Film measurement methods, computer-readable storage media

Abstract

This application discloses a film layer measurement method and a computer-readable storage medium. By predicting the time and path of ultrasonic waves generated by pump light on the surface of the film under test, propagated within the film under test, reflected by the interfaces of the film layers under test, and returning to the surface of the film under test, based on the estimated film layer thickness and acoustic wave propagation velocity, the method obtains the predicted return time and predicted ultrasonic propagation path. During film layer measurement, the method determines the interface corresponding to the peak value in the photoacoustic signal based at least on the predicted return time and predicted ultrasonic propagation path of the ultrasonic waves returning to the surface of the film under test. Based on the peak value in the photoacoustic signal and its corresponding interface, the film properties of each film layer under test are obtained. This method facilitates accurate determination of the interface corresponding to the peak value in the photoacoustic signal, thereby improving the accuracy of film layer measurement.

Description

Technical Field

[0001] This invention relates to the field of photoacoustic measurement technology, specifically to a method for measuring film layers and a computer-readable storage medium. Background Technology

[0002] In the semiconductor field, the photoacoustic principle is widely used to measure film properties such as film thickness, sound velocity, and Young's modulus of metal and dielectric films. Pump light is irradiated onto the film surface to excite the sample, resulting in instantaneous thermoelastic deformation and generating ultrasonic waves on the film surface. These ultrasonic waves propagate into the film layer, and upon encountering the interfaces between different underlying materials, they are reflected and transmitted. The reflected ultrasonic waves form ultrasonic echo signals that return to the surface. For multilayer films, the ultrasonic waves propagate within each layer, and each time they encounter an interface between different layers, reflection and transmission occur, resulting in multiple echo signals returning to the film surface.

[0003] When an ultrasonic echo signal returns to the surface of the film under test, it causes changes in the surface morphology and reflectivity. By illuminating the surface of the film under test with probe light, the probe light reflected from the surface is received, forming a corresponding photoacoustic signal, which can be used to detect changes in the surface of the film. The effect of the ultrasonic echo signal on the surface of the film under test causes a peak in the photoacoustic signal. Based on the timing of the peak in the photoacoustic signal, the film layer can be measured.

[0004] For example, when measuring film thickness, the thickness can be calculated based on the ultrasonic return time (which can be considered the time of peak appearance) and the sound wave propagation speed. Assuming the sound speed in the film being tested is constant, the ultrasonic path length can be calculated using the formula: Ultrasonic path length = Ultrasonic return time × Sound speed. Since the ultrasonic path length is equal to twice the thickness of the film being tested, the thickness can be calculated as: Ultrasonic return time × Sound wave propagation speed within the film / 2. When the film being tested is formed by stacking layers of multiple materials, and the thickness of multiple layers needs to be measured, the formula becomes: Film thickness = (Ultrasonic return time at the interface between the film being tested and the lower layer - Ultrasonic return time at the interface between the film being tested and the upper layer) × Sound wave propagation speed within the film being tested / 2. Therefore, different formulas are used to calculate the film thickness for different film layers.

[0005] However, for thin films with complex stacked structures, it is difficult to identify which interface echo signal the peak in the photoacoustic signal corresponds to, thus making it impossible to accurately measure each film layer. Summary of the Invention

[0006] This invention primarily addresses the technical problem of determining the interface corresponding to the peak value in a photoacoustic signal for thin films with complex layered structures.

[0007] According to a first aspect, one embodiment provides a film layer measurement method, comprising:

[0008] The pump light and probe light are controlled to illuminate the thin film under test, which consists of one or more layers of film under test. The pump light is used to generate ultrasonic waves on the surface of the thin film under test. The ultrasonic waves propagate in the thin film under test, are reflected by the interface of the film under test layer in the thin film under test, and return to the surface of the thin film under test. The probe light is reflected by the surface of the thin film under test to form a probe signal light.

[0009] Acquire the photoacoustic signal formed by the probe signal light;

[0010] The interface corresponding to the peak value in the photoacoustic signal is determined based at least on the predicted return time and predicted ultrasonic propagation path of the ultrasonic wave returning to the surface of the film under test.

[0011] Based on the peak values ​​in the photoacoustic signal and their corresponding interfaces, the film properties of each layer under test are obtained.

[0012] The predicted return time and the predicted ultrasound propagation path are obtained in the following ways:

[0013] Obtain the estimated thickness of each layer of the film under test and the acoustic wave propagation velocity;

[0014] Based on the estimated thickness of each layer of the test film and the acoustic propagation velocity, the time and path of the ultrasonic waves generated by the pump light on the surface of the test film are predicted to propagate within the test film, be reflected by the interfaces of the test layers within the test film, and return to the surface of the test film. The predicted return time and the predicted ultrasonic propagation path are obtained. The ultrasonic propagation path refers to the interfaces that the ultrasonic waves pass through sequentially as they propagate within the test film and return to the surface of the test film.

[0015] In some embodiments, determining the interface corresponding to the peak value in the photoacoustic signal based at least on the predicted return time and predicted ultrasonic propagation path of the ultrasonic wave returning to the surface of the film under test includes:

[0016] Determine the peak value in the photoacoustic signal and its corresponding time;

[0017] For each peak time, it is compared with the predicted return time. If the absolute value of the difference between it and any predicted return time is less than the time difference threshold, then the peak time corresponds to an interface, and the interface corresponding to the peak time is determined according to the predicted ultrasound propagation path corresponding to the predicted return time.

