A method and system for measuring the refractive index and thickness of optical thin films

Abstract

The present invention relates to the technical field of optical film refractive index and thickness measurement, and in particular to a method and system for measuring the refractive index and thickness of an optical film. The method comprises the following steps: a light source generates linearly polarized light and decomposes it into two beams of orthogonal polarized light, which serve as signal light and reference light respectively; the signal light is incident on a sample arm and, after passing through a thin film sample to be measured, forms signal light containing sample information; the reference light passes through the reference arm to form reference light with the same optical path; after forming a combined beam, the two beams are given the same polarization direction; the transmission phase angle of the thin film sample to be measured is calculated; the reference light in the reference arm is blocked, and the intensity of the signal light containing sample information and the intensity of the signal light not containing sample information are measured; the transmittance of the thin film sample to be measured is calculated; and the refractive index and thickness of the thin film sample to be measured are calculated. The method and system provided by the present invention can effectively improve measurement accuracy and expand the scope of application of materials.

Description

Technical Field

[0001] The present invention relates to the technical field of measuring the refractive index and thickness of optical thin films, and in particular to a method and system for measuring the refractive index and thickness of optical thin films. Background Art

[0002] The optical refractive index is a fundamental physical parameter that describes the interaction between a material and light. It also reflects physical properties such as the material's electronic structure and energy band characteristics. 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 ellipsometry, are widely used to measure the refractive index of different materials and across 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 based on non-magnetic response approximations, or even rely on specific dispersion models for modeling and fitting calculations. These techniques present significant limitations in terms of measurement principles, accuracy, spatial resolution, and material applicability. For example, methods using ellipsometry are difficult to directly solve analytically, and numerical fitting algorithms are typically employed, requiring lengthy measurement processes. Methods using scanning electron microscopy (SEM) require destructive measurements. These limitations make it difficult for traditional methods to accurately characterize the optical properties of novel materials, hindering the advancement of basic scientific research and restricting the development 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 optical thin films. By utilizing the clear mathematical relationship between the complex transmission coefficient and the optical constants of the material, the refractive index and thickness of the sample are directly solved through analytical formulas, thereby avoiding the parameter coupling and inversion non-uniqueness problems encountered by traditional methods that rely on complex physical modeling and numerical iterative fitting processes, thereby effectively improving the measurement accuracy.

[0005] A method for measuring the refractive index and thickness of an optical film comprises the following steps:

[0006] S1: The light source forms linearly polarized light after passing through the polarizer, and is decomposed into two orthogonal polarized light beams by the polarization beam splitter, which serve as the signal light and the reference light respectively;

[0007] 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 due to internal reflection on the upper and lower surfaces of the film and the Fabry-Perot cavity effect, forming a signal light containing sample information;

[0008] S3: After passing through the reference arm, the reference light forms a reference light with the same optical path as the signal light containing the sample information;

[0009] S4: After the signal light containing the sample information and the reference light with the same optical path are combined by the polarization beam combiner, After the oriented quarter-wave plate, it passes through the half-wave plate with adjustable angle and the analyzer prism, so that the two beams have the same polarization direction and produce interference light;

[0010] S5: Adjust the fast axis direction of the half-wave plate and obtain the intensity of the corresponding interference light. Then, calculate the transmission phase angle of the thin film sample to be tested based on the intensity of the interference light.

[0011] S6: Block the reference light in the reference arm and adjust the fast axis direction of the half-wave plate to , measuring the intensity of the signal light containing the sample information when there is no interference, then taking out the film sample to be tested, and measuring the intensity of the signal light that does not contain the sample information when there is no interference;

[0012] S7: Calculating the transmittance of the thin film sample to be measured according to the intensity of the signal light containing the sample information when there is no interference and the intensity of the signal light not containing the sample information when there is no interference;

[0013] S8: Calculating 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.

[0014] Optimally, the light source in step S1 is emitted by a coherent supercontinuum laser.

[0015] Furthermore, in step S2, two translationally movable objective lenses are provided 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.

[0016] Furthermore, in step S3, two objective lenses and a delay line are provided in the reference arm.

[0017] Optimally, in step S5, the half-wave plate is rotated at an angle adjusted by an electric turntable.

[0018] Furthermore, in step S5, the fast axis directions of the half-wave plate are adjusted to be , calculate the transmission phase angle of the thin film sample to be tested according to formula (1):

[0019] (1);

[0020] in: represents the transmission phase angle of the thin film sample to be tested, The fast axis direction of the half-wave plate is The intensity of the interference light is The fast axis direction of the half-wave plate is The intensity of the interference light is The fast axis direction of the half-wave plate is The intensity of the interference light is The fast axis direction of the half-wave plate is The intensity of the interference light.

