A resolution-optimized metasurface-based thin-film thickness measurement method and system

Abstract

The application relates to the technical field of micro-nano optics and high-precision thin film thickness measurement, and discloses a resolution-optimized thin film thickness measurement method and system based on a metasurface, to solve the problem that the existing thin film thickness measurement method cannot realize non-contact and high resolution. The system comprises a broadband light source, a composite metasurface measurement probe, a spectrometer and a data processing unit. The composite metasurface measurement probe comprises a composite metasurface, the composite metasurface comprises a multilayer step flat lens and a single-layer metasurface, the single-layer metasurface is composed of a transparent glass substrate and a micro-nano structure array constructed above the transparent glass substrate, and the flat lens is connected with the transparent glass substrate of the single-layer metasurface. The composite metasurface is used for receiving and phase-controlling incident light of different wavelengths. The spectrometer is used for recording the light intensity distribution of reflected light changing with wavelengths, and obtaining a reflected spectrum signal. The data processing unit is used for calculating the thin film thickness according to the reflected spectrum signal. The system can improve the precision of thin film thickness measurement.

Description

Technical Field

[0001] This application relates to the fields of micro-nano optics and high-precision thin film thickness measurement technology, and more specifically, to a resolution-optimized thin film thickness measurement method and system based on metasurfaces. Background Technology

[0002] Precise measurement of thin film thickness plays a crucial role in semiconductor manufacturing, optical coating, biosensing, and precision device inspection. Existing methods for thin film thickness measurement mainly include mechanical stylus method, elliptic polarization method, and white light interferometry. While the mechanical stylus method offers high accuracy, it is a contact measurement method, which can easily damage the sample surface and is unsuitable for measuring flexible or brittle thin films. The elliptic polarization method is a non-contact measurement, but its system structure is complex, costly, and requires a demanding measurement environment. Although the white light interferometry method can achieve high axial resolution, it typically requires a sophisticated scanning device, uniform illumination conditions, and complex data processing procedures.

[0003] In recent years, with the development of metasurface optics, metasurface-based micro and nanostructures have been able to achieve wavefront shaping, dispersion compensation, and high numerical aperture focusing through local phase modulation. Utilizing their anti-dispersion focusing properties, near-diffraction-limited focusing performance can be achieved over a wide spectral range, thereby significantly improving spatial resolution.

[0004] Therefore, there is an urgent need to develop a non-contact, high-resolution thin film thickness measurement method based on metasurfaces. Summary of the Invention

[0005] To address the issue that existing thin film thickness measurement methods cannot achieve non-contact, high-resolution measurement, this invention provides a resolution-optimized thin film thickness measurement method and system based on metasurfaces, thereby improving the accuracy of thin film thickness measurement.

[0006] To achieve the above objectives, according to a first aspect of the present invention, a resolution-optimized thin film thickness measurement system based on metasurfaces is provided, the system comprising:

[0007] Broadband light source, composite metasurface measurement probe, spectrometer and data processing unit; the composite metasurface measurement probe includes a composite metasurface, which includes a multilayer stepped flat lens and a single-layer metasurface. The single-layer metasurface is composed of a transparent glass substrate and a micro-nano structure array built on top of it. The multilayer stepped flat lens is connected to the transparent glass substrate of the single-layer metasurface.

[0008] A broadband light source is used to provide incident light with a continuous spectrum, which is collimated and then irradiates the composite metasurface measurement probe.

[0009] Composite metasurfaces are used to phase-modulate incident light of different wavelengths, so that incident light of different wavelengths is focused at the same focal point to form a focused spot. The focused spot illuminates the surface of the sample containing the thin film structure, and reflected light is generated at the air-thin film interface and the thin film-substrate interface of the sample. The reflected light returns to the composite metasurface through the coaxial optical path and is guided to the spectrometer through the optical fiber.

[0010] A spectrometer is used to collect all reflected light returning from a sample, record the intensity distribution of the reflected light as a function of wavelength, and obtain the reflected spectral signal.

[0011] The data processing unit is connected to the signal output terminal of the spectrometer and is used to calculate the film thickness based on the reflection spectral signal.

[0012] Furthermore, micro- and nanostructures include a variety of centrosymmetric units.

