A Single-Frame White Light Reynolds Interferometry Apparatus and Method Based on Grating Modulated Illumination

Abstract

A single-frame white-light Reynolds interferometry apparatus and method based on grating-modulated illumination is disclosed. This apparatus modulates the light source using a grating to generate a specific illumination pattern and uses the principle of white-light interference to measure the morphology of the measured surface. By analyzing the brightness and position of the interference fringes, the morphological information of the measured surface can be obtained. This invention utilizes grating modulation and the principle of white-light interference to achieve efficient single-frame measurement of thin samples, improving accuracy. When measuring thick samples, the scanning step can be increased and the range reduced, thereby shortening the scanning time and increasing the spatiotemporal throughput of the measurement. Simultaneously, using grating fringes for self-calibration of the scanning step improves measurement stability, compensates for environmental vibrations in real time, and ensures reliable measurement results.

Description

Technical Field

[0001] This invention belongs to the field of optical precision measurement technology, specifically relating to a single-frame white light Reynolds interferometry device and method based on grating modulation illumination. Background Technology

[0002] In the field of precision measurement, white light interferometry surface topography measurement technology occupies a pivotal position, but it still faces a series of technical challenges that urgently need to be solved.

[0003] 1. Traditional white-light interferometry for surface topography measurement relies on longitudinal scanning of the sample or objective lens to gradually change their relative positions, thereby capturing the dynamic process of the interference pattern changing with distance. While effective, this process increases the complexity and time consumption of the measurement. Furthermore, longitudinal scanning can lead to instability in the measurement system, especially when dealing with small or complex surface structures; even minor vibrations or drifts can introduce significant measurement errors.

[0004] 2. A large longitudinal scanning range is required to acquire a complete and sufficiently attenuated interference envelope to achieve high-precision measurement. However, the wide scanning range not only further prolongs the measurement time, but may also increase the interference of unstable factors during the measurement process.

[0005] 3. Smaller longitudinal scanning steps are required to ensure the separation of interference signals in the spatial frequency domain. However, small steps not only increase the difficulty of control but may also introduce additional error sources during the measurement process, especially when dealing with surface structures with small or complex features. Any tiny vibration or drift can have a significant impact on the final measurement results. Summary of the Invention

[0006] To address the shortcomings of the existing technology, this invention provides a single-frame white light Reynolds interferometry device based on grating-modulated illumination. The device utilizes a grating to modulate the light source, generating a specific illumination pattern, and then employs the principle of white light interference to measure the morphology of the surface under test. During the measurement process, the white light is expanded and collimated, then modulated by the grating to form an illumination field with a specific spatial frequency and phase. This illumination field is projected onto the surface under test and a reference mirror, and after reflection, forms two coherent beams, ultimately creating interference fringes on the detector. By analyzing the brightness and position of the interference fringes, the morphological information of the surface under test can be obtained.

[0007] The technical solution of the present invention is as follows:

[0008] A single-frame white light Reynolds interferometric measurement device based on grating modulation illumination is characterized in that it includes an illumination device, an imaging device, and an interferometric device.

[0009] The lighting device includes at least a lighting source and a grating, used to illuminate the grating with white light emitted by the lighting source, and to form an lighting light field with a specific spatial frequency and phase after modulation by the grating;

[0010] The imaging device includes at least an imaging objective and a camera, for receiving coherent light reflected back from the measured surface and the reference objective, and forming interference fringes on the camera;

[0011] The interference device includes a reference objective lens identical to the imaging objective lens, which cooperates with the imaging objective lens to form interference fringes that cover the entire target surface of the camera;

[0012] In this process, illumination light fields of different colors and incident angles form interference fringes with the same spatial frequency after imaging. The phase distribution of the interference pattern is calculated by an off-axis hologram reconstruction algorithm to reconstruct the morphology of the sample.

[0013] Furthermore, the illumination device also includes a collimating lens and a projection lens. The white light emitted by the illumination source is collimated by the collimating lens and then illuminates the grating, generating a multi-order diffraction light field. The 0th order and +1st order diffraction light are focused by the projection lens onto the back focal plane of the imaging objective and the reference objective, respectively, forming interference fringes that cover the entire target surface of the camera.

[0014] Furthermore, the illumination source is a broadband light source.

[0015] Furthermore, the interference device also includes a reference mirror, through which light reflected by the reference mirror is focused onto the camera.

[0016] Furthermore, it also includes a sample stage for placing the sample to be tested, and the sample stage can move along the optical axis.

[0017] Furthermore, when measuring thin samples, the thin sample is placed at the focal length of the imaging objective, an off-axis interferogram is acquired and the phase distribution is calculated. The obtained interference envelope chirp information and the phase distribution of the off-axis interferogram are used to reconstruct the sample morphology.

