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The electro-thermal properties of
integrated circuit microbolometers
* M. du Plessis, J. Schoeman, W. Maclean ** C. Schutte
* Carl and Emily Fuchs Institute for Microelectronics, University of Pretoria* * Detek, Denel Aerospace Systems
CEFIMCarl and Emily Fuchs Institute
for Microelectronics
T & M 2010 - 8 Nov 2010
2
Carl and Emily Fuchs Institute for Microelectronics - CEFIM
CEFIMCarl and Emily Fuchs Institute for Microelectronics
T & M 2010 - 8 Nov 2010
3CEFIMCarl and Emily Fuchs Institute for Microelectronics
Infrared thermal images T & M 2010 - 8 Nov 2010
4CEFIMCarl and Emily Fuchs Institute for Microelectronics
Principal types of infrared (IR) detectors
1) Photon detectors - [ Cooled ] In photon detectors the absorbed photons directly produce free electrons and holes, to generate a photon-induced current or voltage, either in a photoconductive or photovoltaic mode.
2) Thermal detectors - [ Uncooled ] In thermal detectors the absorbed photons produce a temperature change, which is then indirectly detected by measuring a temperature dependent property of the detector material.
T & M 2010 - 8 Nov 2010
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Two classifications of thermal sensors
1) Direct sensors Direct sensors convert thermal signals (temperature or heat) to electrical signals.
2) Indirect sensors Indirect sensors are based on thermal actuation effects, such as thermo-mechanical (thermal expansion) effects.
CEFIMCarl and Emily Fuchs Institute for Microelectronics
T & M 2010 - 8 Nov 2010
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1) Bolometers A bolometer changes its resistance according to the change of the
temperature, and thus a high temperature coefficient of resistance (TCR) is needed for high sensitivity.
2) Pyroelectric effects
The pyroelectric effect is exhibited by ferro-electric crystals that exhibit electric polarization. They have no direct current (DC) response and therefore must employ radiation modulators.
3) Thermoelectric effects Two junctions made of two different materials are at different temperatures, and the magnitude of the voltage generated across the thermopile junction depends on the type of materials and the temperature difference between the junctions.
Three types of direct thermal detectors
CEFIMCarl and Emily Fuchs Institute for Microelectronics
T & M 2010 - 8 Nov 2010
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Thermal sensors based on CMOS technology became feasible when CMOS micromachining (MEMS) was established.
Micromachining makes it possible to remove thermally conducting material for the thermal isolation of heated microstructures.
While thermal effects are intuitively considered to be slow, the small size of CMOS microsensors brings about thermal time constants in the millisecond range.
CMOS integration of microbolometers
CEFIMCarl and Emily Fuchs Institute for Microelectronics
T & M 2010 - 8 Nov 2010
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Sensorabsorbsradiation
Temperatureincreases
Circuit tomeasure
resistance
Infraredradiation
Temperature sensitiveresistive material
thermally isolated from ambient
Principle of bolometer IR detection
ROICReadout
integrated circuit
CEFIMCarl and Emily Fuchs Institute for Microelectronics
T & M 2010 - 8 Nov 2010
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Two approaches used to thermally isolate the sensor structure or part of it from the bulk silicon:
Bulk-machined sensors
Machining the silicon substrate, either from the front or the back. Membranes can be released by surface etching of the silicon
Surface machined sensors Using stacked thin films on the front surface. The mechanical structure is released by removing a sacrificial layer underneath it.
A. Hierlemann,O. Brand, C. Hagleitner and H. Baltes, “Microfabrication techniques for chemical/biosensors”, Proceedings of the IEEE, Vol. 91, No. 6, pp. 839-863, June 2003.
Micro-machined silicon sensors
CEFIMCarl and Emily Fuchs Institute for Microelectronics
T & M 2010 - 8 Nov 2010
10CEFIMCarl and Emily Fuchs Institute for Microelectronics
Bulk machined device - CEFIM, UPT & M 2010 - 8 Nov 2010
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First ever submicron support beamsStiffness enhancement techniquesU-profile – 100 nm sensor thickness – 2.5 ms thermal time constant
SEM picture of 50 x 50 micron polySiGe bolometer
Surface machined bolometer - IMEC, Belgium
CEFIMCarl and Emily Fuchs Institute for Microelectronics
T & M 2010 - 8 Nov 2010
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Important considerations in choosing bolometer material A high TCR, Low noise, especially 1/f noise, Not too high resistivity, and Compatibility with post processing IC
fabrication.
Many materials have been used for bolometers, such as Metals ( Pt, Ti), and Semiconductors (VOx, amorphous silicon).
The semiconductor materials exhibits a TCR ofapproximately –2 %/K, which is 10 times that of metals.