[0018] In some embodiments, determining the interface corresponding to the peak value in the photoacoustic signal based at least on the predicted return time and predicted ultrasonic propagation path of the ultrasonic wave returning to the surface of the film under test includes:

[0019] Determine the peak value in the photoacoustic signal and its corresponding time, as well as the energy parameter or energy ratio of the peak value, wherein the energy ratio of the peak value refers to the ratio of the energy parameter of the peak value to the initial energy parameter of the ultrasonic wave;

[0020] For each peak, if the absolute value of the difference between its corresponding time and any of the predicted return times is less than the time difference threshold, and the absolute value of the difference between its energy parameter and the predicted energy parameter corresponding to the predicted return time is less than the energy difference threshold, or the absolute value of the difference between its energy ratio and the predicted energy ratio corresponding to the predicted return time is less than the energy ratio difference threshold, then the peak is determined to correspond to an interface, and the interface corresponding to the peak is determined according to the predicted ultrasound propagation path corresponding to the predicted return time.

[0021] The predicted energy parameter is the predicted energy parameter of the ultrasonic wave when it propagates within the film under test, is reflected, and returns to the surface of the film under test. The predicted energy ratio is the ratio of the predicted energy parameter of the ultrasonic wave when it returns to the surface of the film under test to the initial energy parameter.

[0022] In some embodiments, obtaining the film properties of each layer under test based on the peak value in the photoacoustic signal and its corresponding interface includes:

[0023] Based on the predicted return time and the corresponding predicted energy parameter or predicted energy ratio, a simulated reflectivity change curve of the surface of the film under test under the action of the ultrasonic wave is generated; wherein, the predicted energy parameter is the predicted energy parameter of the ultrasonic wave when it propagates in the film under test and is reflected back to the surface of the film under test, and the predicted energy ratio is the ratio of the predicted energy parameter of the ultrasonic wave when it returns to the surface of the film under test to the initial energy parameter.

[0024] By fitting the simulated reflectivity variation curve with the photoacoustic signal, the properties of each layer of the film to be tested can be obtained.

[0025] In some embodiments, the predicted energy parameter or the predicted energy ratio is obtained in the following ways:

[0026] The density of each layer of the film to be tested, the density of the substrate of the film to be tested, the propagation speed of the sound wave, and the initial energy parameters of the ultrasound are obtained.

[0027] Based on the density and acoustic wave propagation speed of each layer of the test film, and the density and acoustic wave propagation speed of the substrate, calculate the acoustic impedance of each layer of the test film and the acoustic impedance of the substrate.

[0028] Based on the initial energy parameters of the ultrasonic wave, the acoustic impedance of each layer of the test film, and the acoustic impedance of the substrate, the energy parameters or energy ratio of the ultrasonic wave when it propagates within the test film and is reflected back to the surface of the test film are predicted, thus obtaining the predicted energy parameters or the predicted energy ratio.

[0029] In some embodiments, the energy parameter is an amplitude or an energy value.

[0030] In some embodiments, the step of predicting the time and path of ultrasonic waves generated on the surface of the film under test to propagate within the film under test, be reflected by the interface of the film under test layer, and return to the surface of the film under test based on the estimated thickness of each layer of the film under test and the speed of sound propagation, and obtaining the predicted return time and the predicted ultrasonic propagation path, includes:

[0031] Based on the estimated thickness of each layer of the film to be tested and the propagation speed of the sound wave, an event-driven priority queue algorithm is used to simulate the process of the ultrasonic wave propagating in the film to be tested, being reflected by the interface of the film to be tested, and returning to the surface of the film to be tested, so as to obtain the predicted return time and the predicted ultrasonic wave propagation path.

[0032] Each event includes the event occurrence time, the interface where the event occurs, and the direction of ultrasonic propagation. Whenever the ultrasonic wave reaches an interface, an event is generated to simulate the ultrasonic wave being reflected by that interface. If the ultrasonic wave is also transmitted at that interface, an event is also generated to simulate the ultrasonic wave being transmitted through that interface.

[0033] In some embodiments, whenever an event is generated, the time when the reflected or transmitted ultrasonic wave returns to the surface of the film under test based on the event is determined, and the priority of the event is set according to the time when the reflected or transmitted ultrasonic wave returns to the surface of the film under test, wherein the earlier the time when the reflected or transmitted ultrasonic wave returns to the surface of the film under test, the higher the priority of the event.

[0034] In some embodiments, each event also includes an energy parameter or energy ratio of the reflected or transmitted ultrasound waves based on the event, and the event is ignored if the energy parameter of the generated event is less than an energy parameter threshold or the energy ratio is less than an energy ratio threshold.

[0035] In some embodiments, if the energy parameter of the reflected ultrasonic wave on the first interface is less than the energy parameter threshold or the energy ratio is less than the energy ratio threshold, a prompt message is output.

[0036] According to a second aspect, one embodiment provides a computer-readable storage medium storing a computer program that can be executed by a processor to implement the film measurement method described in the first aspect.

