1
ENE 5400 Spring 2004 1
Lecture 4 : Micromachined Sensors
Sensing Mechanisms Mechanical Sensors
ENE 5400 Spring 2004 2
Transducers ()
Transducers convert one energy domain into another form: Thermal, Mechanical, Magnetic, Electrical, Chemical, Radiant
Transducer Actuators (): convert input energy to a mechanical
motion Sensors (): convert sensed physical quantity to an
electrical signal
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ENE 5400 Spring 2004 3
Terminology
Accuracy: the quality that characterize the capacity of a measuring instrument for giving results closed to the true valueof measured quantity
Precision: the quality that characterize the capability of a measuring instrument for giving repeatable results
Sensitivity: the slope of the measured output with respect to the measured physical domain
(((( ))))oxx
o dxdy
xS====
====
True value×××× ×××××××× ×××××××× ×××××××××××× ××××
×××× ××××
High accuracy, low precision
True value×××××××× ×××××××× ×××××××××××× ×××××××× ×××× ××××
Low accuracy, high precision
ENE 5400 Spring 2004 4
Sensitivity: an Example
A capacitive accelerometer has a detection circuit with a sensitivity of 10 mV/fF. The change of capacitance with respect to the displacement is 100 fF/µµµµm, and the resultant displacement with respect to 1-g force is 100 nm. What is the overall sensitivity of V/g?
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ENE 5400 Spring 2004 5
Sensing Mechanisms
Resistive sensing Capacitive sensing Tunneling-current sensing Self-generating sensing
Thermoelectric Piezoelectric/pyroelectricElectrochemical
ENE 5400 Spring 2004 6
Resistive Sensing
Resistive change can result from: Geometrical change Piezoresistive effect Temperature effect Magnetoresistive effect Chemical effect
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ENE 5400 Spring 2004 7
Geometric Change: Strain Gauge
Resistance R = ρρρρL/A; ρρρρ = resistivity in Ω⋅Ω⋅Ω⋅Ω⋅cm Gauge factor GF = (dR/R) / (dL/L) = 1 + 2v, if ρρρρ unchanged
Thin-film metal GF ≈≈≈≈ 2 Low sensitivity
AD
LL+∆L
ENE 5400 Spring 2004 8
Piezoresistive Sensing
Discovered by Lord Kelvin in 1856; Resistive change due to the change of the carrier mobility or carrier number under applied stress (a quantum effect)
∆ρ/ρ∆ρ/ρ∆ρ/ρ∆ρ/ρ = ππππlσσσσl + ππππtσσσσt
Stresses perpendicular and parallel to the piezoresistor are separately considered
A strong function of temperature Diffused or ion-implanted piezoresistive sensors on silicon is
widely used
Piezoresistor
Orthogonal (or transverse) directionParallel (or longitude) direction
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ENE 5400 Spring 2004 9
Advantages
The gauge factor of semiconductors is at least an order of magnitude higher than that of metals
Silicon is a very robust material The integration of sensor and membrane eliminates the need
for bonding The resistors can be limited to surface where the stresses are
maximal due to bending or torsion A suitable technique for miniaturization of sensors Good matching of resistors can be achieved, which is useful if
Wheatstone bridges are used
ENE 5400 Spring 2004 10
Mathematical Description
For a three-dimensional anisotropic crystal, the electric field vector is related to the current-density vector by a 3××××3 resistivity tensor
For simplicity, we will look at an isotropic conductor with nominal ρρρρ1= ρρρρ2 = ρρρρ3 = ρρρρ, and ρρρρ4 = ρρρρ5 = ρρρρ6 = 0
(((( ))))13
2
1
345
426
561
3
2
1
⋅⋅⋅⋅
====
J
J
J
E
E
E
ρρρρρρρρρρρρρρρρρρρρρρρρρρρρρρρρρρρρ
Ref: (1) S.M. Sze, Semiconductor Sensors, Chap. 4, John Wiley & Sons, 1994(2) W.P. Mason, J. Acoustical Soc. Amer., vol. 29, pp. 1096-1101, 1957
