A Discriminative Imaging FDTD Method Based on Transparent Sources and a Flip Chip System

Abstract

The present invention belongs to the technical field of optical imaging. The present invention proposes a discriminative imaging FDTD method based on a transparent source and a flip-chip system. This method can more accurately establish the FDTD numerical simulation of the "photoacoustic-elastic-optic" process, explain the principle mechanism of two photoacoustic remote sensing signals existing in the flip-chip during the preparation process, and can be used to guide the discriminative imaging of different layer structures of the flip-chip during the preparation process by a photoacoustic remote sensing system.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical imaging, and particularly relates to a discriminative imaging FDTD method based on a transparent source and a flip-chip system. Background Art

[0002] Photoacoustic remote sensing (PARS) technology uses low-coherence light for non-contact photoacoustic detection under the principle of incoherence, which can avoid phase noise in principle, and has extremely high resolution and a higher signal-to-noise ratio than traditional contact photoacoustic microscopy imaging. The characteristics of miniaturization, non-contact and fast imaging of PARS make this technology particularly suitable for the rapid detection of flip chips in principle.

[0003] PARS technology is based on the "photoacoustic - elasto-optic" theory. The "photoacoustic - elasto-optic" theory means that when the excitation light (pulsed laser) irradiates the absorber (flip chip during the manufacturing process), the irradiated position absorbs the light energy and converts all or part of it into heat energy, resulting in thermal expansion at the irradiated position, bringing a huge initial sound pressure (photoacoustic effect); the initial sound pressure will change the refractive index of the flip chip, and the greater the initial sound pressure, the more obvious the change in the refractive index (elasto-optic effect). If a low-coherence continuous detection light beam is focused and irradiated at this position at this time, due to the sudden change in the refractive index, the light intensity of the reflected light of the detection light will change suddenly. According to this change, the image information of the irradiated position of the flip chip can be obtained.

[0004] Currently, the preliminary implementation of the elasto-optic theory is based on an analytical method of a mathematical model. This method has simulated the change in reflectivity of electromagnetic waves in a simple one-dimensional biological model by establishing a detailed mathematical model for one-dimensional sound waves and light waves. However, this analytical method is not suitable for the simulation of flip chips during the manufacturing process with multi-layer dielectrics due to its complex mathematical model. The flip chip during the manufacturing process has different dielectric layers mainly including silicon-metal interfaces and silicon-air interfaces. When using PARS technology to detect the flip chip, the PARS signal generated by the silicon-metal interface has a positive initial slope, while the PARS signal of the silicon-air interface has a negative initial slope, and there has been no research in this field on the reasons for the above different positive and negative initial slopes.

[0005] The Finite-Difference Time-Domain (FDTD) algorithm is a numerical calculation method widely used in the field of electromagnetics. With its advantages of simplicity, high flexibility, high precision, and strong adaptability, it is expected to achieve numerical simulation of the deeper signal mechanism of the "photoacoustic-elastic optic" process. However, all existing FDTD algorithms for "photoacoustic-elastic optic" simulation are time-domain finite-difference methods with soft sources or hard sources as the detected light field sources. The time-domain finite-difference method based on soft sources introduces the field source term in the form of a soft source where s represents the wave source position index, A is the amplitude, and λ is the wavelength. The soft source is equivalent to superimposing a sine wave signal on the original field quantity. The superimposing mechanism does not affect the transmission characteristics of the original field quantity. The reflected electromagnetic wave will pass through the soft source normally, but it will cause the wave transmitted after superposition to be "distorted". The time-domain finite-difference algorithm based on hard sources introduces the field source term in the form of a hard source where the hard source is equivalent to forcing the original field quantity at the wave source to the value of the corresponding sine wave signal. The reflected electromagnetic wave will undergo a secondary reflection when passing through the hard source, directly affecting the transmission characteristics of the original field quantity. However, in the "photoacoustic-elastic optic" FDTD simulation process where it is necessary to obtain the reflected electromagnetic wave signal and ensure that the wave transmitted from the wave source does not appear "distorted", neither the soft source nor the hard source can fully meet its requirements. To sum up, there is no reasonable explanation for the positive and negative initial slope signal mechanisms existing in flip chips in existing research. Summary of the Invention

[0006] The present invention proposes a discriminative imaging FDTD method based on a transparent source and a flip chip system. This method can more accurately establish the FDTD numerical simulation of the "photoacoustic-elastic optic" process, explain the principle mechanisms of two photoacoustic remote sensing signals existing in the flip chip during the preparation process, and can be used to guide the discriminative imaging of different layer structures of the flip chip during the preparation process by the photoacoustic remote sensing system.

