Surface shape correction method of thin film element based on high-gradient optical filter

Abstract

The invention relates to a surface shape correction method of a thin film element based on a high-gradient optical filter, and solves the problem of how to accurately grasp the thickness of a SiO2 film and control the surface shape of the thin film element by using the stress of the SiO2 film during double-sided film coating. According to the method, a mathematical model between SiO2 film thickness and film element deformation after two times of surface shape correction of the same film element is obtained by establishing a surface shape correction mathematical model of the film element, and then a high-gradient optical filter is prepared from two materials with high and low refractive indexes and is plated on the front surface of a substrate; the thickness of the SiO2 film subjected to primary surface shape correction is calculated according to the thickness of the high-gradient optical filter on the front surface of the substrate to carry out primary surface shape correction, and then the thickness of the SiO2 film subjected to secondary surface shape correction is calculated according to the surface shape correction mathematical model of the thin film element to carry out secondary surface shape correction. According to the relationship between the thickness of the SiO2 film and the deformation of the thin film element after the two-time surface shape correction of the same thin film element, the ideal surface shape can be achieved through the two-time surface shape correction.

Description

Technical Field

[0001] The invention relates to a surface shape correction method, in particular to a surface shape correction method of a thin film element based on a high-steepness filter. Background Art

[0002] As an important indicator to measure the surface quality of optical components, surface accuracy has been integrated into every aspect of optical component processing. With the rapid development of my country's aerospace industry, higher requirements have been placed on the functions of optical systems used therein. As an indispensable key component in the optical system, the surface accuracy of optical thin film components is becoming increasingly important.

[0003] When measuring the surface accuracy of thin film components, the sag value, peak-to-valley value and root mean square are generally measured. The sag value is the distance from the center point of the component surface to the corresponding point on the best fitting sphere, which is a representation of the macroscopic deformation of the component. The peak-to-valley value represents the numerical difference between the highest point and the lowest point of the component surface, which reflects the overall processing of the component. The root mean square is the mean square value of the sum of the squares of the optical path difference between the entire measured wavefront (or optical surface) and the best matching reference wavefront, which is a representation of the average error on the entire surface. The smaller the value, the better the surface shape of the entire surface.

[0004] For thin film components with wide spectrum requirements, they are not only required to have good spectral quality, but also to have high surface accuracy. However, such thin film components are coated during production, and the increase in the number and thickness of film layers will inevitably cause the thin film components to bend, resulting in a decrease in surface accuracy, thereby reducing the imaging quality and reliability of the optical system. This phenomenon of decreased surface accuracy is usually attributed to the residual stress of the film layer, which mainly comes from the film preparation process. The first is the substrate stress. Since the substrate material will inevitably introduce residual thermal stress during the melting process, and the substrate will have processing stress after polishing, the two stresses temporarily form a state of equilibrium. As the temperature increases during coating, this state of equilibrium is broken, and the residual stress of the substrate is redistributed irregularly and uncertainly, making the surface of the thin film component uncontrollable. Secondly, during the film formation process, the thermal stress caused by the temperature difference, the intrinsic stress formed by the mutual extrusion or stretching of molecules, and especially the stress development of the multilayer film involve the combination of the thermophysical parameters of multiple thin film materials and the mechanical matching problems of multiple interfaces. The combined influence increases the difficulty of controlling the surface accuracy of the thin film component during the coating process. Although studies have shown that under the conditions of determined substrate, film material and deposition method, the stress of the thin film can be regulated by changing the deposition process parameters, this method has limited control and is only applicable to single-layer films or multilayer films with the same film thickness in each period.

[0005] However, for thin-film components with different film thicknesses, it is impossible to control the surface shape of the thin-film components during the coating process. In addition, external stress will inevitably be introduced into the thin-film components during clamping, testing and transportation. These stresses work together to make it difficult to control the surface shape accuracy of the thin-film components after the coating process.

