2. Introduction to MEMS transducers
MEMS or Micro-Electro-Mechanical Systems are also known as smart
matters.
They are miniaturized mechanical and electromechanical devices.
MEMS are embedded in semiconductor chips using micro-fabrication
techniques.
Why are they important?
It generates continued sustained improvement e.g.
functionality of small microphones, cameras, electrical signal
filters etc.
Generates new kinds of products such as multi-axis inertial
motion sensors.
3. History and current state
■ The origins of what we now know as micro-electromechanical
system (MEMS) technology can arguably be traced back to 1
April 1954, when a paper by Smith (1954), then at the Bell
Telephone Laboratories, was published in Physical Review.
■ Roots were laid by Richard Feynman while delivering a speech at
Caltech in 1959 “There is plenty of room at the bottom.”
■ From 1960s through 1990s development took place at linear
pace.
■ Hit inflation point in 2000s and sustained considerable
momentum into the 2010s.
4. Future of MEMS
The developments in the MEMS include,
Trillions of sensors.
Incorporation of heterogeneous sensors
Improvement on wafer-level packaging technology
Integration with advanced CMOS circuitry
Local environmental monitoring devices and deployment in wearables.
MEMS reliant drones and other small personal robots.
5. Why choose MEMS?
■ Small size and light weight.
■ Enhanced performance and reliability.
■ Low cost
Applications
■ Automotive systems
■ Automated manufacturing
■ Health care
■ Instrumentation
■ Consumer products
■ Aerospace
6. Types of MEMS devices
Typical MEMS Devices;
Sensors
In the broadest definition, a sensor is an object whose purpose is
to detect events or changes in its environment, and then provide a
corresponding output. This category includes:
> Pressure Sensors
> Accelerometers
> Gyroscopes
Actuators
Converts energy into motion or mechanical energy. Actuator is
a motor that actuates or moves something. Actuators include:
> High Aspect Ratio Electrostatic Resonators
> Thermal Actuators
> Magnetic Actuators
> Comb-drives Comb drive actuator
7. MEMS based SENSORS
■ MEMS sensors can be defined as the combination of micro-sensors and electronic
devices integrated on a single chip.
■ That package is a bit like processors, but it includes all the mobile parts of the
device. Technological progress allows more and more sensors to be manufactured
on a microscopic scale as microsensors using MEMS technology.
■ MEMS researchers and developers have demonstrated an extremely large number
of microsensors for almost every sensing modality including temperature,
pressure, inertial forces, chemical species, magnetic fields, radiation, etc.
■ The micromachined version of a sensor usually outperforms a sensor made using
the most precise macroscale level machining techniques.
8. How do they work?
■ Input samples may be:
a) motion of a solid
b) pressurized liquids or gases,
c) biological and chemical substances.
■ Major sensing technologies that can be applied in the
MEMS form include the following:
• piezoresistive
• capacitive
• resonant
• thermoelectric
■ Piezoresistive sensors dominate pressure, acceleration, and force
applications. Typically, four piezoresistors are connected into a
Wheatstone bridge configuration to reduce temperature errors.
9. Applications of MEMS sensors
■ Biotechnology
DNA amplification and identification, biochips for detection of hazardous chemical
and biological agents and drug screening.
■ Communications
Electrical components such as inductors and tunable capacitors can be improved
significantly compared to their integrated counterparts if they are made using
MEMS and Nanotechnology.
■ Inertial Sensing
MEMS accelerometers have displaced conventional accelerometers for crash air-
bag deployment systems in automobiles.
■ Medicine
The first and by far the most successful application of MEMS in medicine are
MEMS pressure sensors, which have been in use for several decades to monitor
patient’s vital signs and used in eye surgery to control vacuum level.
10. Accelerometer
■ Accelerometer measures proper acceleration ("g-force"). which
is the acceleration it experiences relative to freefall and is the
acceleration felt by people and objects.
■ Modern accelerometers are often small micro electro-
mechanical systems (MEMS), and are indeed the simplest
MEMS devices possible.
They can be made using
>) Piezo-electric effect
>) By sensing capacitive changes
>) For very high sensitivities Quantum Tunneling is also used
How are they made?
11. Features of capacitive interface
■ Can operate as both sensor and actuator.
■ Independent of base material.
■ Relies on the variation of capacitance.
Structure of MEMS Accelerometer
■ They consist of MEMS structures suspended by poly-silicon springs above
the substrate in a manner that proof mass(body of sensor) is capable of
moving in both X and Y axes.
■ 32 sets of radical fingers around four sides of proof mass.
■ Fingers are placed between the plates that are fixed to the substrate.
■ Each finger and pair of fixed plate represents a capacitor.
12. Working Calculations
■ Capacitance of capacitor:-
■ Where =
A= area of electrodes
d=distance between the
=permittivity of material separating them
■ For zero acceleration the capacitance of plates remains same.