[0037] According to the film layer measurement method and computer-readable storage medium of the above embodiments, the ultrasonic waves generated by the pump light on the surface of the film under test are predicted based on the estimated film thickness and acoustic wave propagation speed of each film layer. The predicted return time and propagation path are obtained, which can be used as a reference to determine the time of the ultrasonic echo signal and the corresponding interface. During film layer measurement, the interface corresponding to the peak value in the photoacoustic signal is determined based on the predicted return time and propagation path of the ultrasonic waves returning to the surface of the film under test. Furthermore, the film properties of each film layer under test are obtained based on the peak value and its corresponding interface, which helps to accurately determine the interface corresponding to the peak value, thereby improving the accuracy of film layer measurement. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of ultrasound propagation within a multilayer membrane in one embodiment;

[0039] Figure 2 A flowchart of a prediction method according to one embodiment;

[0040] Figure 3 A flowchart of a film measurement method according to one embodiment;

[0041] Figure 4 This is a flowchart of obtaining predicted energy parameters or predicted energy ratios in one embodiment;

[0042] Figure 5 This is a distribution diagram of the predicted energy ratio over time in one embodiment;

[0043] Figure 6 This is a simulated reflectance variation curve in one embodiment;

[0044] Figure 7 This is a schematic diagram of the ultrasonic propagation process in one embodiment. Detailed Implementation

[0045] The present invention will now be described in further detail with reference to specific embodiments and accompanying drawings. Similar elements in different embodiments are referred to by associated similar element reference numerals. In the following embodiments, many details are described to facilitate a better understanding of this application. However, those skilled in the art will readily recognize that some features may be omitted in different situations, or may be replaced by other elements, materials, or methods. In some cases, certain operations related to this application are not shown or described in the specification. This is to avoid obscuring the core parts of this application with excessive description. For those skilled in the art, detailed description of these related operations is not necessary; they can fully understand the related operations based on the description in the specification and general technical knowledge in the art.

[0046] Furthermore, the features, operations, or characteristics described in the specification can be combined in any suitable manner to form various embodiments. At the same time, the steps or actions in the method description can be rearranged or adjusted in a manner obvious to those skilled in the art. Therefore, the various orders in the specification and drawings are only for the clear description of a particular embodiment and do not imply a necessary order, unless otherwise stated that a particular order must be followed.

[0047] The serial numbers assigned to components or physical quantities in this document, such as "first," "second," etc., are used only to distinguish the described objects and have no sequential or technical meaning. They should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. "Multiple" means two or more. Unless otherwise specified, "connection" or "linkage" in this application includes both direct and indirect connections (linkages).

[0048] Since the thin film under test usually consists of multiple layers of different materials, ultrasound undergoes multiple reflections and transmissions between the layers. The ultrasound echo signal returning to the surface will appear at multiple time points, each representing a different interface. It is necessary to estimate the time point of ultrasound return in advance in order to determine the corresponding measurement and calculation methods.

[0049] In existing technologies, a database is constructed based on known film material parameters, and multiple reflectivity change curves caused by ultrasound return to the surface are simulated according to the mechanism of ultrasound generation and propagation, thereby determining the interface corresponding to the ultrasound echo signal. However, this simulation usually requires a large number of material parameters, the construction of the simulation model is usually very complex, and the computational resources consumed are considerable.

[0050] This application provides a prediction method for assisting peak finding of photoacoustic signals, which omits the process of ultrasonic generation and simplifies the ultrasonic propagation process. By estimating the thickness of the membrane layer and the theoretical propagation speed of sound waves in different membrane layers, an ultrasonic propagation model of a multilayer membrane structure is built. The time point at which ultrasonic waves are reflected at the interface of each membrane layer and return to the surface of the uppermost membrane layer under test is calculated. This solves the problem that different ultrasonic echo signals cannot be quickly distinguished and the occurrence time of multilayer membrane signals cannot be quickly determined when measuring multilayer membranes.

[0051] This application is based on Figure 1 The ultrasonic propagation model shown assists in determining the ultrasonic return time. The test film consists of N layers, each represented by a different color. The acoustic propagation velocity of each layer is determined based on its material, and its thickness is estimated. The interfaces of the test layers include the interfaces between adjacent test layers and the interface between the last test layer and the substrate. Figure 1 Interface 0 is the upper surface of the film under test, interface 1 is the interface between the first and second film layers, interface 2 is the interface between the second and third film layers, and so on. Ultrasonic waves are generated at interface 0, propagate within the film under test, and are reflected and transmitted when they encounter the interfaces between the film layers. The reflected ultrasonic waves return to the surface of the film under test.

[0052] Compared to traditional ultrasonic propagation models, this model is simpler, requiring only the estimated thickness of each film layer and the sound wave propagation speed when predicting the time it takes for ultrasound to return to the surface of the film under test and the ultrasonic path.

[0053] Please refer to Figure 2 The prediction method in some embodiments includes steps 110 and 120, which are described in detail below.

[0054] Step 110: Obtain the estimated thickness of each layer of the film to be tested and the acoustic propagation speed.

[0055] The estimated film thickness can be obtained based on experience or previous measurements of the actual film thickness of similar films.

[0056] Step 120: Based on the estimated thickness of each layer of the film to be tested and the acoustic propagation speed, predict the time and path of the ultrasonic waves generated by the pump light on the surface of the film to be tested, the time of propagation of the ultrasonic waves within the film to be tested, the time of reflection by the interface of the film to be tested within the film to be tested, and the return to the surface of the film to be tested. The predicted return time and predicted ultrasonic propagation path are obtained.