[0013] Furthermore, the micro-nano structures include solid cylinders, solid square cylinders, hollow cylinders, hollow square cylinders, hollow cylinders with an outer circle and an inner square, and hollow cylinders with an outer square and an inner circle.

[0014] Furthermore, the step phase and phase change of the multilayer stepped flat lens must both satisfy a condition not exceeding the maximum phase compensation value of the micro / nano structure; the step phase of the multilayer stepped flat lens is calculated based on the equivalent optical path difference between adjacent step sections of the multilayer stepped flat lens; the phase change of the multilayer stepped flat lens is calculated based on the geometric optical path difference at the boundary of adjacent step sections of the multilayer stepped flat lens.

[0015] Furthermore, the boundary radius of each step section of the multilayer stepped flat lens is determined by first determining the feasible range of the boundary radius of each step section of the multilayer stepped flat lens, given the radius of the composite metasurface and the number of step layers of the multilayer stepped flat lens, and ensuring that the step phase and phase change of the composite metasurface are not greater than the maximum phase compensation value of the micro / nano structure, and then using a genetic algorithm to calculate the optimal boundary radius.

[0016] Furthermore, one side of the multi-layer stepped flat lens is a stepped surface, and the other side is a flat surface. The stepped surface or the flat surface is connected to the transparent glass substrate of the single-layer metasurface.

[0017] Furthermore, the stepped surface or plane of the multi-layer stepped flat lens can be adhesively connected to the transparent glass substrate of the single-layer metasurface.

[0018] According to a second aspect of the present invention, a resolution-optimized thin film thickness measurement method based on metasurfaces is also provided, applied to the aforementioned resolution-optimized thin film thickness measurement system based on metasurfaces, the measurement method comprising:

[0019] Calculate the total reflectivity coefficient of all reflected light returned from the sample under test;

[0020] Calculate the reflectance distribution of the sample under test based on the total reflectance coefficient, incident light spectrum, and reflectance spectrum.

[0021] Using a genetic algorithm, the film thickness is calculated based on the refractive index dispersion model of the thin film material and the reflectivity distribution of the sample under test.

[0022] Further, the total reflectivity coefficient of all reflected light returning from the sample under test is calculated, including obtaining all reflected light returning from the sample under test; considering all reflected light returning from the sample under test as the sum of the first-order reflected light to the Nth-order reflected light, where N is an integer greater than 1; determining the reflection coefficients of each order of reflected light from the first-order to the Nth-order reflected light based on the refractive index of air, the refractive index of the thin film medium, the refractive index of the substrate, the reflection and transmission coefficients of the air-thin film interface, and the reflection coefficient of the thin film-substrate interface; and superimposing the reflection coefficients of the first-order to the Nth-order reflected light to obtain the total reflectivity coefficient.

[0023] Furthermore, using a genetic algorithm, the film thickness is calculated based on the refractive index dispersion model of the thin film material and the reflectivity distribution of the sample under test. This includes iteratively calculating the film thickness using a genetic algorithm based on the refractive index dispersion model of the thin film material and the reflectivity distribution of the sample under test. When the preset number of iterations is reached or the fitness improvement rate of the population for multiple consecutive generations is lower than a set threshold, the genetic algorithm stops iterating and takes the film thickness value corresponding to the individual with the highest fitness as the optimal solution for the film thickness.

[0024] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0025] (1) The present invention provides a resolution-optimized thin film thickness measurement system based on metasurfaces. A composite metasurface measurement probe is used as a key optical component. Compared with traditional objectives, the composite metasurface includes multiple stepped flat lenses and a single-layer metasurface. First, the layer-by-layer thickness difference of the multiple stepped flat lenses is used to achieve large-range phase pre-compensation. Then, the micro-nano structure on the single-layer metasurface is used to precisely control the phase of the incident light, achieving achromatic focusing at a specific wavelength, thereby obtaining a focused spot close to the diffraction limit. This system features a compact structure, no aberrations, and customizable phase distribution characteristics, significantly improving the lateral resolution of thin film thickness measurement. It also combines the accuracy of spectral reflectance with the high-resolution characteristics of metasurface focusing, meeting the needs for rapid online detection of thin film materials. The entire measurement process does not require contact with the sample surface, effectively avoiding mechanical damage. The composite metasurface measurement probe can be integrated into the probe tip according to measurement requirements to achieve high-precision measurement of thin film thickness in a confined space.