[0007] The present invention proposes a discriminative imaging FDTD method based on a transparent source, specifically including the following steps:

[0008] Step S1, perform FDTD numerical simulation on photoacoustic waves. Regarding the pulsed excitation light as the heating function of the absorber and meshing it, the time term and space term of the photoacoustic equation are discretized second-order using the three-point difference format, and the specific form is shown in Equation (1):

[0009]

[0010] where v swhere \(c\) is the speed of sound, \(p\) is the photoacoustic pressure distribution at different spatial positions and times, and \(H\) is the thermal energy conversion per unit volume and unit time, i.e., the heating function; the subscript \(i\) represents the discrete point index in space, the superscript \(k\) represents the index in time, \(k + 1\) represents the next moment, and \(k - 1\) represents the previous moment. \(\Delta x\) is the discrete interval in space, and \(\Delta t\) is the discrete interval in time. Taking \(\Delta x = c\Delta t\) simplifies the simulation analysis under the condition of stable calculation. The right side of the equation is the heat source term, which is caused by the short-pulse excitation light.

[0011] Step S2: Set the probe light field source as a transparent source. In previous analyses, neither ordinary hard sources nor soft sources were suitable for the FDTD numerical simulation of the "photoacoustic - elasto - optic" process. Therefore, the present invention proposes a wave source called a transparent source to adapt to the FDTD numerical simulation of the "photoacoustic - elasto - optic" process. The specific form of the transparent source is shown in Equation (2):

[0012]

[0013] where represents the electric field component at the \(k\)th time step, represents the electric field component at the \((k - 1)\)th time step, \(A\) represents the amplitude of the source signal, \(c\) is the speed of light, and \(\lambda\) is the wavelength of the electromagnetic wave. The proposed transparent source of the present invention, based on the original field quantity at the \((k - 1)\)th time step, first superimposes a sine wave signal as the field quantity at the \(k\)th time step. This allows the electromagnetic wave reflected within the \((k - 1)\)th time step to pass through the soft source normally. On this basis, a third compensation term is added to cancel the sine field in the original field quantity and compensate for the distortion caused by the "superposition", thus achieving the effect of neither affecting the normal propagation (reflection process) of the electromagnetic wave nor continuously generating the correct probe light waveform. This ensures the accuracy of the simulation for the "elasto - optic - photoacoustic" process.

[0014] Step S3: Perform FDTD numerical simulation on the electromagnetic wave, i.e., the probe light. In the "photoacoustic - elasto - optic" process, the reflected light intensity needs to be numerically solved finally. Use a sine wave source to propagate according to the following form of the discrete wave equation to simulate the reflection process of the probe light, as shown in Equation (3):

[0015]

[0016] where the subscript \(i\) represents the discrete point index of the electromagnetic wave in space, the superscript \(k\) represents the discrete point index of the electromagnetic wave in time, \(k + 1\) represents the next moment, \(k - 1\) represents the previous moment, and \(c\) represents the speed of light. Taking \(\Delta x = c\Delta t\) simplifies the simulation analysis under the condition of stable calculation.

