MEMS SENSORPresented by

MD. FAIZAN AHMAD

OutlineMEMS IntroductionSensor and its typeApplicationsReferences

What is MEMS?

MEMS or Micro-Electro Mechanical System is a technique of combining Electrical and Mechanical components together on a chip, to produce a system of miniature dimensions.

MEMS is the integration of a number of micro-components on a single chip which allows the microsystem to both sense and control the environment.

The components are integrated on a single chip using micro fabrication technologies.

What is a Sensor?

A device used to measure a physical quantity(such as temperature) and convert it into an electronic signal of some kind(e.g. a voltage), without modifying the environment.

What can be sensed?Almost Everything!!!Commonly sensed parameters are:

Pressure Temperature Flow rate Radiation Chemicals

N

S

EW 2 Axis Magnetic Sensor

2 Axis Accelerometer

Light Intensity Sensor

Humidity Sensor

Pressure Sensor

Temperature Sensor

But why MEMS for sensors?

Smaller in sizeHave lower power consumptionMore sensitive to input variationsCheaper due to mass productionLess invasive than larger devices

Type of Sensors

MechanicalSensors

• Strain Gauges• Accelerometers• Pressure Sensors• Microphones• Gyroscopes(Rotati

on Rate)

OpticalSensors

• Direct Sensors (Light Electronic Signal)

• Indirect Sensors (Light Intermediate Energy Electronic Signal)

• Biological Light Sensors

ThermalSensors

• Thermo mechanical (Dimension)

• Thermo Resistive (Resistance)

• Acoustic (Sound)• Biological

Chemical & Biological Sensors

• Electronic Nose• Electronic Tongue

Mechanical Sensors

AccelerometersPressure SensorsMicrophonesGyroscopes(Rotation

Rate) ,etc.

Accelerometers

Inertial sensor:Newton’s 1st law (Mass of inertia).

Device used to measure:AccelerationDisplacementForce

MEMS-based accelerometer with capacitors is typically a structure that uses two capacitors formed by a moveable plate held between two fixed plates.

Under zero net force the two capacitors are equal but a change in force will cause the moveable plate to shift closer to one of the fixed plates, increasing the capacitance, and further away from the other fixed reducing that capacitance.

This difference in capacitance is detected and amplified to produce a voltage proportional to the acceleration

Pressure Sensors Pressure sensors are required in all walks of life, irrespective of civilian,

defence, aerospace, biomedical, automobile, Oceanography or domestic applications.

Among the various devices, pressure sensors using MEMS technology have received great attention because the pressure sensors find applications in everyday life involving sensing, monitoring and controlling pressure.

Pressure sensors are categorized as

a) Absolute Pressure Sensors

b) Gauge Pressure sensors

c) Differential Pressure Sensors

a) Absolute Pressure Sensors

Measure the pressure relative to a reference vacuum encapsulated within the sensor Such devices are used for atmospheric pressure measurement and as manifold absolute pressure (MAP) sensors for automobile ignition and airflow control systems.

Pressure sensors used for cabin pressure control, launch vehicles, and satellites also belong to this category.

Schematic diagram of Absolute Pressure sensor

b) Gauge Pressure sensors

Measure pressure relative to atmospheric pressure. One side of the diaphragm is vented to atmospheric pressure.

Blood pressure (BP), intra-cranial pressure (ICP), gas cylinder pressure and most of ground-based pressure measurements are gauge pressure sensors.

Schematic diagram of Gauge Pressure sensor

c) Differential Pressure Sensors

Measure accurately the difference ΔP between two pressures P1 and P2 across the diaphragm (with ΔP << P1 or P2 ), and hence need two pressure ports.

They find applications in airplanes used in warfare. They are also used in high pressure oxidation systems.

Schematic diagram of Differential Pressure sensors

In almost all types of pressure sensors, the basic sensing element is the diaphragm, which deflects in response to the pressure.

As the deflections in diaphragm-based sensors are small they cannot be directly measured. This mechanical deflection or the resulting strain in the diaphragm is converted ultimately into electrical signals using suitable transduction mechanisms, namely,

1) Capacitive

2) Piezoresistive or piezo-electric

Capacitive Pressure Sensor

This approach uses the diaphragm as one electrode of a parallel plate capacitor structure and diaphragm displacement causes a change in capacitance with respect to a fixed electrode.

The merits of capacitive pressure sensors are their high sensitivity, which is practically invariant with temperature.

An electronic circuit is used to convert the capacitance change into an electrical output.

Schematic representation of pressure sensors (a) Piezoresistive (b) capacitive (c) Piezoelectric

Microphone

Microphone is transducer that converts acoustic energy into electrical energy. The microphones are widely used in voice communications devices, hearing aids, surveillance and military aims, ultrasonic and acoustic distinction under water, noise and vibration control.

Basically the microphone MEMS sensor is a variable capacitor where the transduction principle is the coupled capacitance change between a fixed plate (back plate) and a movable plate (membrane) caused by the incoming wave of the sound.

Gyroscopes

A gyroscope is a device for measuring or maintaining the orientation, based on the principles of the conservation of momentum.

It uses vibrating mechanical element to sense the rotation.Transfer of energy between two vibrating resonator is by

coriolis acceleration.

Types of Gyroscope

Spinning Gyroscope Optical Gyroscope Vibrating Gyroscope

The rotation of tines causes the Coriolis Force.

Forces detected through either electrostatic, electromagnetic or piezoelectric.

Displacements are measured in the Comb drive.

Applications of Gyroscope

Yaw rate sensor for skid control in antilock braking applications for automobiles.

Inertial navigation systems. Smart cruise control. Guiding gun launched munitions. Detection of roll over detections.

Applications in Medical Science Biocavity Laser : This device

distinguishes cancerous from non cancerous cells thus aiding the surgeons in operations.

Smart Pill : Implanted in the body Automatic drug delivery (on

demand)

Sight for the blind : MEMS based array that may be inserted in the retina of a blind person to provide partial sight

Applications in Marine Science

Sensing in marine environment maybe done for various reasons:

Oil exploration and related applications

Global weather predictions

Monitor water quality for any contamination

Measure parameters detrimental to the “health” of structures in the sea ( like oil rigs and ships )

Study of aquatic plants and animals

In military operations

Applications in Marine Military Operations An array of MEMS sensors spread on the ocean floor

could detect the presence of enemy submarines.

MEMS sensors (pressure sensors, accelerometers etc.) are being used in anti-torpedo weapons on submarines and ships.

MEMS sensors in torpedoes are responsible forDetonating the torpedo at the right timeHitting the target in a crowded environmentPrevent any premature explosion

References

X. Wang, J. Engel, C. Liu, J. Micromech. Microeng. 2003, 13, 628.

Christian A. Zorman, Mehran Mehregany, MEMS Design and Fabrication, 2nd Ed. 2,16.

Ms. Santoshi Gupta, MEMS and Nanotechnology IJSER, Vol 3, Issue 5,2012

Stephen Beeby, MEMS Mechanical Sensor, PP. 7

Lenz, J., Edelstein, A.S., "Magnetic sensors and their applications." IEEE Sensors J. 2006, 6, 631-649.

Sinclair M J 2000 A high force low area MEMS thermal actuator Proc. 7th Intersociety Conf. on Thermal and Thermomechanical Phenomena (Las Vegas, NV) pp 127–32

R. Ghodssi, P. Lin (2011). MEMS Materials and Processes Handbook. Berlin: Springer.

Chang, Floy I. (1995).Gas-phase silicon micromachining with xenon difluoride. 2641. pp. 117.

Thank You