Aluminum nitride thin film, method for preparing the same, and use thereof

Abstract

The application provides an aluminum nitride film, which comprises polycrystalline aluminum nitride; the polycrystalline aluminum nitride is doped with 6.5-7.5 at.% tantalum; the crystal face orientation of the polycrystalline aluminum nitride is (002); and the c value in the cell parameter of the polycrystalline aluminum nitride is 0.512 nm. The application further provides a preparation method of the aluminum nitride film, which specifically comprises the following steps: at normal temperature, magnetron sputtering is performed on a substrate by using an AlTa alloy target under a mixed gas atmosphere of inert gas and nitrogen, so as to obtain the aluminum nitride film. The application further provides an application of the aluminum nitride film in the field of thin film bulk acoustic resonators.

Description

Technical Field

[0001] This invention relates to the field of piezoelectric thin films, and more specifically to a method for preparing aluminum nitride thin films. Background Technology

[0002] With the rapid development of the 5G mobile communication system industry, wireless communication equipment is becoming increasingly miniaturized and lightweight, while also carrying more and more functions. As the operating frequency of wireless communication systems continues to increase, the requirements for system integration are also becoming higher. Furthermore, with the acceleration of transmission rates, increases in frequency and bandwidth, and the miniaturization of systems, wireless communication equipment needs new devices made from novel materials to meet its development needs.

[0003] Thin-film resonators can easily operate at ultra-high frequency bands, especially in the GHz band where they exhibit significant advantages. Thin-film resonators meet the trend of higher frequency communication technologies and have wide applications in sensing and other fields.

[0004] Due to its unique comprehensive properties—high surface acoustic wave velocity, high longitudinal sound velocity, high temperature stability, piezoelectricity, and compatibility with standard CMOS silicon technology—aluminum nitride thin films can be widely used in piezoelectric MEMS devices, especially high-temperature piezoelectric sensors, thin-film bulk acoustic wave resonators, and surface acoustic wave (SAW) devices. Aluminum nitride shows great promise as a piezoelectric thin film material for bulk acoustic wave resonators. However, compared to ZnO and PZT, aluminum nitride thin films have a lower piezoelectric coefficient (d). 33 The piezoelectric response of aluminum nitride (ANT) is ~5.5 pC / N, which limits the widespread application of ANT films. Therefore, improving the piezoelectric response of ANT to meet the demand is of great significance. On the other hand, most high c-axis oriented AlN films are prepared at high temperatures and the process is complex. Therefore, the urgent problems to be solved in preparing high c-axis oriented AlN films are reducing film preparation costs, simplifying the process flow, and lowering the growth temperature. Summary of the Invention

[0005] The first objective of this invention is to provide a high voltage-responsive aluminum nitride thin film.

[0006] The second objective of this invention is to provide a method for preparing a high-voltage electrically responsive aluminum nitride thin film.

[0007] A third objective of this invention is to provide an application of the aforementioned high-voltage electrically responsive aluminum nitride thin film.

[0008] The technical solution adopted in this invention is as follows:

[0009] An aluminum nitride thin film, the aluminum nitride thin film comprising polycrystalline aluminum nitride; the polycrystalline aluminum nitride being doped with 6.5-7.5 at.% tantalum; the crystal orientation of the polycrystalline aluminum nitride being (002); and the c-value of the cell parameter of the polycrystalline aluminum nitride being 0.512 nm.

[0010] The aluminum nitride film has a piezoelectric coefficient of 15.8-16.1 pm / V.

[0011] The method for preparing the aluminum nitride thin film includes a step of magnetron sputtering using an AlTa alloy target at room temperature.

[0012] The atomic percentage of Al atoms to Ta atoms in the AlTa alloy target is 92.5-93.5:6.5-7.5.

[0013] A method for preparing the aluminum nitride thin film includes the step of magnetron sputtering onto a substrate at room temperature using an AlTa alloy target in a mixed atmosphere of inert gas and nitrogen, thereby obtaining the aluminum nitride thin film.

[0014] The inert gas includes argon;

[0015] The purity of the inert gas is greater than 99.999%;

[0016] The purity of the nitrogen gas is greater than 99.999%;

[0017] The flow rate of the inert gas is 15-18 sccm;

[0018] The flow rate of the nitrogen gas is 15-18 sccm.

[0019] The substrate includes a p-type silicon wafer;

[0020] The crystal orientation of the p-type silicon wafer is (111).

[0021] The magnetron sputtering process uses a DC power of 190-210W;

[0022] The magnetron sputtering pressure is 4-5 × 10⁻⁵. -4 Pa;

[0023] The substrate rotates at a speed of 12-15 rpm;

[0024] The purity of the AlTa alloy target is greater than 99.9%.