[0017] Step S4, set the FDTD boundary conditions. Due to the memory limitation of the computer, FDTD generally needs to define a finite space, so boundary conditions need to be added to the space boundary. The so-called boundary conditions are artificially added constraints at the boundaries of the solution range. Generally, an absorbing boundary is added at the boundary, that is, the wave is perfectly absorbed and reflected back as little as possible to be equivalent to the simulation of an infinite space. This boundary is called the perfectly matched layer (PML). The present invention first applies the Mur boundary condition (a kind of PML) to the FDTD numerical simulation in the field of photoacoustic remote sensing, as shown in Equation (4) specifically:

[0018]

[0019] where u is the field quantity (electromagnetic field or acoustic field), v s is the velocity of the field, dt and dx represent the time and space increments, the subscript represents the spatial position index, and the superscript represents the time index. Through experimental verification, the use of the Mur boundary can realize the simulation of photoacoustic waves and probe light under an infinite space, and further realize the simulation of the "photoacoustic - elasto - optic" process.

[0020] Step S5, calculate the reflectivity of the probe light. The reflectivity R is calculated by the following formula (5), where E0 is the intensity of the incident electromagnetic wave, and E r is the intensity of the reflected electromagnetic wave.

[0021]

[0022] The above solution realizes the FDTD simulation of the acoustic pressure field and the probe light field. However, in order to realize the simulation of the "photoacoustic - elasto - optic" process and the electromagnetic field reflection process, a simulation method for combining the above processes is still needed. For this reason, the present invention proposes a nested FDTD simulation method for the "photoacoustic - elasto - optic" process.

[0023] Furthermore, the present invention proposes to use the above - mentioned discriminative imaging FDTD method based on transparent sources to construct a flip - chip system, which can be used to establish a hierarchical elasto - optic discriminative model of the flip - chip, specifically as follows:

[0024] Since there are hierarchical structures mainly composed of silicon - metal interfaces and silicon - air interfaces in the flip - chip, and both will generate PARS signals with different initial slopes, but currently there is no method that can give a reasonable physical mechanism explanation for this slope feature. Therefore, based on the actual flip - chip structure and combined with the practical significance of the model, the present invention establishes a hierarchical elasto - optic discriminative model based on silicon - air interfaces and silicon - metal interfaces, and for the first time gives a physical mechanism explanation for the positive and negative slopes of the above - mentioned characteristic signals based on the aforementioned discriminative imaging FDTD method based on transparent sources.

[0025] The present invention proposes a nested FDTD simulation method for the "photoacoustic - photoelastic" process based on the discriminative imaging FDTD simulation method of a transparent source, which can more accurately simulate the "photoacoustic - photoelastic" process. Further, a flip - chip system is provided. By establishing a hierarchical photoelastic discriminative model and based on the aforementioned discriminative imaging FDTD method of the transparent source, a physical mechanism explanation for the positive and negative slopes of the above - mentioned characteristic signals is given for the first time. It can guide the discriminative imaging of different layer structures of flip - chips during the preparation process by a photoacoustic remote sensing system. Without increasing the complexity of the PARS system, the information dimension of the PARS image is increased, and the discriminative imaging of different layer structures of flip - chips during the preparation process can be realized. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the following - described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0027] Figure 1 is the FDTD method for discriminative imaging based on a transparent source;

[0028] Figure 2 is the simulation flow chart of the nested finite - difference time - domain for the "photoacoustic - photoelastic" process;

[0029] Figure 3 is the structural simulation diagram of the nested finite - difference time - domain for the "photoacoustic - photoelastic" process;

[0030] Figure 4 is the "photoacoustic - photoelastic" model of the silicon - air interface and the silicon - metal interface and its corresponding simulated characteristic signals;

[0031] Figure 5 is the FDTD simulation results of the electromagnetic fields of three field sources;

[0032] Figure 6 is the comparison of the PARS imaging results and bright - field microscopy imaging effects of flip - chips during the preparation process under the guidance of the discriminative imaging model. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] In the following description, specific details such as specific system structures and technologies are presented for the purpose of illustration rather than limitation, so as to thoroughly understand the embodiments of the present invention. However, those skilled in the art should understand that the present invention can also be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to avoid unnecessary details from interfering with the description of the present invention.

