The smaller size of MEMS devices has important effects on the physical processes that govern their operation (Ref. 1). In general, as the length scale (L) of the device is reduced, the scaling of a physical effect with respect to L determines its relative importance (see Table 2-1). The inertial force required to produce a fixed acceleration of a solid body scales volumetrically as L3. The scaling of other forces in comparison to this inertial force has important consequences for MEMS devices. For example, the effective spring constant for a body scales as L1. The spring stiffness therefore decreases much more slowly than the system mass as the size of the system is reduced, resulting in higher resonant frequencies for smaller devices (resonant frequency scales as L−1). This means that micromechanical systems typically have higher operating frequencies and faster response times than macroscopic systems.

Electrostatic forces scale favorably as the device dimensions are reduced (for example, the force between parallel plates with a fixed applied voltage scales as L0) (see Table 2-1). Additionally, electrostatic actuators consume no DC power and can be manufactured using processes that are compatible with standard semiconductor foundries. In comparison, magnetic actuators scale less favorably (for example, the force between wires carrying a fixed current density scales as L4) and can involve materials that are incompatible with standard semiconductor processing. In some cases magnetic actuators or sensors require a DC bias current, which increases power consumption. These factors explain the prevalence of electrostatic actuation and sensing and the absence of magnetically actuated commercial MEMS devices.

Piezoelectric forces also scale well as the device dimension is reduced (the force produced by a constant applied voltage scales as L1). Furthermore, piezoelectric sensors and actuators are predominantly linear and do not consume DC power in operation. Piezoelectrics are more difficult to integrate with standard semiconductor processes, but significant progress has been made with commercial successes in the market (for example, Avago Technologies (formerly Agilent/HP) FBAR filters). High frequency FBAR resonators fabricated from aluminum nitride thin films on silicon wafers are now widely used as filters used in consumer devices such as mobile phones.

Thermal forces scale as L2, assuming that the forces are generated by a fixed temperature change. This scaling is still favorable in comparison to inertial forces, and the thermal time scale also scales well (as L2), making thermal actuators faster on the microscale (although thermal actuators are typically slower than capacitive or piezoelectric actuators). Thermal actuators are also easy to integrate with semiconductor processes although they usually consume large amounts of power and thus have had a limited commercial applicability. Thermal effects play an important role in the manufacture of many commercial MEMS technologies with thermal stresses in deposited thin films being critical for many applications.

The interaction of MEMS devices with a fluid leads to a range of microfluidic devices. COMSOL also has the Microfluidics Module, which is geared to model microfluidic devices, but some functionality to model fluids is included with the MEMS Module. In particular, the interaction of moving components with a fluid can be modeled using the Fluid-Structure Interaction Interface. In addition to modeling microfluidic actuators, this multiphysics interface can be used to model the damping of moving structures in an air ambient. Damping in MEMS devices is often more conveniently modeled by treating the air as a thin layer and solving the Reynolds equations (or a modified form). These equations are also included in this module. The damping forces scale as L1 and are often important for MEMS devices, frequently leading to a requirement for vacuum packaging.