A mechanical loss measuring device for an anisotropic crystal thin film

Abstract

The application provides a mechanical loss measuring device for anisotropic crystal thin film, and relates to the field of material mechanical property testing.The mechanical loss measuring device comprises a clamping mechanism for clamping a first end of a strip-shaped sample; a laser emitting mechanism for emitting probe laser; in the case that a second end is in a stationary mode, a part of the probe laser is incident to a side edge of the strip-shaped sample close to the second end and extending in the length direction; a driving mechanism for driving the second end to vibrate in a first vibration mode, and stopping driving after the amplitude of the second end reaches the maximum, so that the second end vibrates in a first free decay mode; and a measuring mechanism for detecting the optical power of the first reflected laser at multiple time points during the first free decay vibration, obtaining multiple first electric signals, and determining the mechanical loss of the crystal thin film in the first vibration mode. The embodiment of the application realizes the measurement of the mechanical loss of the bending vibration mode of the strip-shaped sample in the width direction.

Description

Technical Field

[0001] This invention relates to the field of material mechanical property testing, and in particular to a device for measuring the mechanical loss of anisotropic crystalline thin films. Background Technology

[0002] The characterization requirements for mechanical losses differ across material systems. For traditional isotropic thin films, such as Ta₂O₅ or SiO₂ amorphous films, mechanical losses can typically be decomposed into two independent components: volumetric deformation-related losses and shear deformation-related losses. However, for novel cubic crystalline thin film materials, such as GaAs or AlGaAs crystalline films, their elastic constants exhibit significant crystal anisotropy. Based on crystal structure symmetry, their mechanical losses should be decomposed into three independent components. Therefore, experiments require obtaining at least three linearly independent vibration modes and combining the proportions of different types of elastic energy in each vibration mode to achieve decoupling analysis of the independent components of mechanical losses in crystalline thin films. However, existing mechanical loss measurement devices based on thinned silicon cantilever and optical lever detection have not yet achieved effective measurement of the bending vibration modes of strip samples in their width direction. Due to the lack of measurement degrees of freedom, accurate characterization of the mechanical losses of this type of anisotropic crystalline thin film is impossible. Summary of the Invention

[0003] In view of this, the present invention provides a device for measuring the mechanical loss of anisotropic crystalline thin films, comprising:

[0004] A clamping mechanism is used to clamp a first end of a strip-shaped sample; the strip-shaped sample includes a substrate and a crystalline thin film formed on the substrate, and a second end of the strip-shaped sample has a stationary mode and a first vibration mode of bending vibration along the width direction of the strip-shaped sample; a laser emitting mechanism is adapted to emit a probe laser, wherein, when the second end is in the stationary mode, a portion of the probe laser is incident on a side edge of the strip-shaped sample extending in the length direction near the second end; a driving mechanism is adapted to drive the second end to vibrate in the first vibration mode at the eigenfrequency of the strip-shaped sample in the first vibration mode, and to stop driving after the amplitude of the second end reaches its maximum, causing the second end to undergo a first free decay vibration; the probe laser is reflected by the second end in the first free decay vibration state and transmitted through the laser emitting mechanism to obtain a first reflected laser; a measuring mechanism is used to detect the optical power of the first reflected laser at multiple moments during the first free decay vibration to obtain multiple first electrical signals; a processing mechanism is configured to determine the mechanical loss of the crystalline thin film in the first vibration mode based on the multiple first electrical signals.

[0005] According to an embodiment of the present invention, the strip sample further has a second vibration mode of torsional vibration about an axis parallel to the length direction of the strip sample and a third vibration mode of bending vibration along the thickness direction of the substrate. For either the second or third vibration mode, the driving mechanism is further adapted to drive the second end to vibrate in the vibration mode at the intrinsic frequency of the strip sample, and to stop driving after the amplitude of the second end reaches its maximum, causing the second end to undergo a second free decay vibration; the second end in the second free decay vibration state reflects the probe laser to obtain a second reflected laser. The measuring mechanism is further adapted to detect the position of the second reflected laser incident on the measuring mechanism at multiple moments during the second free decay vibration, obtaining multiple second electrical signals, which are used to determine the mechanical loss of the crystal thin film in the vibration mode.

[0006] According to an embodiment of the present invention, the processing mechanism is configured to: determine the amplitude decay curve of the strip sample during the first free decay vibration process based on a plurality of first electrical signals, wherein the amplitude decay curve characterizes the vibration amplitude of the strip sample at multiple moments during the first free decay vibration process; determine the mechanical loss of the strip sample in the first vibration mode based on the amplitude decay curve and the eigenfrequency in the first vibration mode; and determine the mechanical loss of the crystal thin film in the first vibration mode based on the mechanical loss of the strip sample in the first vibration mode, the mechanical loss of the substrate in the first vibration mode, and the elastic ratio of the crystal thin film to the substrate. It should be noted that the mechanical loss measuring device of the present invention can also be used to measure the mechanical loss of the substrate in different modes, and the measurement methods are the same.

[0007] According to an embodiment of the present invention, for the first vibration mode, the driving mechanism is further adapted to: drive the second end with an estimated value of the eigenfrequency of the strip sample in the first vibration mode, causing the second end to vibrate in the first vibration mode and reflect the probe laser to obtain a third reflected laser; the measuring mechanism is further adapted to detect the optical power of the third reflected laser to obtain a third electrical signal. The processing mechanism is further adapted to: construct a covariance matrix and determine a noise subspace based on the third electrical signal; and determine the eigenfrequency of the strip sample in the first vibration mode based on the noise subspace.

[0008] According to an embodiment of the present invention, the above-mentioned processing mechanism constructs a covariance matrix and determines a noise subspace based on the third electrical signal using the following method: sampling the third electrical signal multiple times at a preset time interval to obtain multiple first sampled signals; extracting I data segments from the multiple first sampled signals, such that each data segment includes J first sampled signals, and two adjacent data segments have J-1 identical first sampled signals; determining an observation data matrix based on the I data segments, wherein the matrix element located in the i-th column and j-th row of the observation data matrix is ​​the j-th first sampled signal of the i-th observation data segment; I > 1, J > 1, 1 ≤ i ≤ I, 1 ≤ j ≤ J; determining the covariance matrix based on the observation data matrix; performing eigenvalue decomposition on the covariance matrix to obtain multiple eigenvectors; selecting a preset number of eigenvectors from the multiple eigenvectors in ascending order of eigenvalues; wherein the multiple eigenvectors correspond one-to-one with the multiple eigenvalues; and obtaining the noise subspace based on the subspace spanned by the preset number of eigenvectors.

[0009] According to an embodiment of the present invention, the processing mechanism determines the intrinsic frequency of the strip sample in the first vibration mode based on the noise subspace using the following method: determining a plurality of candidate frequencies within a preset frequency range; for each candidate frequency, constructing a frequency steering vector containing J elements, wherein each element is a complex number with unit amplitude, and the phase difference between two adjacent elements is determined according to the candidate frequency and a preset time interval; for each candidate frequency, calculating the inner product of the frequency steering vector corresponding to the candidate frequency with each eigenvector of the noise subspace, and taking the reciprocal of the sum of the squares of the moduli of each inner product as a pseudospectral value; determining the maximum value among the plurality of pseudospectral values, and taking the candidate frequency corresponding to the maximum value as the intrinsic frequency of the strip sample in the first vibration mode.

[0010] According to an embodiment of the present invention, for the first vibration mode, the driving mechanism is further adapted to drive the second end at multiple driving frequencies respectively, causing the second end to vibrate in the first vibration mode and reflect the probe laser to obtain multiple fourth reflected lasers. The processing mechanism is further adapted to: sample the multiple fourth reflected lasers respectively to obtain multiple second sampling signals for each of the multiple fourth reflected lasers; perform fast Fourier transform on the multiple second sampling signals of each fourth reflected laser to obtain multiple amplitude spectra of each fourth reflected laser; and determine the estimated value of the intrinsic frequency of the strip sample in the first vibration mode from all amplitude spectra of the multiple fourth reflected lasers.

[0011] According to an embodiment of the present invention, the laser emitting mechanism includes: a laser emitting unit adapted to emit a probe laser; a first reflecting component adapted to reflect the probe laser, such that a portion of the probe laser, when the second end is in a stationary mode, is incident directly onto a side edge of the strip sample extending in the length direction near the second end, to receive the initial reflected laser obtained after the probe laser is reflected by the strip sample; the laser emitting unit is further adapted to split the initial reflected laser from the first reflecting component into a first reflected laser and a monitoring laser; an imaging component adapted to image the monitoring laser to obtain the incident position of the probe laser on the strip sample; and a second reflecting component adapted to reflect the first reflected laser to the measuring mechanism.

