Microelectromechanical systems [MEMS]

MEMS inductor
MEMS inductor.

Microelectromechanical systems, often referred to as MEMS, are a collection of microscopic devices of different types (mechanical, electrical or electronic) integrated on the same substrate of semiconductor material, e.g. silicon, combining the electrical properties of semiconductor devices with opto-mechanical properties.

MEMS consist of components between 1 and 100 micrometers in size (i.e., 0.001 to 0.1 mm), and MEMS devices generally range in size from 20 micrometers to one millimeter (i.e., 0.02 to 1.0 mm), although components arranged in arrays (e.g., digital micromirror devices) can be more than 1000 mm2 in size. They typically consist of a central unit that processes data (an integrated circuit chip, such as a microprocessor) and several components that interact with the environment (such as microsensors).

These are “smart” systems that combine electronic, fluid management, optical, biological, chemical and mechanical functions in a very small space, integrating sensor and actuator technology and a variety of process management functions. There is also talk of NEMS (nanoelectromechanical systems), which are similar to MEMS but on the nanometer scale.

Principle of operation

The operation of a MEMS can be described by considering the integrated circuit as the “brain” of the system, which makes it possible to monitor the environment through the other devices (“senses” and “arms”) on the same chip. In this way, the system gathers information by measuring mechanical, thermal, biological, optical and magnetic phenomena; the electronics process the information derived from the sensors and react by enabling the actuators to respond by moving, positioning, filtering, pumping or even reverifying, through the same sensors, the changes that have occurred over time in the surrounding environment. In this way, we have a system capable of acquiring information from the environment by translating physical quantities into electrical impulses, processing this information using appropriate logic, and finally responding with certain actions.

Sensors can measure a wide variety of phenomena: mechanical (sound, acceleration, and pressure, to name a few), thermal (temperature and heat flow), biological (cell potential), chemical (pH), optical (light intensity, spectroscopy), and magnetic (flux intensity). MEMS technologies promise to revolutionize entire product categories precisely because they integrate the most diverse functions into the same device. A tiny silicon chip now becomes a pressure sensor, now an accelerometer, now a gyroscope, and so on. The advantage of MEMS is that they can perform the same sensing, processing and actuation functions as much bulkier and more expensive objects.

Applications

Microsystems technology is used in a wide variety of applications, many of which are based on microscopic oscillating mirrors or lenses, in single or arrayed versions, used to make complex optoelectronic devices such as: switches for laser signals, sensors for telescopes, deformable lenses, advanced projectors and displays, as well as inertial sensors, precision accelerometers, retinal scanners, digital shutters, interferometers, and sensors for sophisticated measurements.

In microwave electronics (1 GHz – 100 GHz), the MEMS device is used as a single switch to realize more complex applications such as phase shifters, matching networks, resonant filters, array antenna feed networks and general reconfigurable systems.

MEMS are also being used in chemistry and bioengineering for new solutions. Applications include electric micromotors two millimeters in diameter and tens of meters in length, including planetary gears. The fabrication of MEMS devices is essentially based on the methods and tools used in microelectronics. In fact, electronic parts are fabricated using standard integrated circuit processes; the same processes are used to fabricate mechanical or other components. The integration of mechanical elements, sensors, actuators and electronic circuits on the same substrate opens up new possibilities in a variety of fields.

Some common commercial applications of MEMS are:

