Astigmatic displacement microscopic imaging method and device for axial sub-nanometer resolution

Abstract

The invention discloses an astigmatic displacement microscopic imaging method and an astigmatic displacement microscopic imaging device for axial sub-nanometer resolution, namely an astigmatic displacement microscope (ADM). The astigmatic displacement microscopic imaging method and the astigmatic displacement microscopic imaging device for axial sub-nanometer resolution are applied to an astigmatic displacement microscope (ADM). The ADM technology is based on the astigmatism measurement principle, has the advantage of optical non-contact, and can simultaneously realize four measurement functions: microscopic three-dimensional shape scanning imaging, displacement measurement, vibration analysis and film thickness measurement. The Z-axis resolution of three-dimensional morphology imaging is about 0.1 nm, and the thickness detection of a monatomic layer of a two-dimensional material and the like can be met; the device can be matched with any objective lens, such as working media of air, water immersion and oil immersion; a fast scanner is adopted to replace a traditional sample displacement table for scanning, and the imaging time of a single image is fast to a second level; and in combination with a normalization circuit, the influence of different sample reflectivity on a measurement signal can be eliminated. The astigmatic displacement microscope can meet the test requirements of application scenes such as micro-nano devices, semiconductors, piezoelectric sensing, material morphology measurement, in-situ surface and interface research, precise displacement measurement, microscopic vibration analysis and the like.

Description

Technical Field

[0001] This invention belongs to the field of microscopic optical imaging and micro / nano characterization and measurement technology, and relates to an axial sub-nanometer resolution astigmatic displacement microscopy method and device, namely astigmatic displacement microscopy (ADM). It has advantages such as non-contact optical operation, large imaging field of view, and fast imaging speed, and can simultaneously realize four major measurement functions: microscopic three-dimensional morphology scanning imaging, displacement measurement, vibration analysis, and film thickness measurement. Background Technology

[0002] With the advancement of cutting-edge science, especially in fields such as semiconductors, industrial precision measurement, MEMS devices, piezoelectric sensors, two-dimensional materials, and surface and interface analysis, higher demands are being placed on micro-nano characterization technologies: sub-nanometer three-dimensional morphology measurement, displacement measurement, microscopic vibration analysis, and microscopic material composition analysis, while also possessing the advantages of non-destructive, rapid, and in-situ characterization. However, current single detection methods are still insufficient to simultaneously meet these testing requirements. Scanning probe microscopy (SPM) typically offers sub-nanometer XYZ resolution, but requires maintaining a constant distance between the probe tip and the sample, resulting in slow scanning speed and a small imaging range. Electron microscopy (EM) offers sub-nanometer XY resolution, but has a small imaging field of view and typically lacks Z-axis analysis capabilities. Optical microscopy (OM) offers advantages such as non-contact operation, a large scanning range, and fast imaging speed, but its XYZ resolution is limited by the optical diffraction limit (d). xy = 0.61λ / NA, d z = 0.5λ / NA 2 Typically, the resolution is only a few hundred nanometers. To further improve the axial resolution of optical microscopy, various methods have been developed, such as optical interferometry (laser interferometry, white light interferometry, differential interferometry, etc.), holographic imaging, differential confocal microscopy, and spectral confocal microscopy. However, many measurement methods require specialized objectives, which have certain limitations. The optical astigmatism displacement measurement method involved in this invention is another promising axial super-resolution technique. Its working principle is as follows: Figure 2 As shown, when light passes through the objective lens ( Figure 1 The middle element 9) is focused at different focal planes (I, II, and III) of the sample. The reflected light from the sample returns along the original path and passes through the astigmatism element ( Figure 1 Component 12) is focused on the four-quadrant photodetector. Figure 1 (Component 13), at this time, the imaging spot patterns corresponding to different focal planes are different. Further processing of the output signal of the four-quadrant photodetector yields the output focusing error signal (FES, FES = V). A +V C -V B -V DThe light spot is linear within the design range, thus it can be used as a displacement sensor. By rapidly scanning the sample in the XY direction and obtaining the corresponding FES signal, three-dimensional imaging of the sample can be achieved.

