A longitudinally transversely acoustic wave adjustable lithium niobate piezoelectric coating, a preparation method and application thereof

Abstract

The application discloses a longitudinally-transversely adjustable lithium niobate piezoelectric coating and a preparation method and application thereof, and belongs to the technical field of piezoelectric active materials. Before lithium niobate film deposition, substrate cleaning, target surface cleaning treatment and substrate surface ion beam etching treatment are carried out, so that double cleaning of the target and the substrate surface can be realized, the effect of the bias voltage is enhanced, and the combination between the coating and the substrate is greatly improved. Then, a piezoelectric film with uniform morphology structure, ideal O / Nb atomic ratio in the surface and depth directions and high LiNbO3 purity is prepared by applying a bias voltage between the substrate and the target, and the regulation effect of the bias voltage on the ultrasonic signal amplitude is obtained. The application also regulates the thickness of the coating and the tilt angle of the columnar crystal to regulate the longitudinal wave and transverse wave signal amplitudes. High-temperature annealing treatment is carried out on the prepared film to improve the crystallinity and piezoelectric performance of the film.

Description

Technical Field

[0001] This invention relates to the field of piezoelectric active materials technology, and in particular to a lithium niobate piezoelectric coating with tunable longitudinal and transverse acoustic waves, its preparation method, and its application. Background Technology

[0002] Lithium niobate (LiNbO3) is a well-known perovskite material with room-temperature ferroelectric properties and a high ferroelectric transition temperature (~1483 K). It also exhibits high polarization, approximately 70 μC·cm⁻¹. -2 Furthermore, it possesses a high Curie temperature (1140–1210 °C), exhibiting ferroelectric properties even at room temperature. Due to these superior ferroelectric properties, lithium niobate has potential applications in electro-optic, piezoelectric, pyroelectric, and nonlinear optical devices, such as being used to fabricate piezoelectric micromechanical ultrasonic transducers (PMUTs), transverse wave resonators (YBARs), and SAW temperature sensors. Moreover, ferroelectric materials typically exhibit excellent piezoelectric and electrical properties due to their strong electrostatic dipole motion.

[0003] Although an increasing number of teams are currently dedicated to improving lithium niobate thin film deposition technology and enhancing quality, many research areas remain unexplored. For example, the problem of stoichiometric imbalance caused by Li escape, and the issue of Nb... 5+ -O 2- The bond is larger than Li + -O 2- The bonds are much stronger, therefore LiNbO3 crystals have a severe tendency to be lithium-deficient, such as forming the lithium-deficient phase LiNb3O8. Although lithium-rich targets and lithium diffusion barriers can better control stoichiometry, some scholars have proposed that reducing the O2 content is beneficial to weakening the lithium-deficient phase LiNb3O8, and substrate bias voltage can also effectively transport Li ions from the target substrate to improve film quality.

[0004] Chinese patent CN116732484A discloses a method for preparing a homogeneous lithium niobate piezoelectric coating, the piezoelectric coating itself, and its applications. The coating is a single-layer homogeneous lithium niobate piezoelectric coating. To address the issues of solid particles and elemental segregation, this invention designs a particle flow control device. By setting a controllable and adjustable electric field between the substrate and the target (the substrate potential is lower than the target potential, with a potential difference between 2 and 25V), the direction of particle flow is controlled, allowing Ar... +Bombarding solid particles reduces the formation of wedge-shaped structures while accelerating the movement of gaseous particles to the substrate surface, preventing Li escape. This invention is based on ion flow energy and quantity control, is simple and efficient, requires no additional steps or costs, and is suitable for industrial mass production. However, the above optimization schemes only show partial optimization results, are limited to crystal structure, and only superficially obtain the optimization effect of bias voltage on surface elements, morphology, and structure, obtaining only ultrasonic signals under a single sample preparation parameter. For specific coating morphology, elemental distribution in the depth direction, stoichiometry, LiNbO3 purity, piezoelectric signal, longitudinal and transverse wave amplitudes, and piezoelectric constant d... 33 The lack of corresponding guidance on control methods and optimization techniques limits the application of the process. Summary of the Invention

[0005] In view of the above-mentioned deficiencies of the prior art, in a first aspect of the present invention, a simple and controllable method for preparing a lithium niobate piezoelectric coating is provided, employing a magnetron sputtering process, comprising the following steps:

[0006] (1) Coating preparation: Clean the surface and back of the substrate, and then place the substrate and sputtering target in a vacuum environment for later use;

[0007] (2) Target surface cleaning treatment: Adjust the relative position of the substrate and the sputtering target so that the substrate faces away from the sputtering target surface, and use radio frequency magnetron sputtering to clean the surface of the sputtering target for later use;

[0008] (3) Substrate surface ion beam etching treatment: Argon ion beam is used to bombard the substrate to etch its surface, remove impurities and obtain a clean surface for later use;

[0009] (4) Lithium niobate coating deposition: Adjust the relative position of the substrate and the sputtering target so that the substrate faces the surface of the sputtering target, and perform coating deposition by magnetron sputtering to obtain a lithium niobate coating for later use;

[0010] (5) Heat treatment: The lithium niobate coating is annealed to obtain a lithium niobate piezoelectric coating.

