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Micro and Smart Systems

Lecture - 39

Prof K.N.Bhat,

ECE Department , IISc Bangalore

email: knbhat@gmail.com

(1)Packaging Pressure sensors (Continued from Lecture 38) (2)Micromachined Silicon Accelerometers

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Topics for Discussions• Continued from Lecture -38 on pressure sensor Packaging and testing

•Accelerometer operating principle and types

•SOI based accelerometer – a case study

• Surface micromachined comb type accelerometer structure of ADXL type and the force balance circuit

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Photograph of Integrated pressure sensor chip in a TO39 header

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Integrated pressure sensor in a package

Pressure port

header

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Diced pressure sensor

Metal cap welded to the header

Teflon “O” ring

JigMale part

Pressure portN2 pressure

from cylinderJig

Female part

Metal Washer

Lead from the header

Aluminium wire bonding

Schematic of packaged

pressure sensor mounted in a Jig

for testing

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0 1 2 3 4 5 6 70

500

1000

1500

2000

2500 Sensitivity at the sensor output = 60 mV / bar / 10 VSensitivity at the amplfier output = 270 mV / bar / 10 VDiferential gain of the amplifier = 4.5

Out

put (

mV

)

Pressure ( bar )

Pressure Vs Output Voltage of the packaged Integrated Pressure Sensor

Amplified sensor output

sensor output

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Applications of Pressure sensors• Mapping Pressure on the wings of aircraft

•Automobile industry•Micro-fluidics for Flow sensing

•Biomedical applications –Blood Pressure measurement and Intracranial Pressure (ICP) monitoring

•Oceanography for Depth measurement . Packaging is critical

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Flow Sensor

Pyrex

Pyrex

P1 P2Silicon

Flow rate = P/R Capacitive Pressure sensor

Inlet OutletR - Fluidic resistance P = (P1 – P2 )

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• Despite several engineering challenges, MEMS offer high performance and are small, low power, and relatively cheap.

• Parameters of pressure sensor are sensitivity, nonlinearity , offset voltage and Maximum operating pressure and range

• Micro machined Sensors have been fabricated along with Electronics by the batch process of silicon wafers using SOI approach

• The pressure sensors find wide range of Applications for industrial, automotive, biomedical and aerospace and defense establishments.

• Pressure sensors constitute about 60% of the microsystem market

Summary and Conclusions

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K.N.Bhat, Silicon Micromachined Pressure Sensors”,Journal of Indian Institute of Science , Vol.87,pp210-213, 2007

S.R. Manjula , Teweldebhran Kifle, E. Bhattacharya, and K.N. Bhat 'Physical Model for the Resistivity and Temperature Coefficient of Resistivity in Heavily Doped Polysilicon' IEEE Transactions on Electron Devices (USA) , July 2006

V.Vinoth Kumar, Amitava DasGupta and K.N.Bhat,“Processoptimization for monolithic integration of piezoresistive pressure sensor and MOSFET amplifier with SOI approach” Journal of Physics, Institute of Physics Publishing, Vol.34, pp210-215, 2006

References

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Acceleration Sensors

Steady State condition,

Displacement , , is a measure of acceleration

F Ma kx

Force, F

Spring . Spring constant = k

Reference plane

MASS

Fixed electrodeDisplacement x

x ax Ma kSensitivity =

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• Piezoresistive: by integrating a piezoresistor

on the spring element

• Capacitive by measuring the change in

capacitance between the ‘proof mass’ and a

fixed electrode . The mass is also referred to as ‘seismic mass’

Approaches to Measure Acceleration

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A A’

Seismic Mass

Frame

Beam

1

2

3

ResistorSeismic MassFrame

Beam

Framecross section at A-A’

Top view

Piezoresistive Silicon Micro Accelerometer

1

2

3

Vin

Vo

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Seismic MassFrame Beam Frame

Bumper

Bumper

LeadsTop cap

Bottom cap

Damping gap

Micro structure of an accelerometer with airdamping and bumpers to serve as over-range stopmechanism.

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Capacitive Accelerometer

Beam

Mass

Si (10 μm)

SiO2 (1 μm)

Bulk Si

Metal Contact

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SOI based Accelerometer Fabrication1. SOI or polySi

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2. Patterned mechanical structure (polysi or single crystal ) over the sacrificial SIO2

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Beam

Mass

Si or PoySi

SiO2 (1 μm)

Bulk Si

3. Oxide Etch, Release the mechanical strycture

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SOI accelerometer fabrication

Beam

Mass

Electrodes (Cr / Au)

Si or PolySi

SiO2 (1 μm)

Bulk Si

4. Metallize

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KOH Etching of Si Convex corners

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MICROPHOTOGRAPH OF PATTERENED SAMPLE AFTER ETCHING

50m

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Undercutting Of Convex Corners is present

(411) Planes are exposed

Undercutting Of Convex Corners is Absent

Microphotograph Of Bent Portion Of Beam

(a) 30% KOH at 65C

(b) 30% KOH solution containing 30% tertiary-butanol solution

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Alternate Approach to prevent ender-cutting at convex corners

• The width of the corner beam must be about twice the thickness of the square pattern

•Programs that simulate the structure to achieve by etching with a given mask are commercially available

Modify the etch- mask pattern as shown “Corner compensation” scheme”

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Resonance frequency 35.5 KHz measured using laser dopler vibrometer

Thickness =14.45 m

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0.0 0.5 1.0 1.5 2.015.1015.1515.2015.2515.3015.3515.4015.4515.5015.55

UZ1

Cap

acita

nce(

Pf)

Voltage(volts)

Results obtained by electrostatic actuation of the structure fabricated in the laboratory

Sensitivity=0.2 pF/ V/ g

Released SOI (Mass)

SiO2anchor

Support Beam

Si substrate

oACapaci tance C

d

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Dynamic test result using Laser Doppler vibrometer (Vibration Amplitude versus frequency)

Resonance frequency=35.5KHz

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Vibration test result obtained on silicon micro-accelerometer fabricated with SOI wafer processing fabricated as part of E3-222 course in the lab at CEN IISc Bangalore

Resonance frequency =35.5kHZ frequency,

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Applications of Micromachined Accelerometers

• Front and side airbag crash sensing

• Vehicle and traction control systems

•Inertial Measurements and Navigation

•Human Activity for pacemaker control

(a) Acceleration Measurement

(b) Vibration Measurement• Engine management and condition monitoring

• Monitoring Seismic activity ( Earth quake)

•Shock and impact monitoring

•Security Devices

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Summary

• Pressure sensor packaging and testing

•Accelerometer operating Principle and approaches

• Fabrication of accelerometer with Silicon on Insulator wafer

•Testing by Electrostatic actuation and frequency response for testing capacitive accelerometer

We have discussed the following