(((( ))))2
0
0
0
6
5
4
3
2
1
6
5
4
3
2
1
∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆
++++
====
ρρρρρρρρρρρρρρρρρρρρρρρρ
ρρρρρρρρρρρρ
ρρρρρρρρρρρρρρρρρρρρρρρρ
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ENE 5400 Spring 2004 11
Cont’d
Resistivity change due to pressure is related to 3 variables (36, original) for cubic crystal structure of silicon Substituting Eqn. (2) and (3) into (1)
produce the resulting electric field under stresses
(((( ))))3
00000
00000
00000000
000
000
1
3
2
1
3
2
1
44
44
44
111212
121112
121211
6
5
4
3
2
1
====
∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆
ττττττττττττσσσσσσσσσσσσ
ππππππππ
ππππππππππππππππππππππππππππππππππππππππ
ρρρρρρρρρρρρρρρρρρρρρρρρ
ρρρρ
ττττ1ττττ1
ττττ2
ττττ2
ττττ3
ττττ3
σσσσ3
σσσσ2
σσσσ1
x
y
z
ENE 5400 Spring 2004 12
Cont’d
Vector transformation is used to establish the piezoresistivecoefficients :
The longitudinal and transverse piezoresistive coefficients are (Ref. 2):
For example, ππππl in the <111> direction can be obtained by setting (l1,m1,n1) = (1/√√√√3, 1/√√√√3, 1/√√√√3):
====
z
y
x
nml
nml
nml
z
y
x
333
222
111
*
*
*
(((( )))) (((( ))))(((( )))) (((( ))))2
221
22
21
22
2111124412
21
21
21
21
21
2111124411 2
nnmmll
nmnlml
t
l
++++++++⋅⋅⋅⋅−−−−++++−−−−====
++++++++⋅⋅⋅⋅−−−−++++++++====
ππππππππππππππππππππππππππππππππππππππππ
(((( )))) (((( ))))441211111 2231 ππππππππππππππππ ++++++++====>>>><<<<l
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ENE 5400 Spring 2004 13
ππππl and ππππt for Various Combinations of Directions
(ππππ11+ππππ12-ππππ44)/2(1, -1, 0)(ππππ11+ππππ12+ππππ44)/2(1, 1, 0)
ππππ12(0, 0, 1)(ππππ11+ππππ12+ππππ44)/2(1, 1, 0)
(ππππ11+2ππππ12-ππππ44)/3(1, 1, 1)(ππππ11+ππππ12+ππππ44)/2(1, 1, 0)
(ππππ11+ππππ12-ππππ44)/2(1, -1, 0)(ππππ11+2ππππ12+2ππππ44)/3(1, 1, 1)
ππππ12(1, 1, 0)ππππ11(0, 0, 1)
ππππ12(0, 1, 0)ππππ11(1, 0, 0)
ππππtTransverse directionππππl
Longitudinal direction
ENE 5400 Spring 2004 14
Piezoresistive Coefficients for n- and p-type Silicon
Positive values implies that under tensile stress, the resistance increases Design issue: placement and orientation of resistors
p-type silicon
n-type silicon
Material
7.8
11.7
Resistivity(Ω⋅cm)
138.1-1.16.6
-13.653.4-102.2
π44
(10-11 Pa-1 )π12
(10-11 Pa-1)π11
(10-11 Pa-1)
Source: C.S. Smith, Physical Review, vol. 94, pp. 42-49, 1954
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ENE 5400 Spring 2004 15
Piezoresistance Coefficient ππππl and ππππt
Source: Y. Kanda, IEEE Trans. Electron Device, ED-29, pp. 64-70, 1982
For p-type in the (001) plane(unit: 10-12 cm2/dyne)
For n-type in the (001) plane(unit: 10-12 cm2/dyne)
ENE 5400 Spring 2004 16
Placement of Piezoresistors
R1, R2, and R3, R4 are used to sense transverse stress and longitudinal stress, respectively Resistors are placed in the
middle of side to increase sensitivity
Can use Wheatstone-bridge configuration
To get the same sensitivity, alternative design should consider variation of stress sensitivity towards the center and corners of the membrane; length and number of piezo-resistors may vary in design
membraneR1 R2
R3
R4
R1
R3
R2
R4
Vb
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ENE 5400 Spring 2004 17
Temperature Coefficient of PiezoresistiveSensors
Piezoresistive coefficients depend on doping concentration They decrease with increasing
impurity concentration Temperature sensitivity is a major
concern for piezoresistivesensors The coefficients decrease with
increasing temperature A design trade-off exists between
the sensitivity and the temperature dependency Doping depth is also
important; shallow implanted resistors can have small T.C.