[0025] It also includes the step of bombarding the AlTa alloy target with inert gas ions for at least 10 minutes.

[0026] It also includes the step of cleaning the substrate with hydrofluoric acid.

[0027] One application of the aforementioned aluminum nitride thin film is in the fabrication of a thin-film bulk acoustic resonator.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] The AlN thin film provided by this invention is uniformly doped with 6.5-7.5 at.% Ta. The doping of Ta enables the AlN thin film to preferentially align to the c-axis (002), reduces the surface roughness of the AlN thin film, improves the particle uniformity of the AlN thin film, and increases the piezoelectric coefficient of AlN to 15.8-16.1 pm / V.

[0030] The method for preparing aluminum nitride thin films provided by this invention uses an alloy target for sputtering, which allows for precise control of the tantalum doping content. Moreover, the doping process can be completed using a single target, greatly simplifying the operation and control process flow and parameters. This makes it easier to deposit AlN(002) oriented thin films. This method allows for deposition at room temperature, significantly reducing the deposition temperature. This helps ensure the compatibility of deposited AlN thin films with CMOS manufacturing technology, expands the application range of AlN, and also helps reduce processing costs. Attached Figure Description

[0031] Figure 1 The XRD patterns of aluminum nitride thin films prepared in Reference Example 1, Reference Example 2, Reference Example 3 and Example 1 are shown.

[0032] Figure 2 The images show scanning electron microscope views of the surface and cross-section of the aluminum nitride films prepared in Reference Example 1 and Example 1; Figure 2 (a) is a SEM image of the aluminum nitride film prepared in Reference Example 1. Figure 2 (c) is a SEM cross-sectional image of the aluminum nitride thin film prepared in Reference Example 1. Figure 2 (b) is a SEM image of the aluminum nitride film prepared in Example 1. Figure 2 (d) is a SEM cross-sectional view of the aluminum nitride thin film prepared in Example 1;

[0033] Figure 3 AFM images of aluminum nitride films prepared in Reference Example 1, Reference Example 2, Reference Example 3, and Example 1 are shown; wherein, Figure 3 (a) shows an AFM image of the aluminum nitride thin film prepared in Reference Example 1; Figure 3 (b) shows an AFM image of the aluminum nitride thin film prepared in Reference Example 2; Figure 3 (c) An AFM photograph of the aluminum nitride thin film prepared in Example 1 is shown. Figure 3 (d) shows an AFM image of the aluminum nitride thin film prepared in Reference Example 3;

[0034] Figure 4 A cross-sectional view of the aberration-corrected aluminum nitride thin film prepared in Example 1 is shown using a scanning transmission electron microscope; wherein... Figure 4 (a) shows a cross-sectional STEM-HAADF image of the aluminum nitride thin film prepared in Example 1. Figure 4 (b) shows an HRTEM atomic image of the aluminum nitride thin film prepared in Example 1. Figure 4 (c) shows the SAED diffraction pattern of the aluminum nitride thin film prepared in Example 1;

[0035] Figure 5 The elemental distribution of Si, Al, Ta, and N in the aluminum nitride thin film deposited on a Si substrate prepared in Example 1 is shown.

[0036] Figure 6 The Raman spectra of the aluminum nitride thin films prepared according to Example 1 and Example 1 are shown; wherein, Figure 6 (a) Raman spectra of the aluminum nitride films prepared in Reference Example 1 and Example 1 are shown; Figure 6 (b) is Figure 6 (a) 600-720cm -1 Enlarged view of part of the atlas;

[0037] Figure 7 The diagram shows the AC voltage and amplitude variations of the aluminum nitride thin films prepared in Reference Example 1, Reference Example 2, Reference Example 3 and Example 1 under a piezoelectric force microscope. Detailed Implementation