[0034] As Figure 1 shown, the present invention proposes a discriminative imaging FDTD method based on a transparent source, which specifically includes the following steps:

[0035] Step S1, perform FDTD numerical simulation on photoacoustic waves. Take the pulsed excitation light as the heating function of the absorber, and grid it. Use the three-point difference format to perform second-order discretization on the time term and space term of the photoacoustic equation. The specific form is shown in Equation (1):

[0036]

[0037] where v s is the sound speed, p is the photoacoustic pressure distribution at different spatial positions and times, H is the thermal energy conversion per unit volume and unit time, that is, the heating function; the subscript i represents the discrete point index in space, the superscript k represents the index in time, k + 1 represents the next moment, and k - 1 represents the previous moment. Δx is the discrete interval in space, and Δt is the discrete interval in time. Take Δx = cΔt to simplify the simulation analysis under the condition of stable calculation. The right side of the equation is the heat source term, which is caused by the short-pulse excitation light.

[0038] Step S2, set the detection light field source as a transparent source. In previous analyses, both ordinary hard sources and soft sources are not suitable for the FDTD numerical simulation of the "photoacoustic - elasto - optic" process. Therefore, the present invention proposes a wave source called a transparent source to adapt to the FDTD numerical simulation of the "photoacoustic - elasto - optic" process. The specific form of the transparent source is shown in Equation (2):

[0039]

[0040] where, represents the electric field component at the k - th time step, represents the electric field component at the (k - 1)-th time step, A represents the amplitude of the source signal, c is the speed of light, and λ is the wavelength of the electromagnetic wave. The transparent source proposed by the present invention, based on the original field quantity at the (k - 1)-th time step, first superimposes a sine wave signal as the field quantity at the k - th time step, so that the electromagnetic wave reflected within the (k - 1)-th time step can pass through the soft source normally. On this basis, a third compensation term is added to cancel the original field quantity The original sine field compensates for the distortion caused by "superposition", achieving the goal of neither affecting the normal propagation of electromagnetic waves (reflection process) nor continuously generating the correct detected light waveform. This ensures the accuracy of the simulation of the "photoelastic - photoacoustic" process.

[0041] Step S3: Conduct FDTD numerical simulation on the electromagnetic wave, i.e., the detected light. In the "photoacoustic - photoelastic" process, the reflected light intensity needs to be numerically solved finally. Use a sine wave source to propagate according to the following form of the discrete wave equation to simulate the reflection process of the detected light, as shown in Equation (3) specifically:

[0042]

[0043] Where the subscript i represents the discrete point index of the electromagnetic wave in space, the superscript k represents the discrete point index of the electromagnetic wave in time, k + 1 represents the next moment, k - 1 represents the previous moment, and c represents the speed of light. Take Δx = cΔt to simplify the simulation analysis under the condition of stable calculation.

[0044] Step S4: Set the FDTD boundary conditions. Due to the memory limitation of the computer, FDTD generally needs to delimit a finite space, so boundary conditions need to be added to the space boundary. The so-called boundary conditions are artificially added constraints at the boundaries of the solution range. Generally, an absorbing boundary is added at the boundary, that is, the wave is perfectly absorbed and reflected back as little as possible to be equivalent to the simulation of an infinite space. This boundary is called the perfectly matched layer (PML). The present invention first applies the Mur boundary condition (a kind of PML) to the FDTD numerical simulation in the field of photoacoustic remote sensing, as shown in Equation (4) specifically:

[0045]

[0046] Where u is the field quantity (electromagnetic field or acoustic field), v s is the velocity of the field, dt and dx represent the time and space increments, the subscript represents the space position index, and the superscript represents the time index. Through experimental verification, the use of the Mur boundary can realize the simulation of photoacoustic waves and detected light under an infinite space, and further realize the simulation of the "photoacoustic - photoelastic" process.

[0047] Step S5: Calculate the reflectivity of the detected light. The reflectivity R is calculated by the following formula, where E0 is the intensity of the incident electromagnetic wave, and E r is the intensity of the reflected electromagnetic wave.