[0006] Since it is difficult to control the surface shape during the coating process, a lot of research is now focused on the surface shape correction of thin film components after coating. Studies have shown that double-sided coating technology can achieve surface shape control of thin film components, that is, a complex film system with a thicker film layer is coated on the front surface, and a stress matching film is coated on the back surface. Most complex thin films deposited by ion beam assisted electron beam thermal evaporation are in a compressive stress state. 2 It is the most important low refractive index material. 2 The film has similar optical properties to most glass substrates and can be used as a stress matching film for surface correction. Studies have shown that film stress is sensitive to the selection of process parameters. By controlling the deposition process parameters, SiO 2 The film is always in a compressive stress state, and the change in the curvature of the thin film element caused by stress is related to the film thickness, so it can be controlled by 2 The thickness of the film allows the stresses on the front and back surfaces of the substrate to restrict each other, achieving the effect of utilizing SiO 2 The purpose of controlling the surface shape of thin film components is to control the film stress. 2 The thickness of the film will increase. When using this method, the substrate will be repeatedly polished or coated multiple times, which will increase the risk of substrate damage and plating costs. Summary of the invention

[0007] The purpose of the present invention is to solve the problem of how to accurately grasp the SiO 2 The thickness of the film is as high as that of SiO 2 The invention solves the technical problem of controlling the surface shape of thin-film components by film stress, and provides a surface shape correction method of thin-film components based on high steepness filters.

[0008] In order to achieve the above-mentioned invention object, the present invention provides the following technical solutions: A method for surface correction of a thin film element based on a high-steepness filter is special in that it comprises the following steps: S1. Establishing a mathematical model for surface shape correction of thin film components; S2, using a high refractive index material as a high refractive index layer and a low refractive index material as a low refractive index layer to prepare a high steepness filter, and coating the high steepness filter on the front surface of the substrate; S3, calculate the SiO2 of the first surface correction according to the thickness of the high-steepness filter on the front surface of the substrate 2The film thickness is calculated, and SiO is plated on the rear surface of the substrate according to the calculated thickness. 2 The film undergoes the first surface correction; According to the mathematical model of the surface correction of thin film components, the SiO 2 film thickness, and based on the calculated thickness of SiO 2 The film undergoes a second surface correction; S4, SiO after correction 2 An anti-reflection film is coated on the membrane to complete the surface correction of the thin film element based on the high steepness filter.

[0009] Furthermore, step S1 is specifically as follows: S1.1, through SiO 2 The study of film stress characteristics has revealed that SiO 2 Relationship between film thickness and sag height of thin film element

[0010] in: is the sag of the thin film element, SiO 2 Film thickness, , is a linear factor; S1.2. Residual stress of thin film element obtained based on Stoney's formula modified by beam theory

[0011] in: is the residual stress of the thin film element, is the Young's modulus of the substrate, is the Poisson's ratio of the base, is the base thickness, is the base diameter, SiO 2 Film thickness, is the variation of the vector height of the thin film element; S1.3, establish the SiO 2 Mathematical model between film thickness and deformation of thin film elements

[0012] in: SiO for the first surface modification 2 Film thickness, SiO for the second surface modification 2 Film thickness, SiO for the first surface modification 2 The change in substrate height caused by film thickness, SiO for the second surface modification 2 The change in substrate height caused by film thickness.

[0013] Furthermore, step S2 is specifically as follows: S2.1. Measure the peak-to-valley value PV, root mean square RMS and vector height Power of the base 0 , through Power 0 To observe the deviation of the peak-to-valley value PV and root mean square RMS from the ideal plane; S2.2, use high refractive index Ta 2 O 5 As the high refractive index layer, SiO 2 As a low refractive index layer, a high-steepness filter is prepared by an ion beam assisted electron beam thermal evaporation method, and the high-steepness filter is plated on the front surface of the substrate; S2.3. Measure the surface shape of the thin film element after coating with high steepness filter, and obtain the vector height Power of the thin film element after coating with high steepness filter 1 .

[0014] Furthermore, step S3 is specifically as follows: S3.1. Calculate the SiO2 for the first surface correction based on the thickness of the high-steepness filter on the front surface of the substrate. 2 Film thickness

[0015]

[0016] in: is the actual total thickness of the high refractive index layer in the high steepness filter, is the actual total thickness of the low refractive index layer in the high steepness filter, is a constant; S3.2, SiO based on the calculated first surface correction 2 Film thickness SiO is plated on the rear surface of the substrate 2 Film, perform the first surface correction; S3.3. After coating is completed, measure the vector height Power of the thin film element after the first surface correction 2 ; S3.4. Calculate the SiO after the first surface correction 2 Variation of substrate height caused by film thickness

[0017] ; S3.5, SiO after two surface corrections of the same thin film element 2The mathematical model between film thickness and thin film element deformation is used to calculate the SiO 2 Film thickness

[0018]

[0019] in: It is the change in the vector height of the thin film element when the anti-reflection film acts on the surface of the thin film element; S3.6, SiO2 based on the calculated second surface correction 2 Film thickness SiO plating 2 Film, perform the second surface correction.