[0003] Since the 1970s, feedback control based on astigmatic displacement has been widely used in CD / DVD optical disc readers to control the focusing state of the disc surface in real time to read information recorded on the grooves. However, since the depth of the grooves is on the order of hundreds of nanometers, it has not met the requirement of sub-nanometer measurement accuracy. Since 2000, astigmatic methods have been used for displacement measurement in precision machining processes, with axial displacement resolution as low as 1 nm. They have also been used for amplitude measurement of silicon microcantilever beams and integrated into atomic force microscopy (AFM) feedback to obtain excellent single-atom step-resolution morphological imaging. In addition, it has been used for three-dimensional morphological characterization of CDs, gratings, MEMS devices, and biological cells. However, based on the limited reported results, the axial resolution of its morphological imaging is only about 11 nm, which is insufficient to meet the measurement requirements of sub-nanometer three-dimensional imaging. Furthermore, existing devices suffer from problems such as slow sample displacement stage scanning speed (min-level), refractive index mismatch of objective media in solution, and thickness measurement errors caused by reflectivity differences. Based on the above analysis, this invention proposes an axial sub-nanometer resolution astigmatic displacement microscopy method and device, namely astigmatic displacement microscopy (ADM). It possesses advantages such as 0.1 nm axial resolution, non-contact optical imaging, rapid imaging, and compatibility with any objective lens. It can simultaneously achieve three-dimensional microscopic morphology scanning imaging, displacement measurement, vibration analysis, and film thickness measurement. It is expected to meet the testing needs of applications such as micro / nano devices, semiconductors, piezoelectric sensing, material morphology measurement, in-situ surface and interface research, precision displacement measurement, and microscopic vibration analysis. Summary of the Invention

[0004] This invention discloses an axial sub-nanometer resolution astigmatic displacement microscopy (ADM) imaging method and apparatus. We innovatively employ low-noise circuitry, laser modulation technology, and signal demodulation technology. By eliminating the objective lens built into the optical head and using laser collimation to match arbitrary external objective lenses, and by employing a medium matching method, ADM technology can meet the requirements of in-situ analysis and detection in solutions. A fast scanner replaces the slow sample displacement stage, shortening the imaging time to the second level (e.g., 3 s). By introducing a normalization circuit, we solve problems such as thickness measurement errors caused by differences in sample reflectivity. Ultimately, the Z-axis resolution of the ADM imaging method is better than 0.1 nm, enabling two-dimensional single-atom-layer thickness imaging and layer number identification of materials, and also possessing the functions of three-dimensional morphology scanning imaging, displacement measurement, vibration analysis, and film thickness measurement.

[0005] The specific technical solution adopted in this invention is as follows:

[0006] A method and apparatus for axial sub-nanometer resolution astigmatic displacement microscopy imaging, characterized in that:

[0007] It includes a microscopic illumination system (18), an astigmatism module (19), a 4f scanning system (20), a signal conditioning and control module (28), a mirror (4), a sample (10), and a stage (11). The microscopic illumination system (18) includes a light source (14), a lens (15), a mirror (16), a push-pull module (8), an objective lens (9), and a camera (17); the astigmatism module (19) includes a laser source (1), a beam splitter (2), a collimating lens (3), an astigmatism element (12), and a four-quadrant photodetector (13); the 4f scanning system (20) includes a scanner (5), a lens (6), and a second lens (7); the signal conditioning and control module (28) includes signal processing (21), AC / DC conversion (22), signal acquisition (23), computer software (24), stage control (25), laser control (26), and scanner control (27).

[0008] The aforementioned axial sub-nanometer resolution astigmatic displacement microscopy imaging method and apparatus are characterized in that the instrument and method can have three-dimensional imaging, displacement analysis, vibration analysis and film thickness measurement functions.

[0009] The aforementioned axial sub-nanometer resolution astigmatic displacement microscopy imaging method and apparatus are characterized in that the instrument and method can be matched with any objective lens, and can meet the in-situ characterization of samples under different environments.

