A method of ultrasonic microscopic imaging

Abstract

The present application relates to the technical field of ultrasonic microscopic imaging, and particularly relates to a method for ultrasonic microscopic imaging. In the method, Rayleigh waves generated by a focused ultrasonic transducer are used for blind area defect detection, and reflected longitudinal waves obtained by conversion of the Rayleigh waves are interfered with interface reflected longitudinal waves, and finally the amplitude of ultrasonic waves is superimposed to obtain a high-resolution, high-contrast scanning image reflecting real size and topography information of defects. The method is suitable for a reflected mode high-frequency ultrasonic microscopic imaging detection system, and can realize imaging detection of defects in a blind area of a material or structure, and effectively make up for problems such as poor imaging contrast and difficult defect quantification when a traditional ultrasonic microscopic imaging system detects defects in a blind area.

Description

Technical Field

[0001] This invention relates to the field of ultrasonic microscopy imaging technology, and in particular to a method for ultrasonic microscopy imaging. Background Technology

[0002] Ultrasonic testing, as one of the five conventional non-destructive testing technologies, is widely used in industries such as industry, aerospace, rail transportation, semiconductors, and materials. High-frequency ultrasonic microscopy imaging systems, also known as ultrasonic scanning microscope systems, are crucial tools for achieving microscopic imaging of defects at the millimeter, micrometer, and even submicrometer levels. Utilizing a high-precision three-dimensional scanning platform, a high-frequency focusing ultrasonic transducer, and a high-speed, high-bandwidth data acquisition card, they significantly improve the accuracy and image resolution of ultrasonic scanning imaging. Ultrasonic microscopy imaging systems based on the pulse-echo method reconstruct scanned images using information such as the amplitude, frequency, and phase of reflected longitudinal waves, accurately locating defects and displaying their morphology and depth information, playing a vital role in detecting microscopic defects within materials and structures.

[0003] However, the imaging performance of pulse-echo imaging for detecting near-surface defects is far inferior to that for detecting internal defects. The fundamental reason is that when the acoustic waves emitted by the transducer are focused near the material surface, the reflected or scattered waves from the near-surface defects alias with the interface waves reflected from the material surface. This aliasing severely weakens the amplitude and phase characteristics of the defect echo, resulting in poor contrast in the amplitude- and phase-based imaging results. This is especially true for tiny defects near the center wavelength of the acoustic wave, where the defect image may not even be observable in the scanned image. Therefore, the area containing near-surface defects is often referred to as the blind zone in pulse-echo imaging. Thus, improving the imaging resolution and contrast of the ultrasonic scanning microscope system for detecting defects within the blind zone of the tested sample has become a key bottleneck in improving the performance of this equipment. Summary of the Invention

[0004] To improve the imaging resolution and contrast of defect detection in the blind zone of the tested sample, this invention provides a method for ultrasonic microscopy.

[0005] This invention provides a method for ultrasonic microscopy imaging, applied to an ultrasonic microscopy imaging system. The system includes a focused ultrasonic transducer disposed in a coupling agent and a test sample. The focused ultrasonic transducer is positioned directly above the test sample. The focused ultrasonic transducer includes an acoustic lens and a piezoelectric crystal disposed within the acoustic lens. The method includes:

[0006] Step S1: Based on the longitudinal wave velocity of the ultrasonic longitudinal wave emitted by the piezoelectric wafer propagating in the acoustic lens and the coupling agent, and the transverse wave velocity of the ultrasonic longitudinal wave propagating in the test sample, determine the radius of curvature of the acoustic lens and the diameter of the piezoelectric wafer, so that the ultrasonic longitudinal wave excites Rayleigh waves to be generated on the surface of the test sample.

