A reflective interferometric film thickness measurement method and system for eliminating backscattering on transparent substrates

Abstract

This invention relates to the field of optical measurement, specifically to a reflective interferometric film thickness measurement method and system for eliminating backscattering from transparent substrates. The method includes the following steps: emitting a light beam using a low-coherence-length broadband light source and forming linearly polarized light through polarization modulation; focusing the beam onto the thin film surface using a confocal optical system; collecting reflected interference signals from the upper and lower surfaces of the thin film; synergistically suppressing substrate backscattering using spatial filtering, low-coherence gating, and polarization control; processing the collected interference signals and calculating the thin film thickness information. This invention employs a triple suppression mechanism: spatial filtering: a confocal aperture shields defocused backscattered light; low-coherence gating: naturally suppressing reflected signals exceeding the coherence range using a short coherence length; and polarization control: further weakening stray light interference through polarization state matching, effectively suppressing substrate backscattering.

Description

Technical Field

[0001] This invention relates to the field of optical measurement technology, and in particular to a reflective interferometric film thickness measurement method and system for eliminating backscattering on transparent substrates, which is suitable for non-destructive, high-precision thickness detection of thin films on transparent substrate surfaces. Background Technology

[0002] Reflective interferometry for film thickness measurement, as a non-contact optical inspection method, calculates film thickness by analyzing the interference signals generated by reflected light from the upper and lower surfaces of the thin film. With its high measurement accuracy, non-destructive testing characteristics, and real-time online monitoring capabilities, it has become a core quality control technology in high-end industries such as semiconductor manufacturing, optical coating, and display panels. Its principle is based on the mathematical relationship between optical path difference and interference spectrum, achieving accurate inversion of film thickness through Fourier transform or extremum extraction algorithms, making it particularly suitable for the precision measurement needs of ultrathin films.

[0003] However, when the substrate material is a transparent medium such as glass or quartz, this technology faces a significant backscattering interference problem: some incident light penetrates the thin film and enters the substrate, where it is reflected on the back side (i.e., "backscattering" or "back reflection"). Since the substrate thickness is usually much greater than the thin film thickness, the optical path difference between the backscattered light and the thin film interference signal far exceeds the coherence length of the light source, causing them to superimpose in the spectral domain, leading to baseline drift and signal distortion. Experiments show that unsuppressed backscattering can increase the measurement error by 3-5 times. The interference effect is more severe when the substrate thickness is uneven or the back side is rough, even completely masking the effective interference signal.

[0004] In existing technologies, the following methods are commonly used to attempt to solve this problem: 1. Coating the back of the substrate with light-absorbing material (such as black paint or light-absorbing tape): This method eliminates back reflection through physical absorption, but it damages the integrity of the sample and cannot meet the online detection requirements of industrial production lines. In addition, the uniformity of the light-absorbing coating will introduce new measurement errors. 2. Short coherence length light source system (such as white light interferometer): It uses low coherence to naturally suppress far-field reflected light, but such systems require complex optical path adjustment mechanisms and high-cost spectrometers (a single device costs hundreds of thousands of yuan), making maintenance difficult and large-scale application challenging; 3. Mathematical modeling subtraction method: This method separates the signal from the background noise through an algorithm, but it requires the substrate to have highly uniform thickness and optical properties. In actual production, substrate variations caused by factors such as material stress and processing tolerances can cause the algorithm to fail, resulting in poor robustness.

[0005] Therefore, there is an urgent need for a non-destructive, simple, adaptable, and effective reflective interferometric film thickness measurement system and method that can suppress backscattering interference from transparent substrates, while taking into account both measurement accuracy and engineering practicality. Summary of the Invention

[0006] The purpose of this invention is to provide a reflective interferometric film thickness measurement method and system for eliminating backscattering on transparent substrates. The proposed triple synergistic suppression mechanism (spatial filtering, low coherence gating, and polarization control) significantly improves the backscattering suppression efficiency while maintaining sample integrity through multiple couplings of physical and optical properties, thereby solving the problems of measurement accuracy and stability of backscattering interference on transparent substrates in the prior art.

[0007] The technical solution of this invention is: a reflective interferometric film thickness measurement method for eliminating backscattering on transparent substrates, comprising the following steps: S1. A low-coherence-length broadband light source is used to emit a light beam, and linearly polarized light is formed by polarization modulation. S2. Focus the light beam onto the thin film surface using a confocal optical system; S3. Collect the reflected interference signals from the upper and lower surfaces of the thin film; S4. Spatial filtering, low-coherence gating, and polarization control are used to synergistically suppress substrate backscattering; S5. Process the collected interference signals and calculate the film thickness information.