[0011] The above preparation method has no special requirements on the material and specifications of the substrate. Commonly used commercial products in the field can be used, or appropriate types can be used according to actual application needs. For example, bolts, stainless steel, silicon wafers, cemented carbide, high-temperature alloys, and other materials can all be used as substrates for this invention, improving the applicability of the process.

[0012] To enhance the bias voltage effect, preferably, the substrate, sample holder, and rotating support used for magnetron sputtering are polished. The specific operation is as follows: First, ultrasonically clean with organic solvent for 3-6 minutes to remove oil and visible particles from the surface and back of the substrate; then polish the back of the substrate, sample holder, and rotating support with 1500-2500 grit sandpaper to remove oil and its own oxide layer on the back of the substrate, as well as the coatings and oxides deposited in the previous experiment on the surface of the sample holder and rotating support (total time 10-20 minutes) to enhance the conductivity between the substrate and the sample holder, and between the sample holder and the rotating support; then wipe the entire substrate, sample holder, and rotating support with organic solvent again to remove the impurities remaining after sandpaper polishing; finally, blow with high-purity nitrogen gas for 5-20 seconds to dry the substrate, sample holder, and rotating support and remove residual particles. This operation helps prevent uneven or insufficient contact between the substrate and the sample holder or between the sample holder and the rotating support, reducing the bias effect. In other words, it can enhance the conductivity between the substrate and the sample holder, or between the sample holder and the rotating support, so that the bias voltage can be applied more evenly to the substrate.

[0013] In this field, appropriate sputtering targets can be rationally selected based on the chemical composition and elemental content of the target coating. Sputtering targets containing the target content of Li, Nb, and O elements can be selected as general materials for this invention. In order to prepare lithium niobate (LiNbO3) coatings, the content of Li in general sputtering targets is appropriately increased to make it excessive. While increasing the amount of raw materials, it also reduces the controllability of deposition. In order to solve this technical problem, this method improves the composition of sputtering targets. Preferably, in step (1), the composition of the sputtering target is niobium pentoxide (Nb5O2) and lithium oxide (Li2O), and the molar ratio of the two is 1:1. This sputtering target can be prepared by a simple method, such as mixing, sintering, pressing, and re-sintering the powders of the two in a molar ratio. Without the need for excess Li, an ideal O / Nb stoichiometry or near-stoichiometry lithium niobate film can be prepared.

[0014] To achieve better deposition results, the distance between the target and the substrate (target-substrate distance) needs to be controlled within a suitable range. Preferably, in step (1), the target-substrate distance between the substrate and the sputtering target is 3.5–5.0 cm. Furthermore, magnetron sputtering requires creating a vacuum environment to reduce interference from gas molecules or atoms. Preferably, in step (1), the pressure of the vacuum environment is below 10... -2 Pa.

[0015] For common magnetron sputtering devices, rotating the substrate around the axis on the sample support at an appropriate angle will make the substrate face away from the target surface. Pre-treatment of the target surface using radio frequency magnetron sputtering before thin film deposition is to remove particulate matter from the target surface, preventing these particles from depositing on the substrate surface to form nucleation centers, inducing wedge-shaped structures, and destroying columnar crystal structures. When using radio frequency magnetron sputtering for target surface cleaning, different parameter combinations (e.g., radio frequency power, argon and oxygen ratio, gas pressure, temperature, and time) will affect the actual cleaning effect. To obtain the best cleaning effect without affecting the morphology and performance of the target itself, and to reduce the negative impact of impurities, preferably, in step (2), the surface cleaning process uses a radio frequency power of 800–900 W, an ambient gas pressure of 1.5–2.5 Pa after introducing argon and oxygen, an argon-oxygen flow ratio of 1–3:1, an ambient temperature of 100–200 °C, and a cleaning time of 5–15 min.

[0016] Etching the substrate surface is to remove particulate matter, the inferior lithium niobate coating formed during the target surface cleaning process in step (2), and the surface oxide film inherent to the substrate surface. Removing particulate matter helps to obtain a clean surface and prevent coating bulging. Removing the surface oxide film and the inferior lithium niobate coating helps to increase the thermal expansion coefficient of the substrate, making the expansion coefficients of the substrate and the lithium niobate coating closer, improving adhesion, reducing the risk of peeling off, and improving the structural and performance stability in high-temperature environments. In the etching process, argon ion beams can be obtained by ionizing argon gas. Preferably, the specific method of step (3) is as follows: Argon gas is introduced into the vacuum environment and the arc power supply is turned on, so that Ar is ionized into Ar under the action of the arc. + Ar + The substrate is bombarded under a bias voltage to etch its surface, remove impurities, and obtain a clean surface for later use. To achieve the best cleaning effect, the following are further preferred: the pressure of the introduced argon gas is 0.5-1.5 Pa, the arc power supply is 80-100 A, the bias voltage is 120-160 V, and the etching cleaning time is 0.5-5 min.

