Method and system for measuring refractive index and thickness of optical thin film

Abstract

The invention relates to the technical field of optical film refractive index and thickness measurement, in particular to an optical film refractive index and thickness measurement method and system, and the method comprises the following steps: a light source forms linearly polarized light and decomposes the linearly polarized light into two beams of orthogonal polarized light which are respectively used as signal light and reference light; the signal light enters the sample arm and passes through a film sample to be detected to form signal light containing sample information; the reference light forms reference light with the same optical path after passing through the reference arm; after the combined beam is formed, the two beams of light have the same polarization direction; calculating a transmission phase angle of the to-be-measured film sample; shielding the reference light in the reference arm, and measuring the light intensity of the signal light containing the sample information and the light intensity of the signal light not containing the sample information; calculating the transmissivity of the film sample to be detected; and calculating the refractive index and the thickness of the to-be-measured film sample. The method and the system provided by the invention can effectively improve the measurement precision and expand the application range of the material.

Description

Technical Field

[0001] The present invention relates to the technical field of optical thin film refractive index and thickness measurement, and particularly relates to a method and system for measuring the refractive index and thickness of an optical thin film. Background Art

[0002] The optical refractive index is a basic physical parameter that describes the interaction between materials and light, and simultaneously reflects physical properties such as the electronic structure and energy band characteristics of materials. Accurate refractive index measurement is not only a core requirement for basic scientific research, but also a key support for the design and development of advanced optoelectronic devices. Currently, a variety of refractive index measurement techniques, such as refractometers, interferometers, and ellipsometers, have been widely used in the refractive index measurement of different materials and different spectral ranges, and have been widely applied in fields such as semiconductors, biology, and medicine.

[0003] However, traditional measurement techniques often require samples to have high spatial uniformity, and are all based on non-magnetic response approximations, and even rely on specific dispersion models for modeling and fitting calculations. There are obvious limitations in terms of measurement principle, accuracy, spatial resolution, and material applicability. For example, the method using an ellipsometer is difficult to directly solve analytically, usually using a numerical fitting algorithm, which requires a long measurement process, while the method using SEM requires destructive measurement. These limitations make it difficult for traditional methods to accurately characterize the optical properties of new materials, thereby hindering the in-depth advancement of basic scientific research and restricting the research and application of high-end optoelectronic devices. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method and system for measuring the refractive index and thickness of an optical thin film. By using the clear mathematical relationship between the complex transmission coefficient and the material optical constants, the refractive index and thickness of the sample are directly solved through an analytical formula, thereby avoiding the problems of parameter coupling and non-uniqueness of inversion encountered in the traditional method that relies on complex physical modeling and numerical iterative fitting processes, and effectively improving the measurement accuracy.

[0005] A method for measuring the refractive index and thickness of an optical thin film, comprising the following steps: S1: The light source forms linearly polarized light after passing through a polarizer, and is decomposed into two orthogonally polarized lights by a polarization beam splitter, which are used as the signal light and the reference light respectively; S2: The signal light is incident on the sample arm, and after passing through the thin film sample to be measured placed in the sample arm, the amplitude and phase of the signal light are changed based on the internal reflection on the upper and lower surfaces of the thin film and the Fabry-Perot cavity effect, forming a signal light containing sample information; S3: The reference light forms a reference light with the same optical path as the signal light containing sample information after passing through the reference arm; S4: After the signal light containing sample information and the reference light with the same optical path are combined by a polarization beam combiner and then pass through a quarter-wave plate with an orientation, they pass through a half-wave plate with an adjustable angle and an analyzer prism, so that the two beams of light have the same polarization direction to generate interference light; S5: Adjust the fast axis direction of the half-wave plate, obtain the light intensity of the corresponding interference light, and then calculate the transmission phase angle of the thin film sample to be measured according to the light intensity of the interference light; S6: Block the reference light in the reference arm, and adjust the fast axis direction of the half-wave plate to , measure the light intensity of the signal light containing sample information without interference, then take out the thin film sample to be measured, and measure the light intensity of the signal light without sample information without interference; S7: Calculate the transmittance of the thin film sample to be measured according to the light intensity of the signal light containing sample information without interference and the light intensity of the signal light without sample information without interference; S8: Calculate the refractive index and thickness of the thin film sample to be measured based on the transmittance of the thin film sample to be measured.