[0048]

[0049] The above - mentioned solution realizes the FDTD simulation of the sound - pressure field and the probe - light field. However, in order to realize the simulation of the "photoacoustic - elasto - optic" process and the electromagnetic - field reflection process, a simulation method for combining the above - mentioned processes is still needed. For this purpose, the present invention proposes a nested FDTD simulation method for the "photoacoustic - elasto - optic" process, specifically as Figure 2 shown. The nested time - domain finite - difference simulation structure diagram of the "photoacoustic - elasto - optic" process is as Figure 3 shown. In the PARS excited by a finite - short pulse, during the propagation of the initial sound pressure, the sound - field is a time - varying stepped distribution. The sound - field at each moment is stepped, and under the action of the elasto - optic effect, a stepped refractive - index distribution will also be generated. Therefore, to obtain the reflectivity of the probe light at this moment in this stepped refractive - index structure, the specific steps are as follows:

[0050] Step S11, perform the photoacoustic - wave FDTD simulation of step S1 at the k - th moment. The time - varying sound - pressure field is obtained, and according to the formula the refractive - index change caused by the sound - pressure field is obtained for the electromagnetic FDTD simulation. Where ε is the elasto - optic coefficient, n0 is the static refractive index, that is, the intrinsic refractive index of the material without being modulated by the pressure, ρ is the density of the material, v s is the sound speed, p(z,t) is the photoacoustic - pressure distribution at different spatial positions and times, and δn(z,t) is the refractive - index change amount modulated by the pressure. Therefore, at the moment when the material is excited (t = 0 + ), its refractive index is n0+δn;

[0051] Step S12, perform a complete FDTD electromagnetic - field simulation of the probe light at the k - th moment. The required reflectivity signal is obtained, and the simulation of the "photoacoustic - elasto - optic" process at this moment is completed;

[0052] Step S13, when the photoacoustic - field changes to the next moment, repeat the above steps S11 and S12 until the "photoacoustic - elasto - optic" simulation at all moments is completed.

[0053] Furthermore, the present invention proposes to use the above - mentioned discrimination - imaging FDTD method based on the transparent source to construct a flip - chip system, which can be used to establish a hierarchical elasto - optic discrimination model of the flip - chip, specifically as follows:

[0054] Since there are hierarchical structures mainly composed of silicon - metal interfaces and silicon - air interfaces in the flip - chip, and both will generate PARS signals with different initial slopes, but currently there is no method that can give a reasonable physical - mechanism explanation for this slope feature. Therefore, based on the actual flip - chip structure and combined with the actual meaning of the model, the present invention establishes a hierarchical elasto - optic discrimination model based on the silicon - air interface and the silicon - metal interface, and for the first time gives a physical - mechanism explanation for the positive and negative slopes of the above - mentioned characteristic signals based on the foregoing discrimination - imaging FDTD method of the transparent source.

[0055] Figure 4 For the established flip-chip system, the system includes an excitation light, a detection light, a silicon-air composite structure, and a silicon-metal composite structure, that is, a layered photoelastic discrimination model based on the silicon-air interface and the silicon-metal interface.

[0056] The excitation light is the heating function in the FDTD simulation process to generate a photoacoustic effect to obtain an initial sound pressure, and then modulate the refractive index of the medium.

[0057] The detection light is a transparent source in the FDTD simulation process to generate a sinusoidal electromagnetic wave as a detection signal. Using the methods in the foregoing steps S1 - S5, by continuously monitoring the reflectivity of the detection light, the PARS signal can be obtained. According to the positive or negative of its initial slope, different model interfaces can be distinguished.

[0058] In the layered photoelastic discrimination model, the silicon-metal interface can generate a positive initial slope in the "photoacoustic - photoelastic" process, while the silicon-air interface generates a negative initial slope in the "photoacoustic - photoelastic" process; the silicon layer, the metal layer, and the air layer all have different initial refractive indices, densities, photoelastic coefficients, and sound velocities. In the "photoacoustic - photoelastic" process, the three media will generate different refractive index changes, and finally generate PARS signals with completely different initial slopes at the silicon-metal interface and the silicon-air interface.

