# MEMS Module Glossary

This glossary contains finite-element modeling terms specific to the MEMS Module and its applications. For mathematical terms as well as geometry and CAD terms specific to the COMSOL Multiphysics software, please see the Multiphysics Glossary.

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### absorption

Uptake of a gas into the bulk of a liquid. Gas absorption takes place, for example, in the liquid of a scrubber tower where an up-streaming gas is washed by a down-going flow of a scrubber solution.

### actuator

A device that by means of an agent, such as electric field or heat, produces a desired effect on a target or on the surroundings.

Attachment of a molecule or atom to a solid surface. Adsorption involves a chemical bond between the adsorbed species and the surface.

### ALE

See arbitrary Lagrangian-Eulerian method.

### arbitrary Lagrangian-Eulerian (ALE) method

A technique to formulate equations in a mixed kinematical description. An ALE referential coordinate system is typically a mix between the material (Lagrangian) and spatial (Eulerian) coordinate systems.

### aspect ratio

The ratio of the dimensions of a device in different directions, for example, the ratio of height to width. See also lateral aspect ratio and vertical aspect ratio.

### biosensor

A general term for sensor devices that either detect biological substances or use antibodies, enzymes, or other biological molecules in their operation. Biosensors are a subcategory of chemical sensors.

### bonding

A process by which one type of substrate is firmly attached to the surface of another.

### buckling

The sudden collapse or reduction in stiffness of a structure under a critical combination of applied loads.

### cantilever beam

A beam with one end fixed and one end free.

### capacitive sensor

A sensor that produces a signal due to a change in its capacitance.

### comb drive

A MEMS device consisting of inter-digitated fingers similar to a comb.

### convection

Transport of molecules or heat due to the movement of the surrounding medium.

### damping

An effect that attenuates or reduces mechanical oscillations or vibrations.

### dielectrophoresis

Migration of polarizable particles in an electrolyte in a nonuniform applied electric field.

### diffusion

Transport of material resulting from the random motion of molecules in the presence of a concentration gradient.

### EDL

See electric double layer.

### elastic deformation

A nonpermanent deformation that recovers its shape completely upon the release of an applied stress.

### electric double layer

(EDL) At the contact of a solid and a polar fluid (such as water), the solid acquires an electric charge. This charge attracts ions within the fluid, and a narrow fluid layer of opposite charge, the Stern layer, forms on the boundary. In addition, adjacent to the Stern layer, a wider layer with the same charge as in the Stern layer forms in the fluid. Together, the Stern layer and the wider layer (called the diffuse or Gouy-Chapman layer) form the electric double layer. Due to the close distance between the charges, the Stern layer is fixed on the surface, but the more distant diffuse layer can move.

### electrokinetics

Study of the motion of charged particles under an applied electric field in moving substances such as water.

### electrokinetic flow

Transport of fluid or charged particles within a fluid by means of electric fields. See also electroosmosis, electrophoresis, electrothermal flow, and dielectrophoresis.

### electrolyte

A solution that can carry an electric current through the motion of ions.

### electroosmosis

Fluid flow in a narrow channel produced by the movement of the electric double layer (EDL) along the channel boundary under the influence of an applied electric field. Also, fluid flow through a membrane under the influence of an applied electric field. See also electric double layer.

### electroosmotic flow

See electroosmosis.

### electrophoresis

Migration of charged electrolyte ions in an applied electric field.

### electrothermal flow

Fluid flow resulting from an applied non-uniform AC electric field on a fluid. The Joule heating changes the fluid’s electrical properties locally, and that effect, together with the power gradient of the AC electric field, results in fluid motion.

### Eulerian

Model described and solved in a coordinate system that is fixed (spatial). See also Lagrangian and arbitrary Lagrangian-Eulerian method.

### frequency-response analysis

An analysis solving for the steady-state response from a harmonic excitation. Typically a frequency sweep is performed, solving for many excitation frequencies.

### fully developed laminar flow

Laminar flow along a channel or pipe that has velocity components only in the main direction of the flow. The velocity profile perpendicular to the flow does not change downstream in the flow.