[0006] Optimized, the light source described in step S1 is emitted by a coherent supercontinuum laser.

[0007] Further, two translatable objective lenses are arranged in the sample arm, and the thin film sample to be measured is located on the focal planes of the two objective lenses and the rotation angle is adjusted by an electric turntable.

[0008] Further, two objective lenses and a delay line are arranged in the reference arm.

[0009] Optimized, the rotation angle of the half-wave plate in step S5 is adjusted by an electric turntable.

[0010] Further, the fast axis directions of the half-wave plate in step S5 are respectively adjusted to , and the transmission phase angle of the thin film sample to be measured is calculated according to formula (1): (1); Where: represents the transmission phase angle of the thin film sample to be measured, represents the light intensity of the interference light when the fast axis direction of the half-wave plate is , represents the light intensity of the interference light when the fast axis direction of the half-wave plate is , represents the light intensity of the interference light when the fast axis direction of the half-wave plate is , represents the light intensity of the interference light when the fast axis direction of the half-wave plate is .

[0011] Further, in step S7, the transmittance of the thin film sample to be measured is calculated according to formula (2): (2); Where: represents the transmittance of the thin film sample to be measured, represents the light intensity of the signal light containing sample information without interference, represents the light intensity of the signal light without sample information without interference.

[0012] Further, in step S8, the refractive index and thickness of the thin film sample to be measured are calculated according to formula (3): (3); Where: represents the refractive index of the thin film sample to be measured, represents the thickness of the thin film sample to be measured, represents the transmission coefficient phase of the thin film sample to be measured, represents the refractive index of the substrate, represents the transmittance of the thin film sample to be measured, represents the wavelength of the light emitted by the light source.

[0013] An optical thin film refractive index and thickness measurement system for performing an optical thin film refractive index and thickness measurement method as described in any one of the above, comprising a light source module, an optical interference module, an optical detection module, and a data acquisition and processing module; The light source module includes a coherent supercontinuum laser and a polarizer; The optical interference module includes a polarization beam splitter, a sample arm, a reference arm, and a polarization beam combiner. Two translatable objective lenses are provided in the sample arm, and two objective lenses and a delay line are provided in the reference arm; The optical detection module includes an oriented quarter-wave plate, an adjustable-angle half-wave plate, and an analyzer prism; The data acquisition and processing module includes a spectrometer and a data processing unit.

[0014] Optimally, the two objective lenses in the sample arm are controlled to translate by a high-precision electric displacement stage.

[0015] Advantages of the invention: The optical thin film refractive index and thickness measurement method and system provided by the present invention have the following advantages: 1. It can realize the full automation of the measurement process, significantly improving the measurement efficiency.

[0016] 2. It is applicable to the application requirements of various complex material structures, not limited to uniform thin film materials, and can also be applicable to anisotropic materials, expanding the applicable range of the materials to be measured.

[0017] 3. Directly analyze and calculate the refractive index and thickness of the material based on the transmittance, avoiding the dependence on iterative fitting of the physical model, thereby improving the calculation accuracy and convergence stability. Description of the Drawings

[0018] Figure 1 It is a schematic flowchart of the present invention.

[0019] Figure 2 It is a schematic structural diagram of the system of the present invention. Detailed Embodiments

[0020] A method for measuring the refractive index and thickness of an optical thin film, which includes the following steps, and its flowchart is as Figure 1 shown: S1: The light source forms linearly polarized light after passing through a polarizer, and is decomposed into two orthogonally polarized lights by a polarization beam splitter, which are used as the signal light and the reference light respectively; Specifically, the light source can be emitted by a coherent supercontinuum laser, and is decomposed into two orthogonally polarized lights by a polarization beam splitter, which are used as the signal light and the reference light respectively, which can take into account the spectral width and coherence length, while ensuring the wide-spectrum measurement and the visibility of the interference signal, improving the measurement accuracy of the transmission coefficient, and realizing the analysis of the material dispersion effect.

[0021] S2: The signal light is incident on the sample arm. After passing through the thin film sample to be measured placed in the sample arm, the amplitude and phase of the signal light are changed based on the internal reflection on the upper and lower surfaces of the thin film and the Fabry-Perot cavity effect, forming a signal light containing sample information; Specifically, two translatable objective lenses are arranged in the sample arm, and the thin film sample to be measured is located on the focal planes of the two objective lenses and the rotation angle is adjusted by an electric turntable.