[0057] The ultrasonic propagation path refers to the interfaces that ultrasonic waves pass through sequentially as they propagate within the film under test and return to its surface. Algorithms such as quicksort or priority queues can be used to simulate the process of ultrasonic waves propagating within the film under test, being reflected by the interfaces of the film layers, and returning to the surface, thus obtaining the predicted return time and predicted ultrasonic propagation path.

[0058] The time it takes for a sound wave to propagate through each membrane layer depends on the membrane thickness d. i and the speed of sound c i , where d i Indicates the first i The thickness of the film layer, c i Indicates the first i The speed of sound wave propagation in the membrane layer is as follows: the ultrasonic wave propagates in the first... i The one-way propagation time of the test film layer is τ i =d i / c i The total time for the ultrasonic wave to return to the surface of the film under test, also known as the predicted return time, is the sum of the one-way propagation time of the ultrasonic wave in each segment of the propagation path, including the one-way propagation time of the ultrasonic wave in each film under test during its propagation to the lower layer and the one-way propagation time of the ultrasonic wave in each film under test during its return to the surface of the film under test.

[0059] Each interface of the thin film under test can be assigned a number or index. Based on the film structure, the interfaces through which the ultrasonic waves are generated, propagated, and return to the surface of the thin film under test are recorded sequentially to obtain the predicted ultrasonic propagation path. For example... Figure 1 In the middle, the predicted ultrasonic propagation path of the ultrasonic wave reflected from interface 2 can be denoted as 01210.

[0060] The predicted results provide guidance for optimizing film layer measurements. Based on the predicted return time and the predicted ultrasonic propagation path, it is possible to help determine the peak time of the ultrasonic echo signal in the photoacoustic signal obtained during actual measurement, thus assisting measurement personnel in using peak-finding algorithms to obtain film layer properties. It can also help measurement personnel rationally adjust the control parameters of the measurement equipment, such as determining the scanning time range during measurement. Based on the predicted ultrasonic propagation path, measurement personnel can reconstruct the entire ultrasonic propagation process.

[0061] Based on the above prediction method, this application provides a film measurement method, which includes the following steps:

[0062] Step 130: Control the pump light and probe light to illuminate the thin film to be tested, which consists of one or more layers of the film to be tested.

[0063] Pump and probe beams can be generated using lasers. The laser emits a pulsed beam, which is split into pump and probe beams by a beam splitter. The pulsed beam generated by the laser can be an ultrashort pulse beam, etc.

[0064] The pump light and probe light are ultimately incident at the same location on the thin film under test (TFT). The pump light generates ultrasonic waves on the surface of the TFT, which propagate within the TFT and are reflected back to the TFT surface by the interfaces of the TFT layers. The probe light is reflected from the TFT surface to form a probe signal light, which characterizes the change in the reflectivity of the TFT surface.

[0065] Step 140: Acquire the photoacoustic signal formed by the probe signal light.

[0066] A photoacoustic signal can be formed by receiving the detection signal light through a detector, such as a photodetector.

[0067] Step 150: Determine the interface corresponding to the peak value in the photoacoustic signal based at least on the predicted return time and predicted ultrasonic propagation path of the ultrasonic waves returning to the surface of the film under test.

[0068] Specifically, step 150 may include: determining the peak value in the photoacoustic signal and its corresponding time; for each peak value, comparing it with the predicted return time; if the absolute value of the difference between the peak value and any predicted return time is less than the time difference threshold, then determining that the peak value corresponds to an interface, and determining the interface corresponding to the peak value according to the predicted ultrasonic propagation path corresponding to the predicted return time.

[0069] The peak value in the photoacoustic signal and its corresponding time can be determined using a peak-finding algorithm. This involves identifying an interface corresponding to the peak value, meaning the peak is caused by ultrasonic waves reflected back to the surface of the film under test. The bottommost interface in the predicted ultrasonic propagation path is the interface that causes the reflection, which is also the interface corresponding to the peak value.

[0070] Step 160: Based on the peak values ​​in the photoacoustic signal and their corresponding interfaces, obtain the membrane properties of each layer to be tested.

[0071] In some embodiments, the film properties include the film thickness of the film to be tested.

[0072] The film measurement method provided in this application utilizes the predicted return time and predicted ultrasonic propagation path of the ultrasonic waves returning to the surface of the film under test to help determine the interface corresponding to the peak value in the photoacoustic signal. Based on the peak value in the photoacoustic signal and its corresponding interface, the film properties of each layer under test are obtained, which is beneficial for accurately determining the interface corresponding to the peak value, thereby improving the accuracy of film measurement. Furthermore, the prediction of the return time and ultrasonic propagation path only requires the estimated film thickness of the film under test and the acoustic wave propagation speed, which can reduce computational resources and improve computational speed.

[0073] In some embodiments, step 150 includes: determining a peak in the photoacoustic signal and its corresponding time, as well as the energy parameter or energy ratio of the peak; for each peak, if the absolute value of the difference between its corresponding time and any predicted return time is less than a time difference threshold, and the absolute value of the difference between its energy parameter and the predicted energy parameter corresponding to the predicted return time is less than an energy difference threshold, or the absolute value of the difference between its energy ratio and the predicted energy ratio corresponding to the predicted return time is less than an energy ratio difference threshold, then the peak corresponds to an interface, and the interface corresponding to the peak is determined according to the predicted ultrasonic propagation path corresponding to the predicted return time.