[0026] (2) The present invention provides a resolution-optimized thin film thickness measurement system based on metasurfaces, which introduces a multi-layer stepped flat lens as a macroscopic phase pre-compensation layer. Negative phase dispersion compensation is introduced by accumulating the radially varying optical thickness in different annular zones. Subsequently, the micro-nano structure of the phase library is reused in each stepped partition to achieve local fine phase modulation. Thus, continuous focusing is achieved in the white light band and the range of designable aperture and focal length is significantly expanded.

[0027] (3) The metasurface-based resolution-optimized thin film thickness measurement method provided by the present invention not only improves the accuracy and stability of thin film thickness measurement, but also expands the application range of the metasurface-based resolution-optimized thin film thickness measurement system in micro-nano scale structures and complex space environments. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 A structural block diagram of a resolution-optimized thin film thickness measurement system based on metasurfaces is provided for embodiments of this application;

[0030] Figure 2 A schematic diagram of the structure of a composite metasurface measurement probe in a resolution-optimized thin film thickness measurement system based on metasurfaces, provided for an embodiment of this application;

[0031] Figures 3A-3F This is a schematic diagram of the shape of the micro / nano structure provided in the embodiments of this application;

[0032] Figure 4 A schematic flowchart illustrating a resolution-optimized thin film thickness measurement method based on metasurfaces, provided for an embodiment of this application;

[0033] Figure 5 This is a schematic diagram of multiple reflections of incident light inside a sample containing a thin film structure, provided in an embodiment of this application. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0035] The terms "first," "second," "third," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish different objects, not to describe a specific order. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or apparatuses.

[0036] like Figure 1 As shown, this invention provides a resolution-optimized thin film thickness measurement system based on metasurfaces, including a broadband light source, a composite metasurface measurement probe, a spectral acquisition unit (also called a spectrometer), and a data processing unit. The composite metasurface measurement probe is integrated into the probe tip, adapting to the needs of thin film thickness measurement in confined spaces. The composite metasurface measurement probe includes a composite metasurface. During measurement, the broadband light source illuminates the surface of the sample to be measured (hereinafter referred to as the thin film sample) containing the thin film structure through the composite metasurface measurement probe. The spectrometer collects all reflected light returning from the sample to be measured, recording the reflection spectral signal as a function of wavelength, showing the intensity of the reflected light. The reflection spectral signal can be represented by a reflection spectrum diagram, which is a curve plotted with wavelength as the abscissa and light intensity as the ordinate. Finally, the reflection spectral signal is fitted based on reflectance by the data processing unit to accurately calculate the thin film thickness. Reflectance is the ratio of the intensity of the reflected light to the intensity of the incident light.

[0037] like Figure 2 As shown, the composite metasurface includes a multilayer stepped flat lens and a single-layer metasurface. The single-layer metasurface consists of a transparent glass (e.g., quartz glass) substrate and an array of micro / nano structures constructed on top of it. One side of the multilayer stepped flat lens is a stepped surface, and the other side is a plane. This stepped surface or platform can be connected to the transparent glass substrate of the single-layer metasurface, preferably in an adhesive manner.

[0038] A broadband light source is used to provide incident light with a continuous spectrum, preferably a white light source or a supercontinuum source, to ensure sufficient spectral width and stability. The incident light is transmitted via a coupled optical fiber and collimated before illuminating the composite metasurface measurement probe. The composite metasurface performs phase modulation on incident light of different wavelengths, focusing all wavelengths of incident light at the same focal point, thus obtaining a focused spot close to the diffraction limit. This focused spot then illuminates the surface of the sample containing the thin film structure, generating reflected light at the air-film interface and the film-substrate interface, respectively, forming multi-beam interference.

[0039] The reflected light returns to the composite metasurface measurement probe via a coaxial optical path and is then guided to the spectral acquisition unit through a beam splitter or optical fiber. The spectral acquisition unit preferably employs a high-resolution array spectrometer to record the intensity distribution of the reflected light as a function of wavelength, thereby obtaining the reflected spectral signal.

