Transcript Infrared Radiation (IR) sensors

Device-level vacuum-packaged
infrared sensors on flexible
substrates
Aamer Mahmood
Advisor:
Prof. Donald P. Butler
Microsensors Laboratory
Department of Electrical Engineering
University of Texas at Arlington
Arlington, TX 76019
Outline
– Introduction
–
–
–
–
MEMS
Infrared radiation and detection
Bolometers
Flexible substrates
– Bolometers on flexible substrates
– Device-level vacuum-packaged microbolometers
– Fabry Perot cavity based tunable infrared
microspectrometer
2
Microelectromechanical Systems
(MEMS)
• Micro-Electro-Mechanical Systems
(MEMS) is the integration of mechanical
elements, sensors, actuators, and
electronics on a common substrate
through microfabrication technology.
3
Infrared radiation
Planck’s law
1.2
Normalized exitance
1
Blackbody
Graybody
0.8
0.6
0.4
0.2
0
-0.2
0
5
10
15
20
25
30
35
Wavelength(m)
4
Infrared Detectors
• Photon Detectors
– Incident radiation generates photo carriers
• Photovoltaic detectors
• Photoconductive detectors
• Photoemissive detectors
• Thermal Detectors
– Incident radiation causes change in temperature that
causes a change in a detector property e.g.
• Bolometers (change in temperature causes the
detector resistance to change)
• Pyroelectric detectors (change in temperature causes
the detector capacitance to change)
• Thermocouples (use Seebeck effect)
5
Bolometers and IR detection
Tac 
Psignal
Geff 1  ( th ) 2
Cth
 th 
Geff
6
Bolometers and IR detection
• Temperature induces a change in the detector
resistance
IRb
amp
I
V  
P
Rb
signal
Geff (1   2 2 )1/ 2
Rb
V
I  
P
2
2 2 1/ 2 signal
( R  Rb ) Geff (1    )
Rb
V
R
amp
η = absorptivity,  = angular frequency of incident radiation, τ = detector thermal
time constant, Psignal = the magnitude of the incident flux fluctuation
7
Bolometer Figures of Merit
Temperature coefficient of resistance
Normalized change in resistance w.r.t. temperature
  TCR 
1 dR
R dT
Responsivity
Output/Input
IRb
V
V 

Psignal
Geff (1   2 2 )1/ 2
Rb
I
V
I 

Psignal
( R  Rb ) 2 Geff (1   2 2 )1/ 2
8
Bolometer Figures of Merit
Detectivity
Signal-to-noise ratio normalized to the detector area and frequency bandwidth
DV  V
DI   I
f  A
Vn
f  A
In
9
Bolometer materials
Material
TCR (300K)
Salient features
YBCO
-3 to -3.5 %K-1
Room temp sputtering, no
heat treatment
VOx
-2 %K-1
Low noise
-2.7 %K-1
High doping with
impurities, Crystalization
by high temp annealing
A-Si
P-Si
P-Si_Ge alloy
-1 to -2
~ -2
%K-1
%K-1
High temperature
annealing
High temperature
deposition
10
Flexible substrates
Property
Units
Polyimide
Polyester
Thickness Range
mils
0.3 - 5
0.25 - 14
Dielectric Constant
1 Mhz
3.4
3.2
Volume Resistivity
W-cm
10 18
10 18
Tensile Strength (at 25oC)
psi
40 000
27 000
Tear Initiation Strength
gms
800
1200
Operational Temperature
Coefficient of Thermal
Expansion (at 20 oC)
min/max oC .-200 to +300
.-60 to +105
1/oC
10x10-6
20x10-6
Change in Linear
Dimension(150 oC, 30min)
%
>0.15
>1.5
Acid Resistance
_
Good
Good
Alkali Resistance
_
Poor
Poor
Grease/Oil Resistance
_
Good
Good
Organic Solvent
Resistance
_
Good
Good
% (24 hrs.)
3
>0.8
Water Absorbtion