[0038] This invention provides an aluminum nitride thin film, comprising polycrystalline aluminum nitride. A single crystal is a crystal in which particles are arranged regularly and periodically in three-dimensional space, with a uniform overall orientation. Polycrystalline aluminum nitride is an aggregate of single crystals with numerous oriented grains. Obviously, the fabrication cost of polycrystalline aluminum nitride thin films is much lower than that of single-crystal aluminum nitride thin films. The presence of grains with other orientations will lead to variations in the d-axis of polycrystalline aluminum nitride. 33 The piezoelectricity is lower than that of single-crystal aluminum nitride. Doping polycrystalline aluminum nitride with tantalum atoms can increase the piezoelectric coefficient of the aluminum nitride film and significantly improve its piezoelectric response. On the one hand, the larger radius of Ta atoms compared to Al atoms leads to an increase in lattice volume. On the other hand, theoretical calculations show that the enthalpy of formation of TaN is lower than that of AlN. Therefore, doping AlN with tantalum atoms causes local instability in the AlN lattice structure, promoting the elastic softening of the AlN lattice. The competition between Ta and Al atoms for the coordination of nitrogen atoms leads to Ta... x Al 1-xStrain forms in the AlN thin film, increasing its internal strain sensitivity. Furthermore, the elastic softening and increased lattice volume of AlN make it easier for the electric dipoles to flip, thus improving the piezoelectric response of the aluminum nitride piezoelectric thin film. Excessive tantalum doping leads to too many crystal defects, resulting in a decrease in the piezoelectric coefficient; therefore, the tantalum doping amount is controlled between 6.5 and 7.5 at.%. Further, when the crystal orientation of polycrystalline aluminum nitride is (002), its piezoelectric properties are even better. AlN has a hexagonal wurtzite crystal structure, and the c-axis (002) orientation is a sixth rotation axis of the hexagonal wurtzite AlN crystal. The AlN crystal has a high degree of symmetry in the plane perpendicular to the c-axis, but a lower degree of symmetry in the plane parallel to the c-axis. This makes the AlN crystal more likely to exhibit piezoelectricity along the c-axis direction. Meanwhile, since the spontaneous polarization direction of AlN material is along the c-axis, and the interatomic spacing of AlN crystals is relatively large along the c-axis, the change in electric dipole moment of the AlN film is most significant when deformation or an electric field is applied along the c-axis, resulting in more pronounced positive or negative piezoelectric effects. Tantalum doping can also ensure that polycrystalline aluminum nitride is oriented as much as possible (002) under relatively mild conditions during its preparation. Furthermore, when the tantalum doping amount is between 6.5 and 7.5 at.%, the c-value in the unit cell parameter of the polycrystalline aluminum nitride reaches 0.512 nm. At this point, the change in electric dipole moment of the AlN film is the largest, and the positive or negative piezoelectric effects are most pronounced. The piezoelectric coefficient of the AlN film can reach 15.8-16.1 pm / V. Therefore, this invention provides a low-cost method for preparing aluminum nitride films with high piezoelectric coefficients.

[0039] Specifically, the aluminum nitride thin film can be prepared by magnetron sputtering at room temperature using an AlTa alloy target. Using an AlTa alloy target ensures a more uniform distribution of sputtered atoms during the reaction, makes it easier to control the Ta doping amount, and simplifies the process. Preferably, the atomic percentage of Al atoms to Ta atoms in the AlTa alloy target is 92.5-93.5:6.5-7.5. Insufficient doping has little impact on the crystal structure and does not significantly improve piezoelectric performance, while excessive doping causes too many crystal defects, severely damaging the crystal structure and leading to a decrease in piezoelectric performance.

[0040] Specifically, this invention also provides a method for preparing the aluminum nitride thin film. Surprisingly, an (002) oriented aluminum nitride thin film can be obtained by magnetron sputtering a substrate at room temperature using an AlTa alloy target in a mixed atmosphere of inert argon and working nitrogen. The reason this method can prepare a highly (002) oriented aluminum nitride thin film is that, under the same sputtering conditions, the energy of Al atoms sputtered using an aluminum target is lower. When the energy of Al atoms is lower, they react more readily with nitrogen to form B1 bonds, thus more easily forming a (100) oriented aluminum nitride thin film. However, if the sputtering method using an AlTa alloy target provided by this invention is used, since the energy of Ta atoms is greater than that of Al atoms, Ta replacing Al sites will more easily form B2 bonds, suitable for forming a (002) orientation. Therefore, under the same conditions, doping with Ta can change the orientation of the aluminum nitride thin film, resulting in a polycrystalline (002) oriented aluminum nitride thin film. This invention utilizes an AlTa alloy target for sputtering to achieve (002) oriented aluminum nitride thin films via magnetron sputtering at room temperature. This is because the Ta atoms sputtered using an AlTa alloy target have relatively high energy, making it easier for Ta to replace Al sites and form B2 bonds, which is suitable for (002) orientation. High temperatures are not required to provide energy to the atoms, thus allowing (002) oriented AlN thin films to be obtained at room temperature. Magnetron sputtering at room temperature helps ensure the compatibility of these deposits with CMOS manufacturing technology and also reduces processing costs. In contrast, the paper "Enhancement of piezoelectric response of diluted Ta doped AlN" requires sputtering at 400°C to form (002) oriented aluminum nitride. Lowering the temperature may result in only polycrystalline (100) aluminum nitride. This is because high temperatures provide energy to the atoms. Lower atomic energy tends to lead to (100) oriented aluminum nitride.