[0022] The two translatable objective lenses can be controlled to translate by a high-precision electric displacement stage, and the thin film sample to be measured can be adjusted to rotate by an electric turntable, so as to realize the precise rotation and electric translation adjustment of the thin film sample to be measured, optimize the angle between the sample orientation and the beam polarization, and realize the independent measurement of ordinary light and extraordinary light; at the same time, ensure that the thin film sample to be measured is always in the best object plane position, thereby improving the measurement accuracy.

[0023] S3: The reference light forms a reference light with the same optical path as the signal light containing sample information after passing through the reference arm; Specifically, two objective lenses and a delay line can be arranged in the reference arm, and the delay line can be adjusted by a precision electric optical path adjustment device.

[0024] Two objective lenses are arranged in the reference arm to eliminate chromatic aberration. By adjusting the delay line, the optical path difference between the sample arm and the reference arm can be finely compensated, so that the reference light after passing through the reference arm forms a reference light with the same optical path as the signal light containing sample information, thereby eliminating the background phase and improving the measurement accuracy of the phase signal.

[0025] S4: After the signal light containing sample information and the reference light with the same optical path are combined by a polarization beam combiner, they pass through the quarter-wave plate with the specified orientation, and then pass through the half-wave plate with an adjustable angle and the polarization analyzer prism, so that the two beams of light have the same polarization direction to generate interference light; The quarter-wave plate with the specified orientation, the half-wave plate with an adjustable angle and the polarization analyzer prism form an optical detection module, which can realize the polarization projection interference of the optical signals of the sample arm and the reference arm, improve the signal-to-noise ratio of the interference spectral signal, and ensure the stability and accuracy of the measurement data.

[0026] S5: Adjust the fast axis direction of the half-wave plate, obtain the light intensity of the corresponding interference light, and then calculate the transmission phase angle of the thin film sample to be measured according to the light intensity of the interference light; Specifically, the half-wave plate can adjust the rotation angle through an electric turntable, so that the control of the fast axis direction of the half-wave plate is automated and relatively precise.

[0027] Specifically, the fast axis direction of the half-wave plate can be adjusted to be respectively, and the transmission phase angle of the thin film sample to be measured is calculated according to formula (1): (1); Where: represents the transmission phase angle of the thin film sample to be measured, represents the light intensity of the interference light when the fast axis direction of the half-wave plate is ; represents the light intensity of the interference light when the fast axis direction of the half-wave plate is ; represents the light intensity of the interference light when the fast axis direction of the half-wave plate is ; represents the light intensity of the interference light when the fast axis direction of the half-wave plate is .

[0028] S6: Block the reference light in the reference arm, adjust the fast axis direction of the half-wave plate to , measure the light intensity of the signal light containing sample information without interference, then take out the thin film sample to be measured, and measure the light intensity of the signal light without sample information without interference; S7: Calculate the transmittance of the thin film sample to be measured according to the light intensity of the signal light containing sample information without interference and the light intensity of the signal light without sample information without interference; Specifically, the transmittance of the thin film sample to be measured can be calculated according to Equation (2): (2); Where: represents the transmittance of the thin film sample to be measured, represents the light intensity of the signal light containing sample information without interference, represents the light intensity of the signal light without sample information without interference.

[0029] S8: Calculate the refractive index and thickness of the thin film sample to be measured based on the transmittance of the thin film sample to be measured.

[0030] Specifically, the refractive index and thickness of the thin film sample to be measured can be calculated according to Equation (3): (3); Where: represents the refractive index of the thin film sample to be measured, represents the thickness of the thin film sample to be measured, represents the phase of the transmission coefficient of the thin film sample to be measured, represents the refractive index of the substrate, represents the transmittance of the thin film sample to be measured, represents the wavelength of the light emitted by the light source.

[0031] It can be seen from the above measurement method that the measurement method of the refractive index and thickness of the optical thin film provided by the present invention can realize the full automation of the measurement process, significantly improving the measurement efficiency. And its spectrum is from 600 nm to 1100 nm, which can be extended to deep ultraviolet or mid- and far-infrared according to specific requirements, covering key bands such as visible light and near-infrared, and is applicable to the characterization of various material systems such as semiconductors, metals, dielectrics, and metamaterials.