[0040] The data processing unit is connected to the output of the spectral acquisition unit and is used to analyze and process the acquired reflectance spectral signals. First, a high signal-to-noise ratio reflectance spectral curve (i.e., reflectance spectrum) is obtained through spectral normalization and noise filtering. Second, combined with the refractive index dispersion model of the thin film material (e.g., Cauchy dispersion formula, Sellmeier dispersion formula, etc.), the film thickness parameters are calculated using a reflectance fitting method based on a genetic algorithm, achieving a high-precision solution for the film thickness.

[0041] Limited by the maximum phase dispersion achievable by micro / nano structures, large-scale designs of single-layer metasurfaces are difficult to realize. To address this, this invention introduces a multi-layer stepped flat lens as a macroscopic phase pre-compensation layer, achieving progressively accumulated negative phase dispersion compensation through radially varying optical thickness. Subsequently, micro / nano structures reusing the phase library are used within each stepped zone to achieve localized fine phase modulation. Thus, under broadband achromatic focusing conditions, the aperture size bottleneck of single-layer metasurfaces can be overcome.

[0042] To realize the structural design of composite metasurfaces, a phase response library (referred to as the phase library) of micro / nano structures is first constructed. Finite-difference time-domain (FDTD) electromagnetic simulation is then used to scan the micro / nano structures under periodic boundary conditions. The micro / nano structures can be various centrosymmetric units, such as... Figures 3A-3F The diagram shows solid cylinders, solid square cylinders, hollow cylinders, hollow square cylinders, hollow cylinders with an outer circle and inner square, and hollow cylinders with an outer square and inner circle. Six types of original phase libraries are obtained by changing the key geometric parameters of the micro / nanostructure (cylinder diameter or square cylinder side length). The phase of the complex amplitude of the transmitted field is extracted within the target wavelength range. After unwrapping the phases of each library, to eliminate the overall phase bias between different libraries, the micro / nanostructure with the "smallest average phase" among all phase libraries is selected as the global benchmark, and phase leveling (normalization) is performed on all libraries. The maximum phase compensation value in the normalized phase library is then used as the basis for the calculation. (Maximum phase compensation for all micro / nano structures at the minimum design / target wavelength).

[0043] Due to the maximum phase compensation value of micro / nano structures Limited by their inherent limitations, single-layer metasurfaces struggle to meet the phase requirements of wide-spectrum achromatic focusing under large aperture or long focal length conditions. To overcome this limitation, this invention introduces a multi-layer stepped flat lens as a macroscopic phase pre-compensation layer. Negative phase dispersion compensation is introduced through radially varying optical thicknesses accumulating in different annular zones. Subsequently, micro / nano structures of the phase library are reused within each stepped zone to achieve localized fine phase modulation. This enables continuous focusing in the white light band and significantly expands the range of designable apertures and focal lengths.

[0044] Given a composite metasurface with radius R ≈ 200 μm and a designed focal length f = 5 mm, divide the multi-layer stepped planar lens into N partitions, with the partition boundary radius... for:

[0045] (1)

[0046] Where N represents the number of steps in a multi-step stepped flat lens.

[0047] Let the equivalent thickness of the i-th step partition (relative to the outermost layer, i.e., the N-th layer) be... And define the difference between the refractive index of a multi-layered stepped flat lens and the refractive index of air as . Then the equivalent optical path difference (OPD) between adjacent partitions can be written as:

[0048] (2)

[0049] The corresponding step phase is:

[0050] (3)

[0051] in, The wavelength of incident light is any wavelength in the continuous spectrum.

[0052] To achieve focusing at a focal length of f, the geometric optical path difference at the boundary of adjacent zones can be written as:

[0053] (4)

[0054] The corresponding layer-by-layer phase changes are:

[0055] (5)

[0056] In engineering design, to ensure that the "residual phase" (i.e., the difference between the target phase of the composite metasurface and the phase provided by each partition of the multilayer stepped flat lens) borne by the micro-nano structure in each partition does not exceed the maximum phase compensation value of the micro-nano structure in the phase library. The phase library constraint conditions must be met:

[0057] (6)

[0058] Therefore, the target phase of the composite metasurface at any radius r It can be broken down into:

[0059] (7)

[0060] in, Local phase modulation provided for micro / nano structures. The step phase provided for a multi-layered stepped flat lens.