[0074] The predicted energy parameter is the predicted energy parameter of the ultrasonic wave propagating within the film under test and returning to the surface of the film after reflection. The predicted energy ratio is the ratio of the predicted energy parameter of the ultrasonic wave returning to the surface of the film under test to the initial energy parameter. The peak energy ratio refers to the ratio of the peak energy parameter to the initial energy parameter of the ultrasonic wave. The energy parameter can be amplitude or energy value, etc., and the energy ratio is the ratio of the energy parameter of the ultrasonic wave returning to the surface of the film under test to the initial energy parameter.

[0075] In this embodiment, the accuracy is further improved by adding an energy parameter or energy ratio parameter to determine the interface corresponding to the peak.

[0076] In some embodiments, step 160 includes: generating a simulated reflectance change curve of the surface of the film under test under ultrasonic action based on the predicted return time and the corresponding predicted energy parameter or predicted energy ratio; and fitting the simulated reflectance change curve with the photoacoustic signal to obtain the film properties of each layer of the film under test.

[0077] The simulated reflectivity variation curve generated in one embodiment is as follows: Figure 6 As shown.

[0078] The detection of ultrasonic echo signals is essentially a measurement of the change in reflectivity of the surface at the measurement point. The timing of the reflectivity change depends on the time it takes for the ultrasonic wave to return to the surface of the film under test. The amplitude and direction of the reflectivity change depend on the energy parameters of the ultrasonic wave returning to the surface of the film under test and the optical properties of the film surface material itself. By simulating the return time and corresponding predicted energy parameters or predicted energy ratios, a simulated reflectivity change curve is generated. This curve is then fitted with the photoacoustic signal to calculate the film properties of each layer under test, allowing for convenient and accurate acquisition of the film properties of the tested layers.

[0079] Please refer to Figure 4 In some embodiments, the predicted energy parameters or predicted energy ratios are obtained in the following ways:

[0080] Step 210: Obtain the density of each layer of the film to be tested, the density of the substrate of the film to be tested, the acoustic propagation speed, and the initial energy parameters of the ultrasound.

[0081] Step 220: Calculate the acoustic impedance of each layer of the test film and the acoustic impedance of the substrate based on the density and acoustic velocity of each layer of the test film and the density and acoustic velocity of the substrate.

[0082] Acoustic impedance can be calculated using the formula Z. i = ρ i ×c i Calculate, where Z i Indicates the first i The acoustic impedance of the membrane layer to be tested ρ i Indicates the first i Density of the film layer.

[0083] Step 230: Based on the initial energy parameters of the ultrasonic wave, the acoustic impedance of each layer of the film to be tested, and the acoustic impedance of the substrate, predict the energy parameters or energy ratio of the ultrasonic wave when it propagates within the film to be tested and is reflected back to the surface of the film to be tested, and obtain the predicted energy parameters or predicted energy ratio.

[0084] Specifically, if the energy parameter is amplitude, the reflection amplitude coefficient and transmission amplitude coefficient of each interface can be calculated. The initial amplitude of the ultrasonic wave is then multiplied by the reflection amplitude coefficient or transmission amplitude coefficient of each interface encountered during propagation (multiply by the reflection amplitude coefficient if reflection occurs, multiply by the transmission amplitude coefficient if transmission occurs) to obtain the predicted amplitude. The reflection amplitude coefficient and transmission amplitude coefficient are determined by the acoustic impedance of the films on both sides of the interface, and the calculation formula is:

[0085] ,

[0086] in R ij and T ij The first i The test film layer and the first layer j The reflection amplitude coefficient and transmission amplitude coefficient of the interface between the layers of the film to be tested, if the first layer... i If the test layer is the last test layer, then the... j The test film layer refers to the substrate, Z. j The acoustic impedance of the substrate.

[0087] If the energy parameter is an energy value, the reflected energy ratio and transmitted energy ratio at each interface can be calculated. The initial energy value of the ultrasonic wave is then multiplied by the reflected energy ratio or transmitted energy ratio at each interface encountered during propagation (multiply by the reflected energy ratio if reflection occurs, multiply by the transmitted energy ratio if transmission occurs) to obtain the predicted energy value. The formulas for calculating the reflected energy ratio and transmitted energy ratio are:

[0088] ,

[0089] in P ij and Q ij The first i The test film layer and the first layer j The ratio of reflected energy and the ratio of transmitted energy at the interface between the layers of the film to be tested.

[0090] The initial energy parameters of the ultrasound can be set by the user and can be any value. To obtain the actual energy returning to the surface of the film under test, it can be set to the actual estimated value of the initial energy parameters of the ultrasound. To obtain the predicted energy ratio, it can be set to 1. For example... Figure 1 In this scenario, assuming the initial energy parameter of the ultrasonic wave is 1, and the ultrasonic wave is generated from the uppermost interface 0, incident on interface 1, reflected, and returns to interface 0, then the predicted return time is... t =2× d 1 / c 1. The predicted energy ratio is: 1×| R 12 | 2 .

[0091] In some embodiments, step 120 includes: based on the estimated thickness of each layer of the film to be tested and the speed of sound propagation, using an event-driven priority queue algorithm to simulate the process of ultrasonic waves propagating in the film to be tested, being reflected by the interface of the film to be tested within the film to be tested, and returning to the surface of the film to be tested, thereby obtaining the predicted return time and the predicted ultrasonic propagation path.