[0032] By rotating the thin film sample to be measured to control the angle between the optical axis direction of the sample and the polarization direction of the signal light, independent measurement of ordinary light and extraordinary light can be realized, which is applicable to anisotropic materials.

[0033] This method directly analyzes and calculates the refractive index and thickness of the material based on the transmittance, avoiding the dependence on the iterative fitting of the physical model, thereby improving the calculation accuracy and convergence stability.

[0034] Specifically, through experiments, it can be verified that the measurement results of the thickness of the optical thin film obtained by using this method and the thickness results obtained by using the existing method are shown in Table 1: Table 1 In Table 1, SEM represents the method measured by the existing scanning electron microscope, and ellipsometer represents the method measured by the existing ellipsometer.

[0035] As can be seen from Table 1, the measurement result of the optical film thickness finally obtained by this method is closest to the nominal value. Therefore, the optical film thickness obtained by this method is the most accurate. The method using an ellipsometer has obvious measurement deviations for transparent sample materials with small thicknesses. And it is difficult to directly solve analytically using the ellipsometer method. Usually, a numerical fitting algorithm is adopted, which requires a long measurement process. While the method using SEM requires destructive measurement. Compared with these two methods, this method does not require a fitting process and can achieve non-destructive, non-contact, fast, and high-precision measurement of the optical film thickness.

[0036] An optical film refractive index and thickness measurement system for performing an optical film refractive index and thickness measurement method as described in any one of the above, the system structure schematic diagram is as Figure 2 shown, which includes a light source module, an optical interference module, an optical detection module, and a data acquisition and processing module; The light source module includes a coherent supercontinuum laser and a polarizer. The coherent supercontinuum laser is used to emit light, and the polarizer is used to form linearly polarized light from the light source; The optical interference module is designed based on the Mach-Zehnder interferometer structure. It includes a polarization beam splitter, a sample arm, a reference arm, and a polarization beam combiner. Two translatable objective lenses are arranged in the sample arm, and two objective lenses and a delay line are arranged in the reference arm; The polarization beam splitter is used to decompose the linearly polarized light into two orthogonally polarized lights, which are used as the signal light and the reference light respectively. The sample arm is used to change the amplitude and phase of the signal light based on the internal reflection of the upper and lower surfaces of the film and the Fabry-Perot cavity effect; The reference arm is used to form a reference light with the same optical path as the signal light containing sample information; The polarization beam combiner is used to combine the signal light containing sample information and the reference light with the same optical path; The optical detection module includes an oriented quarter-wave plate, an adjustable-angle half-wave plate, and an analyzer prism. The optical detection module is used to make the two beams of light have the same polarization direction and generate interference light; The data acquisition and processing module includes a spectrometer and a data processing unit, which are used to measure the light intensity of the interference light and finally obtain the refractive index and thickness of the optical film through data processing.

[0037] Optimally, the two objective lenses in the sample arm are controlled to translate by a high-precision electric displacement stage, and the thin film sample to be measured can be adjusted to rotate the angle by an electric turntable, so as to realize automatic control, accurately rotate and electrically translate the thin film sample to be measured, ensure that the thin film sample to be measured is always in the best object plane position, and thus improve the measurement accuracy.

[0038] In summary, the method and system for measuring the refractive index and thickness of an optical thin film provided by the present invention utilize the clear mathematical relationship between the complex transmission coefficient and the optical constants of the material, and directly solve the refractive index and thickness of the sample through an analytical formula, avoiding the complex physical modeling and numerical iterative fitting processes relied on by traditional methods, effectively improving the measurement speed and accuracy, and having a relatively wide application range.

