Device and method for detecting ferroelectric direction of thin film under action of stress

Abstract

The invention discloses a device for detecting the ferroelectric direction of a thin film under the action of stress. The device is mainly composed of the following modules: an excitation light module which is used for polarization initialization and regulation of incident fundamental frequency light and stray light filtering; the excitation light source-femtosecond pulse laser is used for generating an incident excitation light beam with a proper frequency; the polarization module comprises a polaroid and is used for initializing polarization of incident light; the focusing module comprises a lens A and is used for focusing incident laser onto a to-be-detected sample; the half-wave plate comprises an electrically-controlled rotatable half-wave plate and is used for regulating and controlling polarization of incident light. The ferroelectric direction of the thin film under the stress is detected through nonlinear optics, the corresponding testing system and method are provided, the problems that in an existing measuring technology, sensitivity to the environment is high, and the sample preparation process is difficult are solved, and the problem that the sample is damaged in the measuring process and cannot be further researched is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of ferroelectric direction measurement, and in particular to a device and method for detecting the direction of thin film ferroelectricity under stress. Background Art

[0002] Ferroelectric thin film materials have broad application prospects in information storage, sensors, optoelectronic devices and other fields due to their unique dielectric properties and ferroelectric domain structure. Stress, as a common influencing factor and regulation method, has an important impact on the performance and stability of thin films and their devices. Accurately measuring the ferroelectric direction under stress is crucial to achieving high-performance devices.

[0003] At present, traditional methods for measuring ferroelectric direction include transmission electron microscopy (TEM), X-ray diffraction (XRD), and piezoelectric stress microscopy (PFM). However, these methods not only require complex sample preparation processes and expensive experimental equipment, but the test results are highly dependent on the preparation process of the equipment parameter samples. In addition, the requirements for film size and thickness are high, and there is a risk that the sample will be damaged and cannot be used again. More importantly, these integrated devices cannot be coupled with stress application devices to realize the detection of the ferroelectric direction of the film under stress.

[0004] As a non-destructive and highly sensitive measurement method, second harmonic generation (SHG) technology is an important tool for detecting the ferroelectric orientation of thin films under stress. Second harmonic generation is a nonlinear optical process in which the frequency of the incident light wave is doubled to generate a second harmonic signal. For ferroelectric materials, their non-centrosymmetric structure enables them to generate second harmonics. When the polarization direction of the incident light matches the polarization direction of the ferroelectric material, the intensity of the second harmonic signal reaches its maximum value. Therefore, by measuring the intensity and phase of the second harmonic signal, the ferroelectric orientation of the ferroelectric material can be determined. In addition, the SHG measurement system can be better compatible with various stress application devices, assisting in studying the dynamic process of the change of the ferroelectric orientation of thin films under stress.

[0005] In recent years, methods for detecting the distribution of ferroelectric domains in thin films using the second harmonic method have gradually emerged. For example, in the patent "A system and method for determining the domain orientation of ferroelectric thin films using optical second harmonics" in 2017 (CN 107144550 A), the distribution of ferroelectric domains in thin films is detected by rotating the sample and the polarization of the incident light. Although this technology overcomes the problems of complex sample preparation process and the destructiveness of the measurement to the sample. However, since the sample is rotated by a rotating stage, the center of the measured sample (i.e., the measurement position) must be strictly located in the center of the optical path and the rotating stage during the measurement process. Once the measurement position is slightly offset from the center, it will cause it to deviate from the optical path during the process of rotating the sample, resulting in the need to readjust the sample position for each angle measured. In addition, there are also great challenges in the process of coupling the rotating stage with the stress application device. Once the stress is applied and the sample deviates from the center of the stage, the sample position needs to be moved again. The entire measurement process is complex to adjust and has high restrictions. Summary of the invention

[0006] In order to solve the problems mentioned in the above background technology, the present invention provides a device and method for detecting the direction of thin film ferroelectricity under stress.

