Packaging structure topography optical coherence tomography method based on bessel beam

Abstract

The application discloses a packaging structure topography optical coherence tomography detection method based on a Bessel beam. A broadband beam is modulated by an axicon prism and a reflective spatial light modulator to form a ring-shaped long focal depth Bessel beam, and a two-dimensional scanning of pupil conjugation is realized through a double-vibration mirror and a two-stage relay lens group. A frequency domain optical coherence tomography system is used to collect spectral interference signals at each transverse scanning position, and depth information is obtained through wave number linearization, windowing and Fourier transform. Based on the three-dimensional point spread function of the Bessel beam, deconvolution is carried out to suppress the sidelobe crosstalk, and the three-dimensional topography of the packaging structure is reconstructed in combination with the refractive index correction. The method utilizes the non-diffraction and self-healing physical characteristics of the Bessel beam, and does not need to perform additional Z-axis focal point tracking, so that the resolution, accuracy and efficiency of the three-dimensional topography detection of the complex packaging structure can be improved.

Description

Technical Field

[0001] This invention belongs to the field of optical micro-interference three-dimensional topography detection, specifically relating to an optical coherence tomography detection method for packaged structure topography based on Bessel beams. Background Technology

[0002] Packaging is a crucial step in semiconductor manufacturing, microelectromechanical systems (MEMS), and the production of various sensors. By placing the chip in a compatible packaging structure, a stable and reliable operating environment can be provided, reducing external interference such as dust and moisture. However, in the packaging process, the difference in the coefficients of thermal expansion between different materials (such as silicon, resin, and transparent substrates) can generate residual stress under temperature changes, leading to thermal deformation and consequently, warping or microcracks on the device surface. To facilitate input / output responses and non-destructive optical inspection, transparent media are generally used for packaging in industry. Therefore, high-precision non-destructive testing of the three-dimensional surface morphology of transparent packaged devices and their internal structures is not only helpful in the timely detection of potential defects but also of great significance for improving packaging yield and optimizing process parameters.

[0003] Currently, several representative methods have been developed for the inspection of packaged devices, such as scanning acoustic microscopy and infrared thermography. However, the former is limited by edge effects and struggles to identify minute defects, while the latter can only identify thermally induced defects and is sensitive to environmental noise, failing to meet the high-resolution inspection requirements of packaged devices. Micro-interferometry is widely used due to its non-contact and high-resolution advantages; however, when measuring deep packaged structures, the packaged layer, as a thick transparent medium, introduces significant monochromatic aberrations and dispersion, resulting in phase delay, severely reducing the contrast of the interferogram, and causing systematic errors or even failure in morphology reconstruction.

[0004] As complex packaging structures continue to evolve, the limitations of traditional Gaussian beams in optical coherence tomography (OCT) for three-dimensional topography inspection are becoming increasingly apparent. Gaussian beams are limited by the Rayleigh diffraction limit, resulting in an irreconcilable contradiction between lateral resolution and depth of focus: increasing the numerical aperture of the system can achieve higher lateral resolution, but it leads to a drastic reduction in depth of focus. When dealing with packaged devices with significant depth or thick transparent media, the short depth of focus makes it impossible for Gaussian beams to maintain clear focus throughout the entire package depth. Bessel beams, with their unique "diffraction-free" and "self-healing" properties, offer an ideal solution to this problem. The non-diffraction properties of Bessel beams enable them to maintain an extremely fine central main lobe without significant divergence over extremely long axial transmission distances. Within a limited aperture and set propagation distance range, Bessel beams can extend the effective depth of focus relative to Gaussian beams and maintain a relatively stable lateral main lobe size at multiple depth interfaces of the encapsulation structure. At the same time, their self-healing properties allow the beam to quickly reconstruct its own optical field morphology behind obstacles after penetrating complex encapsulation interfaces or encountering scattering particles inside the medium, exhibiting excellent anti-scattering and anti-interference capabilities.

[0005] Traditional physical optical elements (such as fixed-axis prisms or slits) face limitations in flexibility and modulation accuracy when generating Bessel beams. Spatial light modulators (SLMs), as programmable microelectronic optical devices, can perform high-resolution spatial dynamic modulation of incident broadband beams and can adaptively optimize the pupil according to sample characteristics, significantly improving the flexibility of optical path design and the signal-to-noise ratio of deep detection signals.

[0006] The inherent concentric annular sidelobes of the Bessel beam are a major physical limitation when applied to the detection of dense microstructures. In OCT detection systems, the off-focus illumination of the Bessel beam sidelobes generates overlapping backscattered signals, leading to severe background "hazing" and spatial crosstalk artifacts in topography reconstruction, thus reducing measurement accuracy. To overcome this limitation, a three-dimensional deconvolution filter must be introduced. This algorithm, relying on the system's point spread function, mathematically reconverges the diffused sidelobe energy back to the main lobe, effectively eliminating crosstalk artifacts. It achieves a deep fusion of the hardware advantages of long focal depth and the advantages of computational imaging artifact removal, and ensures that the topography at the bottom of the encapsulation layer is always within the high-resolution focal coverage without sacrificing imaging depth, thereby significantly improving the detection accuracy and imaging quality of deep three-dimensional topography.

[0007] Existing methods have significant limitations in the inspection of complex packaged 3D morphology. The patent "Measuring Lenses and Lens Molds Using Optical Coherence Tomography" proposes a method using traditional optical coherence tomography (OCT), but the traditional Gaussian beam is limited by the Rayleigh diffraction limit, making it impossible to achieve clear focusing across the entire depth when probing thick, transparent media. The patent "Super-Resolution Bessel Microscopy Imaging Device and Method Based on Multifocal Illumination" proposes an imaging method using Bessel beams combined with deconvolution filtering to eliminate sidelobe interference. However, in practical applications, both multifocal Bessel microscopy and light-sheet fluorescence microscopy rely on fluorescence signals generated by sample stimulation, making them unsuitable for non-destructive probing of non-fluorescent semiconductor transparent packages. Furthermore, the system ultimately reconstructs a super-resolution 2D image of the sample; to extract the internal structure of packages with large aspect ratios, it still cannot fully utilize the advantages of long depth of field.

[0008] Meanwhile, to address the challenge of detecting the morphology and thickness of large-area complex structures in advanced packaging technologies, Julia Brueckner et al. proposed a method using traditional spectral coherent interferometry in their paper "Effective Inspection Methods for Advanced Packaging Technologies." However, this method has significant limitations in practical applications: when dealing with high-gradient microstructures with large slopes (such as those at bump sidewalls), the reflected signal from the material surface attenuates sharply, making feature extraction difficult; furthermore, deviations in the local refractive index of actual packaging materials can directly lead to inaccurate measurement results. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this invention provides an optical coherence tomography method for detecting the morphology of packaged structures based on Bessel beams. This method enables non-destructive testing of the interior of packaged structures while effectively eliminating crosstalk artifacts, thereby significantly improving the detection efficiency and accuracy of the three-dimensional morphology of complex packaged structures.

