The basic mechanism of a resonant pressure sensor is to use a pressure sensitive element to sense pressure, change the resonant frequency of the associated resonator, and detect the pressure by measuring the change in the resonator frequency. Compared with common pressure sensors such as piezoresistive and capacitive pressure sensors, resonant pressure sensors are small in size and low in power consumption; the characteristic of frequency as the final output makes it have higher accuracy and stability, and is easy and large-scale integration Circuit compatible.
In recent years, with the advancement of micro-machining technology and the advancement of weak signal detection technology, the research and production of resonant pressure sensors based on MEMS technology have received more and more attention. This paper introduces a resonant pressure sensor with a differential detection structure. The resonator adopts the electromagnetic excitation-electromagnetic pickup excitation method and the closed-loop self-excited oscillation detection method to detect pressure.
1. Working principle
The overall structure of the sensor is composed of a single crystal Si pressure film and a single crystal Si beam resonator. The two are combined into a whole through bonding technology. The upper resonator is encapsulated in vacuum, and the lower side of the lower Si film is in contact with the pressure source to be measured. The periphery of the film is in fixed contact with the bottom of the packaged tube socket. When the Si film is subjected to pressure, the film will deform. The resonant beam struts in contact with the membrane will also deform with the deformation of the membrane. In this way, the beam resonator located at the upper end of the strut will be subjected to axial stress due to the deformation of the strut, thereby changing its own natural frequency. The change in frequency is approximately linear with the change in axial stress and the pressure on the membrane, so the purpose of pressure detection can be achieved by detecting the change in the natural frequency of the resonant beam. This sensor uses a differential detection method, which is divided into three groups of upper, middle and lower resonant beams for detection. Through the differential detection of the middle beam and any group of upper and lower beams, the sensitivity of the entire sensor can be improved, and the influence of temperature drift on the frequency drift of the resonance beam can be greatly reduced.
The structure of the sensor's resonant beam, the two beams and the bridge connected in the middle constitute the sensor's resonator. When working, a magnetic field perpendicular to the upper surface of the resonance beam is applied. When a periodic alternating voltage is applied between the excitation electrodes A and B, the excitation beam is subjected to Lorentz force due to the generation of current. With the change of the voltage direction, Luo The direction of the Lenz force also changes periodically, so that the excitation beam vibrates due to the force of the periodically changing direction, and the upper vibration pickup beam is driven to vibrate through the middle bridge. When the vibrating beam vibrates, an induced electromotive force is generated between the vibrating electrodes C and D due to the cutting of the magnetic field lines, and the frequency is the same as the voltage applied to the vibrating beam. When the frequency of the applied voltage is close to or equal to the natural frequency of the entire resonant beam, the resonant beam will resonate, and the amplitude of the pickup beam will reach the maximum, so that the amplitude of the induced electromotive force between the pickup electrodes will also reach the maximum. By detecting the magnitude of the induced electromotive force generated by the vibrating beam, the natural frequency of the resonance beam is determined to achieve the purpose of detecting pressure. This structure is called "H-shaped" beam structure.
2. Production and packaging
The production of the sensor uses bulk silicon micromachining technology. First, the resonator and the silicon pressure-sensitive film are fabricated on the Si chip respectively, and then the two Si chips are combined by Si-Si bonding to form a chip with a three-dimensional structure, and finally the entire device is completed by vacuum packaging.
After the chip is made, stick it on the tube socket, ultrasonically weld the lead, cover the tube cap, open the vent on both sides, seal the side with the resonance beam in a vacuum (10-3 Pa), and the pressure film on the other end The pressure source to be tested is connected, thus completing the packaging of the pressure sensor.
3. Test
The pressure sensor test system consists of a pressure sensor, a lock-in amplifier, a signal excitation source, and a vibration pickup constant current source. Using pure AC excitation voltage to excite the vibration, using a lock-in amplifier, by controlling frequency scanning, data acquisition, and detecting amplitude and phase, the frequency characteristics of the resonator can be detected.
The pressure sensor device was dynamically analyzed and tested. The frequency characteristics of the device in the air were tested by an open-loop scanning system composed of a frequency scanner. The peak value of the resonant beam was about 72.30 kHz, and the -3 dB bandwidth was about 50 Hz. It is calculated that the quality factor Q of the resonant beam is greater than 1,200.