Piezoelectric materials are functional materials that realize the conversion between mechanical energy and electrical energy(2)

Method for preparing piezoelectric film

The preparation methods of piezoelectric thin films are mainly traditional vacuum coating methods, including vacuum evaporation coating, sputtering coating, chemical vapor deposition coating is prepared in a thickness of 0 ~ 18μm, new sol-gel method, hydrothermal method, electrophoretic deposition method is prepared 10-100μm piezoelectric thick film material.

Thick piezoelectric film usually refers to a piezoelectric film  piezoelectric hemisphere transducer 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 The 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) The heating and evaporation process including the edging process from the condensed phase to the 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 vaporized 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 piezo ceramics piezoelectric transducer 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.

①Resistance heating source: made of refractory metals such as tungsten and tantalum made of boat foil or filament, passing current to heat the evaporation material above it or placed in the crucible (resistance heating source is mainly used to evaporate Cd, Pb, Ag, Al, Cu, Cr, Au, Ni and other materials.

② High frequency induction heating source: heating the crucible and evaporating material with high frequency induction current.

③ Electron beam heating source: suitable for materials with high evaporation temperature (not less than 2000), that is, bombard the material with electron beam to make it evaporate.

In order to deposit a high-purity single crystal film, molecular beam epitaxy can be used. The jet furnace is equipped with a molecular beam source. When it is heated to a certain temperature under ultra-high vacuum, the  piezo disk transducer elements in the furnace are directed toward the substrate as a beam of molecular flow. The substrate is heated to a certain temperature, the molecules deposited on the substrate can migrate, and the crystals are grown in the order of the substrate lattice. The molecular beam epitaxy method can obtain a single crystal film of high purity compound with the required stoichiometric ratio, and the film grows the slowest. The speed can be controlled at 1 single layer/second. By controlling the baffle, single piezo crystal thin films with desired composition and structure can be made accurately. Molecular beam epitaxy is widely used to manufacture various optical integrated devices and various superlattice structure films

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 research on sputter coating is magnetron sputtering 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 target cathode surface, 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 restrain the movement path of electrons, increase the ionization rate, and increase the coating rate.

4. New solution gel method

The new sol-gel method is to add the prepared powder (same composition as the sol) to the sol, and then add a certain organic solvent to the solution as a dispersant, and add other organic solvents to adjust the viscosity and pH of the solution. Continuous ultrasonic vibration disperses the nano-powder in the solution, and finally obtains a uniform powder solution, and deposits the required film 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 which do not require high-temperature sintering. 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

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.

Hydrothermal preparation of thick films has many advantages:

① The process is completed in the liquid phase at one time, and no post-crystallization heat treatment is required, thus avoiding defects such as cracking, grain coarsening, reaction with the substrate or atmosphere that may be caused during the heat treatment process;

②Inorganic materials are used as precursors, and water is used as the reaction medium. The raw materials are easily available, which reduces the cost of film preparation and has less environmental pollution;

③ The equipment is simple, and the temperature of hydrothermal treatment is low, which avoids the interdiffusion of the film and substrate components before and after hydrothermal treatment. The resulting film has high purity and good uniformity. In addition, when this method is used to prepare thick films, thick films can be deposited on substrate surfaces of various complex shapes. The resulting thick films have certain advantages of spontaneous polarization, low hysteresis, and good bonding with substrates. . At present, this method has attracted more and more attention.

6. Electrophoretic deposition method

Electrophoretic deposition (EPD) refers to dispersing the prepared fine powder with the same composition as the thick film in the suspension to form a suspension with different concentrations, and adjusting the pH value of the suspension with acid-base solution. Stable suspension is obtained through ultrasonic dispersion and magnetic stirring, and under constant pressure, the charged particles move directionally under the action of the electric field, thereby obtaining a thick film with a certain thickness. The thick film prepared by this method has the advantages of simple equipment, fast film formation, unlimited shape of the plated parts, uniform and controllable film thickness, etc. The resulting thick film can reach tens of microns, and the composition is uniform and dense.