什麼是『微機電系統』 MEMS ？在此引用 State University of New York 之 Mohammad I. Younis 先生所寫
《 MEMS Linear and Nonlinear Statics and Dynamics 》
by
Mohammad I. Younis
書中之一段說法︰
1.1 What Are MEMS and Why They Are Attractive?
The easiest way to introduce MEMS is to refer to the acronym MEMS itself and what it means. MEMS stands for micro-electro-mechanical-systems. Hence, they are devices in the “micro” scale, in which one or more of their dimensions are in the micrometer range. The “electro” part indicates that they use electric power, for example for actuation and detection, or electronics, for instance for amplifying and filtering signals and for controlling purposes. “Mechanical” means these devices rely on some sort of mechanical motion, action, or mechanism. The word “system” refers to the fact that they function, are designed, and are fabricated as integrated systems and not as individual components. In addition to these features, there are some basic aspects of MEMS that are hidden in the acronym. These can be revealed in this more formal definition of MEMS, which is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through micro-fabrication technology [1].
This concise definition emphasizes important features of MEMS. The first is the fact that most MEMS are basically sensors and actuators. Examples of MEMS sensors are inertia sensors (accelerometers, gyroscopes), pressure sensors, gas and mass sensors, temperature sensors, force sensors, and humidity sensors. Almost for every physical quantity, there is a MEMS sensor that is developed or being developed to measure it. Examples of actuators are micromirrors to deflect lights in flat-screen TVs, RF switches and microrelays, microgrippers, and generic force and displacement actuators, such as thermal bimorph actuators and comb-drive electrostatic actuators. Thus, historically speaking, the early generation of MEMS researchers
has relied on sensors and actuators journals and conferences to disseminate their research on miniature devices before the introduction of specialized MEMS conferences and journals in the early 1990s. Today, several major MEMS meetings and journals still hold the words sensors, actuators, or transducers in their titles.
The second feature is that silicon represents the core material of this technology. Silicon substrates are commonly used as the platform where MEMS components are built and electrically bonded, although recently other materials, such conductive polymers have been utilized [2, 3]. The fabrication of MEMS devices usually starts with single crystal silicon wafers, which come in many standard sizes (4 in, 8 in, and 12 in). Silicon is the preferred material because of its excellent thermal and mechanical properties (small thermal expansion, high melting point, high toughness, and brittleness with no plastic behavior or hysteresis). In addition, silicon has been used
for microelectronics long before the MEMS technology. Hence, many of the well-established processes to fabricate microelectronics from silicon have been adopted directly or modified slightly for MEMS. MEMS made of silicon can be integrated easily with other electronics components, which are also made of silicon, on the same chip. Besides silicon, a number of materials are used to realize MEMS structures, such as silicon-oxide, silicon-nitride, polysilicon, gallium arsenide (GaAs), aluminum, and gold. These are grown or deposited as thin-films over the silicon substrate, which are then etched or processed by micro-fabrication techniques [4].
Another key aspect of MEMS devices is the fact that they are made through the micro-fabrication technology, which enables fabricating numerous numbers of them at the same time (batch fabrication). Many of the micro-fabrication processes, such as material deposition, evaporation, and etching, can be applied on multiple silicon wafers at the same time. Each wafer can produce hundreds of MEMS devices. This means that each fabrication batch can produce thousands of MEMS devices all at once. Of course, reaching this level of production is not trivial; micro-fabrication processes need extensive research and optimization for each step to reach stable and reliable level of production. However, once this critical stage is passed, the payoff is thousands of devices at very low cost. MEMS devices have replaced many expensive devices for fractions of the cost. For instance, the Analog Devices airbag accelerometers in cars, which today costs less than a dollar, has replaced bulkier more expensive accelerometers, which cost more than US$ 50 apiece.
Another important feature of MEMS is the fact that they are systems. This implies that the components of MEMS have to be designed during the design of the whole system. Assembly of individual MEMS components is expensive, cumbersome, and impractical [5]. Also, when designing a microsystem, its fabrication process must be designed too, otherwise the design many not be feasible or cannot be fabricated. Another implication is that system issues, such as packaging, system partitioning into components, stability, and reliability of the products, must be analyzed and taken into consideration during the design and development cycles.
The fascination in the MEMS technology comes from their distinguished characteristics. MEMS are characterized by low cost, which is a direct consequence of the batch fabrication. They have lightweight and small size, which is desirable for compactness and convenience reasons. In addition, this has opened the gates for new possibilities of implementing MEMS in many places where large devices do not fit, such as engine of cars and inside the human body. Moreover, they consume very low power, which not only does reduce the operational cost but also enables the development of long-life and self-powered devices that can harvest the small amount of energy they need from the environment during their operation [6, 7]. Furthermore, MEMS devices have enabled many superior performances, smart functionalities, and complicated tasks that cannot be achieved in other technologies. Ultra-sensitive mass detectors, high isolation and low-insertion-loss RF switches, lab-on-a-chip bio-sensors, tiny directional microphones for hearing aids (Fig. 1.1), high-temperature pressure sensors for automobile engines (Fig. 1.2), and precise controlled liquid droplets for ink-jet printers are just few examples.
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再輔之以
《 Microsensors, MEMS, and Smart Devices 》
by
Julian W. Gardner
University of Warwick, UK
Vijay K. Varadan
Osama O. Awadelkarim
Pennsylvania State University, USA
書中的『分類圖解』 Classification scheme 和 MEMS 可以量測哪些『物理量』，以及『裝置構造』概要︰
8.4 MECHANICAL SENSORS
8.4.1 Overview
Mechanical microsensors are, perhaps, the most important class of microsensor because of both the large variety of different mechanical measurands and their successful application in mass markets, such as the automotive industry. Table 8.4 lists some 50 or so of the numerous possible mechanical measurands and covers not only static and kinematic parameters, such as displacement, velocity, and acceleration, but also physical properties of materials, such as density, hardness, and viscosity.
Figure 8.19 shows a classification scheme for mechanical microsensors together with an example of a device type.
Table 8.4 List of mechanical measurands. Adapted from Gardner (1994)
Figure 8.19 Classification scheme for mechanical microsensors. From Gardner (1994)
The most important classes of mechanical microsensors to date is a subset of only six or so and these constitute the majority of the existing market for micromechanical sensors. Thus, the main measurands of mechanical microsensors are as follows in alphabetical order:
• Acceleration/deceleration
• Displacement
• Flow rate
• Force/torque
• Position/angle
• Pressure/stress
Therefore, we describe in detail here four of the most important types of mechanical microsensors, namely,
• Pressure microsensors (Section 8.4.5)
• Microaccelerometers (Section 8.4.6)
• Microgyroscopes (Section 8.4.7)
• Flow microsensors (Section 8.4.8)
在這杜鵑呼嘯即將過境，或仍可借著今日『中秋佳月』之明，略窺 MEMS 之面貌的耶！！