is low, the grains (BB') are separated from each other more greatly, so that a microscopic attraction force is generated between the grains, and the reaction force of the attraction force is a macroscopic tensile stress (see fig. 8 b). Perhaps more easily understood by a theoretical explanation of mechanical springs: when the crystal grain structure of the film is loose, the spring is under the action of tension, so the spring tends to generate a contraction reaction force, namely the film generates tensile stress opposite to the contraction direction; and vice versa for compressive stresses.

A low-stress multi-layer film filter is composed of substrate made of optical glass and multi-layer film filter made of narrow-band interference filter G (H L)³H2LH(LH)³L(HL)³H2LH(LH)³I Air, which is a typical double half-wave narrow-band interference filter, wherein G denotes a glass substrate, H denotes a high refractive index film of quarter-center wavelength thickness, L denotes a low refractive index film of quarter-center wavelength thickness, whose center wavelength is 1060nm, the total number of film layers is 31, i.e., the number of high and low refractive index films is 16 and 15, respectively, and the total physical film thickness is 4800nm, the substrate employs 3.3 borosilicate glass of low thermal expansion coefficient, whose thermal linear expansion coefficient is about 3.3 × 10^-6Degree, its refractive index at a wavelength of 1060nm is 1.46. The high-refractive-index film is titanium dioxide (TiO)₂) The film, the low refractive index film is silicon dioxide (SiO)₂) Film, TiO₂The refractive index of the film at 1060nm wavelength is 2.26, SiO₂The film had a refractive index of 1.45 at a wavelength of 1060 nm. The TiO is₂The film has a coefficient of thermal linear expansion of 2 to 2.5 × 10^-6Degree of SiO₂The film had a coefficient of thermal linear expansion of about 0.7 × 10^-6Degree/deg.

Example one

As a first embodiment, the narrow-band interference filter is prepared by a conventional process. If the ion source is not installed on the coating apparatus, the optical filter can be manufactured only by a conventional process. Controlling the temperature of the substrate at 250 ℃, and alternately preparing a high-refractive-index film and a low-refractive-index film on the substrate by adopting coating equipment. The coating equipment adopts titanium dioxide (Ti)₂O₅) As an initial evaporation material of the high refractive index film, a coating apparatus using silicon dioxide (SiO)₂) As the initial evaporation material for the low refractive index film. Since the substrate temperature is very sensitive to stress in the conventional process, the present invention attempts to achieve TiO by adjusting the substrate temperature₂Film and SiO₂The stress values of the films are similar and opposite. Since the increase in the substrate temperature not only increases the thermal stress but also changes the internal stress, the internal stress can be expressed in the form of tensile stress or compressive stress depending on the concentration density of the film and the sensitivity of the film to temperature. In general, as the concentration density is gradually increased, the tensile stress is gradually reduced and even converted into compressive stress, and thus it is possible to adjust the substrate temperature to make TiO possible₂Film and SiO₂The absolute values of the total stress of the films are relatively close and opposite in direction. For example, TiO when the substrate temperature is 250 deg.C₂The film has a bulk density of approximately 1 and a compressive stress of approximately-60 MPa, while SiO₂The film had an aggregate density of only about 0.92 and exhibited a tensile stress of about 78 MPa. When the two films are used for alternately plating the narrow-band interference filter, the compressive stress and the tensile stress are expected to be mutually counteracted to greatly reduce the accumulated stress of the whole filter, fig. 9 is a curvature test curve of the narrow-band interference filter at 250 ℃ measured by a step profiler, and can be obtained from fig. 9, the deformation h of the whole narrow-band interference filter is 12.5nm, a is 5mm, and d is_s＝1mm、E_s＝64GPa、ν_s0.2 and d_fSubstituting 4800nm into the formula (4) above to obtain the whole narrow-band interference filter with junction tensile stress σ_{Accumulation in the body}10.8 MPa. Compared with other conventional process conditionsThe accumulation stress of the narrow-band interference filter is mostly in GPa magnitude and even higher, and the accumulation stress of 10.8MPa is quite small, which is mainly due to the fact that the compressive stress and the tensile stress are mutually counteracted.