[0007] In order to solve the problems of difficulty in measuring the ferroelectric direction of thin films under stress, destructiveness, environmental sensitivity, and high measurement complexity and restrictions, the present invention provides a device and a method for using nonlinear optical technology to detect the ferroelectric direction of thin films under stress. Specifically, a device for measuring the ferroelectric direction of thin films under stress is established by using nonlinear second harmonic generation technology. The basic principle is: When a laser with a sufficiently large electric field strength (generally higher than 100 kV / cm) irradiates a dielectric material, the strong electric field will affect the relative positions of atoms in the dielectric and the motion state of electrons, causing nonlinear polarization of the dielectric, which in turn causes the frequency of the incident light to change and energy to transfer. Second harmonic generation is a second-order nonlinear optical phenomenon. The incident light electric field satisfies: , in is the second-order nonlinear polarizability tensor. The polarization generated by the second-order nonlinear process will correspondingly radiate a second harmonic signal light with a frequency twice the frequency of the incident light, which is the second harmonic generation process. It should be noted that the second harmonic is only generated in materials with broken centrosymmetry. Ferroelectric thin film is a typical non-centrosymmetric material, and the intensity of its second harmonic strongly depends on the angle between the polarization direction of the incident light and the ferroelectric polarization direction. By changing the angle, the polarization-dependent SHG intensity change can be obtained, and then the ferroelectric direction in the film can be calculated.

[0008] The device for detecting the direction of thin film ferroelectricity under stress proposed by the present invention mainly consists of the following modules: Excitation light module: used for polarization initialization, regulation and stray light filtering of incident fundamental frequency light. It includes an excitation light source - a femtosecond pulse laser, which is used to generate an incident excitation light beam of suitable frequency; a polarizer, which is used to initialize the polarization of the incident light; a lens A, which is used to focus the incident laser onto the sample to be tested; an electrically controlled rotatable half-wave plate, which is used to regulate the polarization of the incident light; and a filter A, which is a bandpass filter. The corresponding central wavelength and bandpass width are selected according to the frequency of the incident light, and it is used to filter the stray frequency light generated by the optical components in the excitation light module to ensure that only the light of the incident laser frequency is irradiated on the sample.

[0009] Imaging module: includes a white light source, an imaging CCD, a push-pull beam splitter and an objective lens, which are used to realize imaging of the surface of the sample to be measured. The push-pull beam splitter and the objective lens are located on the optical path system during imaging and can be pulled away from the optical path system after observing the sample.

[0010] Pressure module: compatible with a variety of sample mounting and pressure application modules, used to apply sample stress; includes a translation stage for fine-tuning the measurement position of the sample; Signal receiving module: used for collecting the second harmonic signal of the sample. It includes filter B, which is a bandpass filter. It is a bandpass filter with the second harmonic wavelength of the sample (usually half the wavelength of the incident light) as the central wavelength, and is used to filter the incident light and only pass the second harmonic signal light; it includes lens B, which is used to convert the second harmonic generated at the focal point of the sample into parallel light; it includes an electrically controlled rotatable Glan Taylor prism, which is used to realize the selection of the polarization of the detection signal light. It includes lens C, which is used to focus the second harmonic signal on the detector. It includes a detector, which is used to complete the detection of the second harmonic signal intensity.

[0011] The present invention also discloses a method for detecting the direction of thin film ferroelectricity under stress, comprising the following steps: S1: After transferring the sample to a flexible stress-applying substrate, install it on the stress-applying device, and install the device on the translation stage in the stress module. During the installation process, pay attention to making the edge of the device as parallel as possible to the horizontal direction of the light path in the laboratory. Adjust the optical system, observe the sample to be tested through the imaging system, find the required measurement area, focus the incident light on the measurement area of ​​the sample to be tested, and focus the signal light on the detector sensing window.

