Open-loop characteristic test of resonant silicon microstructure pressure sensor

In 1954, Smith of Bell Labs discovered the silicon piezoresistive effect, which led to the birth and rapid development of micro-electromechanical system (MEMS) technology, and first achieved success in the application of silicon micromechanical sensors , bringing a new revolution in sensor technology. Pressure sensors measure by reading the deformation of the membrane under the measured pressure. In the early 1980s, the use of low creep and low hysteresis single-crystal silicon membranes to replace traditional metal membranes made a breakthrough in pressure sensors, while achieving the goals of miniaturization and mass production. Among the many silicon micromechanical pressure sensors, the resonant silicon microstructure pressure sensor that directly outputs frequency quantity has the highest measurement accuracy and has become the main development direction of pressure sensors. China has independently tracked this international advanced technology since the 1990s and began to study resonant silicon micromechanical pressure sensors. The resonator is the core component of the resonant sensor, and its quality largely determines the overall accuracy of the sensor. To obtain a high-quality resonator, it depends on design on the one hand and processing on the other. The mechanical quality factor (Q value) is usually used to characterize the quality of the resonator, which is defined as the ratio of the total energy stored in the vibration to the energy consumed per cycle. The Q value cannot be measured directly, but is usually obtained from the steady-state frequency response curve of the resonator with constant amplitude sinusoidal excitation. In addition, the resonant sensor must work in a closed-loop self-excitation state to maintain resonance, and the closed-loop design also needs to be based on the amplitude-frequency and phase-frequency characteristics of the resonator. Since the amplitude of the silicon micromechanical resonator is very weak and the signal-to-noise ratio is very low. It cannot be detected by a general spectrum analyzer, and a measuring instrument with weak signal processing function is required to obtain its frequency response curve. There are two common practices internationally. One is to use general equipment to build, such as a signal generator and a lock-in amplifier, or a signal generator and a Doppler vibrometer; the other is to use a network analyzer for testing. The former system is large and inconvenient to carry; the latter is expensive, and only a small part of the functions are used, which is a bit wasteful. Therefore, it is particularly necessary to develop a dedicated open-loop characteristic tester for resonant silicon micromechanical sensors. Working principle of open-loop characteristic tester The dedicated open-loop characteristic tester for resonant silicon micromechanical sensors uses a single-point steady-state frequency scanning method to obtain the frequency response characteristics of the resonator. It mainly includes the following functional modules: excitation signal generation unit, weak signal processing unit, frequency scanning control unit, and output display unit, as shown in Figure 1. Among them, the weak signal processing unit is its core component.

Figure 1. Functional diagram of a dedicated open-loop characteristic tester for resonant silicon microstructure sensors

Its working process is that the frequency scanning control unit sends a control instruction to the excitation signal generating unit to generate a sinusoidal excitation signal of a certain frequency. The resonator is forced to vibrate under the excitation signal. After it reaches a steady-state response, the weak signal processing unit detects and processes the weak vibration output signal, and then sends it to the output display unit to draw the frequency response curve and calculate related parameters, such as Q value, resonant frequency, resonant phase, etc.

The first generation of open-loop characteristic tester

In the weak signal processing technology in the open-loop characteristic test of resonant silicon micromechanical sensors, phased breakthroughs and progress have been gradually achieved. Based on the weak signal processing technology of each stage, three generations of open-loop characteristic testers have been developed respectively.

Correlation detection is an effective means to extract weak periodic signals under strong noise background, usually composed of multipliers and integrators. The existing analog multiplier has large input equivalent noise, DC offset and nonlinearity, and cannot be directly used in the output signal processing of resonant silicon micromechanical sensors. Therefore, based on the principle of correlation detection, a direct correlation algorithm based on Ohm phase discrimination is proposed. By using Ohm's law, the pickup resistor is directly used as a multiplier, which effectively overcomes the defects of analog multipliers and successfully breaks through the technical bottleneck of weak signal detection. In 1999, the first generation of open-loop characteristic tester was successfully developed, and the open-loop characteristic test of the self-developed resonant silicon micromechanical pressure sensor was carried out. The test results show that the resonant frequency of the early sensor sample is 71.5889kHz and the Q value is about 500.