[0092] Among them, the event-driven priority queue algorithm can be the ray tracing method, etc. Each event includes the event occurrence time, the interface where the event occurs, and the direction of ultrasonic propagation. Whenever the ultrasonic wave reaches an interface, an event is generated to simulate the ultrasonic wave being reflected by the interface. If the ultrasonic wave is also transmitted at the interface, an event is also generated to simulate the ultrasonic wave being transmitted by the interface.

[0093] The event-driven priority queue algorithm has a loop system. It starts from the generation of ultrasonic waves on the surface of the film under test and propagates downwards. According to the two propagation directions (upward or downward) and the two situations after encountering the interface (transmission or reflection), multiple events are generated and iterated in a loop. The complete iteration of an ultrasonic propagation event starts from the generation of ultrasonic waves on the surface of the film under test and ends when the ultrasonic waves return to the surface of the film under test. Thus, the predicted return time and the predicted ultrasonic propagation path are obtained.

[0094] In some embodiments, whenever an event is generated, the time when the reflected or transmitted ultrasonic wave returns to the surface of the film under test based on the event is determined, and the priority of the event is set according to the time when the reflected or transmitted ultrasonic wave returns to the surface of the film under test, wherein the earlier the time when the reflected or transmitted ultrasonic wave returns to the surface of the film under test, the higher the priority of the event.

[0095] Computational efficiency can be improved by prioritizing events that return to the surface of the film under test earlier.

[0096] In some embodiments, each event also includes an energy parameter or energy ratio of the reflected or transmitted ultrasonic wave generated based on the event. If the energy parameter or energy ratio of the generated event is less than an energy parameter threshold or less than an energy ratio threshold, the event is ignored, i.e., the algorithm stops iterating over the event. In other words, when ultrasonic waves are reflected or transmitted at an interface, if the energy parameter or energy ratio of the reflected or transmitted ultrasonic waves is less than an energy parameter threshold or less than an energy ratio threshold, it is ignored.

[0097] By ignoring events where the energy parameter is less than an energy parameter threshold or the energy ratio is less than an energy ratio threshold, ultrasonic echo signals with too low an energy level can be removed. The energy parameter threshold and energy ratio threshold can be set according to the signal energy resolution of the equipment used for the actual measurement, in order to remove ultrasonic echo signals that cannot be recognized by the equipment.

[0098] In some embodiments, if the energy parameter of the reflected ultrasonic wave at the first interface is less than an energy parameter threshold or the energy ratio is less than an energy ratio threshold, a prompt message is output, such as an empty output or a pop-up reminder window. The prompt message is used to indicate that the film under test cannot be measured.

[0099] The ultrasonic echo signal on the first interface is the ultrasonic echo signal with the shortest path. If its energy parameter is less than the energy parameter threshold or the energy ratio is less than the energy ratio threshold, it means that even the ultrasonic echo signal with the shortest path cannot be measured, and a prompt message is output. This provides guidance for the measurement feasibility assessment and measurement parameter optimization, and can help measurement personnel assess whether the film under test can be measured by the photoacoustic measurement principle, and whether the equipment and / or measurement parameters need to be adjusted.

[0100] To help understand the technical solution of this application, a specific example is introduced below.

[0101] A film structure model is constructed for a thin film under test with four layers, and the parameters are shown in the table below:

[0102] membrane Estimated film thickness ( ) Speed ​​of sound / ps) density Membrane 1 1000 60 14.5 Membrane 2 2000 90 4.5 Membrane 3 5000 80 8.9 Membrane 4 5000 60 2.2

[0103] in The unit of length is angstrom (Å), where 1 Å = 0.1 nm. Layer 1 is the uppermost layer, and layer 4 is the lowermost layer. Above layer 1 is air, and the reflection amplitude coefficient at the interface between air and layer 1 is -1. Below layer 4 is a rigid boundary with a reflection amplitude coefficient of 1. The indices 0, 1, 2, 3, and 4 are used for each interface.

[0104] The initial amplitude is set to 1, the amplitude threshold to 1%, and the time range to [0ps, 250ps]. Based on an event-driven priority queue algorithm, the final propagation node of one incident ultrasonic wave after propagating within the four membrane layers is calculated. The propagation node refers to the point where the ultrasonic wave returns to the surface of the uppermost membrane layer 1. The predicted return time (Time), predicted amplitude coefficient (Amp) (i.e., the energy proportion calculated using amplitude), and ultrasonic propagation paths (Paths) are output. The results are shown in the table below:

[0105] Ultrasonic propagation events Time (ps) Amp Paths 1 33.33 -0.365 010 2 66.67 -0.133 01010 3 77.78 0.238 01210 4 202.78 -0.551 0123210 5 236.11 -0.201 012321010 6 236.11 -0.201 010123210

[0106] Based on the predicted return time (Time) and predicted ultrasonic propagation paths (Paths), the measurement personnel can determine the time point when the peak value occurs during the actual measurement. For example, if the ultrasonic propagation path (Paths) corresponding to ultrasonic propagation event 1 is 010, then the ultrasonic wave propagates downward from interface 0 (the upper surface of the film under test) to interface 1 (the lower surface of film layer 1), is reflected, and returns to interface 0. In this case, the ultrasonic wave travels back and forth twice inside film layer 1. According to the formula distance = time × velocity, the time point corresponding to the peak value near 33 ps represents the thickness value of film layer 1. That is, the thickness value of film layer 1 = the time corresponding to the peak value near 33 ps × the sound velocity value of film layer 1 / 2.