[0021] Furthermore, in step S7, the transmittance of the thin film sample to be tested is calculated according to formula (2):

[0022] (2);

[0023] in: Indicates the transmittance of the film sample to be tested, Indicates the intensity of the signal light containing sample information when there is no interference, Indicates the intensity of signal light that does not contain sample information when there is no interference.

[0024] Furthermore, in step S8, the refractive index and thickness of the thin film sample to be tested are calculated according to formula (3):

[0025] (3);

[0026] in: represents the refractive index of the thin film sample to be tested, Indicates the thickness of the film sample to be tested, represents the transmission coefficient phase of the thin film sample to be tested, represents the substrate refractive index, Indicates the transmittance of the film sample to be tested, Indicates the wavelength of light emitted by a light source.

[0027] A system for measuring the refractive index and thickness of an optical film, for performing any of the above-mentioned methods for measuring the refractive index and thickness of an optical film, comprising a light source module, an optical interference module, an optical detection module, and a data acquisition and processing module;

[0028] The light source module includes a coherent supercontinuum laser and a polarizer;

[0029] The optical interference module includes a polarization beam splitter, a sample arm, a reference arm and a polarization beam combiner. The sample arm is provided with two translationally movable objective lenses, and the reference arm is provided with two objective lenses and a delay line.

[0030] Optical detection module includes Oriented quarter-wave plates, angle-adjustable half-wave plates, and analyzer prisms;

[0031] The data acquisition and processing module includes a spectrometer and a data processing unit.

[0032] The two objectives in the optimized sample arm are translated by a high-precision motorized stage.

[0033] Beneficial effects of the invention:

[0034] The present invention provides a method and system for measuring the refractive index and thickness of an optical thin film, which has the following advantages:

[0035] 1. It can realize full automation of the measurement process and significantly improve the measurement efficiency.

[0036] 2. It is applicable to the application requirements of various complex material structures. It is not limited to uniform thin film materials, but can also be applied to anisotropic materials, expanding the scope of application of the materials to be tested.

[0037] 3. Directly calculate the refractive index and thickness of the material based on the transmittance, avoiding dependence on iterative fitting of the physical model, thereby improving the calculation accuracy and convergence stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic flow chart of the present invention.

[0039] Figure 2 It is a schematic diagram of the system structure of the present invention. DETAILED DESCRIPTION

[0040] A method for measuring the refractive index and thickness of an optical film comprises the following steps, and its flow chart is as follows: Figure 1 As shown:

[0041] S1: The light source forms linearly polarized light after passing through the polarizer, and is decomposed into two orthogonal polarized light beams by the polarization beam splitter, which serve as the signal light and the reference light respectively;

[0042] Specifically, the light source can be emitted by a coherent supercontinuum laser and decomposed into two beams of orthogonal polarized light by a polarization beam splitter, which serve as signal light and reference light respectively. This can take into account both the spectral width and the coherence length, while ensuring the visibility of wide-spectrum measurement and interference signals, improving the measurement accuracy of the transmission coefficient, and realizing the analysis of the dispersion effect of the material.

[0043] 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 due to internal reflection on the upper and lower surfaces of the film and the Fabry-Perot cavity effect, forming a signal light containing sample information;

[0044] Specifically, two translationally movable objective lenses are provided 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.

[0045] The two translatable objective lenses can be controlled for translation via a high-precision motorized translation stage, and the rotation angle of the thin film sample to be measured can be adjusted via a motorized turntable, thereby achieving precise rotation and motorized translation adjustment of the thin film sample to be measured, thereby optimizing the angle between the sample orientation and the polarization of the light beam, and realizing independent measurement of ordinary light and extraordinary light; at the same time, ensuring that the thin film sample to be measured is always in the optimal object plane position, thereby improving measurement accuracy.

[0046] S3: After passing through the reference arm, the reference light forms a reference light with the same optical path as the signal light containing the sample information;

[0047] Specifically, two objective lenses and a delay line can be arranged in the reference arm, and the delay line can be adjusted by a precise electrically controlled optical path adjustment device.

[0048] Two objective lenses installed in the reference arm can also 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 the sample information, thereby eliminating the background phase and improving the phase signal measurement accuracy.

[0049] S4: After the signal light containing the sample information and the reference light with the same optical path are combined by the polarization beam combiner, After the oriented quarter-wave plate, it passes through the half-wave plate with adjustable angle and the analyzer prism, so that the two beams have the same polarization direction and produce interference light;

[0050] The oriented quarter-wave plate, the angle-adjustable half-wave plate, and the analyzing prism form an optical detection module, which can realize the polarization projection interference of the light signals of the sample arm and the reference arm, improve the signal-to-noise ratio of the interference spectrum signal, and ensure the stability and accuracy of the measurement data.