[0017] In the deposition process of lithium niobate coating, the selection of deposition parameters determines the quality of the lithium niobate film. In order to improve the performance of the finished product, preferably, in step (4), the radio frequency power of magnetron sputtering is 800-900W, the ambient gas pressure after introducing argon and oxygen is 2.0-3.0Pa, the argon-oxygen flow ratio in the gas is 2-8:1, the ambient temperature is between 100-200℃, the deposition time is 6-10h, the target-substrate distance is 3.5-5.0cm, and the radial distance is ≤8.0cm (the radial distance refers to the distance from the sputtering center). In addition, based on radio frequency magnetron sputtering, a piezoelectric film with a smooth morphology, uniform crystal structure, and excellent O / Nb atomic ratio can be prepared by applying a bias voltage between the substrate and the target. The effects of the bias voltage and other deposition parameters on the ultrasonic signal amplitude and piezoelectric constant d can be obtained. 33 The regulation effect. Preferably, in step (4), during the coating deposition process of magnetron sputtering, a bias voltage is applied between the substrate and the sputtering target (the substrate is at a low potential), the bias voltage is 2 to 100V, the current is 0.6 to 1.5A, and the duty cycle is 40% to 60%.

[0018] The annealed lithium niobate coating exhibits improved crystallinity, piezoelectric properties, and piezoelectric constant d. 33 Both can be improved. To obtain the best annealing effect, preferably, in step (5), the annealing temperature is 650-750℃ and the treatment time is 4-6h.

[0019] Unlike existing technologies, this invention employs additional cleaning techniques before coating deposition, including polishing the substrate (including the front and back sides), sample holder, and rotating support used for magnetron sputtering; pre-cleaning the target surface using RF magnetron sputtering; and etching the substrate surface. These processes result in a cleaner sample preparation environment, optimized process parameters suitable for the coating, such as a wider range of argon-oxygen flow ratios, and an increased argon-oxygen flow ratio also contributes to improved coating quality. Simultaneously, these processes result in a purer coating with better substrate adhesion, and a wider range of bias voltages, allowing the invention to achieve its objectives even at 50V or 100V.

[0020] The lithium niobate piezoelectric coating prepared by this method exhibits excellent piezoelectric and electrical properties. Unlike existing technologies, this invention achieves varying longitudinal wave signal amplitude, transverse wave signal amplitude, and piezoelectric constant d by controlling the columnar crystal tilt angle, coating thickness, and O / Nb atomic ratio. 33The tilt angle is controlled as follows: Preferably, the thickness of the lithium niobate piezoelectric coating is 3.5–40.0 μm, and the tilt angle is the angle between the columnar crystal orientation and the normal to the substrate surface (the tilt angle of columnar crystals grown perpendicular to the substrate surface is 0°). More preferably, the tilt angle of the columnar crystals in the lithium niobate piezoelectric coating is 0–30°. The tilt angle is affected by the target-substrate distance and the radial distance; therefore, the corresponding parameters can be adjusted to keep the tilt angle within the required range. To obtain a strong transverse wave signal, the substrate should not be placed too close or too far from the sputtering center; the radial distance should be between 2.5 and 4.5 cm, and the tilt angle of the columnar crystals should be 10–20°.

[0021] The method of this invention can optimize the uniformity of coating morphology and structure, the uniformity of Li, Nb, and O elemental densities on the surface and inside, and control the O / Nb atomic ratio in both the surface and interior layers to be between 2.79 and 3.03, achieving excellent stoichiometry. This significantly increases the content of the LiNbO3 phase in the coating, reduces the proportion of the lithium-deficient LiNb3O8 phase, and precisely controls the longitudinal and transverse wave ultrasonic signals, thereby increasing the piezoelectric constant d. 33 .

[0022] In a second aspect of the invention, a lithium niobate piezoelectric coating with excellent piezoelectric and electrical properties is provided, which is prepared by the method provided in the first aspect of the invention.

[0023] In a third aspect of the invention, the application of the lithium niobate piezoelectric coating of the second aspect of the invention is provided, specifically its application as a piezoelectric active material in the fields of electro-optics, piezoelectricity, pyroelectricity, and optics.

[0024] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0025] (1) This invention provides a method for preparing a lithium niobate piezoelectric coating with adjustable longitudinal and transverse acoustic waves. The method is simple and flexible, can improve the bonding between the coating and the substrate, and can easily achieve the control of multiple parameters.

[0026] (2) The technology of this invention can optimize the uniformity of coating morphology and structure, the uniformity of Li, Nb, and O element distribution on the surface and inside, control the O / Nb atomic ratio on the surface and inside to 2.79-3.03, with excellent stoichiometry, greatly increase the content of LiNbO3 phase in the coating, reduce the proportion of lithium-deficient LiNb3O8, and precisely control the longitudinal and transverse wave ultrasonic signals and improve the piezoelectric constant d. 33 .

[0027] (3) The present invention provides a lithium niobate piezoelectric coating with adjustable longitudinal and transverse acoustic waves, which has good piezoelectric and electrical properties.

[0028] (4) The present invention also provides the application of lithium niobate piezoelectric coating, which can meet the needs of piezoelectric materials in many fields and has broad application prospects. Attached Figure Description

[0029] Figure 1 In the figure, (a) to (d) show the microstructure of the surface and cross-section of the lithium niobate coating at different selected locations prepared in Example 1, (e) shows the X-ray diffraction (XRD) pattern at the corresponding location, and (f) shows the longitudinal and transverse dual-wave ultrasonic signal pattern of the lithium niobate coating after annealing.