[0012] S2: Use the motion controller to make the starting position of the half-wave plate fast axis and the starting position of the Glan Taylor prism optical axis parallel to the horizontal direction of the laboratory. Link the controller and the detector to make the half-wave plate and Glan Taylor prism move at an angle of (1 / 2) and Turn and detect each The SHG intensity at different angles can be obtained, and then the SHG intensity changes under different polarizations can be obtained. This process has the same effect as rotating the sample, which is equivalent to fixing the polarization of the incident light and the outgoing light. By rotating the sample at an angle and performing SHG measurements, the dependence of the SHG intensity on the ferroelectric polarization direction in the sample can be established. This operation avoids the problem of the sample being offset from the center of the optical path when the sample is not in the center of the rotating stage during rotation.

[0013] S3: Stress can be applied to the film to be tested by stretching the film or changing the shape of the sample, and the imaging module can be used to observe the morphological changes of the sample under each stress. The changes in ferroelectric direction under stress are then used to obtain the dynamic process of ferroelectric direction evolution under stress.

[0014] Data analysis method: According to the above, the device of the present invention simultaneously Rotate the polarization of the incident light and the signal light to rotate the optical path instead of using the angle Rotating the sample can build up the SHG intensity With Angle The dependencies between With Angle The formula for the dependency relationship between them is introduced as follows: For the measurement system, there is currently a known laboratory coordinate system ( ), sample coordinate system ( ), and the crystal coordinate system ( ). According to the above, the second harmonic, as a second-order nonlinear optical phenomenon, will produce the corresponding polarization intensity The incident light field is expressed in the laboratory coordinate system, where and are the electric field components along the horizontal and vertical directions of the laboratory, respectively. Since the light is incident vertically on the sample surface, The second-order nonlinear coefficient is expressed in the lattice coordinate system. It is a third-order tensor element with 27 elements. Since the two incident lights have the same frequency, ,at this time The components are simplified to 18.

[0015] Therefore, in order to describe the SHG intensity With Angle The dependence between them requires the second-order nonlinear coefficient Converting from Lattice Coordinates To the laboratory coordinate system Since the second-order nonlinear polarization coefficient tensor is usually known in the crystal coordinate system as , so it is necessary to first transform it from the crystal coordinate system to the sample coordinate system and then to the laboratory coordinate system. The transformation matrix can be expressed as: ,in To transform the lattice coordinate system to the sample coordinate system, The sample coordinate system is transformed into the laboratory coordinate system by using the transformation matrix , the second-order nonlinear coefficient in the laboratory coordinate system can be expressed as: In the laboratory coordinate system, the electric field component of the incident light can be expressed as , then the second-order nonlinear polarization generated in the film It can be expressed as: Polarization intensity of the second harmonic light radiated Then Since the polarization direction of the incident light is parallel to the polarization direction of the signal light, the SHG intensity With Angle The dependency relationship between them can be expressed as: in, and is the Fresnel factor. From the lattice coordinate system to the laboratory coordinate system, the SHG intensity under different stresses can be fitted by the above equation With Angle changes, among which The angle corresponding to the maximum value It reflects the main direction of the ferroelectric direction within the film plane to a certain extent.

[0016] In addition, the total SHG intensity can be obtained as: in, In-plane The proportion of ferroelectric polarization in the direction.

[0017] The device of the present invention can analyze the influence of stress on the ferroelectric orientation of thin films quickly, accurately and non-destructively. By utilizing nonlinear optical technology, the complexity, time consumption, destructiveness and environmental sensitivity of traditional methods are avoided, and a reliable means of detecting the ferroelectric orientation of thin films under stress is provided.

[0018] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention utilizes nonlinear optics to detect the ferroelectric orientation of thin films under stress, and provides a corresponding testing system and method, which not only overcomes the problems of high sensitivity to the environment and difficult sample preparation process in the existing measurement technology, but also solves the problem that the sample is damaged during the measurement process and cannot be further studied.