[0061] In one embodiment, to balance fabrication feasibility and structural quantity control, i.e., to control the radius R to be no greater than a set value; and to require that the width of each partition be increased as much as possible to reduce the number of steps and alignment difficulty, i.e., in order to control the number of layers to be no greater than a set number of layers, the final radius of the composite metasurface is R=196um, and the number of steps of the multi-layer stepped flat lens is N=6.

[0062] Given R=196um, N=6, and [other conditions] Under the aforementioned phase library constraints, the radius of each partition boundary can be determined first. The feasible interval is given (defined by the condition that "the remaining phase borne by the micro / nano structure within each partition does not exceed the maximum phase compensation value"). (derived), and then a genetic algorithm is used to... (Right now{ Joint optimization is performed using discrete sampling points along the lens diameter direction. Multiple sampling wavelengths The objective function is the "weighted sum of errors between the remaining phase and the matching structure phase", where m and k are integers.

[0063] (8)

[0064] in, This represents the micro / nano structure parameter in the phase library that best matches the remaining phase of the discrete sampling point. This is the wavelength weighting coefficient. For the remaining phase, To match the structure phase, by minimizing this error, we can obtain the optimal set of partition boundary radii that satisfy wide-spectrum achromatic focusing and are fabrication-friendly. , as the final structural parameters of the composite metasurface.

[0065] Finally, the minimum error and the corresponding set of partition boundary radii obtained through genetic algorithm optimization are obtained. The possible values ​​are shown in the table below:

[0066]

[0067] In one embodiment, assembling a multilayer stepped flat lens and a single-layer metasurface into a composite metasurface measurement probe includes the following steps: The multilayer stepped flat lens is moved to the center of the single-layer metasurface using a vacuum pickup system. During lateral alignment, the center point of the single-layer metasurface is first marked on the camera. Then, the multilayer stepped flat lens is illuminated with a vertically incident laser beam until its focal point is clearly visible on the camera. Next, the position of the flat lens is fine-tuned until its focal point completely coincides with the previously marked center point of the single-layer metasurface. After this step, the multilayer stepped flat lens is vertically moved to approximately 50 μm above the single-layer metasurface and released. The multilayer stepped flat lens then rests on a U-shaped double-sided adhesive tape. A vacuum chuck is then used to gently press the top of the multilayer stepped flat lens to ensure it adheres firmly to the tape. That is, one side of the double-sided tape is the stepped surface or plane of the multilayer stepped flat lens, and the other side is the transparent glass substrate of the single-layer metasurface.

[0068] like Figure 4 As shown, a resolution-optimized thin film thickness measurement method based on metasurfaces is provided, applied to the aforementioned resolution-optimized thin film thickness measurement system based on metasurfaces. This method can be executed by a data processing unit and includes the following steps:

[0069] Step 401: Calculate the total reflectivity coefficient of all reflected light returned from the sample to be tested.

[0070] Step 402: Calculate the reflectance distribution of the sample under test based on the total reflectance coefficient, incident light spectrum and reflectance spectrum.

[0071] Step 403: Using a genetic algorithm, calculate the film thickness based on the refractive index dispersion model of the thin film material and the reflectivity distribution of the sample to be tested.

[0072] The aforementioned metasurface-based resolution-optimized thin film thickness measurement method avoids mechanical damage by eliminating the need for contact with the sample surface containing the thin film structure. It enables rapid and high-precision online detection of thin film thickness, solving the limitations of traditional optical thickness measurement methods in terms of resolution and spatial adaptability. This provides an efficient and reliable solution for thin film thickness measurement in micro-nano manufacturing, semiconductor processes, and precision optical components.

[0073] In one embodiment, such as Figure 5 As shown, for a test sample containing a thin film structure, the perpendicularly incident light will undergo multiple reflections at the air-thin film interface (i.e., the upper interface of the test sample) and the thin film-substrate interface (i.e., the lower interface of the test sample).