[0020] Furthermore, step S3.2 is as follows: the background vacuum is reduced to 8.5×10 -4 Pa, set the baking temperature to 160℃, the ion beam anode voltage to 130V, the current to 50A, the gas volume to 50sccm, and the SiO 2 Film thickness SiO is plated on the rear surface of the substrate 2 Film, perform the first surface correction; Step S3.6 is as follows: the background vacuum is reduced to 8.5×10 -4 Pa, set the baking temperature to 160℃, the ion beam anode voltage to 130V, the current to 50A, the gas volume to 50sccm, and the SiO 2 Film thickness SiO plating 2 Film, perform the second surface correction.

[0021] Furthermore, in step S2.1, the material of the substrate is K9 glass.

[0022] Furthermore, in step S2.1, an interferometer is used to measure the peak-to-valley value PV, root mean square RMS and vector height Power of the substrate. 0 ; In step S2.3, the surface shape of the thin film element after being coated with the high-steepness filter film is measured by an interferometer.

[0023] Further, in step S3.1, the The value is 0.3.

[0024] Furthermore, in step S3.2, SiO is plated on the rear surface of the substrate. 2 Before coating, the substrate is cleaned by ion beam.

[0025] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a method for surface correction of a thin film element based on a high-steepness filter. First, a mathematical model for surface correction of the thin film element is established to obtain the SiO2 surface correction of the same thin film element after two surface corrections. 2 The mathematical model between film thickness and deformation of thin film elements is constructed. Then, high-steepness filters are prepared by using two materials with high and low refractive indexes and plated on the front surface of the substrate of the thin film element. The SiO2 surface correction for the first time is calculated according to the thickness of the high-steepness filter on the front surface of the substrate. 2 The first surface correction is performed based on the film thickness, and then the second surface correction SiO2 is calculated based on the surface correction mathematical model of the thin film element. 2 The film thickness is used for the second surface correction. 2 The mathematical model between film thickness and thin film element deformation shows that for the same thin film element, two surface corrections can achieve the ideal surface shape. 2 The anti-reflection film is coated on the film to complete the surface correction of the thin film element based on the high steepness filter, so as to achieve the use of SiO 2 The present invention can accurately calculate the SiO2 required for surface correction. 2 The film thickness can accurately correct the surface shape of the thin film element, avoiding damage to the thin film element caused by excessive correction. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A demonstration diagram of surface shape correction of a thin film element in an embodiment of a surface shape correction method of a thin film element based on a high steepness filter of the present invention; Figure 2 It is a flow chart of a method for surface shape correction of a thin film element based on a high steepness filter according to the present invention; Figure 3 It is a transmittance spectrum curve diagram of a thin film element after double-sided coating in an embodiment of a surface shape correction method of a thin film element based on a high steepness filter of the present invention; Figure 4 In the embodiment of the present invention, SiO 2 Relationship between membrane surface shape change and film element sagittal height; Figure 5 The figure is a schematic diagram of film-substrate deformation in an embodiment of a method for surface shape correction of a thin film element based on a high-steepness filter of the present invention. DETAILED DESCRIPTION

[0027] The present invention will be further described below in conjunction with the accompanying drawings and exemplary embodiments.

[0028] Reference Figure 1-Figure 5 The present invention provides a method for correcting the surface shape of a thin film element based on a high-steepness filter, comprising the following steps: S1. Establish a mathematical model for surface shape correction of thin film components.

[0029] S1.1, through SiO 2 The study of film stress characteristics has revealed that SiO 2 Relationship between film thickness and sag height of thin film element

[0030] in: is the sag of the thin film element, SiO 2 Film thickness, , is a linear factor; , The value of is related to the substrate characteristics and coating process. When the substrate material and coating process are determined, the two linear factors , Is a fixed value.