Leakage current occurs at high temperature (pn junction)
ENE 5400 Spring 2004 18
TCO and TCS
Two temperature-dependent coefficients are considered: Temperature Coefficient of Offset
(TCO) Temperature Coefficient of Sensitivity
(TCS) For the TCO case: an offset voltage exists
due to unmatched resistors during fabrication, but can be made insensitive to temperature
Vb
R
RR + ∆∆∆∆r
R + ∆∆∆∆r
(((( ))))(((( ))))
etemperaturtoinsentiveisoffsettheRRrrifTherefore
rRRrRr
TO
sorR
rVV
Ob
o
,//
22
,2
2
&
&
&
&
====++++−−−−====
∂∂∂∂∂∂∂∂
++++========
Vo
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ENE 5400 Spring 2004 19
Temperature Coefficient of Sensitivity (TCS)
TCS can be compensated if a constant-bridge-current scheme, instead of constant bridge voltage, is employed
Let’s derive for the constant-bridge-voltage case first; we know that:
For p-type resistors:
Case I: constant-voltage bridge
ttllRR ππππσσσσππππσσσσ ++++====
∆∆∆∆
(((( )))) (((( ))))12
,
44
12441144
tlRR σσσσσσσσππππ
ππππππππππππππππ
−−−−====∆∆∆∆
>>>>>>>>>>>>>>>>Vb
R + ∆∆∆∆R
R + ∆∆∆∆RR - ∆∆∆∆R
R - ∆∆∆∆R
∆∆∆∆V
( ) bar/V/mVunitRP
R
VPV
SySensitivit
VRR
V
b
b
=∆∆=
∆∆=
∆=∆
21
1
ENE 5400 Spring 2004 20
Cont’d
Combining (1) and (2), the TCS expression says that the TC of the pressure sensitivity is essentially the same as the TC of ππππ44
Case II: constant-current bridge
Low TCS can be achieved by matching of the two temperature coefficients
(((( ))))(((( )))) (((( ))))
(((( ))))(((( ))))
TTTS
STCS
TPTPTS
PS
tl
tl
tltl
tl
∂∂∂∂−−−−∂∂∂∂
−−−−++++
∂∂∂∂∂∂∂∂====
∂∂∂∂∂∂∂∂====
∂∂∂∂−−−−∂∂∂∂
∆∆∆∆++++
∂∂∂∂∂∂∂∂
∆∆∆∆−−−−====
∂∂∂∂∂∂∂∂
−−−−∆∆∆∆
====
σσσσσσσσσσσσσσσσ
ππππππππ
σσσσσσσσππππππππσσσσσσσσ
σσσσσσσσππππ
11122
21
44
44
4444
44
R + ∆∆∆∆R
R + ∆∆∆∆RR - ∆∆∆∆R
R - ∆∆∆∆R
∆∆∆∆V
Ib
(((( ))))
(((( ))))444 3444 21
32143421
neglected
tl
tlplusus
tlb
b
TTR
RTTS
STCS
RPP
RPI
VSRIV
∂∂∂∂−−−−∂∂∂∂
−−−−++++
∂∂∂∂∂∂∂∂++++
∂∂∂∂∂∂∂∂====
∂∂∂∂∂∂∂∂====
−−−−∆∆∆∆
====∆∆∆∆∆∆∆∆====
∆∆∆∆∆∆∆∆====∆∆∆∆====∆∆∆∆
σσσσσσσσσσσσσσσσ
ππππππππ
σσσσσσσσππππ
1111
211
,
min
44
44
44
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ENE 5400 Spring 2004 21
Self-heating Effect
Happens in thermal resistive sensors by the biasing current; measured resistance value is affected Self-heating versus SNR Can induce thermal instability for material with negative
TCR Can be avoided by using short heating pulse in measurement
Detection circuit?