[0008] Preferably, the confocal optical system forms an aperture stop.

[0009] Preferably, the optical path length of the light reflected from the back of the substrate is greater than the coherence length, so that no interference is formed.

[0010] Preferably, in step S4, before collecting the interference signal, a polarization filter is formed to suppress backscatter by forming an orthogonal polarization pair with the initial polarization modulation.

[0011] A reflective interferometric film thickness measurement system for eliminating backscattering from transparent substrates includes: a low coherence length broadband light source module that provides low coherence length broadband light; The polarization modulation module modulates the low-coherence-length broadband light into linearly polarized light. A beam splitter module is used to separate incident light from reflected light; The confocal detection module includes a confocal lens assembly and a confocal aperture. The confocal lens assembly focuses the incident light beam onto the surface of the thin film to form a focal point. The confocal aperture is located at the conjugate position of the focal point to form a confocal structure. The confocal structure collects only the reflected light from the focal region while shielding the reflected light from the back of the substrate. The spectral acquisition module includes an optical signal processing component and a spectrometer. The optical signal processing component and the polarization modulation module form an orthogonal polarization pair to further attenuate the back-reflected light. The spectrometer receives the optical signal processed by the optical signal processing component and forms a spectral signal. The data processing module performs data calculations on the spectral signal to obtain the film thickness.

[0012] Preferably, the beam splitting module includes an incident path and an exit path, the incident path being used to guide light emitted from the light source to the thin film surface, and the exit path being used to guide reflected light to the spectral acquisition module.

[0013] Preferably, the film thickness d is calculated as follows: ; The spectral signal exhibits periodic oscillations, where λ1 and λ2 are the wavelengths of adjacent peaks / troughs, and n is the refractive index.

[0014] Compared with the prior art, the advantages of the present invention are: (1) High measurement accuracy: The triple suppression mechanism effectively eliminates substrate backscattering interference, achieving sub-nanometer measurement accuracy, which is significantly better than the traditional reflective interferometry method; (2) Non-destructive testing, no need to coat the back of the substrate with light-absorbing material, maintaining the integrity of the sample, especially suitable for online testing and product quality control, meeting the real-time measurement needs of industrial sites, and providing an efficient and reliable solution for the accurate measurement of film thickness in high-end manufacturing fields; (3) It is highly adaptable and can be applied to transparent substrates of different thicknesses and roughnesses. It has low requirements for substrate consistency and good robustness. (4) The system is simple and low in cost. Combining low coherence interference and confocal detection technology, the system structure is relatively simple, the cost is controllable, and it is easy to promote and apply in industrial fields. (5) The triple suppression mechanism works synergistically, including: spatial filtering: the confocal aperture shields the back reflection light from defocus; low coherence gating: the short coherence length is used to naturally suppress the reflection signal that exceeds the coherence range; polarization control: the stray light interference is further weakened by polarization state matching, and the substrate backscattering is effectively suppressed. Attached Figure Description

[0015] The present invention will be further described below with reference to the accompanying drawings and embodiments: Figure 1 This is a schematic diagram of the system structure for reflective interferometric film thickness measurement for eliminating backscattering from transparent substrates as described in this invention; Figure 2 This is a schematic diagram of the defocus coupling efficiency described in this invention.

[0016] The components include: light source module 1, polarization modulation module 2, beam splitting module 3, confocal lens assembly 4, confocal aperture 5, thin film 6, transparent substrate 7, optical signal processing assembly 8, and spectrometer 9. Detailed Implementation

[0017] The embodiments of this application are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this application, and should not be construed as limiting this application.

[0018] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0019] The present invention will be further described in detail below with reference to specific embodiments: This invention is applied in industries such as optical coating, semiconductor packaging, display device manufacturing, and photovoltaic materials, enabling the thickness detection of thin films on the surface of transparent substrates (such as glass, quartz, and transparent conductive films), and is particularly suitable for measurement scenarios with significant backscattering interference. By synergistically combining low-coherence interferometry and confocal detection techniques, and integrating spatial filtering, low-coherence gating, and polarization control—a triple suppression mechanism—the industry challenge of backscattering interference from transparent substrates is solved. Spatial filtering physically shields defocused backscattered light through a confocal aperture, experimentally verified to suppress approximately 92.8%; low-coherence gating uses a coherence length light source to suppress reflection signals exceeding the coherence range; polarization control reduces backscattered light transmittance by approximately 80% through orthogonal polarization pairs. The solution of this invention maintains sample integrity while stably controlling the measurement error within 0.5 nm, significantly outperforming traditional methods. Specifically: like Figure 1 As shown, a reflective interferometric film thickness measurement method and system for eliminating backscattering on transparent substrates includes the following steps and employs the following core module components: S1. A low-coherence-length broadband light source is used to emit a light beam, and linearly polarized light is formed by polarization modulation.