[0019] 2. The system of the present invention can overcome the requirement that the sample must be placed on a rotating table and can adapt to the application of various stress conditions. In addition, it overcomes the problem that the sample measurement position must be placed at the center of the rotating table and the inconsistent measurement position caused by rotation, reduces the sample placement requirements, and can achieve more accurate in-situ detection by rotating the optical path instead of rotating the sample.

[0020] 3. The device and method of the present invention is a highly sensitive, non-destructive means for detecting the ferroelectric orientation of a thin film under stress, which is of great significance for in-depth research on the dynamic process of the change of the ferroelectric orientation of a thin film under stress. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0022] Figure 1 It is a schematic structural diagram of a thin film ferroelectric direction detection device under stress of the present invention; Figure 2 A schematic diagram of a self-made stress-applying sample holder in Example 3 of the present invention; Figure 3 Schematic diagram of the evolution of the surface morphology of a 40 nm thick BFO film with stress in Example 3 of the present invention; Figure 4 Schematic diagram of the evolution of SHG intensity diagram of a 40 nm thick BFO film under different polarizations with stress in Example 3 of the present invention (where and represent the stress magnitudes applied in the x and y directions, respectively and ); Figure 5 (a)-(c) are , , Evolution of the second harmonic intensity of the BFO film under stress with the rotation angle, where and Represent the stress magnitudes applied in the x and y directions, respectively. The black circles are experimental data, and the red curve is the fitting result of the formula in Example 4. (d) is the evolution process of the ferroelectric polarization direction under stress obtained by the fitting result; In the figure: 1 excitation light module, 11 excitation light source, 12 polarizer, 13 lens A, 14 half wave plate, 15 filter A, 2 imaging module, 21 white light source, 22 imaging CCD, 23 beam splitter, 24 objective lens, 3 pressure module, 4 signal receiving module, 41 filter B, 42 lens B, 43 Glan Taylor prism, 44 lens C, 45 detector, 46 stress application sample holder, 47 translation stage. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0024] Example 1 Reference Figure 1 , a device for detecting the direction of thin film ferroelectricity under stress, comprising an excitation light module 1, an imaging module 2, a pressure module 3 and a signal receiving module 4; the excitation light module 1, the imaging module 2, the pressure module 3 and the signal receiving module 4 work together to achieve non-contact, high-precision detection of the direction of thin film ferroelectricity under stress; The excitation light module 1 is used to generate incident light of specific frequency and energy, and to precisely control its polarization and filter stray light to ensure that the incident light meets the requirements of thin film ferroelectric direction detection; The imaging module 2 is used to perform high-resolution imaging of the sample surface to assist in determining the measurement area and to monitor in real time the changes in the sample surface morphology under stress, which are associated with the evolution of the ferroelectric direction; The pressure module 3 includes a stress application sample holder 46, on which four displacement stages 47 are mounted. The sample is stretched by moving the displacement stages 47, thereby applying a controllable biaxial stress to the sample. The measurement position of the sample can be finely adjusted during the stress application process, and the stability of the sample position can be maintained, so as to accurately study the relationship between stress and ferroelectric direction. The signal receiving module 4 is used to efficiently and accurately collect the second harmonic signal generated by the sample under the action of stress and incident light, and the signal is the key basis for determining the ferroelectric direction.

[0025] The excitation light module 1 includes an excitation light source, a polarizer 12, a lens A13, a polarization control element and a filter A15, and the excitation light source can generate high-energy incident light.

[0026] The imaging module 2 includes an illumination light source, an imaging CCD 22 , a spectroscopic element and an objective lens 24 , and the imaging element has high resolution and high sensitivity.

[0027] The signal receiving module 4 includes a filter B41, a lens B42, a polarization selection element, a focusing element and a detection element. The detection element can accurately detect second harmonic signals of different intensities.

[0028] The excitation light source is a femtosecond pulse laser 11, which can generate laser light of high intensity and a specific frequency range.

[0029] The polarization regulating element is an electrically controlled rotatable half wave plate 14 .

[0030] The illumination light source is a white light source 21 , and the light splitting element is a push-pull beam splitter 23 .