[0010] The technical solution to achieve the objective of this invention is as follows: a method for optical coherence tomography detection of packaged structure morphology based on Bessel beams, comprising the following steps:

[0011] Step 1: Set up the Bessel beam illumination system and the 2D scanning system separately:

[0012] The Bessel beam illumination system includes a light source, a first lens, an axon prism, a second lens, a first beam splitter, and a reflective spatial light modulator arranged sequentially along a first optical axis.

[0013] The two-dimensional scanning system includes a first single-axis galvanometer, a third lens, a fourth lens, a second single-axis galvanometer, a fifth lens, and a sixth lens.

[0014] The third and fourth lenses, when confocal, form the first-stage relay lens group for beam relay. The fifth and sixth lenses, when confocal, form the second-stage relay lens group for beam relay.

[0015] The first single-axis galvanometer, the third lens, the fourth lens, and the second single-axis galvanometer are arranged sequentially along the second optical axis. The first single-axis galvanometer receives the reflected light beam from the first beam splitter, and the third lens is located on the reflected light path of the first single-axis galvanometer. The first single-axis galvanometer is used to control the one-dimensional scanning of the light beam.

[0016] A second single-axis galvanometer, a fifth lens, and a sixth lens are sequentially arranged along the third optical axis. The fifth lens is located on the reflected light path of the second single-axis galvanometer. The second single-axis galvanometer, in conjunction with the first single-axis galvanometer, achieves two-dimensional XY plane scanning of the beam. The Bessel beam emitted from the sixth lens, after undergoing two-dimensional scanning control, enters the optical coherence tomography system located behind the second-stage relay lens group composed of the fifth to sixth lenses.

[0017] Step 2: Set up the optical coherence tomography detection system:

[0018] The optical coherence tomography (OCT) system includes a second beam splitter, a first microscope objective, a plane mirror, a second microscope objective, a packaged structure under test (DUT), a seventh lens, a single-mode fiber, a spectrometer, and a computer. The second beam splitter, second microscope objective, and DUT are arranged sequentially along the third optical axis and behind the second-stage relay mirror group, with the second microscope objective and DUT forming the test optical path. Along the fourth optical axis, the seventh lens, second beam splitter, first microscope objective, and plane mirror are arranged sequentially, with the first microscope objective and plane mirror forming the reference optical path. The seventh lens is connected to the spectrometer via a single-mode fiber, and the three components form the interference signal acquisition optical path. The spectrometer is connected to the computer.

[0019] Step 3: The light source emits a broadband Gaussian beam, which is collimated by the first lens and then passes to the axial prism. The axial prism modulates the phase of the incident beam to generate a long focal depth Bessel beam. The generated Bessel beam passes through the second lens to the first beam splitter. After being transmitted through the first beam splitter, it passes to the reflective spatial light modulator. The reflective spatial light modulator modulates the incident beam with a ring pupil and reflects it. The modulated beam is reflected by the first beam splitter to the first single-axis galvanometer. After being deflected by the first single-axis galvanometer, the beam passes through the first-stage relay mirror group to the second single-axis galvanometer. After being deflected by the second single-axis galvanometer, the beam passes through the second-stage relay mirror group to the second beam splitter.

[0020] The second beam splitter separates the light into test and reference beams. The test beam is transmitted to the second microscope objective, focused, and illuminates the packaged structure under test with a long-depth-of-focal-length Bessel beam. The signal light, carrying information about the internal morphology of the packaged structure, is backscattered by the packaged structure and returns to the second beam splitter. It is then reflected by the second microscope objective and enters the seventh lens. The seventh lens couples the signal light, carrying information about the internal morphology of the packaged structure, into a single-mode fiber and then into the spectrometer. The reference beam is reflected to the first microscope objective, illuminates a plane mirror, and returns to the second beam splitter. It is then transmitted through the second beam splitter into the seventh lens, which couples the reference beam into a single-mode fiber and then into the spectrometer. Simultaneously, the reference beam and the signal beam interfere, and the spectrometer acquires the interference spectral signal.

[0021] Step 4: By continuously deflecting the first and second single-axis galvanometers, a two-dimensional lateral point-by-point scan of the probe beam on the package structure under test is achieved. During scanning, the deflection of the first single-axis galvanometer produces a first-dimensional angular displacement, which is precisely conjugately imaged onto the surface of the second single-axis galvanometer by the first-stage relay mirror group. Subsequently, the second single-axis galvanometer undergoes a second-dimensional deflection, and the beam center, superimposed with the two-dimensional angular deflection information, is further conjugately imaged onto the entrance pupil of the second microscope objective by the second-stage relay mirror group. Relying on the pupil conjugate transmission effect of the two relay mirror groups, the angular displacement of the two single-axis galvanometers is accurately converted into a lateral linear translation of the Bessel beam illuminating the package structure under test, thereby effectively suppressing beam wandering during two-dimensional scanning across the entire field of view.

[0022] Step 5: At each spatial pixel point of the two-dimensional scan, due to the extremely long depth of focus of the Bessel beam, the system does not need to move the focus or sample along the Z-axis. The broadband interference spectrum signal formed by the superposition of backscattered light from different depth layers of the package structure under test and the reference light is focused by the seventh lens and coupled into the single-mode fiber. Subsequently, the high-purity broadband interference light is transmitted along the single-mode fiber into the spectrometer.

[0023] Step 6: The computer receives the interference spectral signal acquired by the spectrometer, removes the DC background, and obtains the equally spaced wavenumber domain signal after wavenumber linearization and resampling. For wavenumber domain signals Apply window functions A fast Fourier transform is then performed to extract the magnitude of the complex signal, thereby obtaining the interference envelope signal in the depth direction of a single point.

[0024] Step 7: Based on the three-dimensional point spread function of the Bessel beam, perform deconvolution filtering on the data formed by sequentially arranging the above single-point depth signals to obtain the three-dimensional topography reconstruction data of the package structure under test after eliminating crosstalk artifacts.

[0025] The significant advantages of this invention compared to existing methods are as follows:

[0026] (1) The optical coherence tomography detection method for packaged structure morphology based on Bessel beams described in this invention introduces Bessel beams with "self-healing" characteristics, overcoming the physical defects of traditional Gaussian beams, which are prone to divergence and obstruction in complex packaging materials (such as encapsulants containing particles) and high-density microstructures. When the central main lobe encounters a small obstacle or suffers loss due to scattering, its outer ring energy can quickly converge to the center and reconstruct itself, effectively bypassing local obstruction and reaching the bottom interface, greatly improving the integrity of the interference signal of deep complex microstructures.

[0027] (2) The optical coherence tomography detection method for packaging structure based on Bessel beam described in this invention combines the long focal depth of Bessel beam with the high axial resolution of tomography. When acquiring signals, there is no need to move the focal plane, and the high lateral resolution can be maintained in a very large depth range. This eliminates the cumbersome Z-axis mechanical scanning when measuring structures with large height differences (such as deep holes and high bumps), greatly improves the three-dimensional detection efficiency of packaging structures with high aspect ratio and eliminates mechanical errors.