[0007] Step S2: Excite the focused ultrasound transducer and adjust the height of the focused ultrasound transducer so that the test sample is located in the negative defocusing area of ​​the focused ultrasound transducer;

[0008] Step S3: Keep the height of the focused ultrasonic transducer constant and scan the test sample in the horizontal direction to collect ultrasonic signals at different positions of the test sample; wherein, the ultrasonic signal is a signal of interference between the interface wave signal and the longitudinal wave reflection signal obtained by Rayleigh wave conversion, and the interface wave signal is a signal generated by the ultrasonic longitudinal wave emitted by the piezoelectric crystal and reflected by the surface of the test sample.

[0009] Step S4: For the ultrasonic signal at each location, based on the target time when the longitudinal wave reflection signal converted from Rayleigh wave begins to appear in the current ultrasonic signal, a scanning image with an interference ring is obtained; wherein, the center of the interference ring is the location of the defect in the blind zone of the tested sample, and the target time is used to determine the pixel value of the scanning image;

[0010] Step S5: For the location of the defect in the blind zone of the test sample, the signal amplitudes of the Rayleigh waves generated at different locations are superimposed to the center of the interference ring to obtain an ultrasonic image containing the true size and morphology information of the defect in the blind zone.

[0011] In one possible design, step S1 includes:

[0012] The radius of curvature of the acoustic lens and the diameter of the piezoelectric wafer are determined using the following formulas:

[0013]

[0014]

[0015] In the formula, θ1 and θ2 represent the incident angle of the ultrasonic longitudinal wave emitted by the piezoelectric wafer relative to the concave surface of the acoustic lens and the refraction angle of the ultrasonic longitudinal wave after transmission from the surface of the acoustic lens, respectively. c The second critical angle represents the surface of the tested sample, D and R represent the diameter of the piezoelectric wafer and the radius of curvature of the acoustic lens, respectively, and c L1 c L2c represents the longitudinal wave velocity of the ultrasonic longitudinal wave propagating in the acoustic lens and the coupling agent, respectively. S3 The transverse wave velocity represents the longitudinal ultrasonic wave propagating in the test sample.

[0016] In one possible design, after step S2 and before step S3, the following is also included:

[0017] The focusing depth of the focused ultrasound transducer is changed, and the focusing depth at which the amplitude of the ultrasonic signal at the target time reaches its maximum is determined as the optimal focusing depth.

[0018] In one possible design, the superposition of signal amplitudes propagated from Rayleigh waves generated at different locations to the center of the interference ring includes:

[0019] The sum of all signal amplitudes at the center of the interference ring is obtained using the following formula:

[0020]

[0021] In the formula, A(x) i ,y i f is the sum of the amplitudes of all signals at the center of the interference ring. m,n (t b ) represents the amplitude of the ultrasonic signal received by the focused ultrasonic transducer at the m-th row and n-th column position during the scanning process, and t represents the amplitude of the ultrasonic signal received by the focused ultrasonic transducer at the m-th row and n-th column position. b Let x be the time required for a Rayleigh wave signal emitted at position m in row n in column n to propagate to its current position. j ,y j (x) represents the location where the Rayleigh wave is emitted. i ,y i (x) represents the location where Rayleigh waves are received. m,n ,y m,n ) represents the position in the m-th row and n-th column, c R The speed at which the Rayleigh wave propagates in the test sample.

[0022] This invention provides a method for ultrasonic microscopy imaging. It utilizes Rayleigh waves generated by a focused ultrasonic transducer to detect defects within a blind zone. The reflected longitudinal waves obtained from the Rayleigh wave conversion are then interfered with the reflected longitudinal waves from the interface. Finally, the amplitudes of the ultrasonic signals are superimposed to obtain a high-resolution, high-contrast scanning image reflecting the true size and morphology of the defect. This method is applicable to high-frequency ultrasonic microscopy imaging systems using reflection mode, enabling imaging detection of defects within the blind zone of materials or structures. It effectively overcomes the problems of poor imaging contrast and difficulty in quantifying defects in traditional ultrasonic microscopy imaging systems when detecting defects within the blind zone. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a schematic flowchart of an ultrasonic microscopy imaging method provided in an embodiment of the present invention;

[0025] Figure 2 This is a schematic diagram illustrating the principle of Rayleigh wave excitation and two-wave interference provided in an embodiment of the present invention;