[0012] According to an embodiment of the present invention, the first reflective component includes: a first motorized reflector and a second motorized reflector; when the strip sample is in a static mode, in response to a change in the ambient temperature of the strip sample, the processing mechanism controls the pose of the second motorized reflector and the first motorized reflector to change the transmission direction of the detection laser, so that the detection laser is incident on the side edge of the strip sample that extends in the length direction near the second end.

[0013] According to an embodiment of the present invention, the measuring mechanism is a four-quadrant detector; the second reflecting component includes a third motorized reflector, which forms a negative feedback closed loop with the four-quadrant detector and the processing mechanism, so that when the strip sample is in a static mode, the detection laser is reflected by the strip sample and incident on the center of the four-quadrant detector.

[0014] According to an embodiment of the present invention, the laser emitting unit includes: a laser for emitting an initial laser; a modulation module for adjusting the optical power of the initial laser; a beam splitting module for splitting the initial laser into a probe laser and a calibration laser; and a detection module for detecting the calibration laser, obtaining a detection result, and generating an adjustment signal using the detection result to act on the modulation module to keep the optical power of the adjusted initial laser stable; wherein the beam splitting module is further adapted to split the initial reflected laser transmitted through the first reflection component into the first reflected laser and the monitoring laser.

[0015] According to an embodiment of the present invention, the strip sample includes: a strip portion having the first end; and a protrusion extending outward from the first end along the width direction of the strip portion.

[0016] The embodiments of the present invention have the following beneficial effects: When the strip sample is stationary, a portion of the detection laser is incident on the strip sample, that is, a partial blocking method is adopted, so that the blocking area of ​​the strip sample to the detection laser changes sensitively with the vibration of the strip sample, thereby improving the detection sensitivity of the vibration displacement of the strip sample. Even a small amplitude of the strip sample can produce a significant change in optical power, which can accurately capture the slowly decaying weak vibration signal of the strip sample with ultra-low mechanical loss, and is suitable for detection scenarios with high precision and low loss crystal thin films. Attached Figure Description

[0017] The above and other objects, features and advantages of the present invention will become more apparent from the following description of embodiments of the invention with reference to the accompanying drawings, in which:

[0018] Figure 1 A schematic diagram of a mechanical loss measuring device for anisotropic crystalline thin films provided according to an embodiment of the present invention is shown.

[0019] Figure 2 A schematic diagram of a clamping mechanism for clamping a strip sample according to an embodiment of the present invention is shown.

[0020] Figure 3 The diagram shows the energy percentage of the clamping mechanism at different vibration frequencies when using strip samples from embodiments of the present invention and strip samples from the prior art.

[0021] Figure 4A A schematic diagram of a first vibration mode provided according to an embodiment of the present invention is shown.

[0022] Figure 4B A schematic diagram of a second vibration mode provided according to an embodiment of the present invention is shown.

[0023] Figure 4C A schematic diagram of a third vibration mode provided according to an embodiment of the present invention is shown.

[0024] Figure 5 A flowchart of a method for determining the mechanical loss of a strip sample in a first vibration mode according to an embodiment of the present invention is shown.

[0025] Figure 6 A flowchart of an adjustment method for a first, second, and third electric reflector according to an embodiment of the present invention is shown.

[0026] Figure 7A The illustration shows an automatic alignment without opening the optical path according to an embodiment of the present invention, in which a laser beam received by a four-quadrant detector is at the incident position of the four-quadrant detector.

[0027] Figure 7BThe illustration shows an automatic optical path alignment provided according to an embodiment of the present invention, in which a laser beam received by a four-quadrant detector is positioned at the incident position of the four-quadrant detector.

[0028] Figure 7C The image shows a spot image of the probe laser obtained by the imaging component with the optical path automatically aligned according to an embodiment of the present invention.

[0029] Figure 8 The short-time FFT spectrum of the drive signal frequency when it is updated to 92Hz according to an embodiment of the present invention is shown.

[0030] Figure 9 The results of the high-resolution spectral estimation algorithm provided according to an embodiment of the present invention and the traditional long-time FFT algorithm are shown.

[0031] Explanation of reference numerals in the attached figures

[0032] 1. Laser emitting mechanism; 2. Measuring mechanism; 3. Processing mechanism; 4. Strip sample; 5. Clamping mechanism; 11. Laser emitting unit; 12. First reflection component; 13. Imaging component; 14. Second reflection component; 111. Laser; 112. Modulation module; 113. Beam splitting module; 114. Detection module; 1121. Acousto-optic modulator; 1122. Fiber optic coupler; 1123. Direct digital frequency synthesis source; 1131. Polarization beam splitter; 1132. Quarter-wave plate; 1133. Beam splitting cube; 1141. First lens; 1142. Photodetector; 1143. Proportional-integral-differential controller; 121. First motorized reflector; 122. Second motorized reflector; 141. Third motorized reflector; 142. Reflector; 143. Second lens; 31. Lock-in amplifier; 32. Data acquisition card; 33. Host computer. Detailed Implementation

[0033] In the process of realizing this invention, it was discovered that a key step in detecting the mechanical loss of a crystal thin film is to accurately measure the overall mechanical loss of the strip sample (i.e., the substrate coated with the crystal thin film). This is because the crystal thin film itself is extremely thin, and its mechanical loss cannot be directly measured alone; it is often necessary to indirectly deduce it by comparing the mechanical losses of the substrate and the strip sample. Furthermore, traditional methods for measuring the mechanical loss of strip samples or substrates mainly rely on measuring the positional change of the reflected laser spot after reflection from the strip sample or substrate. However, for the first vibration mode of the strip sample bending along its width, the above measurement method cannot effectively respond to the weak vibration displacements in the first vibration mode. Therefore, embodiments of this invention provide a mechanical loss measurement device capable of highly sensitive detection of mechanical loss under multiple vibration modes.

[0034] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the invention. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of the invention for ease of explanation. However, it will be apparent that one or more embodiments may be practiced without these specific details. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0035] Figure 1 A schematic diagram of a mechanical loss measuring device for anisotropic crystalline thin films provided according to an embodiment of the present invention is shown.

[0036] Figure 2 A schematic diagram of a clamping mechanism for clamping a strip sample according to an embodiment of the present invention is shown.

[0037] like Figures 1-2 As shown, the mechanical loss measuring device includes: a clamping mechanism 5, a laser emitting mechanism 1, a driving mechanism, and a measuring mechanism 2.

[0038] A clamping mechanism 5 is used to clamp the first end of a strip sample 4. The strip sample 4 includes a substrate and a crystalline thin film formed on the substrate. The second end of the strip sample 4 has a stationary mode and a first vibration mode of bending vibration along the width direction of the strip sample 4. A laser emitting mechanism 1 is adapted to emit a probe laser. When the second end is in the stationary mode, a portion of the probe laser is incident on one side edge of the strip sample extending in the length direction near the second end, and a portion of the probe laser is incident outside the strip sample. A driving mechanism is adapted to drive the second end to vibrate in the first vibration mode at the eigenfrequency of the strip sample 4 in the first vibration mode, and to stop driving after the amplitude of the second end reaches its maximum, causing the second end to undergo a first free decay vibration. The driving mechanism includes, for example, a signal generator, a high-voltage amplifier, and an electrostatic driving plate. Specifically, the driving signal (e.g., an alternating signal) output by the signal generator is amplified by the high-voltage amplifier and applied to the electrostatic driving plate, causing the electrostatic driving plate to drive the second end to vibrate in the first vibration mode at the eigenfrequency of the strip sample 4 in the first vibration mode. The probe laser is reflected by the second end in the first free decay vibration state and transmitted through the laser emitting mechanism 1 to obtain a first reflected laser. The measuring mechanism 2 is adapted to detect the optical power of the first reflected laser at multiple moments under the first free decay vibration state, and obtain multiple first electrical signals; the processing mechanism 3 is configured to determine the mechanical loss of the crystal thin film under the first vibration mode based on the multiple first electrical signals.

