ELEC-E3220 Semiconductor devices
Spring 2016
Lecture 8
Silicon sensors for mechanical signals
Ville Vähänissi
Department of Micro and Nanosciences
Outline
• Introduction to mechanical sensors
• Basic physical principles – conventional lecture
–
–
–
–
Piezoresistivity
Piezojunction
Piezoelectricity
Capacitance
• Applications – group working
– Pressure sensors
– Accelerometers
– …
2
Introduction to mechanical sensors
• Mechanical sensors rely on some physical principle to transform
a mechanical signal into an electrical signal
• Can be used to measure
– Motion-related measurands
– Force-related measurands


Material generally silicon
Tandem transducer



position, displacement, surface
roughness, velocity, flow, speed
of rotation
weight, pressure, acceleration,
torque, strain, attitude, vibration
Mechanical signal → Radiant signal → Electrical signal
No mechanical contact with detector
Most often used transduction techniques based on:


piezoresistivity
capacitance
3
Piezoresistive effect
A conductor’s resistance changes when it is subjected to
mechanical strain.
Confidential
Piezoresistivity
• Electrical resistance of any conductor
• There are two modes of resistance change when a conductor is
deformed/strained:
Mode 1: Physical change in dimensions (L, A (Δl, Δd))
Mode 2: Resistivity  is a function of strain 
• Gauge factor
R / R
 / 
K
 1  2 
l / l
l / l
5
d / d
 
