Piezoelectrics for MEMS
Exhibiting strong electro-mechanical coupling, piezoelectrics are among the most suited functional materials for electromechanical systems.
Piezoelectricity is the ability of some materials to generate an electric potential in response to applied mechanical stress. This may take the form of a separation of electric charge across the crystal lattice. If the material is not short-circuited, the applied charge induces a voltage across the material. The piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect (the production of electricity when stress is applied) also exhibit the converse piezoelectric effect (the production of stress and/or strain when an electric field is applied).
|The direct piezoelectric effect: electric field as a function
of stress. E: electric field, g: piezoelectric coefficients, σ: stress.
|The converse piezoelectric effect: strain as a function of electric field. x: strain, d: piezoelectric coefficient, E: electric field.|
The effect finds useful applications such as the production and detection of sound, generation of high voltages, electronic frequency generation, microbalances, and ultra fine focusing of optical assemblies. It is also the basis of a number of scientific instrumental techniques with atomic resolution, the scanning probe microscopies such as STM, AFM, SNOM etc, as well as more mundane uses including acting as the ignition source for lighters.
In a number of MEMS applications, piezoelectric materials offer several advantages over other principles like piezoresistive and capacitive technologies:
- Strong forces per voltage: the use of high electric fields across thin films (of the order 100 kV/cm) results in considerable piezoelectric stresses of over 100 MPa in the transverse direction when the film is clamped to a flexible passive structure.
- The voltage is small compared to electrostatic actuation.
- The electric field can be concentrated in the material and easily generated by a parallel plate capacitor or interdigital electrodes. Both structures are planar and not 3-dimensional as needed for electromagnetic actuation.
- The ideal piezoelectric effect is strictly linear. The real effect is linear at small fields, but may deviate from linearity in ferroelectrics at higher fields. No collapse as in the case of electrostatic actuation.
- Low power consumption as compared to thermal bilayer actuation or current controlled magnetic actuation.
- The effect works in both directions: actuator and sensor mode.
- High energy-conversion efficiency from electrical to mechanical, as well from mechanical to electric energy in properly designed structures.
- High frequency operation. In materials with ferroelectric domains, the frequency limit for low-loss operation is expected to be around 100 MHz. If no such domains are present, the limit is well in the 10 to 100 GHz range.
- The excursion range is not inherently limited as is the case of electrostatic actuators (limited by gap), but depends on the length/diameter of the beam/plate structure.
- No sticking and pull-in failure as in the case of electrostatic actuation