[0041] Specifically, the inert gas includes argon; obviously, other inert gases can also be used to achieve the present invention. Preferably, the purity of the inert gas is greater than 99.999%; the higher the gas purity, the purer the reaction process, the purer the product, and the absence of other impurities.

[0042] Specifically, the purity of the nitrogen gas is greater than 99.999%. The higher the purity of the gas, the purer the atoms participating in the reaction process, the purer the products, and the absence of other impurities.

[0043] Specifically, the flow rate of the inert gas is 15-18 sccm. If the gas flow rate is too high, too many sputtered atoms will be generated, and the reaction will not have enough time to proceed. If the gas flow rate is too low, too few sputtered atoms will be generated, and the deposition rate will be too slow. Therefore, choosing an appropriate flow rate can ensure a certain deposition rate while allowing the reaction to proceed in a timely manner.

[0044] Specifically, the nitrogen flow rate is 15-18 sccm. Excessive flow rate leads to an overabundance of reactants during the reaction process, while insufficient flow rate results in a shortage of reactants, affecting the deposition rate and causing an excess of Al atoms.

[0045] Specifically, the substrate includes a p-type silicon wafer; since CMOS processes typically use p-type substrates, this makes it more suitable for applications of other devices.

[0046] Specifically, the p-type silicon wafer has a crystal orientation of (111). Since there is a large lattice mismatch between the (100) oriented Si substrate and AlN(002), the (111) oriented Si substrate is more likely to deposit AlN(002).

[0047] Specifically, the magnetron sputtering uses a DC power of 190-210W. The sputtering power provides energy for the deposited particles. It's not enough to simply bombard the Al atoms in the target material; they must also possess sufficient energy to ensure a smooth chemical reaction with nitrogen. If the power is too low, the sputtered atoms will lack energy, resulting in an amorphous state. If the power is too high, the sputtered atoms will have excessive kinetic energy, leading to more sputtered particles that cannot migrate in time, hindering grain nucleation and deteriorating the preferred orientation growth of the thin film.

[0048] Specifically, the purity of the AlTa alloy target is greater than 99.9%. Using a high-purity target reduces impurity atoms during the sputtering process, ensuring the purity of the deposited thin film.

[0049] Specifically, the magnetron sputtering pressure is 4-5 × 10⁻⁵. -4 Pa. The lower the vacuum pressure, the less residual other gases such as oxygen remain in the chamber, ensuring that only the working gas and the reactant gas are present during the reaction, thus reducing the generation of other substances during the reaction.

[0050] Specifically, before sputtering, the AlTa alloy target should be bombarded with inert gas ions for at least 10 minutes to remove contaminants and oxides from the AlTa alloy target surface and ensure atomic purity during the sputtering reaction.

[0051] Specifically, the substrate rotates at a speed of 12-15 rpm. This speed ensures film uniformity; too high a speed will affect film deposition and nucleation, while too low a speed will result in uneven film deposition.

[0052] Specifically, before sputtering, the p-type silicon wafer should be cleaned with hydrofluoric acid. The purpose of this step is to remove oxides from the surface of the silicon wafer.

[0053] This invention also provides an application of the aforementioned aluminum nitride thin film. Specifically, the aluminum nitride thin film can be used to prepare thin-film bulk acoustic resonators. Wurtzite aluminum nitride has different orientations, and different orientations affect its physical properties, thus determining different applications. Aluminum nitride (100) oriented films are advantageous for transverse acoustic wave applications, while the (002) oriented aluminum nitride provided by this invention is more suitable for longitudinal acoustic wave applications, and can therefore be used to prepare thin-film bulk acoustic resonators. This is because in (100) oriented aluminum nitride, acoustic waves mainly propagate along the surface of the aluminum nitride thin film. In (002) oriented aluminum nitride, acoustic waves mainly propagate along the longitudinal direction of the aluminum nitride thin film. After Ta doping, the piezoelectric coefficient of the aluminum nitride thin film provided by this invention increases, and the electroacoustic conversion efficiency also increases, thereby increasing the frequency of the bulk acoustic resonator. This makes the aluminum nitride thin film provided by this invention very suitable for preparing bulk acoustic resonators.

[0054] The present invention will be further described below with reference to reference examples and embodiments.