l / l
Metal strain gauges
•
Resistivity change of metallic conductors solely due to Geometrical PE
6
Piezoresistive effect in silicon
• Resistivity change of silicon due to both GPE and strain
dependency of the specific resistivity ρ
The phenomenon is much larger in silicon than in
metals (gauge factor more than an order of magnitude
higher)
The use of silicon enables high signal-to-noise ratio &
the measurement of low strain levels
R / R
 / 
K
 1  2 
l / l
l / l
7
Piezoresistivity : N-type silicon
The application of anisotropic stress affects the band structure
-> Applied stress changes the position of the energy minima and electrons move
from higher energies to lower energies until equilibrium is obtained
a) Upon compression in the [100] direction, the [100] energy minimum is
lowered and the [010] and [001] minima are raised
b) Electrons flow from the [010] and [001] minima to the [100] minimum
(high impurity concentration)
c) Electrons flow from the [010] and [001] minima to the [100] minimum
(low impurity concentration)
8
Doping concentration dependence
•
•
The piezoresistive gauge factor K depends on the impurity concentration
With high impurity concentration, many electrons are available in the conduction
band
relatively small changes in the situation
small gauge factor
•
With low impurity concentration, only few electrons are available in the
conduction band
relatively large changes in the situation
large gauge factor
The gauge factor increases when the
impurity concentration decreases
9
Doping concentration dependence
10
Temperature dependence
• The piezoresistive gauge factor K is a strong function
of temperature
• For low concentration samples the dependency is high
– The relative effect of thermal excitation to the number of
electrons in the conduction band is large
– Rising temperature decreases gauge factor
• For high concentration samples the dependency is
negligible
– The relative effect of thermal excitation to the number of
electrons in the conduction band is negligible
– Temperature changes do not affect the gauge factor
11
P-type silicon
•
The gauge factor in p-type silicon is larger than in n-type silicon
most applications fabricated from p-type
•
The band structure of the valence band is very different from that of the
conduction band
situation much more complicated
•
In the [100] direction the difference between the two bands is rather small
small gauge factor
•
In the [111] direction the difference between the two bands is significant
large gauge factor
•
The gauge factor of p-type silicon is positive (opposite to n-type)
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P-type silicon
Gauge factor of p-type silicon as a function of temperature and
impurity concentration
13
P-type / N-type silicon
Effective masses and gauge factors for n- and p-type silicon
14
Piezoresistive sensor
• Operates on a sensor principle whereby an electrical
resistor will change its resistance when it is
subjected to a strain (deformation).
• Piezoresistive sensors are used as part of many MEMS
devices including:
Pressure sensors
Accelerometers
Flow Sensors
• In other words, these “Piezoresistive-based”
applications are sensitive to phenomena that cause
beams or thin plates to deform and this deformation can
be measured by resistance change.
15
Example of piezoresistive sensor
16
Location of maximum strain
17
Pressure sensors
•
By varying the diameter and thickness of the diaphragms wide range of
operating pressures can be obtained (0-200 MPa)
•
Temperature dependency causes problems
electronic compensation circuits needed
•
Advantages
–
–
–
–
–
•
Mass-production
Large operation range
Easily integrated to electronics
Good linearity and stability
Vibration and shock insensitive
low cost
“smart” sensors
Application areas: medical and automotive markets
(blood pressure gauges, tire-leak detectors, fuel metering, barometry, etc.)
18
Piezojunction effect
•
Stress changes the characteristics of semiconductor devices
•
Example: Planar diode
– Applied stress changes IV-behaviour
– With constant current voltage decreases as the stress increases
– Saturation current increases as the stress increases (band gap decreases and
diffusion length increases)
The current-voltage curve of a
planar np diode as a function of
compressive stress
19
Piezojunction effect
• Example: Pressure sensitive MOSFET
– Saturation current depends on the mobility in the channel
– When subjected to stress, mobility changes and thus saturation
current changes
Influence of strain on the
characteristics of a MOSFET
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Piezoelectricity
A mechanical stress produces
electricity and reciprocally an applied
electric field generates mechanical
strain.
•
Can be used to convert mechanical or acoustical signals into electrical signals
Microphones, accelerometers, roughness indicators
21
Piezoelectricity (PE)
• Caused by the irregular molecular structure of the crystals
• Under stress the structure of the molecule changes
– different charges spread out unevenly
22
Piezoelectricity (PE)
• Silicon is nonpiezoelectric
– Good thing for ICdesigner
– Bad thing for sensor
designer
• GaAs, ZnO and CdS
• Due to redistribution
of surface charge
– Ionic bonding
– Acentric crystal
structure
Basic
physical principle of
23
silicon sensors for
mechanical signals
Use of piezoelectric layers
Applications
of silicon
24
sensors for mechanical
signals
Capacitive sensors
• A change in measurand is converted into a change of capacitance
• Structure of a capacitor: two electrodes separated by a dielectric
• Capacitance change can be caused by
• Motion of one of the electrodes with respect to the other electrode
• Changes in the dielectric between two fixed electrodes
A
C 
d
25
Capacitive sensors
•
•
Capacitance between two parallel plates:
A
C 
d
After displacement ∆d (plates still parallel):
C
A
  2
d
d
high sensitivity obtained by maximizing
plate area and minimizing gap distance
•
Tradeoffs between sensitivity and sensor dimensions, fabrication
accuracy and reproducibility and damping of the movement of the
electrode
26
Capacitive Pressure sensors
•
General structure of most
pressure sensors:
– Silicon membrane
(acting as one electrode)
– Metallized substrate
(counter electrode)