[0028] (3) The optical coherence tomography detection method for encapsulated structures based on Bessel beams described in this invention introduces a three-dimensional point spread function deconvolution mechanism at the algorithm layer, turning physical disadvantages into computational advantages. Addressing the weakness of traditional Gaussian beam algorithms, which are susceptible to strong sidelobe interference and crosstalk artifacts when processing Bessel beams, deconvolution filtering converges the diffuse sidelobe energy to the main lobe, eliminating background "fogging" and artifacts from the bottom layer, significantly improving the three-dimensional imaging contrast and signal-to-noise ratio of dense and complex microstructures. Attached Figure Description

[0029] Figure 1 This is an experimental optical path diagram of the optical coherence tomography detection method for the packaging structure morphology based on Bessel beams described in this invention.

[0030] Figure 2 This is a flowchart illustrating the steps of an optical coherence tomography method for detecting the morphology of a packaged structure based on a Bessel beam, as described in this invention.

[0031] Figure 3 These are schematic diagrams illustrating the axial depth of field of Gaussian and Bessel beams. The left image shows the axial depth of field of a Gaussian beam, and the right image shows the axial depth of field of a Bessel beam.

[0032] Figure 4 The image shows the two-dimensional point spread function of a Bessel beam. The left image is the two-dimensional point spread function of an ideal Bessel beam, and the right image is the one-dimensional radially normalized intensity cross-section.

[0033] Figure 5 This is a diagram showing the conjugate relationship between the two uniaxial galvanometers and the two sets of relay mirrors.

[0034] Figure 6 This is the standard scanning trajectory for two-dimensional scanning.

[0035] Figure 7 The image shows the results of wavenumber linearization, windowing, and Fourier transform processing of a single-point spectral interference signal.

[0036] Figure 8 This is a comparison of two-dimensional tomographic images before and after deconvolution using a three-dimensional point spread function.

[0037] Figure 9 Comparison of the top surface morphology reconstruction results of the encapsulation structure. Detailed Implementation

[0038] 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 only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0039] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indication will also change accordingly.

[0040] Furthermore, in this invention, descriptions involving "first," "second," etc., are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0041] In this invention, unless otherwise explicitly specified and limited, the terms "connection," "fixing," etc., should be interpreted broadly. For example, "fixing" can mean a fixed connection, a detachable connection, or an integral part; "connection" can mean a mechanical connection or an electrical connection. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0042] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0043] This invention addresses the limitation of high lateral resolution and long depth of field in traditional Gaussian beam tomography by proposing a method for detecting the three-dimensional topography of encapsulated structures based on Bessel beams. This method utilizes the long depth of field and self-healing properties of Bessel beams, combined with three-dimensional deconvolution filtering, to achieve high-depth, high-resolution imaging without the need for cumbersome Z-axis mechanical focusing. This invention effectively overcomes sidelobe artifacts in traditional detection methods, significantly improving the detection efficiency and absolute accuracy of the three-dimensional topography of complex encapsulated structures.

[0044] Combination Figure 1 , Figure 2 An optical coherence tomography method for detecting the morphology of a packaged structure based on a Bessel beam includes the following steps:

[0045] Step 1: Set up the Bessel beam illumination system and the 2D scanning system separately:

[0046] The Bessel beam illumination system includes a light source 1, a first lens 2, an axial prism 3, a second lens 4, a first beam splitter 5, and a reflective spatial light modulator 6, arranged sequentially along the first optical axis.

[0047] The two-dimensional scanning system includes a first single-axis galvanometer 7, a third lens 8, a fourth lens 9, a second single-axis galvanometer 11, a fifth lens 12, and a sixth lens 13.

[0048] The third lens 8 and the fourth lens 9 are confocal to form the first-stage relay lens group 10 for beam relay transmission. The fifth lens 12 and the sixth lens 13 are confocal to form the second-stage relay lens group 14 for beam relay transmission.

[0049] The first single-axis galvanometer 7, the third lens 8, the fourth lens 9, and the second single-axis galvanometer 11 are arranged sequentially along the second optical axis. The first single-axis galvanometer 7 receives the reflected beam from the first beam splitter 5. The third lens 8 is located on the reflected light path of the first single-axis galvanometer 7. The first single-axis galvanometer 7 is used to control the one-dimensional scanning of the beam.

[0050] A second single-axis galvanometer 11, a fifth lens 12, and a sixth lens 13 are sequentially arranged along the third optical axis. The fifth lens 12 is located on the reflected light path of the second single-axis galvanometer 11. The second single-axis galvanometer 11, in conjunction with the first single-axis galvanometer 7, realizes two-dimensional XY plane scanning of the beam. The Bessel beam emitted from the sixth lens 13, after undergoing two-dimensional scanning control, enters the optical coherence tomography system located behind the second-stage relay lens group 14, which consists of the fifth lens 12 to the sixth lens 13.

[0051] Step 2: Set up the optical coherence tomography detection system:

[0052] The optical coherence tomography (OCT) system includes a second beam splitter 15, a first microscope objective 16, a plane mirror 17, a second microscope objective 18, a packaged structure under test 19, a seventh lens 20, a single-mode fiber optic cable 21, a spectrometer 22, and a computer 23. The second beam splitter 15, the second microscope objective 18, and the packaged structure under test 19 are arranged sequentially along the third optical axis and behind the second-stage relay mirror group 14, with the second microscope objective 18 and the packaged structure under test 19 forming the test optical path. The seventh lens 20, the second beam splitter 15, the first microscope objective 16, and the plane mirror 17 are arranged sequentially along the fourth optical axis, with the first microscope objective 16 and the plane mirror 17 forming the reference optical path. The seventh lens 20 is connected to the spectrometer 22 via the single-mode fiber optic cable 21, and the three components form the interference signal acquisition optical path. The spectrometer 22 is connected to the computer 23.

[0053] Step 3: The light source 1 emits a broadband Gaussian beam, which is collimated by the first lens 2 and then passes to the axial prism 3. The axial prism 3 modulates the phase of the incident beam to generate a long focal depth Bessel beam. The generated Bessel beam passes through the second lens 4 to the first beam splitter 5. After being transmitted through the first beam splitter 5, it passes to the reflective spatial light modulator 6. The reflective spatial light modulator 6 modulates the incident beam with a ring pupil and reflects it. The modulated beam is reflected by the first beam splitter 5 to the first single-axis galvanometer 7. After being deflected by the first single-axis galvanometer 7, the beam passes through the first-stage relay mirror group 10 to the second single-axis galvanometer 11. After being deflected by the second single-axis galvanometer 11, the beam passes through the second-stage relay mirror group 14 to the second beam splitter 15.

