Micro Electro Mechanical Systems ( MEMS)

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Measuring systems have become a integral part in human existence. And since the dawn of the electronics age with prevalence of computers that are interactive, real time sensing has been the norm for an intuitive system. With ever-increasing demand for electronics that gets smaller and smarter every day, the opportunities to develop compact high efficient sensors and actuators have risen exponentially.
Miniaturization of the measurement systems and incorporation of signal processing elements with relevant actuating system is termed as Micro electro mechanical system.


These include detection of motion, force, fluid flow, pressure, temperature, heat etc that regular sensors are designed to do.

With the event of multi device integration and development of IoT, The market for MEMS is potentially rising. Though it is a developing field, there are quite a hand full of commercial applications available in the market at present.

These MEMS devices use the same operation principles that we are familiar with. They do have 3-dimensional delicate structure development; MEMS fabrication is based on the tried and tested IC (integrated chips) fabrication. Batch processing of MEMS structure does decrease the cost per device, it is wrought with structural failure especially with movable actuator components.

Broad classification of MEMS devices

Most MEMS devices can not be strictly classified as there is always correlation between systems, some basic classifications can be arrived at.

Mechanical MEMS Transducer

Sensor Principles Description
Piezoresistive Change in resistivity of the material with applied strain. This change due to dimensional change is more prevalent in semiconductors than in metals. Material: Silicon
Piezoelectric Applied force on a piezoelectric crystal results in a potential difference across the crystal and vice versa. Material: Quartz, lead zirconate titanate (PZT), polyvinylidene fluoride (PVDF) and ZnO,
Capacitive Most commonly used in MEMS, based on the principle of parallel plate capacitors. Changes in any of the parameter contributes to sensor input.
Resonant Made of micromachined beams that vibrate at their resonant frequency which can change when attached to a measurand and is detected by piezoresistive elements

Common mechanical type Sensor:

  1. Strain gauge: For detailed description click here

  2. Accelerometer: Sensing acceleration by a mass suspended to a cantilever or beam on which acceleration acts to produce force (F= ma) that is detected using piezoresistive elements
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  3. Gyroscope: Measure rotational force. Works on measuring influence of the Coriolis force utilizing vibrational structures due to the difficulty in fabricating rotational elements.

  4. Pressure sensor: Stretched thin film that is sealed or in vacuum will have pressure difference on either side of the membrane. Like macro sensors based on diaphragm, the membrane deforms on force which is then measured using piezoresistive or capacitive methods

Actuator Principles Description
Electrostatic actuation Based on the capacitive effect of two charged parallel plates. The force exerted is proportional to the Capacitance.
Piezoelectric actuation Depending on the piezoelectric effect, a low voltage can produce large amount of force on the system. Used in tips of AFM or STM

Radiation MEMS Transducer

Sensor Principles Description
Photodiodes measuring light intensity based on the photoconductive effect where charge carriers are generated due to incidence of light resulting in current flow.
Charge-coupled devices These are metal oxide semiconductor (MOS) capacitor, the charge generated from photogenerated carriers. They are used in handheld video recorders
Pyroelectric sensors Capacitors whose charge can be altered by illumination or temperature changes. Pyroelectric sensors use piezoelectric and ferroelectric materials i.e they have varying dielectric constant with applied voltage. Material: ZnO . Used in surveillance, military, human motion detectors
Actuator Principles Description
light emitting diodes active devices that emit light on applying electric field across it.
light modulators Passive devices. Light modulation due to change in the polarization

optical MEMS components includes:

  • optical waveguides to route optical energy from one region to another

  • fibre-optic couplers

  • micromirrors


Thermal MEMS Transducer

Sensor Principles Description
Thermo-mechanical sensors All materials have a coefficient of thermal expansion that is unique to the material. Applying head on a sandwiched pair of materials results in the bending of the pair due to compression and tension. Bimetallic sensors are good examples.
Thermoresistive sensors Thee resistivity, ρ , of most materials changes with temperature. Thermistor with negative temperature coefficient wich exhibit a decrease in resistance with increasing temperature. Thought the output is not linear they are cheaper to fabricate
Thermocouples Working on the thermoelectric effect. Thermopiles are used in the MEMS Sensor application
Actuator Principles Description
Shape memory alloy actuation They show changes in their length (contraction) when heated. Material: titanium/nickel alloys, once mechanically deformed, returns to its original state when heated. Being conductive they are heated by passing current.

