Micro-Electro-Mechanical Systems (MEMS) technology uses semiconductor fabrication processes to produce miniaturized mechanical and electro-mechanical elements that range in size from less than one micrometer to several millimeters. MEMS devices can vary from relatively simple structures with no moving elements, to complex electromechanical systems with multiple moving elements.
MEMS fabrication process & techniques
Fabricating a MEMS device requires many of the same techniques used to make other semiconductor circuits: oxidation, diffusion, ion implantation, low pressure chemical vapor deposition (LPCVD), sputtering, and others. In addition, MEMS uses specialized processes such as micromachining.
MEMS fabrication, technology, and devices are discussed in much greater detail in this article.
Advantages & disadvantages of MEMS vs. competitive technologies
MEMS devices are much smaller, cost less, and consume less power than earlier methods of implementing the same functions. They’re also highly sensitive and extremely accurate. MEMS devices also benefit from the tight tolerances inherent in semiconductor process technology, as they exhibit excellent repeatability.
On the down side, although production costs per part are very low, there’s a large investment associated with designing, qualifying, and manufacturing a MEMS product. As a result, manufacturers are less likely to develop parts for low-volume applications.
MEMS device types & MEMS applications
A typical MEMS sensor employs a mechanical structure that moves in a controlled manner in response to a mechanical or electrical stimulus – pressure, motion, acceleration, magnetic field, etc. The mechanical movement is converted to an electrical form; a typical technique is to use the movement to change the distance between the plates of a variable capacitance.
The output can take many forms: an analog voltage; a standard serial bus such as SPI or I2C; or a specialized protocol such as DSI or PSI5, popular in automotive airbag applications. Wireless connectivity options include Bluetooth Low Energy (BLE).
MEMS devices are available as single-function sensors; modules that bundle several MEMS categories in the same package; and highly-integrated system-on-chip (SoC) devices that combine MEMS devices, signal conditioning electronics, and even embedded processors in a single part.
Figure 1: A gyroscope requires multiple MEMS structures to measure angular motion. (Image Source: Analog Devices)
MEMS technology has been applied to measure many different quantities by measuring their effects on an appropriate MEMS structure.
A MEMS gyroscope (Figure 1) measures angular rotation by taking advantage of the Coriolis acceleration that induces a force on the MEMS frame as a mass moves towards and away from the center of rotation. Gyroscopes are available in single-, dual- and three-axis versions, suitable for different applications: for example, a dual-axis gyroscope is used in gaming and optical image stabilization, and a three axis fits the needs of automotive telematics and navigation.
Accelerometers also use a mass in a frame to measure both static acceleration (i.e., gravity) and dynamic accelerations such as vibration, motion, tilt, shock, etc. Devices grouped under accelerometers include inclinometers, shock sensors, concussion sensors, tilt sensors, and motion sensors. Accelerometers also come in different combinations of axes: single-axis devices are found in automotive crash sensors, and three-dimensional units appear in robotics, vibration-monitoring, and anti-tampering applications.
Pressure sensors measure pressure by the deflection it induces in a MEMS structure. There are versions that measure pressure relative to atmospheric, or absolute pressure referenced to a vacuum-sealed chamber. MEMS pressure sensors can also indirectly measure other quantities such as fluid flow, altitude, and water level.
Magnetometers use a variety of physical phenomena, such as the Hall effect, and to measure the mechanical effects induced by magnetic fields. Single- and three-axis versions are available.
An inertial measurement unit (IMU) measures both linear and angular acceleration by combining a three-axis accelerometer and gyroscope into a single unit; an IMU can also include a magnetometer and a pressure sensor to provide information about the unit’s three-dimensional orientation and motion: acceleration in the x-, y-, and z-axes; pitch, roll, and yaw; altitude; and heading. Applications include unmanned autonomous vehicles (UAVs), robotics and factory automation, avionics, smartphones and tablets, virtual reality, and gaming.
MEMS microphones operate by measuring the change in capacitance when a sound wave hits a variable capacitive element composed of a movable membrane and a fixed backplate. They’re widely used in space-constrained consumer applications such as smartphones and tablets.