• Polyester and Polyimide
used as flexible
substrates
• Polyimide is thermally
stable
• Polyimide has a Tg of
~400°C
• Polyimide is chemically
resistant to most clean
room etchants
http://www-ee.uta.edu/zbutler/Smart_skin_for_web.ppt
11
Flexible systems
• Advantages of flexible substrate micro
sensors
– Low cost
– Lightweight
– Conformable to non planar surfaces
– Software based printed IC processes
– High degree of redundancy
12
Flexible systems
• Flexible electronics for personal
communication (flexible electronic paper)
• Smart clothing (Wireless communications with
smart sensors and actuators in the ambient)
• BioMEMS (flexible electrodes for neural prostheses,
vision prosthesis)
• Conductive polymers (compound eye,
piezoresistive strain sensors)
• Flexible energy sources (photovoltaic cells,
organic solar cells)
13
Sensors on flexible substrates
(Smart Skin)
• Sensor Arrays on flexible substrates
– Infrared sensors
– Pressure/Tactile Sensors
– Flow sensors
– Humidity sensors
– Velocity sensors
14
Evolution of “smart skin” in the
micro sensors lab
• First generation (2001-2002)
– Used solid Kapton sheets pasted on to wafers
• Second generation (2003-2004)
– Spin on Kapton used (no micromachining, not
separated from carrier wafer)
• Third generation (2004)
– Spin on Kapton used (micromachined devices, peeled
off carrier wafer)
• Fourth generation (2005)
– Vacuum packaging at the device level
15
Microbolometers on
flexible substrates
16
Microbolometer Fabrication
Trench Geometry
Au
SrTiO3
YBCO
Ti
Si3N4
PI2610
SrTiO3
Al
Si3N4
PI5878
Si
17
Microbolometers on a flexible
substrate
18
Temperature Coefficient of
Resistance (TCR)
30
-2
25
-2.5
-3
-3.5
10
-4
5
0
240
-4.5
250
260
270
280
290
300
310
320
Temperature (K)
19
-1
1 dR
TCR   
R dT
15
%TCR (K )
Resistance (MW)
20
Effects of Joule Heating
3
R(T )  R0 exp( Ea kT )
Voltage (V)
PI  VI  I b2 R(T )
G eff (T )  PI T  I b2 R(T ) (T - T0 )
0
Geff  Gth  Grad - PI
-3
-2
0
Current ()
2
20
Responsivity/Detectivity
10
Responsivity (V/W)
10
6
10
5
10
4
f  A
10
5
10
4
Vn
1/2
V
IR
Rv 