Bolometer thermosensitive materials
CEFIMCarl and Emily Fuchs Institute for Microelectronics
T & M 2010 - 8 Nov 2010
13CEFIMCarl and Emily Fuchs Institute for Microelectronics
CfG
PT O
22241
= fraction IR power absorbedPO = incident IR power, WG = thermal conductance, W/K = GGAS + GSOL
f = frequency of modulation, Hz
= thermal time constant = H/G, secH = thermal capacitance, J/K
GSOL
Solid thermalconductance
of supporting leg
Gaseous thermalconductance
of suspended plateGGAS
Thermal capacityof suspended plate
H
TB
TSTB = Plate (bolometer) temperatureTS = Substrate (ambient) temperature
T
Heat
PO
Membrane or Plate
Bolometer thermal analysis
CTTT SB
T & M 2010 - 8 Nov 2010
14CEFIMCarl and Emily Fuchs Institute for Microelectronics
Bolometer electrical analysis
KdT
Rd
dT
dR
RTCR
B
B
B
B
B
%)(ln1IB
VB
IB
tton <<
WV
GRI
dP
dT
dT
dR
dR
dV
dP
dVR BB
O
B
B
B
B
B
O
BV
Voltage sensitivity RV :
For RV and NETD , we need , , G , Ad
KA
GvNETD
d
n
With noise voltage vn in system, we candefine the system performance parameter NETD,the Noise Equivalent Temperature Difference.
with Ad = bolometer area, m2
Trade-off
T & M 2010 - 8 Nov 2010
15CEFIMCarl and Emily Fuchs Institute for Microelectronics
A. Rogalski, “Optical detectors for focal plane arrays”, Opto-Electronics Review, Vol. 12, No. 2, pp. 221-245, 2004.
Thermal conductivity vs. fill factor trade-offSingle level surface machining
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Advanced detector structuresSurface machining
Higher fill factors than bulk machining
Double level machiningHigher fill factors than single level machining
Single level Double levelSurface micromachining
D. Murphy et al, “Resolution and sensitivity improvements for VOx microbolometer FPAs”, Proceedings of SPIE Vol. 5074, Infrared Technology and Applications XXIX, pp. 402-413, 2003.
Surface micromachining
CEFIMCarl and Emily Fuchs Institute for Microelectronics
T & M 2010 - 8 Nov 2010
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Comparing single and double level devices
D. Murphy et al, “Resolution and sensitivity improvements for VOx microbolometer FPAs”, Proceedings of SPIE Vol. 5074, Infrared Technology and Applications XXIX, pp. 402-413, 2003.
Single level , 50 m pixel Double level , 25 m pixel
CEFIMCarl and Emily Fuchs Institute for Microelectronics
T & M 2010 - 8 Nov 2010
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D. Murphy et al, “Resolution and sensitivity improvements for VOx microbolometer FPAs”, Proceedings of SPIE Vol. 5074, Infrared Technology and Applications XXIX, pp. 402-413, 2003.
Single level2 m design rules
Single level1 m design rules
Double level2 m design rules
Double level1 m design rules
10 20 30 40 50 60Pixel dimension (m)
1000
100
10
1
Rel
ativ
e pe
rfor
man
ceConstant thickness, TCR, absorption and ROIC design
Bolometer performance vs. pixel dimension
CEFIMCarl and Emily Fuchs Institute for Microelectronics
T & M 2010 - 8 Nov 2010
19CEFIMCarl and Emily Fuchs Institute for Microelectronics
C.M. Hanson et al, “Small pixel a-Si/a-SiGe bolometer focal plane array technology atL-3 Communications”, Proc. of SPIE Vol. 7660, 76600R-2, 19 May 2010.
Surface machined bolometer atL-3 Communications, USA
T & M 2010 - 8 Nov 2010
20CEFIMCarl and Emily Fuchs Institute for Microelectronics
L-3 Communications, USATCR = 3.9 % / K 17μm pixel technology 1024 768NETD8-12μm ~ 35mK Thermal time constant ~10ms
Gth ~ 5 nW/K 0.35μm photolith IR absorptance ~ 90%
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60 nm Ti600 nm Au
100 nm SiO2
200 nm SiO2
900 nm Si 3N4
2 μm Cavity 2 μm Al
Si bulk
1 μm SiO2
Ti
5 µm
Cavity
Membrane
73 μm
97 μm
Si 3N4
29 μm
9 μm i i
iiiSOL l
dWG 2
Wi = width of supporting legdi = thickness of supporting legli = length of supporting legi = thermal conductivity of supporting leg material
s
dgasGAS d
AG
Ad = device areads = cavity separationgas = gas thermal conductivity = 0.026 W/mK , 1 atm N2
= 0 in vacuum
Our experimental device
CEFIMCarl and Emily Fuchs Institute for Microelectronics
T & M 2010 - 8 Nov 2010
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GSOL
G
10-7
10-6
10-5
10-4
10-4 10-2 100 102 104
Pressure (Torr)
The
rmal
con
duct
ance
(W
/K)
Thermal conductance vs. pressure
M. Ou-Yang and J. Shie, “Measurement of effective absorbance on microbolometers”,IEEE Tran. Instr. and Meas., Vol. 55, No. 3, pp.1012-1016, June 2006.