[0054] The second beam splitter 15 splits the light into test light and reference light. The test light is transmitted to the second microscope objective 18, and after being focused by the second microscope objective 18, it illuminates the packaged structure under test 19 with a long focal depth Bessel beam. The signal light carrying the internal topography information of the packaged structure after backscattering from the packaged structure under test 19 returns to the second beam splitter 15 through the second microscope objective 18. After being reflected by the second beam splitter 15, it enters the seventh lens 20. The seventh lens 20 couples the signal light carrying the internal topography information of the packaged structure into the single-mode fiber 21 and then sends it to the spectrometer 22. The reference light is reflected to the first microscope objective 16, and after being reflected by the first microscope objective 16, it illuminates the plane mirror 17. The reference light reflected by the plane mirror 17 returns to the second beam splitter 15 through the first microscope objective 16, and is transmitted into the seventh lens 20 through the second beam splitter 15. The seventh lens 20 couples the reference light into the single-mode fiber 21 and then sends it to the spectrometer 22. At the same time, the reference light and the signal light interfere, and the spectrometer 22 collects the interference spectrum signal.

[0055] Step 4: By continuously deflecting the first single-axis galvanometer 7 and the second single-axis galvanometer 11, a two-dimensional lateral point-by-point scan of the probe beam on the package structure under test 19 is achieved. During scanning, the deflection of the first single-axis galvanometer 7 generates a first-dimensional angular displacement. The first-stage relay mirror group 10 precisely conjugates the deflection center of the first single-axis galvanometer 7 onto the surface of the second single-axis galvanometer 11. Subsequently, the second single-axis galvanometer 11 undergoes a second-dimensional deflection, and the second-stage relay mirror group 14 further conjugates the beam center, which is superimposed with two-dimensional angular deflection information, to the entrance pupil of the second microscope objective 18. Relying on the pupil conjugate transmission effect of the two relay mirror groups, the angular displacement of the two single-axis galvanometers is accurately converted into a lateral linear translation of the Bessel beam illuminating the package structure under test 19, thereby effectively suppressing beam wandering during two-dimensional scanning across the entire field of view.

[0056] Step 5: At each spatial pixel point of the two-dimensional scan, due to the extremely long depth of focus of the Bessel beam, the system does not need to move the focus or sample along the Z-axis. The broadband interference spectrum signal formed by the superposition of backscattered light from different depth layers of the package structure under test 19 and the reference light is focused by the seventh lens 20 and coupled into the single-mode fiber 21. Subsequently, the high-purity broadband interference light is transmitted along the single-mode fiber 21 into the spectrometer 22.

[0057] Step 6: Computer 23 receives the interference spectrum signal acquired by spectrometer 22 and removes the DC background. After wavenumber linearization and resampling, it obtains the equally spaced wavenumber domain signal. For wavenumber domain signals Apply window functions A fast Fourier transform is then performed to extract the magnitude of the complex signal, thereby obtaining the interference envelope signal in the depth direction of a single point.

[0058] Step 7: Based on the three-dimensional point spread function of the Bessel beam, perform deconvolution filtering on the data formed by sequentially arranging the above single-point depth signals to obtain the three-dimensional topography reconstruction data of the package structure under test after eliminating crosstalk artifacts.

[0059] Combination Figure 1 , Figure 2 The method for optical coherence tomography detection of the package structure morphology based on Bessel beams includes a first lens 2, an axon prism 3 and a second lens 4 arranged in sequence to form a Bessel beam generation group, a plane mirror 17 located at the back focal plane of the first microscope objective 16, and the package structure 19 to be tested located near the object-side focal plane of the second microscope objective 18.

[0060] Combination Figure 5The deflecting surfaces of the first single-axis galvanometer 7 and the second single-axis galvanometer 11 are conjugate with respect to the first-stage relay mirror group 10. The deflecting surface of the second single-axis galvanometer 11 is conjugate with respect to the entrance pupil surface of the first microscope objective 16 and the entrance pupil surface of the second microscope objective 18 with respect to the second-stage relay mirror group 14, ensuring that the light beam is always incident at the center of the pupil of the microscope objective during two-dimensional scanning. The focal planes of the first microscope objective 16 and the second microscope objective 18 are set with equal optical path lengths with respect to the second beam splitter prism 15. The incident and outgoing light of the first single-axis galvanometer 7 and the incident and outgoing light of the second single-axis galvanometer 11 are perpendicular to each other. The probe optical axis of the seventh lens 20 is perpendicular to the optical axis of the test optical path. The optical structure of the test optical path is consistent with that of the reference optical path, wherein the position of the test package structure 19 in the test optical path is optically conjugate with the position of the plane mirror 17 in the reference optical path.

[0061] Light source 1 is a near-infrared light source with a center wavelength of 850 nm and a spectral bandwidth of 50 nm. All seven lenses are achromatic lenses, and the base angle of the axial prism 3 is 1°. The long-focal-depth Bessel beam generated by phase modulation by the axial prism 3 has diffraction-free and self-healing properties. When the test beam penetrates the interior of the package structure under test 19 and encounters impurity particles or scattering obstacles, the light field can restore the original beam cross-sectional intensity distribution of the central main lobe after passing through the obstacle, thereby maintaining the high resolution and high signal-to-noise ratio of the deep structure detection signal.

[0062] Combination Figure 1 The aforementioned method for optical coherence tomography detection of encapsulated structure morphology based on Bessel beams uses an amplitude-type reflective spatial light modulator 6 to spatially modulate the incident Bessel beam to form a ring beam. The blocking ratio m of the ring pupil is defined as follows:

[0063] ,

[0064] In the above formula, This represents the radius of the circular occlusion area at the center of the annular pupil. This represents the outer radius of the incident circular light field.

[0065] Combination Figure 6 The aforementioned method for optical coherence tomography detection of packaged structure morphology based on Bessel beams involves driving the first uniaxial mirror 7 and the second uniaxial mirror 11 to perform point-by-point scanning of the XY two-dimensional plane during the measurement process.

[0066] At each transverse scanning point, the relative mechanical positions of the reference and test optical paths remain unchanged. Due to the long focal depth of the Bessel beam, no mechanical displacement scanning along the Z-axis depth direction is required. The spectrometer 22 synchronously records the broadband interference spectral signal generated by the superposition of the test and reference beams in the frequency domain.

[0067] The aforementioned method for optical coherence tomography detection of encapsulated structure based on Bessel beams uses a single-mode fiber 21 that not only serves as a light guiding medium but also constitutes a spatial filter. This filter is used to remove and suppress multiple backscattered stray lights deviating from the fourth optical axis and system background noise, thereby enhancing the signal-to-noise ratio of the interference signal and the confocal characteristics of the system.

[0068] Combination Figure 3 , Figure 4 An optical coherence tomography method for detecting the morphology of encapsulated structures based on Bessel beams, wherein the generated long-depth-of-focus Bessel beam has a complex amplitude in three-dimensional free space for a wavefield with a center wavelength of λ. Satisfies the scalar Helmholtz equation:

[0069] ,

[0070] In the above formula, For the Laplace operator, Given spatial coordinates in a three-dimensional rectangular coordinate system, the diffraction-free exact solution of the scalar Helmholtz equation is as follows:

[0071] ,

[0072] In the above formula, The amplitude of the light field. is the longitudinal wave vector component, and i is the imaginary unit.