[0031] The polarization selection element is an electrically controlled rotatable Glan Taylor prism 43 , the focusing element is a lens C44 , and the detection element is a high-performance detector 45 .

[0032] Example 2 A method for detecting the direction of thin film ferroelectricity under stress comprises the following steps: S1: The sample is transferred to a flexible stress-applying substrate, and then mounted on a stress-applying device, and the device is mounted on a translation stage (47) in a stress module, and during the mounting process, the edge of the device is parallel to the horizontal direction of the optical path in the laboratory as much as possible; S2: Adjust the optical system, observe the sample to be tested through the imaging system, find the required measurement area, focus the incident light on the measurement area of ​​the sample to be tested, and focus the signal light on the detector sensing window; S3: Use the motion controller to make the starting position of the half-wave plate fast axis and the starting position of the Glan Taylor prism optical axis parallel to the horizontal direction of the laboratory, link the controller and the detector, and make the half-wave plate and Glan Taylor prism move at an angle of (1 / 2) at the same time. and Turn and detect each The SHG intensity at different angles is obtained, and then the SHG intensity changes under different polarizations are obtained, and the dependence between the SHG intensity and the ferroelectric polarization direction in the sample is established. This operation avoids the problem of the sample offsetting the center of the optical path due to the sample position not being at the center of the rotating stage when rotating the sample; S4: Stress can be applied to the film to be tested by stretching the film or changing the shape of the sample. The imaging module is used to observe the morphological changes of the sample under each stress. The SHG intensity under each stress changes with the angle. The change of , the dynamic process of the evolution of ferroelectric direction under stress is obtained.

[0033] Example 3 Embodiments of a thin film ferroelectric direction detection device under stress tension and a method of using the same.

[0034] The sample measured in this example is a 40 nm thick BiFeO3 (BFO) film. The BFO film to be measured is transferred to a stress application sample holder 46. In this embodiment, the stress application sample holder 46 is a self-assembled stress stretching device. Four displacement stages 47 are fixed on a bracket, and the sample is stretched by moving the displacement stages 47. The actual figure is as shown in FIG. Figure 2 It should be noted that when installing the film, the bottom edge of the sample should be kept parallel to the horizontal direction of the laboratory (that is, the horizontal direction of the incident light) as much as possible.

[0035] The central wavelength of the excitation light source 11 used in this embodiment is 800 nm, the repetition frequency is 80 MHz, and the pulse width is 35 fs. The excitation light power is attenuated to 10 mW by the attenuation plate. The optical path system is adjusted to allow the 800 nm fundamental frequency light to pass through the center of the optical components such as the polarizer 12, lens A 13, half-wave plate 14, and filter A 15 with a central wavelength bandpass of 800 nm and a passband of 10 nm and then focus on the sample to be tested. The direction of the polarizer 12 is set to horizontal to ensure that the incident fundamental frequency light is horizontal linear polarized light. The half-wave plate 14 is mounted on an electric rotating seat, and its initial angle and rotation can be accurately controlled by a motion controller. The half-wave plate 14 in this embodiment is a true zero-order wave plate of 800 nm, which can avoid changes in the excitation light power during the rotation of the wave plate. The 800 nm filter A 15 can remove the stray light of other bands generated in front of the sample and only allow the incident light of 800 nm to pass. By moving the slidable beam splitter 23 and the objective lens 24, the light is allowed to pass through the center of the aperture of the objective lens 24, and the sample surface and the position of the light spot can be observed through the imaging CCD 22. By moving the sample holder, the 800 nm incident light is allowed to irradiate the sample to be measured.

[0036] The 400 nm second harmonic signal and 800 nm fundamental frequency light signal generated by the sample will be incident on the filter B 41 with a 400 nm central wavelength bandpass of 10 nm. Then the 800 nm fundamental frequency light is filtered, leaving only the 400 nm signal light, which passes through the lens B 42 and becomes horizontal light. After passing through the center of the Glan Taylor prism 43 and the lens C 44, it is focused on the photomultiplier diode detector 45. The Glan Taylor prism 43 is installed on an electric rotating seat, and its angle initialization and rotation can be controlled by a motion controller.