In a MEMS biosensor, biomolecular interactions cause a measurable movement in a MEMS structure. In tuberculosis (TB) detection, for example, a MEMS cantilever coated with TB antibodies deflects when an infected blood sample is placed on it.
A MEMS gas sensor detects the presence of a gas by measuring the resistance change it induces in the surface of a coated sensor. The sensor can detect low concentrations of the target gas with a typical response time of less than one second. A humidity sensor is optimized to detect water vapor.
An RF MEMS switch uses electrostatically-actuated cantilever beams in conjunction with a separate driver IC to replace unreliable, bulky electromechanical relays in RF switching applications. A variety of switch configurations are available: Analog Device’s ADGM1304, for example, comes in a SP4T configuration and can handle signals from DC to 14GHz.
A MEMS optical actuator, such as the Digital Micromirror Device (DMD) from Texas Instruments, uses MEMS technology to form a large array of individually-controllable mirrors. Each mirror can be tilted under electronic control to switch between “on” and “off” states. When on, the pixel reflects light from a projector bulb into a lens, making it appear bright. In the off state, the light is directed elsewhere, making the pixel appear dark.
MEMS oscillators contain a resonator that vibrates under electrostatic excitation from an analog driver chip. MEMS oscillators can generate frequencies from 1Hz to hundreds of MHz, with excellent stability, low power consumption, and high resistance to electromagnetic interference (EMI).
MEMS in IoT: A closer look
We discuss other MEMS applications elsewhere, but to illustrate the wide variety of MEMS applications let’s look more closely at MEMS use in the Internet of Things (IoT), also known as Factory 4.0.
The IoT has an enormous requirement for tiny, low-cost sensors to monitor all aspects of production; these sensors must communicate the information to other nodes in the factory network and must operate reliably in the harsh electrical and mechanical environment of the factory. MEMS devices are tailor-made for this purpose: they’re small, rugged, and lend themselves to the inclusion of additional circuit blocks in the same package for wired or wireless connectivity.
Industrial robots use MEMS-based 3-D gyroscopes and accelerometers to continuously measure changes in angular rate and direction, replacing expensive rotary sensors and encoders. They can also detect excessive vibration in joints and actuators that might be omens of premature failure.
MEMS accelerometers also detect unwanted vibration in other industrial machines or sense unwanted shocks. Pressure sensors measure water flow and gas pressure; gas sensors check for toxic emissions; and temperature sensors are a key part of many processes.
In the IoT network infrastructure, MEMS oscillators find a welcome in the programmable logic controllers (PLCs) that supervise the operations of robots and other units. And optical devices are suitable for use in human-machine interface (HMI) display screens.
The factory building itself uses MEMS in multiple ways. Pressure, temperature, and humidity sensors help control the HVAC system; anti-tampering sensors are installed in smart meters; and MEMS shock sensors can help shut off gas supplies if seismic tremors occur.
Calibrated, temperature-compensated MEMS sensors measure gas pressure in LPG- and CNC-powered vehicles transporting the products to the loading dock. And once the product leaves the factory, asset tracking systems use MEMS to monitor shipments for shock and vibration.
Arrow suppliers of MEMS devices
A variety of semiconductor suppliers offer MEMS products. Arrow MEMS suppliers include:
- ST Microelectronics: Pressure sensors, single & multi-axis accelerometers, 6-axis inertial modules, magnetometers, microphones, scalable modules with up to 6 axes
- NXP Semiconductors: Pressure sensors, single & multi-axis accelerometers, gyroscopes
- Bosch: Multi-axis accelerometers, gyroscopes, pressure sensors, microphones, inertial modules
- Analog Devices: Single & multi-axis accelerometers, gyroscopes, RF-switches, magnetometers, inertial modules
- Texas Instruments: MEMS micromirror arrays
- Silicon Labs: Oscillators
- SiTime: Oscillators
- Murata Manufacturing: Single & multi-axis accelerometers, inertial sensors
- Microchip Technology: Oscillators
Conclusion: MEMS is everywhere
This article has reviewed the basics of MEMS technology and its use in IoT applications. Additional articles in the series will discuss other current and emerging MEMS applications, as well as review the MEMS fabrication and the operation of MEMS devices in more detail.