[0026] Figure 3 These are waveforms of ultrasonic signals at different focusing depths provided in embodiments of the present invention;

[0027] Figure 4 These are scanning images at different focusing depths provided in the embodiments of the present invention;

[0028] Figure 5 This is a schematic diagram illustrating the principle of dual-wavelength interferometric imaging provided in an embodiment of the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0030] As mentioned earlier, the traditional longitudinal wave pulse reflection method has a detection blind zone when detecting defects in the shallow surface of the test sample. This not only makes it difficult to detect near-surface defects, but also fails to effectively detect internal defects in thin-layered samples where the thickness is less than the depth of the blind zone. Traditional methods for extracting defect signals within the blind zone based on signal processing lose the amplitude and frequency information of the defect echo signal, making it difficult to accurately determine the defect size and type. Furthermore, the complexity of the algorithm affects the real-time performance of the scanning imaging. Methods that reduce or eliminate the blind zone, such as pulse coding, inverse superposition, and reducing the number of pulse cycles, reduce the sensitivity of defect detection.

[0031] To address the aforementioned issues, the inventors discovered during the research and development process that Rayleigh waves are essentially a special type of sound wave that propagates along the free surface of a solid. The wave amplitude decays exponentially below the surface of the sample, with most of its energy concentrated within a depth range of one wavelength below the surface. Therefore, when using an ultrasonic microscopy system to detect near-surface defects in materials, the focused ultrasonic transducer can be moved down to the negative defocusing region of the large-aperture focused ultrasonic transducer on the upper surface of the sample. The Rayleigh waves generated by the longitudinal waves emitted by the transducer, converted onto the surface of the material, can be used to image and detect shallow surface defects. This improves the performance of ultrasonic microscopy systems in defect microscopic imaging and quantitative characterization within the blind zone, playing a crucial role in the semiconductor industry, metal additive manufacturing, thermal barrier coatings, and various rolling contact structures where surface or shallow surface defects need to be evaluated.

[0032] Please see Figure 1 This invention provides a method for ultrasonic microscopy imaging, applied to an ultrasonic microscopy imaging system. The system includes a focused ultrasonic transducer disposed in a coupling agent and a test sample. The focused ultrasonic transducer is positioned directly above the test sample. The focused ultrasonic transducer includes an acoustic lens and a piezoelectric crystal disposed within the acoustic lens. The method includes:

[0033] Step S1: Based on the longitudinal wave velocity of the ultrasonic longitudinal wave emitted by the piezoelectric crystal propagating in the acoustic lens and coupling agent, and the transverse wave velocity of the ultrasonic longitudinal wave propagating in the test sample, determine the radius of curvature of the acoustic lens and the diameter of the piezoelectric crystal, so that the ultrasonic longitudinal wave can generate Rayleigh waves on the surface of the test sample.

[0034] Step S2: Excite the focused ultrasound transducer and adjust the height of the focused ultrasound transducer so that the test sample is located in the negative defocusing area of ​​the focused ultrasound transducer.

[0035] Step S3: Keep the height of the focused ultrasonic transducer constant and scan the test sample in the horizontal direction to collect ultrasonic signals at different positions of the test sample; wherein, the ultrasonic signal is the interference signal of the interface wave signal and the longitudinal wave reflection signal obtained by Rayleigh wave conversion, and the interface wave signal is the signal generated by the ultrasonic longitudinal wave emitted by the piezoelectric crystal reflected by the surface of the test sample.

[0036] Step S4: For the ultrasonic signal at each location, based on the target time when the longitudinal wave reflection signal converted from Rayleigh wave begins to appear in the current ultrasonic signal, a scanning image with an interference ring is obtained; wherein, the center of the interference ring is the location of the defect in the blind zone of the tested sample, and the target time is used to determine the pixel value of the scanning image;

[0037] Step S5: For the location of the defect in the blind zone of the tested sample, the signal amplitudes of the Rayleigh waves generated at different locations are superimposed to the center of the interference ring to obtain an ultrasonic image containing the true size and morphology information of the defect in the blind zone.

