An Introduction to mems (Micro-electromechanical Systems)
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| An Introduction to MEMS
Prime Faraday Technology Watch – January 2002 31
4.1 Mechanical Transducers 4.1.1 Mechanical Sensors
There is a tremendous variety of direct mechanical sensors that have been or could be micromachined depending on their sensing mechanism (usually piezoresistive, piezoelectric or capacitive) and the parameters sensed (typically strain, force and displacement).
i) Piezoresistive sensors As a result of the piezoresistive effect (defined as the change in resistivity of the material with applied strain), changes in gauge dimension result in proportional changes in resistance in the sensor. The piezoresistive effect in semiconductors is considerably higher than in traditional metals, making silicon an excellent strain sensor. MEMS piezoresistors are readily manufactured using bulk silicon doped with p-type or n-type impurities.
ii) Piezoelectric sensors Piezoelectric sensors utilize the piezoelectric effect in which an applied strain (or force) on a piezoelectric crystal results in a potential difference across the crystal. Similarly, if the crystal is subjected to a potential difference, a displacement, or strain, is produced. The effect can be used to sense mechanical stress (i.e. displacement) and as an actuation mechanism, although displacements are small even for large voltages. Common piezoelectric materials used for MEMS applications include quartz, lead zirconate titanate (PZT), polyvinylidene fluoride (PVDF) and ZnO, PVDF and ZnO being the most common. Silicon is not piezoelectric; hence a thin film of a suitable material must be deposited on the devices.
iii) Capacitive sensors Capacitive (or electrostatic) sensing is one of the most important (and widely used) precision sensing mechanisms and includes one or more fixed conducting plates with one or more moving conducting plates. Capacitive sensing relies on the basic parallel-plate capacitor equation shown below. As capacitance is inversely proportional to the distance between the plates, sensing of very small displacements is extremely accurate.
A C r ε ε 0 =
where: 0 ε = permittivity of free space = 8.854x10 -12
Fm -1
r ε = relative permittivity of material between the plates A = overlapping plate area (m) d = plate separation (m)
iv) Resonant sensors MEMS resonant sensors consist of micromachined beams or bridges which are driven to vibrate at their resonant frequency. They can be attached to membranes or designed to adhere to a particular substance (as in the case of a biosensor). Movement of the membrane or increased build-up of the binding substance will affect the resonant frequency and can be monitored using implanted piezoresistors.
Types of mechanical sensor include: a) Strain gauge - a strain gauge is a conductor or semiconductor that is fabricated on or bonded directly to the surface to be measured. An example of a polysilicon strain sensor |