[0018] Furthermore, when measuring thick samples, the thick sample is placed at the focal length of the imaging objective lens, an off-axis interferogram sequence is acquired, the complex amplitude encoded in the off-axis interferogram is calculated, the extrema at each point in space are found, the phase at the extrema is obtained, and the morphology is reconstructed using the location of the interference extrema, the phase at the extrema, and the known interference envelope chirp at the focal point.

[0019] On the other hand, the present invention provides a method for measurement using the above-mentioned single-frame white light Reynolds interferometry device based on grating modulation illumination, characterized in that it includes:

[0020] a) Calibration steps: Instrument calibration to obtain the interference envelope chirp information at the focal position (i.e., by performing a measurement on a perfect plane mirror sample with a known longitudinal displacement, the proportional coefficient between the interferogram encoding phase and the step at the focal position is obtained).

[0021] b) Sample measurement steps:

[0022] i) For thin samples, place the sample at the focal length of the objective lens, acquire off-axis interferograms and calculate the phase distribution, and use the interference envelope chirp information and phase distribution to achieve rapid reconstruction of the sample morphology.

[0023] ii) For thick samples, place the sample near the focal length of the objective lens, acquire a series of off-axis interferograms by scanning the sample stage, calculate the interference extremum position and phase at each spatial point, and use the interference extremum position, phase at the extremum and interference envelope chirp information at the focal point to reconstruct the morphology.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] 1) It can perform single-frame measurement on thin samples with surface morphology undulations less than one wavelength, without the need for a complex longitudinal scanning process, thus greatly improving measurement efficiency and accuracy.

[0026] 2) When measuring thicker samples, the measurement accuracy is improved by increasing the minimum scan step to four times the original value. The scanning range is reduced, and only the undulating area of ​​the sample needs to be covered. There is no need to collect the complete interference envelope, which further shortens the measurement time and reduces the system complexity.

[0027] 3) The use of grating fringes to achieve self-calibration measurement of scanning steps not only improves measurement stability, but also enables real-time sensing and compensation for the impact of environmental vibrations, ensuring the reliability of measurement results. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the single-frame white light Reynolds interferometry device based on grating modulation illumination according to the present invention;

[0029] In the diagram: 2. Camera; 3. Imaging lens; 4. Beam splitter; 5. Imaging objective; 6. Sample under test; 7. Projection lens; 8. Grating; 9. Collimating lens; 10. Illumination source; 11. Reference objective; 12. Reference mirror.

[0030] Figure 2 This is a flowchart of the process of measuring thin samples according to the present invention;

[0031] Figure 3 This is a flowchart of the process of measuring thick samples according to the present invention. Detailed Implementation

[0032] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments, but this should not be construed as limiting the scope of protection of the present invention.

[0033] Please see Figure 1 , Figure 1 This is a schematic diagram of a single-frame white light Reynolds interferometry apparatus based on grating-modulated illumination. As shown in the figure, it includes three parts: an illumination device, an imaging device, and an interferometer. The illumination device includes an illumination source 10, a collimating lens 9, a grating 8, and a projection lens 7; the imaging device includes an imaging objective lens 5, an imaging lens 3, and a camera 2; and the interferometer includes a reference objective lens 11 identical to the imaging objective lens 5.

[0034] The illumination source 10 is collimated by the collimating lens 9 and then illuminates the grating 8, generating a multi-order diffraction light field. The 0th order and +1st order diffraction light are focused by the projection lens 7 onto the back focal plane of the imaging objective 5 and the reference objective 11, respectively, forming interference fringes that cover the entire target surface of the camera 2.

[0035] Because illumination fields of different colors and incident angles form interference fringes with the same spatial frequency, the interference fringes can cover the entire target surface of the camera after imaging.

[0036] The phase distribution of the interferogram is calculated using an off-axis hologram reconstruction algorithm, which contains the phase information of the scattered field. When the sample is thin, a direct linear relationship can be established between the phase of the scattered field and the morphology of the sample, enabling the reconstruction of the sample's morphology using a single frame of the interferogram. When the sample morphology has large variability, the extreme interference positions at each point in the field of view can be found by scanning the sample or the imaging objective. Then, the phase at the maximum interference point is calculated using the off-axis hologram phase reconstruction algorithm. Finally, the sample height is calculated point by point using the extreme interference positions and this phase.

[0037] Example 1: Calibration and Thin Sample Measurement

[0038] Use a broadband light source as the illumination source 10;

[0039] A perfect mirror (sample 6) is placed at the focal length of the imaging objective 5.