[0073] Combining the integral formula of the Bessel function, we obtain a completely universal scalar expression for the non-diffraction field in the form of a superposition of Bessel functions, which is the basic form of the Bessel beam:

[0074] When two zero-order Bessel beams of the same frequency propagate along the Z-axis and interfere, their interference intensity distribution Represented as:

[0075] ,

[0076] This represents the reference beam of the zeroth-order Bessel beam. This represents the zero-order Bessel beam test beam. Its basic form is:

[0077] ,

[0078] Will Expanding, we get:

[0079] ,

[0080] in, z is the radial coordinate, and z is the axial coordinate. It is a zeroth-order Bessel function of the first kind; and These represent the amplitudes of the two interfering beams; and For radial wave vector components; and Let be the longitudinal wave vector component; k is the spatial wave vector, and the wave vector satisfies the following relation:

[0081] ,

[0082] Where λ is the center wavelength of light source 1.

[0083] Computer 23 receives the interference spectral signal acquired by spectrometer 22, removes the DC background, and obtains an equally spaced wavenumber domain signal after wavenumber linearization and resampling. By applying the window function W(k) and performing a fast Fourier transform, the magnitude of the complex signal is extracted to obtain the interference envelope signal in the depth direction of a single point. :

[0084] ,

[0085] In the above formula, This represents the forward Fast Fourier Transform.

[0086] By using two-dimensional scanning, the discrete one-dimensional single-point interference envelope signal in the depth direction is obtained. To aggregate in space:

[0087] ,

[0088] This represents the ordered arrangement and combination of discrete signals on a spatial grid according to a two-dimensional scan path. The resulting arrangement forms the original three-dimensional tomographic data. It includes crosstalk artifacts caused by the side lobes of the Bessel beam and geometric distortions caused by the refractive index of the medium.

[0089] In step 7, the three-dimensional point spread function of the Bessel beam is used. right Perform deconvolution filtering as follows:

[0090] Mathematical model based on system imaging:

[0091] ,

[0092] In the formula, For spatial convolution operation, N represents the system background noise.

[0093] The true morphology of the package structure 19 under test was obtained by deconvolution filtering. The calculation formula is as follows:

[0094] ,

[0095] in, This is the optical transfer function. is the conjugate optical transfer function. This indicates a forward Fast Fourier Transform. This indicates the inverse fast Fourier transform. These represent frequencies in three-dimensional space. This is a regularization parameter used to suppress the amplification of high-frequency noise and ensure the stability of the algorithm.

[0096] The deconvolution operation described above converges the diffuse sidelobe energy to the main lobe, eliminating crosstalk artifacts. Finally, the three-dimensional topographic information of the package structure 19 under test is obtained.

[0097] In the aforementioned optical coherence tomography method for detecting the morphology of a packaged structure based on Bessel beams, the packaged structure 19 under test is a multilayer medium with different refractive indices in different layers. The method utilizes the three-dimensional data after artifact removal via deconvolution. The single horizontal coordinate Top-level optical path difference Extracting the absolute height of the top surface :

[0098] ,

[0099] in, The refractive index is given by air, and the geometric thickness of each internal layer is measured by the optical path difference at each interface. After extracting the refractive index deviation, for each internal layer, correction is performed based on the measured optical path difference between adjacent interfaces and the group refractive index of the corresponding material to obtain the true geometric thickness of the j-th layer along the optical axis. :

[0100] ,

[0101] and This represents the optical path difference between two adjacent interfaces. Let be the group refractive index of the j-th layer of encapsulation material. Finally, a high-precision three-dimensional morphology of the encapsulation structure with true physical dimensions is reconstructed based on the absolute spatial depth.

[0102] Example

[0103] Combination Figure 1 In this embodiment, the light source 1 is a broadband light source in the near-infrared band with a center wavelength of 850 nm and a spectral bandwidth of 50 nm. The first lens 2 has a focal length of 50 mm, and the second lens 4 has a focal length of 50 mm. The bottom angle of the axial prism 3 is 1°. The first single-axis galvanometer 7 and the second single-axis galvanometer 11 are single-axis scanning galvanometers GVS011( / M) from Thorlabs. The third lens 8 and the fourth lens 9, which constitute the first-stage relay lens group 10, and the fifth lens 12 and the sixth lens 13, which constitute the second-stage relay lens group 14, all have a focal length of 100 mm to strictly form a 4f spatial conjugate system. The first microscope objective 16 and the second microscope objective 18 have a magnification of 10×, a numerical aperture of 0.28, and a working distance of 3.2 mm. The liquid crystal spatial light modulator 6 is an amplitude-type reflective liquid crystal spatial light modulator. The spectrometer 22 is a high-resolution linear array CMOS spectrometer matched to the 850 nm near-infrared band. However, the device described in this invention is not limited thereto.

[0104] For the design of the two-dimensional scanning and relay lens group in the system, the primary consideration is to strictly avoid beam wandering during the scanning process in long-focal-depth Bessel beam imaging. Relying on the 4f conjugate transfer effect of the first-stage relay lens group 10 and the second-stage relay lens group 14, the system can precisely conjugate the angular deflection centers of the first single-axis galvanometer 7 and the second single-axis galvanometer 11 to the entrance pupil of the second microscope objective 18. This design ensures that the beam is always perpendicularly incident on the sample during full-field two-dimensional lateral scanning, minimizing aberrations at the edges of the field of view and improving the quality of tomographic imaging.

[0105] Combination Figure 1 , Figure 2 The method for optical coherence tomography detection of the packaging structure morphology based on Bessel beams comprises the following steps:

[0106] Step 1: Set up the Bessel beam illumination system and the 2D scanning system separately:

[0107] The Bessel beam illumination system includes a light source 1, a first lens 2, an axial prism 3, a second lens 4, a first beam splitter 5, and a reflective spatial light modulator 6, arranged sequentially along the first optical axis.

[0108] The two-dimensional scanning system includes a first single-axis galvanometer 7, a third lens 8, a fourth lens 9, a second single-axis galvanometer 11, a fifth lens 12, and a sixth lens 13.

[0109] The third lens 8 and the fourth lens 9 are confocal to form the first-stage relay lens group 10 for beam relay transmission. The fifth lens 12 and the sixth lens 13 are confocal to form the second-stage relay lens group 14 for beam relay transmission.

[0110] The first single-axis galvanometer 7, the third lens 8, the fourth lens 9, and the second single-axis galvanometer 11 are arranged sequentially along the second optical axis. The first single-axis galvanometer 7 receives the reflected beam from the first beam splitter 5. The third lens 8 is located on the reflected light path of the first single-axis galvanometer 7. The first single-axis galvanometer 7 is used to control the one-dimensional scanning of the beam.