[0039] According to an embodiment of the present invention, the strip sample 4 is placed in a vacuum thermostat to provide a stable temperature environment for the strip sample 4. When the probe laser irradiates the second end of the strip sample 4 and the second end is stationary, a portion of the probe laser (e.g., half of the probe laser beam) is incident on the side edge of the strip sample near the second end and extending along its length, causing the strip sample to partially block the probe laser (i.e., a portion of the probe laser irradiates one side of the side edge of the strip sample near the second end and extending along its length, or a portion of the probe laser spot is near the second end and covers a portion of the side edge of the strip sample near the second end and extending along its length). When the strip sample 4 enters the first free decay vibration from the first vibration mode (the vibration mode is still the first vibration mode), the change in the area of ​​the strip sample 4 blocking the probe laser carries the vibration information of the strip sample 4 in the first free decay vibration state. The change in the area of ​​the strip sample 4 blocking the probe laser in the first free decay vibration state can also be reflected in the change in the optical power of the first reflected laser. Therefore, based on the optical power of the first reflected laser at multiple moments, the mechanical loss of the strip sample in the first vibration mode can be calculated, and then the mechanical loss of the crystal thin film in the first vibration mode can be further determined.

[0040] According to an embodiment of the present invention, when the strip sample 4 is in a static state, a portion (e.g., half) of the detection laser is incident on the strip sample, i.e., a partial blocking (e.g., half blocking) method is adopted. This method makes the blocking area of ​​the strip sample 4 sensitive to changes in the detection laser intensity with the vibration of the second end, thereby improving the detection sensitivity of the vibration displacement of the strip sample 4 in the first vibration mode. That is, even a small amplitude of the strip sample 4 can produce a significant change in optical power. Accordingly, the measurement mechanism of this embodiment accurately captures the slowly decaying weak vibration signal of the ultra-low mechanical loss strip sample and obtains the mechanical loss of the strip sample based on this signal. The mechanical loss measurement device of this embodiment is adapted to detection scenarios with high precision and low loss crystal thin films.

[0041] According to embodiments of the present invention, reference continues. Figure 2 The strip sample 4 includes a strip-shaped portion 41 and a protrusion 42. The strip-shaped portion 41 has a first end. The protrusion 42 protrudes outward from the first end along the width direction of the strip-shaped portion. The first end of the strip-shaped portion 41 and the protrusion 42 form a clamping section, and the second end of the strip-shaped portion is the vibrating end of the strip sample. In the prior art, the strip sample used for measuring low-temperature mechanical loss is a thinned silicon cantilever with the same width at both the vibrating end and the clamping end. Its shape is simple, but when measuring in the first vibration mode, there is no gradient in the vibration direction (width) between the clamping end and the second vibrating end, and the free vibration energy of the vibrating end will be coupled into the clamping mechanism 5 in a large amount. Since the clamping mechanism is usually made of metal (stainless steel, oxygen-free copper, etc.), its mechanical loss is relatively large (often in...). (on the order of magnitude), which will cause the measured mechanical loss data of the strip sample to be distorted and significantly overestimated.

[0042] Figure 3 The diagram shows the energy percentage of the clamping mechanism at different vibration frequencies when using strip samples from embodiments of the present invention and strip samples from the prior art.

[0043] like Figure 3 As shown, the resonant frequency of the first-order lowest vibration mode of the strip sample in the prior art is 7060.5Hz, at which point the proportion of vibration energy leaking to the clamping mechanism reaches 9.5×10⁻⁶. -3 The resonant frequency of the first-order lowest vibration mode corresponding to the strip sample of the present invention is reduced to 2320.4Hz, and the vibration energy ratio corresponding to the clamping mechanism is only 8.8×10⁻⁶. -5 Compared to traditional strip structures, the strip sample of this invention can reduce the additional loss introduced by anchor coupling in the first-order vibration mode to 1% of the original level, reduce the measurement interference caused by the additional loss of the clamping mechanism, and after realizing this first-order vibration mode, the mechanical loss of anisotropic crystal thin films can be detected. According to embodiments of the present invention, refer to... Figure 1 The laser emitting mechanism 1 includes: a laser emitting unit 11, a first reflecting component 12, an imaging component 13, and a second reflecting component 14.

[0044] Laser emitting unit 11 is adapted to emit a probe laser. First reflecting component 12 is adapted to reflect the probe laser, such that a portion of the probe laser, with its second end in a stationary mode, is incident perpendicularly onto the longitudinally extending edge of the strip sample near its second end, to receive the initial reflected laser obtained after reflection from the strip sample. Laser emitting unit 11 is also adapted to split the initial reflected laser from first reflecting component 12 into a first reflected laser and a monitoring laser. Imaging component 13 is adapted to image the monitoring laser to obtain the incident position of the probe laser on the strip sample. Second reflecting component 14 is adapted to reflect the first reflected laser to measuring mechanism 2. Measuring mechanism 2 is, for example, a four-quadrant detector; preferably, second reflecting component 14 is adapted to direct the first reflected laser to the center of the four-quadrant detector to provide a reference zero point for detection.

[0045] Specifically, the laser emitting unit 11 includes: a laser 111, a modulation module 112, a beam splitting module 113, and a detection module 114.

[0046] Laser 111 is used to emit an initial laser beam. Modulation module 112 is used to adjust the optical power of the initial laser beam. Modulation module 112 includes fiber optic acousto-optic modulator 1121, fiber optic coupler 1122, and direct digital frequency synthesizer 1123. Fiber optic acousto-optic modulator 1121 is used for high-speed and precise control of the initial laser beam power. Laser 111 outputs laser beam through an optical fiber, and its output fiber is connected to the input fiber of fiber optic acousto-optic modulator 1121. Fiber optic coupler 1122 is used to collimate the laser beam after passing through fiber optic acousto-optic modulator 1121 and output it into free space. Direct digital frequency synthesizer 1123 outputs an amplitude-adjustable radio frequency signal, which, in conjunction with fiber optic acousto-optic modulator 1121, completes the adjustment of the initial laser beam power.

[0047] Beam splitting module 113 is suitable for splitting an initial laser beam into a probe laser and a calibration laser beam. Beam splitting module 113 includes a polarization beam splitter 1131, a quarter-wave plate 1132, and a beam splitting cube 1133. Polarization beam splitter 1131 is suitable for converting the initial laser beam into a linearly polarized state with a first polarization direction. Quarter-wave plate 1132 is suitable for converting the initial laser beam from a linearly polarized state to a circularly polarized state. Beam splitting cube 1133 is suitable for splitting the circularly polarized initial laser beam into a probe laser and a calibration laser beam. Beam splitting module 113 is also suitable for splitting the initial reflected laser beam transmitted via the first reflection component 12 into a first reflected laser beam and a monitoring laser beam. Specifically, the initial reflected laser beam is split into a first reflected laser beam and a monitoring laser beam after passing through beam splitting cube 1133. The first reflected laser beam is converted from a circularly polarized state to a linearly polarized state with a second polarization direction after passing through quarter-wave plate 1132. The first reflected laser beam from quarter-wave plate 1132 is reflected by polarization beam splitter 1131 to measurement mechanism 2.

[0048] The detection module 114 is used to detect the calibration laser, obtain the detection result, and generate an adjustment signal using the detection result. The adjustment signal acts on the modulation module 112 to keep the optical power of the adjusted initial laser stable. Furthermore, the detection module 114 includes a first lens 1141, a photodetector 1142, and a proportional-integral-differential controller 1143. The first lens 1141 focuses the calibration laser onto the photodetector 1142, the photodetector 1142 detects the calibration laser, and the proportional-integral-differential controller 1143 calculates the adjustment signal based on the deviation between the detection result of the photodetector 1142 and the target value, and outputs it to the direct digital frequency synthesizer 1123, thereby ensuring that the optical power of the initial laser remains stable near the set calibration standard value, achieving closed-loop automatic control.

[0049] Imaging component 13 may be, for example, a CCD camera, used to detect the size and shape of the spot of the first reflected laser to determine the incident position of the detection laser on the strip sample.

[0050] Based on the principle of mechanical loss measurement: Under high vacuum, the intrinsic vibration mode of the strip sample 4 is excited. After the excitation is turned off, the vibration of the strip sample 4 decays freely. The decay process of its amplitude is detected and collected, and the mechanical loss data of the strip sample can be obtained by exponentially fitting the amplitude decay curve. The processing mechanism 3 is configured to: determine the amplitude decay curve of the strip sample in the first free decay vibration state based on multiple first electrical signals. The amplitude decay curve characterizes the vibration amplitude of the strip sample at multiple moments during the first free decay vibration process. The mechanical loss of the strip sample in the first vibration mode is determined based on the amplitude decay curve and the intrinsic frequency in the first vibration mode; and the mechanical loss of the crystal film in the first vibration mode is determined based on the mechanical loss of the strip sample in the first vibration mode, the mechanical loss of the substrate in the first vibration mode, and the elastic ratio between the crystal film and the substrate. The processing mechanism 3 includes a lock-in amplifier 31, a data acquisition card 32, and a host computer 33. The lock-in amplifier 31 is used to filter and lock the first electrical signal to obtain an amplitude signal. The acquisition card 32 is used to acquire the output signal of the lock-in amplifier. The host computer 33 is used to process the acquired signal to obtain the mechanical loss of the strip sample in the first vibration mode, and to obtain the mechanical loss of the crystal film in the first vibration mode based on the mechanical loss of the strip sample in the first vibration mode.

