Transcript Document

Applications:
Pressure Sensors, Mass Flow Sensors, and
Accelerometers
CSE 495/595: Intro to Micro- and Nano- Embedded Systems
Prof. Darrin Hanna
From last time…
Differential pressure sensor
Absolute pressure sensor
From last time…
High temp.
pressure
sensor using
Silicon-oninsulator
(SOI)
processes
Mass flow sensors
The flow of
gas over the
surface of a
heated
element
produces
convective
heat loss at
a rate
proportional
to mass flow.
Mass flow sensors
• Deposit a thin layer of silicon nitride
• approximately 0.5 µm in thickness
• Deposit & pattern thin-film heaters and sense elements
• chemical vapor deposition of a heavily doped layer of
polysilicon
• Deposit & pattern an insulating layer to protect heating & sense
elements
• silicon nitride again but keep contacts
exposed
• Etch silicon in KOH anisotropic etch
solution to form the deep cavity
Mass flow sensors
Two Wheatstone bridges
• The 2 heating resistors form the two legs of the first bridge
• The 2 sensing resistors form the two legs of the second bridge
Mass flow sensors
For equilibrium
R1/R2 = R4/R3
Heating
Sensing
In either case two of the bridge
resistor pairs are fixed and equal
such as R2 and R3. R2 = R3 = RB

R1 RB  RB R4
Vin
( R1  RB )( R4  RB )