[0059] Figure 5 Are the FDTD simulation results of the electromagnetic fields of three field sources. (a)-(c) are the simulation results of the soft source, (d)-(f) are the hard source, and (g)-(i) are the transparent source. Three important moments are selected from the simulation results of the three different field sources: the moment when the electromagnetic wave transmits to the reflection interface (t = 0), the time point when the reflected electromagnetic component reaches the sound source (t0), and the subsequent moment after the reflected component reaches the sound source (t0 + Δt). In the simulation, the initial amplitude of the electromagnetic wave is set to 1. As Figure 5 Shown in (a)-(c), the initial amplitude of the soft source becomes an unrealistic 10. Figure 5 (d)-(f) intuitively shows that the hard source has a reverse effect on the reflected electromagnetic wave, thus affecting the transmission characteristics of the initial electromagnetic field. As Figure 5 Shown in (f), the amplitude of the reflected component after passing through the field source is different from that before reaching the field source. As Figure 5 (g)-(i) shows that when using a transparent field source, it has no effect on the propagation of the electromagnetic wave, neither generating an abnormal initial amplitude nor affecting the reflected component, realizing the FDTD simulation of the "photoacoustic - photoelastic" process.

[0060] The present invention establishes a hierarchical simulation model of flip chips during the manufacturing process, which actually restores the structure of flip chips during the manufacturing process, can distinguish different layer structures of flip chips during the manufacturing process, and realizes PARS discriminative imaging of different layer structures of flip chips during the manufacturing process. The PARS imaging results of flip chips during the manufacturing process under the guidance of the discriminative imaging model are compared with the bright-field microscopic imaging effects as Figure 6 shown.

[0061] It should be understood that the magnitudes of the sequence numbers of the steps in the above embodiments do not imply the order of execution. The order of execution of each process should be determined by its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of the present invention.

[0062] Those skilled in the art can clearly understand that for the convenience and brevity of description, only the above division of each functional unit and module is used as an example. In actual applications, the above functions can be allocated to different functional units and modules according to needs, that is, the internal structure of the device is divided into different functional units or modules to complete all or part of the functions described above. Each functional unit and module in the embodiment can be integrated into a processing unit, or each unit can exist physically alone, or two or more units can be integrated into one unit. The above integrated unit can be implemented in the form of hardware or in the form of a software functional unit. In addition, the specific names of each functional unit and module are only for the convenience of mutual distinction and do not limit the protection scope of the present application. The specific working processes of the units and modules in the above system can refer to the corresponding processes in the foregoing method embodiments and will not be elaborated herein.

[0063] In the above embodiments, the descriptions of the respective embodiments have their own emphases. For the parts not described in detail or recorded in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0064] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments or perform equivalent replacements for some of the technical features. These modifications or replacements 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 and should all be included in the protection scope of the present invention.

Claims

1. A discriminative imaging FDTD method based on a transparent source, characterized in that, The discriminative imaging FDTD method includes the following steps: Step S1, perform FDTD numerical simulation on photoacoustic waves; use the pulsed excitation light as the heating function of the absorber, and grid it, and perform second-order discretization on the time term and space term of the photoacoustic equation using the three-point difference format; Step S2, set the detection light field source as a transparent source, and the transparent source is adapted to the FDTD numerical simulation of the "photoacoustic - elasto - optic" process. The specific form of the transparent source is shown in Equation (2): wherein, represents the electric field component at the k-th time step, represents the electric field component at the (k - 1)-th time step, A represents the amplitude of the source signal, c is the speed of light, and λ is the wavelength of the electromagnetic wave; Step S3, perform FDTD numerical simulation on electromagnetic waves, that is, detection light; in the "photoacoustic - elasto - optic" process, finally, the reflected light intensity needs to be numerically solved; Step S4, set the FDTD boundary conditions, and apply the Mur boundary conditions to the FDTD numerical simulation in the field of photoacoustic remote sensing, specifically shown in Equation (4): where u is the field quantity, i.e., representing the electromagnetic field or the sound field, and v s is the velocity of the field, dt and dx represent the time and space increments, the subscript represents the spatial position index, and the superscript represents the time index; Step S5, calculate and obtain the reflectivity of the detection light.