[0051] According to an embodiment of the present invention, the amplitude decay curve is expressed as Equation (1).

[0052] (1);

[0053] in, This represents the amplitude of the first electrical signal output by the measuring mechanism at time t. The amplitude of the first electrical signal output by the measuring mechanism represents the initial moment of the first free decaying vibration. This represents the decay time of the first free decaying oscillation.

[0054] After obtaining the amplitude decay curve, the eigenfrequency of the bar sample in the first vibration mode can be used as a basis. Decline Time Calculated mechanical losses Mechanical wear It is represented by equation (2).

[0055] (2).

[0056] According to an embodiment of the present invention, in addition to the first vibration mode, the strip sample 4 also has a second vibration mode of torsional vibration about an axis parallel to the length direction of the strip sample and a third vibration mode of bending vibration along the thickness direction of the substrate.

[0057] For either the second or third vibration mode, the process for determining mechanical vibration is similar: the driving mechanism uses the intrinsic frequency of the strip sample 4 corresponding to that vibration mode to drive the second end of the strip sample 4, causing the second end of the strip sample 4 to undergo forced vibration according to that vibration mode; after the vibration amplitude of the second end of the strip sample 4 reaches its maximum value, the driving mechanism stops outputting the driving signal, and the second end of the strip sample 4 is no longer subject to external excitation, maintaining the current vibration mode and entering the second free decay vibration state. The second end in the second free decay vibration state continues to vibrate, reflecting the probe laser and forming a second reflected laser. In the second or third vibration mode, the second end will change the position of the reflected laser incident on the measuring mechanism 2. In this second free decay vibration state, the measuring mechanism 2 continuously detects the position of the second reflected laser incident on the measuring mechanism 2 at multiple different times, generating multiple second electrical signals accordingly.

[0058] In this embodiment, the method for determining the mechanical loss of the crystal thin film corresponding to the second and third vibration modes is similar to the method for determining the mechanical loss of the crystal thin film under the first vibration mode. Multiple sets of second electrical signals completely record the entire process of the position of the second reflected laser spot changing over time under the second free decay vibration state. By using the multiple second electrical signals acquired under the second or third vibration mode, the mechanical loss of the strip sample under the corresponding vibration mode can be calculated. Specifically, for the second or third vibration mode, the processing mechanism determines the mechanical loss of the strip sample under the second or third vibration mode based on multiple second electrical signals. For the second or third vibration mode, the processing mechanism determines the mechanical loss of the crystal thin film under the vibration mode based on the mechanical loss of the strip sample under that vibration mode, the mechanical loss of the substrate under that vibration mode, and the elastic ratio between the crystal thin film and the substrate. According to the embodiments of the present invention, the following is combined with Figures 4A-4C The vibration modes of the embodiments of the present invention will be described in detail.

[0059] Figure 4A A schematic diagram of a first vibration mode provided according to an embodiment of the present invention is shown.

[0060] like Figure 4A As shown, (a) represents the bending vibration of the strip sample along the width direction of the strip sample, and (b) represents the first electrical signal output by the measuring mechanism, that is, the first electrical signal output by the measuring mechanism is the change in the optical power of the first reflected laser.

[0061] Figure 4B A schematic diagram of a second vibration mode provided according to an embodiment of the present invention is shown.

[0062] like Figure 4BAs shown, (a) represents the torsional vibration of the strip sample around its length axis, and (b) represents the second electrical signal output by the measuring mechanism. The second electrical signal output by the measuring mechanism is the change of the second reflected laser along the Y direction at the incident position of the measuring mechanism, expressed as... .

[0063] Figure 4C A schematic diagram of a third vibration mode provided according to an embodiment of the present invention is shown.

[0064] like Figure 4C As shown, (a) represents the bending vibration of the strip sample along its thickness direction, and (b) represents the second electrical signal output by the measuring mechanism. The second electrical signal output by the measuring mechanism is the change of the second reflected laser along the X-direction at the incident position of the measuring mechanism, expressed as follows: .

[0065] The three vibration modes share the same set of mechanical loss measurement device. The measurement mechanism 2 detects the optical power of the first reflected laser or the incident position of the second reflected laser, thereby realizing the measurement of the mechanical loss of the strip sample under each of the three vibration modes. The measurement of mechanical loss under the three vibration modes does not interfere with each other and can be realized independently.

[0066] According to embodiments of the present invention, determining the mechanical loss of a strip sample under any vibration mode typically requires finding the intrinsic frequency of the strip sample under that vibration mode. The decay time during mechanical loss measurement in low-temperature regions is often tens of thousands of seconds, requiring the driving frequency of the drive mechanism to deviate from the intrinsic frequency of that vibration mode within the mHz range or even less to avoid off-resonance. There are generally two methods to find the intrinsic frequency: one is to calculate the intrinsic frequency by changing the driving frequency and measuring the amplitude response of the strip sample at different driving frequencies. However, due to the long relaxation time of the strip sample and the high accuracy required for frequency finding, this method is not suitable. The other method is to apply a white noise signal to the strip sample to excite its various intrinsic vibrations, and then obtain the intrinsic frequency through a Fast Fourier Transform (FFT). However, white noise signals often only excite low-order vibrations, and FFT in principle has an inverse relationship between frequency resolution and sampling time; a frequency resolution in the mHz range means a sampling time on the order of 1000s, resulting in low efficiency. Neither of the above two methods can accurately and efficiently obtain the eigenfrequency, resulting in low efficiency in measuring the mechanical loss of the strip sample. The mechanical loss measuring device of this invention employs other methods to measure the eigenfrequency of the strip sample under any vibration mode.

[0067] According to an embodiment of the present invention, for the first vibration mode, the driving mechanism is further adapted to: drive the second end with an estimated value of the intrinsic frequency of the strip sample 4 in the first vibration mode, so that the second end vibrates in the first vibration mode and reflects the probe laser to obtain a third reflected laser. The measuring mechanism 2 is further adapted to detect the optical power of the third reflected laser to obtain a third electrical signal. The processing mechanism 3 is further adapted to: construct a covariance matrix and perform eigenvalue decomposition based on the third electrical signal, and then scan the orthogonality of the frequency steering vector and the noise subspace to determine the intrinsic frequency of the strip sample in the first vibration mode.

[0068] According to an embodiment of the present invention, specifically, the processing mechanism constructs a covariance matrix and determines a noise subspace based on a third electrical signal using the following method: The third electrical signal is sampled multiple times at a preset time interval to obtain multiple first sampled signals; I data segments are extracted from the multiple first sampled signals, such that each data segment includes J first sampled signals, and adjacent data segments have J-1 identical first sampled signals; an observation data matrix is ​​determined based on the I data segments, wherein the matrix element located in the i-th column and j-th row of the observation data matrix is ​​the j-th first sampled signal of the i-th observation data segment, where I > 1, J > 1, 1 ≤ i ≤ I, 1 ≤ j ≤ J; a covariance matrix is ​​determined based on the observation data matrix; eigenvalue decomposition is performed on the covariance matrix to obtain multiple eigenvectors; a preset number of eigenvectors are selected from the multiple eigenvectors in ascending order of eigenvalues, wherein the multiple eigenvectors correspond one-to-one with the multiple eigenvalues; and a noise subspace is obtained based on the subspace spanned by the preset number of eigenvectors. Furthermore, the subspace spanned by the remaining eigenvectors can be used to obtain the signal subspace.

[0069] The processing mechanism determines the eigenfrequency of the strip sample in the first vibration mode based on the noise subspace using the following method:

[0070] Multiple candidate frequencies are determined within a preset frequency range. For each candidate frequency, a frequency steering vector containing J elements is constructed, where each element is a complex number with unit amplitude, and the phase difference between two adjacent elements is determined according to the candidate frequency and a preset time interval. For each candidate frequency, the inner product of the frequency steering vector corresponding to that candidate frequency with each eigenvector of the noise subspace is calculated, and the reciprocal of the sum of the squares of the magnitudes of each inner product is taken as the pseudospectral value. The maximum value among the multiple pseudospectral values ​​is determined, and the candidate frequency corresponding to the maximum value is taken as the eigenfrequency of the bar sample in the first vibration mode.