[0030] Figure 2 In the image, (a) shows the microstructure of the lithium niobate coating prepared in Example 2, (b) shows the X-ray diffraction (XRD) patterns at different selected locations, and (c) shows the acoustic waveform excited by the coating.

[0031] Figure 3 In the table, (a) represents the O / Nb atomic ratio of the lithium niobate coating prepared in Example 3, and (b) represents the O / Nb atomic ratio, crystal orientation peak intensity ratio, ultrasonic signal amplitude, and piezoelectric constant d. 33 (c) is the X-ray diffraction (XRD) pattern at the sputtering center, and (d) is the acoustic waveform.

[0032] Figure 4 Images (a) through (c) show the ultrasonic signal amplitude and piezoelectric constant d of the lithium niobate coating sensor prepared in Example 4. 33 A graph showing the relationship between coating thickness and bias voltage;

[0033] Figure 5 In the figure, (a) to (d) show the microstructure of the surface and cross-section of the lithium niobate coating prepared in Example 5 at different selected locations, and (e) and (f) show the relationship between signal amplitude, film thickness and tilt angle and radial distance.

[0034] Figure 6 In the figure, (a) to (d) are the graphs showing the relationship between the longitudinal and transverse wave signals of the lithium niobate coating prepared in Example 6 and the radial distance under different conditions;

[0035] Figure 7 In the figure, (a) and (b) are graphs showing the relationship between the longitudinal wave signal amplitude and the film thickness of the lithium niobate coating prepared in Example 7 before and after annealing, and (c) is a graph showing the relationship between the longitudinal wave amplitude and the tilt angle of the columnar crystal.

[0036] Figure 8 In the figure, (a) and (b) are graphs showing the relationship between the transverse wave signal amplitude of the lithium niobate coating prepared in Example 8 and the tilt angle of the columnar crystal and the film thickness.

[0037] Figure 9The amplitude of longitudinal and transverse waves and the piezoelectric constant d of the lithium niobate coating prepared in Example 9 before and after annealing. 33 Numerical value;

[0038] Figure 10 In the figures, (a) to (d) show the changes in the density of Li, Nb, and O elements with coating depth before and after annealing of the lithium niobate coating prepared in Example 10, as well as the amplitude of longitudinal and transverse waves and the piezoelectric constant d. 33 The numerical values ​​and the changes in the O / Nb atomic ratio are shown in (e), which represents the O / Nb atomic ratio at a depth of 2 μm on the surface and inside of the lithium niobate coating prepared in Example 10 after annealing at 700 °C for 5 h.

[0039] Figure 11 The cross-sectional morphology of the coating prepared for Comparative Example 1 is shown.

[0040] In the above figures, L and LW represent longitudinal waves, S and SW represent transverse waves, L / L0 refers to the ratio of the longitudinal wave signal amplitude at each location under the condition to the longitudinal wave signal amplitude at the sputtering center, and S / Sh refers to the ratio of the transverse wave signal amplitude at each location under the condition to the amplitude of the strongest transverse wave signal. Detailed Implementation

[0041] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0042] In the following embodiments:

[0043] The sputtering target is made by mixing Nb5O2 and Li2O powders in a molar ratio of 1:1, sintering, pressing, and then sintering again.

[0044] In all embodiments and comparative examples, SEM (TESCAN MIRA3, Czech Republic) was used to observe changes in the microstructure of the coating surface and cross-section. An Aztec energy-X-Max 20 X-ray energy dispersive spectroscopy (EDS) instrument was used to detect elemental distribution and the O / Nb atomic ratio. The structural orientation of the coating was tested using a Tongda TDM-10 X-ray diffractometer. A DPR300 ultrasonic pulse receiver (JSRUltrasonics) was used to collect feedback signals and test the acoustic waveform and longitudinal and transverse acoustic amplitudes. A quasi-static d 33 The tester measures the piezoelectric constant d. 33 The changes in the density of Li, Nb, and O elements with coating depth were measured using a focused ion beam electron beam dual-beam electron microscope coupled with a secondary ion mass spectrometer (SIMS).

[0045] Example 1

[0046] Preparation method of lithium niobate piezoelectric coating:

[0047] (1) Coating preparation: First, ultrasonically clean with acetone for 4 minutes to remove oil and visible particles from the surface and back of the substrate; then, polish the back of the substrate, sample holder, and rotating support with 2000-grit sandpaper to remove oil and oxide layers from the back of the substrate, as well as coatings and oxides deposited in previous experiments on the surfaces of the sample holder and rotating support (total time 15 minutes), to enhance the conductivity between the substrate and sample holder, and between the sample holder and rotating support; then, wipe the entire substrate, sample holder, and rotating support with acetone again to remove impurities remaining after sandpaper polishing; finally, blow with high-purity nitrogen for 10 seconds to dry the substrate, sample holder, and rotating support and remove residual particles. Place the substrate and sputtering target in the chamber of the magnetron sputtering instrument, evacuate, and obtain a 6×10⁻⁶ m³ / h sputtering target. -3 A vacuum environment of Pa was established, and the distance between the sputtering target and the substrate (target-substrate distance) was controlled to be 5.0 cm for later use.