Theoretically, high refractive index TiO₂Total stress σ of the membrane_HSiO with low refractive index of-60 MPa₂Total stress σ of the membrane_L78MPa and number N of layers of high refractive index film_HNumber of layers N of 16 and low refractive index film_LSubstituting 15 into σ'_{Accumulation in the body}＝(σ_HN_H+σ_LN_L)/(N_H+N_L) The theoretical resultant tensile stress of the entire narrow-band interference filter is then σ'_{Accumulation in the body}6.8 Mpa. This value is compared with the above-mentioned test value σ_{Accumulation in the body}10.8MPa is very close.

Example two

In the second embodiment, the narrow-band interference filter is prepared by weak ion assistance. Most of the existing inlet coating equipment is provided with an ion source, and because ion assistance is very sensitive to the internal stress due to the concentration density of a film, the invention tries to realize the effect of enabling TiO to be subjected to ion assistance parameters₂Film and SiO₂The stress value of the film is small and the direction is opposite. A coating device with an ion source is adopted, and weak ions are adopted for assisting to prepare high-refractive-index films and low-refractive-index films on a substrate alternately. The coating equipment with ion source adopts titanium pentoxide (Ti)₂O₅) As an initial evaporation material for the high refractive index film, a coating apparatus with an ion source using silicon dioxide (SiO)₂) As the initial evaporation material for the low refractive index film.

The invention selects the ion energy to be 350eV and keeps TiO₂Film and SiO₂Ratio J of number of auxiliary ions from film to number of deposited molecules reaching substrate_I/J_MAbout 0.15 and about 0.05 respectively, under the assistance of weak ions, TiO₂The film still has an aggregate density close to 1, but exhibits a lower compressive stress of about-26 MPa, SiO₂The film has an aggregation density of about 0.96 and a lower tensile stress of about 18MPa, and the two films are used for alternately plating a narrow-band interference filterThe stress and the tensile stress are small and can be mutually counteracted, and the accumulated stress of the whole narrow-band interference filter is greatly reduced. FIG. 10 is a curvature test curve of a weakly ion-assisted deposited narrow-band interference filter measured by a step profiler, and it can be derived from FIG. 10 that the deformation h of the narrow-band interference filter is 8nm, and a is 5mm and d is known_s＝1mm、E_s＝64GPa、ν_s0.2 and d_fSubstituting 4800nm into the formula (4) above to obtain the whole narrow-band interference filter with junction stress σ_{Accumulation in the body}This junction accumulated compressive stress should be said to be very low at-7 MPa. Generally, multilayer thin film filters using ion assisted deposition often have high compressive stress because the concentration density of strong ion assisted films is generally very high, and thus both high and low refractive index films exhibit high compressive stress, which ultimately results in unacceptable cumulative compressive stress of the multilayer thin film filters due to loss of stress compensation. That is, although the improvement of the film concentration density is very advantageous for the improvement of the optical stability, mechanical strength and adhesion of the film, it also brings great troubles such as stress of the filter and resistance to laser damage, so that it is necessary to adjust the ion assist energy and the ratio of the number of assist ions reaching the substrate to the number of deposited molecules J in accordance with specific specifications in consideration of various properties of the filter in practical use of the ion source_I/J_MSo as to take account of various optical and mechanical properties of the multilayer thin film filter.

Also, theoretically, high refractive index TiO₂Total stress σ of the membrane_HSiO with low refractive index under-26 MPa₂Total stress σ of the membrane_L18MPa and number N of layers of high refractive index film_HNumber of layers N of 16 and low refractive index film_LSubstituting 15 into σ'_{Accumulation in the body}＝(σ_HN_H+σ_LN_L)/(N_H+N_L) The theoretical accumulated compressive stress of the whole narrow-band interference filter is obtained to be sigma'_{Accumulation in the body}-4.7 Mpa. This value is compared with the above-mentioned test value σ_{Accumulation in the body}Also very close is-7 MPa.