[0071] For the first vibration mode, the driving mechanism is also suitable for driving the second end at multiple driving frequencies respectively, causing the second end to vibrate in the first vibration mode and reflect the probe laser to obtain multiple fourth reflected lasers. The processing mechanism is also suitable for: sampling the multiple fourth reflected lasers respectively to obtain multiple second sampled signals for each of the multiple fourth reflected lasers; performing fast Fourier transform on the multiple second sampled signals of each fourth reflected laser to obtain multiple amplitude spectra of each fourth reflected laser; and determining the estimated value of the intrinsic frequency of the strip sample in the first vibration mode from all the amplitude spectra of the multiple fourth reflected lasers.

[0072] The following provides a detailed description of the process for determining the intrinsic frequency of a strip sample using the mechanical loss measuring device according to an embodiment of the present invention.

[0073] Once the temperature in the thermostat reaches the set temperature and the optical path calibration is completed (with the second end in stationary mode, a portion of the probe laser is incident on the edge of the strip sample extending along its length near the second end, and the laser reflected by the strip sample is incident on the center of the four-quadrant detector), it is necessary to find the intrinsic frequencies of the strip sample for different vibration modes, excite the corresponding intrinsic vibrations, and then turn off the excitation to allow the vibration of the strip sample to decay freely. The amplitude decay curve measured by the measuring mechanism is then analyzed. Finding the intrinsic frequencies is the most critical step. The signal generator operates at a preset frequency. Output drive signal, scan from the drive signal as the center to low frequency and high frequency, each time the frequency of the drive signal is switched Then, a short sampling time is performed to obtain the time-domain discrete sequence. , This represents the number of sampling points. The number of sampling points is then calculated using the Fast Fourier Transform (FFT) method according to equation (3). Amplitude spectrum at each frequency point Where j is the imaginary unit, Indicates frequency index.

[0074] (3);

[0075] The FFT short-time spectral analysis method can quickly obtain low-resolution FFT spectral information with short sampling times, making it suitable for the initial search of intrinsic frequencies. When a significant peak (exceeding the set threshold) is found in the vicinity, it is considered that the intrinsic frequency has been initially found.

[0076] After completing the coarse frequency localization based on the FFT short-time spectrum analysis method, this invention introduces a high-resolution spectrum estimation algorithm based on subspace decomposition to further improve the accuracy of intrinsic frequency identification while avoiding the reduction in experimental efficiency caused by long sampling times. Unlike traditional long-time FFT spectrum analysis methods, the high-resolution spectrum estimation algorithm does not rely on the sampling time length to directly improve frequency resolution. Instead, it achieves a fine analysis of the signal frequency structure by constructing statistical features of the observed signal. Specifically, the acquired vibration time series is segmented and reconstructed to form multiple sets of correlated observation data vectors, and a sample covariance matrix is ​​constructed accordingly. By performing eigenvalue decomposition on this matrix, the observed signal is decomposed into a signal subspace and a noise subspace. Since the signal components corresponding to the system's intrinsic vibration frequencies have significant energy distributions in the signal subspace, and are orthogonal to it in the noise subspace, this invention further constructs a frequency scanning model to iterate through candidate frequencies. When the scanning frequency matches the true intrinsic frequency, the orthogonality between its corresponding vector and the noise subspace reaches an extreme value, thus manifesting as a sharp peak in the spectrum function, thereby achieving high-resolution intrinsic frequency finding.

[0077] Set a preset time interval The third electrical signal is sampled N times to obtain a time-domain discrete first sampled signal of length N. The specific process of the high-resolution spectral estimation algorithm is as follows.

[0078] from Extract I data segments, each containing J first sampled signals, and the i-th data segment. This is expressed as equation (4). Where I > 1, J > 1, 1 ≤ i ≤ I, and satisfies... .

[0079] (4);

[0080] The observation data matrix is ​​defined according to equation (5). :

[0081] (5)

[0082] Calculate the covariance matrix according to equation (5). And by performing eigenvalue decomposition on the covariance matrix, we obtain equation (6).

[0083] (6).

[0084] It is an eigenvalue square matrix. This represents a square matrix of eigenvectors, ordered from largest to smallest by their corresponding eigenvalues. The feature vectors can be obtained as follows: The eigenvectors corresponding to the first K largest eigenvalues ​​span the signal subspace. The eigenvectors corresponding to the remaining JK smaller eigenvalues ​​span the noisy subspace. The signal subspace and the noise subspace are orthogonal to each other, and H represents the conjugate transpose.

[0085] Scan within a preset frequency range of interest, where the preset frequency range includes multiple candidate frequencies.

[0086] For each candidate frequency Construct frequency steering vector Where j is the imaginary unit, the pseudospectrum is calculated according to equation (7).

[0087] (7);

[0088] Because the noise subspace is orthogonal to the signal subspace, and the steering vector corresponding to the signal frequency lies within the signal subspace. When the scanning frequency... When approaching the true frequency of the signal, with the noise subspace Nearly orthogonal, the denominator is theoretically close to zero, pseudo-spectrum A spike will appear, which is the precise eigenfrequency. Exciting the sample's eigenvibration at this frequency and collecting the amplitude signal as a function of time, then fitting it to an exponential decay function, yields the ring-down time. satisfy Therefore, it can be determined by the vibration frequency. Decline Time Calculated mechanical losses .

[0089] In summary, this method can achieve high-precision estimation of intrinsic frequencies within a short sampling time, avoiding the limitation that frequency resolution is inversely proportional to sampling time in traditional long-time FFT spectrum analysis methods, and can achieve frequency resolution on the order of mHz within a second-level sampling time.

[0090] Figure 5 A flowchart illustrating a method for determining the mechanical loss of a strip sample in a first vibration mode according to an embodiment of the present invention is shown. The determination of the intrinsic frequencies of the strip sample is performed at any temperature.

[0091] like Figure 5 As shown, the determination method includes operations S501 to S510.

[0092] When operating S501, the signal generator sends a drive signal;

[0093] When operating S502, the drive signal is amplified by the high-voltage amplifier and then applied to the electrostatic drive board, causing the electrostatic drive board to drive the second end of the strip sample at the frequency of the drive signal, so that the second end vibrates in the first vibration mode.

[0094] In operation S503, the processing mechanism samples the electrical signal detected by the measuring mechanism under the drive signal with a short sampling time, and performs a fast Fourier transform on the sampling result to obtain the amplitude spectrum.

[0095] In operation S504, it is determined whether there are frequencies in the amplitude spectrum with amplitudes higher than a preset threshold.

[0096] In operation S505, if there is no frequency with an amplitude higher than the preset threshold, the frequency of the drive signal is changed, and the operation returns to S502~S504 until it is determined that there is a frequency with an amplitude higher than the preset threshold in the amplitude spectrum.

[0097] In operation S506, if there is a frequency with an amplitude higher than the preset threshold, the frequency higher than the preset threshold is used as the estimated value of the intrinsic frequency of the bar sample in the first vibration mode.

[0098] In operation S507, the frequency of the drive signal emitted by the signal generator is switched to the estimated value of the intrinsic frequency, so that the second end vibrates in the first vibration mode and reflects the probe laser to obtain the third reflected laser.

[0099] During operation of S508, the processing mechanism samples the third electrical signal corresponding to the third reflected laser for a relatively long time and executes a high-resolution spectrum estimation algorithm.

[0100] In operation S509, the frequency of the pseudo-spectral peak determined by the high-resolution spectral estimation algorithm is used as the intrinsic frequency of the bar sample in the first vibration mode.

[0101] In operation S510, the frequency of the drive signal emitted by the signal generator is switched to the intrinsic frequency. When the amplitude at the second end reaches its maximum, the signal generator is turned off, and the amplitude decay curve of the bar sample in the first free decay vibration process is collected. The amplitude decay curve is then subjected to exponential fitting to obtain the mechanical loss of the bar sample in the first vibration mode.

[0102] According to embodiments of the present invention, an optical thin film with ultra-high reflectivity is a thin film capable of reflecting light of a specific wavelength band with extremely high reflectivity (above 99.999%), and it is currently a core component of various precision measurement systems. The most advanced measurement systems, such as laser interferometry gravitational wave detectors and ultra-stable optical atomic clocks, require operation in low-temperature environments to ensure low thermal noise. For example, ultra-stable optical atomic clocks based on single-crystal silicon cavities operate at 4K, 16K, or 124K, while the next-generation gravitational wave detector, the Einstein Telescope, operates at 10K or 20K. Therefore, conducting thin film mechanical loss measurements at different temperatures in the low-temperature region is of great significance. To address the need for thin film mechanical loss measurements at both room temperature and low temperatures, related research has developed testing techniques based on thinned silicon cantilever, nodal support methods, and dual-propeller oscillators. However, existing mechanical loss measurement devices can only perform mechanical loss measurements under a single constant temperature condition. If the temperature inside the thermostat is changed by adjusting the power of the heater, the strip sample and the clamping mechanism will undergo slight deformation due to thermal expansion and contraction. This will cause the probe laser's optical path to shift, resulting in inaccurate spot position. Each temperature adjustment requires manual recalibration of the optical path, making fully automated continuous temperature-changing measurements impossible and leading to low testing efficiency. To improve experimental efficiency, automatic optical path correction is needed during temperature changes, and continuous measurement of the mechanical loss of the crystal thin film should be conducted at different temperature points.