ENE 5400 Spring 2004 22
Single Heart Cell Force Measurement
Piezoresistive sensing can be used in solutions
before
after
Lin et al., Int. Conf. on Solid-State Sensors and Actuators(Transducers 97’), pp. 199-200 (UCLA and UC Berkeley)
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ENE 5400 Spring 2004 23
Atomic Force Microscopy (AFM)
J.A. Harley et al., “High sensitivity piezoresistive cantilevers under 1000 Å thick,” Appl. Phys. Lett., vol. 75, p. 289, 1999.
Y. Liang et al., “Performance characteristics of ultra-thin n-type peizoresistive cantilevers,” Transducers 01’, p. 998, 2001.
Force resolution of fN/√√√√Hz can be achieved by piezoresistivesensing Reduction of beam thickness t is a must
wt
∆F
piezoresistor
ENE 5400 Spring 2004 24
source: web.mit.edu/cortiz
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ENE 5400 Spring 2004 25
Temperature Sensor
Can be built based on the thermoresistive effect: RT = Ro [1 + αααα(T – To)]
αααα = Temperature Coefficient of Resistance (TCR) in K-1; can be positive or negative Metal thin film < 1%; aluminum = 0.36%, gold = 0.83%
» Low SNR and long-interconnect issue Semi-conducting metal oxide > 1%; e.g. V2O5 ≈≈≈≈ 2% Semiconductor thermistor : ρρρρ = 1/[niq(µµµµn + µµµµp)], where the
intrinsic carrier concentration ni and mobility µµµµ are both temperature dependent
» Must reduce thermally excited electrons for low band-gap materials in cryogenic temperature sensing
ENE 5400 Spring 2004 26
Magnetoresistive Sensor
Widely used as magnetic read heads (∆∆∆∆R/R from 2 to 6%); higher SNR than that of inductive heads
Average traveled path varies due to different path lengths of carriers of different velocities affected by the Lorentz force and counter-balancing Hall field (Edwin Hall, 1879) Result: conductivity σσσσ changes with applied magnetic flux
density: σσσσ = σσσσo (1 – r2µµµµ2Bz2)
x
z
y
Ix
Bz
e-
+
-
VH
+ -
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ENE 5400 Spring 2004 27
Capacitive Sensing: I-V Relation
Micromechanical capacitance is time and displacement dependent; C = C(x, t)
Current has a electrical component and a mechanical motional component
dt
dCV
dt
dVC
dt
CVd
dt
dQI +=== )(
g
AC orεε= +
_
V
+ ++
++
++++
++Q
__ _
__
_
___
-Q
I
ENE 5400 Spring 2004 28
Parallel-Plate Motion
Nonlinear capacitance vs. plate displacement
Motion limited to initial gap separation, g May need feedback to
increase dynamic range More sensitive at smaller gaps
2)(
)(
)(
xg
A
dx
xdC
xg
AxC
o
o
−=
−=
ε
ε
g x
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ENE 5400 Spring 2004 29
Lateral-Plate Motion
Linear capacitance vs. plate displacement
Motion not limited to initial gap, but overlap L
Constant sensitivity
g
x
L
g
W
dx
xdC
g
xLWxC
o
o
ε
ε
−=
−=
)(
)()(
ENE 5400 Spring 2004 30
Tunneling-Current Sensing
Scanning Tunneling Microscopy (STM) was first used by IBM Zurich Lab to obtain atomic resolution image of surface Tunneling of electrons from an ultra-sharp tip through a very
narrow gap at high vacuum; constant gap maintained by feedback control
Tunneling current )( zoeII Φ−= β
Scaling factor
Conversion factor, ~10.25 eV-1/2/nm
Tunneling barrier height, ~ 0.5 eV
Separation, ~ 1 nm
z
Piezoelectric scanner
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ENE 5400 Spring 2004 31
Scanning Tunneling Microscopy
Source: www.eng.yale.edu/reedlab/research/spm
