《MEMS Micro-Electromechanical Systems》-Overview

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With the rise of the Internet of Things and smart wearable devices, MEMS sensors are also a hot topic in the IC industry. Of course, our MEMS sensors will be used in not only smart terminals, but also automotive electronics and Industry 4.0. Future terminals will have thinking and perception, and this thinking and perception will rely on MEMS sensors.

On smartphones, MEMS sensors provide applications in sound performance, scene switching, gesture recognition, direction positioning, and temperature/pressure/humidity sensors. In cars, MEMS sensors enhance vehicle performance with the help of airbag collision sensors, tire pressure monitoring systems (TPMS), and vehicle stability control. In the medical field, MEMS sensors have been successfully developed to produce miniature insulin injection pumps, and heart bypass transplants and artificial cell tissues have become practical treatment methods. In wearable applications, MEMS sensors can achieve motion tracking, heart rate measurement, etc.

1. What is MEMS sensor?

The full name of MEMS is Micro-ElectroMechanical System. A micro-electromechanical system is a micro-device or system that integrates micro-mechanisms, micro-sensors, micro-actuators, signal processing and control circuits, communications and power supplies. So it contains two parts: one is the Electronic circuit part, and the other is the Mechanical part. In a narrow sense, it can be said to be a technology that uses micron technology to manufacture micro-machines on chips and integrates them with corresponding application-specific circuits (ASICs) into a whole. So it is an advanced manufacturing technology platform developed based on semiconductor manufacturing technology. Of course, MEMS in a broad sense is not just the process of semiconductor materials, but also other things such as optical fiber detection. I am a semiconductor person, so I only focus on MEMS technology based on semiconductor technology.

Why do we need MEMS sensors? Mainly because it originated from IC manufacturing technology. Compared with traditional mechanical structures, it is more compact and easier to control in size (CD and OVL manufactured by IC should be too easy for MEMS manufacturing). In addition, it is easier to integrate and reduce the interconnection loss of AC/DC conversion during the conversion of mechanical and electrical signals. The material properties of silicon are similar to those of metals, such as hardness similar to iron, density similar to aluminum, and thermal conductivity similar to tungsten, so it is suitable for manufacturing components with mechanical properties.

MEMS sensors actually consist of two parts: sensors and actuators. Sensors are used to detect and sense physical and chemical phenomena (temperature, vibration, light, magnetic field, pH value, ion concentration, etc.), while actuators sense physical and chemical signals and generate mechanical motion (force and torque). Therefore, they can also be called transducers, which convert signals from one energy to another (electrical energy, mechanical energy, chemical energy, radiation energy, magnetic energy, thermal energy, etc.).

The inkjet printers that everyone has in the office are based on silicon machining technology. Due to the tiny structure of semiconductor manufacturing technology, the nozzle array is very dense (>300/print head), realizing high-density resolution printing technology (1000dpi). Since the inkjet cavity is small (40ng), the small heater can quickly heat up and cool the ink to achieve high-speed printing. This technology was first proposed by HP in 1978 and has become a cheap alternative to laser printers.

In 1989, UC-Berkeley in the United States developed a polysilicon electrostatic motor with a diameter of less than 120um and a thickness of 1um. It could reach a speed of 500rpm at a voltage of 350V. This was also the first step in arousing the industry's enthusiasm for MEMS.

2. Development history of MEMS sensors:

It should be said that the origin of MEMS technology should be IC manufacturing technology, and the earliest person to propose this concept should be "Silicon as a mechanical material" published by Petersen in 1984. In the following years, the industry has the term MEMS, and it has gradually been widely accepted in the industry. This name includes the size (um), implementation method (combination of electronics and mechanics), and research topic (system) of the research in this field. The 20 years before 1984 were almost all in the embryonic stage, mainly studying some scattered components and three-dimensional processing technology of anisotropic etching, etc.