[0010] The method and apparatus for axial sub-nanometer resolution astigmatic displacement microscopy is characterized in that the laser source (1) can be a laser diode or a commercial laser.

[0011] The method and apparatus for axial sub-nanometer resolution astigmatic displacement microscopy is characterized by using a scanner (5), a lens (6), and a second lens (7) to form a 4f optical path system, and using a high-speed scanner to achieve rapid scanning imaging and positioning of the light spot.

[0012] The axial sub-nanometer resolution astigmatic displacement microscopy imaging method and apparatus is characterized in that the astigmatic element (12) can be a cylindrical lens or the astigmatic effect of other elements.

[0013] The axial sub-nanometer resolution astigmatic displacement microscopy imaging method and apparatus is characterized by the integration of a microscopic illumination system (18), which facilitates sample positioning and observation.

[0014] The axial sub-nanometer resolution astigmatic displacement microscopy imaging method and apparatus is characterized in that the astigmatic module (19) can be implemented by discrete components or by using an optical pickup unit (OPU).

[0015] The method and apparatus for axial sub-nanometer resolution astigmatic displacement microscopy is characterized in that the signal processing (21) includes gain, addition, subtraction, filtering, bias adjustment and normalization processing of the four output signals of the four-quadrant photodetector to obtain measurement results with high sensitivity, low noise and no influence from the difference in sample reflectivity.

[0016] The method and apparatus for axial sub-nanometer resolution astigmatic displacement microscopy is characterized in that the AC-DC conversion (22) can be achieved by phase-locked loop and RMS-DC.

[0017] The axial sub-nanometer resolution astigmatic displacement microscopy imaging method and apparatus is characterized in that the laser control (26) has the functions of continuous output, modulation output and power control. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of an axial sub-nanometer resolution astigmatic displacement microscopy imaging method and apparatus disclosed in this invention.

[0019] Figure 2 This is a schematic diagram illustrating the working principle of the astigmatic displacement microscopic imaging method.

[0020] Figure 3 The results are the three-dimensional morphology measurement results corresponding to Example 1, and the sample is a gold nanosheet.

[0021] Figure 4 The results are the three-dimensional morphology measurement results corresponding to Example 2. The sample is a thin two-dimensional material.

[0022] Figure 5 The results are the displacement analysis of the piezoelectric displacement stage and the vibration analysis of the crystal oscillator corresponding to Example 3.

[0023] Figure 6 The results are the polyurethane (TPU) film thickness measurement results corresponding to Example 4. Detailed Implementation

[0024] To better illustrate the purpose and advantages of this invention, the following description, in conjunction with the accompanying drawings and examples, further clarifies the invention. The specific description below provides a more detailed explanation of the invention's purpose, technical solution, and beneficial effects. It should be understood that the following descriptions are merely specific embodiments of this invention and are not intended to limit the scope of protection of this invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

[0025] like Figure 1 As shown in the figure, the working method of the axial sub-nanometer resolution astigmatic displacement microscopy imaging method and apparatus disclosed in this embodiment is as follows:

[0026] Optical path design: After being modulated by laser control (26), the laser beam emitted by the laser source (1) passes through the polarization beam splitter (2) to the collimating lens (3) for collimation, and then passes through the reflector (4) to the scanner (5), lens (6) and second lens (7) before being focused onto the surface of the sample (10) by the objective lens (9). The beam reflected from the sample surface returns along the same path, passes through the polarization beam splitter (2) and astigmatism element (12) to the four-quadrant photodetector (13) to generate a focusing error signal, which is then processed (21).

[0027] Obtain and calibrate the S-curve: The computer software (24) controls the precision displacement stage (11) to move up and down in a certain step size (e.g., 300 nm), records the focusing error signal value and displacement, and obtains and calibrates the S-curve. For example, for a laser with a wavelength of 650 nm and an objective lens of 100 x NA = 0.8, its linear range is generally 2-3 μm. By changing the objective lens NA and wavelength, the effective range of the linear range can be adjusted.