■ Displacement is approximately proportional to the capacitive difference.
■ The differential capacitance is measured using synchronous
modulation/demodulation techniques.
■ Output signals are;
* Voltage proportional to acceleration
* PWM proportional to acceleration
13. Applications of MEMS
Accelerometers
■ Personal devices such as media players, gaming
devices and smart phones.
■ Camcorders and still cameras.
■ Detecting car crashes and deploying air bags.
■ Controlling and monitoring military and aerospace
systems.
14. Gyroscopes
■ MEMS gyroscope reliably sense and measure the
angular rate of an object using the Coriolis Effect.
■ MEMS gyroscopes are making significant progress
towards high performance and low power
consumption.
■ When a mass (m) is moving in direction v→ and
angular rotation velocity Ω→ is applied, then the mass
will experience a force in the direction of the arrow as a
result of the Coriolis force. And the resulting physical
displacement caused by the Coriolis force is then read
from a capacitive sensing structure.
15. Features
■ Measure rotation
■ Couple energy from one vibrational axis to another due to Coriolis Effect
■ Two micromachined modes: Open loop vibration and Force-to rebalance
mode
■ Vibrating prismatic beams
■ Beam driven in one direction, deflection measured in orthogonal
direction
How are they made?
MEM gyroscopes are printed onto circuit boards using
photolithography.
Some parts incorporate multiple gyroscopes and accelerometers,
to achieve output that has six full degrees of freedom.
There are many types, but they all rely on the same principle, that
of vibrating objects undergoing rotation.
16. Structure
Internally, MEMS gyroscopes use lithographically constructed versions of one or more of the
mechanisms outlined below:
• Tuning forks
– This type of gyroscope uses a pair of test masses driven to resonance. Their displacement from the
plane of oscillation is measured to produce a signal related to the system's rate of rotation.
• Piezoelectric gyroscopes
–A piezoelectric material can be induced to vibrate, and lateral motion due to Coriolis force can be
measured to produce a signal related to the rate of rotation.
• Vibrating wheel gyroscope
– A wheel is driven to rotate a fraction of a full turn about its axis. The tilt of the wheel is measured to
produce a signal related to the rate of rotation.
17. Applications of MEMS Gyroscope
■ Spacecraft orientation
The oscillation can be induced and controlled in the vibrating structure gyroscope for the positioning
of spacecraft such as Cassini-Huygens. They provide accurate 3 axis positioning of the spacecraft
and are highly reliable over the years as they have no moving parts.
■ Automotive
These are used to detect error states in yaw compared to a predicted response when connected as
an input to electronic stability control systems in conjunction with a steering wheel sensor.
■ Entertainment
Game Boys and most modern smartphones use a piezoelectric gyroscope to detect rotational
movement. The Sony SIXAXIS PS3 controller uses a single MEMS gyroscope to measure the sixth
axis (yaw).
■ Photography
Many image stabilization systems on video and still cameras employ vibrating structure gyroscopes.
■ Industrial robotics
Epson Robots uses a quartz MEMS gyroscope, called QMEMS, to detect and control vibrations on
their robots. This helps the robots position the robot end effector with high precision in high speed
and fast-deceleration motion.
18. Actuators
■ An actuator is a type of motor that is responsible for moving
or controlling a mechanism or system, a device that
actuates or moves something.
■ Converts Energy into motion or mechanical energy.
MEMS Actuators
■ Also known as micro-actuators, micro-systems or micro-
machines.
■ Produced by assembling extremely small functional parts
around 1-15 mm.
19. Classification of MEMS Actuators
■ Electrostatic: attraction between oppositely charged conductors.
■ Thermal: Displacement due to thermal expansion.
■ Piezoelectric: Displacement is due to strain induced by an electric field.
■ Magnetic: Displacement is due to interaction among various magnetic elements
i.e. permanent magnets, external magnetic fields, magnetizable material and
current carrying conductor.
■ On the basis of movement micro-actuators are:
1. Translational
2. Rotational
20. Features
Features of MEMS actuators are:
■ Light weight
■ Conformable
■ Precision device
■ One of the basic building blocks in MEMS processing is the ability to deposit thin
films of material with a thickness anywhere between a few nanometers to about
100 micrometers.
■ Patterning in MEMS is the transfer of a pattern into a material.
21. Applications of MEMS Actuators
■ The applications of micro-actuators include:
■ Digital Micro-mirror Device (DMD) chip in a projector based
on DLP technology, which has a surface with several
hundred thousand micro-mirrors or single micro-scanning-
mirrors also called micro-scanners.
■ Optical switching technology, which is used for switching
technology and alignment for data communications.
■ Fluid acceleration such as for micro-cooling.
■ Micro-surgical applications.
■ Data reading and recording control.
■ RF signal limiting.