[0074] Step 401 above calculates the total reflectivity coefficient of all reflected light returning from the sample under test, including the following steps: All reflected light returning from the sample under test (also called total reflected light) can be considered as the sum of reflected beams 1, 2, 3, 4, and higher-order (Nth-order) reflected light (i.e., first-order reflected light, second-order reflected light, third-order reflected light, fourth-order reflected light... Nth-order reflected light). Let the refractive index of air be n0, the refractive index of the thin film medium be n1, the refractive index of the substrate be n2, and the reflectivity coefficient and transmittance coefficient of the air-thin film interface be r... 01 t 01 The reflectivity and transmittance coefficients of the thin-film-air interface are r and r, respectively. 10 t 10 The reflectivity coefficient of the thin film-substrate interface is r. 12 Then the reflectivity coefficients of the reflected light at each level can be expressed as:

[0075] (9)

[0076] in, This refers to the phase delay that occurs when light propagates in a thin film medium.

[0077] (10)

[0078] Where d is the physical thickness of the film. Let be the angle of incidence of incident light of any wavelength.

[0079] The reflection coefficients are respectively:

[0080] (11)

[0081] (12)

[0082] By superimposing all the reflected light, we can obtain the total reflectivity coefficient:

[0083] (13)

[0084] According to Stokes' theorem and The total reflectivity coefficient can be expressed as:

[0085] (14)

[0086] Since reflectivity is equal to the square of the reflectivity coefficient, once the incident light spectrum and the collected reflection spectrum are known, the reflectivity distribution of the sample can be calculated. By combining the refractive index dispersion model (or reflectivity model) of the thin film material and fitting the reflectivity, the film thickness can be accurately obtained.

[0087] The thin film thickness measurement method used in this invention is based on a global optimization genetic algorithm. This algorithm originates from the theory of biological evolution in nature, simulating the natural selection process of "survival of the fittest" and has the advantages of strong global optimization ability, insensitivity to initial values, and high computational stability. It is especially suitable for complex optimization problems with nonlinearity, multiple peaks, and multiple parameters.

[0088] In thin film thickness measurement, there is a highly nonlinear mapping relationship between the reflectance spectral signal and the film thickness. Traditional gradient descent or local minimization algorithms are prone to getting trapped in local optima. Using a genetic algorithm for global search can effectively avoid this problem, thus obtaining the optimal thickness fitting result. The algorithm stops iterating when a preset number of iterations is reached or the fitness improvement rate of the population for several consecutive generations falls below a threshold. At this point, the film thickness value corresponding to the individual with the highest fitness is the optimal solution.

[0089] It should be noted that, for the sake of simplicity, the foregoing method embodiments are all described as a series of actions. However, those skilled in the art should understand that this application is not limited to the described order of actions, as some steps may be performed in other orders or simultaneously according to this application. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to this application.

[0090] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0091] The above description is merely an exemplary embodiment of this disclosure and should not be construed as limiting the scope of this disclosure. Any equivalent changes and modifications made in accordance with the teachings of this disclosure shall still fall within the scope of this disclosure. Those skilled in the art will readily conceive of embodiments of this disclosure upon considering the specification and practicing the disclosure herein. This application is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art not described herein. The specification and embodiments are to be considered exemplary only, and the scope and spirit of this disclosure are defined by the claims.

[0092] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0093] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A resolution-optimized thin film thickness measurement system based on metasurfaces, characterized in that, It includes a broadband light source, a composite metasurface measurement probe, a spectrometer, and a data processing unit; the composite metasurface measurement probe includes a composite metasurface, which includes a multi-layer stepped flat lens and a single-layer metasurface. The single-layer metasurface is composed of a transparent glass substrate and a micro-nano structure array constructed on top of it. The multi-layer stepped flat lens is connected to the transparent glass substrate of the single-layer metasurface. A broadband light source is used to provide incident light with a continuous spectrum, which is collimated and then irradiates the composite metasurface measurement probe. Composite metasurfaces are used to phase-modulate incident light of different wavelengths, so that incident light of different wavelengths is focused at the same focal point to form a focused spot. The focused spot illuminates the surface of the sample containing the thin film structure, and reflected light is generated at the air-thin film interface and the thin film-substrate interface of the sample. The reflected light returns to the composite metasurface through the coaxial optical path and is guided to the spectrometer through the optical fiber. A spectrometer is used to collect all reflected light returning from a sample, record the intensity distribution of the reflected light as a function of wavelength, and obtain the reflected spectral signal. The data processing unit is connected to the signal output terminal of the spectrometer and is used to calculate the film thickness based on the reflected spectral signal.