ENE 5400 Spring 2004 32
Micromachined Tunneling-Based Golay Cell as an Infrared Sensor T.W. Kenny et al., Applied Physics Letters, pp. 1820-1822, 1991
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ENE 5400 Spring 2004 33
Self-Generating Sensing: Thermoelectric
Thermoelectric sensing is based on the Seebeck effect A flux of carriers carries charges, and “energy” as well; for
metals, the “hot” electrons on the hot side migrate or diffuse to the cold side, setting up an electric field that willoppose the diffusion of any additional electrons (analogous to the built-in electric field in a diode)
The Peltier effect: use the applied voltage to create hot and cold points
∆∆∆∆V = ααααa⋅⋅⋅⋅(Tcold – Thot) + ααααb⋅⋅⋅⋅(Thot – Tcold ) = (ααααb - ααααa)⋅⋅⋅⋅ (Thot – Tcold )
ThotTcold
conductor A
conductor BV
+
-
Measurement of Seebeck voltage
ThotTcold
conductor A
conductor B
+
-
The Peltier effect
(heatabsorbed)
(heatreleased)
ENE 5400 Spring 2004 34
Thermoelectric Sensing
The Seebeck Coefficients relative to Platinum Seebeck coefficients: metal < semiconductor < doped
semiconductor
7.47.67.611.2-450270
AgCuZnW
n-poly (2600 ΩΩΩΩ/€€€€)p-poly (400 ΩΩΩΩ/€€€€)
-14.80
4.24.24.46.5
NiPtAlSnMgIr
µµµµV/KµµµµV/K
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ENE 5400 Spring 2004 35
Micromachined Thermopile IR Detector
Series-connected thermocouples (doped-polysilicon/gold) to increase responsivity Very simple to implement, yet at the cost of using a large area
Hot junction on the diaphragm and cold junction in the surrounding silicon
silicon (cold junctionregion)
thermocouplesinfrared radiation
IR-absorbing material (hot junction region)
Choi and Wise, IEEE. Trans. on Electron Devices, v.33, 1986
diaphragm
ENE 5400 Spring 2004 36
Piezoelectric Sensing
Piezoelectric effect was discovered by brothers Jacques and Pierre Curie A reversible effect: stress (mech.) ⇒ polarization (elec.); voltage
(elec.) ⇒ strain (mech.) Large sensed voltage and very precise displacement
F
piezoelectricmaterial
∆∆∆∆Q
conductingplates
A
t A
tFd
C
QV
FdQ
jiji
jiji
ε∆
=∆=∆
∆=∆1
2
3
19
ENE 5400 Spring 2004 37
Piezoelectric Sensing
A crystal possessing a center of symmetry can not be piezoelectric because the net polarization (ΣΣΣΣpi) is zero
Piezoelectric coefficients are temperature-sensitive; Piezoelectric effect disappears above the “Curie temperature”
The impedance of the signal source is capacitive ⇒ no d.c. response
+
++
--
-
+-
-
+
+-
+
++
--
-
+
++
--
-
Σpi = 0
+
++
--
-
+
++
--
-
+
+- +
-
-+
-
-
+
+-
pipi
Σpi ≠ 0
ENE 5400 Spring 2004 38
Piezoelectric Materials
Yes (sputter)1235.71,400d31 = 5.2d33 = 246
ZnO
Yes (sputter or sol-gel)
537.71,700d31 = -171d33 = 370
PZT
No (ceramics)5.71,700d31 = 78d33 = 190
BaTiO3
No (ceramics)2454.628d31 = -4d33 = 23
LiNbO3
Yes (spin-on polymer, need
“poling”)
31.7812d31 = 23d33 = -33
Polyvinyledene (PVDF)
Substrate1072.654.5d33 = 2.31Quartz
Thin-film form? Young’s Modulus
(GPa)
Density (g/cm3)
Relative permittivity
Piezoelectric constant (10-12
C/N)
Material
20
ENE 5400 Spring 2004 39
Pyroelectric Sensing
The change in temperaturetemperature causes change in spontaneous polarization and electric charge
Many piezoelectric materials are also pyroelectric, for example, PVDF
The pyroelectric properties disappear at Curie temperature
ENE 5400 Spring 2004 40
Summary of Sensing Mechanisms
Yes
No
Yes
Yes
DC response?
