Microelectromechanical systems (MEMS)

Micro machines

It’s been almost 60 years since the first MEMS were manufactured. Semiconductor engineers capitalised on the manufacturing approaches developed for computer chips and used them to make these microscopic devices. 

These days, MEMS are fashioned from silicon, polymers, metals or ceramics. Their internal parts, for example, a spring or weight, can be as small as one micrometre, with overall dimensions ranging from 20 micrometres to a few millimetres. High surface-area-to-volume ratios make them ideally suited to detecting marginal changes in magnetism, pressure, chemical composition, or temperature. 

While their functions can differ enormously, what all MEMS have in common is their size and ability to convert electrical signals into movement, or vice versa. A MEMS sensor generates an electrical output when its components register changes in its surroundings, often by bending, stretching or contacting another part. On the other hand, a MEMS actuator converts an electrical input into mechanical movement. 

So, what’s actually happening inside? A MEMS sensor might contain a mass suspended between capacitive plates. When the suspended mass tilts towards one plate, it shows up as a change in capacitance. Movement of 0.0001° can be enough to trigger a reading on a tilt sensor. This type of device is known as a capacitive transducer. 

Often, the MEMS that come to mind are sensors, from accelerometers, microphones and gyroscopes, to devices detecting pressure and flow. However, MEMS can also work as switches and oscillators – for instance in communication systems. Meanwhile, micronozzles are common in inkjet printers. 

Built to last

While impressive in their own right, MEMS do not work in isolation. They must be bridged somehow with the outside world, typically by integration into electronic circuitry on a microchip. This also enables functions such as signal filtering or amplification. 

MEMS – particularly sensors – often need some degree of access to their external environments. Yet they must also be packaged to protect them from contamination, for instance from dust, and some MEMS have to be hermetically sealed. Packaging also dissipates heat and offers their sensitive components protection from mechanical shocks and other unwanted physical stresses. 

All of this is key when you consider the conditions MEMS often operate under: extreme temperatures, protective systems in vehicles crashing at high speeds, and being fired into space are just a few examples.