Device level vacuum packaged micromachined infrared detectors on flexible substrates Aamer Mahmood Donald P. Butler Zeynep Çelik-Butler Microsensors Laboratory.
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Device level vacuum packaged micromachined infrared
detectors on flexible substrates
Aamer MahmoodDonald P. Butler
Zeynep Çelik-Butler
Microsensors Laboratory Department of Electrical Engineering
University of Texas at Arlington,Arlington, TX 76019
2
Outline
Microbolometers Flexible substrates
Device level vacuum packaging Design and fabrication
Characterization Future work
3
Bolometers Bolometers are thermal detectors YBCO is used as the detector material Change in temperature induces a change in the detector
resistance
η = absorptivity, β = TCR, = angular frequency of incident radiation, τ = detector thermal time constant, ΔΦ = the magnitude of the incident flux fluctuation, Geff = thermal conductivity
21221 /
eff )τω(G
βIRηΔV
21221 /
eff )τω(RG
βVηΔI
4
Sensors on flexible substrates
PI 5878G (liquid Kapton) is used as the flexible substrate
Sensor Arrays on flexible substrates (Smart skins) Infrared sensors Pressure/Tactile Sensors Flow sensors Humidity sensors Velocity sensors Accelerometers
Advantages of flexible substrate micro sensors Low cost Lightweight Conformable to non planar surfaces High degree of redundancy
Vacuum packaging brings the best out of many MEMS devices
17
Section of vacuum cavity before micromachining
Al2O3
Sacrificial PI2737 mesa
Sacrificial PI2610
Al mirrorNitride
Nitride
23
Design considerations Transmission through optical window Structural integrity of vacuum element
Lateral dimensions Cavity resonant wavelength
Axial dimensions
25
Al2O3 stress analysis
100
101
102
103
104
0.1 1 10 100
Al2O
3 stress vs. radius of curvature
Mises stress (MPa)Tensile strength (MPa)Compressive strength (MPa)
Str
es
s (M
Pa
)
r (cm)
27
Fabrication of encapsulated devices
Partially micromachined
device
Fully micromachined
device
SEM graph of an unsealed
micromachined device
28
Fabrication of encapsulated devices
Sealed device SEM graph of sealed device SEM graph of
cross section of vacuum cavity
Vacuum cavity
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VI curve
-40
-30
-20
-10
0
10
20
30
40
-0.6 -0.4 -0.2 0 0.2 0.4 0.6
Vol
tage
(V
)
Current (A)
)()(1
)( 20 TRI
dT
dR
GRTR b
th
Gth=3.36x10-6 W/K
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Temperature Coefficient of Resistance (TCR)
40
60
80
100
120
140
160
180
-5
-4
-3
-2
-1
0
280 285 290 295 300 305 310 315
Res
ista
nce
(M
)
TC
R (%
K-1)
Temperature (K)
dT
dR
RTCR
1
R(300K)=53.4 MΩ
TCR(300K)=-3.4%/K
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Current Responsivity (RI)
10-1
100
101
102
1 10 100 1000
10.09V7.20V5.48V3.66V
Res
po
ns
ivit
y (A
/W)
Frequency (Hz)
2/122 )1(
eff
I RG
VR
RI=6.13x10-5 A/W
@ 5Hz
Current Responsivity (RI)
=Output current/Input power
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Detectivity (D*)
102
103
104
105
106
1 10 100 1000
10.09V7.20V5.48V3.66V
Det
ec
tiv
ity
(cm
Hz1/
2 /W)
Frequency (Hz)
nV
AfRD
*
D* = 1.76x105 cm-Hz1/2/W
Detectivity = D*
= Area normalized signal to noise ratio
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Conclusion
Device level vacuum encapsulated microbolometers on flexible substrates have been fabricated
Theoretical thermal conductivity in vacuum is 5x10-6 W/K
Measured thermal conductivity is 3.36x10-6 W/K (Intact Vacuum cavity)
Measured room temperature TCR is -3.4%/K, resistance is 53.4MΩ
Measured RI is 6.13x10-5 A/W, D*=1.76x105cm-Hz1/2/W
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Future work
Incorporating more sensors in the smart skins e.g. pressure/tactile sensors, flow sensors, accelerometers
Cavity design to improve/tune optical response
True integrated flexible system
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