CEFIMCarl and Emily Fuchs Institute for Microelectronics
GGAS
Conventional model: G = GSOL + GGAS
T & M 2010 - 8 Nov 2010
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We
de
ds
Cross section
Substrate
Heat flow
Our improved analytical model 1,2
Sidewall thermalgaseous conduction
P
T1 T2 T3 T4 T(x)
x
exp(-x/Lth)
x
rsol x
ggas x
x
Equivalent thermallength Lth
mddgr
L segas
e
gassolth
131
1.
2.
CEFIMCarl and Emily Fuchs Institute for Microelectronics
T & M 2010 - 8 Nov 2010
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Our improved analytical model 3,4
x
T1 T3T2
xx=L
PE
ppp p
PS
PS+PE
T0T(L)
L
rsol x
x
OEESsol
TxTL
xPxPP
LGxT
)(
2
1)(
2
T(x)
Distrubuted thermalconduction in legs
4
1ln2
1
1ln
11 2
eeC d
R
p
e
W
W
N. Topaloglu, P.M. Nieva, M. Yavuz, J.P. Huissoon, “Modeling of thermal conductance in an uncooled microbolometer pixel”, Sensors and Actuators A, Vol. 157, 2010, pages 235 to 245.
Wp
We
Spreading resistance
Plate and leg
3.
4.
CEFIMCarl and Emily Fuchs Institute for Microelectronics
RC
T & M 2010 - 8 Nov 2010
25CEFIMCarl and Emily Fuchs Institute for Microelectronics
0
10
20
30
40
50
0 20 40 60 80 100 120 140
Distance x μm
K
Long section Plate
Short section
Conventional model
Modified model
CoventorWaresimulation
Lth
RC
Fsw
T(x)
Thermal modeling and simulation at atmospheric pressure
T & M 2010 - 8 Nov 2010
26CEFIMCarl and Emily Fuchs Institute for Microelectronics
Thermal modelling and simulation under vacuum conditions
0
10
20
30
40
50
0 20 40 60 80 100 120 140
Conventional modelModified modelCoventorWare
Long section Plate
Short section
Distance μm
K
T(x)
T & M 2010 - 8 Nov 2010
27CEFIMCarl and Emily Fuchs Institute for Microelectronics
5.5
5.6
5.7
5.8
5.9
6.0
6.1
0 0.5 1.0 1.5 2.0
IB2 A2 ( 106)
RB1
1
( 1
04 )
a / G = 17.5
a = 0.1 % / K)1()( TRTR BOB
CG
RI
G
PT BB
2
G
I
RRB
BOB
211
Experimental determination of the thermal conductance at atmospheric pressure
IB
VB
RB = VB / IB
DC
Self heating of device:
G = 60 μW/K
T & M 2010 - 8 Nov 2010
28CEFIMCarl and Emily Fuchs Institute for Microelectronics
Experimental determination of the thermal time constant at atmospheric pressure
Bolometer current iB
IH
Time
IL
Bolometer voltage vB Time
tf
iB
vB
Bolometer
VL = IL RL
VT = IH RL
VH = IH RH
V = IH ( RH RL ) = IH R
V(RL)
(RH)
The bolometer resistance will rise exponentially during tf withthe exponential time constant equal to the thermal time constant
T & M 2010 - 8 Nov 2010
29CEFIMCarl and Emily Fuchs Institute for Microelectronics
Thermal time constant transient curve
V = 80 mV
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0 100 200 300 400 500 600
Time μs
stt
BB eevtv 16008.0)0()(
Thermal time constant = 160 μsThermal capacitance H = G = 9.5 nJ/K
(Atmospheric pressure)
vB(t)
T & M 2010 - 8 Nov 2010
30CEFIMCarl and Emily Fuchs Institute for Microelectronics
Predicted thermal parameters under vacuum conditions for our device
Atmospheric pressure Vacuum
G 60 μW/K 600 nW/K
160 μs 16 ms
H 9.5 nJ/K 9.5 nJ/K
T & M 2010 - 8 Nov 2010
31CEFIMCarl and Emily Fuchs Institute for Microelectronics
“Acid test” for IR room temperature system
State of the art microbolometer NETD ≈ 30 mK
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Conclusions
Theory/modeling and design of IR bolometers well understood
Improvements to analytical modeling – atmospheric pressure
Experimental determination of thermal parameters
THANKS TO AMTS (TIA)
CEFIMCarl and Emily Fuchs Institute for Microelectronics
T & M 2010 - 8 Nov 2010