RB  R1  R4 
( R1  RB )( R4  RB )
Vin
R1 > R4  + Pol
R4 > R1  - Pol
Flow direction
Mass flow sensors
1
1
2
2
Heat2 – some heat, H, transferred to gas
Heat1 – very little heat transferred from H
Sense1 – some heat transferred from H
Mass flow sensors
Heat2 – some heat, H, transferred to gas
Heat1 – very little heat transferred from H
Sense1 – some heat transferred from H
1
2
1
2
1
2
Mass flow sensors
Heat1 – some heat, H, transferred to gas
Heat2 – very little heat transferred from H
Sense2 – some heat transferred from H
1
2
1
2
1
2
Mass flow sensors
• 0 – 1000 std cubic cm
• 75 mV max output
• time < 3 ms
• power ~ 30 mW
Acceleration sensors
Acceleration sensors
The primary specifications of an accelerometer are
• full-scale range (often given in Gs <9.81 m/s2)
• sensitivity (V/G)
• resolution (G)
• bandwidth (Hz)
• cross-axis sensitivity
• immunity to shock
Acceleration sensors
• Airbag crash sensing
• full range of ±50G
• bandwidth of about one kilohertz
• Measuring engine knock or vibration
• range of about 1G
• small accelerations (<100 µG)
• large bandwidth (>10 kHz)
• Modern cardiac pacemakers
• multi-axis accelerometers
• range of ±2G
• bandwidth of less than 50 Hz
• require extremely low power consumption
• Military applications
• range of > 1,000G
Acceleration sensors
F = m∙a
Acceleration sensors
Q and Bandwidth
• The quality factor (Q) is a measure of the rate at which a
vibrating system dissipates its energy into heat
, of heat dissipation
• A higher Q indicates a lower rate
• When the system is driven, its resonant behavior depends
strongly on Q
• Q factor is defined as the number of oscillations required for a
freely oscillating system's energy to fall off to 1/535 of its
original energy, where 535 = e2π
Resonant frequency
Bandwidth
Acceleration sensors
Q and Bandwidth
• Bandwidth is defined as the "full width at half maximum".
• width in frequency where the energy falls to half of its peak
,
value
dB level
Ratio
Voltage and Current is 20
Power and Intensity is 10
−30 dB
1/1000
−20 dB
1/100
−10 dB
1/10
−3 dB
0.5 (approx.)
3 dB
2 (approx.)
10 dB
10
20 dB
100
30 dB
1000
Acceleration sensors
Q and Bandwidth
Example: Q of a radio receiver
A radio receiver used in the FM band, needs to be tuned in to within
about 0.1 MHz for signals at about 100 MHz. What is its Q?
Ans: Q=fres/FWHM=1000. This is an extremely high Q compared
to most mechanical systems.
Acceleration sensors
Q and Bandwidth
Example: Decay of a saxophone tone
If a typical saxophone setup has a Q of about 10, how long will it take
for a 100-Hz tone played on a baritone
, saxophone to die down by a
factor of 535 in energy, after the player suddenly stops blowing?
Ans: A Q of 10 means that it takes 10 cycles for the vibrations to die
down in energy by a factor of 535. Ten cycles at a frequency of 100
Hz would correspond to a time of 0.1 seconds, which is not very long.
This is why a saxophone note doesn't “ring” like a note played on a
piano or an electric guitar.
Acceleration sensors
Q and Bandwidth
Resonant frequency
Bandwidth
,
The lower the bandwidth, the higher Q and vice versa
The higher the bandwidth, the lower Q and vice versa
Acceleration sensors
F = m∙a
time
power
amplitude
Freq.
Brownian noise is the integral of
white noise 
Brownian noise
The change in noise with
time is random whereas
white noise is random
noise
Acceleration sensors
Piezoresistive Bulk Micromachined
Accelerometer
Acceleration sensors
Piezoresistive Bulk Micromachined
Accelerometer
• Inertial mass sits inside a frame suspended by the spring
• Two thin boron-doped piezoresistive elements
• Wheatstone bridge configuration
• Piezoresistors are only 0.6 µm thick and 4.2 µm long
• very sensitive
• Inertial mass
• Output in response to 1G is 25mV for a Wheatstone bridge
excitation of 10V.
Acceleration sensors
Piezoresistive Bulk Micromachined
Accelerometer
• 6,000G for the inertial mass to touch the frame
• The device can survive shocks in excess of 10,000G
• Holes in inertial mass reduce weight and provide a high
resonant frequency of 28 kHz
Acceleration sensors
Piezoresistive Bulk Micromachined
Accelerometer
• {110} Silicon for center
• {111} plane is perpendicular to the surface, therefore an
anisotropic wet etchant can be used
Acceleration sensors
Piezoresistive Bulk Micromachined
Accelerometer
• Boron implantation and diffusion to form highly doped
p-type piezoresistors
• the piezoresistors are aligned along a <111> dir
• A silicon oxide or silicon nitride layer masks the silicon
in the form of the inertial mass and hinge during the
subsequent anisotropic etch in EDP
Acceleration sensors
Piezoresistive Bulk Micromachined
Accelerometer
• Deposit and pattern aluminum electrical contacts
• Pattern and etch shallow recesses in base & lid substrates
• Bond together using adhesive
Acceleration sensors
Capacitive Bulk Micromachined
Accelerometer
Acceleration sensors
Capacitive Bulk Micromachined
Accelerometer
•
•
•
•
•
Measuring range from ±0.5G to ±12G
Electronic circuits sense changes in capacitance using voltages
Bandwidth is up to 400 Hz for the ±12G accelerometer
Cross-axis sensitivity is less than 5%
Shock immunity is 20,000G
Acceleration sensors
Capacitive Bulk Micromachined
Accelerometer
Timed etching
Acceleration sensors
Capacitive Bulk Micromachined
Accelerometer
Contacts On side
of wafer  Postprocessed
Acceleration sensors
Capacitive Surface Micromachined
Accelerometer
Acceleration sensors
Capacitive Surface Micromachined
Accelerometer
• The overall capacitance is small, typically on the order of 100 fF
• (1 fF = 10-15 F)
• ADXL105 (programmable at either ±1G or ±5G)
• the change in capacitance in response to 1G is 100 aF
• (1 aF = 10-18 F).
• Two-phase oscillator
• 0 DC offset
Acceleration sensors
Capacitive Surface Micromachined
Accelerometer
• Range from ±1G (ADXL 105) up to ±100G (ADXL 190)
• Bandwidth (typically, 1 to 6 kHz)
• The small change in capacitance and the relatively small mass
combine to give a noise floor that is relatively large
• ADXL105 - the mass is approximately 0.3 µg and noise floor
is dominated by Brownian noise
• Bulk-micromachined sensor can exceed 100 µg
Acceleration sensors
Capacitive Surface Micromachined
Accelerometer
• Open loop measurement
• Voltage generated at sense
contacts
• Close loop measurement
• Applying a large-amplitude
voltage at low frequency—
below the natural frequency
of the sensor—between the
two plates of a capacitor gives
rise to an electrostatic force
that tends to pull the two
plates together.
Acceleration sensors
Capacitive Deep-Etched Micromachined
Accelerometer
Acceleration sensors
Capacitive Deep-Etched Micromachined
Accelerometer
• Two sets of stationary fingers attached
directly to the substrate form the
capacitive half bridge.
• Structures 50 to 100 µm deep
• sensor gains a larger inertial mass,
up to 100 µg,
• larger capacitance, up to 5 pF.
• Larger mass reduces Brownian noise
and increases resolution.
Measuring Capacitance
Improving the Circuit
• Design an accurate sensing
circuit
• + Wheatstone Bridge
• + Differential Amplifier
• = Sensitivity (1nF ~ 3mV)
More accurate
sensor
modelon sensor
Wheatstone
Bridge
–
+ Based
Differential
Amplifier
model and
10x Gain
optimized using PSPICE
Experimentally determined
that the biosensor behaves
like a capacitor in parallel
with a resistor
1.0Vpp
1.0Vpp
3.5kHz
3.5kHz
Measuring Capacitance
Sensor Attach Point
Differential Amplifier
(10x Gain)
Variable Capacitor
(0-2.1 uF)
Variable Resistor
(0-210 Ohms)
Measuring Capacitance
Sensor Attach Point
Differential Amplifier
(10x Gain)
Variable Capacitor
(0-2.1 uF)
Variable Resistor
(0-210 Ohms)