Piezoelectric ceramics materials are functional materials that realize the conversion between mechanical energy and electrical energy (1)

Piezoelectric materials are functional materials that realize the conversion between mechanical energy and electrical energy. Its development has a long history. Since the discovery of the piezoelectric effect on quartz crystals by the CURIE brothers in the 1880s, piezoelectric materials have attracted widespread attention. With the deepening of research, a large number of piezoelectric materials, such as piezoelectric functional ceramic materials, Piezo film, piezoelectric composite materials, etc. These materials have a very wide range of uses, and play an important role in functional conversion devices such as electricity, magnetism, sound, light, heat, humidity, gas, and force.

PVDF piezoelectric film
PVDF piezoelectric film is polyvinylidene fluoride piezoelectric film. In 1969, the Japanese discovered the polymer material polyvinylidene fluoride (polyvinylidene fluoride polymer) referred to as PVDF, which has a very strong piezoelectric effect. The PVDF film mainly has two piezo crystal types, namely, α type and β type. The α type piezo crystal does not have piezoelectricity, but after the PVDF film is rolled and stretched, the original α type crystal in the film becomes a β type crystal structure. When the stretched and polarized PVDF film is subjected to external force or deformation in a certain direction, the polarized surface of the ultrasonic level transducer will generate a certain electric charge, that is, the piezoelectric effect.

Compared with piezoelectric ceramics and piezoelectric crystals, piezoelectric films have the following advantages:

(1) Light weight, its density is only a quarter of the commonly used piezoelectric ceramic PZT, pasted on the measuring object has almost no effect on the original structure, high elastic flexibility, can be processed into a specific shape The measuring surface is completely fitted, with high mechanical strength and impact resistance;
(2) High voltage output, under the same stress conditions, the output voltage is 10 times higher than piezoelectric ceramics;
(3) High dielectric strength which can withstand the effect of strong electric field (75V / um), at this time most piezoelectric ceramics have been depolarized;
(4) The acoustic impedance is low, only one tenth of the piezoelectric ceramic PZT, close to water, human tissue and viscous body;
(5) The frequency response is wide, and the electromechanical effect can be converted from 10-3Hz to 109, and the vibration mode is simple.

Piezo film properties
1. Dielectric constant
Although the piezoelectric film is a single crystal film or a polycrystalline film with preferred orientation, the atomic packing in it is not as dense and ordered as in the crystal, so the dielectric constant value of the piezoelectric film is different from the value of the crystal . In addition to this, there are often large residual internal stresses in the film and the reasons for the measurement, which also cause the dielectric constant value of the film to be different from the corresponding value of the crystal.
Existing studies have shown that the dielectric constant of the piezoelectric film is not only related to the crystal orientation, but also depends on the test conditions. The dielectric constant of the piezoelectric film has a considerable dispersion. In addition to the difference in internal stress and test conditions, the difference between the chemical composition ratio and the film thickness of the film composition is generally believed to decrease with the thickness of the film. Thin and small. In addition, the dielectric constant of the piezoelectric thin film will also change significantly with temperature and frequency.

2. Volume resistivity
From the perspective of reducing the dielectric loss and relaxation frequency of the piezoelectric film, it is expected that it has a very high resistivity, at least ρv≥108Ω • cm. The resistance of AlN film is 2 × 1014 ~ 1 × 1015Ω · cm, which is much higher than 108Ω · cm, so in this respect, AlN is a very excellent film. In addition, the change in electrical conductivity of AlN piezoelectric films with temperature also follows the 1nσ∝1 / T law. None of the crystals with piezoelectric effect have a center of symmetry, so their electron mobility is also anisotropic and their electrical conductivity is also different. The conductivity of the AlN piezoelectric film along the C-axis direction is different from the direction perpendicular to the C-axis. The former is about 1 to 2 orders of magnitude smaller.

3. Loss angle tangent
The dielectric loss tangent of the AlN piezoelectric film is tanδ = 0.003 ～ 0.005, and the tanδ of the ZnO film is larger, which is 0.005 ～ 0.01. The reason why the tanδ of these films is so large is that in addition to the conductance process, these films also have significant relaxation phenomena. Similar to the dielectric thin film, the tan δ of the piezoelectric thick film increases gradually with the increasing of temperature and frequency and the increase of humidity. In addition, as the film thickness decreases, tan δ tends to increase. Obviously, the increase in tan δ with temperature is due to the increase in conductance and the increase in relaxors. It increases with frequency because the number of relaxation times in time increases.