[0103] According to an embodiment of the present invention, the first reflective component includes a first motorized reflector and a second motorized reflector. When the strip sample is in a static mode, in response to a change in the ambient temperature of the strip sample, the processing mechanism controls the pose of the first and second motorized reflectors to change. The second motorized reflector cooperates with the first motorized reflector to change the transmission direction of the detection laser, so that the detection laser is incident on the edge of the strip sample extending in the length direction near the second end.

[0104] The measuring mechanism is a four-quadrant detector. The second reflection assembly includes a third motorized mirror, which forms a negative feedback closed loop with the four-quadrant detector and the processing mechanism. When the strip sample is in a static mode, the probe laser is reflected by the strip sample and then incident on the center of the four-quadrant detector. The second reflection assembly also includes a mirror and a second lens disposed between the third motorized mirror and the four-quadrant detector. The mirror is used to further reflect the first reflected laser light reflected by the third motorized mirror, and the second lens is used to focus the first reflected laser light onto the four-quadrant detector.

[0105] When the temperature changes, the optical path of the first reflected laser will also drift. Adjusting the position of the third motorized reflector ensures that the first reflected laser always strikes the center of the four-quadrant detector. It should be noted that... Figure 5The flowchart shown is also applicable to determining the mechanical loss of the crystal thin film under the second or third vibration mode. Additionally, the flowchart also applies to the measurement process of the substrate's mechanical loss under any vibration mode.

[0106] Figure 6 A flowchart of an adjustment method for a first, second, and third electric reflector according to an embodiment of the present invention is shown.

[0107] The following combination Figure 1 and Figure 6 The adjustment process of the first, second, and third motorized reflectors under temperature changes during the measurement of mechanical losses is described in detail. It should be noted that the adjustment of the first motorized reflector, specifically its pitch angle, is crucial. The rotation angle of the first electric reflector about the vertical direction Adjustments are then made. Similarly, the second motorized reflector is adjusted, specifically its pitch angle. The rotation angle of the second electric reflector about the vertical direction Adjustments are made to the third motorized reflector, specifically its pitch angle. The rotation angle of the third electric reflector about the vertical direction Adjustments are required. The specific adjustment steps are as follows.

[0108] In operation S601, the strip sample is brought to a standstill. The first and second motorized reflectors are manually adjusted so that the spot of the probe laser illuminates the second end of the strip sample.

[0109] In operation S602, the position of the probe laser after reflection from the strip sample and incident on the four-quadrant detector is established. 3. The negative feedback between 3 ensures that the probe laser, after being reflected by the strip sample, always strikes the center of the four-quadrant detector.

[0110] Next, the laser spot is centered. The spot of the probe laser is adjusted to the midpoint between the two edges extending along the length of the strip sample. This adjustment process includes operations S603 to S606.

[0111] When operating S603, adjust Record the SUM value. The optical power detected by the four-quadrant detector, i.e., the SUM value, varies with... Change. In During the process of change, it was found that when Increase to The SUM value at that time relative to The SUM value decreases significantly when the initial value is used. Reduce to The SUM value at that time relative to The SUM value is significantly reduced when the initial value is used.

[0112] When operating S604, adjust Record the SUM value. The optical power detected by the four-quadrant detector, i.e., the SUM value, varies with... The changes. In During the process of change, it was found that when Increase to SUM value relative to The SUM value decreases significantly when the initial value is used. Reduce to SUM value relative to The SUM value is significantly reduced when the initial value is used.

[0113] When operating S605, set , This ensures that the spot of the probe laser is positioned at the midpoint between the two edges extending along the length of the strip sample.

[0114] Disconnect when operating S606. 3、 Negative feedback between 3.

[0115] Next, the normal incident angle of the probe laser is determined. This process includes operations S607 to S609.

[0116] When operating S607, make exist ~ Scan between, each time it changes Simultaneously adjust the pitch angle of the second motorized reflector. This ensures that the position of the probe laser, after being reflected by the strip sample and incident on the four-quadrant detector, returns to the center of the four-quadrant detector. The SUM value at this point is recorded, i.e., SUM( ), and based on all recorded SUM ( )Sure , SUM (time) () is a maximum value.

[0117] When operating S608, make exist ~ Scan between, each time it changes Simultaneously adjust the pitch angle of the second motorized reflector. This ensures that the probe laser, after being reflected from the strip sample, returns to the center of the four-quadrant detector, and the SUM value at this point is recorded. And based on all recorded SUM Sure , SUM It is a maximum value.

[0118] When operating S609, set , , , .

[0119] The following operations S610 to S611 are to move the spot of the probe laser to a semi-obstructed position, that is, half of the probe laser is incident on the strip sample.

[0120] During operation S610, the second establishment of the position where the detection laser, after being reflected from the strip sample, enters the four-quadrant detector and... 3、 The negative feedback between the three ensures that the probe laser, after being reflected by the strip sample, always strikes the center of the four-quadrant detector.

[0121] During operation S611, adjustment is performed at the same rate and in opposite directions. and While keeping the incident angle of the probe laser constant, the position of the light spot on the strip sample is moved until the SUM value is reduced by half.

[0122] In operation S612, the intrinsic frequencies of the strip sample under each vibration mode are found, and the mechanical losses of the strip sample under each vibration mode are determined.

[0123] By operating S613 and changing the temperature of the thermostat, the position of the probe laser after reflection from the strip sample and incident on the four-quadrant detector was established for the third time. 3、 The negative feedback between points 3 ensures that the probe laser, after being reflected by the strip sample, always strikes the center of the four-quadrant detector. Then, operations S603~S612 are executed to obtain the mechanical loss of the strip sample under each vibration mode after temperature changes. Figure 6 The adjustment method (i.e., optical path alignment procedure) for the first, second, and third motorized mirrors is applicable to any of the three vibration modes: the first vibration mode, the second vibration mode, and the third vibration mode. Additionally... Figure 6 The automatic optical path alignment process is also applicable to the alignment of the optical path during the measurement of mechanical loss of the substrate.

[0124] To make the objectives, technical solutions, and advantages of this invention clearer, the following example illustrates the anisotropic mechanical loss measurement and analysis of a thinned silicon cantilever sample (a 90μm thick single-crystal silicon cantilever with a 500nm thick GaAs crystal film transferred to the substrate) at a temperature of 4K. The crystal coordinate systems of the crystal film and the substrate are consistent, with the thickness along the

[001] crystal direction, the length along the

[110] crystal direction, and the width along the [-110] crystal direction. The invention will be further described in detail with reference to the accompanying drawings.

[0125] First, the function and implementation process of the automatic optical path alignment system are explained: The experiment uses an uncoated thinned silicon cantilever as a substrate for testing. It is installed in a low-temperature thermostat. Before cooling, the optical path is pre-aligned. Then, the cooling equipment is started, and the low-temperature thermostat is gradually cooled to 4K.

[0126] Figure 7A The illustration shows an automatic alignment without opening the optical path according to an embodiment of the present invention, in which a laser beam received by a four-quadrant detector is at the incident position of the four-quadrant detector.

[0127] Figure 7B The illustration shows an automatic optical path alignment provided according to an embodiment of the present invention, in which a laser beam received by a four-quadrant detector is positioned at the incident position of the four-quadrant detector.

[0128] Figure 7C The image shows a spot image of the probe laser obtained by the imaging component with the optical path automatically aligned according to an embodiment of the present invention.

[0129] like Figure 7A As shown, without automatic optical path alignment, the beam received by the quadrant detector deviates from its center. Simultaneously, the detection laser is not incident orthogonally on the silicon cantilever, resulting in a loss of optical power in the reflected laser received by the quadrant detector, and the optical power value is also lower. If automated measurement is initiated, the system first aligns the third motorized reflector with the quadrant detector during the cooling process to avoid losing the spot position signal due to excessive offset. Once the thermostat temperature drops to 4K, automatic optical path alignment is performed. The laser beam received by the quadrant detector at its incident position is as follows: Figure 7B As shown. When the temperature of the thermostat drops to 4K, automatic optical path alignment is performed, and the spot image of the probe laser in the imaging component becomes semi-circular, as shown. Figure 7C As shown, a semi-occluded state was achieved where half of the probe laser spot was incident on the strip sample and the other half was located outside the strip sample. This completed the automatic alignment of the optical path, allowing the process to proceed to the intrinsic frequency search and mechanical loss measurement stages.