[0111] A second single-axis galvanometer 11, a fifth lens 12, and a sixth lens 13 are sequentially arranged along the third optical axis. The fifth lens 12 is located on the reflected light path of the second single-axis galvanometer 11. The second single-axis galvanometer 11, in conjunction with the first single-axis galvanometer 7, realizes two-dimensional XY plane scanning of the beam. The Bessel beam emitted from the sixth lens 13, after undergoing two-dimensional scanning control, enters the optical coherence tomography system located behind the second-stage relay lens group 14, which consists of the fifth lens 12 to the sixth lens 13.

[0112] Step 2: Set up the optical coherence tomography detection system:

[0113] The optical coherence tomography (OCT) system includes a second beam splitter 15, a first microscope objective 16, a plane mirror 17, a second microscope objective 18, a packaged structure under test 19, a seventh lens 20, a single-mode fiber optic cable 21, a spectrometer 22, and a computer 23. The second beam splitter 15, the second microscope objective 18, and the packaged structure under test 19 are arranged sequentially along the third optical axis and behind the second-stage relay mirror group 14, with the second microscope objective 18 and the packaged structure under test 19 forming the test optical path. The seventh lens 20, the second beam splitter 15, the first microscope objective 16, and the plane mirror 17 are arranged sequentially along the fourth optical axis, with the first microscope objective 16 and the plane mirror 17 forming the reference optical path. The seventh lens 20 is connected to the spectrometer 22 via the single-mode fiber optic cable 21, and the three components form the interference signal acquisition optical path. The spectrometer 22 is connected to the computer 23.

[0114] Step 3: The light source 1 emits a broadband Gaussian beam, which is collimated by the first lens 2 and then passes to the axial prism 3. The axial prism 3 modulates the phase of the incident beam to generate a long focal depth Bessel beam. The generated Bessel beam passes through the second lens 4 to the first beam splitter 5. After being transmitted through the first beam splitter 5, it passes to the reflective spatial light modulator 6. The reflective spatial light modulator 6 modulates the incident beam with a ring pupil and reflects it. The modulated beam is reflected by the first beam splitter 5 to the first single-axis galvanometer 7. After being deflected by the first single-axis galvanometer 7, the beam passes through the first-stage relay mirror group 10 to the second single-axis galvanometer 11. After being deflected by the second single-axis galvanometer 11, the beam passes through the second-stage relay mirror group 14 to the second beam splitter 15.

[0115] The second beam splitter 15 splits the light into test light and reference light. The test light is transmitted to the second microscope objective 18, and after being focused by the second microscope objective 18, it illuminates the packaged structure under test 19 with a long focal depth Bessel beam. The signal light carrying the internal topography information of the packaged structure after backscattering from the packaged structure under test 19 returns to the second beam splitter 15 through the second microscope objective 18. After being reflected by the second beam splitter 15, it enters the seventh lens 20. The seventh lens 20 couples the signal light carrying the internal topography information of the packaged structure into the single-mode fiber 21 and then sends it to the spectrometer 22. The reference light is reflected to the first microscope objective 16, and after being reflected by the first microscope objective 16, it illuminates the plane mirror 17. The reference light reflected by the plane mirror 17 returns to the second beam splitter 15 through the first microscope objective 16, and is transmitted into the seventh lens 20 through the second beam splitter 15. The seventh lens 20 couples the reference light into the single-mode fiber 21 and then sends it to the spectrometer 22. At the same time, the reference light and the signal light interfere, and the spectrometer 22 collects the interference spectrum signal.

[0116] Step 4: By continuously deflecting the first single-axis galvanometer 7 and the second single-axis galvanometer 11, a two-dimensional lateral point-by-point scan of the probe beam on the package structure under test 19 is achieved. During scanning, the deflection of the first single-axis galvanometer 7 generates a first-dimensional angular displacement. The first-stage relay mirror group 10 precisely conjugates the deflection center of the first single-axis galvanometer 7 onto the surface of the second single-axis galvanometer 11. Subsequently, the second single-axis galvanometer 11 undergoes a second-dimensional deflection, and the second-stage relay mirror group 14 further conjugates the beam center, which is superimposed with two-dimensional angular deflection information, to the entrance pupil of the second microscope objective 18. Relying on the pupil conjugate transmission effect of the two relay mirror groups, the angular displacement of the two single-axis galvanometers is accurately converted into a lateral linear translation of the Bessel beam illuminating the package structure under test 19, thereby effectively suppressing beam wandering during two-dimensional scanning across the entire field of view.

[0117] Step 5: At each spatial pixel point of the two-dimensional scan, due to the extremely long depth of focus of the Bessel beam, the system does not need to move the focus or sample along the Z-axis. The broadband interference spectrum signal formed by the superposition of backscattered light from different depth layers of the package structure under test 19 and the reference light is focused by the seventh lens 20 and coupled into the single-mode fiber 21. Subsequently, the high-purity broadband interference light is transmitted along the single-mode fiber 21 into the spectrometer 22.

[0118] Step 6: Computer 23 receives the interference spectrum signal acquired by spectrometer 22 and removes the DC background. After wavenumber linearization and resampling, it obtains the equally spaced wavenumber domain signal. For wavenumber domain signals Apply window functions A Fast Fourier Transform is then performed to extract the magnitude of the complex signal, obtaining the interference envelope signal along the depth direction at a single point. Combined with... Figure 7It is known that, under the conditions of a center wavelength of 850 nm and a spectral bandwidth of 50 nm, the theoretical air axial resolution of the system is approximately 6.37 μm. After sequentially performing DC removal, wavenumber linearization, windowing, and fast Fourier transform on the acquired spectral interference signal, a clear interface response peak can be obtained in the depth direction, achieving effective separation of reflection interfaces at different depths.

[0119] Step 7, Combining Figure 8 It can be seen that, based on the three-dimensional point spread function of the Bessel beam, the data formed by sequentially arranging the above single-point depth signals are subjected to deconvolution filtering to obtain the three-dimensional topography reconstruction data of the package structure under test after eliminating crosstalk artifacts.

[0120] Depend on Figure 9 It can be seen that after interface extraction and refractive index correction, the root mean square errors of the top surface height, the first layer thickness and the second layer thickness are approximately 0.93 μm, 0.90 μm and 1.23 μm, respectively. The reconstruction results are in good agreement with the set real morphology.

[0121] The above results demonstrate that the spectral signal processing, three-dimensional deconvolution artifact removal, and refractive index correction methods proposed in this invention can achieve the extraction of interface information and three-dimensional morphology reconstruction of multi-layer encapsulation structures, verifying the feasibility of the proposed detection method and algorithm.