### Green-Lagrange strain

A measure of nonlinear strain used in large-deformation analysis. In a small-strain large rotation analysis, the Green-Lagrange strain corresponds to the engineering strain with the strain values interpreted in the original directions. The Green-Lagrange strain is a natural choice when formulating a problem in the undeformed state.

### Helmholtz-Smoluchowski equation

Gives the velocity of a parallel electroosmotic flow for an applied electric field.

### Joule heating

The increase in temperature of a medium as a result of resistance to an electric current flowing through it.

### Knudsen number

A dimensionless number that provides a measure of how rarefied a gas flow is, in other words, the average distance between the gas molecules compared to the length scale of the flow. The following equation defines the Knudsen number Kn where λ is the mean free path of the molecules and L is a length scale characteristic to the flow.

### Lagrangian

Model described and solved in a coordinate system that moves with the material. See also Eulerian and arbitrary Lagrangian-Eulerian method.

### large deformation

The deformations are so large so the nonlinear effect of the change in geometry or stress stiffening need to be accounted for.

### lateral aspect ratio

The ratio of the length of a structure in the plane of a wafer to its width in that plane. See also aspect ratio and vertical aspect ratio.

### MEMS

Short for microelectromechanical systems, an acronym derived from the words Micro Electro Mechanical System. More generally, MEMS refers to systems, devices, and components in microscale size and where the physics are not limited only to electrical or mechanical phenomena.

### microfluidics

Study of the behavior of fluids at the micro scale. Also refers to MEMS fluidic devices.

See MEMS.

### migration

The nonrandom movement of particles under an external force.

### mobility

The relation between the drift velocity of a molecule within a fluid and the applied electric field.

### piezoelectricity

The ability of certain crystalline materials to produce an electric voltage when subjected to mechanical stress (direct piezoelectric effect). Inversely, the material’s ability to change shape when an external voltage is applied (reverse piezoelectric effect).

### plane strain

An assumption on the strain field where all out-of-plane strain components are assumed to be zero.

### plane stress

An assumption on the stress field where all out-of-plane stress components are assumed to be zero.

### proof mass

A predetermined test mass in a measurement device or machine that serves as the reference mass for the quantity to be measured.

### residual strain

Strain remaining in a structure after some operation, for example, the strain resulting from cooling a system after high-temperature bonding. It often appears as the initial strain for any consequent tasks. See also strain and initial strain.

### residual stress

Stress remaining in a structure after some operation, for example, stress resulting from cooling a system after high-temperature bonding. It often appears as the initial stress for any consequent tasks. See also stress and initial stress.

### Reynolds number

A dimensionless number classifying how laminar or turbulent a flow is. The Reynolds number Re is a measure of the relative magnitude of the flow’s viscous and inertial forces. It is defined by the following equation where ρ is the fluid density, η is the dynamic viscosity, ν is its kinematic viscosity, u is a velocity characteristic to the flow, and L is a length scale characteristic to the flow.

### sensor

A device that measures a physical variable such as temperature or pressure and converts it (usually) to an electrical signal.

### slide-film damping

The damping effect of a fluid between two solid surfaces when the distance between them is small compared to their area and there is mostly tangential movement (lubrication). Slide-film damping is a type of thin-film damping.

### squeezed-film damping

The damping effect of a fluid between two solid surfaces when the distance between them is small compared to their area and there is mostly normal movement. Squeezed-film damping is a type of thin-film damping.

### strain

Relative change in length, a fundamental concept in structural mechanics.

### stress

Internal forces in a material. Normal stresses are defined as forces/area normal to a plane, and shear stresses are defined as forces/area in the plane. A fundamental concept in structural mechanics.

### transducer

A device that converts one type of energy to another. It often refers to a device that responds to a physical parameter and converts it to an electrical signal.

### thin-film damping

Damping that occurs in a thin channel of fluid (fluid film) between moving structures. See squeeze-film damping and slide-film damping.

### vertical aspect ratio

The ratio of the height of a structure perpendicular to a wafer’s surface to its depth in the wafer’s plane.