MEMS Classification: Principle Analysis of 10 Mainstream MEMS Devices

In recent years, pressure sensors in smartphones have gradually become standard, mainly used to measure atmospheric pressure. The purpose of measuring the atmospheric pressure is to calculate the altitude through the air pressure at different altitudes, and cooperate with the GPS positioning signal to achieve more accurate three-dimensional positioning, which is a very friendly use for outdoor hiking enthusiasts.

MEMS pressure sensor

MEMS pressure sensors, as the name suggests, measure pressure.

When it comes to pressure measurement, the most mentioned application scenario is tire pressure measurement, that is, measuring the degree of tire inflation.

In recent years, pressure sensors in smartphones have gradually become standard, mainly used to measure atmospheric pressure. The purpose of measuring the atmospheric pressure is to calculate the altitude through the air pressure at different altitudes, and cooperate with the GPS positioning signal to achieve more accurate three-dimensional positioning, which is a very friendly use for outdoor hiking enthusiasts.

The principle of MEMS pressure sensor is also very simple. The core structure is a thin film element, which deforms when subjected to pressure, and the deformation will cause the electrical properties (resistance, capacitance) of the material to change. Therefore, piezoresistive strain gauges can be used to measure this deformation, and then calculate the pressure.

MEMS Classification: Principle Analysis of 10 Mainstream MEMS Devices

This illustration shows the principle of a capacitive MEMS pressure sensor. When subjected to pressure, the distance between the upper and lower diaphragms (the upper part of the sensor diaphragm and the lower part of the sensor) changes, resulting in a change in the capacitance between the diaphragms. Based on this, it can be calculated out pressure.

(2) MEMS acceleration sensor

A MEMS accelerometer, as the name suggests, is a MEMS device that can measure acceleration.

The core application of acceleration sensor is to use acceleration to sense motion and vibration, such as the most extensive somatosensory detection in consumer electronics, which is widely used in game control, handle vibration and shaking, gesture recognition, etc.

The principle of MEMS accelerometer is very easy to understand, that is, Newton’s second law, which is the most basic high school physics. Force is the cause of acceleration, and the magnitude of acceleration is proportional to the external force and inversely proportional to the mass of the object: F=ma.

Therefore, the MEMS acceleration sensor is essentially a pressure sensor. To calculate the acceleration, it is also to calculate the inertial force generated due to the change of state. Common acceleration sensors include piezoresistive, capacitive, piezoelectric, resonant and so on.

MEMS Classification: Principle Analysis of 10 Mainstream MEMS Devices

Taking the resonant accelerometer as an example, the principle is similar to that of a taut guitar string. Due to the different degree of tension, the sound frequency played is also different. In the resonant accelerometer, the vibrating beam connecting the “detection mass” acts as a guitar string. When the mass block is accelerated by the inertial force, the tension of the vibrating beam will be different. A certain vibration is detected, and the vibration frequency is detected on the comb teeth of the vibrating beam, and then the acceleration is calculated.

(3) MEMS gyroscope (angular velocity sensor)

The gyroscope is relatively complicated. It is a device that measures the angular velocity. Let’s first introduce the ordinary gyroscope.

It is not easy to measure the angular velocity. It is necessary to find a stationary anchor in the moving object – this anchor is the gyro. The property used is the gyro in high-speed rotation. The angular momentum is very large, and the rotation axis does not change with the change of the external motion state, and will always point in one direction stably.

The chicken with the best stability in the animal world, so many people joke that there must be an advanced gyroscope in the head of the chicken, no matter how you move it, the head will not move.

As for the structure of a gyroscope, the core is a spinning rotor that acts as a stationary anchor for other moving objects.

MEMS Classification: Principle Analysis of 10 Mainstream MEMS Devices

Going back to the MEMS gyroscope, it is different from the working principle of the traditional gyroscope, because it is not an easy task to process a rotatable three-dimensional rotor on the silicon substrate by the “micro-engraving” technology.