Displacement
Strain/pressure
Displacement
Strain/pressure
Parameter sensed
No
No
Yes
No
Local circuit?
Complex fabrication and implementation Sensitive to surface stateVery high sensitivity
Tunneling
Fabrication can be complexHigh sensitivityTemperature-sensitive
Piezoelectric
Low cost and simple fabricationLow temperature dependenceHigh sensitivitySensitive to parasitic cap. And EMI
Capacitive
Low cost and simple fabricationHigh temperature dependenceRelatively low sensitivityGood linearity
Piezoresistive
Advantages and IssuesMechanism
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ENE 5400 Spring 2004 41
Micromachined Mechanical Sensors
Accelerometers Gyroscopes Pressure sensors Flow sensors Tactile sensors
ENE 5400 Spring 2004 42
Micromachined Accelerometers
Applications: Automotive: air bag, alarm
system» The old technology was bulky
and expensive (~$100 /car)» MEMS accelerometers more
reliable, smaller, and less expensive (~$5 /car)
Home appliances: washing machines, subwoofers
Computer peripherals: wireless game controller, mouse
Automotive: air bag, alarm system
Machine health: vibration monitoring (e.g., disk drives)
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ENE 5400 Spring 2004 43
Accelerometer
2n
inin
total
aa
km
xωωωω
========
)()()()( tmatxktxbtxm intotaltotal ====++++++++ &&&
Static:
Dynamic:
substrateain
Resonant frequency = mktotal
ππππ21
ktotal = 2kbtotal = 2b
ENE 5400 Spring 2004 44
Primary Specifications
Measuring range (in G, G = 9.81 m/s2) Sensitivity (in V/g) Bandwidth (Hz)
Mechanical bandwidth (resonant frequency) Signal bandwidth
Resolution (in G) Related to the magnitude of noises
Dynamic range (in dB) Cross-axis sensitivity (in %)
23
ENE 5400 Spring 2004 45
Commercial Products
Image courtesy: Analog Devices, Inc. (http://www.analog.com)
100 millions shipped by Sept. 2002
Proof mass
Finger electrodes
ENE 5400 Spring 2004 46
A Three-Axis Piezoresistive Accelerometer
H. Takao et al., Proc. of Transducers 97’, pp. 683-686 Doesn’t require on-chip detection circuits (e.g., CMOS) Allow high-temperature operation by placing the piezeresistors
directly on top of SiO2 (fabrication starts with a SOI wafer); no traditional pn junction to cause leakage current
The resistor R5 and its associated voltage is used as a reference to eliminate the thermal drift of piezoresistors
24
ENE 5400 Spring 2004 47
Cont’d
All resistors are series-connected and biased by a constant current source
Sensed voltages Vs due to the x, y, and z accelerations:
42,
31,
4
1,
5
VVV
VVV
VV
VVV
ys
xs
nnzs
nn
∆∆∆∆−−−−∆∆∆∆====∆∆∆∆−−−−∆∆∆∆====
∆∆∆∆====
−−−−====∆∆∆∆
∑====
better
R5 doesn’t experience stress
ENE 5400 Spring 2004 48
Thermal Accelerometer
Two-axis sensing of acceleration and tilt No moving part; Based on the moving of “hot air bubble” which
is sensed by temperature sensors
A.M. Leung et al., MEMS 98’, pp. 627 - 630
(1) Cross section (2) After heating (w/omotion)
(3) Motion applied
25
ENE 5400 Spring 2004 49
Gyroscopes
Measure the rotation rate or the whole angle rotation
Macroscopic gyroscopes fall into the optical and mechanical types
Most micromachinedgyroscopes use vibrating structures to enhance sensitivity (no good bearings is another reason); the vibrational energy is coupled nto another axis due to the CoriolisCoriolis effecteffect:
vmFc
rvv
××××ΩΩΩΩ==== 2
m
Drive
Input rotation ΩΩΩΩ
Coriolis acceleration
macro gyro
ENE 5400 Spring 2004 50
Main Specifications
Full-scale range: °°°°/s or °°°°/hr Sensitivity: V / (°°°°/s) Noise (angle random walk): °°°°/s/√√√√Hz Bandwidth: Hz Resolution: °°°°/s Dynamic range: dB Drift: °°°°/s or °°°°/hr
Source: Analog Device, Inc.