[0048] (2) Target surface cleaning treatment: Rotate the substrate 180° around the axis on the sample support frame so that the substrate faces away from the sputtering target surface. Then, use radio frequency magnetron sputtering to clean the surface of the sputtering target. The cleaning parameters are as follows: radio frequency power is 900W, gas pressure after argon and oxygen are introduced is 2.0Pa, argon-oxygen flow ratio in the gas is 2:1, cavity temperature is 150℃, and cleaning time is 10min.

[0049] (3) Substrate surface ion beam etching: High-purity Ar gas at 1.0 Pa is introduced, the arc power supply is turned on, and the arc power supply is set to 90 A. Ar is ionized into Ar under the action of the arc. + Ar is biased by a 140V voltage. + The substrate surface was cleaned by ion bombardment for 2 minutes;

[0050] (4) Lithium niobate coating deposition: The substrate is rotated back to its initial position so that the substrate surface faces the sputtering target surface, and then the coating is deposited. The deposition parameters are: RF power of 900W, gas pressure after argon and oxygen are introduced of 2.0Pa, argon-oxygen flow ratio of 2:1, cavity temperature of 150℃, deposition time of 8h, target-substrate distance of 5cm, and radial distance ≤7.0cm. In addition, a bias voltage is applied between the substrate and the sputtering target during the entire deposition process to prepare a piezoelectric thin film with uniform structure and excellent stoichiometry. The bias voltage between the substrate and the sputtering target is 35V, the current is 1.5A, and the duty cycle is 50%. The lithium niobate coating is obtained by deposition and is ready for use.

[0051] (5) Heat treatment: The prepared lithium niobate coating was subjected to high-temperature annealing in an annealing furnace to improve the crystallinity and piezoelectric properties of the film. The annealing temperature was 700℃ and the time was 5h. After the annealing was completed, the lithium niobate piezoelectric coating was obtained.

[0052] The surface morphology, cross-sectional morphology, O / Nb atomic ratio, acoustic waveform, and structural orientation of the lithium niobate coating in this embodiment before and after different processes were characterized, and the corresponding test results are as follows: Figure 1 As shown. Figure 1 (a) to (d) show the surface and cross-sectional morphology of the coating prepared in Example 1 before annealing. It can be seen that the grain size of the coating surface is uniform, the columnar crystal morphology of the cross section is good, and no peeling phenomenon is observed. The O / Nb atomic ratio measured by EDS is 2.88 / 1, which is close to the ideal ratio of 3 / 1. Figure 1 (e) indicates that the crystal orientation of the coating is completely consistent at different locations, and the peak intensity of LiNbO3(012) is much higher than that of LiNb3O8(116), which means that the content of the lithium-deficient phase LiNb3O8 is extremely low; at the same time, after annealing at 700℃ for 5h, Figure 1 (f) The coating exhibits both longitudinal and transverse waves, with the longitudinal wave signal at the sputtering center reaching 0.13V. These results demonstrate that the proposed method significantly improves the bonding between the coating and the substrate. A lithium niobate piezoelectric coating with uniform morphology, excellent O / Nb atomic ratio, high LiNbO3 purity, and capable of generating both longitudinal and transverse waves can still be prepared using a 35V bias voltage.

[0053] Example 2

[0054] The preparation method and process parameters of this embodiment are basically the same as those of Example 1. The only difference is that, based on Example 1, the bias voltage of step (4) is adjusted to 50V to prepare a lithium niobate piezoelectric coating.

[0055] The surface morphology and structural orientation of the lithium niobate coating of this embodiment were characterized using the method described in Example 1, and the corresponding test results are as follows: Figure 2 As shown. Figure 2 The surface morphology and structural orientation of the lithium niobate coating before annealing are shown. It can be seen that even with a bias voltage increased to 50V, columnar and spherical surface morphologies were still prepared, indicating good coating adhesion and no peeling. The O / Nb atomic ratio is 2.91, close to the ideal ratio of 3 / 1. The XRD crystal orientation is uniform, and the LiNbO3(012) peak intensity is much higher than the LiNb3O8(116) peak, indicating an extremely low content of the lithium-deficient LiNb3O8 phase. An ultrasonic signal of 0.045V can still be excited. These results demonstrate that this preparation method greatly improves the bonding between the coating and the substrate. A lithium niobate piezoelectric coating with uniform morphology and structure, excellent O / Nb atomic ratio, high LiNbO3 purity, and the ability to excite ultrasonic waves can still be prepared using a 50V bias voltage.

[0056] Example 3

[0057] In this embodiment, different batches of lithium niobate piezoelectric coatings were prepared. The preparation method and process parameters were basically the same as those in Example 1. The only difference was that in step (1) of this embodiment, the target-substrate distance was adjusted to 4 cm, and in step (4), the argon-oxygen flow ratio was 4:1, 6:1, and 8:1 respectively, so as to prepare three batches of lithium niobate coatings with different argon-oxygen flow ratios.