[0038] In this embodiment, Rayleigh waves generated by a focused ultrasonic transducer are used for defect detection within the blind zone. The reflected longitudinal waves obtained from the Rayleigh wave conversion are then interfered with the reflected longitudinal waves from the interface. Finally, the amplitudes of the ultrasonic signals are superimposed to obtain a high-resolution, high-contrast scanning image that reflects the true size and morphology of the defect. This method is applicable to high-frequency ultrasonic microscopy imaging systems using reflection mode, enabling imaging detection of defects within the blind zone of materials or structures. It effectively overcomes the problems of poor imaging contrast and difficulty in quantifying defects in traditional ultrasonic microscopy imaging systems when detecting defects within the blind zone.

[0039] The steps are described below.

[0040] Regarding step S1:

[0041] In some embodiments, the coupling agent can be water or other liquids, and this is not limited thereto.

[0042] Please see Figure 2 In one embodiment of the present invention, step S1 may specifically include:

[0043] The radius of curvature of the acoustic lens and the diameter of the piezoelectric crystal are determined using the following formula:

[0044]

[0045] In the formula, θ1 and θ2 represent the incident angle of the ultrasonic longitudinal wave emitted by the piezoelectric crystal relative to the concave surface of the acoustic lens and the refraction angle of the ultrasonic longitudinal wave after transmission from the surface of the acoustic lens, respectively. c The second critical angle represents the surface of the tested sample, D and R represent the diameter of the piezoelectric crystal and the radius of curvature of the acoustic lens, respectively, and c L1 c L2 c represents the longitudinal wave velocity of the ultrasound propagating in the acoustic lens and the coupling agent, respectively. S3 The transverse wave velocity represents the longitudinal wave velocity of the ultrasound propagating in the test sample.

[0046] In this embodiment, to detect near-surface defects, Rayleigh waves are excited on the surface of the test sample by designing the geometric parameters of a focused ultrasonic transducer. The condition for Rayleigh wave excitation on the sample surface is that the incident angle of the longitudinal wave signal emitted along the outermost radial direction of the transducer on the test sample surface must be greater than the second critical angle. The incident angle of the outermost longitudinal wave is related to the transducer's geometric parameters, as well as the sound velocity of the acoustic lens and coupling agent, and must satisfy the aforementioned inequality condition.

[0047] By designing the diameter D of the piezoelectric crystal and the radius of curvature R of the acoustic lens to make the above inequality hold, a Rayleigh wave signal can be excited on the surface of the test sample. As this Rayleigh wave propagates along the surface of the test sample, its longitudinal wave component continuously leaks into the water, where it is received by the focused ultrasonic transducer. Therefore, the focused ultrasonic transducer receives a signal resulting from the interference of the interface wave signal and the longitudinal wave reflection signal converted from the Rayleigh wave. The phase or amplitude information of the interference signal can then be used to achieve imaging detection of defects within the blind zone.

[0048] For steps S2 and S3:

[0049] also, Figure 2 The propagation paths of different modes of acoustic waves under dual-wave interference conditions are also presented. When the outermost acoustic wave of the transducer is incident on the surface of the test sample, because the incident angle is greater than the second critical angle, both the transmitted longitudinal wave and the transmitted transverse wave propagate along the sample surface and merge into a Rayleigh wave signal. Simultaneously, the Rayleigh wave continuously leaks its longitudinal component into the water during its propagation along the surface, thus forming a reflected longitudinal wave converted from the Rayleigh wave. On the other hand, near the transducer axis, because the incident angle of the longitudinal wave is less than the first critical angle, the incident longitudinal wave is directly reflected by the sample surface, forming an interface wave signal (i.e., a reflected longitudinal wave). The interface wave is essentially a longitudinal wave signal, which interferes with the reflected longitudinal wave signal converted from the Rayleigh wave, resulting in the final dual-wave interference signal. Since both ultrasonic signals originate from the same transducer, they have identical frequency band characteristics, thus generating a stable interference signal. The phase of the reflected longitudinal wave and the Rayleigh-converted longitudinal wave signal are shown. and The calculation formula is:

[0050]

[0051] Where x represents the distance the Rayleigh wave travels to the transducer axis, such as... Figure 2 As shown. k L and k R Let F and z represent the wavenumbers of the longitudinal wave and Rayleigh wave, respectively, and let F and z represent the focal length of the focused ultrasound transducer and the focusing depth of the transducer on the sample surface, respectively. The phase difference between the reflected longitudinal wave and the Rayleigh wave-converted longitudinal wave signal can be calculated from the above formula:

[0052]

[0053] Different interference signal waveforms can be obtained by changing the focusing depth z of the transducer. Figure 3 The waveform comparison results of the interference signal under deep focusing and shallow focusing conditions are presented. From Figure 3As can be seen, different focusing depths do not change the waveform of the reflected P-wave, but a significant phase change occurs in the waveform of the interference between the tail of the reflected P-wave and the Rayleigh wave-converted P-wave signal. Unlike traditional defect echoes that are submerged in interface waves, the Rayleigh wave signal carrying defect information has a longer delay time than the interface reflected P-wave. Therefore, the P-wave signal converted from the Rayleigh wave interferes with the tail of the interface reflected P-wave. The amplitude difference between the two is relatively small, resulting in a more obvious interference signal. This is also... Figure 3 The fundamental reason for the significant differences in the interference signals.

[0054] In one embodiment of the present invention, after step S2 and before step S3, the method further includes:

[0055] By changing the focusing depth of the focused ultrasound transducer, the focusing depth at which the amplitude of the ultrasonic signal at the target moment reaches its maximum is determined as the optimal focusing depth.

[0056] In this embodiment, under different focusing depth conditions, the amplitude characteristics of the dual-wave interference signal are used to image near-surface defects in the titanium alloy additive manufacturing part. The resulting scanning imaging results are as follows: Figure 4 As shown, when the focused ultrasonic transducer is focused on a surface or near-surface defect, the amplitude imaging results do not provide a clear image of the defect, and the imaging contrast is relatively poor. However, when the transducer is focused on the negative defocus region and imaging is performed using a dual-wavelength interference signal, a clear interference ring pattern can be observed, such as... Figure 4 As shown in (b). With Figure 4 (a) In comparison, Figure 4 The presence of more interference rings in (b) indicates the presence of a defect at the center of the ring, thus demonstrating that the two-wavelength interferometry method can image defects that traditional amplitude imaging methods cannot display. However, it should be noted that the two-dimensional interference pattern obtained by this imaging does not represent the true morphology of the defect within the blind zone, nor can it quantitatively characterize the defect. Figure 4 (c) and Figure 4 (d) represents the interference patterns of the same scanning area at different focusing depths. As can be seen from the figure, the radius of the interference ring gradually increases with the increase of focusing depth, and the degree of radius increase is positively correlated with the propagation distance x of the Rayleigh wave on the surface of the test sample.

[0057] As can be seen from the above analysis, after determining the optimal focusing depth, the height of the focused ultrasound transducer can be kept constant, and the focused ultrasound transducer can be made to scan the test sample in the horizontal direction to collect the ultrasonic signals of the test sample at different positions.

[0058] Regarding step S4:

[0059] Please continue reading. Figure 3Because the Rayleigh wave signal carrying defect information has a longer delay time than the longitudinal wave reflected from the interface, the longitudinal wave signal converted from the Rayleigh wave interferes with the tail of the longitudinal wave reflected from the interface. Therefore, the target time (i.e., t) when the longitudinal wave reflection signal converted from the Rayleigh wave begins to appear in the ultrasonic signal at each location can be determined. a (at the end of the time), thus obtaining a scanned image with interference rings.