Diaphragm forms an electrical capacitance with its surroundings
 As pressure is increased, the membrane bends towards the substrate causing
the capacitance to increase
 Due to this effect accurate pressure sensors can be designed
 Capacitive effect 10 to 20 times more sensitive than piezoresistive effect
 Problems with stray capacitance of the leads
27
Summary
•
Mechanical sensors transform a mechanical signal into an electrical signal
•
Material generally silicon
•
Most common transduction techniques based on piezoresistivity and
capacitance
•
Piezoresistive effect
•
•
•
Highly anisotropic
Doping concentration and temperature dependent
Piezoresistive sensors vs. capacitive sensors
complicated structure
simple structure
high temperature coefficient low temperature coefficient
easy read-out
stray capacitances
•
Main applications strain gauges, pressure sensors and accelerometers
28
Group working
• 6 groups – 3 applications
– Piezoresistive sensors
– Piezoelectric sensors
– Capacitive sensors
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Surface acoustic wave transducers
(SAW)
• Transduction from electrical into mechanical energy
(acoustic) at the input of device
• Then acoustic signal can be affected by light, stress,
temperature etc.
• Output: Transduction from mechanical into electrical
energy
• SAW can be used as chemical sensor and gas sensor
Applications
of silicon
30
sensors for mechanical
signals
SAW
• Different acoustic waves
– Longitudinal waves
– Transverse waves
– Rayleigh or surface acoustic waves
Applications
of silicon
31
sensors for mechanical
signals
SAW
• Features
– Travel along the surface and decay at a depth
below the surface comparable to their
wavelength
– Remains accessible at the surface
– SAW can be easily created and detected by
interdigital transducers
32
Applications of silicon sensors for mechanical signals
SAW
•
•
•
•
Delay lines are made of piezoelectric substrate
Two transducers on top of substrate
Velocity of an acoustic wave is 3.4 km/s on quartz
Time delay 300 microseconds per meter
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SAW
• RF-voltage is applied
• The piezoelectric material is alternately compressed and
expanded and two acoustic waves are generated
• The acoustic waves will propagate
• Absorbent material is placed at the edges to prevent
unwanted reflections of the acoustic waves
• When the frequency is chosen correctly, the amplitudes
of the wave increases due to the interference
• SAW arrives to output transducer -> RF signal can be
detected
Applications
of silicon
34
sensors for mechanical
signals
SAW
• Delay-line oscillator
– The output signal is electronically amplified and applied to the
input transducer
– SAW can be affected by physical parameters, such as stress,
light…
– The delay increases -> decrease in oscillation frequency
• SAW Gas sensors
– Gas-sensitive material -> electrical properties and/or mass is
affected -> delay time changes
– Good selectivity for NO2 using ZnO layer
Applications
of silicon
35
sensors for mechanical
signals
Applications: Pressure sensors
• General structure of most pressure sensors:
• a stainless steel housing
• one or two pressure ports
• Silicon diaphragm with diffused or implanted piezoresistor

Four different types: absolute, sealed, gauge, differential
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Pressure sensors
• Absolute pressure sensors
– Use right port only
– Sensor cavity is sealed in a vacuum and used as reference
• Sealed sensors
– Similar to absolute pressure sensors except that the cavity is
sealed in 100 kPa
• Gauge pressure sensors
– Uses only the left port
– Upper surface is in the ambient atmosphere
• Differential pressure sensors
– Uses usually both ports
Applications
of silicon
37
sensors for mechanical
signals
Pressure sensors
• Operation range of pressure sensors
– Very accurate sensors 0-10 kPa
– Range of 0-200 MPa are also available
•
Bridge voltages are 5-10 V
– Sensitivity 10 mV/kPa for low pressure
– Sensitivity 0,001 mV/kPa for high pressure
• If the temperature range is 0-50 C degrees
– Accuracy is better than 1 %, if compensative electronics is used
Applications
of silicon
38
sensors for mechanical
signals
Capacitive pressure sensors
• CPS pressure response is not linear
• CPS are 20 times more sensitive than the sensors
based on piezoresistive effect
• Sensitivity depends on the
– diaphragm radius
– diaphragm thickness
– distance between capacitor plates
• Problem of CPS is the stray capacitances caused by
leads connecting the sensor to the outside
Applications
of silicon
39
sensors for mechanical
signals
Applications: Accelerometers
• Simple accelerometers consist of
– A piezoelectric crystal placed between two capacitor plates,
which is fastened to a mass
• Operation principle
–
–
–
–
Mass presses on crystal, when the device is accelerated
Crystal generates a small voltage on the plates
Voltage is proportional to the acceleration
Do not need external power supply
Applications
of silicon
40
sensors for mechanical
signals
Another type of accelerometer
• Consist of
– Cantilever+silicon mass
– Silicon piezoresistor
• Operation principle
– Acceleration causes cantilever bending
– Piezoresistor is on top of the cantilever and when the cantilever
bends, the resistance is changed
– The resistor is part of a Wheatstone Bridge
– More accurate than the preceding accelerator
• Range from 0.01g to 100g
Applications
of silicon
41
sensors for mechanical
signals
Applications: Accelerometers
•
General structure of most
accelerometers:
– Inertial mass suspended
between two substrates
– Two fixed electrodes
– Inertial mass acts as a
movable electrode
acceleration
movement of the mass
No inherent temperature sensitivity
42
change of the capacitance
Capacitive accelerator
Applications
of silicon
43
sensors for mechanical
signals