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Delta Pressure Sensors
M. Chitteboyina, D. Butler
and Z. Celik-Butler,
Nanotechnology Research and Teaching Facility
University of Texas at Arlington
http://www.uta.edu/engineering/nano/
1
Delta Pressure Sensor Model
Delta Pressure Sensor Final Model
Packaged piezo
resistors ~ 25 nm
Unsealed
cavity ~
7.5 µm
Passivation
layer ~ 600 nm
packaging layer ~
1.35 µm
Z-scale exaggerated 5 times
 Delta pressure (differential pressure) sensor is a type of pressure
sensor which is used to detect pressure changes
 Delta pressure measures the pressure between two different points
 Here the pressure sensor is not sealed
2
Delta Pressure Sensor Results
STRAIN YY PLOT
DISPLACEMENT PLOT
MISES STRESS PLOT
STRAIN XX PLOT
3
Delta Pressure Sensor Fabrication
STEP 1
 On a clean wafer spin-coat ~ 40 µm flexible polyimide as the
substrate layer followed by 600 nm passivation layer
Passivation
layer ~ 600 nm
Piezo resistors
~ 25 nm
STEP 2
 Spin polyimide and cure
STEP 3
 Deposit ~ 2 µm membrane layer
STEP 4
 Deposit ~ 25 nm thick Piezoresistors
Silicon nitride
membrane layer ~ 2 µm
Sacrificial layer
~ 7.5µm
Packaged piezo
resistors ~ 25 nm
Unsealed
cavity ~
7.5 µm
NEXT STEPS
 Deposit ~ 500 nm thick aluminum as the metallization layer
 Deposit ~ 100 nm thick packaging layer to package the pressure
sensors
packaging layer
~ 0.1 µm
 Ash the sacrificial layer using oxygen plasma to suspend the
membrane
 Etch the silicon wafer from the back side to get access to the
bond pads and characterize the delta pressure sensors
Passivation
layer ~ 600 nm
membrane layer ~ 2
µm
Z-scale exaggerated 5 times
4
Delta Pressure Sensor Fabrication Steps
Membrane Layer + Packaging
Layer 1.35 µm
Piezoresistors
Metallization Layer Aluminum
0.5 µm
Polyimide completely gone
Polyimide completely gone
5
Delta Pressure Sensor Characterization
4
Vin
3
Vout
2
Vin
1
Vout
Vin
R2
R4
R1
Vout
R3
R2
R3
R4
R1
 The electrical circuit is complete with two active piezoresistors (R1 and R3) and two passive
piezoresistors (R2 and R4) in a Wheatstone bridge configuration
 The current-voltage characteristics is plotted and the true resistances are found
6
Delta Pressure Sensor Characterization
IV_P1
Voltage (V)
-0.6
-0.4
IV_A1
1.0E-04
8.0E-05
y = 0.0001x + 1E-07
6.0E-05
4.0E-05
2.0E-05
0.0E+00
-0.2
0.2
0.4
-2.0E-05 0.0
-4.0E-05
-6.0E-05
-8.0E-05
Current (A)
Voltage (V)
0.6
-0.6
-0.4
IV_P2
Voltage (V)
-0.6
-0.4
8.0E-05
y = 0.0001x - 3E-07
6.0E-05
4.0E-05
2.0E-05
0.0E+00
-0.2
0.2
0.4
-2.0E-05 0.0
-4.0E-05
-6.0E-05
-8.0E-05
Current (A)
0.6
IV_A2
8.0E-05
y = 0.0001x - 6E-08
6.0E-05
4.0E-05
2.0E-05
0.0E+00
-0.2
0.2
0.4
-2.0E-05 0.0
-4.0E-05
-6.0E-05
-8.0E-05
Current (A)
1.5E-04
1.0E-04
y = 0.0002x - 4E-09
5.0E-05
0.6
Voltage (V)
-0.6
-0.4
0.0E+00
-0.2
0.0
-5.0E-05
0.2
0.4
0.6
-1.0E-04
-1.5E-04
Current (A)
 P1 and P2 are passive piezoresistors whereas A1 and A2 are active piezoresistors
 RP1=9.386 KΩ
RA1=9.987 KΩ
RP2=9.824 KΩ
RA2=6.625 KΩ
7
İsmail Erkin Gönenli
Advisor:
Zeynep Çelik-Butler
Department of Electrical Engineering
The University of Texas, Arlington
•
•
Membrane serves to amplify the pressure, convert into mechanical strain and then transfer
it to the piezoresistor.
Piezoresistors convert strain into resistance change which is transferred into
electrical signal by the Wheatstone bridge.
Piezoresistors
Wheatstone
Bridge
R1
Vin
Membrane
R4
Vout
R2
R3
Passive
R4
R3
Membrane
R2
Active
R1
• Linear coupling between mechanical stress and change in resistivity given by:
ΔR
R
 Πlσl  Π t σ t
where R/R is the change in resistance, l and t and l and t are
longitudinal and transverse piezoresistive coefficients and applied
stresses respectively.