[0055] Reference Example 1

[0056] Substrate treatment. The p-type (111) silicon wafer (size: 1.5cm × 1.5cm) was ultrasonically cleaned in deionized water, acetone and anhydrous ethanol for 15 minutes in sequence, then immersed in diluted 5% hydrofluoric acid to remove oxides on the Si surface, and then rinsed in deionized water for 10 minutes to remove organic contaminants.

[0057] Install the substrate and target. Place the processed p-type (111) silicon wafer on the sample stage in the sputtering chamber of the magnetron sputtering coating machine, and install an aluminum target with a purity of 99.999% and a diameter of 3 inches on the magnet target head in the sputtering chamber.

[0058] Evacuate the sputtering chamber of the magnetron sputtering system. After installation, close the chamber door and sequentially open the mechanical pump and bypass valve to evacuate the chamber to below 5 Pa. Then, close the bypass valve and sequentially open the forestage valve, molecular pump, and gate valve to continue evacuating the chamber to 5 x 10 Pa. -4 Pa.

[0059] The target material is cleaned. After vacuuming, the argon flow rate is set, and the argon shut-off valve is opened to introduce high-purity argon (99.999%) at a flow rate of 40 sccm into the mixing chamber. Then, the inlet valve is opened, and the gas flows into the sputtering chamber. The gate valve is adjusted to maintain the vacuum level in the sputtering chamber at 0.5 Pa. The target baffle is then opened, the DC power is set to 190W, and the DC power supply is turned on to generate Ar.+ The target is bombarded with ions for 10 minutes to remove contaminants and surface oxides. After that, the DC power supply is turned off, the argon gas shut-off valve is closed, and finally the inlet valve is closed.

[0060] Thin film deposition was performed. After the target cleaning was completed, the flow rates of argon and nitrogen were set, and the shut-off valves of argon and nitrogen were opened. High-purity argon (99.999%) and nitrogen (99.999%) were introduced into the mixing chamber at a flow rate of 18 sccm, respectively. After waiting for 2 minutes to allow the argon and nitrogen to mix evenly, the inlet valve was opened to introduce the evenly mixed gas into the sputtering chamber. The gate valve was adjusted to keep the vacuum degree (sputtering pressure) in the sputtering chamber at 0.5 Pa. The sample baffle was opened, and the rotation speed of the sample stage was set to 15 revolutions per minute (rpm) to ensure the uniformity of the thin film. The DC power was set to 190W and the power was turned on to start the reaction. The thin film was deposited for 1 hour to obtain an AlN thin film deposited on a Si(111) substrate.

[0061] See Example 2

[0062] Al 0.97 Ta 0.03 Preparation of N-type piezoelectric thin film: The target material used is an AlTa alloy target with a purity of 99.9% and a composition of Al:Ta = 97:3 at.% manufactured by powder metallurgy sintering process. The sputtering pressure is 0.5 Pa. Other steps and conditions are the same as in Reference Example 1.

[0063] Example 1

[0064] Al 0.93 Ta 0.07 Preparation of N-type piezoelectric thin film: The target material used is an AlTa alloy target with a purity of 99.9% and a composition of Al:Ta = 93:7 at.% manufactured by powder metallurgy sintering process. The sputtering pressure is 0.5 Pa. Other steps and conditions are the same as in Reference Example 1.

[0065] Example 2

[0066] Al 0.935 Ta 0.065 Preparation of N-type piezoelectric thin film: The target material used is an AlTa alloy target with a purity of 99.9% and a composition of Al:Ta = 93.5:6.5 at.% manufactured by powder metallurgy sintering process. The sputtering pressure is 0.5 Pa. Other steps and conditions are the same as in Reference Example 1.

[0067] Example 3

[0068] Al 0.925 Ta 0.075Preparation of N-type piezoelectric thin film: The target material used is an AlTa alloy target with a purity of 99.9% and a composition of Al:Ta = 92.5:7.5 at.% manufactured by powder metallurgy sintering process. The sputtering pressure is 0.5 Pa. Other steps and conditions are the same as in Reference Example 1.

[0069] See Example 3

[0070] Al 0.88 Ta 0.12 Preparation of N-type piezoelectric thin film: The target material used is an AlTa alloy target with a purity of 99.9% and a composition of Al:Ta = 88:12 at.% manufactured by powder metallurgy sintering process. The sputtering pressure is 0.5 Pa. Other steps and conditions are the same as in Reference Example 1.

[0071] Example 4

[0072] Substrate treatment. The p-type (111) silicon wafer (size: 1.5cm × 1.5cm) was ultrasonically cleaned in deionized water, acetone and anhydrous ethanol for 15 minutes in sequence, then immersed in diluted 5% hydrofluoric acid to remove oxides on the Si surface, and then rinsed in deionized water for 10 minutes to remove organic contaminants.