[0028] ADM Function Module:

[0029] 1) Scanning imaging:

[0030] The sample to be tested (10) is placed on the XYZ precision displacement stage (11), and the light spot is focused by means of microscopic observation (18). A suitable laser modulation frequency and power (26) are set, and the light spot is scanned on the sample by the scanner (5). The focusing error signal is processed by AC-DC conversion (22) and then collected and recorded by dedicated computer software (24). By using the calibrated relationship between the focusing error signal and the displacement, the three-dimensional surface morphology image of the micro-area sample can be obtained.

[0031] 2) Displacement measurement:

[0032] The sample to be tested (10) is placed on the XYZ precision displacement stage (11), and the light spot is focused by means of microscopic observation (18). The sample is driven to move up and down, causing light spots of different shapes to be generated on the four-quadrant photodetector (13). The four-quadrant photodetector (13) performs photoelectric conversion and outputs photoelectric signals, which are transmitted to the signal processing (21). The signal processing (21) performs gain, addition, subtraction, filtering, bias adjustment and normalization on the output signal to obtain the focusing error signal. After AC-DC conversion (22) and signal acquisition (23), the signal is recorded and displayed by dedicated computer software (24). The sample displacement is obtained by using the calibrated relationship between the focusing error signal and the displacement.

[0033] 3) Vibration analysis:

[0034] The vibrating sample (10) to be tested is placed on the XYZ precision displacement stage (11), and the light spot is focused by means of microscopic observation (18). Driving the sample to vibrate is equivalent to the sample producing a rapidly changing displacement, causing different shaped light spots to be generated on the four-quadrant photodetector (13) and outputting photoelectric signals, which are transmitted to the signal processing (21). The signal processing (21) performs gain, addition, subtraction, filtering, bias adjustment and normalization on the output signal to obtain the focusing error signal. After AC-DC conversion (22) and data acquisition (23), the signal is recorded and displayed by the dedicated computer software (24). Combined with the excitation frequency scan, the focusing error signal of the sample vibration can be obtained. Using the calibrated relationship between the focusing error signal and the displacement, the amplitude, frequency and phase characteristics of the sample vibration can be obtained, and other mechanical parameters of the sample can be further calculated.

[0035] 4) Film thickness measurement:

[0036] The measurement method is similar to the displacement measurement described above, but this time the focusing error signal corresponding to the light spot moving from the lower surface of the sample to the upper surface of the sample is fully recorded. By reading the displacement between the two S-curves of the upper and lower surfaces and combining it with the sample's refractive index information, the thickness of the transparent thin film sample can be calibrated, thus realizing film thickness measurement.

[0037] Example 1

[0038] In this embodiment, Figure 3 This invention demonstrates the ADM method for XYZ resolution characterization of three-dimensional morphology imaging of gold nanosheets. The imaging results show that ADM imaging exhibits excellent signal-to-noise ratio and sharp image edges. The gold sheet thickness is 21 nm. Cross-sectional information reveals an XY spatial resolution of approximately 470 nm (corresponding to signal variations from 10% to 90%) and a Z-axis spatial resolution of approximately 0.1 nm. The ADM method demonstrates superior three-dimensional imaging performance.

[0039] Example 2

[0040] In this embodiment, Figure 4 The ADM method of this invention is used to image the three-dimensional morphology of 1-2 layer WSe2 two-dimensional materials, with a single layer thickness of approximately 0.6 nm. While white light images (OM) can distinguish between 2D materials with fewer layers by color, the color differences are small, making it difficult to accurately determine the number of layers. However, the ADM imaging results show higher contrast for 1-2 layer WSe2 two-dimensional materials, and the number of layers can be accurately obtained by measuring the sample's cross-sectional information.