Claims

1. A method for optical coherence tomography detection of packaged structure morphology based on Bessel beams, characterized in that: Step 1: Set up the Bessel beam illumination system and the 2D scanning system separately: The Bessel beam illumination system includes a light source (1), a first lens (2), an axial prism (3), a second lens (4), a first beam splitter (5), and a reflective spatial light modulator (6) arranged sequentially along a first optical axis. The two-dimensional scanning system includes a first single-axis galvanometer (7), a third lens (8), a fourth lens (9), a second single-axis galvanometer (11), a fifth lens (12), and a sixth lens (13); Among them, the third lens (8) and the fourth lens (9) are confocal to form the first-stage relay lens group (10) for beam relay transmission; the fifth lens (12) and the sixth lens (13) are confocal to form the second-stage relay lens group (14) for beam relay transmission. The first single-axis galvanometer (7), the third lens (8), the fourth lens (9), and the second single-axis galvanometer (11) are arranged sequentially along the second optical axis. The first single-axis galvanometer (7) receives the reflected beam from the first beam splitter (5), and the third lens (8) is located on the reflected light path of the first single-axis galvanometer (7). The first single-axis galvanometer (7) is used to control the one-dimensional scanning of the beam. The third optical axis is sequentially arranged with a second single-axis galvanometer (11), a fifth lens (12), and a sixth lens (13). The fifth lens (12) is located on the reflected light path of the second single-axis galvanometer (11). The second single-axis galvanometer (11) works in conjunction with the first single-axis galvanometer (7) to achieve XY two-dimensional plane scanning of the beam. The Bessel beam emitted from the sixth lens (13) and controlled by two-dimensional scanning enters the optical coherence tomography detection system located behind the second-stage relay mirror group (14) composed of the fifth lens (12) to the sixth lens (13). Step 2: Set up the optical coherence tomography detection system: The optical coherence tomography system includes a second beam splitter (15), a first microscope objective (16), a plane mirror (17), a second microscope objective (18), a packaged structure under test (19), a seventh lens (20), a single-mode fiber (21), a spectrometer (22), and a computer (23); wherein, the second beam splitter (15), the second microscope objective (18), and the packaged structure under test (19) are arranged sequentially along the third optical axis and behind the second-stage relay mirror group (14). The microscope objective (18) and the package structure under test (19) form the test optical path; the seventh lens (20), the second beam splitter (15), the first microscope objective (16), and the plane mirror (17) are arranged sequentially along the fourth optical axis. The first microscope objective (16) and the plane mirror (17) form the reference optical path; the seventh lens (20) is connected to the spectrometer (22) through a single-mode fiber (21), and the three form the interference signal acquisition optical path; the spectrometer (22) is connected to the computer (23); Step 3: The light source (1) emits a broadband Gaussian beam, which is collimated by the first lens (2) and then passes to the axial prism (3). The axial prism (3) modulates the phase of the incident beam to generate a long focal depth Bessel beam. The generated Bessel beam passes through the second lens (4) to the first beam splitter (5). After being transmitted through the first beam splitter (5), it passes to the reflective spatial light modulator (6). The reflective spatial light modulator (6) modulates the incident beam with a ring pupil and reflects it. The modulated beam is reflected by the first beam splitter (5) to the first single-axis galvanometer (7). After being deflected by the first single-axis galvanometer (7), the beam passes through the first-stage relay mirror group (10) to the second single-axis galvanometer (11). After being deflected by the second single-axis galvanometer (11), the beam passes through the second-stage relay mirror group (14) to the second beam splitter (15). The second beam splitter (15) splits the light into a test beam and a reference beam. The test beam is transmitted to the second microscope objective (18) and focused by the second microscope objective (18) to illuminate the packaged structure under test (19) with a long focal depth Bessel beam. The signal light carrying the internal topography information of the packaged structure after backscattering from the packaged structure under test (19) returns to the second beam splitter (15) through the second microscope objective (18) and is reflected by the second beam splitter (15) into the seventh lens (20). The seventh lens (20) couples the signal light carrying the internal topography information of the packaged structure into a single-mode beam. The reference light is reflected by the first microscope objective (16) and then illuminates the plane mirror (17). The reference light reflected by the plane mirror (17) returns to the second beam splitter (15) through the first microscope objective (16) and is transmitted into the seventh lens (20). The seventh lens (20) couples the reference light into the single-mode fiber (21) and then into the spectrometer (22). At the same time, the reference light and the signal light interfere with each other, and the spectrometer (22) collects the interference spectrum signal. Step 4: By driving and controlling the continuous deflection of the first single-axis galvanometer (7) and the second single-axis galvanometer (11), the probe beam is scanned point-by-point laterally on the package structure under test (19) in two dimensions. During the scan, the deflection of the first single-axis galvanometer (7) generates an angular displacement in the first dimension. The deflection center of the first single-axis galvanometer (7) is strictly conjugately imaged onto the surface of the second single-axis galvanometer (11) by the first-level relay mirror group (10). Subsequently, the second single-axis galvanometer (11) undergoes a second-dimensional deflection, and the beam center superimposed with the two-dimensional angular deflection information is further conjugated to the entrance pupil of the second microscope objective (18) by the second-level relay mirror group (14). Relying on the pupil conjugate transmission effect of the two relay mirror groups, the angular displacement of the two single-axis galvanometers is accurately converted into a lateral linear translation of the Bessel beam illuminating the package structure under test (19), thereby effectively suppressing beam wandering during the two-dimensional scanning process in the entire field of view. Step 5: At each spatial pixel point of the two-dimensional scan, due to the extremely long focal depth of the Bessel beam, the system does not need to move the focus or sample along the Z-axis; the broadband interference spectrum signal formed by the superposition of the backscattered light of the different depth layers of the package structure under test (19) and the reference light is focused by the seventh lens (20) and coupled into the single-mode fiber (21); then, the high-purity broadband interference light is transmitted along the single-mode fiber (21) into the spectrometer (22); Step 6: The computer (23) receives the interference spectrum signal collected by the spectrometer (22) and removes the DC background. After wavenumber linearization and resampling, it obtains the equally spaced wavenumber domain signal. For wavenumber domain signals Apply window functions A fast Fourier transform is then performed to extract the magnitude of the complex signal, thereby obtaining the interference envelope signal in the depth direction of a single point. Step 7: Based on the three-dimensional point spread function of the Bessel beam, perform deconvolution filtering on the data formed by sequentially arranging the above single-point depth signals to obtain the three-dimensional topography reconstruction data of the package structure under test after eliminating crosstalk artifacts.