Transverse stress
t
Longitudinal stress
Direction of
Current Flow
Direction of Stress
l
• S. Sze, “Semiconductor Sensors”
, John Wiley & Sons, Inc., New York
, 1994
Direction of stress and current flow are
the same
Device 1
Device 2
42.43 µm x 21.21 µm
90 µm x 90 µm
35.36 µm x 21.21 µm
4.5 m
x-axis
4 m
Device 3
16.5 m
4.5 m
4 m
Device 4
6.8 m
2.6 m
90 µm x 90 µm
16.5 m
y-axis
110 µm x 110 µm
Device 5
90 µm x 90 µm
3.8 m
9 m
90 µm x 90 µm
4 m
2.5 m
42.43 µm x 21.21 µm
4 m
13.5 m
35.36 µm x 21.21 µm
16.5 m
4.5 m
Device 6
Device 7
42.43 µm x 21.21 µm
90 µm x 90 µm
14.14 µm x 14.14 µm
4.5 m
3.8 m
Device 9
80 µm x 80 µm
80 µm x 80 µm
2.5 m
4 m
Device 10
80 µm x 80 µm
21.21 µm x 14.14 µm
4 m
9 m
2.5 m
2.5 m
21.21 µm x 14.14 µm
28.28 µm x 14.14
µm
4 m
13.5 m
Device 8
2.6 m
11 m
4 m
16.5 m
x-axis
6.8 m
9 m
y-axis
110 µm x 110 µm
• Simulations were performed using CoventorwareTM
• Twenty one pressure sensors with different membrane and piezoresistor
dimensions and shapes were simulated among which ten showing best
properties were selected.
• The normalized change in resistance is found to be 0.84-3.5% assuming a
gauge factor of 50.
• Differential output voltages between 4.18-17.20 mV were obtained for 1 V
input with a sensitivity range of 0.08 mV/kPa-0.34 mV/kPa.
50 kPa load
applied to the top
of the membrane
fixed
fixed
fixed
fixed
a- Calculation of the Average Strain
Strain AVG
StrainTOTAL


AreaTOTAL
 [
XX
  YY ]dxdy
 dxdy
 x   xx cos2    yy sin 2    xy sin  cos
Axis transformation
has been applied
εyy used for these
parts
εxx used for these parts
Axis transformation
b- Calculation of the Output Voltage and Percent Change in Resistance
R
R
 Strain AVG * 50
where 50 is the gauge factor for polysilicon
Vout 
R
R
2  RR
Vin
for half bridge wheatstone structure
• Si/ Si3N4 passivation layer/ flexible substrate (50 µm)/ Si3N4/ sacrificial layer/
Si3N4 membrane layer (~1.9 µm).
• Deep reactive ion etch to etch Si3N4 membrane and open trenches.
• Trenches on flexible substrate were fabricated by lift-off
Trenches
• Al layer (~0.5 µm)/ a-Si layer (~0.5 µm).
• Piezoresistor fabrication.
Piezoresistors
•
•
•
Al metallization layer (~0.5 µm).
Patterning metallization layer.
Al bond pads (~0.5 µm).
Bond pads
Metallization
• Oxygen plasma ashing to release the structure.
• Non-linear I-V characteristics with resistances tens to hundreds of k were
obtained
Before wire bonding
After wire bonding
Probe to apply pressure to the center.
R1
R2
R3
R4
• A maximum differential
output voltage of 13.7 mV
has been obtained for 1V
bias at full membrane
deflection.
Vout
Output voltage is
measured here.
-Vin +
Input voltage is applied here.
•
Four devices (Device 1, Device 2, Device 3 and Device 6) giving the highest
voltage output in response to pressure were selected for determination of
the actual piezoresistive gauge factor.
Device 1
Device 2
Device 3
Device 6
110 µm x 110 µm
90 µm x 90 µm
90 µm x 90 µm
110 µm x 110 µm
Before removal of sacrificial layer
A’
A’
A
A
Membrane (1.9 µm)
Sacrificial
layer (3.1 µm)
Si substrate
A’
A
After removal of sacrificial layer
3.1 µm gap after
removal of sacrificial
layer which is used
to support the
structure during
fabrication
The height given as
around 5 µm includes
the membrane which
is around 1.9 µm thick.
The remainder is the
distance taken at full
membrane deflection.
Device 1
110 µm x 110 µm
Applied Pressure: 78 MPa
Device 2
90 µm x 90 µm
Applied Pressure: 120 MPa
Device 3
90 µm x 90 µm
Applied Pressure: 128 MPa
Device 6
110 µm x 110 µm
Applied Pressure: 73 MPa
Device 1
Results in Response to 78 MPa :
Average Strain: 7.05 10-4
Change in Resistance (%) : 3.52
Output Voltage: 17.30 mV for 1 V input
Device 2
Results in Response to 120 MPa :
Average Strain: 9.9 10-4
Change in Resistance (%) : 4.95
Output Voltage: 24.15 mV for 1 V input
Device 3
Results in Response to 128 MPa :
Average Strain: 9.46 10-4
Change in Resistance (%) : 4.73
Output Voltage: 23.10 mV for 1 V input
Device 6
Results in Response to 73 MPa :
Average Strain: 7.53 10-4
Change in Resistance (%) : 3.76
Output Voltage: 18.45 mV for 1 V input
Sample
Actual Gauge Factor
Type of Device
1
6.36
Device 3
2
19.998
Device 1
3
18.533
Device 3
4
24.121
Device 6
5
22.878
Device 1
6
25.764
Device 1
7
13.615
Device 6
8
17.766
Device 3
9
6.079
Device 2
10
29.366
Device 3
11
14.903
Device 3
12
7.992
Device 6