1. The electro-thermal properties of integrated circuit microbolometers
T & M Nov 2010
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
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Carl and Emily Fuchs Institute
for Microelectronics

2. Carl and Emily Fuchs Institute for Microelectronics - CEFIM
T & M Nov 2010
Carl and Emily Fuchs Institute for Microelectronics - CEFIM
CEFIM
Carl and Emily Fuchs Institute for Microelectronics

3. Carl and Emily Fuchs Institute for Microelectronics
T & M Nov 2010
Infrared thermal images
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Carl and Emily Fuchs Institute for Microelectronics

4. Carl and Emily Fuchs Institute for Microelectronics
T & M Nov 2010
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.
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5. Carl and Emily Fuchs Institute for Microelectronics
T & M Nov 2010
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.
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6. Carl and Emily Fuchs Institute for Microelectronics
T & M Nov 2010
Three types of direct thermal detectors
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.
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Carl and Emily Fuchs Institute for Microelectronics

7. CMOS integration of microbolometers
T & M Nov 2010
CMOS integration of microbolometers
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.
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Carl and Emily Fuchs Institute for Microelectronics

8. Principle of bolometer IR detection
T & M Nov 2010
Principle of bolometer IR detection
Infrared
radiation
Sensor
absorbs
radiation
Temperature
increases
Circuit to
measure
resistance
Temperature sensitive
resistive material
thermally isolated from ambient
ROIC
Readout
integrated
circuit
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Carl and Emily Fuchs Institute for Microelectronics

9. Carl and Emily Fuchs Institute for Microelectronics
T & M Nov 2010
Micro-machined silicon sensors
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 , June 2003.
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Carl and Emily Fuchs Institute for Microelectronics

10. Carl and Emily Fuchs Institute for Microelectronics
T & M Nov 2010
Bulk machined device - CEFIM, UP
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Carl and Emily Fuchs Institute for Microelectronics

11. Carl and Emily Fuchs Institute for Microelectronics
T & M Nov 2010
Surface machined bolometer - IMEC, Belgium
First ever submicron support beams
Stiffness enhancement techniques
U-profile – 100 nm sensor thickness – 2.5 ms thermal time constant
SEM picture of 50 x 50 micron polySiGe bolometer
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Carl and Emily Fuchs Institute for Microelectronics

12. Carl and Emily Fuchs Institute for Microelectronics
T & M Nov 2010
Bolometer thermosensitive materials
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 of
approximately –2 %/K, which is 10 times that of metals.
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13. Carl and Emily Fuchs Institute for Microelectronics
T & M Nov 2010
Bolometer thermal analysis
 = fraction IR power absorbed
PO = incident IR power, W
G = thermal conductance, W/K
= GGAS + GSOL
f = frequency of modulation, Hz
 = thermal time constant = H/G, sec
H = thermal capacitance, J/K
Gaseous thermal
Heat
conductance
GGAS
of suspended plate PO
Membrane or Plate
GSOL TB
Solid thermal H
conductance T
of supporting leg
Thermal capacity
of suspended plate TS
TB = Plate (bolometer) temperature
TS = Substrate (ambient) temperature
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Carl and Emily Fuchs Institute for Microelectronics

14. Carl and Emily Fuchs Institute for Microelectronics
T & M Nov 2010
Bolometer electrical analysis IB VB
Voltage sensitivity RV : IB
With noise voltage vn in system, we can
define the system performance parameter NETD,
the Noise Equivalent Temperature Difference. t
ton << 
with Ad = bolometer area, m2
Trade-off
For RV and NETD , we need  ,  , G , Ad
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Carl and Emily Fuchs Institute for Microelectronics

15. Thermal conductivity vs. fill factor trade-off
T & M Nov 2010
Thermal conductivity vs. fill factor trade-off
Single level surface machining
A. Rogalski, “Optical detectors for focal plane arrays”, Opto-Electronics Review, Vol. 12, No. 2, pp , 2004.
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Carl and Emily Fuchs Institute for Microelectronics

16. Advanced detector structures
T & M Nov 2010
Advanced detector structures
Surface machining
Higher fill factors than bulk machining
Double level machining
Higher fill factors than single level machining
Single level
Double level
Surface micromachining
Surface micromachining
D. Murphy et al, “Resolution and sensitivity improvements for VOx microbolometer FPAs”, Proceedings of SPIE Vol. 5074, Infrared Technology and Applications XXIX, pp , 2003.
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Carl and Emily Fuchs Institute for Microelectronics

17. Comparing single and double level devices
T & M Nov 2010
Comparing single and double level devices
Single level , 50 m pixel
Double level , 25 m pixel
D. Murphy et al, “Resolution and sensitivity improvements for VOx microbolometer FPAs”, Proceedings of SPIE Vol. 5074, Infrared Technology and Applications XXIX, pp , 2003.
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Carl and Emily Fuchs Institute for Microelectronics

18. Bolometer performance vs. pixel dimension
T & M Nov 2010
Bolometer performance vs. pixel dimension
Constant thickness, TCR, absorption and ROIC design
1000
Double level
1 m design rules
Double level
2 m design rules
Single level
1 m design rules
100
Single level
2 m design rules
Relative performance 10 1
Pixel dimension (m)
D. Murphy et al, “Resolution and sensitivity improvements for VOx microbolometer FPAs”, Proceedings of SPIE Vol. 5074, Infrared Technology and Applications XXIX, pp , 2003.
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Carl and Emily Fuchs Institute for Microelectronics

