What are the applications of MEMS sensors in the field of ocean observation?
Compared with traditional sensors, MEMS sensors have smaller volume, higher sensitivity, faster response speed, lower power consumption, and can make sensor arrays and integrate with processing circuits. It can fully meet the requirements of high integration, miniaturization, intelligence, and low power consumption in the ocean observation field. At present, there are some researches on ocean observation technology based on MEMS at home and abroad.
⒈ MEMS-based CTD equipment
CTD equipment is a commonly used measuring instrument in ocean observation, which can measure the temperature, salinity and depth of seawater. Usually CTD equipment includes three sensor parts: temperature sensor, conductivity sensor and pressure sensor. MEMS-based CTD sensors have been studied extensively.
In the MEMS-based CTD device, the temperature sensor usually adopts a resistance thermometer made of platinum or gold doped on a silicon substrate. This type of MEMS temperature sensor has the advantages of low cost, simple structure, and easy packaging. At the same time, because it is directly fabricated on a silicon substrate, it can be integrated with a processing circuit or other sensors fabricated on a silicon substrate. The platinum resistance thermometer can reach a very high accuracy, in the range of 0～50℃, the accuracy can reach 10-3℃～10-4℃.
MEMS-based conductivity sensors usually use a parallel plate structure to measure the conductivity of seawater between the plates. In order to improve the measurement accuracy and eliminate errors and other influences, many improvements have been made to the design of the parallel plate conductivity sensor. When the parallel plate conductivity sensor is working, the electric field will be distributed in a larger area, and any conductor or insulator entering the electric field will interfere with the measurement result. In order to reduce the influence of the external electric field, the electric field can be limited to the required area by design. For conductivity sensors with electrodes located on the surfaces of two parallel plates, the external electric field only appears in the inter-plate area around the opposite electrode. Usually, a protection ring can be added around one electrode to reduce or eliminate the external electric field. Another type of parallel plate conductivity sensor, the electrode is located on one of the plates. Although this structure has higher resolution, the external electric field generated by it has a larger distribution range, not only in the area between the plates, but also on the outside of the plates. An external electric field is distributed. For this structure, people limit the electric field by adding electrodes and adding auxiliary circuits.
A complete CTD system usually also includes a pressure sensor, but in some systems, the pressure sensor is also omitted. The pressure sensor can be a piezoresistive or piezoelectric sensor. At present, the piezoresistive sensor technology is relatively mature, and the pressure sensor in CTD equipment often adopts a piezoresistive sensor. The basic structure of the pressure sensor is a fixed diaphragm structure. The diaphragm structure acts as an elastic element to sense pressure and convert the pressure into a change in resistance through piezoresistive characteristics. Monocrystalline silicon itself is a good piezoresistive material, which can be directly used as a structural material. During processing, bulk processing is usually performed on a monocrystalline silicon substrate to form a diaphragm, and then a small amount of doping on the edge of the diaphragm to form a p-type or n-type resistor for easy measurement .
⒉ MEMS-based acoustic transducer
Acoustic transducers can convert sound waves and electrical signals into each other, and have the functions of receiving and emitting sound waves. Acoustic transducers play an important role in ocean observation, and can be used for hydrophones, underwater sonar detection devices, or for underwater communication to build underwater wireless sensor networks. The MEMS-based acoustic transducer has a structure similar to that of the pressure sensor. There are acoustic transducers based on piezoelectric effect and capacitance detection respectively.
The piezoelectric effect-based acoustic transducer uses the piezoelectric effect and the reverse piezoelectric effect. The piezoelectric effect can convert the deformation of the diaphragm into the movement of electric charges, which is used to induce the vibration of sound waves and generate electrical signals. The reverse piezoelectric effect is the opposite. By applying a longitudinal external electric field to the piezoelectric material, the piezoelectric material generates a single-direction lateral stress, which causes the diaphragm to bend. By controlling the periodic change of the electric field, sound waves of a certain frequency can be generated. Capacitive acoustic transducers generally adopt a parallel plate capacitor structure, and can also adopt relatively complex structures such as interdigital capacitors. The upper plate of the parallel plate capacitor structure is a movable (deformable) diaphragm, and the lower plate is a fixed silicon substrate, which can contain multiple electrodes. When used as a sound wave launching device, a DC bias voltage is applied between the upper and lower plates to form an electrostatic field, and then an AC voltage signal is applied to drive the diaphragm to vibrate to generate sound waves. When used as a sound wave receiving device, the diaphragm structure induces vibration and transforms it into a change in capacitance between parallel plates.
The MEMS acoustic transducer is small in size and highly integrated, and can integrate a signal processing circuit on-chip to compensate for the nonlinearity of the transducer itself. Highly integrated MEMS devices can realize a two-dimensional array structure closely arranged in micron size, which can be applied to three-dimensional imaging.
Inertial (acceleration) sensor is a relatively mature technology and widely used MEMS sensor, mainly used for wave observation. At present, wave measuring buoys at home and abroad usually use traditional acceleration sensors. These sensors are large, heavy, and expensive, which increases the cost of making the buoy. In recent years, as the technology of MEMS-based inertial sensors has gradually matured, MEMS inertial sensors have gradually been popularized for wave observation. The basic principle of an inertial sensor is similar to that of a pressure sensor. It uses the piezoelectric effect or piezoresistive effect or capacitive displacement detection principle. The difference is that it does not sense the deformation caused by external force, but adds a suspended mass to the elastic structure. When the sensor is accelerating or decelerating, due to inertia, the motion of the mass has a certain lag relative to the motion of the substrate, which causes the elastic diaphragm connecting the mass and the substrate to deform, thereby sensing acceleration. Depending on the structure, a single inertial sensor can also detect acceleration changes in multiple directions. Since the principle of wave measurement is the same as before, it has promoted the replacement of foreign wave measurement buoys.
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