[0130] First, the laser spot of the probe is centered on the strip sample, and then the normal incident angle with the highest power is found. The process of high-speed, high-precision finding of the intrinsic frequency is then explained, taking a 4K temperature as an example: After the optical path is aligned, the signal generator produces a drive signal to drive the strip sample to vibrate. First, the first order of the third vibration mode is measured, and the readings are... Signal. Based on the finite element simulation results, 90Hz is set as the preset value of the intrinsic frequency, and it is used as the driving signal frequency. The frequency is then symmetrically changed towards lower and higher frequencies, with a step size of 1Hz. After each change in the driving signal, a 1s vibration signal is acquired and a short-time FFT is performed to obtain the spectrum.

[0131] Figure 8 The short-time FFT spectrum of the drive signal frequency when it is updated to 92Hz according to an embodiment of the present invention is shown.

[0132] like Figure 8 As shown, when the driving signal frequency is updated to 92Hz, a signal peak with a driving frequency of 92Hz and a signal peak with an intrinsic frequency of 100Hz are observed in the short-time FFT spectrum. At this time, the intrinsic frequency is roughly found.

[0133] A comparison between the high-resolution spectral estimation algorithm of this invention and the traditional long-time FFT algorithm is shown in the figure. Figure 9 See Table 1.

[0134] The electrical signal output by the measuring mechanism was sampled for 2.5 seconds at a sampling rate of 5120 samples / s. Using a high-resolution spectrum estimation algorithm, with K=600 and J=2048, the intrinsic frequency was calculated to be 100.367Hz, and the calculation time was 11.6s. In comparison, under the same conditions, sampling was performed for 1000 seconds at the same 5120 samples / s sampling rate, and the intrinsic frequency was calculated using a traditional long-time FFT algorithm, yielding an intrinsic frequency of 100.367Hz in less than 1 second. Compared to the traditional long-time FFT algorithm, the high-resolution spectrum estimation algorithm used in this invention reduces the total time for data acquisition and calculation to 1% of the traditional long-time FFT algorithm in the intrinsic frequency search stage for mechanical loss measurement, thus improving efficiency.

[0135] Table 1

[0136]

[0137] Figure 9 The results of the high-resolution spectral estimation algorithm provided according to an embodiment of the present invention and the traditional long-time FFT algorithm are shown.

[0138] like Figure 9As shown, a comparison of the processing results of the high-resolution spectrum estimation algorithm used in this embodiment of the invention and the traditional long-time FFT algorithm shows that both methods achieve a frequency resolution of 1 mHz, and the frequencies corresponding to their respective spectral peaks are consistent. This verifies that the high-resolution spectrum estimation algorithm used in this invention can obtain equivalent frequency estimation accuracy under conditions that are much shorter than the sampling time required by the traditional FFT.

[0139] Finally, the implementation process of multimodal (exciting and measuring multiple vibration modes of a strip sample by switching the intrinsic frequencies corresponding to different vibration modes to obtain the mechanical loss of anisotropic crystalline thin films in different directions) is explained: The drive signal of the signal generator is switched to the intrinsic frequency corresponding to any vibration mode obtained by the high-resolution spectrum estimation algorithm, the intrinsic vibration of the strip sample is excited, and the excitation is turned off when the amplitude is at its maximum. The amplitude decay curve is then acquired to obtain the signal. The mechanical loss is calculated according to equations (1) to (2). .

[0140] For the measurement of the third vibration mode of bending vibration along the thickness direction of the substrate, read... Signal obtained The decay time and mechanical loss are obtained by performing an exponential decay fitting. For the measurement of the second vibration mode of torsional vibration about an axis parallel to the length direction of the strip sample, the readings are... Signal obtained The ringing time and mechanical loss are obtained by performing exponential decay fitting on the vibration; for the measurement of the first vibration mode of bending vibration along the width direction, the SUM signal value is read to obtain... The ring-down time and mechanical loss were obtained by performing an exponential decay fitting. The results obtained above represent the mechanical loss of the strip sample coated with the crystalline thin film. Then, the 500nm GaAs layer was removed by wet etching and cleaning, and the test was repeated to obtain the mechanical loss of the substrate itself. The statistical results are shown in Table 2.

[0141] Table 2

[0142]

[0143] For cubic crystal thin films, their mechanical losses should be decomposed into three independent components according to the symmetry of the crystal thin film, namely: Mechanical loss components of symmetrical volume deformation Symmetric tetragonal distortion mechanical loss components and The mechanical loss components of symmetrical shear distortion are denoted as follows: , and The elastic properties of the crystalline thin film and the substrate were calculated using the finite element method, and the elastic property ratios are shown in Table 3.

[0144] Table 3

[0145]

[0146] in It is the elastic property of the substrate. It is the total elastic property of a crystalline thin film (or simply a thin film). It is the symmetric volumetric deformation component of the elastic energy of the crystalline thin film. It is the tetragonal distortion component of the elastic properties of the crystalline thin film. It is the symmetric shear distortion component of the elastic properties of the crystalline thin film.

[0147] In the third vibration mode, equations (8) to (9) are obtained by measurement and calculation.

[0148] (8);

[0149] (9).

[0150] In the second vibration mode, equations (10) to (11) are obtained by measurement and calculation.

[0151] (10);

[0152] (11).

[0153] In the first vibration mode, equations (12) to (13) are obtained by measurement and calculation.

[0154] (12);

[0155] (13).

[0156] Among them, equations (8), (10), and (12) are in is established under the conditions, This indicates the mechanical loss of the crystalline thin film corresponding to the first vibration mode. This indicates the mechanical loss of the crystalline thin film corresponding to the second vibration mode. This indicates the mechanical loss of the crystalline thin film corresponding to the third vibration mode. This represents the mechanical loss of the uncoated strip sample under the first vibration mode. This indicates the mechanical loss of the uncoated strip sample under the second vibration mode. This represents the mechanical loss of the uncoated strip sample under the third vibration mode. This indicates the mechanical loss of the coated strip sample under the first vibration mode. This indicates the mechanical loss of the coated strip sample under the second vibration mode. This indicates the mechanical loss of the coated strip sample in the third vibration mode.

[0157] The anisotropic mechanical loss components of a 500 nm GaAs crystal thin film are obtained by solving equations (9), (11) and (13) simultaneously, as shown in Table 4.

[0158] Table 4

[0159]

[0160] This invention employs three sets of motorized mirror frames and a four-quadrant detector to form a closed-loop feedback control system, achieving automatic alignment of the optical path during temperature variations. It can automatically correct the incident direction of the probe laser and the position of the probe laser after reflection from the strip sample onto the four-quadrant detector, even when temperature changes cause thermal expansion and contraction of the sample and sample stage, and optical path deflection. This improves the continuity, stability, and automation level of temperature-dependent mechanical loss measurement.

[0161] This invention proposes a hierarchical frequency search strategy combining short-time FFT with a high-resolution spectral estimation algorithm, which can achieve high-precision intrinsic frequency finding within a short sampling time. Compared with traditional FFT methods that rely on long sampling times to improve frequency resolution, this invention can achieve frequency identification accuracy on the order of mHz within a second-level sampling time.

[0162] This invention achieves the identification and measurement of three independent vibration modes—a first vibration mode, a second vibration mode, and a third vibration mode—by setting the position of the probe laser spot on the sample and utilizing different output channels of a four-quadrant detector. Furthermore, the measurement of all three independent vibration modes can be completed using the same probe laser path structure, without requiring changes to the probe device, adjustments to the probe principle, or rearrangement of the optical path for different modes; only the excitation frequency needs to be switched to the intrinsic frequency of the corresponding mode.

[0163] This invention provides three independent modal measurement results, thus satisfying the decoupling requirement for the three independent components of mechanical loss in anisotropic crystalline thin films. Compared to existing technologies that typically only obtain information in two independent modes and struggle to fully characterize the mechanical loss of anisotropic crystalline thin films, this invention enables more complete and accurate measurement and analysis of the mechanical loss of anisotropic crystalline thin film materials, and is particularly suitable for the study of mechanical loss in cubic crystalline materials such as GaAs, AlGaAs, and silicon.