[0058] The O / Nb atomic ratio, peak intensity ratio, structural orientation at the sputtering center, acoustic waveform, and piezoelectric constant d of lithium niobate coatings prepared under different argon-oxygen flow ratios in this embodiment are analyzed. 33 Characterization was performed, and the results are as follows: Figure 3 As shown. By Figure 3 It is evident that as the argon-oxygen flow ratio increases, the O / Nb atomic ratio gradually increases. When the argon-oxygen ratio is 8:1, the O / Nb atomic ratio is 3, achieving the ideal and optimal O / Nb atomic ratio. This is further supported by the XRD crystal structure. Figure 3 (c) and the peak intensity ratio of (012) of LiNbO3 / (116) of LiNb3O8 Figure 3 (b) The lithium niobate film prepared under a high argon-oxygen flow ratio has purer LiNbO3. When the argon-oxygen ratio is 8:1, the (012) / (116) peak intensity ratio of LiNbO3 is the highest, indicating that the proportion of the lithium-deficient phase LiNb3O8 is the lowest; at the same time, see Figure 3 (b) Figure 3 (d) When the argon-oxygen ratio is 4:1, 6:1, and 8:1, the acoustic wave amplitudes excited at the sputtering centers of each thin film are 0.8V, 1.7V, and 9.3V, respectively, and the piezoelectric constant d 33 The ratios are 0.7 PC / N, 1.1 PC / N, and 2.5 PC / N, respectively. Therefore, increasing the argon-oxygen flow ratio can produce LiNbO3 films with higher purity and a more ideal O / Nb atomic ratio, resulting in stronger acoustic signals and a gradually enhanced piezoelectric effect.

[0059] Example 4

[0060] The preparation method and process parameters of this embodiment are basically the same as those of Example 1. The only difference is that, based on Example 1, the bias voltage in step (4) is adjusted to be 0V, 2V, 4V, 6V, 8V, 10V, 15V, 25V, 35V, 50V, and 100V, respectively, in order to study the effect of bias voltage on the amplitude of ultrasonic signal.

[0061] Figure 4 The amplitude and piezoelectric constant d of the ultrasonic signal excited by the lithium niobate coatings prepared under different bias voltages in this embodiment are shown. 33Therefore, the desired longitudinal and transverse wave signals and piezoelectric constant d can be obtained by adjusting the bias voltage. 33 Among them, the longitudinal wave signal and the piezoelectric constant d 33 The shear wave gradually weakens as the bias voltage increases, while it reaches its maximum value at a bias voltage of 4V.

[0062] Example 5

[0063] The preparation method and process parameters of this embodiment are basically the same as those of Example 1. The only difference is that the argon-oxygen flow ratio is adjusted to 6:1 in step (4) to study the effect of radial distance on morphology, thickness, tilt angle and ultrasonic signal amplitude under this condition.

[0064] The surface morphology, cross-sectional morphology, film thickness, and tilt angle of the lithium niobate coating before and after different processes in this embodiment are characterized. Figure 5 The figure illustrates the relationship between the surface morphology, cross-sectional morphology, film thickness, and tilt angle of the lithium niobate coating prepared in this embodiment before annealing and the radial distance, as well as the relationship between the amplitudes of longitudinal and transverse waves and the radial distance after annealing. As can be seen from the figure, the coating morphology remains uniform, the columnar crystal thickness decreases with increasing radial distance, and the tilt angle gradually increases with increasing radial distance. The measured longitudinal wave signal weakens with increasing radial distance; the transverse wave signal reaches its maximum value at a radial distance of 3.0 cm. In sputtering regions with a radial distance greater than 3.0 cm, the transverse wave amplitude weakens with increasing radial distance.

[0065] Example 6

[0066] Based on the preparation method and process parameters of Example 5, the gas pressure, argon-oxygen flow ratio, and bias voltage after argon and oxygen are introduced in step (4) are adjusted. The range of the above parameters is 2.0-3.0 Pa, 2-6:1, and 2-50 V, respectively. Multiple batches of lithium niobate piezoelectric coatings are prepared to study the effect of radial distance on the control of ultrasonic signals.

[0067] Figure 6 The relationship between the longitudinal and transverse wave signals of the lithium niobate coating prepared in this embodiment and radial distance is shown. Figure 6 (a) represents the average value under the corresponding conditions before annealing. Figure 6 (b) shows the average values ​​of each condition after annealing at 700℃ for 5 hours. It can be seen that regardless of the deposition conditions, the changes in longitudinal and transverse wave signals with radial distance are consistent before and after annealing; that is, the longitudinal wave signal always weakens as the radial distance increases, while the transverse wave signal reaches its maximum value at 3.5 cm. Therefore, with fixed deposition parameters, to obtain a strong transverse wave signal, the substrate should not be placed too close or too far from the sputtering center; the radial distance should be between 2.5 and 4.5 cm.