[0037] The initialization angles of the half wave plate 14 and the Glan Taylor prism 43 are set to be horizontal by the motion controller, and the detector and the motion controller are linked to simultaneously rotate the half wave plate 14 in the excitation light module 1 and the Glan Taylor prism 43 in the signal collection module 4 (ensuring that the rotation angle of the Glan Taylor prism 43 is The half wave plate 14 rotation angle This process is equivalent to fixing the polarization of the incident and outgoing light while changing the angle By rotating the sample, the SHG intensity under different polarizations can be obtained.

[0038] Then, by pulling the four displacement stages 47 on the sample holder, tensile stress in the horizontal and vertical directions can be applied respectively (the stress part can be changed to other devices that can apply stress. If the stress part is an automated device, it can also be linked with the detector and the motion control module). By moving the imaging module 2, the surface morphology of the sample can be observed in real time under each stress. The surface morphology of the 40 nm thick BFO film in this embodiment changes with the stress in the horizontal and vertical directions as shown in FIG. Figure 3 By linking the detector and the motion controller, the evolution of the polarization-dependent SHG intensity map with stress can be obtained, as shown in Figure 4 As shown in the figure, the direction of the arrows shows the change of ferroelectric direction under stress.

[0039] Example 4 An embodiment of detecting and analyzing the ferroelectric direction distribution through SHG intensity results with different polarizations under stress.

[0040] This embodiment is a method for analyzing the change of ferroelectric direction by using the evolution of SHG intensity in 40 nm thick BFO measured by the device in Example 3 with stress.

[0041] According to the initial polarization angles of the incident fundamental frequency light and the signal light, the measurement configuration is divided into parallel and perpendicular configurations. Among them, for the parallel configuration, the measured SHG signal intensity under different polarizations is It can be expressed by the following equation: in is the rotation angle of the Glan-Taylor prism, and The SHG intensity diagrams under different pressures in Example 3 are fitted using this equation to obtain the Fresnel factors under different stresses. With Angle Changes. Figure 5 The red curves in (a)-(c) show , , Second harmonic intensity of BFO film under stress Evolution with rotation angle. In the absence of stress, The angle corresponding to the maximum value of (( Figure 5 (a)), which reflects to some extent the main direction of the ferroelectric polarization projection in the sample without stress. By fitting, the evolution of the ferroelectric direction in the film plane with applied stress can be obtained, such as Figure 5 (d) as shown.

[0042] In addition, the total SHG intensity can be obtained as: in, In-plane To a certain extent, it reflects the proportion of ferroelectric polarization in different directions within the plane in the material.

[0043] In summary, by analyzing the SHG intensity measured under different stresses in the device of the present invention, the directional distribution and corresponding ratio of the in-plane projection of the ferroelectric polarization in the measured sample can be obtained.

[0044] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0045] In the present invention, unless otherwise clearly specified and limited, the terms "set", "install", "connect", "connect", "fix" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral one; it can be a mechanical connection, a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0046] The control method of the present invention is automatic control through a controller. The control circuit of the controller can be implemented by simple programming by technicians in this field. The provision of power is also common knowledge in this field. The present invention is mainly used to protect mechanical devices, so the present invention will no longer explain the control method and circuit connection in detail.