  • Inkjet printers, which use piezoelectrics or thermal bubble ejection to deposit ink on paper.
  • Accelerometers in modern automobiles for a variety of purposes, including airbag deployment and electronic stability control.
  • Inertial measurement units (IMUs):
  • MEMS accelerometers
  • MEMS gyroscopes in remote-controlled or autonomous helicopters, airplanes, and multi-rotor aircraft (also known as drones) used to automatically sense and compensate for roll, pitch, and yaw flight characteristics.
  • MEMS magnetic field sensors (magnetometers) may also be incorporated into such devices to provide directional heading.
  • MEMS inertial navigation systems (INS) of modern cars, airplanes, submarines, and other vehicles to detect yaw, pitch, and roll; for example, the autopilot of an airplane.
  • Accelerometers in consumer electronics devices such as game controllers (Nintendo Wii), personal media players/cell phones (virtually all smartphones, various HTC PDA models), augmented reality (AR) and virtual reality (VR) devices, and a number of digital cameras (various Canon Digital IXUS models). Also used in PCs to park the hard disk head when free fall is detected to prevent damage and data loss.
  • MEMS Barometer
  • MEMS microphones in portable devices such as cell phones, headsets and laptops. The market for smart microphones includes smartphones, wearable devices, smart home and automotive applications.
  • Precision temperature-compensated resonators in real-time clocks
  • Silicon pressure sensors, such as tire pressure sensors for cars and disposable blood pressure sensors.
  • Displays, such as the Digital Micromirror Device (DMD) chip in a projector based on DLP technology, which has a surface with several hundred thousand micromirrors or individual micro-scanning mirrors, also known as microscanners.
  • Optical switching technology used in switching and alignment for data communications
  • Bio-MEMS applications in medical and health-related technologies, including lab-on-a-chip (taking advantage of microfluidics and micropumps), biosensors, chemosensors, and embedded components of medical devices such as stents.
  • Interferometric modulator display (IMOD) applications in consumer electronics (primarily displays for mobile devices), used to create interferometric modulation-reflective display technology as found in Mirasol displays.
  • Fluid acceleration, e.g. for micro-cooling
  • Micro-scale energy harvesting, including piezoelectric, electrostatic, and electromagnetic microharvesters
  • Micromachined ultrasonic transducers
  • MEMS-based loudspeakers, focusing on applications such as in-ear headphones and hearing aids
  • MEMS oscillators
  • MEMS-based scanning probe microscopes, including atomic force microscopes
  • LiDAR (Light Detection and Ranging)

Sensors and actuators

  • MEMS pressure sensors are used in the military, aerospace, automotive and biomedical sectors.
  • The accelerometers and gyroscopes are mainly used in airbag systems for cars, smartphones, drones, and even in the games console industry.
  • MEMS magnetic field sensor (magnetometer) may also be incorporated in such devices to provide directional heading.
  • Speed sensors were introduced in the 1990s and are used both in the automotive industry (stability control systems, tire pressure sensors, GPS receivers) and in consumer electronics products (cameras stability control, GPS receivers for mobile phones).
  • MEMS are also used in Inertial navigation systems (INSs) of modern cars, airplanes, submarines and other vehicles to detect yaw, pitch, and roll; for example, the autopilot of an airplane.
  • Weight and force sensors are used for the construction of measurement and control devices.
  • Actuators are used to generate movement or force capable of moving other MEMS components. They can be divided into electrostatic and thermal type actuators.
  • Micro-fluid MEMS devices are designed to operate with fluids at a microscopic level. Typical applications of this type of MEMS are valves, pumps, injectors (used for example for the realization of inkjet printers).
  • Bio-MEMS are similar to micro-fluid MEMS, with the difference that they are designed to work with biological fluids, typically blood. Bio-MEMS are mainly used in the biomedical and health sectors.

Uses in radio frequency

They are mainly used in mobile phones, cordless phones, and GPS receivers. They present remarkable properties in terms of extremely reduced dimensions, wide bandwidth, low cost, and a very high signal-to-noise ratio, fundamental in all radio frequency applications. This type of MEMS is spreading rapidly and is replacing the traditional solutions realized in solid-state technology.

Optics and optoelectronics

MEMS optical devices are used to guide, amplify or attenuate, and reflect an optical signal of a specific wavelength. They are mainly used in the production of optical switches and optical modules for fiber optic transmission systems.

In conventional optical switches and systems for the cross-connection of DWDM channels, it is necessary to modify and control the optical path of the light. This is achieved through the use of micromirrors made with MEMS technology; used also to create other sophisticated optoelectronic devices.

For example switches for laser signals, sensors for telescopes, deforming lenses, projectors, and advanced displays, but also inertial sensors, precision accelerometers, retinal scanners, digital shutters, interferometers, sensors for sophisticated measurements.

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