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12.5. MEMS Testing

MEMS is the acronym for a microelectromechanical system [Hsu 2002]. The prefix “micro” indicates the most important feature of MEMS: its extremely small size. The typical size of MEMS components is in the range of between 1 micron (μm) and 1 millimeter (mm). This means that the key feature size of a MEMS device is usually smaller than the diameter of human hair. For feature size below 1 μm, the quantum effect cannot be ignored. It belongs to the recently emerged concept of a nanoelectromechanical system (NEMS). Thus, MEMS devices primarily concentrate on the feature sizes from 1 ~ 1000 μm. Further, the electronic and mechanical parts of a MEMS device interact with each other, so it can be called a “system.” For example, in a MEMS system, the signals in a mechanical sensor can be sensed by an electronic circuit, while the actuation instructions from the electronic circuit can be implemented by a mechanical actuator. Thus, MEMS can incorporate the environment data collection, signal processing, and actuation in the same “smart” system. When compared with conventional electromechanical products, MEMS has the following specific features and corresponding advantages: (1) small volume, low weight, and high resolution; (2) high reliability; (3) low energy consumption and high efficiency; (4) multifunction capabilities and intelligentization; and (5) low cost. Typical examples of commercial MEMS devices are the ADXL series accelerometers [Chau 1998] which have been widely used in the world’s automobile market.

12.5.1. Basic Concepts for Capacitive MEMS Devices

A typical MEMS differential capacitance structure is shown in Figure 12.11 where M represents the movable plate, F1 and F2 denote fixed plates, and B1 and B2 are both beams of the MEMS device. As shown in Figure 12.11, movable plate M is anchored to the substrate through two flexible beams, B1 and B2. It constitutes differential capacitances C1 and C2 with the top and bottom fixed plates (F1 and F2). In the static mode, the movable plate M is located in the center between F1 and F2, thus:


  

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