Silicon Micro Resonant Sensor

Resonant sensors are a type of quasi-digital sensors that utilize the measured physical quantity to change the resonant characteristics of the resonant sensitive structure and directly output frequency signals. These sensors operate in the mechanical resonant state of the resonant sensitive structure (also known as a resonator or a resonant element), are less affected by changes in external circuit parameters, and possess relatively high resolution, stability, and anti-interference ability.

In the early stage, resonant sensors mainly used materials such as metal or quartz to prepare resonant sensitive structures, like resonant cylinders, resonant diaphragms, and compound tuning forks. Correspondingly, the sizes of the relevant sensor products were large and their power consumption was high. Since the late 1980s, some well-known international companies have taken advantage of the excellent physical properties of silicon materials and combined with MEMS (Micro-Electro-Mechanical Systems) processing techniques to fabricate silicon micro-structured resonant sensors. The characteristic dimensions of these sensors can reach the micron or even sub-micron level. The typical representatives of this type of sensors are silicon micro-resonant pressure sensors and silicon micro-resonant accelerometers.

Silicon micro-resonant sensors not only possess the excellent performance of general resonant sensors but also have the characteristics of small size, low power consumption, fast dynamic response, easy integration, and mass production. Therefore, they are widely used in fields such as industrial control, consumer electronics, and aerospace. With the continuous development of MEMS processing technology and the continuous increase in practical application requirements, micro-resonant sensors continue to develop towards high performance, high sensitivity, miniaturization, and even the direction of Nano-Electromechanical Systems (NEMS). However, since silicon micro-structures are prone to defects when reduced to a few hundred nanometer sizes, it is difficult to further reduce the characteristic size of the corresponding sensors, which limits the measurement performance and application fields of silicon micro-resonant sensors. Therefore, exploring new materials that can be used for excellent performance and small size and developing new types of resonant sensors has naturally become a potential development trend of micro-resonant sensors.

Fundamental Theories of Silicon Micro - Resonant Sensors

Resonant Sensitive Mechanism

The working principle of resonant sensors lies in the utilization of the positive - feedback principle to form a closed - loop self - excited system that includes a resonator, an excitation/detection unit, and an amplification unit, as shown in the figure below. Among them, the resonant - sensitive structure is the core part of the closed - loop system and operates in its own natural vibration mode. The excitation unit generates an excitation signal to cause the resonant - sensitive structure to produce mechanical vibration. The detection unit picks up its vibration signal and converts it into an electrical signal. After being processed by the amplification unit, it is converted into an excitation force through the excitation unit and positively fed back to the resonator to maintain the resonator's stable - frequency vibration at its resonant frequency. The measured quantity modulates the resonant state of the resonator through a certain way. By measuring the output - frequency signal, the magnitude of the measured quantity can be calculated. For micro - resonant sensors, their resonant - sensitive structures are prepared by micro - machining technology, and their geometric dimensions can reach the order of several hundreds or even tens of micrometers. Through the design of a reasonable resonant - sensitive structure, combined with multiple sensitive parameters such as the vibration frequency, phase, and amplitude of the resonator, the measurement of various physical quantities such as force, acceleration, and angular velocity can be realized.

Design of Resonant-Sensitive Structures

The resonant-sensitive structure is the core component of various resonant sensors and is responsible for directly or indirectly sensing the quantity to be measured. Its design will directly affect the measurement accuracy, sensitivity, dynamic performance and other indicators of the sensor. In terms of structural forms, the commonly used micro-sensitive structures in micro-resonant sensors include resonant membranes, resonant beams, double-ended fixed tuning forks and so on. Among them, the resonant beam and vibrating tuning fork structures are most widely used in micro-resonant pressure sensors and accelerometer sensors.

In silicon micro-resonant pressure sensors, the resonant-sensitive structure is usually divided into two classic implementation methods according to whether the quantity to be measured is in direct contact with it:

One is the resonant membrane structure, as shown in the figure below. In this structure, the pressure directly acts on the resonant diaphragm, changing its equivalent stiffness, and the vibration is excited by the excitation elements set on the diaphragm itself. This structure has simple process requirements. However, since the diaphragm itself is in direct contact with the measured medium, for diaphragm structures at the micron or even nanometer level, the problem of vibration energy dissipation caused by the quantity to be measured needs to be considered.

Another approach is a composite sensitive structure composed of a pressure-sensitive diaphragm and a resonator. In this structure, the resonant sensitive element is usually placed at an appropriate position on the pressure-sensitive diaphragm and is responsible for indirectly sensing the quantity to be measured. Under the action of the pressure load, the diaphragm deforms, resulting in a change in the axial stress of the sensitive element and thus altering its resonant frequency. The outstanding advantage of the composite sensitive structure is that the resonant sensitive element is isolated from the measured medium, avoiding the direct influence of the latter. Moreover, the sensitive element can work in a vacuum environment, which is beneficial for maintaining a relatively high quality factor. In addition, the measurement range can be changed by appropriately adjusting the structural parameters of the pressure-sensitive diaphragm.

Resonant Sensitive Materials

Currently, with the continuous development of MEMS technology and the changes in the application environmental conditions of sensors, the requirements for the size of micro-resonant sensors are gradually increasing. Among them, the size of the resonant-sensitive structure is gradually transitioning from the micron level to the nanometer level. However, the physical properties of silicon materials are not flawless. When its thickness is reduced to several hundred nanometers, defects are prone to occur, and problems such as difficulty in controlling device quality and poor uniformity are likely to arise. Therefore, it is quite necessary to seek new solutions.

With the active exploration of domestic and foreign researchers, quite a number of nanomaterials, such as diamond and carbon nanotubes, have been applied in the field of micro/nano-electromechanical sensors. However, there are relatively few literature reports related to resonant sensors. In the past few years, graphene, an emerging nanomaterial, has attracted widespread attention from experts and scholars in the sensor field due to its unique mechanical, electrical, optical and other properties. It has brought new research ideas and opportunities for the development of new types of micro-resonant sensors and even nano-electromechanical resonant sensors, and is expected to replace silicon materials and trigger revolutionary changes in the field of resonant sensors.