[0031] The derivation process of the above formula is as follows: In the study, it was found that the substrate of the thin film element was plated with SiO 2 The film will cause the change of the sag height of the thin film element. For the same substrate, SiO2 is plated twice under the same coating process. 2 The change in the vector height caused by the membrane is approximately

[0032] in: Indicates the second SiO plating 2 The sag of the film element behind the film, For the first SiO plating 2 The sagittal height of the film element behind the film; After a lot of experiments, we found Figure 4 The relationship diagram shown, Figure 4 The 1# and 2# lines represent the same thin film element plated with different thicknesses of SiO under the same process. 2 The film element sag changes caused by the film. It can be seen that the film element sag changes with SiO 2 The relationship between the film thickness and the ; S1.2. Residual stress of thin film element obtained based on Stoney's formula modified by beam theory

[0033] in: is the residual stress of the thin film element, is the Young's modulus of the substrate, is the Poisson's ratio of the base, is the base thickness, is the base diameter, is the variation of the vector height of the thin film element; The derivation process of the above formula is as follows: First, the relationship between film stress and substrate bending is established by Stoney's formula, which is expressed as

[0034] in: is the radius of curvature of the thin film element after coating.

[0035] The beam theory uses the biaxial elastic modulus instead of the Young's modulus of the substrate and proposes on this basis that when evaluating residual stress, the curvature should be expressed as the inverse of the radius of curvature of the thin film element. Subtract the inverse of the initial radius of curvature of the base , so the Stoney formula is corrected to

[0036] Depend on Figure 5 The geometric relationship and approximate relationship in

[0037]

[0038] in: is the diameter of the thin film element, It is half of the central angle corresponding to the arc length of the bent thin film element.

[0039] This gives the relationship:

[0040] Then the change of the vector height of the thin film element can be expressed as

[0041] Combined with Stoney's formula, the residual stress of the thin film element is

[0042] S1.3, establish the SiO 2 Mathematical model between film thickness and deformation of thin film elements

[0043] in: SiO for the first surface modification 2Film thickness, SiO for the second surface modification 2 Film thickness, SiO for the first surface modification 2 The change in substrate height caused by film thickness, SiO for the second surface modification 2 The change in substrate height caused by film thickness.

[0044] Will Substituting into the residual stress formula of thin film element, SiO 2 The stress of the membrane can be expressed as

[0045] It can be seen from the above formula that for the same substrate, when the linear factor a is a certain value, SiO 2 The stress of the membrane is a certain value, that is,

[0046]

[0047] From the above formula, we can see that using SiO 2 When the membrane is correcting the surface shape, in order to achieve an ideal change in the vector height, that is, , it is necessary to combine the first surface correction SiO 2 Variation of substrate height caused by film thickness And the first surface-modified SiO 2 Film thickness Therefore, in this method, for the same thin film element, two surface corrections can be performed to achieve the ideal surface accuracy.

[0048] S2. Use a high refractive index material as a high refractive index layer and a low refractive index material as a low refractive index layer to prepare a high steepness filter, and plate the high steepness filter on the front surface of the substrate.

[0049] S2.1. Use interferometer to measure the peak-to-valley value PV, root mean square RMS and vector height Power of the substrate 0 , through Power 0 To observe the deviation of the peak-to-valley value PV and root mean square RMS from the ideal plane; The substrate is made of K9 glass material, which has good thermal stability, hardness, wear resistance and impact resistance.

[0050] S2.2, use high refractive index Ta 2 O 5 As the high refractive index layer, SiO 2As a low refractive index layer, a high-steepness filter is prepared by an ion beam assisted electron beam thermal evaporation method, and the high-steepness filter is plated on the front surface of the substrate; The film structure of the high steepness filter is (HL) 25 , where H is a high refractive index material, L is a low refractive index material, Ta 2 O 5 The refractive index of SiO is 2.3. 2 The refractive index is 1.46.

[0051] S2.3. Use interferometer to measure the surface shape of the thin film element after coating with high steepness filter, and obtain the vector height Power of the thin film element after coating with high steepness filter. 1 .

[0052] The surface shape of the thin film element before and after coating the high steepness filter is shown in the following table:

[0053] Note: In the above table, λ represents the laser wavelength of the interferometer, and its value is 632.8nm.