[0020] The light beam is emitted by a low coherence length broadband light source module, which can be a white LED, SLED, xenon lamp, halogen tungsten lamp, deuterium lamp, etc., to provide low coherence length broadband light. The coherence length of the light source is relatively short, typically 10-20μm.

[0021] The polarization modulation module that realizes polarization modulation can be a polarizer, which modulates low-coherence-length broadband light into linearly polarized light, in preparation for subsequent polarization suppression backscattering.

[0022] A beam splitter module is used for light transmission. The beam splitter module includes an incident path and an exit path. The incident path guides the light emitted from the light source to the thin film surface, and the exit path guides the reflected light to the spectral acquisition module. In this embodiment, the beam splitter module is a Y-type optical fiber, which has both transmitting and receiving functions. Its incident path is an incident fiber bundle, and its exit path is an exit fiber bundle. The incident fiber bundle guides the light emitted from the light source, and the exit fiber bundle guides the reflected light. The incident fiber bundle includes a first incident end connected to the polarization adjustment module and a first exit end connected to the confocal detection module. The exit fiber bundle includes a second incident end connected to the confocal detection module and a second exit end connected to the spectral acquisition module. The first exit end and the second incident end are on the same cross-section. Linearly polarized light enters the incident fiber bundle through the first incident end and is transmitted to the confocal lens assembly from the first exit end, forming a collimated and focused optical path. The characteristics of the beam at this time are linear polarization, low coherence, and unidirectional transmission. Of course, in other embodiments, the beam splitting module can also use devices such as free-space beam splitters and fiber optic couplers to separate the incident light from the reflected light.

[0023] S2. The beam is focused onto the thin film surface using a confocal optical system. The confocal optical system forms an aperture at the conjugate position of the focal point, achieving spatial filtering and shielding against defocused substrate back reflection.

[0024] A confocal detection module is employed, comprising a confocal lens assembly and a confocal aperture. The confocal lens assembly focuses the incident light beam onto the thin film surface, forming a focal point. The confocal aperture is located at the conjugate position of the focal point, forming an aperture stop. Only reflected light from the focal point can return along its original path and pass through the confocal aperture, while reflected light from the substrate backside at non-focal points is shielded. This creates spatial filtering, achieving suppression of the first backscattered light.

[0025] S3. Collect the reflected interference signals from the upper and lower surfaces of the thin film to form a low-coherence gating system, further suppressing backscattering.

[0026] The focused light is reflected from both the upper and lower surfaces of the thin film. The light reflected from the upper surface is reflected directly, carrying information about the film's surface; the light reflected from the lower surface penetrates the film and is reflected back, carrying information about its thickness. The optical path difference between the two beams is 2n·d (where n is the film's refractive index and d is its thickness). Due to the short coherence length of the light source, only beams with an optical path difference within this coherence length can interfere. The light reflected from the back of the substrate, however, cannot participate in interference because its optical path difference is much greater than the coherence length, and its signal is naturally suppressed. This creates a low-coherence gating mechanism, suppressing the second backscattered light.

[0027] S4. Process the collected interference signals and calculate the film thickness information.

[0028] The spectral acquisition module includes an optical signal processing component and a spectrometer. The optical signal processing component and the polarization modulation module form an orthogonal polarization pair to further attenuate the back-reflected light. The spectrometer receives the processed optical signal from the optical signal processing component and generates a spectral signal. The reflected light from the upper and lower surfaces of the thin film returns along its original path, passes through the confocal lens assembly, enters the output fiber bundle via the second incident end, and then enters the optical signal processing component from the second exit end. Since the back-reflected light has undergone two suppressions, the reflected light at this point mainly consists of the interference signal from the thin film.

[0029] The optical signal processing component can be an adjustable polarizer or a polarization beam splitter, which forms an orthogonal polarization-to-polarization alignment with the initial polarization modulation module. Even if the backscattered light undergoes multiple reflections and changes its polarization state, it will be attenuated again when passing through the optical signal processing component. This forms a polarization filter, suppressing the third backscattered light. This results in the interference light signal that finally enters the spectrometer being pure, stable, and highly resistant to interference.