4. Breakdown strength
Because the dielectric breakdown field strength belongs to the strength parameter, and various defects are unavoidable in the film, the breakdown field strength of the piezoelectric film has considerable dispersion; the dielectric breakdown theory .The breakdown field strength should gradually increase as the film thickness decreases. But in fact, because the film contains many defects, the effect of the defect is more significant as the thickness is smaller, so when the thickness is reduced to a certain value, the breakdown field strength of the film becomes sharply smaller. The breakdown field strength of the film, in addition to the reasons of the film itself, also has the influence of the edge of the electrode during the test. Since the thicker the film, the more uneven the electric field at the edge of the electrode, so as the film thickness increases, its breakdown field strength gradually decreases. In addition to the above factors, the breakdown field strength of the dielectric film also depends on the film structure. For the piezoelectric film, the breakdown field strength also depends on the direction of the electric field, that is, it is also anisotropic in the breakdown field strength. Due to the existence of grain boundaries in the polycrystalline film, its breakdown field strength is lower than that of the amorphous film; for similar reasons, the breakdown field strength of the preferentially oriented piezoelectric film in the grain orientation direction is higher than that in the perpendicular direction .The breakdown field strength is lower.

Like other dielectric films, the breakdown field strength of the piezoelectric film also depends on some external factors, such as voltage waveform, frequency, temperature, and electrodes. Because the breakdown field strength of the piezoelectric film is related to many factors, for the same film, the breakdown field strength values reported in the relevant literature are often inconsistent, and even vary greatly. For example, the breakdown field strength of the ZnO film is 0.01 ~ 0.4MV / cm, AlN film is 0.5 ~ 6.0MV / cm.

5. Bulk acoustic wave performance
The most important characteristic parameters of bulk acoustic wave piezoelectric transducers are resonance frequency f0, acoustic impedance Za and electromechanical coupling coefficient K, so the sound velocity υ and temperature coefficient of piezoelectric film, acoustic impedance and electromechanical coupling coefficient are particularly strict. These properties of the film not only depend on the elasticity, dielectric, piezoelectric and thermal properties of the crystal grains in the film, but also are closely related to the structure of the piezoelectric film such as the degree of compactness of the grains and the degree of preferred orientation. In the piezoelectric film, due to the defects and strain of the crystal grain, it is not a perfect single crystal, so the physical constant of the film is slightly different from the crystal value. Because the structure of the piezoelectric film is closely related to the preparation process, even for the same piezoelectric film, the performance values reported in the various literatures are often inconsistent. Among all inorganic non-ferrous piezoelectric films, the AlN film has a large elastic constant, but a low density, and the highest sound velocity. Therefore, the film is most suitable for UHF and microwave devices.

6. Surface acoustic wave performance
When the surface acoustic wave propagates in the piezoelectric cylinder transducer, its particle displacement amplitude attenuates rapidly as the distance from the surface of the medium increases, so the surface acoustic wave energy is mainly concentrated in the next two wavelengths on the surface. The surface acoustic wave performance of the film material can be expressed as the following functional formula: surface acoustic wave performance = F (raw material, substrate, film structure, wave mode, propagation direction, interdigital electrode form, thickness wave number product) A table of sound wave performance parameters cannot be represented by a single value. Another acoustic wave property of piezoelectric films is transmission loss. Because piezoelectric films are often used as acoustic transmission media in surface wave devices, the source of transmission loss is mainly the scattering of acoustic waves in the piezoelectric film and the substrate.

Method for preparing piezoelectric film
The preparation methods of piezoelectric thin films mainly include traditional vacuum coating methods, including vacuum evaporation coating, sputter coating, and chemical vapor deposition coating with a thickness of 0-18 μm, and new sol-gel method, hydrothermal method, and electrophoretic deposition method 10 ～ 100μm piezoelectric thick film material.
Thick piezoelectric film usually refers to a piezoelectric film with a thickness of 10 to 100 μm. Compared with the thin film, its piezoelectric and ferroelectric properties are less affected by the interface and surface; because of its relatively large thickness, this kind of PZT material can also generate a large driving force, and has a wider operating frequency; compared with the bulk material, its operating voltage is low, the frequency of use is high, and it is compatible with semiconductor processes.