[0073] Install the substrate and target. Place the processed p-type (111) silicon wafer on the sample stage in the sputtering chamber of the magnetron sputtering coating machine, and install an aluminum target with a purity of 99.999% and a diameter of 3 inches on the magnet target head in the sputtering chamber.

[0074] Evacuate the sputtering chamber of the magnetron sputtering system. After installation, close the chamber door and sequentially open the mechanical pump and bypass valve to evacuate the chamber to below 5 Pa. Close the bypass valve and sequentially open the fore-stage valve, molecular pump, and gate valve to continue evacuating the chamber to 4 x 10 Pa. -4 Pa.

[0075] The target material is cleaned. After vacuuming, the argon flow rate is set, and the argon shut-off valve is opened to introduce high-purity argon (99.999%) at a flow rate of 40 sccm into the mixing chamber. Then, the inlet valve is opened, and the gas flows into the sputtering chamber. The gate valve is adjusted to maintain the vacuum level in the sputtering chamber at 0.5 Pa. The target baffle is then opened, the DC power is set to 210W, and the DC power supply is turned on to generate Ar. + The target is bombarded with ions for 10 minutes to remove contaminants and surface oxides. After that, the DC power supply is turned off, the argon gas shut-off valve is closed, and finally the inlet valve is closed.

[0076] Thin film deposition was performed. After the target cleaning was completed, the flow rates of argon and nitrogen were set, and the shut-off valves of argon and nitrogen were opened. High-purity argon (99.999%) and nitrogen (99.999%) were introduced into the mixing chamber at a flow rate of 15 sccm, respectively. After waiting for 2 minutes to allow the argon and nitrogen to mix evenly, the inlet valve was opened to introduce the evenly mixed gas into the sputtering chamber. The gate valve was adjusted to keep the vacuum degree (sputtering pressure) in the sputtering chamber at 0.5 Pa. The sample baffle was opened, and the rotation speed of the sample stage was set to 12 revolutions per minute (rpm) to ensure the uniformity of the thin film. The DC power was set to 210W and the power was turned on to start the reaction. The thin film was deposited for 1 hour to obtain an AlN thin film deposited on a Si(111) substrate.

[0077] The physicochemical characteristics of the AlN piezoelectric films prepared in Reference Examples 1-3 and Examples 1-3 are further analyzed below.

[0078] First, the present invention compares the crystal orientation of the AlN piezoelectric films prepared in Reference Examples 1-3 and Example 1 by performing XRD tests on the AlN piezoelectric films prepared in Reference Examples 1-3 and Example 1.

[0079] Figure 1 The XRD patterns of the AlN piezoelectric thin films prepared in Reference Example 1, Reference Example 2, Reference Example 3, and Example 1 are shown. From... Figure 1 As can be seen from the image of the undoped AlN film, diffraction peaks at 33.22°, 36.05°, and 37.94° are observed, which matches well with PDF#25-1133. The results indicate that the AlN film possesses polycrystalline properties with (100), (002), and (101) planes, and the strongest peak is at 36.04°, corresponding to the (002) plane of the hexagonal wurtzite structure of AlN. This confirms the preferred (002) orientation of the AlN film on the Si(111) substrate. In the Ta-doped AlN film, with increasing Ta concentration, the positions of the diffraction peaks of the AlN piezoelectric films prepared in Reference Example 2, Reference Example 3, and Example 1 gradually shift to lower angles. This is because the doped Ta replaces the Al sites, and... The radius of the atom is greater than that of Al. According to Bragg's law, as the interplanar spacing increases, the angle shifts towards lower angles, causing varying degrees of lattice distortion. This indicates the preferred c-axis (002) orientation for the successful deposition of Ta-doped AlN films on the surface of Si(111) substrates.

[0080] This invention compares the grain size and surface morphology of the AlN piezoelectric films prepared in Reference Example 1 and Example 1 by performing SEM tests on the AlN piezoelectric films prepared in Reference Example 1 and Example 1.

[0081] Figure 2 SEM images of the AlN piezoelectric thin films prepared in Reference Example 1 and Example 1 are shown. Figure 2 (a) and Figure 2 (c) shows the surface and cross-sectional view of Reference Example 1. Figure 2 (b) and Figure 2 (d) shows the surface and cross-sectional view of Example 2. Compared with Reference Example 1, the grain size of Example 1 is smaller. This is because the deposition rate of Ta atoms in Example 1 is faster, and the grains migrate directly, forming a dense surface morphology with smaller grains. In contrast, in Reference Example 1, grains merge, that is, secondary grain growth occurs, resulting in a larger grain size. Figure 2 (c) and Figure 2 (d) It can be observed that the piezoelectric films of Reference Example 1 and Example 1 both exhibit a typical (002) oriented columnar structure arrangement perpendicular to the Si (111) surface.