[0068] Example 7

[0069] Thickness-longitudinal wave signal amplitude were measured one-to-one for the lithium niobate coatings prepared under the conditions in Example 6 to investigate the modulating effect of coating thickness and columnar crystal tilt angle on the longitudinal wave ultrasonic signal. The relationship between longitudinal wave amplitude and film thickness was obtained as follows: Figure 7 Where 7(a) is the value before annealing, Figure 7 (b) shows the values ​​after annealing at 700℃ for 5 hours. It can be seen that, regardless of whether it is before or after annealing, the amplitude of the longitudinal wave signal increases exponentially with the increase of the film thickness. Figure 7 (c) shows the relationship between the longitudinal wave amplitude and the tilt angle of the columnar crystal. For the same coating thickness, the smaller the tilt angle, the stronger the longitudinal wave signal amplitude.

[0070] Example 8

[0071] Thickness, columnar crystal tilt angle, and transverse wave signal amplitude were measured one-to-one for the lithium niobate coatings prepared under various conditions in Example 6 to investigate the relationship between transverse wave amplitude and film thickness and tilt angle. Figure 8 As stated above, 8(a) represents the numerical value relating the transverse wave amplitude to the tilt angle. Figure 8 (b) shows the values ​​of transverse wave amplitude and film thickness. It can be seen that when the film thickness is constant, the transverse wave amplitude excited by the lithium niobate film is related to the tilt angle of the columnar crystals. Around 15° is most favorable for transverse wave excitation. When the angle is below 15°, the transverse wave amplitude increases with increasing angle; when the angle is above 15°, the transverse wave amplitude decreases with increasing angle. Similarly, when the tilt angle of the columnar crystals is constant, the transverse wave amplitude excited by the lithium niobate film is related to the film thickness; the greater the thickness, the stronger the transverse wave signal. Therefore, to obtain a strong transverse wave signal, the tilt angle of the columnar crystals should be controlled between 10° and 20°.

[0072] Therefore, based on the test results of Examples 7 and 8, it is demonstrated that the required longitudinal wave signal amplitude and transverse wave signal amplitude can be obtained by adjusting the film thickness and tilt angle, thereby achieving the purpose of the present invention.

[0073] Example 9

[0074] The preparation method and process parameters in this embodiment are basically the same as those in Example 1, except that the argon-oxygen flow ratio in step (4) is adjusted to 6:1 to study the effect of annealing on piezoelectric properties and piezoelectric constant d. 33 Its optimization effect.

[0075] The results are as follows Figure 9 It is evident that the annealing conditions of 700℃ and 5h significantly improve the piezoelectric properties and the piezoelectric constant d. 33The longitudinal wave amplitude increased from 0.14V to 0.51V, the transverse wave amplitude increased from 0V to 0.15V, and the piezoelectric constant d... 33 Increased from 0.58 PC / N to 1.41 PC / N.

[0076] Example 10

[0077] The preparation method and process parameters in this embodiment are basically the same as those in Example 1. The only difference is that the argon-oxygen flow ratio in step (4) is adjusted to 4:1, which is used to study the effect of annealing on elemental distribution, stoichiometry, piezoelectric properties, and piezoelectric constant d. 33 The optimization effect. To more intuitively compare and analyze the distribution of Li, Nb, and O elements in the coating depth direction before and after annealing, the elemental proportions obtained by SIMS-TOF were converted, that is, the content of each element inside the coating was set to 1, and the distribution curves of the relative content of each element with depth were obtained, such as... Figure 10 (a)-(c), for example, the relative content of Li element at the coating surface before annealing is 12, which means that the Li element on the coating surface is 12 times that inside the coating.

[0078] The results are as follows Figure 10 :from Figure 10 (d) It can be seen that the annealing condition of 700℃ and 5h significantly improves the piezoelectric properties and the piezoelectric constant d. 33 The longitudinal wave amplitude increased from 0.10V to 0.92V, the transverse wave amplitude increased from 0.06V to 0.32V, and the piezoelectric constant d... 33 Increased from 0.88 PC / N to 1.48 PC / N; from Figure 10 As can be seen from (a)-(c), before annealing, the three elements Li, Nb, and O exhibit non-uniformity, meaning that the content of Li and Nb on the coating surface is much higher than that inside the coating (the content of Li on the surface is 12 times that inside the coating, and the content of Nb on the surface is 5.7 times that inside the coating). After annealing at 700℃ for 5 hours, the distribution of Li and Nb elements becomes relatively uniform, and their content is almost unaffected by the depth from the coating surface. As for O element, before annealing, in the near-surface region, the O element content first increases and then decreases, eventually stabilizing. After annealing at 700℃... After annealing for 5 hours, the O element distribution was relatively uniform. Simultaneously, the 700℃, 5-hour annealing condition significantly optimized the O / Nb atomic ratio in both the surface and interior of the lithium niobate coating. Before annealing, the surface O / Nb atomic ratio was 2.68, and the interior O / Nb atomic ratio was 2.49. After annealing at 700℃ for 5 hours, the surface O / Nb atomic ratio was 2.83, and the interior O / Nb atomic ratio at 2 μm was 2.79. This indicates that annealing resulted in a LiNbO3 piezoelectric coating with a better stoichiometry, while also improving the uniformity of the O / Nb atomic ratio between the surface and interior (e.g., ...). Figure 10 (e) The middle box shows the lithium niobate coating after annealing at 700℃ for 5 hours, treated with Ga+ The 10μm × 10μm region with a depth of 2μm left after ion bombardment had an O / Nb atomic ratio of 2.79, compared to 2.83 in the unbombarded region. Figure 10 (e) It can also be seen that the coating surface still has uniform grains after annealing, and the morphology is not affected by annealing. That is, the annealing treatment at 700℃ for 5h is conducive to promoting the interaction between elements, generating a lithium niobate coating with better stoichiometry and higher elemental uniformity, so as to excite stronger acoustic signals and higher piezoelectric constant d. 33 value.