[0051] S5: Adjust the fast axis direction of the half-wave plate and obtain the intensity of the corresponding interference light. Then, calculate the transmission phase angle of the thin film sample to be tested based on the intensity of the interference light.

[0052] Specifically, the rotation angle of the half-wave plate can be adjusted by an electric turntable, so that the fast axis direction control of the half-wave plate is automated and the control is relatively precise.

[0053] Specifically, the fast axis direction of the half-wave plate can be adjusted to , calculate the transmission phase angle of the thin film sample to be tested according to formula (1):

[0054] (1);

[0055] in: represents the transmission phase angle of the thin film sample to be tested, The fast axis direction of the half-wave plate is The intensity of the interference light is The fast axis direction of the half-wave plate is The intensity of the interference light is The fast axis direction of the half-wave plate is The intensity of the interference light is The fast axis direction of the half-wave plate is The intensity of the interference light.

[0056] S6: Block the reference light in the reference arm and adjust the fast axis direction of the half-wave plate to , measuring the intensity of the signal light containing the sample information when there is no interference, then taking out the film sample to be tested, and measuring the intensity of the signal light that does not contain the sample information when there is no interference;

[0057] S7: Calculating the transmittance of the thin film sample to be measured according to the intensity of the signal light containing the sample information when there is no interference and the intensity of the signal light not containing the sample information when there is no interference;

[0058] Specifically, the transmittance of the film sample to be tested can be calculated according to formula (2):

[0059] (2);

[0060] in: Indicates the transmittance of the film sample to be tested, Indicates the intensity of the signal light containing sample information when there is no interference, Indicates the intensity of signal light that does not contain sample information when there is no interference.

[0061] S8: Calculating 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.

[0062] Specifically, the refractive index and thickness of the thin film sample to be tested can be calculated according to formula (3):

[0063] (3);

[0064] in: represents the refractive index of the thin film sample to be tested, Indicates the thickness of the film sample to be tested, represents the transmission coefficient phase of the thin film sample to be tested, represents the substrate refractive index, Indicates the transmittance of the film sample to be tested, Indicates the wavelength of light emitted by a light source.

[0065] As can be seen from the above measurement method, the method for measuring the refractive index and thickness of optical thin films provided by the present invention can fully automate the measurement process, significantly improving measurement efficiency. Furthermore, its spectrum covers 600 nm to 1100 nm, and can be expanded to the deep ultraviolet or mid-to-far infrared, as required, covering key wavelengths such as the visible and near-infrared. It is suitable for characterizing a variety of material systems, including semiconductors, metals, dielectrics, and metamaterials.

[0066] By rotating the thin film sample to be tested to control the angle between the sample optical axis and the polarization direction of the signal light, independent measurement of ordinary light and extraordinary light can be achieved, which is suitable for anisotropic materials.

[0067] This method directly calculates the refractive index and thickness of the material based on transmittance, avoiding dependence on iterative fitting of the physical model, thereby improving calculation accuracy and convergence stability.

[0068] Specifically, through experiments, it can be verified that the measurement results of the optical film thickness obtained by this method are the same as the thickness results obtained by the existing method as shown in Table 1:

[0069] Table 1

[0070]

[0071] In Table 1, SEM represents a method of measuring using an existing scanning electron microscope, and ellipsometry represents a method of measuring using an existing ellipsometry.

[0072] As can be seen in Table 1, the optical film thickness measurement results obtained by this method are closest to the nominal value. Therefore, the optical film thickness obtained by this method is the most accurate. However, the method using ellipsometer has significant measurement deviation for transparent sample materials with thin thickness. In addition, the method using ellipsometer is difficult to directly solve analytically. Numerical fitting algorithms are usually used, which requires a long measurement process. 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, rapid, and high-precision measurement of optical film thickness.

[0073] A system for measuring the refractive index and thickness of an optical film, used to perform a method for measuring the refractive index and thickness of an optical film as described above, the system structure diagram is as follows Figure 2 As shown, it includes a light source module, an optical interference module, an optical detection module and a data acquisition and processing module;

[0074] 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 the light source into linearly polarized light.

[0075] The optical interferometer module is based on the Mach-Zehnder interferometer structure design, which includes a polarization beam splitter, a sample arm, a reference arm and a polarization beam combiner. Two translational objective lenses are set in the sample arm, and two objective lenses and a delay line are set in the reference arm.

[0076] The polarization beam splitter is used to decompose linearly polarized light into two orthogonal polarized beams, which serve as signal light and 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.