The low-stress thin film filter technology can be widely applied to systems such as medium high-reflection films, cut-off filters, band-pass filters, polarization and depolarization filters and the like in various photoelectric instruments.

Claims

1. A low stress multi-layer film filter, comprising a substrate and a multi-layer film arranged on the substrate, characterized in that, the low stress multi-layer film filter is a narrow-band interference filter, adopting G | (H L)³H2LH(LH)³L(HL)³H2LH(LH)³The film system structure of | Air, wherein G represents a substrate, H represents a high refractive index film of quarter center wavelength thickness, L represents a low refractive index film of quarter center wavelength thickness, Air represents Air, the high refractive index film is a titanium dioxide film, and the low refractive index film is a silicon dioxide film.

2. The low stress multilayer thin film filter according to claim 1, wherein the multilayer thin film has a total number of layers of 31, the number of layers of the high refractive index film is 16, and the number of layers of the low refractive index film is 15.

3. The low-stress multilayer thin film filter according to claim 2, wherein the 1 st, 3 rd, 5 th, 7 th, 9 th, 11 th, 13 th, 15 th, 17 th, 19 th, 21 th, 23 th, 25 th, 27 th, 29 th, 31 st layers from the substrate to the air direction are high refractive index films, and the 2 nd, 4 th, 6 th, 8 th, 10 th, 12 th, 14 th, 16 th, 18 th, 20 th, 22 th, 24 th, 26 th, 28 th, 30 th layers from the substrate to the air direction are low refractive index films.

4. The low stress multilayer thin film filter according to claim 1, wherein the center wavelength is 1060 nm;

the refractive index of the titanium dioxide film at the wavelength of 1060nm is 2.26, and the coefficient of thermal linear expansion of the titanium dioxide film is 2-2.5 × 10^-6Degree/degree;

the silica film has a refractive index of 1.45 at a wavelength of 1060nm, and a thermal linear expansion coefficient of about 0.7 × 10^-6Degree/deg.

5. The low stress multilayer thin film filter of claim 1, wherein the substrate is a low cte 3.3 borosilicate glass having a thermal linear expansion coefficient of about 3.3 × 10^-6Degree, its refractive index at a wavelength of 1060nm is 1.46.

6. The method of manufacturing a low stress multilayer thin film filter according to any one of claims 1 to 5, comprising the steps of:

controlling the temperature of the substrate at 245-255 ℃, and alternately preparing a high-refractive-index film and a low-refractive-index film on the substrate by adopting coating equipment;

the coating equipment adopts titanium pentoxide as an initial evaporation material of the high-refractive-index film, and the coating equipment adopts silicon dioxide as an initial evaporation material of the low-refractive-index film.

7. The method of manufacturing a low-stress multilayer thin film filter according to claim 6, wherein the temperature of the substrate is controlled to 250 ℃.

8. The method of manufacturing a low stress multilayer thin film filter according to any one of claims 1 to 5, comprising the steps of:

adopting a coating device with an ion source, and adopting weak ions to assist in alternately preparing a high-refractive-index film and a low-refractive-index film on a substrate;

the coating equipment with the ion source adopts titanium pentoxide as an initial evaporation material of the high-refractive-index film, and the coating equipment with the ion source adopts silicon dioxide as an initial evaporation material of the low-refractive-index film.

9. The method of claim 8, wherein the ion source has an ion energy of 350 ± 50eV, and maintains a ratio J of the number of auxiliary ions reaching the substrate per unit time and unit area to the number of deposited molecules_I/J_M: to titanium dioxide film0.15. + -. 0.03, and 0.05. + -. 0.01 for the silica film.

10. The method of claim 9, wherein the ion source has an ion energy of 350 ± 20eV, and maintains a ratio J of the number of auxiliary ions reaching the substrate per unit time and unit area to the number of deposited molecules_I/J_M: 0.15. + -. 0.015 for titanium dioxide films and 0.05. + -. 0.005 for silicon dioxide films.