 Geff (1   2 2 )1/ 2
1000
970 na
730 na
540
540 na
na
1000
390 na
214 na
136
136 na
na
100
1
10
Detectivity (cm Hz /W)
Dv  Rv
6
100
100
1000
Frequency (Hz)
21
Effects of substrate heating
22
Area scans of bolometers
Device 1b4 (Trench Geometry)
23
Area scans of bolometers
Device DD15 (Mesa Geometry)
24
Conclusion
• Microbolometers on flexible substrates have been
fabricated
•
•
•
•
•
Mean measured thermal conductance = 5.61x10-7 W/K
Max room temperature responsivity RV = 7.4x103 V/W
Max room temperature detectivity D*= 6.6x105 cmHz1/2/W
Measured room temperature TCR = -2.63%/K
Measured room temperature resistance = 3.76MΩ
25
Device-level
vacuum-packaging
26
Device-level vacuum packaging
OTMS
Si3N4
Au
YBCO
Ti
PI2610
SrTiO3
Al
Si3N4
PI5878
Si
27
Device-level vacuum packaging
Optical Window
Detector
Al Mirror
Bond Pad
28
Device-level vacuum packaging
Design Considerations
–
–
–
–
Optical window transmission characteristics
Optical window structural analyses
Cavity vacuum
Thermal analyses
29
Optical Transmission
Characteristics
Transmission characteristics of thin Aluminum Oxide film
M. Aguilar-Frutis, M. Garcia, C. Falcony, G. Plesch and S. Jimenez-Sandoval, “A study of the dielectric characteristics of aluminum
oxide thin films deposited by spray pyrolysis from Al(acac)3,” Thin Solid Films, vol 389, Issues 1-2, pp 200-206, 15 June 2001.
30
6
5
5
4
4
3
3
2
2
1
1
0
40
"
'
Complex relative permittivity of
Al2O3
-1
0
5
10
15
20
25
30
35
40
30
35
40
Wavelength(m)
3
tan
2
1
0
-1
0
5
10
15
20
25
Wavelength(m)
31
Optical Transmission
Characteristics of polyimide
Transm ission
Verification
Transmission (%)
PI 5878G
120
Noise
Verification
100
5 per. Mov. Avg.
(Transm ission
Verification)
80
60
40
20
0
0.9
1.9
2.9
3.9
4.9
5.9
6.9
7.9
Wavelength(um)
8.9
9.9
10.9
11.9
12.9
32
Structural analysis of vacuum
element
• Mechanical Strength
– Ceramic Al2O3 has a tensile strength of 260
MPa
– ZnSe has an apparent elastic limit of 55.1
MPa
33
Structural integrity of vacuum
element
34
Al2O3 stress analysis
Stress (MPa)
Al O stress vs. radius of curvature
10
4
10
3
10
2
10
1
10
0
2
3
Mises stress (MPa)
Tensile strength (MPa)
Compressive strength (MPa)
0.1
1
10
100
r (cm)
35
• Permeability is the flow rate
through a specimen once
steady state has been
achieved
• He Permeability through Al2O3
at room temperature is
~100-1000 atoms/s/cm/atm
•
nRT
P
V
n=number of moles
R=universal gas constant=8.314J/(mole.K)
Pressure (mT)
Permeability through Al2O3
10
-1
10
-2
10
-3
10
-4
10
-5
Permeability=100atoms/s/cm/atm
Permeability=1000atoms/s/cm/atm
10
-6
1
10
100
1000
10000
Time (Days)
36
Thermal analysis (analytic)
Heat source ((300+ΔT)K)
Au
Au
Au
Ti
patch
Ti arm
Si3N4
Si3N4
Si3N4
Top air
Al2O3
PI2610
Ti
patch
Si3N4
Si3N4
Lower
air
Al2O3
Al2O3
GA
Lower
air
GB
GC
GD
GE
GF
GG
Heat sink (300K)
Complete
micromachining
Incomplete
micromachining
Ruptured cavity
37
Thermal analysis (numeric)
Gth ≈ 5x10-6 W/K
≈10-4 W/K
(Vacuum)
(air)
38
Microbolometer fabrication
Trench Geometry
(Not to scale)
39
Fabrication
(Silicon wafer)
40
Fabrication
(PI 5878G)
41
Fabrication
(Nitride)
42
Fabrication
(Al)
43
Fabrication
(Sacrificial Polyimide PI 2610)
44
Fabrication
(Support Nitride)
45
Fabrication
(Ti arms)
46
Fabrication
(Au contacts)
47
Fabrication
(YBCO detector pixel)
48
Fabrication
(Photodefinable PI2737 sacrificial mesa)
49
Fabrication
(Al2O3)
50
Section of vacuum cavity before
micromachining
Al2O3
Sacrificial
PI2737 mesa
Sacrificial
PI2610
Nitride
Al mirror
Nitride
51
Fabrication
(Partially micromachined)
52
Fabrication
(Fully micromachined)
53
Fabrication
(Sealed vacuum cavity)
54
Fabrication
(Superstrate PI 5878G)
55
Single microbolometer
56
Fabrication of encapsulated
devices
Partially
micromachined
device
Fully
micromachined
device
SEM graph of an
unsealed
micromachined
device
57
Fabrication of encapsulated
devices
Vacuum cavity
Sealed device
SEM graph of
sealed device
SEM graph of
cross section of
vacuum cavity
58
VI curve
40
30
Voltage (V)
20
10
Measured Gth=3.73x10-6 W/K
0
-10
-20
-30
-40
-0.6
-0.4
-0.2
0
0.2
Current (A)
0.4
0.6
59
Temperature Coefficient of
Resistance (TCR)
180
TCR(300K)=-3.7%/K
-1
140
120
-2
100
-3
-1
80
60
TCR (%K )
R(300K)=53.4 MΩ
Resistance (MW)
160
0
-4
40
-5
280 285 290 295 300 305 310 315
Temperature (K)
60
Current Responsivity (RI)
2
Responsivity (A/W)
10
Current Responsivity (RI)
=Output current/Input power
1
10
RI=61.3 μA/W
@ 5Hz
0
10
10.09V
7.20V
5.48V
3.66V
-1
10
1
10
100
Frequency (Hz)
1000
61
Detectivity (D*)
6
Detectivity (D*)
Detectivity (cm Hz
1/2
/W)
10
= Area and frequency normalized
signal to noise ratio
5
10
D* = 1.76x105 cm-Hz1/2/W
4
10
10.09V
7.20V
5.48V
3.66V
3
10
2
10
1
10
100
Frequency (Hz)
1000
62
Conclusion
• Device level vacuum encapsulated microbolometers on
flexible substrates have been fabricated
• Theoretical thermal conductance in vacuum is 5x10-6
W/K
• Measured thermal conductance is 3.73x10-6 W/K (Intact
Vacuum cavity)
• Measured room temperature TCR is -3.7%/K, resistance
is 53.4MΩ
• Measured RI is 61.3 μA/W, D*=1.76x105cm-Hz1/2/W
63
• This work is supported by the National Science Foundation
The End
64