microelectromechanical(-ish) sensors using them...

1 © 2007 Spring_2007_MEMS_JKa.ppt / 2007-03-07

Company Confidential

Microelectromechanical(-ish) sensors &

using them for something useful

S-108.4010 Licentiate course in measurement science and technology JKa 07.03.2007



“MEMS-like” sensors

• MEMS general• Displacement

• Capacitive• Basic equations

• Resistive• Properties• Inertial

• Acceleration• Bulk• surface

• Angular acceleration• examples

• Gyro• Theory• structures

• Gas• Resistive/capacitive, thin/thick film• Fabry perot• Combination



MEMS inertial sensors

• Simple pendulum analogy• Inertia deflection • Properties:

• Spring constant• Mass• Damping

deflection

mass

movement

Bandwidth etc.



Capacitive measurement

• Movement detected as a change in capacitance (Si~ideal spring)• Plate capacitor: C~ A, 1/d

• Noise limits resolution

• Base capacitance can be increased by• Increasing surface area A• Decreasing electrode separation d• Paralleling several plate capacitors

dA

C

ε



Linear acceleration sensor

• Inertial mass• Spring(s)• Mass position measured

• capacitively

C1

C2

C1

C2

C1=C2 C1<C2



Angular acceleration sensor

• For rotation measurement• Measures angular acceleration (dou..)• Double integration required for position• Structure simpler than Gyro…

C1 C3

C4C2

C1C3

C4C2



Bulk micromachining

• Etch structure out of bulk material • Anisotropic etching

• +: big mass high performance• -: thick

• VTI Technologies Oy



Surface micromachining

• Often comb-like structures• +: thin, process combination, sophisticated structures possible• -: somewhat more complex



Surface micromachining

• Grow structures on silicon substrate• Mask, grow, etch, • Thin layers



Performance

• Mass• Small mass for bandwidth• Big mass for low noise

• Filling gas• Type• Pressure• Damping

• Bandwidth• Sensor• asic

• Structural rigidity• Calibration



Acceleration sensor uses

• Automotive industry• Sports• Cameras (automatic image rotation)• Shock detection• UI control• HDD free fall detection

• Some math• ∫ acceleration velocity + noise• ∫ ∫ a ∫ (velocity+noise) position + noise2

• Double integration noise accumulates



Foot trajectory tracking

• Foot position by double integration (fs ~ 1kHz)• Noise accumulation problem avoided by zeroing on each step• Speed, distance,…

Dynastream Innovations



Gyroscope

• Rotation measurement

• Measures angular velocity• Rad/s

• Some math• ∫ velocity position + noise• Only Single integration

needed for position!Mechanical flywheel gyro

T=Ω*ω*IT=Ω*ω*IT=torqueΩ=turning rateω=angular rate I=spinning mass inertia



Gyroscopes

• Based on coriolis force• excitation in Y direction + rotation in Z signal in X direction

x

y

drive

rotation

signal



Gyros…

• Vibrating wheel (Berkeley et.al.)• Two axis• Excitation around axis of symmetry• Rotation tilting



Vibrating bar gyro

• Murata



Tuning fork gyro

• Tuning for structure• Commonly micromachined

from quartz

microcomponents.ch



Yet another gyro sensor…•Analog Devices

•Sensor + excitation•Signal conditioning



Gyro uses• Image stabilization

• Cameras

• Automotive industry• Platform stabilization• Navigation (support for GPS)



Image stabilization

• By active compensation• Optics• Image sensor position

• By calculation



Image stabilization…

• Mobile phones:• Small sensors• Optics• Shaky platform• Mega pixel cameras

Low light sensitivity



Benchmarking: Fiber-optic gyro

• Optical phase tracking• Extremely high performance

• Drift <0.01°/hr



Fabry-perot interferometer

• Tunable band-pass filter• Transmission range depends on separation between reflecting surfaces• Separation d altered electrostatically



Optical gas measurement

• Two-point measurement

Sample gas Detector V

Lightsource

F-P filter

Tuning voltage

λ1λ2



Example of a commercial product

• Vaisala Carbocap



Other MEMS based sensors

• Sensing element• Thermal• SAW• Capacitive• Etc.

• Manufacturing steps often similar



Microbolometer array, aka…

• Bolometer == instrument for measuring thermal radiation• black body radiation electrical signal• Miniature array of temperature sensors• Uncooled (or cooled)

• Array:



…thermal imaging sensor

• False color maps of “thermal field of view”



Ammonicap

• Thick film ammonia sensor• Not a “real” MEMS, but some interesting means used…• Water vapor usually a major problem• Heating-cooling cycles

• H2O and NH3 diffuse at different rates hysteresis in C vs. T graph

vaisala.com



Problems with MEMS

• Design• Manufacturing

• Cost• Yield important

• Fragility• Small sensor >100000g!!!

• Stiction etc.



That’s it.What was covered?• MEMS• Bulk micromachining vs. surface micromachining• Acceleration• Angular acceleration• Angular velocity (gyro)

• Some gas sensing examples



Other stuff…

• Image sources:• Murata, ADI, Kionix, Seiko-Epson, Vaisala Oyj, Wikipedia, Aerospace Corporation,

Dynastream, Nike, Apple, Coyotair, Microcomponents…

microelectromechanical(-ish) sensors using them...

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