[0039] The foregoing are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for measuring the refractive index and thickness of an optical thin film, characterized in that: It includes the following steps: S1: The light source forms linearly polarized light after passing through a polarizer and is decomposed into two orthogonally polarized lights by a polarization beam splitter, which are used as the signal light and the reference light respectively; S2: The signal light is incident on the sample arm. After passing through the thin film sample to be measured placed in the sample arm, the amplitude and phase of the signal light are changed based on the internal reflection on the upper and lower surfaces of the thin film and the Fabry-Perot cavity effect, forming a signal light containing sample information; S3: The reference light forms a reference light with the same optical path as the signal light containing sample information after passing through the reference arm; S4: After the signal light containing sample information and the reference light with the same optical path are combined by a polarization beam combiner, they pass through a quarter-wave plate with an orientation of and then pass through a half-wave plate with an adjustable angle and an analyzer prism, so that the two beams of light have the same polarization direction and produce interference light; S5: Adjust the fast axis direction of the half-wave plate, obtain the light intensity of the corresponding interference light, and then calculate the transmission phase angle of the thin film sample to be measured according to the light intensity of the interference light; S6: Block the reference light in the reference arm and adjust the fast axis direction of the half-wave plate to , measure the intensity of the signal light containing sample information without interference, then take out the thin film sample to be measured, and measure the intensity of the signal light without sample information without interference; S7: Calculate the transmittance of the thin film sample to be measured according to the light intensity of the signal light containing sample information when there is no interference and the light intensity of the signal light without sample information when there is no interference; S8: Calculate the refractive index and thickness of the thin film sample to be measured based on the transmittance of the thin film sample to be measured; 2. The method for measuring the refractive index and thickness of an optical thin film according to claim 1, characterized in that: In step S1, the light source is emitted by a coherent supercontinuum laser; 3. The method for measuring the refractive index and thickness of an optical thin film according to claim 1, wherein: In step S2, two translatable objective lenses are arranged in the sample arm, and the thin film sample to be measured is located on the focal planes of the two objective lenses and the rotation angle is adjusted by an electric turntable; 4. A method for measuring the refractive index and thickness of an optical thin film according to claim 1, characterized in that: In step S3, two objective lenses and a delay line are arranged in the reference arm; 5. The method for measuring the refractive index and thickness of an optical thin film according to claim 1, wherein: In step S5, the rotation angle of the half-wave plate is adjusted by an electric turntable; 6. The measuring method for the refractive index and thickness of an optical thin film according to claim 1, wherein: In step S5, the fast axis directions of the half-wave plates are adjusted respectively to , and then the transmission phase angle of the thin film sample to be measured is calculated according to Equation (1): （1）； Wherein: represents the transmission phase angle of the thin film sample to be measured, represents that the fast axis direction of the half-wave plate is the light intensity of the interference light at this time, represents that the fast axis direction of the half-wave plate is the light intensity of the interference light at this time, represents that the fast axis direction of the half-wave plate is the light intensity of the interference light at this time, represents that the fast axis direction of the half-wave plate is the light intensity of the interference light at this time.

7. The measuring method for the refractive index and thickness of an optical thin film according to claim 6, characterized in that: In step S7, the transmittance of the thin film sample to be measured is calculated according to Equation (2); （2）； Wherein: represents the transmittance of the thin film sample to be measured, represents the light intensity of the signal light containing sample information without interference, represents the light intensity of the signal light not containing sample information without interference.

8. A method for measuring the refractive index and thickness of an optical thin film according to claim 1, characterized in that: In step S8, the refractive index and thickness of the thin film sample to be measured are calculated according to Equation (3); （3）； Wherein: represents the refractive index of the thin film sample to be measured, represents the thickness of the thin film sample to be measured, represents the transmission coefficient phase of the thin film sample to be measured, represents the refractive index of the substrate, represents the transmittance of the thin film sample to be measured, represents the wavelength of the light emitted by the light source.

9. An optical thin film refractive index and thickness measurement system for implementing a method for measuring the refractive index and thickness of an optical thin film according to any one of claims 1 to 8, characterized in that: It includes a light source module, an optical interference module, an optical detection module, and a data acquisition and processing module; The light source module includes a coherent supercontinuum laser and a polarizer; The optical interference module includes a polarization beam splitter, a sample arm, a reference arm, and a polarization beam combiner. Two translatable objective lenses are arranged in the sample arm, and two objective lenses and a delay line are arranged in the reference arm; The optical detection module includes an oriented quarter-wave plate, an adjustable-angle half-wave plate, and an analyzer prism; The data acquisition and processing module includes a spectrometer and a data processing unit; 10. The measurement system for the refractive index and thickness of an optical thin film according to claim 9, characterized in that: The two objective lenses in the sample arm are controlled to translate by a high-precision electric displacement stage.

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