[0082] The present invention performs AFM tests on the AlN piezoelectric thin films prepared in Reference Examples 1-3 and Example 1.

[0083] Figure 3 The surface roughness and morphology of the AlN piezoelectric films prepared in Reference Examples 1-3 and Example 1 are shown. The surface morphology of the film has a significant impact on the performance of the device. When the surface roughness of the film is large, sound waves will be scattered, which will seriously affect the performance of the device. Figure 3 (a)-(d) are surface morphology undulation diagrams of Reference Example 1, Reference Example 2, Example 1, and Reference Example 3, respectively. The roughness of Reference Example 1 is higher than that of Reference Example 2, Example 1, and Reference Example 3. This is because the film deposition rate is faster in Reference Example 2, Example 1, and Reference Example 3, and the sputtering produces uniform and dense grain growth with low roughness. In contrast, the secondary growth of grains in Reference Example 1 leads to larger grains and higher roughness.

[0084] The AlN piezoelectric thin film prepared in Example 1 was subjected to STEM testing in this invention.

[0085] Figure 4 The images show cross-sectional STEM-HAADF images, HRTEM atomic images, and SAED diffraction images of the AlN piezoelectric thin film prepared in Example 1, all with spherical aberration correction. Figure 4 (a) is a cross-sectional STEM-HAADF image, which shows that the thin film is mainly composed of many parallel columns perpendicular to the substrate surface. (Cross-section) Figure 4(b) shows an HRTEM atomic image of the AlN piezoelectric thin film prepared in Example 1. The image reveals different texture orientations in each region, which is because the aluminum nitride thin film prepared in Example 1 is composed of single crystals with different orientations. The planar spacing is 0.256 nm, and the lattice constant c is 0.512 nm, which is consistent with Ta... 0.07 Al 0.93 The XRD results for the N thin film are consistent. Figure 4 (c) shows the SAED diffraction pattern of the AlN piezoelectric thin film prepared in Example 1. The SAED pattern is a diffraction ring, confirming the polycrystalline nature of this region and indicating that the Ta-doped AlN piezoelectric thin film prepared in Example 1 is polycrystalline. The brightest diffraction point marked by the white solid line corresponds to the c-axis orientation (002) diffraction pattern of the hexagonal wurtzite AlN piezoelectric thin film, while other diffraction points correspond to... Figure 1 The smaller diffraction peaks (100) and (101) in the XRD indicate that it is a polycrystalline hexagonal wurtzite structure, and that a preferred c-axis (002) orientation is formed in the hexagonal wurtzite AlN piezoelectric film, which is consistent with... Figure 1 The diffraction peaks in the XRD are consistent.

[0086] The present invention performed an energy spectrum analysis of the elemental distribution of the AlN piezoelectric thin film prepared in Example 1 using scanning transmission electron microscopy.

[0087] Figure 5 The elemental composition of the aluminum nitride thin film deposited on the Si(111) substrate in Example 1 is shown. Figure 4 (a) Elemental distribution in the region. The distribution of Si, Ta, N, and Al in the figure shows that Ta is uniformly distributed in the piezoelectric film. This clearly demonstrates that Ta doping into the AlN piezoelectric film causes lattice changes, which is consistent with… Figure 1 Changes in XRD diffraction peaks and Figure 4 The lattice constant changes in (b) are consistent.

[0088] The present invention performed Raman tests on the AlN piezoelectric thin films prepared in Reference Example 1 and Example 1.

[0089] Figure 6 (a) shows the Raman spectra of the aluminum nitride thin films prepared in Reference Example 1 and Example 1. Figure 6 (b) is Figure 6 (a) 600-720cm -1 Partial magnified spectra; in the samples of Reference Example 1 and Example 1, approximately 520 cm⁻¹ -1 Approximately 620cm -1 The strong peak at 665 cm⁻¹ is clearly visible; this is the signal from the Si substrate. -1The peak centered on the AlN film is related to E, which corresponds to E 2 high The photophonon mode indicates that the AlN thin film crystal structure prepared in Reference Example 1 and Example 1 is a wurtzite structure, which is consistent with... Figure 1 The XRD diffraction peaks are consistent. To better explain the Raman mode characteristics of the AlN piezoelectric films prepared in Reference Example 1 and Example 1, software can be used to fit the Lorentz function at a wavenumber of 620 cm⁻¹. -1 and 665cm -1 Two overlapping peaks centered on the center. Figure 6 (b) shows the E after fitting the sample. 2 high A magnified image of the visible spectral region of photophonons, from which E can be observed. 2 high Higher wavenumber, i.e. E 2 high It exhibits a blue shift phenomenon. Generally, the Raman shift is related to the structural defects and stress of the material under study. On the one hand, according to theoretical calculations, the enthalpy of formation of wurtzite TaN is lower than that of wurtzite AlN. Therefore, doping AlN with tantalum atoms will cause local instability in the AlN lattice structure, promoting the elastic softening of the AlN lattice. On the other hand, due to the competition between Ta and Al atoms for coordinating nitrogen atoms, Ta... x Al 1-x Strain formation occurs in N thin films, causing changes in the lattice constant, and... Figure 4 The results in (b) are consistent.