[0164] The mechanical loss measurement device of this invention realizes full automation of multi-mode mechanical loss measurement under variable temperature conditions. This method and device have significant advantages in improving experimental efficiency, reducing the complexity of manual operation, enhancing test stability, and improving the mechanical loss characterization capability of anisotropic crystalline thin films. It can achieve unattended, fully automated variable temperature mechanical loss measurement after sample loading.

[0165] The embodiments of the present invention have been described above. However, these embodiments are merely illustrative and not intended to limit the scope of the invention. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. The scope of the invention is defined by the appended claims and their equivalents. Various substitutions and modifications can be made by those skilled in the art without departing from the scope of the invention, and all such substitutions and modifications should fall within the scope of the invention.

Claims

1. A device for measuring the mechanical loss of anisotropic crystalline thin films, characterized in that, include: A clamping mechanism for clamping a first end of a strip sample; the strip sample includes a substrate and a crystalline thin film formed on the substrate, and a second end of the strip sample has a stationary mode and a first vibration mode of bending vibration along the width direction of the strip sample; A laser emitting mechanism is suitable for emitting a probe laser, wherein, when the second end is in a stationary mode, a portion of the probe laser is incident on one side edge of the strip sample that extends in the length direction near the second end; The driving mechanism is adapted to drive the second end of the strip sample to vibrate in the first vibration mode at the intrinsic frequency of the first vibration mode, and to stop driving after the amplitude of the second end reaches the maximum, so that the second end undergoes a first free decay vibration; the detection laser is reflected by the second end in the first free decay vibration state and transmitted through the laser emitting mechanism to obtain a first reflected laser. The measuring mechanism detects the optical power of the first reflected laser at multiple moments during the first free decay vibration process, and obtains multiple first electrical signals; The processing mechanism is configured to determine the mechanical loss of the crystal thin film in the first vibration mode based on a plurality of first electrical signals.

2. The mechanical loss measuring device according to claim 1, characterized in that, The strip sample also has a second vibration mode of torsional vibration about an axis parallel to the length direction of the strip sample and a third vibration mode of bending vibration along the thickness direction of the substrate. For any of the second and third vibration modes, The driving mechanism is also adapted to drive the second end to vibrate in the vibration mode at the intrinsic frequency of the strip sample, and to stop driving after the amplitude of the second end reaches its maximum, so that the second end undergoes a second free decay vibration; the second end in the second free decay vibration state reflects the detection laser to obtain a second reflected laser. The measuring mechanism is also adapted to detect the position of the second reflected laser incident on the measuring mechanism at multiple moments during the second free decay vibration process, and obtain multiple second electrical signals, wherein the multiple second electrical signals are used to determine the mechanical loss of the crystal thin film in the vibration mode.

3. The mechanical loss measuring device according to claim 1, characterized in that, The processing unit is configured as follows: The amplitude decay curve of the strip sample during the first free decay vibration process is determined based on multiple first electrical signals. The amplitude decay curve characterizes the vibration amplitude of the strip sample at multiple moments during the first free decay vibration process. The mechanical loss of the strip sample under the first vibration mode is determined based on the amplitude decay curve and the eigenfrequency under the first vibration mode. The mechanical loss of the crystal film in the first vibration mode is determined based on the mechanical loss of the strip sample in the first vibration mode, the mechanical loss of the substrate in the first vibration mode, and the elastic ratio of the crystal film to the substrate.

4. The mechanical loss measuring device according to claim 3, characterized in that, For the first vibration mode, The driving mechanism is also adapted to: drive the second end with an estimated value of the intrinsic frequency of the strip sample in the first vibration mode, so that the second end vibrates in the first vibration mode and reflects the probe laser to obtain a third reflected laser; The measuring mechanism is also suitable for detecting the optical power of the third reflected laser to obtain a third electrical signal; The processing mechanism is also suitable for: Based on the third electrical signal, construct the covariance matrix and determine the noise subspace; The intrinsic frequencies of the strip sample in the first vibration mode are determined based on the noise subspace.

5. The mechanical loss measuring device according to claim 4, characterized in that, The processing mechanism constructs a covariance matrix and determines the noise subspace based on the third electrical signal using the following method: The third electrical signal is sampled multiple times at a preset time interval to obtain multiple first sampled signals; I data segments are extracted from multiple first sampled signals, such that each data segment includes J first sampled signals, and two adjacent data segments have J-1 identical first sampled signals; An observation data matrix is ​​determined based on I data segments, wherein the matrix element located in the i-th column and j-th row of the observation data matrix is ​​the j-th first sampled signal of the i-th observation data segment; I > 1, J > 1, 1 ≤ i ≤ I, 1 ≤ j ≤ J; The covariance matrix is ​​determined based on the observed data matrix. The covariance matrix is ​​then decomposed into multiple eigenvectors. A predetermined number of eigenvectors are selected from the multiple eigenvectors in ascending order of eigenvalues. Each of the multiple eigenvectors corresponds one-to-one with a single eigenvalue. The noise subspace is obtained by spanning a subspace of a preset number of feature vectors.

6. The mechanical loss measuring device according to claim 5, characterized in that, The processing mechanism determines the intrinsic frequencies of the strip sample in the first vibration mode based on the noise subspace using the following method: Multiple candidate frequencies are determined within a preset frequency range; For each candidate frequency, a frequency steering vector containing J elements is constructed, where each element is a complex number with unit amplitude, and the phase difference between two adjacent elements is determined according to the candidate frequency and a preset time interval. For each candidate frequency, the inner product of the frequency steering vector corresponding to the candidate frequency and each feature vector of the noise subspace is calculated, and the reciprocal of the sum of the squares of the magnitudes of each inner product is used as the pseudospectral value. The maximum value among multiple pseudo-spectral values ​​is determined, and the candidate frequency corresponding to the maximum value is taken as the intrinsic frequency of the bar sample in the first vibration mode.

7. The mechanical loss measuring device according to claim 5, characterized in that, For the first vibration mode, The driving mechanism is also suitable for driving the second end at multiple driving frequencies respectively, causing the second end to vibrate in the first vibration mode and reflect the detection laser to obtain multiple fourth reflected lasers; The processing mechanism is also suitable for: Multiple fourth-reflection lasers are sampled separately to obtain multiple second-sample signals for each of the multiple fourth-reflection lasers; Fast Fourier transform is performed on the multiple second sampling signals of each fourth reflected laser to obtain multiple amplitude spectra of each fourth reflected laser. Estimates of the intrinsic frequencies of the strip sample in the first vibration mode were determined across all amplitude spectra of multiple fourth-reflection lasers.

8. The mechanical loss measuring device according to claim 3, characterized in that, The laser emitting mechanism includes: Laser emitting unit, suitable for emitting detection laser; The first reflecting component is adapted to reflect the detection laser, such that a portion of the detection laser, while the second end is in a stationary mode, is incident directly onto the side edge of the strip sample extending in the length direction near the second end, to receive the initial reflected laser obtained after the detection laser is reflected by the strip sample; the laser emitting unit is further adapted to split the initial reflected laser from the first reflecting component into a first reflected laser and a monitoring laser. An imaging component is adapted to image the monitoring laser to obtain the incident position of the detection laser on the strip sample; A second reflective component is adapted to reflect the first reflected laser to the measuring mechanism.

9. The mechanical loss measuring device according to claim 8, characterized in that, The first reflective component includes: First and second motorized reflectors; when the strip sample is in a static mode, in response to a change in the ambient temperature of the strip sample, the processing mechanism controls the pose of the second and first motorized reflectors to change the transmission direction of the detection laser, so that the detection laser is incident on the side edge of the strip sample that extends in the length direction near the second end.

10. The mechanical loss measuring device according to claim 8, characterized in that, The measuring mechanism is a four-quadrant detector; The second reflective component includes: The third electric reflector forms a negative feedback closed loop with the quadrant detector and the processing mechanism. When the strip sample is in a static mode, the detection laser is reflected by the strip sample and then incident on the center of the quadrant detector.

11. The mechanical loss measuring device according to claim 8, characterized in that, The laser emitting unit includes: Laser, suitable for emitting initial laser light; A modulation module is used to adjust the optical power of the initial laser. A beam splitting module is used to split the initial laser beam into the probe laser and the calibration laser; The detection module is used to detect the calibration laser, obtain the detection result, and use the detection result to generate an adjustment signal to act on the modulation module so that the optical power of the adjusted initial laser remains stable. The beam splitting module is also adapted to split the initial reflected laser transmitted through the first reflective component into the first reflected laser and the monitoring laser.

12. The mechanical loss measuring device according to claim 1, characterized in that, The strip sample includes: The strip-shaped portion has the first end; The protrusion extends outward from the first end along the width direction of the strip portion.

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