[0041] Example 3

[0042] In this embodiment, Figure 5 This invention is used for displacement analysis of a P-527.3CD XYZ piezoelectric triaxial displacement stage (PI) and characterization of the resonant frequency of a passive crystal oscillator. In the left figure, as the P-527.3CD displacement stage moves in 300 nm steps, the ADM signal increases linearly with the displacement. The right figure shows the test spectrum of the ADM on an 8 MHz cylindrical passive crystal oscillator. During the test, the crystal oscillator's outer shell was removed, altering the symmetry of the resonance peak, but the measured resonant frequency remained consistent with the nominal value. It is foreseeable that the ADM will be a powerful tool for micro / nano analysis, piezoelectric sensing, and other characterization techniques, playing a significant role in these related fields in the future.

[0043] Example 4

[0044] In this embodiment, Figure 6 This invention is used to measure the thickness of a 25 μm thick polyurethane (TPU) film. Theoretically, the zero points of the two S-shaped curves correspond to the upper and lower surfaces of the film, respectively, and the distance between the two zero points is the film thickness. However, in actual testing, due to the difference in refractive index between air and the film, the refractive effect can cause testing errors. Therefore, it is necessary to consider the factors of the incident angle and the refractive index of the medium and perform calibration. In this embodiment, the test results are close to the nominal value.

Claims

1. An axial sub-nanometer resolution astigmatic displacement microscopy method and device, characterized in that: It comprises a microscopic illumination system (18), an astigmatic module (19), a 4f scanning system (20), a signal conditioning and control module (28), a mirror (4), a sample (10) and a displacement stage (11). The microscopic illumination system (18) comprises a light source (14), a lens (15), a mirror (16), a push-pull module (8), an objective lens (9) and a camera (17); the astigmatic module (19) comprises a laser source (1), a beam splitter (2), a collimating lens (3), an astigmatic element (12) and a four-quadrant photodetector (13); the 4f scanning system (20) comprises a scanner (5), a lens (6) and a second lens (7); the signal conditioning and control module (28) comprises signal processing (21), AC-DC conversion (22), signal acquisition (23), computer software (24), displacement stage control (25), laser control (26) and scanner control (27).

2. An axially sub-nanometer resolved astigmatic displacement microscopy method and apparatus, characterized by The instrument method can have three-dimensional imaging, displacement analysis, vibration analysis and film thickness measurement functions.

3. An axially sub-nanometer resolved astigmatic displacement microscopy method and apparatus, characterized by The instrument method can match any objective lens and meet the in-situ characterization of samples in different environments.

4. An axially sub-nanometer resolved astigmatic displacement microscopy imaging method and apparatus according to claim 1, characterized in that The laser source (1) can be a laser diode or a commercial laser.

5. An axially sub-nanometer resolved astigmatic displacement microscopy imaging method and apparatus according to claim 1, characterized in that The 4f optical path system is composed of the scanner (5), the lens (6) and the second lens (7), and the high-speed scanner realizes the functions of fast scanning imaging and positioning.

6. An axially sub-nanometer resolved astigmatic displacement microscopy imaging method and apparatus according to claim 1, characterized in that The astigmatic element (12) can be a cylindrical lens or other elements with astigmatic effect.

7. An axially sub-nanometer resolved astigmatic displacement microscopy imaging method and apparatus according to claim 1, wherein The microscopic illumination system (18) is integrated, which is convenient for sample positioning and observation.

8. An axially sub-nanometer resolved astigmatic displacement microscopy imaging method and apparatus according to claim 1, wherein The astigmatic module (19) can be realized by discrete elements or by using an optical pickup head (OPU).

9. An axially sub-nanometer resolved astigmatic displacement microscopy imaging method and apparatus according to claim 1, wherein The signal processing (21) includes gain, addition, subtraction, filtering, bias adjustment and normalization processing of the four output signals of the four-quadrant photodetector, so as to obtain measurement results with high sensitivity, low noise and no influence of sample reflectivity difference.

10. The method and apparatus of axially sub-nanometer resolved astigmatic displacement microscopy according to claim 1, wherein The AC-DC conversion (22) can be realized by phase-locked loop and RMS-DC.

11. An axially sub-nanometer resolved astigmatic displacement microscopy imaging method and apparatus according to claim 1, characterized in that The laser control (26) has the functions of continuous output, modulated output and power control.

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