What Are Microelectromechanical Devices?

Microelectromechanical devices are integrated structures that combine electrical circuitry with micron-scale mechanical components on a single substrate, typically silicon, to perform sensing, actuation, or signal processing functions. Commonly abbreviated as MEMS (microelectromechanical systems), these devices operate at length scales ranging from a few micrometers to several millimeters, occupying a domain where mechanical forces, electrostatic fields, and electronic signals can be fabricated and controlled with the same photolithographic batch processes used to manufacture integrated circuits. The result is a class of transducers that can be produced in large volumes at low cost and integrated directly with the electronics that process their signals.

MEMS technology draws from semiconductor manufacturing, mechanical engineering, and materials science. Silicon is the dominant substrate because its mechanical properties, including high elastic modulus and low fatigue susceptibility, are well characterized, and because the silicon microfabrication toolchain is mature and globally accessible. NIST maintains an active program in micro- and nanoelectromechanical systems measurement science, developing metrology for mechanical properties, resonant frequencies, and surface interactions at the microscale.

Fabrication Processes

MEMS devices are fabricated by surface micromachining, bulk micromachining, or a combination of both. Surface micromachining deposits and selectively removes thin film layers on a wafer to build three-dimensional structures above the substrate surface; polysilicon is the most widely used structural material in this approach. Bulk micromachining etches material from the silicon wafer itself, using wet etchants such as potassium hydroxide or dry processes such as deep reactive ion etching (DRIE) to form membranes, beams, and cavities of precise geometry. A review of actuation and sensing mechanisms in MEMS-based sensor devices describes how these fabrication choices determine the mechanical compliance, mass, and damping of the resulting structures, which in turn govern sensitivity and frequency response. Wafer bonding seals cavities at controlled internal pressures, enabling the vacuum-sealed resonant structures used in gyroscopes and high-Q filters.

Sensing Mechanisms and Microsensors

MEMS microsensors convert physical quantities into electrical signals by exploiting the coupling between mechanical deformation and electrical properties. Piezoresistive sensors measure stress-induced changes in resistivity; capacitive sensors measure changes in the gap or overlap area between electrodes as the structure deflects. Inertial sensors based on these principles are found in smartphones and automotive safety systems: a MEMS accelerometer measures the capacitance between a suspended proof mass and fixed electrodes, detecting the displacement caused by an applied acceleration. MEMS gyroscopes use the Coriolis effect in vibrating ring or tuning-fork structures to measure angular rate. Pressure sensors form a thin diaphragm over a reference cavity, and piezoelectric sensors convert membrane deflection to a voltage. The MEMS Exchange provides an accessible overview of what MEMS technology is and how these sensors work across the range of transduction principles in production devices.

Actuation and Mechanical Systems

MEMS actuators move or position mechanical elements in response to electrical signals. Electrostatic actuation, using the attractive force between charged plates, is common in optical switches and mirror arrays (as in digital micromirror devices for projectors). Thermal actuators exploit differential thermal expansion of dissimilar materials. Electromagnetic actuators use interaction between a current-carrying conductor and an external magnetic field; magnetic particle manipulation for biological assays employs MEMS structures to guide magnetic beads through microfluidic channels. Piezoelectric actuators using PZT or aluminum nitride films deliver high force at low voltage and are used in ultrasonic MEMS transducers and RF filters.

Applications

Microelectromechanical devices have applications in a wide range of fields, including:

• Inertial navigation in automotive airbag systems, consumer electronics, and aerospace attitude control
• Microsensors for environmental monitoring including pressure, humidity, and gas concentration
• RF MEMS switches, filters, and resonators in wireless communication hardware
• Biomedical diagnostics, including microfluidic lab-on-chip devices and implantable pressure sensors
• Optical MEMS for tunable lasers, optical coherence tomography, and LiDAR scanning mirrors