MEMS gyroscopes Gyroscopes utilize the principle of Coriolis force – the tangential force experienced by a rotating object when it has radial motion. This force is beyond the author’s high school physics level. How to describe this Coriolis force? You can imagine the rotating magic disk in the playground. People are the most stable near the rotation axis, but when the speed of the large disk increases, the person will automatically slide to the edge of the disk, as if being pushed by a force toward the trailing edge of the disk. The direction is gradually accelerated, and this force is the Coriolis force.

Just think of him as a special kind of “Oligi”.

MEMS Classification: Principle Analysis of 10 Mainstream MEMS Devices

Therefore, the structure of the MEMS gyroscope is an object block on a disk, which is driven to make radial motions or oscillations back and forth. Due to the radial motion in the rotating state, Coriolis forces are generated. MEMS gyroscopes usually measure Coriolis force through capacitive changes with movable capacitive plates in two directions.

(4) MEMS inertial combined sensor

The inertial combined sensor is not a new device, but a combination of acceleration sensors, gyroscopes, magnetic sensors, etc., such as three-axis, six-axis, nine-axis, etc., mainly to achieve omnidirectional and three-dimensional motion detection.

A well-known application area of ‚Äč‚Äčinertial sensors is inertial navigation, such as control applications such as aircraft/missile flight control, attitude control, yaw damping, and guidance applications such as medium-range missile guidance and inertial GPS navigation.

MEMS Classification: Principle Analysis of 10 Mainstream MEMS Devices

(5) MEMS microfluidic system

The pressure sensors, accelerations, gyroscopes, etc. mentioned above belong to the category of sensors, while microfluidics belong to the actuators.

The so-called microfluidics is flow control, which is a device for precise control and manipulation of liquid flow. It uses pipes with a scale of tens to hundreds of microns, generally for trace fluids, and is used for high precision and high sensitivity in the field of biomedical diagnosis. It has the advantages of less sample consumption, fast detection speed, simple operation, multi-function integration, small size and easy portability.

MEMS microfluidics is a purely mechanical structure, and the main materials for making microfluidic chips include silicon, glass, quartz, polymers, ceramics, paper, etc.

(6) RF MEMS (including FBAR)

Radio frequency is a field that is closely related to everyone. As long as wireless communication (2345G, Wi-Fi, Bluetooth…) is involved, radio frequency technology must be used.

Because one of the core components in radio frequency is power amplification PA (silicon, gallium arsenide, gallium nitride device), many people default that radio frequency devices belong to the field of semiconductor integrated circuits.

But in fact, in the entire radio frequency front end (RFFE), MEMS devices account for the main proportion, including radio frequency switches (Switch), filters (SAW, BAW, FBAR, etc.), oscillators/resonators (Oscillator/Resonator), etc.

RF switch (Switch) is not a simple switch, but a switcher, which is mainly used for switching and processing signals in different directions (receiving or transmitting) and different frequencies in RF equipment to realize channel multiplexing .

The filter (SAW, BAW, FBAR, etc.) is responsible for filtering the RF signal of the receiving channel, outputting the signal of a specific frequency in the received multiple RF signals, and filtering out other frequency signals. Taking the SAW surface acoustic wave as an example, undesired frequency signals are filtered out through two conversions of electromagnetic signal-sound wave-electromagnetic signal.

MEMS Classification: Principle Analysis of 10 Mainstream MEMS Devices

Oscillator/Resonator (Oscillator/Resonator), the oscillator is the process of converting DC power into AC power, which is used to generate an AC signal of a certain frequency, which is an active device. A resonator is a circuit that resonates a signal of a certain frequency. It is mainly used to filter out a certain frequency and belongs to a passive device.

MEMS Classification: Principle Analysis of 10 Mainstream MEMS Devices

Quartz crystal oscillator is a kind of resonant device made by the piezoelectric effect of quartz crystal. Its basic structure is to cut a thin slice from a quartz crystal at a certain azimuth angle, and apply silver layers as electrodes on its two corresponding surfaces.