26
ENE 5400 Spring 2004 51
Tuning Fork Structure for Angular Sensing
The Coriolis effect transfers the energy from a primary, flexural mode to a secondary, torsional mode
Courtesy of N. Maluf
ENE 5400 Spring 2004 52
A Tuning-Fork Vibratory Gyroscope
Two tines are driven differentially and the Coriolis force is detected as a twisting force at the base
Piezoelectric actuation and piezoresistive sensing at the base
R. Voss et al,”Silicon angular rate sensor for Automotive applications with piezoelectric Drive and piezoresistive readout,” Transducers 97’, pp. 879-882. (Daimler Benz AG, Germany)
27
ENE 5400 Spring 2004 53
Inertia Measurement Unit (IMU) at Carnegie Mellon University
Video courtesy: Hao Luo, Carnegie Mellon University (now with Hewlett Packard, CA)
ENE 5400 Spring 2004 54
Pressure Sensors
Mechanical and thermal sensing (Pirani-type) mechanisms The deflection of the center for a circular clamped membrane
under pressure P: (r: radius, h: thickness, v: Poisson’s ratio, y: deflection, E: Young’s modulus)
vacumm
P1
absolute
ambient
P1
gauge
Preference
P1
Sealed gauge
P2
P1
differential
3
24
4
)1(37
)1(316Pr
−−−−−−−−++++
−−−−====
hy
vv
hy
vEh
28
ENE 5400 Spring 2004 55
Commercial Products
Image courtesy: NovaSensor(http://www.novasensor.com) Acquired by GE
Applications: Medical: angioplasty,
blood pressure, respiratory
Automotive: tire pressure, manifold air pressure (MAP), fuel and engine control systems
Industrial: portable gauge, water depth
Before package
After package
ENE 5400 Spring 2004 56
A Typical Pressure Sensor
Sensitivity = (mV / V) / Pa Need good resistor match to
avoid zero offset Piezoresistive sensing is a
convenient, cheaper solution Shallow diffusions are
prone to surface-charge effect that can cause long-term drift
Deep diffusions degrade the sensitivity
Source: N. Maluf
29
ENE 5400 Spring 2004 57
Corrugated membranes
Provide both increased net deflection for equivalent loads and linear operating range
C.J. van Mullem et al., “Large deflection performance of surfaceMicromachined corrugated diaphragms,” Transducers91’, pp.1014-1017
ENE 5400 Spring 2004 58
Balloon Angioplasty
Source Microsoft
30
ENE 5400 Spring 2004 59
Cardiovascular Pressure Sensor
E. Kalvesten et al., MEMS 2000(Royal Institute of Technology, Sweden and RADI Medical Systems)
A commercial product Used in balloon angioplasty to
sense the pressure gradient during the operation
Piezoresistive sensing; Lower sensitivity (2 µµµµV/V/mmHg) than capacitive sensing, but no on-chip detection circuit required
ENE 5400 Spring 2004 60
High-Temperature Piezoresistive Pressure Sensor
Allow high-temperature operation (300 °°°°C) by placing the p+ piezeresistors directly on top of SiO2; avoid p-njunction leakage current at high temperature Adjacent leakage avoided
by shallow trenches
Source: Lucas NovaSensor
31
ENE 5400 Spring 2004 61
Cont’d: Fabrication
Use SOI (silicon-on-insulator) wafer
SiO2 is thermally grown The top silicon etch uses
EDP (toxic) which stops on the heavily doped p+ silicon
Backside silicon etch KOH Need front-side
protection of p+ silicon
Courtesy of N. Maluf
ENE 5400 Spring 2004 62
Thermal Gas Pressure Sensors (Pirani Type)
Use a heated resistor that loses heat to the external gas; monitor the resistance of the wire to know its temperature, and the associated thermal conductivity and gas pressure
Often used in vacuum instruments CMOS integrated: Use a diode-connected transistor as the
temperature sensor, and polysilicon wire as the heater
Suspended cantilever
*Klaassen et al., “Integrated thermal conductivity vacuum sensor,”Sensors and Actuators A, vol. 58, no. 1, pp. 37-42, 1997 (Stanford Univ.)