19. Surface machined bolometer at
T & M Nov 2010
Surface machined bolometer at
L-3 Communications, USA
C.M. Hanson et al, “Small pixel a-Si/a-SiGe bolometer focal plane array technology at
L-3 Communications”, Proc. of SPIE Vol. 7660, 76600R-2, 19 May 2010.
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Carl and Emily Fuchs Institute for Microelectronics

20. Carl and Emily Fuchs Institute for Microelectronics
T & M Nov 2010
L-3 Communications, USA
TCR = 3.9 % / K μm pixel technology  768
NETD8-12μm ~ 35mK Thermal time constant ~10ms
Gth ~ 5 nW/K μm photolith IR absorptance ~ 90%
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Carl and Emily Fuchs Institute for Microelectronics

21. Our experimental device
T & M Nov 2010
Our experimental device
600 nm Au
60 nm Ti
5 µm
Cavity
100 nm SiO2
Membrane
900 nm Si
3N4 Si
3N4
200 nm SiO2
2 μm Cavity
2 μm Al
1 μm SiO2 Ti
Si bulk
9 μm
73 μm
Wi = width of supporting leg
di = thickness of supporting leg
li = length of supporting leg
i = thermal conductivity of
supporting leg material
Ad = device area
ds = cavity separation
gas = gas thermal
conductivity
= W/mK , 1 atm N2
= 0 in vacuum
29 μm
97 μm
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Carl and Emily Fuchs Institute for Microelectronics

22. Thermal conductance vs. pressure
T & M Nov 2010
Thermal conductance vs. pressure
10-4
Conventional model: G = GSOL + GGAS
10-5 G
Thermal conductance (W/K)
GSOL
10-6
GGAS
10-7
10-4
10-2
100
102
104
Pressure (Torr)
M. Ou-Yang and J. Shie, “Measurement of effective absorbance on microbolometers”,
IEEE Tran. Instr. and Meas., Vol. 55, No. 3, pp , June 2006.
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Carl and Emily Fuchs Institute for Microelectronics

23. Our improved analytical model 1,2
T & M Nov 2010
Our improved analytical model 1,2 W e 1.
Cross section d e
Sidewall thermal
gaseous conduction
Heat flow d s
Substrate 2. T T T T
T(x) 1 2 3 4
Equivalent thermal
length Lth
rsol x P
exp(-x/Lth)
ggas x x x x
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Carl and Emily Fuchs Institute for Microelectronics

24. Our improved analytical model 3,4
T & M Nov 2010
Our improved analytical model 3,4 3. T T T T
T(x) 1 2 3
T(L)
rsol x
Distrubuted thermal
conduction in legs P E P +P S E x p p p p x L P
x=L x S 4.
Plate and leg Wp
Spreading resistance RC We
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.
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Carl and Emily Fuchs Institute for Microelectronics

25. Thermal modeling and simulation at atmospheric pressure
T & M Nov 2010
Thermal modeling and simulation at atmospheric pressure 50
Conventional model F sw 40 R C K
Modified model 30 L th
CoventorWare 20
simulation
T(x) 10
Short section
Long section
Plate 20 40 60 80
100
120
140
Distance x μm
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Carl and Emily Fuchs Institute for Microelectronics

26. Thermal modelling and simulation under vacuum conditions
T & M Nov 2010
Thermal modelling and simulation under vacuum conditions 50
Conventional model
Modified model
CoventorWare 40 K 30 20
T(x)
Short section 10
Long section
Plate 20 40 60 80
100
120
140
Distance μm
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Carl and Emily Fuchs Institute for Microelectronics

27. Carl and Emily Fuchs Institute for Microelectronics
T & M Nov 2010
Experimental determination of the thermal conductance at atmospheric pressure IB
6.1 VB
6.0 DC
G = 60 μW/K
5.9
RB  1 ( 104)
5.8
RB = VB / IB
/ G = 17.5
= 0.1 % / K
5.7
Self heating of device:
5.6
5.5
0.5
1.0
1.5
2.0
IB A2 ( 106)
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Carl and Emily Fuchs Institute for Microelectronics

28. Carl and Emily Fuchs Institute for Microelectronics
T & M Nov 2010
Experimental determination of the thermal time constant at atmospheric pressure
Bolometer current iB
Bolometer voltage vB
Time I
(RH)
Bolometer H
VL =  IL RL v
VT =  IH RL B V i
VH =  IH RH B I
(RL) L t f
Time
V = IH ( RH  RL ) = IH R
The bolometer resistance will rise exponentially during tf with
the exponential time constant equal to the thermal time constant 
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Carl and Emily Fuchs Institute for Microelectronics

29. Thermal time constant transient curve
T & M Nov 2010
Thermal time constant transient curve
vB(t)
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
100
200
300
400
500
600
Time μs
Thermal time constant  = 160 μs
Thermal capacitance H =   G = 9.5 nJ/K
(Atmospheric pressure)
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Carl and Emily Fuchs Institute for Microelectronics

30. Predicted thermal parameters under vacuum conditions for our device
T & M Nov 2010
Predicted thermal parameters under vacuum conditions for our device
Atmospheric pressure Vacuum
G μW/K nW/K
 μs ms
H nJ/K nJ/K
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Carl and Emily Fuchs Institute for Microelectronics

31. “Acid test” for IR room temperature system
T & M Nov 2010
“Acid test” for IR room temperature system
State of the art microbolometer NETD ≈ 30 mK
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Carl and Emily Fuchs Institute for Microelectronics

32. Carl and Emily Fuchs Institute for Microelectronics
T & M Nov 2010
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)
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Carl and Emily Fuchs Institute for Microelectronics