(7) MEMS silicon microphone

Everyone knows the microphone, the “Shouting Mai” on Kuaishou refers to the microphone, a subculture of hysterical hysteria at the microphone.

The silicon microphone refers to the use of MEMS technology to manufacture miniature microphones on silicon substrates, catering to the current trend of miniaturization and integration of 3C products, so TWS headsets and mobile phone microphones can achieve such an integrated effect.

The principle of the microphone, whether it is a traditional electret microphone or the current miniaturized silicon microphone, is the use of acoustoelectric conversion.

The key element of the electret microphone’s acousto-electric conversion is the electret diaphragm – a very thin plastic diaphragm, which forms a capacitor with the metal back electrode after being electreted by a high-voltage electric field. When the electret diaphragm encounters sound wave vibration, the electric field at both ends of the capacitor changes, thereby generating an alternating voltage that changes with the sound wave.

MEMS Classification: Principle Analysis of 10 Mainstream MEMS Devices

Compared with traditional electret microphones, MEMS microphones have the characteristics of small size, low power consumption, high reliability, strong anti-interference ability, and high product consistency, and have gradually replaced electret microphones as the microphone in these consumer Electronic products. It is a mainstream device that realizes various functions such as voice acquisition, elimination of environmental noise, and improvement of the resolution of voice commands.

MEMS Classification: Principle Analysis of 10 Mainstream MEMS Devices

(8) MEMS inkjet print head

The MEMS inkjet print head is similar to the microfluidic system mentioned above, but the microfluidic system mainly focuses on the detection and analysis of microfluidics, while the MEMS inkjet printhead realizes it according to the controller. command to eject ink outward.

In general, the function of the inkjet print head is to squeeze out the ink. Some use the piezoelectric film to vibrate to squeeze the ink, and some use the heating bubble to expand and squeeze out the ink in the cavity.

MEMS Classification: Principle Analysis of 10 Mainstream MEMS Devices

(9) DMD (Digital Micromirror Device)

DMD (DigitalMicromirrorDevice, digital micromirror device) is mainly used in the field of DLP (DigitalLightProcessing, digital light processing), that is, the projection of images.

Projection, simply understood, is the process of projecting digital picture signals to the outside through a series of convergence and reflection.

MEMS Classification: Principle Analysis of 10 Mainstream MEMS Devices

In the projection system, the DMD chip is one of the core components. On the small chip between this square inch, there are densely arranged micro-mirrors (precise, micro-mirrors) matrices of the order of millions. Each mirror can reverse the movement independently, flip the positive and negative directions, and flip the number of times per second. Up to tens of thousands of times.

Each micro-mirror controls a pixel in the projection screen, and by means of the reversal of the micro-mirror device, the required light is reflected, and the unwanted light is absorbed by the light absorber to realize the projection of the image, forming different brightness, grayscale and contrast. image.

MEMS Classification: Principle Analysis of 10 Mainstream MEMS Devices

(10) Thermopile

In the early days of the new crown epidemic, in addition to masks, another kind of rare commodity is the thermopile sensor. The price soared more than 10 times, and a bunch of bad guys in the WeChat group went around asking if there was any supply.

A thermopile is a pyroelectric infrared sensor composed of a series of thermocouples in series. It is a temperature detection device. Its main function is to achieve non-contact infrared temperature measurement, such as non-contact forehead thermometers and ear thermometers.

MEMS Classification: Principle Analysis of 10 Mainstream MEMS Devices

The thermopile infrared sensor utilizes the Seebeck effect and is composed of a series of thermocouples in series. The two ends of the thermocouple are composed of two different materials. When one end contacts the hot end and the other end contacts the cold end, the two different materials There will be a potential difference between them, and the magnitude of the potential difference represents the temperature difference between the two different materials.

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