Magnetic MEMS Transducer

Sensor Principles Description
Hall effect Potential is developed across an current carrying conductor in a magnetic field that is perpendicular to the direction of flow of current
magneto-optical effect A electromagnetic wave that propagates in a medium is altered by a quasi-magnetic state of the medium. Mediums that exhibit shape, stress or crystalline anisotropy are used.

SQUIDS - superconducting quantum interference devices are the most common magnetic sensors in MEMS that can detect small magnetic flux in the heart and brain tissues due to electric pulses generated in them.

Actuator Principles Description
Magnetostrictive Actuators This is the change In shape of the material when magnetic field acts on it. The materials are ferromagnetic in nature to exhibit this property,

While Solenoids, and other 3D coils are used in the macroscale as magnetic actuators, it is difficult to mimic these structures in the microscale due to fabrication constraints.


Chemical & Bio MEMS Transducer

The devices need direct physical contact with the measurand which can be in anyone of the phases (solid, liquid or gas). The direct chemical reaction between the constituents give an appropriate electric signal that is indicative of the reaction. They use a variety of principles for operation. Some of which can be grouped as below.

Sensor Principles Description
Chemiresistors Resistance of a chemically reactive layer is measured.
Chemicapacitors Here capacitance is measured. Applications in humidity sensor.
Chemomechanical Chemical to mechanical transduction where in the material deforms on absorption of the analyte
Calorimetric sensors Chemical reactions are exothermic or endothermic in nature, they release or absorb heat, this is measured to give an idea of the measurand.

Ion Sensitive Field Effect Transistor

These are based on MOSFET (Metal oxide Semiconductor Field Effect Transducer) which can act as switches where the MOSETS turn on or off and the current flow in them depends on the Gate voltage applied. MOSFETS are also very sensitive to the impurities present in between their junctions.

The design of MOSFETS include a Silicon substrate which is doped followed by deposition of source and drain region which is also silicon doped. The source and drain are over the Silicon substrate separated by a gap of insulating layer over which an electrical contact is established which is the gate. The flow of electron in the system is dependent on the channel that forms at the silicon substrate close to the insulator layer which is depended on the metal contact gate (Think Capacitor like) . Such system can be very sensitive to any impurities on its surface which can lead to change in its workfunction. This phenomenon is exploited by the ISFET where instead of a gate, the device is in direct contact with the analyte and the current flow is depended on the potential of the electrolyte (analyte environment) and the insulator.

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Microfluidics MEMS Transducer

This is one instance where the underlying principle and applications get blurred because microfluidics are used in one way or the other for many different applications. Chemical analysis, drug delivery, biological sensing, environmental monitoring and many other applications typically incorporate MEMS microfluidic devices.

The design of these systems depends on the type of flow (Turbulent or laminar), capillary forces, fluid resistance and capacitance, bubble formation and cracking.

  1. Fluid channels


    Channels design for the fluids that it might come in contact with.

  2. Flow sensors:

Flow sensors can work in any one of the ways stated below

  • Being fluid dependent, they can heat up the fluid on one end and then measure the temperature of the fluid at other end. The measured temperature will be proportional to the flow rate and the distance travelled.
  • Being fluid independent they can measure the pressure excerted on the device channels by the fluid.
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  1. Valves
    Depending on the opening mechanism they can be active or passive i.e requirement of power to open. Most valves are passive and open with the pressure from the fluid. Active valves are opened by thermoelectric, piezoelectric, shape alloy, electrostatic principles.
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  2. Pumps
    MEMS pumps are very sensitive to fine particles which often cause contamination and leakage of the device. Membrane, rotary and ultrasonic pumps are the most common types of MEMS pumps. In rotary pumps the fluid is moved by gear like mechanism. Made of NiFe electroplated on the gear.
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  3. Droplet generators
    Used as inkjet printer heads they work on thermal or piezoelectric actuation to eject ink droplets. Thermal actuation is most common. Piezoelectrically actuated valves offer the advantage of very high forces, but very small movement for even very large voltages. They also find use in automotive fuel-injection valves and spray nozzles.

Application of MEMS

  • Automotive airbag sensor – Accelerometer

  • Medical pressure sensor - piezoelectric sensor

  • Inkjet printer head - Microfluidics

  • Overhead projection display - Radiation mems

  • BioMEMS - Chmical & biological Sensor

  • MOEMS - Radiation MEMS . MOEMS stands for Micro opto electro mechanical systems

  • RF MEMS - Mechanical sensor with focus on the vibrational frequency of the material

The last three are in various developmental stages.

See also

Actuators

Thermoelectric effect

Thermistor

Capacitive sensor

Strain gauge

Piezoresistive effect


Instrument selection for flow measurement based on application