2. The resolution-optimized thin film thickness measurement system based on metasurfaces as described in claim 1, characterized in that, Micro- and nanostructures include various centrosymmetric units.

3. The resolution-optimized thin film thickness measurement system based on metasurfaces as described in claim 2, characterized in that, The micro / nano structures include solid cylinders, solid square cylinders, hollow cylinders, hollow square cylinders, hollow cylinders with an outer circle and an inner square, and hollow cylinders with an outer square and an inner circle.

4. The resolution-optimized thin film thickness measurement system based on metasurfaces as described in claim 1, characterized in that, The step phase and phase change of the multi-layer stepped flat lens must both satisfy a value not greater than the maximum phase compensation value of the micro-nano structure. The step phase of a multi-step flat lens is calculated based on the equivalent optical path difference between adjacent step sections of the multi-step flat lens. The phase change of a multi-step flat lens is calculated based on the geometric optical path difference at the boundary of adjacent step sections of the multi-step flat lens.

5. The resolution-optimized thin film thickness measurement system based on metasurfaces as described in claim 4, characterized in that, The boundary radius of each step section of the multilayer stepped flat lens is determined by first determining the feasible range of the boundary radius of each step section of the multilayer stepped flat lens, and then calculating the optimal boundary radius using a genetic algorithm, given the radius of the composite metasurface, the number of step layers of the multilayer stepped flat lens, and the step phase and phase change of the composite metasurface must not exceed the maximum phase compensation value of the micro / nano structure.

6. The resolution-optimized thin film thickness measurement system based on metasurfaces as described in claim 1, characterized in that, One side of the multi-layer stepped flat lens is a stepped surface, and the other side is a flat surface. The stepped surface or the flat surface is connected to a transparent glass substrate with a single-layer metasurface.

7. The resolution-optimized thin film thickness measurement system based on metasurfaces as described in claim 6, characterized in that, The stepped surface or plane of the multi-layer stepped flat lens can be adhesively connected to a transparent glass substrate with a single-layer metasurface.

8. A resolution-optimized thin film thickness measurement method based on metasurfaces, characterized in that, The measurement method, applied to the metasurface-based resolution-optimized thin film thickness measurement system as described in any one of claims 1-7, comprises: Calculate the total reflectivity coefficient of all reflected light returned from the sample under test; Based on the total reflectance coefficient, incident light spectrum, and reflectance spectrum, calculate the reflectance distribution of the sample to be tested; The thickness of the thin film is calculated using a genetic algorithm based on the refractive index dispersion model of the thin film material and the reflectivity distribution of the sample under test.

9. The resolution-optimized thin film thickness measurement method based on metasurfaces as described in claim 8, characterized in that, The calculation of the total reflectivity coefficient of all reflected light returned from the sample to be tested includes: Acquire all reflected light returning from the sample under test; All reflected light returning from the sample to be tested is considered as the sum of the first-order reflected light to the Nth-order reflected light, where N is an integer greater than 1; Based on the refractive index of air, the refractive index of the thin film medium, the refractive index of the substrate, the reflection and transmission coefficients of the air-thin film interface, and the reflection coefficient of the thin film-substrate interface, the reflection coefficients of each level of reflected light from the first order to the Nth order are determined. The total reflectivity coefficient is obtained by superimposing the reflection coefficients of the first-order reflected light to the Nth-order reflected light.

10. The resolution-optimized thin film thickness measurement method based on metasurfaces as described in claim 8, characterized in that, The calculation of the film thickness using a genetic algorithm, based on the refractive index dispersion model of the thin film material and the reflectivity distribution of the sample under test, includes: Using a genetic algorithm, the film thickness is iteratively calculated based on the refractive index dispersion model of the thin film material and the reflectivity distribution of the sample under test; When the preset number of iterations is reached or the fitness improvement rate of the population is lower than the set threshold for multiple consecutive generations, the genetic algorithm stops iterating and takes the membrane thickness value corresponding to the individual with the highest fitness as the optimal solution for the membrane thickness.

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