[0079] Comparative Example 1

[0080] The preparation method and process parameters of this comparative example are basically the same as those of Example 1, except that this comparative example did not perform backside polishing, sputtering target surface cleaning, or substrate surface ion beam etching. The results are as follows: Figure 11 As shown, the prepared lithium niobate coating exhibits cracking, lacks columnar crystal morphology, and has no acoustic signal.

[0081] The experimental results of Examples 1, 2 and Comparative Example 1 show that backside grinding of the substrate, surface cleaning of the target material and ion beam etching of the substrate surface are beneficial to obtaining a uniform lithium niobate film, improving the adhesion of the film and enabling the excitation of ultrasonic signals.

[0082] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method for preparing a lithium niobate piezoelectric coating with tunable longitudinal and transverse acoustic waves, employing a magnetron sputtering process, characterized in that, Includes the following steps: (1) Coating preparation: Clean the surface and back of the substrate, and then place the substrate and sputtering target in a vacuum environment for later use; (2) Target surface cleaning treatment: Adjust the relative position of the substrate and the sputtering target so that the substrate faces away from the sputtering target surface, and use radio frequency magnetron sputtering to clean the surface of the sputtering target for later use; (3) Substrate surface ion beam etching treatment: Argon ion beam is used to bombard the substrate to etch its surface, remove impurities and obtain a clean surface for later use; (4) Lithium niobate coating deposition: Adjust the relative position of the substrate and the sputtering target so that the substrate faces the surface of the sputtering target, and deposit the coating by magnetron sputtering to obtain a lithium niobate coating for later use; The magnetron sputtering process employs an RF power of 800-900W, an ambient gas pressure of 2.0-3.0Pa after introducing argon and oxygen, an argon-oxygen flow ratio of 4-8:1, an ambient temperature of 100-200℃, a deposition time of 6-10h, a target-substrate distance of 3.5-5.0cm, and a radial distance ≤8.0cm. During the magnetron sputtering coating deposition process, a bias voltage of 2-35V is applied between the substrate and the sputtering target, with a current of 0.6-1.5A and a duty cycle of 40%-60%. (5) Heat treatment: The lithium niobate coating is annealed to obtain a lithium niobate piezoelectric coating; The annealing temperature is 650~750℃ and the treatment time is 4~6h; the thickness of the lithium niobate piezoelectric coating is 3.5~40.0μm and the columnar crystal tilt angle is 0~30°.

2. The method according to claim 1, characterized in that, In step (1), the substrate, sample holder, and rotating support used for magnetron sputtering are polished. The specific operation is as follows: first, use organic solvent to ultrasonically clean for 3-6 minutes to remove oil and visible particles from the surface and back of the substrate; then, use 1500-2500 grit sandpaper to polish the back of the substrate, the sample holder, and the rotating support to remove oil and its own oxide layer on the back of the substrate, as well as the coating and oxide deposited on the surface of the sample holder and the rotating support in the previous experiment. The polishing time is 10-20 minutes to enhance the conductivity between the substrate and the sample holder, and between the sample holder and the rotating support; then, use organic solvent to wipe the entire substrate, sample holder, and rotating support again to remove the impurities remaining after sandpaper polishing; finally, use high-purity nitrogen to blow for 5-20 seconds to dry the substrate, sample holder, and rotating support and remove the remaining particles.

3. The method according to claim 1, characterized in that: In step (1), the sputtering target is composed of niobium pentoxide and lithium oxide, with a molar ratio of 1:1; the distance between the substrate and the sputtering target is 3.5~5.0 cm.

4. The method according to claim 1, characterized in that: In step (2), the surface cleaning process uses a radio frequency power of 800~900W, an ambient gas pressure of 1.5~2.5Pa after introducing argon and oxygen, an argon-oxygen flow ratio of 1~3:1, an ambient temperature of 100~200℃, and a cleaning time of 5~15min.

5. The method according to claim 1, characterized in that, The specific method of step (3) is as follows: Argon gas is introduced into the vacuum environment, the gas pressure is controlled at 0.5~1.5Pa, and the arc power supply is turned on. The arc power supply is 80~100A and the bias voltage is 120-160V, so that Ar is ionized into Ar under the action of the arc. + Ar + The substrate is bombarded under a bias voltage to etch its surface. The etching cleaning time is 0.5~5 minutes to remove impurities and obtain a clean surface for later use.

6. A lithium niobate piezoelectric coating, characterized in that: It is prepared by the method described in any one of claims 1 to 5.

7. An application of the lithium niobate piezoelectric coating as described in claim 6, characterized in that: Applications of piezoelectric active materials in electro-optics, piezoelectricity, pyroelectricity, and optics.

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