1. Vacuum evaporation coating
Vacuum evaporation coating is to evaporate a substance by heating and deposit it on a solid surface, which is called evaporation coating. This method was first proposed by M. Faraday in 1857, and modernization has become one of the commonly used coating technologies.
Vacuum evaporation coating includes the following three basic processes:
(1) Heating and evaporation process, including the edging process of changing from condensed phase to gas phase (solid phase or liquid phase → gas phase). Each evaporating substance has a different saturated vapor pressure at different temperatures. When evaporating a compound, its components react, and some of them enter the evaporation space in gaseous state or vapor.
(2) The transportation of vaporized atoms or molecules between the evaporation source and the substrate, and the flight process of these examples in the ambient atmosphere. The number of collisions with residual gas molecules in the vacuum chamber during flight depends on the average free path of the evaporated atoms and the distance from the evaporation source to the substrate, often called the source-base distance.
(3) The precipitation process of evaporated atoms or molecules on the surface of the substrate, and the vapor condensation, nucleation, nuclear growth, and the formation of a continuous film. Since the temperature of the substrate is much lower than the temperature of the evaporation source, the phase transition process of the deposit molecules on the substrate surface will occur directly from the gas phase to the solid phase.
When a substance evaporates, it is important to know the saturated vapor pressure, evaporation rate, and average free path of the evaporated molecules. There are three types of evaporation sources.

2. Vacuum sputtering coating
An example with an kinetic energy of more than a few hundred electron volts or an ion beam bombards the solid surface, so that the atoms close to the solid surface obtain a part of the energy of the incident particles and leave the solid to enter the vacuum. This phenomenon is called sputtering. The sputtering phenomenon involves a complex scattering process and is accompanied by various energy transfer mechanisms. It is generally believed that this process is mainly the so-called collision cascade process, that is, the incident ions collide elastically with the target atoms, so that the target atoms obtain sufficient energy to overcome the potential barrier formed by the surrounding atoms and leave the original position, and further and nearby Atoms collide. When this collision cascade reaches the surface of the target atom so that the atoms obtain energy higher than the surface binding energy, these atoms will leave the surface of the target atom and enter a vacuum. Now more researches on sputter coating are magnetron sputter coating. Magnetron sputtering is to perform high-speed sputtering under low pressure, and it is necessary to effectively increase the ionization rate of the gas. By introducing a magnetic field on the surface of the target cathode, the magnetic field is used to restrain the charged particles to increase the plasma density to increase the sputtering rate. Use an external magnetic field to capture electrons, extend and constrain the movement path of electrons, increase the ionization rate, and increase the coating rate.

3. Chemical vapor deposition coating
Chemical vapor deposition is a chemical vapor growth method, referred to as CVD (Chemical Vapor Deposition) technology. In this method, the elemental gas containing one or several compounds constituting the thin film element is supplied to the substrate, and the required thin film is formed by gas phase or chemical reaction on the surface of the substrate by using energy sources such as heating, plasma, ultraviolet light or even laser light. Since the CVD method uses various gas reactions to prepare the thin film, the composition of the thin film can be arbitrarily controlled, so that many new film materials can be produced. When the CVD method is used to prepare a thin film, its growth temperature is significantly lower than the melting point of the thin film constituent material, the resulting film layer has good uniformity, has step coverage, and is suitable for substrates with complex shapes. Because of its advantages such as high deposition rate, few pinholes, high purity, compactness, and few crystal-forming defects, the application range of chemical vapor deposition is very wide. The CVD method can be used to prepare piezoelectric thick film materials with dense, smooth surface, thickness of 0 ～ 18μm and excellent performance. Therefore, in the preparation of piezoelectric thick films, the CVD method has developed rapidly and has been adopted by many researchers.

4. New solution gel method
The new sol-gel method is to add the prepared powder (same composition as the sol) to the sol, then add a certain organic solvent to the solution as a dispersant, add other organic solvents to adjust the viscosity and pH of the solution, and finally do not continuous ultrasonic vibration disperses the nano-powders in the solution, and finally obtains a uniform powder solution. The required film is deposited on the substrate by the sol-gel method. In this deposition process, the powder particles act as seed crystals.
In this way, a thick film with a thickness of tens of microns can be produced. It avoids the problem of cracking or even film shedding caused by the thick film prepared by the traditional sol-gel method. The prepared thick film components are uniformly mixed and high in purity, and do not require high temperature sintering, and the resulting thick film is compatible with the semiconductor preparation process. And the equipment is simple, the cost is low, and the membrane composition can be controlled, so this method is currently used more often.

5. Hydrothermal method
The hydrothermal method refers to the use of an aqueous solution as a reaction medium in a specially-made closed reaction vessel (autoclave). By heating the reaction vessel, a high-temperature, high-pressure reaction environment is created, so that normally insoluble or insoluble substances are dissolved and recrystallized. The thick film prepared by this method is to stoichiometrically mix some compounds in the thick film component to be prepared into a saturated solution in a certain alkaline medium and adjust the PH value. After that, the solution is transferred into an autoclave, and a certain thickness can be grown on the substrate after a certain reaction time.