[0090] The effective piezoelectric coefficient d of the AlN piezoelectric thin films prepared in Reference Example 1, Reference Example 2, Example 1, and Reference Example 3 was tested by piezoelectric force microscopy (PFM). 33 .

[0091] Figure 7 The diagrams showing the AC voltage versus amplitude variations under a piezoelectric force microscope (PFM) for Reference Example 1, Reference Example 2, Example 1, and Reference Example 3, illustrate the effective piezoelectric coefficient d. 33，eff The effective piezoelectric coefficients d of the AlN piezoelectric thin films prepared in Reference Examples 2, 1, and 3 relative to Reference Example 1 are shown. 33，effThe effective piezoelectric coefficient increases significantly with increasing Ta content, reaching a maximum of 16.1 pm / V at a Ta doping content of 7 at.%. Subsequently, the effective piezoelectric coefficient decreases, consistent with XRD and Raman results. Ta substitution at Al sites alters the lattice parameters due to the larger atomic radius of Ta atoms compared to Al, causing local instability in the lattice structure and promoting elastic softening of AlN. Simultaneously, competition between Ta and Al atoms for nitrogen atom coordination leads to stress, increasing internal strain sensitivity and thus improving the effective piezoelectric coefficient of AlN. However, excessive Ta doping results in more lattice defects, leading to a decrease in the effective piezoelectric coefficient of the AlN piezoelectric film prepared in Example 3. The effective piezoelectric coefficients d in Examples 2, 3, and 4 are... 33 The values ​​are 15.8 pm / V, 15.9 pm / V, and 15.8 pm / V (not shown in the figure), respectively, which indicates that the electric dipole moment of the AlN piezoelectric film is the largest when the Ta doping content is 7 at.%.

Claims

1. A method for preparing an aluminum nitride thin film, the method comprising the step of magnetron sputtering an AlTa alloy target on a substrate in a mixed atmosphere of inert gas and nitrogen at room temperature, wherein: the direct current power used in the magnetron sputtering is 190-210 W; the atomic percentage of Al atoms to Ta atoms in the AlTa alloy target is 92.5-93.5: 6.5-7.5; the aluminum nitride thin film comprises polycrystalline aluminum nitride; the polycrystalline aluminum nitride is doped with 6.5-7.5 at.% of tantalum; the crystal face orientation of the polycrystalline aluminum nitride is (002); the value of c in the unit cell parameters of the polycrystalline aluminum nitride is 0.512 nm; and the piezoelectric coefficient of the aluminum nitride thin film is 15.8-16.1 pm / V.

2. The method for preparing an aluminum nitride thin film according to claim 1, wherein: the inert gas comprises argon; the purity of the inert gas is greater than 99.999%; the purity of the nitrogen is greater than 99.999%; the flow rate of the inert gas is 15-18 seem; and the flow rate of the nitrogen is 15-18 seem.

3. The method for preparing an aluminum nitride thin film according to claim 1, wherein: the substrate comprises a p-type silicon wafer; and the crystal face orientation of the p-type silicon wafer is (111).

4. The method for preparing an aluminum nitride thin film according to claim 1, wherein: the rotation speed of the substrate is 12-15 rpm; and the purity of the AlTa alloy target is greater than 99.9%.

5. The method for preparing an aluminum nitride thin film according to claim 1, further comprising the step of ion bombarding the AlTa alloy target with inert gas for at least 10 minutes.

6. The method for preparing an aluminum nitride thin film according to claim 1, further comprising the step of cleaning the substrate with hydrofluoric acid.

7. The use of an aluminum nitride thin film according to claim 1, wherein: the aluminum nitride thin film is used to prepare a film bulk acoustic resonator. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The pressure of the magnetron sputtering is 4-5x10 -4 Pa; ​ ​ ​ ​ ​ ​ ​ ​

Images