[0107] Based on the predicted return time and the corresponding predicted amplitude coefficient, a distribution diagram of the amplitude over time when the ultrasound returns to the surface of the film under test can be generated, such as... Figure 5 As shown in the figure. The sign of the amplitude coefficient represents the direction of ultrasonic vibration. The amplitude coefficient and the optical properties of the surface material of the film under test determine the direction of reflectivity change (whether it increases or decreases) when the ultrasound returns to the surface of the film under test. Furthermore, a simulated reflectivity change curve can be constructed based on the time point of ultrasound return and the amplitude coefficient, as shown in the figure. Figure 6 As shown.

[0108] Furthermore, the ultrasound propagation process can be visualized based on the predicted return time (Time), predicted ultrasound propagation paths (Paths), and predicted amplitude coefficient (Amp), generating a schematic diagram of the ultrasound propagation process, such as... Figure 7 As shown in the diagram, different colored areas represent different membrane layers, and the dashed lines represent the predicted ultrasound propagation paths. The horizontal direction represents the membrane thickness, the zero-thickness point is the surface of the uppermost membrane layer, and the vertical direction is the time axis. Assuming the ultrasound wave is incident at the surface of the uppermost membrane layer (zero-thickness point) at 0 ps, ​​the propagation path of the ultrasound wave within the membrane layer can be considered as following the dashed lines in the diagram (actual propagation speed will be uneven due to the non-uniformity of the membrane layers). The different colored dashed lines represent the propagation paths of the ultrasound wave in different membrane layers. When the dashed line representing the ultrasound propagation path returns to the uppermost surface, the predicted return time (Time) and the predicted amplitude coefficient (Amp) are marked to facilitate the determination of the location and amplitude of the ultrasound echo signal.

[0109] By visualizing the ultrasonic propagation process and converting it into a graphical display, measurement personnel can gain a deeper understanding of the photoacoustic measurement principle, making it easier for them to determine the physical meaning represented by the measured signal peaks, quickly determine the film thickness calculation formula, and build a peak-finding algorithm.

[0110] The simplified ultrasound propagation model described above only includes three parameters: estimated film thickness, density, and sound velocity. The reflection and transmission coefficients can be calculated based on acoustic impedance to obtain the time points and energy parameters or energy ratios of ultrasound reflection, transmission, and propagation back to the surface or any interface within the multilayer film. Furthermore, a more comprehensive simulation model can be built by combining parameters such as the optical properties of the film surface, the attenuation coefficient within the film, and the thermal conductivity of the film. By continuously adjusting the estimated film thickness in the model, simulated photoacoustic signals or simulated reflectivity variation curves under different thickness estimates can be obtained. These simulations can then be fitted with actual photoacoustic signals to obtain the measured film thickness value.

[0111] Those skilled in the art will understand that all or part of the functions of the various methods in the above embodiments can be implemented by hardware or by computer programs. When all or part of the functions in the above embodiments are implemented by computer programs, the program can be stored in a computer-readable storage medium, which may include: read-only memory, random access memory, disk, optical disk, hard disk, etc., and the program is executed by a computer to achieve the above functions. For example, the program can be stored in the memory of a device, and when the program in the memory is executed by the processor, all or part of the above functions can be achieved. In addition, when all or part of the functions in the above embodiments are implemented by computer programs, the program can also be stored in a server, another computer, disk, optical disk, flash drive, or external hard drive, etc., and can be downloaded or copied to the memory of a local device, or the system of the local device can be updated. When the program in the memory is executed by the processor, all or part of the functions in the above embodiments can be achieved.

[0112] The above examples illustrate the present invention only to aid in understanding it and are not intended to limit the scope of the invention. Those skilled in the art can make various simple deductions, modifications, or substitutions based on the principles of this invention.

Claims

1. A method for measuring film layers, characterized in that, include: The pump light and probe light are controlled to illuminate the thin film under test, which consists of one or more layers of film under test. The pump light is used to generate ultrasonic waves on the surface of the thin film under test. The ultrasonic waves propagate in the thin film under test, are reflected by the interface of the film under test layer in the thin film under test, and return to the surface of the thin film under test. The probe light is reflected by the surface of the thin film under test to form a probe signal light. Acquire the photoacoustic signal formed by the probe signal light; The interface corresponding to the peak value in the photoacoustic signal is determined based at least on the predicted return time and predicted ultrasonic propagation path of the ultrasonic wave returning to the surface of the film under test. Based on the peak values ​​in the photoacoustic signal and their corresponding interfaces, the film properties of each layer under test are obtained. The predicted return time and the predicted ultrasound propagation path are obtained in the following ways: Obtain the estimated thickness of each layer of the film under test and the acoustic wave propagation velocity; Based on the estimated thickness of each layer of the test film and the speed of sound propagation, the time and path of ultrasonic waves generated by pump light on the surface of the test film are predicted to propagate within the test film, be reflected by the interface of the test film layer within the test film, and return to the surface of the test film. The predicted return time and the predicted ultrasonic propagation path are then obtained. The ultrasonic propagation path refers to the interfaces through which the ultrasonic wave propagates within the film under test and returns to the surface of the film under test.

2. The film measurement method as described in claim 1, characterized in that, The step of determining the interface corresponding to the peak value in the photoacoustic signal based at least on the predicted return time and predicted ultrasonic propagation path of the ultrasonic wave returning to the surface of the film under test includes: Determine the peak value in the photoacoustic signal and its corresponding time; For each peak time, it is compared with the predicted return time. If the absolute value of the difference between it and any predicted return time is less than the time difference threshold, then the peak time corresponds to an interface, and the interface corresponding to the peak time is determined according to the predicted ultrasound propagation path corresponding to the predicted return time.