[0054] S3, calculate the SiO2 of the first surface correction according to the thickness of the high-steepness filter on the front surface of the substrate 2 The film thickness is calculated, and SiO is plated on the rear surface of the substrate according to the calculated thickness. 2 The film undergoes the first surface correction; According to the mathematical model of the surface correction of thin film components, the SiO 2 film thickness, and based on the calculated thickness of SiO 2 The film undergoes a second surface correction.

[0055] S3.1. Calculate the SiO2 for the first surface correction based on the thickness of the high-steepness filter on the front surface of the substrate. 2 Film thickness

[0056]

[0057] in: is the actual total thickness of the high refractive index layer in the high steepness filter, is the actual total thickness of the low refractive index layer in the high steepness filter, is a constant; S3.2, draw the background vacuum to 8.5×10 -4 Pa, the baking temperature is set to 160 ° C, the ion beam anode voltage is set to 130 V, the current is set to 50 A, the filling volume is set to 50 sccm, and the substrate is first cleaned by ion beam before coating, and then the SiO 2Film thickness SiO is plated on the rear surface of the substrate 2 Film, perform the first surface correction; S3.3. After coating is completed, measure the vector height Power of the thin film element after the first surface correction 2 ; S3.4. Calculate the SiO after the first surface correction 2 Variation of substrate height caused by film thickness

[0058] ; S3.5, SiO after two surface corrections of the same thin film element 2 The mathematical model between film thickness and thin film element deformation is used to calculate the SiO 2 Film thickness

[0059]

[0060] in: It is the change in the vector height of the thin film element when the anti-reflection film acts on the surface of the thin film element; S3.6, reduce the background vacuum to 8.5×10 -4 Pa, set the baking temperature to 160℃, the ion beam anode voltage to 130V, the current to 50A, the gas volume to 50sccm, and the SiO 2 Film thickness SiO plating 2 Film, perform the second surface correction.

[0061] In this embodiment, is 3078.69nm, is 4390.21nm, is a constant, which is related to the film deposition process. In this embodiment, its value is 0.3, which can be obtained by calculation. The thickness is 3719.67nm, so the first surface correction is performed based on this thickness, and then the interferometer is used to measure the vector height Power of the thin film element after the first surface correction. 2 = 1.222λ, then according to = Power 2 - Power 1 Calculate is 1.245λ.

[0062] In calculation SiO 2 Film thickness When the antireflection film acts on the surface of the thin film element, the change in the vector height of the thin film element is is a known quantity, which is 0.228λ in this embodiment, and the ideal surface shape, that is , is the final surface shape minus the surface shape value when the antireflection film acts on the surface of the thin film element, and we can get , we can calculate is 0.966λ, according to , calculate =2886.11nm.

[0063] The surface shapes of the thin film components before and after surface correction are as follows:

[0064] Note: In the above table, λ represents the laser wavelength of the interferometer, and its value is 632.8nm.

[0065] S4, SiO after correction 2 An anti-reflection film is coated on the membrane to complete the surface correction of the thin film element based on the high steepness filter.

[0066] like Figure 3 As shown, the spectral characteristics of the thin film element corrected by the method provided by the present invention ensure high quality. It can be seen that for the thin film element requiring double-sided coating, the present invention can accurately correct its surface shape and avoid damage to the element caused by excessive correction or multiple corrections.

[0067] The embodiments described above are merely descriptions of specific implementation methods of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for surface correction of a thin film element based on a high steepness filter, characterized in that: The following steps are involved: S1. Establishing a mathematical model for surface shape correction of thin film components; S2, using a material with a high refractive index as a high refractive index layer and a material with a low refractive index as a low refractive index layer to prepare a high steepness filter, and coating the high steepness filter on the front surface of the substrate; S3, calculating the thickness of the SiO2 film for the first surface shape correction according to the thickness of the high steepness filter on the front surface of the substrate, and coating the SiO2 film on the rear surface of the substrate according to the calculated thickness for the first surface shape correction; According to the mathematical model of the surface correction of the thin film element, the thickness of the SiO2 film for the second surface correction is calculated, and the SiO2 film is plated according to the calculated thickness for the second surface correction; S4. An anti-reflection film is deposited on the corrected SiO2 film to complete the surface shape correction of the thin film element based on the high steepness filter.

2. The method for surface correction of a thin film element based on a high steepness filter according to claim 1, characterized in that: Step S1 is specifically as follows: S1.