2. The discriminative imaging FDTD method of the transparent source according to claim 1, wherein the specific form of the second - order discretization of the time term and space term of the photoacoustic equation using the three - point difference format is shown in Equation (1): where v s is the speed of sound, p is the photoacoustic pressure distribution at different spatial positions and times, H is the thermal energy conversion per unit volume and per unit time, i.e., the heating function; the subscript i represents the discrete point index in space, the superscript k represents the index in time, k + 1 represents the next moment, and k - 1 represents the previous moment; Δx is the discrete interval in space, and Δt is the discrete interval in time. Take Δx = cΔt to simplify the simulation analysis under the condition of stable calculation; the right side of the equation is the heat source term, which is caused by the short-pulse excitation light.

3. The discriminative imaging FDTD method of the transparent source according to claim 1, in step S3, use a sine - wave source to propagate in the form of the following discrete wave equation to simulate the reflection process of the detection light, specifically shown in Equation (3): where the subscript i represents the discrete point index of the electromagnetic wave in space, the superscript k represents the discrete point index of the electromagnetic wave in time, k + 1 represents the next moment, k - 1 represents the previous moment, c represents the speed of light; take Δx = cΔt to simplify the simulation analysis under the condition of stable calculation.

4. The discriminative imaging FDTD method for a transparent source as claimed in claim 1, wherein the reflectivity R is calculated by the following formula (5), where E0 is the intensity of the incident electromagnetic wave and E r is the intensity of the reflected electromagnetic wave; 5. A nested FDTD simulation method for the "photoacoustic - photoelastic" process, which utilizes the discriminative imaging FDTD method based on a transparent source according to any one of claims 1 - 4, characterized in that, The nested FDTD simulation method includes the following steps: Step S11, perform the photoacoustic FDTD simulation of Step S1 at the k-th moment; a time-varying sound pressure field is obtained through the photoacoustic FDTD simulation of Step S1, and according to the formula the refractive index change caused by the sound pressure field is obtained for the electromagnetic FDTD simulation; where ε is the photoelastic coefficient, n0 is the static refractive index, i.e., the intrinsic refractive index of the material without stress modulation, ρ is the density of the material, v s is the sound velocity, p(z,t) is the photoacoustic pressure distribution at different spatial positions and times, and δn(z,t) is the refractive index change due to stress modulation; thus, at the instant when the material is excited, i.e., t = 0 + the refractive index is n0 + δn; Step S12, perform a complete FDTD electromagnetic field simulation of the detection light at the k - th moment; obtain the required reflectivity signal through the FDTD electromagnetic field simulation of the detection light, and complete the simulation of the "photoacoustic - elasto - optic" process at this moment; Step S13, when the photoacoustic field changes to the next moment, repeat the above steps S11 and S12 until the "photoacoustic - elasto - optic" simulation at all moments is completed.

6. A flip - chip system established based on the discriminative imaging FDTD method of the transparent source according to any one of claims 1 - 4, the system includes excitation light, detection light, silicon - air composite structure and silicon - metal composite structure, that is, use the excitation light, detection light, silicon - air composite structure and silicon - metal composite structure to construct a layered elasto - optic discriminative model of "photoacoustic - elasto - optic" based on the silicon - air interface and silicon - metal interface.

7. The flip - chip system according to claim 6, wherein the excitation light is the heating function in the FDTD simulation process, to generate a photoacoustic effect to obtain the initial sound pressure, and then modulate the refractive index of the medium.

8. The flip - chip system according to claim 6, wherein the detection light is the transparent source in the FDTD simulation process, generates a sine electromagnetic wave as the detection signal, and using the method in steps S1 - S5, by continuously monitoring the reflectivity of the detection light, the PARS signal can be obtained, and different model interfaces can be distinguished according to the positive or negative of its initial slope.

9. As for the flip-chip system described in claim 6, in the layered photoelastic discrimination model, the silicon-metal interface can generate a positive initial slope during the "photoacoustic-photoelastic" process, while the silicon-air interface generates a negative initial slope during the "photoacoustic-photoelastic" process; the silicon layer, metal layer, and air layer all have different initial refractive indices, densities, photoelastic coefficients, and sound velocities. During the "photoacoustic-photoelastic" process, the three media will produce different refractive index changes, and finally generate PARS signals with completely different initial slopes at the silicon-metal interface and the silicon-air interface.

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