[0040] The sample stage is moved along the optical axis by a known small step dz. The phase difference between the two interferograms is calculated to obtain the interference envelope chirp information at the focal position, and this information is saved. This step is part of the instrument calibration process and does not need to be performed for each measurement.

[0041] A thin sample is placed at the focal length of imaging objective 5, an off-axis interferogram is acquired, and the phase distribution is calculated.

[0042] By utilizing the obtained interference envelope chirp information and the phase distribution of the off-axis interferogram, the morphology of the sample can be rapidly reconstructed.

[0043] Example 2: Thick Sample Measurement

[0044] Use a broadband light source as the illumination source 10;

[0045] The thick sample was placed near the focal length of imaging objective 5, and an off-axis interferogram sequence was acquired by scanning the sample stage to ensure that each point in space crossed the interference extremum.

[0046] Calculate the complex amplitude encoded in the off-axis interferogram, find the extrema at each point in space, and obtain the phase at the extrema.

[0047] Morphology reconstruction is performed using the location of the interference extrema, the phase at the extrema, and the known interference envelope chirp at the focal point.

[0048] This invention utilizes a grating-modulated white light source to form an illumination field with specific spatial frequency and phase. This not only improves the controllability of the light field but also allows illumination fields of different colors and incident angles to form interference fringes with the same spatial frequency after imaging. This simplifies the processing and analysis of interference fringes and improves the accuracy and efficiency of measurements. The introduction of collimating and projection lenses enables precise control of the white light source, allowing the 0th and +1st order diffracted beams to be focused onto the back focal planes of the imaging and reference lenses, respectively, forming interference fringes covering the entire target surface of the camera. This invention proposes different measurement methods for thin and thick samples. For thin samples, off-axis interferograms are acquired and the phase distribution is calculated, using the interference envelope chirp information and phase distribution for rapid reconstruction. For thick samples, a series of off-axis interferograms are acquired by scanning the sample stage, and the extreme positions and phases at these extremes are calculated for morphology reconstruction. This method is applicable to a wider range of sample types, improving the applicability and flexibility of measurements.

Claims

1. A method for measurement using a single-frame white light Reynolds interferometer based on grating-modulated illumination, wherein the single-frame white light Reynolds interferometer based on grating-modulated illumination includes an illumination device, an imaging device, and an interferometer. The lighting device includes at least a lighting source (10) and a grating (8) for illuminating the white light emitted by the lighting source (10) onto the grating (8) and forming an lighting light field with a specific spatial frequency and phase after being modulated by the grating (8). The imaging device includes at least an imaging objective (5) and a camera (2) for receiving coherent light reflected from the surface under test and the reference objective (11) and forming interference fringes on the camera (2); The interference device includes a reference objective (11) identical to the imaging objective (5), which, in conjunction with the imaging objective (5), forms interference fringes that cover the entire target surface of the camera (2); wherein, The grating (8) enables illumination light fields of different colors and incident angles to form interference fringes with the same spatial frequency after imaging. The phase distribution of the interference pattern is calculated by the off-axis hologram reconstruction algorithm to realize the morphology reconstruction of the sample. It also includes a sample stage for placing the sample to be tested, and the sample stage can move along the optical axis; The features include: a) Calibration steps: instrument calibration to obtain interference envelope chirp information at the focal position, i.e., by performing one or several measurements on a perfect plane mirror sample under known longitudinal displacement conditions, the proportional coefficient between the interferogram encoding phase and the step at the focal position is obtained; b) Sample measurement steps: i) For thin samples, the sample is placed at the focal length of the imaging objective (5), an off-axis interferogram is acquired and the phase distribution is calculated, and the rapid reconstruction of the sample morphology is achieved by using the interference envelope chirp information and the phase distribution; ii) For thick samples, place the sample near the focal length of the imaging objective (5), collect a series of off-axis interferograms by scanning the sample stage, calculate the interference extreme position and phase at each spatial point, and use the interference extreme position, phase at the extreme point and interference envelope chirp information at the focal point to reconstruct the morphology.

2. The method according to claim 1, characterized in that, The illumination device also includes a collimating lens (9) and a projection lens (7). The white light emitted by the illumination source (10) is collimated by the collimating lens (9) and then illuminates the grating (8), generating a multi-level diffraction field. The 0th and +1st order diffraction lights are focused by the projection lens (7) onto the back focal plane of the imaging objective (5) and the reference objective (11), respectively, forming interference fringes that cover the entire target surface of the camera (2).

3. The method according to claim 2, characterized in that, The lighting source (10) is a broadband light source.

4. The method according to claim 1, characterized in that, The interference device also includes a reference mirror (12), through which light reflected by the reference mirror (12) is focused onto the camera (2).

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