[0077] The reference arm is used to form a reference light having the same optical path as the signal light containing the sample information;

[0078] The polarization beam combiner is used to combine the signal light containing sample information with the reference light with the same optical path length;

[0079] Optical detection module includes The oriented quarter-wave plate, the angle-adjustable half-wave plate and the analyzer prism, and the optical detection module are used to make the two beams of light have the same polarization direction and generate interference light;

[0080] The data acquisition and processing module includes a spectrometer and a data processing unit, which are used to measure the intensity of the interference light and finally obtain the refractive index and thickness of the optical film through data processing.

[0081] Optimized, the two objective lenses in the sample arm are controlled by a high-precision electric translation stage. The rotation angle of the thin film sample to be measured can be adjusted by the electric turntable, thereby realizing automatic control, making the thin film sample to be measured accurately rotated and electrically translated, ensuring that the thin film sample to be measured is always in the optimal object plane position, thereby improving measurement accuracy.

[0082] In summary, the present invention provides a method and system for measuring the refractive index and thickness of optical thin films. By utilizing the clear mathematical relationship between the complex transmission coefficient and the optical constants of the material, the refractive index and thickness of the sample are directly solved through analytical formulas. This avoids the traditional method's reliance on complex physical modeling and numerical iterative fitting processes, effectively improving the measurement speed and accuracy, and having a relatively wide range of applications.

[0083] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for measuring the refractive index and thickness of an optical thin film, characterized by: The steps include: S1: The light source forms linearly polarized light after passing through the polarizer, and is decomposed into two orthogonal polarized light beams by the polarization beam splitter, which serve 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 due to internal reflection on the upper and lower surfaces of the film and the Fabry-Perot cavity effect, forming a signal light containing sample information; S3: After passing through the reference arm, the reference light forms a reference light with the same optical path as the signal light containing the sample information; S4: After the signal light containing the sample information and the reference light with the same optical path are combined by the polarization beam combiner, After the oriented quarter-wave plate, it passes through the half-wave plate with adjustable angle and the analyzer prism, so that the two beams have the same polarization direction and produce interference light; S5: Adjust the fast axis direction of the half-wave plate and obtain the intensity of the corresponding interference light. Then, calculate the transmission phase of the thin film sample to be tested based on the 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 , measuring the intensity of the signal light containing the sample information when there is no interference, then taking out the film sample to be tested, and measuring the intensity of the signal light that does not contain the sample information when there is no interference; S7: Calculating the transmittance of the thin film sample to be measured according to the intensity of the signal light containing the sample information when there is no interference and the intensity of the signal light not containing the sample information when there is no interference; S8: Calculate the refractive index and thickness of the thin film sample to be tested based on the transmittance of the thin film sample to be tested according to formula (3): （3）； in: represents the refractive index of the thin film sample to be tested, Indicates the thickness of the film sample to be tested, represents the transmission phase of the thin film sample to be measured, represents the substrate refractive index, Indicates the transmittance of the film sample to be tested, Indicates the wavelength of light emitted by a light source.

2. The method for measuring the refractive index and thickness of an optical thin film according to claim 1, wherein: The light source in step S1 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 translationally movable objective lenses are provided 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. The method for measuring the refractive index and thickness of an optical thin film according to claim 1, wherein: 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 half-wave plate is rotated at an angle adjusted by an electric turntable.

6. The method for measuring 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 plate are adjusted to be , and then calculate the transmission phase of the thin film sample to be tested according to formula (1): （1）； in: The fast axis direction of the half-wave plate is The intensity of the interference light is The fast axis direction of the half-wave plate is The intensity of the interference light is The fast axis direction of the half-wave plate is The intensity of the interference light is The fast axis direction of the half-wave plate is The intensity of the interference light.

7. The method for measuring the refractive index and thickness of an optical thin film according to claim 6, wherein: In step S7, the transmittance of the thin film sample to be tested is calculated according to formula (2): （2）； in: Indicates the transmittance of the film sample to be tested, Indicates the intensity of the signal light containing sample information when there is no interference, Indicates the intensity of signal light that does not contain sample information when there is no interference.

8. A system for measuring the refractive index and thickness of an optical thin film, for performing the method for measuring the refractive index and thickness of an optical thin film according to any one of claims 1 to 7, characterized in that: It includes light source module, optical interference module, optical detection module and 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 translationally movable 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 Oriented quarter-wave plates, angle-adjustable half-wave plates, and analyzer prisms; The data acquisition and processing module includes a spectrometer and a data processing unit.

9. The optical thin film refractive index and thickness measurement system according to claim 8, characterized in that: The two objective lenses in the sample arm are translated by a high-precision motorized stage.

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