[0030] The data processing module performs frequency domain analysis on the interference signal to obtain the film thickness. The spectrometer receives the interference light signal and acquires the reflectivity versus wavelength curve. Due to thin-film interference, periodic oscillations (similar to a sine wave) appear in the spectrum. The interference peaks corresponding to the reflections from the upper and lower surfaces of the film are extracted. Based on the position and phase difference of the interference peaks, the relationship between the oscillation period Δλ and the film thickness d is as follows:

[0031] Where λ1 and λ2 are the wavelengths of adjacent peaks / troughs.

[0032] To further illustrate the beneficial effects of the present invention, the following description is based on actual testing: like Figure 2 The figure shows the defocus coupling efficiency without a polarizer. The point with the highest coupling efficiency is 0, and the positive or negative sign describes the distance of the sample from the 0 point. It can be seen that the coupling efficiency drops sharply when the sample position deviates from the optimal coupling efficiency point. With a transparent substrate of 1 mm and a refractive index of 1.6, the backscattering intensity is only 0.63 / 8.71 ≈ 7.2% of the signal from the surface film layer, corresponding to a loss of: That is, the backscattered signal was suppressed by about 92.8%.

[0033] It should be noted that the experimental data mentioned above were achieved solely through spatial filtering and low-coherence gating dual backscatter suppression, which provides a basis for further optimization by introducing polarization control. Moreover, the suppression effect is increased by approximately 80% after introducing orthogonal polarizers.

[0034] The above embodiments are merely illustrative of the technical concept and features of the present invention, intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and should not be construed as limiting the scope of protection of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects. The scope of the present invention is defined by the appended claims rather than the foregoing description, and thus all changes falling within the meaning and scope of the equivalents of the claims are intended to be included within the present invention.

Claims

1. A method for measuring film thickness using reflective interferometry to eliminate backscattering from transparent substrates, characterized in that... Includes the following steps: S1. A low-coherence-length broadband light source is used to emit a light beam, and linearly polarized light is formed by polarization modulation. S2. A confocal optical system is used to focus a light beam onto the surface of a thin film. The confocal optical system includes a confocal aperture located at the focal conjugate position on the surface of the thin film, such that light reflected from the surface of the thin film converges and passes through the confocal aperture, while reflected light from the back of the transparent substrate that is off-center is blocked by the confocal aperture. S3. Collect the reflected interference signals from the upper and lower surfaces of the thin film; S4. Spatial filtering, low coherence gating, and polarization control are used to synergistically suppress substrate backscattering; wherein, the spatial filtering is achieved by the confocal aperture, the low coherence gating utilizes the short coherence length of the low coherence length broadband light source to make the optical path difference between the reflected light from the back of the transparent substrate and the reflected light from the upper surface of the thin film greater than the coherence length of the light source, thereby preventing interference; the polarization control further weakens stray light whose polarization state changes after backscattering by setting a polarizer orthogonal to the linearly polarized light in the optical path for collecting interference signals; S5. Process the collected interference signals and calculate the film thickness information.

2. A reflective interferometric film thickness measurement system for eliminating backscattering from transparent substrates, characterized in that, include: Low coherence length broadband light source module, providing low coherence length broadband light; The polarization modulation module modulates the low-coherence-length broadband light into linearly polarized light. A beam splitter module is used to separate incident light from reflected light; A confocal detection module includes a confocal lens assembly and a confocal aperture. The confocal lens assembly focuses the incident light beam onto the surface of the thin film to form a focal point. The confocal aperture is located at the focal conjugate position, forming a confocal structure. The confocal structure collects only the reflected light from the focal region, while shielding the reflected light from the back of the substrate. The spectral acquisition module includes an optical signal processing component and a spectrometer. The optical signal processing component and the polarization modulation module form an orthogonal polarization pair to further attenuate the back-reflected light. The spectrometer receives the optical signal processed by the optical signal processing component and forms a spectral signal. The data processing module performs data calculations on the spectral signal to obtain the film thickness.

3. The reflective interferometric film thickness measurement system for eliminating backscattering from transparent substrates according to claim 2, characterized in that, The beam splitting module includes an incident path and an exit path. The incident path is used to guide the light emitted by the light source to the surface of the thin film, and the exit path is used to guide the reflected light to the spectral acquisition module.

4. The reflective interferometric film thickness measurement system for eliminating backscattering from transparent substrates according to claim 2, characterized in that, The film thickness d is calculated as follows: ； The spectral signal exhibits periodic oscillations, where λ1 and λ2 are the wavelengths of adjacent peaks / troughs, and n is the refractive index.

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