[0060] Regarding step S5:

[0061] In one embodiment of the present invention, the step of "superimposing the signal amplitudes of Rayleigh waves generated at different locations propagated to the center of the interference ring" may specifically include:

[0062] The sum of all signal amplitudes at the center of the interference ring is obtained using the following formula:

[0063]

[0064] In the formula, A(x) i ,y i f is the sum of the amplitudes of all signals at the center of the interference ring. m,n (t b Let t be the amplitude of the ultrasonic signal received by the focused ultrasonic transducer at the m-th row and n-th column position during the scanning process. b Let x be the time required for a Rayleigh wave signal emitted at position m in row n in column n to propagate to its current position. j ,y j (x) represents the location where the Rayleigh wave is emitted. i ,y i (x) represents the location where Rayleigh waves are received. m,n ,y m,n ) represents the position in the m-th row and n-th column, c R The speed at which the Rayleigh wave propagates in the test sample.

[0065] In this embodiment, since the obtained scan image with interference rings cannot truly reflect the size and shape information of the defects in the blind zone, the two-dimensional ultrasonic signal data matrix composed of interference signals is further processed (i.e., signal amplitude superposition processing) based on the scan image, thereby realizing the true reconstruction of the image of the defects in the blind zone.

[0066] Please see Figure 5 For a certain spatial location A(x) i ,y i The transducer is located at different positions in two-dimensional space, such as (x... j ,y jRayleigh waves emitted by any of these locations may propagate to this point and be scattered to different locations as shown in (x1,y1) and (x2,y2) for reception. Therefore, the annular location shown in Figure 5 will receive scattered Rayleigh wave signals from different locations after being scattered by the defect. Therefore, spatial location A(x1,y1) will receive scattered Rayleigh wave signals after being scattered by the defect. i ,y i The signal amplitude at point t is the superposition of the signal amplitudes of Rayleigh waves propagating from different locations in two-dimensional space to that location. b The reference time is the peak time of the first zero-crossing point on the left side of the reflected longitudinal wave, such as... Figure 3 As shown, by extracting the amplitude of interference signals at different locations in two-dimensional space for image reconstruction, high-resolution and high-contrast imaging of defects within the blind zone can be achieved.

[0067] Thus, high-frequency ultrasonic microscopic imaging detection of defects within the basic blind zone has been achieved. Taking a focused ultrasonic transducer with a center frequency of 100MHz as an example, the wavelength of Rayleigh waves in titanium alloys is approximately 30μm. Therefore, theoretically, defects with a size of around 30μm within the blind zone can be detected, while the detection limit of traditional longitudinal wave reflection-based methods is approximately 60μm. Therefore, the defect detection method within the blind zone proposed in this invention can improve the defect detection resolution by more than 50%.

[0068] In summary, the solution provided by this invention has the following advantages:

[0069] (1) A method for extracting defect signal features in the blind zone based on dual-wave interference is proposed, and the geometric parameters for Rayleigh waves excited by the focused ultrasonic transducer are given. This effectively makes up for the problem that the defect echo signal is difficult to extract accurately in the blind zone due to the interface wave being submerged in the traditional reflected ultrasonic detection mode.

[0070] (2) To address the problem that high-resolution imaging of defects in the blind zone cannot be achieved by using the amplitude of dual-wave interference signals, a two-dimensional scanning image is reconstructed based on the superposition of Rayleigh wave signal amplitudes, which improves image contrast and enables quantitative characterization of defects.

[0071] (3) Since the Rayleigh wave signal interacts with the defects in the blind zone, the defect imaging resolution is more than 50% higher than that of the traditional longitudinal wave reflection imaging method at the same frequency.

[0072] (4) The Rayleigh wave signal generated on the surface of the test sample by the focused ultrasonic transducer has a propagation characteristic that converges in the axial direction of the transducer, which allows the longitudinal wave signal converted from the Rayleigh wave to be fully received by the transducer, thereby increasing the amplitude of the final Rayleigh wave to longitudinal wave signal and helping to enhance the effect of dual-wave interference.

[0073] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus.