2. The method for optical coherence tomography detection of packaged structure morphology based on Bessel beams according to claim 1, characterized in that: The first lens (2), the axon prism (3) and the second lens (4) are arranged in sequence to form a Bessel beam generating group. The plane mirror (17) is located at the back focal plane of the first microscope objective (16), and the package structure to be tested (19) is located near the object-side focal plane of the second microscope objective (18). The deflecting and reflecting surfaces of the first single-axis galvanometer (7) and the second single-axis galvanometer (11) are conjugate with respect to the first-stage relay mirror group (10); the deflecting and reflecting surfaces of the second single-axis galvanometer (11) are conjugate with respect to the incident pupil surface of the first microscope objective (16) and the incident pupil surface of the second microscope objective (18) with respect to the second-stage relay mirror group (14), so that during the two-dimensional scanning process, the light beam is always incident at the center of the pupil of the microscope objective; the focal plane of the first microscope objective (16) and the focal plane of the second microscope objective (18) are set with equal optical path with respect to the second beam splitter (15); the incident light and the outgoing light of the first single-axis galvanometer (7) and the incident light and the outgoing light of the second single-axis galvanometer (11) are perpendicular to each other; the probe optical axis of the seventh lens (20) is perpendicular to the optical axis of the test optical path. The optical structure of the test optical path is consistent with that of the reference optical path. The position of the package structure under test (19) in the test optical path is optically conjugate to that of the plane mirror (17) in the reference optical path.

3. The method for optical coherence tomography detection of packaged structure morphology based on Bessel beams according to claim 2, characterized in that: The light source (1) is a near-infrared light source with a center wavelength of 850 nm and a spectral bandwidth of 50 nm. All seven lenses are achromatic lenses, and the bottom angle of the axial prism (3) is 1°. The long focal depth Bessel beam generated by the phase modulation of the axial prism (3) has diffraction-free and self-healing characteristics. When the test beam penetrates the interior of the package structure under test (19) and encounters impurity particles or scattering obstacles, the light field can restore the original beam cross-sectional intensity distribution of the central main lobe after passing through the obstacle, thereby maintaining the high resolution and high signal-to-noise ratio of the deep structure detection signal.

4. The method for optical coherence tomography detection of packaged structure morphology based on Bessel beams according to claim 3, characterized in that: The reflective spatial light modulator (6) is an amplitude-type reflective spatial light modulator used to spatially modulate the incident Bessel beam to form a ring beam. The blocking ratio m of the ring pupil is defined as: ， In the above formula, This represents the radius of the circular occlusion area at the center of the annular pupil. This represents the outer radius of the incident circular light field.

5. The method for optical coherence tomography detection of packaged structure morphology based on Bessel beams according to claim 4, characterized in that, Step 4 is detailed below: During the measurement process, the first single-axis galvanometer (7) and the second single-axis galvanometer (11) are driven to perform point-by-point scanning of the XY two-dimensional plane; At each transverse scanning point, the relative mechanical position of the reference optical path and the test optical path remains unchanged. Due to the long focal depth characteristic of the Bessel beam, there is no need to perform mechanical displacement scanning in the Z-axis depth direction. The spectrometer (22) synchronously records the broadband interference spectrum signal generated by the superposition of the test light and the reference light in the frequency domain.

6. The method for optical coherence tomography detection of packaged structure morphology based on Bessel beams according to claim 5, characterized in that: The single-mode fiber (21) not only serves as a light guiding medium, but also constitutes a spatial filter to filter out and suppress multiple backscattered stray light deviating from the fourth optical axis and system background noise, thereby enhancing the signal-to-noise ratio of the interference signal and the confocal characteristics of the system.

7. The method for optical coherence tomography detection of packaged structure morphology based on Bessel beams according to claim 6, characterized in that: The generated long-focal-depth Bessel beam, with respect to the complex amplitude of the wavefield at its center wavelength λ in three-dimensional free space. Satisfies the scalar Helmholtz equation: ， In the above formula, For the Laplace operator, Given spatial coordinates in a three-dimensional rectangular coordinate system, the diffraction-free exact solution of the scalar Helmholtz equation is as follows: ， In the above formula, The amplitude of the light field. Let i be the longitudinal wave vector component and i be the imaginary unit. Combining this with the integral formula for the Bessel function, we obtain a completely universal scalar expression for the non-diffraction field in the form of a superposition of Bessel functions, which is the basic form of the Bessel beam: When two zero-order Bessel beams of the same frequency propagate along the Z-axis and interfere, their interference intensity distribution Represented as: ， This represents the reference beam of the zeroth-order Bessel beam. This represents the zero-order Bessel beam test beam. Its basic form is: ， Will Expanding, we get: ， in, z is the radial coordinate, and z is the axial coordinate; It is a zeroth-order Bessel function of the first kind; and These represent the amplitudes of the two interfering beams; and For radial wave vector components; and Let be the longitudinal wave vector component; k is the spatial wave vector, and the wave vector satisfies the following relation: ， Where λ is the center wavelength of the light source (1).

8. The method for optical coherence tomography detection of packaged structure morphology based on Bessel beams according to claim 7, characterized in that, In step 6, the computer (23) receives the interference spectrum signal collected by the spectrometer (22) and removes the DC background. After wavenumber linearization and resampling, it obtains the equally spaced wavenumber domain signal. By applying the window function W(k) and performing a fast Fourier transform, the magnitude of the complex signal is extracted to obtain the interference envelope signal in the depth direction of a single point. : ， In the above formula, This represents the forward Fast Fourier Transform; By using two-dimensional scanning, the discrete one-dimensional single-point interference envelope signal in the depth direction is obtained. To aggregate in space: ， This represents the ordered arrangement and combination of discrete signals on a spatial grid according to a two-dimensional scanning path; the resulting original three-dimensional tomographic data. It includes crosstalk artifacts caused by the side lobes of the Bessel beam and geometric distortions caused by the refractive index of the medium.

9. The method for optical coherence tomography detection of packaged structure morphology based on Bessel beams according to claim 8, characterized in that: In step 7, the three-dimensional point spread function of the Bessel beam is used. right Perform deconvolution filtering as follows: Mathematical model based on system imaging: ， In the formula, For spatial convolution operation, N is the system background noise; The true morphology of the package structure under test (19) is obtained by deconvolution filtering. The calculation formula is as follows: ， in, Optical transfer function; For conjugate optical transfer functions; This indicates a forward Fast Fourier Transform. This indicates the inverse fast Fourier transform; They represent frequencies in three-dimensional space, respectively. For regularization parameters; Through the above deconvolution operation, the diffuse sidelobe energy is converged to the main lobe, eliminating crosstalk artifacts; finally, the three-dimensional morphology information of the packaging structure (19) under test is obtained.

10. The method for optical coherence tomography detection of packaged structure morphology based on Bessel beams according to claim 9, characterized in that: The package structure under test (19) is a multilayer dielectric with different refractive indices in different layers; the three-dimensional data after deconvolution artifact removal is used. The single horizontal coordinate Top-level optical path difference Extracting the absolute height of the top surface : ， in, The refractive index is given by air, and the geometric thickness of each internal layer is measured by the optical path difference at each interface. After extracting the refractive index deviation, for each internal layer, correction is performed based on the measured optical path difference between adjacent interfaces and the group refractive index of the corresponding material to obtain the true geometric thickness of the j-th layer along the optical axis. : ， and This represents the optical path difference between two adjacent interfaces. Let be the group refractive index of the j-th layer of encapsulation material; finally, a high-precision three-dimensional morphology of the encapsulation structure with real physical dimensions is reconstructed based on the absolute spatial depth.

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