It was not until 1990 that the world began to truly industrialize MEMS sensors. The earliest should be ADI, which mainly engaged in inertial sensors for automobile airbags, and TI's digital optical processing chips (light sensors). The airbag inertial sensor that everyone is more familiar with should be the airbag inertial sensor. When the car collides, the airbag will automatically pop up to protect the people in the car. This is more interesting. Its principle is to use two groups of forked electrodes, one group is fixed to the anode, and the other group is suspended on a longer support beam. In this way, when there is acceleration outside, the suspended support beam will drift, causing the distance between the adjacent group of fixed forks to change instantly, causing the capacitance to change. By detecting the change in capacitance, it can sense whether there is acceleration caused by external force collision. (Of course, the mechanical capriciousness of this fork finger is more important, otherwise it will break when acceleration comes, and its suspension drift characteristics are also very important F=m*a, so the weight of the suspension bar depends on the length and thickness)

The third active phase of MEMS development should be after 2000, when the entire industrial system should have been formed, such as HP's inkjet printer (sales of $500 million), Epson/Lexmark's inkjet printing achieved $600 million, TI's optical digital processing (DLP) achieved $900 million, ADI's airbag sensor also achieved $150 million, Freescale's pressure sensor achieved $200 million, and so on.

Later, accelerometers from MEMSIC and STMicro, gyroscopes from InvenSense, crystal oscillators from SiTime, etc., became well-known sensor companies. Significant progress has also been made in the medical field, such as capsule endoscopes, MEMS injection needles, DNA recognition, etc. What I feel more is that with capsule endoscopes, are we still afraid of doing gastroscopy and colonoscopy? MEMS injection needles are more advanced, and injections will be painless in the future. The pain of injections is because they touch the subcutaneous nerve endings, which are located 70um deep under the skin, while our injections only need to be completed in the 50um cortex. We can use MEMS injections to control the depth at 50~70um to achieve painless injections.

3. Process challenges of MEMS sensors:

Because electrical properties have been very mature in the IC manufacturing era, MEMS technology should mainly focus on mechanical and physical properties, such as the rigidity of the cantilever beam and the resonance characteristics of the membrane layer, etc. Typical MEMS devices are at the um~cm level. Due to the principle of proportional reduction, the smaller the size, the higher the hardness, resonance frequency, and sensitivity. For example, the shorter the stick, the harder it is to break. Another example is that fleas can jump more than ten times their height, but can elephants do that? This is why MEMS is needed to achieve mechanical properties. However, not everything is better the smaller it is. There is a concept called proportional reduction (Scanable) in the semiconductor IC manufacturing industry. There is also a scale law in MEMS. Many effects that can be ignored at large sizes are very prominent at small sizes, such as the elastic constant (sensitivity) of the cantilever beam, the resonance frequency (frequency bandwidth) of the support beam, and the total capacitance (sensitivity), etc., all change with size.

Its manufacturing process is mainly divided into bulk silicon (Bulk-Si) and surface silicon (Surface-Si) process. It is a bit difficult to explain. People who have studied semiconductor ICs should all know the Mesa process, which is the table process. This is our Bulk-Si process, which is to eat grooves on the Si substrate to achieve the mechanical structure (MESA eats grooves to achieve isolation effect).

So it was developed into Surface Silicon process, mainly because it is compatible with mature CMOS technology, so it is also called CMOS-MEMS. It mainly relies on thin film growth (Poly, PSG, Metal) to obtain the desired structure through etching. The biggest difference between it and CMOS is that the film of CMOS process is attached to the substrate layer by layer, but the film layer of MEMS is suspended and stands on the substrate (Free Standing) after etching, which is very strict for the physical and mechanical research of the cantilever beam under force and vibration.

4. Development trend of MEMS sensors:

MEMS is a very beautiful industry, but at best it is small and beautiful. It is difficult for a company engaged in the MEMS industry alone to survive. First of all, it is linked to the semiconductor industry, which is a money-burning industry with high risks. Secondly, sensors are complex and ever-changing in design. Each product is an independent event. There is no standard process and standard parameters like CMOS, so many FABs do not provide MEMS manufacturing services. This makes MEMS companies have to have their own FABs and go back to the IDM route of the semiconductor manufacturing industry before 1987, such as China's MEMSIC. How to standardize MEMS manufacturing and follow the semiconductor foundry route will be the focus of industry research and the most important driving force.

Later, we will offer specific courses to study CMOS-Based MEMS technology and market, as well as the principles and implementation methods of various MEMS.

Reprinted from: Xinyuan.com

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