[0047] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A device for detecting the direction of a thin film ferroelectric under stress, characterized in that: It comprises an excitation light module (1), an imaging module (2), a pressure module (3) and a signal receiving module (4); the excitation light module (1), the imaging module (2), the pressure module (3) and the signal receiving module (4) work together to achieve non-contact, high-precision detection of the ferroelectric direction of the thin film under stress; The excitation light module (1) is used to generate incident light of specific frequency and energy, and to precisely control its polarization and filter stray light, so as to ensure that the incident light meets the requirements of thin film ferroelectric direction detection; The imaging module (2) is used to perform high-resolution imaging of the sample surface to assist in determining the measurement area and to monitor in real time the change in the sample surface morphology under stress, the change being associated with the evolution of the ferroelectric direction; The pressure module (3) comprises a stress application sample holder (46), on which four displacement stages (47) are mounted, and the sample is stretched by moving the displacement stages (47), thereby applying controllable biaxial stress to the sample, and the measurement position of the sample can be finely adjusted during the stress application process, so as to maintain the stability of the sample position, so as to accurately study the relationship between stress and ferroelectric direction; The signal receiving module (4) is used to efficiently and accurately collect the second harmonic signal generated by the sample under the action of stress and incident light, and the signal is the key basis for determining the ferroelectric direction.

2. The device for detecting the direction of a thin film ferroelectric under stress according to claim 1, characterized in that: The excitation light module (1) comprises an excitation light source, a polarizing plate (12), a lens A (13), a polarization regulating element and a filter A (15); the excitation light source is capable of generating high-energy incident light.

3. The device for detecting the direction of a thin film ferroelectric under stress according to claim 1, characterized in that: The imaging module (2) comprises an illumination light source, an imaging CCD (22), a light splitting element and an objective lens (24); the imaging element has high resolution and high sensitivity.

4. The device for detecting the direction of a thin film ferroelectric under stress according to claim 1, characterized in that: The signal receiving module (4) comprises a filter B (41), a lens B (42), a polarization selection element, a focusing element and a detection element, wherein the detection element is capable of accurately detecting second harmonic signals of different intensities.

5. The device for detecting the direction of thin film ferroelectric under stress according to claim 2, characterized in that: The excitation light source is a femtosecond pulse laser (11) and is capable of generating laser light of high intensity and a specific frequency range.

6. The device for detecting the direction of thin film ferroelectric under stress according to claim 2, characterized in that: The polarization regulating element is an electrically controlled rotatable half-wave plate (14).

7. The device for detecting the direction of a thin film ferroelectric under stress according to claim 3, characterized in that: The illumination light source is a white light source (21), and the light splitting element is a push-pull beam splitter (23).

8. The device for detecting the direction of thin film ferroelectric under stress according to claim 4, characterized in that: The polarization selection element is an electrically controlled rotatable Glan Taylor prism (43), the focusing element is a lens C (44), and the detection element is a high-performance detector (45).

9. A method for detecting the direction of a thin film ferroelectric under stress, implemented by a device for detecting the direction of a thin film ferroelectric under stress as described in any one of 1 to 8, characterized in that: The following steps are involved: S1: The sample is transferred to a flexible stress-applying substrate, and then mounted on a stress-applying device, and the device is mounted on a translation stage (47) in a stress module, and during the mounting process, the edge of the device is parallel to the horizontal direction of the optical path in the laboratory as much as possible; S2: Adjust the optical system, observe the sample to be tested through the imaging system, find the required measurement area, focus the incident light on the measurement area of ​​the sample to be tested, and focus the signal light on the detector sensing window; S3: Use the motion controller to make the starting position of the half-wave plate fast axis and the starting position of the Glan Taylor prism optical axis parallel to the horizontal direction of the laboratory, link the controller and the detector, and make the half-wave plate and Glan Taylor prism move at an angle of (1 / 2) at the same time. and Turn and detect each The SHG intensity at different angles is obtained, and then the SHG intensity changes under different polarizations are obtained, and the dependence between the SHG intensity and the ferroelectric polarization direction in the sample is established. This operation avoids the problem of the sample offsetting the center of the optical path due to the sample position not being at the center of the rotating stage when rotating the sample; S4: Stress can be applied to the film to be tested by stretching the film or changing the shape of the sample. The imaging module is used to observe the morphological changes of the sample under each stress. The SHG intensity under each stress changes with the angle. The change of , the dynamic process of the evolution of ferroelectric direction under stress is obtained.

Images