After etch
32
ENE 5400 Spring 2004 63
Cont’d: Diode Temperature Sensors
The current of a forward-biased diode is given by the Shockley equation:
When the diode is operated in a constant-current mode, the forward diode voltage in directly proportional to the absolute temperature, and the sensitivity S is a constant depending on the driving current
≈≈≈≈−−−−==== TnkqV
sTnk
qV
sbb eIeII )1(
========
====
s
b
s
b
II
qnk
dTdV
S
II
qTnk
V
ln
ln
ENE 5400 Spring 2004 64
Cont’d
Pirani type: the resistance of an electrically heated wire is a function of temperature, which, in turn, is related to the surrounding pressure and the thermal conductivity κκκκ
vcvnmλλλλκκκκ31≈≈≈≈ 22 δδδδππππ
λλλλPTkb====
Mean free path
pressureAverage molecularmass
# of molecules per unit volume
Mean velocity
Specific heat, K/(J⋅⋅⋅⋅kg)
Molecular diameter
Heated filament(polysilicon with nitride coating)
C.H. Mastrangelo, IEEE Trans. Electron Devices, vol. 26,no. 12, pp. 1998-2007, 1991
33
ENE 5400 Spring 2004 65
Flow Sensors
Measurement of the flow rate of a gas (or liquid) is important for a number of fields (automotive, aerospace, chemical industries, laboratory-on-a-chip)
Sensor types: Conventional: Pitot tube, Venturi
tube Novel: thermal flow microsensor
Pitot tube
Venturi tube
ENE 5400 Spring 2004 66
Thermal Flow Sensor from Honeywell, Inc.
Use heaters and measure the resistive change (wired in a Wheatstone bridge) due to the cooling from forced convection of measured gas or liquid flow
Known as the “hot-wire anemometer” Great thermal isolation; tiny heated volume provides a fast time
constant (< 3 ms) Sensitive to operating temperature variation; require thermal
compensation
silicon
silicon nitride
upstreamresistor
downstreamresistor
heaters
Johnson and Higashi, Sensors and Actuators A, v.11, no.1, 1987
Thermal isolation
34
ENE 5400 Spring 2004 67
Tactile Sensor: Sweeping-Mode Fingerprint Sensor
Integrated biometric devices in PDAs, mobile phones, etc Two types of minutiae: ending () and bifurcation ( ) Full matrix or partial matrix?
Courtesy of B. Courtois
ENE 5400 Spring 2004 68
Cont’d
35
ENE 5400 Spring 2004 69
Capacitive Tactile Sensor
32 ×××× 32 element array (1.6 ×××× 1.6 cm2)
Fabricated by boron diffusion and the subsequent anodic bonding and release steps
Sensitivity of 0.27 pF/g/element Dielectric film to prevent short
Suzuki et al., IEEE Trans. ElectronDevice, vol. 37, no. 8, pp. 1852-18601990