3. The film measurement method as described in claim 1, characterized in that, The step of determining the interface corresponding to the peak value in the photoacoustic signal based at least on the predicted return time and predicted ultrasonic propagation path of the ultrasonic wave returning to the surface of the film under test includes: Determine the peak value in the photoacoustic signal and its corresponding time, as well as the energy parameter or energy ratio of the peak value, wherein the energy ratio of the peak value refers to the ratio of the energy parameter of the peak value to the initial energy parameter of the ultrasonic wave; For each peak, if the absolute value of the difference between its corresponding time and any of the predicted return times is less than the time difference threshold, and the absolute value of the difference between its energy parameter and the predicted energy parameter corresponding to the predicted return time is less than the energy difference threshold, or the absolute value of the difference between its energy ratio and the predicted energy ratio corresponding to the predicted return time is less than the energy ratio difference threshold, then the peak is determined to correspond to an interface, and the interface corresponding to the peak is determined according to the predicted ultrasound propagation path corresponding to the predicted return time. The predicted energy parameter is the predicted energy parameter of the ultrasonic wave when it propagates within the film under test, is reflected, and returns to the surface of the film under test. The predicted energy ratio is the ratio of the predicted energy parameter of the ultrasonic wave when it returns to the surface of the film under test to the initial energy parameter.

4. The film measurement method as described in claim 1, characterized in that, The process of obtaining the film properties of each layer under test based on the peak value in the photoacoustic signal and its corresponding interface includes: Based on the predicted return time and the corresponding predicted energy parameter or predicted energy ratio, a simulated reflectivity change curve of the surface of the film under test under the action of the ultrasonic wave is generated; wherein, the predicted energy parameter is the predicted energy parameter of the ultrasonic wave when it propagates in the film under test and is reflected back to the surface of the film under test, and the predicted energy ratio is the ratio of the predicted energy parameter of the ultrasonic wave when it returns to the surface of the film under test to the initial energy parameter. By fitting the simulated reflectivity variation curve with the photoacoustic signal, the properties of each layer of the film to be tested can be obtained.

5. The film measurement method as described in claim 3 or 4, characterized in that, The predicted energy parameter or the predicted energy ratio is obtained in the following way: The density of each layer of the film to be tested, the density of the substrate of the film to be tested, the propagation speed of the sound wave, and the initial energy parameters of the ultrasound are obtained. Based on the density and acoustic wave propagation speed of each layer of the test film, and the density and acoustic wave propagation speed of the substrate, calculate the acoustic impedance of each layer of the test film and the acoustic impedance of the substrate. Based on the initial energy parameters of the ultrasonic wave, the acoustic impedance of each layer of the test film, and the acoustic impedance of the substrate, the energy parameters or energy ratio of the ultrasonic wave when it propagates within the test film and is reflected back to the surface of the test film are predicted, thus obtaining the predicted energy parameters or the predicted energy ratio.

6. The film measurement method as described in claim 3 or 4, characterized in that, The energy parameter is either amplitude or energy value.

7. The film measurement method as described in claim 1, characterized in that, The process of predicting the time and path of ultrasonic waves generated on the surface of the film under test, propagating within the film under test, being reflected by the interface of the film under test layer, and returning to the surface of the film under test based on the estimated thickness of each layer and the speed of sound propagation, and obtaining the predicted return time and the predicted ultrasonic propagation path, includes: Based on the estimated thickness of each layer of the film to be tested and the propagation speed of the sound wave, an event-driven priority queue algorithm is used to simulate the process of the ultrasonic wave propagating in the film to be tested, being reflected by the interface of the film to be tested, and returning to the surface of the film to be tested, so as to obtain the predicted return time and the predicted ultrasonic wave propagation path. Each event includes the event occurrence time, the interface where the event occurs, and the direction of ultrasonic propagation. Whenever the ultrasonic wave reaches an interface, an event is generated to simulate the ultrasonic wave being reflected by that interface. If the ultrasonic wave is also transmitted at that interface, an event is also generated to simulate the ultrasonic wave being transmitted through that interface.

8. The film measurement method as described in claim 7, characterized in that, Whenever an event is generated, the time when the reflected or transmitted ultrasonic wave returns to the surface of the film under test is determined based on the event. The priority of the event is set according to the time when the reflected or transmitted ultrasonic wave returns to the surface of the film under test, wherein the earlier the time when the reflected or transmitted ultrasonic wave returns to the surface of the film under test, the higher the priority of the event.

9. The film measurement method as described in claim 7 or 8, characterized in that, Each event also includes the energy parameter or energy ratio of the reflected or transmitted ultrasound waves based on the event. If the energy parameter of the generated event is less than the energy parameter threshold or the energy ratio is less than the energy ratio threshold, the event is ignored.

10. The film measurement method as described in claim 9, characterized in that, Also includes: If the energy parameter of the reflected ultrasonic wave on the first interface is less than the energy parameter threshold or the energy ratio is less than the energy ratio threshold, a prompt message will be output.

11. A computer-readable storage medium, characterized in that, The medium stores a computer program that can be executed by a processor to implement the film measurement method as described in any one of claims 1 to 10.

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