1. The relationship between SiO2 film thickness and film element sag is obtained through the study of SiO2 film stress characteristics: ； in: is the sag of the thin film element, is the SiO2 film thickness, , is a linear factor; S1.

2. The residual stress of the thin film element is obtained based on the Stoney formula modified by beam theory: ； in: is the residual stress of the thin film element, is the Young's modulus of the substrate, is the Poisson's ratio of the base, is the base thickness, is the base diameter, is the variation of the vector height of the thin film element; S1.

3. Establish a mathematical model between the SiO2 film thickness and the deformation of the thin film element after two surface corrections of the same thin film element: ； in: is the SiO2 film thickness for the first surface correction, is the thickness of SiO2 film after the second surface correction, is the change in substrate vector height caused by the thickness of the SiO2 film during the first surface correction, is the change in substrate vector height caused by the SiO2 film thickness during the second surface correction.

3. The method for surface correction of a thin film element based on a high steepness filter according to claim 2, characterized in that: Step S2 is specifically as follows: S2.

1. Measure the peak-to-valley value PV, root mean square RMS and vector height Power0 of the substrate, and use Power0 to observe the deviation of the peak-to-valley value PV and root mean square RMS from the ideal plane; S2.2, using high refractive index Ta2O5 as the high refractive index layer, using low refractive index SiO2 as the low refractive index layer, and preparing a high steepness filter by ion beam assisted electron beam thermal evaporation method, and coating the high steepness filter on the front surface of the substrate; S2.

3. Measure the surface shape of the thin film element after being coated with a high-steepness filter, and obtain the vector height Power1 of the thin film element after being coated with a high-steepness filter.

4. The method for surface correction of a thin film element based on a high steepness filter according to claim 3, characterized in that: Step S3 is specifically as follows: S3.

1. Calculate the SiO2 film thickness for the first surface correction based on the thickness of the high-steepness filter on the front surface of the substrate : ； in: is the actual total thickness of the high refractive index layer in the high steepness filter, is the actual total thickness of the low refractive index layer in the high steepness filter, is a constant; S3.2, SiO2 film thickness calculated based on the first surface correction A SiO2 film is deposited on the rear surface of the substrate to perform the first surface correction; S3.3, after coating is completed, measure the vector height Power2 of the thin film element after the first surface correction; S3.

4. Calculate the change in substrate vector height caused by the SiO2 film thickness after the first surface correction : ； S3.

5. Calculate the SiO2 film thickness of the second surface correction based on the mathematical model between the SiO2 film thickness and the deformation of the thin film element after two surface corrections of the same thin film element. : ； in: It is the change in the vector height of the thin film element when the anti-reflection film acts on the surface of the thin film element; S3.6, SiO2 film thickness based on the calculated second surface correction Coat SiO2 film and perform the second surface correction.

5. The method for surface correction of a thin film element based on a high steepness filter according to claim 4, characterized in that: Step S3.2 is as follows: the background vacuum is reduced to 8.5×10 -4 Pa, set the baking temperature to 160℃, the ion beam anode voltage to 130V, the current to 50A, the gas volume to 50sccm, and the SiO2 film thickness calculated for the first surface correction A SiO2 film is deposited on the rear surface of the substrate to perform the first surface correction; Step S3.6 is as follows: the background vacuum is reduced to 8.5×10 -4 Pa, set the baking temperature to 160℃, the ion beam anode voltage to 130V, the current to 50A, the gas volume to 50sccm, and the SiO2 film thickness calculated for the second surface correction Coat SiO2 film and perform the second surface correction.

6. The method for surface correction of a thin film element based on a high steepness filter according to claim 3, characterized in that: In step S2.1, the material of the substrate is K9 glass.

7. The method for surface correction of a thin film element based on a high steepness filter according to claim 3, characterized in that: In step S2.1, an interferometer is used to measure the peak-to-valley value PV, root mean square RMS and vector height Power0 of the substrate; In step S2.3, the surface shape of the thin film element after being coated with the high-steepness filter film is measured by an interferometer.

8. The method for surface correction of a thin film element based on a high steepness filter according to claim 4, characterized in that: In step S3.1, the The value is 0.

3.

9. The method for surface correction of a thin-film element based on a high-steepness filter according to claim 5, characterized in that: In step S3.2, before coating the SiO2 film on the rear surface of the substrate, the substrate is first cleaned by an ion beam.

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