The minimum frequency scanning step of the first generation of open-loop characteristic tester is 0.01Hz, the weak signal test accuracy is 110nVp-p, it has a friendly interactive graphical interface, simple operation, and intuitive results; the disadvantage is that the measurement speed is slow, and the measurement time for each point takes 120ms; in addition, the tester is not intelligent enough, and the sweep range, scanning step and reference phase need to be manually adjusted until the resonant frequency is accurately searched, and the initial reference phase needs to be manually adjusted each time the measurement is performed until the curve is symmetrical, and the phase information at the resonant frequency point cannot be directly obtained.

Second-generation open-loop characteristic tester

Based on the existing technology, the second-generation open-loop characteristic tester was developed in 2005 after improvement and optimization to address the shortcomings of the first-generation open-loop characteristic, as shown in Figure 2. The weak signal detection method uses the direct correlation algorithm based on Ohm phase discrimination, but proposes the concept of time-sharing orthogonal differential, that is, applying reference signals with a phase difference of 90° to the pickup resistor at four adjacent moments to obtain the corresponding output, using two pairs of anti-phase signals for differential, eliminating common-mode interference, and then performing vector operations on the orthogonal signals obtained after this set of differentials, so that the amplitude and phase of the frequency point can be obtained at the same time. This method not only improves the detection signal-to-noise ratio, but also can independently solve the phase. Figure 2 (right) shows that the resonant frequency of the recent sensor sample is 57.5258 kHz, the phase is 8°, and the Q value is about 3000.

Figure 2. Photo of the second-generation open-loop characteristic tester in operation (left), and test results of a silicon microstructure resonant sensitive element (right).

The advantage of the second-generation open-loop characteristic tester is that the frequency sweep control algorithm is intelligent. It can not only automatically adjust the frequency sweep range and step length to search for the resonant frequency, but also increase the control interface and algorithm of the pressure calibrator, which can perform a series of overall characteristic test analysis based on the open-loop characteristics of the resonant silicon micromechanical pressure sensor, such as sensitivity, repeatability, time drift and temperature drift; the test interface is friendly and easy to operate. Its disadvantage is that it can only be used for resonant sensors with resistance pickup.

The third-generation open-loop characteristic tester

In order to broaden the scope of application of the instrument, the third-generation open-loop characteristic tester is being further developed recently. The instrument adopts a board-card circuit architecture design, and integrates the piezoresistive, capacitive, and magnetoelectric pickup detection signal processing modules into the same test platform in the form of a board card, making the instrument very open and flexible. At present, there has been a new breakthrough in the weak signal processing technology for piezoresistive pickup. A fast cross-correlation detection method has been proposed and has been initially implemented. With the help of the display software of the first-generation open-loop characteristic tester, it has been experimentally verified, as shown in Figure 3. The resonant frequency of the sensor is 71.0402kHz, and the Q value is about 3000.

Figure 3. Fast cross-correlation detection results for a silicon microstructure resonant sensitive element.

The capacitance pickup module and electromagnetic detection module of the third-generation open-loop characteristic tester have not yet been completed, but the detection accuracy of the piezoresistive detection module has been improved to 50nVp-p, which is higher than that of the first-generation open-loop characteristic tester, and the single-point measurement time has been reduced to 10ms, which is shortened to 1/12 of the first-generation tester, greatly improving the test efficiency.

Summary

The open-loop characteristic tester introduced in this article provides a necessary test method for studying resonant silicon micromechanical sensors. However, China's research on this type of high-performance resonant sensor with direct output frequency quantity is still in the laboratory stage, and no products have been launched. Analyzing the reasons, in addition to the gap between domestic processing technology level, signal conditioning circuit design and implementation technology and foreign countries, the lack of in-depth theoretical research on resonator structure, mechanism, characteristics and corresponding testing and evaluation methods is also one of the important reasons. Therefore, on the basis of the existing weak signal detection technology and open-loop characteristic test technology, it is necessary to further carry out research on comprehensive test and analysis instruments specifically for resonant silicon micromechanical sensors.