[0074] Those skilled in the art will understand that all or part of the steps of the above method embodiments can be implemented by hardware related to program instructions. The aforementioned program can be stored in a computer-readable storage medium. When the program is executed, it performs the steps of the above method embodiments. The aforementioned storage medium includes various media that can store program code, such as ROM, RAM, magnetic disk, or optical disk.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method of ultrasonic microscopic imaging, characterized by, The application is applied to an ultrasonic microscopic imaging system, the system comprises a focused ultrasonic transducer arranged in a coupling agent and a test sample, the focused ultrasonic transducer is arranged directly above the test sample, the focused ultrasonic transducer comprises an acoustic lens and a piezoelectric wafer arranged in the acoustic lens, and the method comprises the following steps: In step S1, the curvature radius of the acoustic lens and the diameter of the piezoelectric wafer are determined based on the longitudinal wave speed of the ultrasonic longitudinal wave emitted by the piezoelectric wafer propagating in the acoustic lens and the transverse wave speed of the ultrasonic longitudinal wave propagating in the test sample, so that the ultrasonic longitudinal wave can excite Rayleigh wave on the surface of the test sample; In step S2, the focused ultrasonic transducer is excited and the height of the focused ultrasonic transducer is adjusted, so that the test sample is located in the negative defocusing area of the focused ultrasonic transducer; In step S3, the height of the focused ultrasonic transducer is kept unchanged, and the focused ultrasonic transducer is scanned along the horizontal direction to the test sample to collect ultrasonic signals of the test sample at different positions; wherein the ultrasonic signal is a signal obtained by interference of an interface wave signal and a longitudinal wave reflection signal converted from Rayleigh wave, and the interface wave signal is a signal generated by reflection of the ultrasonic longitudinal wave emitted by the piezoelectric wafer on the surface of the test sample; In step S4, for the ultrasonic signal at each position, a scanning image with an interference annulus is obtained based on a target time point at which the longitudinal wave reflection signal converted from Rayleigh wave starts to appear in the current ultrasonic signal; wherein the center of the interference annulus is the position of the defect in the blind area of the test sample, and the target time point is used to determine the pixel value of the scanning image; In step S5, for the position of the defect in the blind area of the test sample, the signal amplitude of the Rayleigh wave generated at different positions propagating to the center of the interference annulus is superimposed to obtain an ultrasonic image containing the real size and topographic information of the defect in the blind area; The curvature radius of the acoustic lens and the diameter of the piezoelectric wafer are determined by the following formula: wherein θ1 and θ2 represent the incident angle of the ultrasonic longitudinal wave emitted by the piezoelectric wafer to the concave surface of the acoustic lens and the refraction angle of the ultrasonic longitudinal wave transmitted from the surface of the acoustic lens, respectively, θ c represents the second critical angle of the surface of the sample under test, D and R represent the diameter of the piezoelectric wafer and the radius of curvature of the acoustic lens, respectively, c L1 , c L2 represent the longitudinal wave velocities of the ultrasonic longitudinal wave propagating in the acoustic lens and the coupling agent, respectively, c S3 represents the transverse wave velocity of the ultrasonic longitudinal wave propagating in the sample under test; The sum of all signal amplitudes at the center of the interference annulus is obtained by the following formula: In the formula, A(x i ,y i ) is the sum of all signal amplitudes at the center of the interference annulus, f m,n (t b ) is the amplitude of the ultrasonic signal received by the focused ultrasonic transducer at the mth row and nth column position during scanning, t b is the time required for the Rayleigh wave signal transmitted at the mth row and nth column position to propagate to the current position, (x j ,y j ) is the position from which the Rayleigh wave is emitted, (x i ,y i ) is the position at which the Rayleigh wave is received, (x m,n ,y m,n ) is the mth row and nth column position, and c R is the speed at which the Rayleigh wave propagates in the sample being tested.

2. The method of claim 1, wherein, After step S2 and before step S3, the following steps are further included: The focusing depth of the focused ultrasonic transducer is changed, and the focusing depth at which the amplitude corresponding to the target time point in the ultrasonic signal reaches the maximum is determined as the optimal focusing depth.

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