d etectivity o ptimization of hts b olometers mehdi fardmanesh supercondutor electronics research...
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DETECTIVITY OPTIMIZATION OF HTS BOLOMETERS
Mehdi FardmaneshSupercondutor Electronics Research Laboratory (SERL) Electrical Engineering DepartmentSharif university of technology
Superconductor Electronics Research Laboratory(SERL) 1
Out lines
Part I : Theoretical optimizationOperation principleTheoretical LimitDetectivity parameters
Substrate thicknessModulation frequencyThermal conductivity
Part II :Experimental ResultsSubstrate thinningAbsorber effectDetectivity calculationConclusion and Summary
Superconductor Electronics Research Laboratory(SERL) 2
Bolometer - Principles
4
A device which transforms a change in the incident input power into a change of electrical resistance.
A bolometer in general consists of: 1- Absorber 2- Thermometer 3- A thermal link to the temperature reservoir
Superconductor Electronics Research Laboratory(SERL)
5
Operation Mechanism
Transition curve and bolometric response Comparison of different types of infrared
detectors [SERL]
Cut off frequency for semiconductor detectors:
cc
hc
U
Superconductor Electronics Research Laboratory(SERL)
Detectivity
A criterion to determine device performance
Different noise sources
1/2*
n
V
A cmHzD
NEP WV W
NEPr Hz
1/22 2 2 2 2 2 2. . 1/ . .nTot n Phot Phon R f Amp ExtV V V V V V V V
Superconductor Electronics Research Laboratory(SERL) 6
Theoretical limit
Considering only background noise
It can be enhanced by reducing field of view of detector
1/22
1/2* 11
2
8
2.56 10 cm Hz /W8
v B B
Photon
photon
B Bphoton
r Ak TV
DK AT
NEP
Superconductor Electronics Research Laboratory(SERL) 7
Important terms in total NEP
Phonon NEP
Johnson NEP
1/f NEP
4 bPhonon B
TNEP k G
4 B b bJohnson
v
k T RNEP
r
1b bH
fv
I RNEP
NAdf r
Superconductor Electronics Research Laboratory(SERL) 8
Maximum Response & Total NEPMaximum Optical Response
Considering linear transition
Total NEP
,max 22 2
v
GdR
dRdTrdTG fC
2222
222 2
2484
2 2b B bB B H
total B
T G fCT k Tk AT T WNEP k G HzG NAdf
Superconductor Electronics Research Laboratory(SERL) 10
2bR T
dRdT
Detectivity vs. thermal conductance & Normal resistance(1mm thickness)
Almost independent of device resistanceStrong variation versus thermal conductanceDecreases at low and high value of thermal conductance
Superconductor Electronics Research Laboratory(SERL)
Detectivity versus thermal conductance and normal resistance of a bolometer over SrTiO3 substrate with thickness of 1mm at 80Hz modulation frequency.
Detectivity vs. thermal conductance & Normal resistance(10µm thickness)
Lower values of optimum thermal conductanceHigher detectivity Decreases at low and high value of thermal conductance
Superconductor Electronics Research Laboratory(SERL)
Detectivity versus thermal conductance and normal resistance of a bolometer over SrTiO3 substrate with thickness of 10µm at 80Hz modulation frequency.
Detectivity Vs. thermal conductance & Normal resistance(10µm thickness)
Ignoring 1/f noiseDetectivity as high as 2×1010
Very strong dependence on thermal conductanceDecreases at low and high value of thermal conductance
Superconductor Electronics Research Laboratory(SERL)
Detectivity versus thermal conductance and normal resistance of a bolometer over SrTiO3 substrate with thickness of 10µm at 2Hz modulation frequency.
Substrate thickness effectsThinning the substrate would increase device detectivityThickness independent at high thermal conductanceThere is an optimum value for thermal conductance
0.0E+00
1.0E+09
2.0E+09
3.0E+09
4.0E+09
0 0.04 0.08 0.12 0.16 0.2
Det
ectiv
ity(C
m.H
z^0.
5/W
)
Thermal conductance(W/K)
1000µm
200µm
100µm
50µm
20µm
Detectivity versus thermal conductance of a bolometer over SrTiO3 substrate with device area of 9 mm2, surface absorption of 20% and modulation frequency of 30Hz.Superconductor Electronics Research Laboratory(SERL) 14
Modulation frequency effectVery low detectivity at low value of modulation frequency Frequency independent at high value of thermal conductanceIncreasing frequency decreases the detectivityThere is an optimum value of thermal conductance
0.0E+00
1.0E+09
2.0E+09
3.0E+09
0 0.04 0.08 0.12 0.16 0.2
Det
ectiv
ity(C
mH
z^0.
5/W
)
Thermal conductance(W/K)
2Hz
12Hz
30Hz
90Hz
200Hz
Detectivity versus thermal conductance of a bolometer over SrTiO3 substrate with device area of 9 mm2, surface absorption of 20% and substrate thickness of 100µm.Superconductor Electronics Research Laboratory(SERL) 15
Analytical optimizationOptimum thermal conductance
Optimum NEP
16
2 2
2
(2 ) 10 0
2
22
total s
optimal s
NEP T fCT
G G
TG fC
T
222
22
42 2
22 2
2
16
2
Boptimum s s
s
Boptimum s
K T T TNEP T fC fC
T TfC
T
K T TNEP fC T
Superconductor Electronics Research Laboratory(SERL)
Optimum thermal conductance
Increases by increasing modulation frequencyDecreases by decreasing substrate thickness
Optimum thermal conductance versus modulation frequency and substrate thickness
Superconductor Electronics Research Laboratory(SERL)
Maximum detectivity
Decrease by increasing modulation frequencyincreases by decreasing substrate thickness
Maximum Detectivity versus modulation frequency and substrate thickness
Superconductor Electronics Research Laboratory(SERL)
Optimum detectivity
19
0
5E+09
1E+10
1.5E+10
2E+10
2.5E+10
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140
Max
imum
Det
ecti
vity
(Cm
.Hz^
0.5/
W)
Substrate Thickness(µm)
12Hz
30Hz
90Hz
3Hz
Maximum detectivity versus substrate thickness of a bolometer over SrTiO3 substrate with device area of 9 mm2 and surface absorption of 20%.
Superconductor Electronics Research Laboratory(SERL)
For detectivity in the range of 1010, substrate thickness should be as low as 30µm whereas modulation frequency should be as low as possible .
Surface Conductance Limitation“Effect of Substrate Thickness on Responsivity of Free-Membrane Bolometric Detectors,” To appear in IEEE Journal of Sensors�
Device fabrication
200nm thin YBCO film deposited on 1mm thick SrTiO3 substrate using PLD
YBCO film was patterned using standard photolithography process
Au coated pads
Silver paste for contacts
Superconductor Electronics Research Laboratory(SERL)
Sample ‘A’
Sample ‘B’
New design of cold headReducing thermal conductance is essential in order to achieve high detectivity
Reducing thermal contact between substrate and cold headFree standing bolometer
Superconductor Electronics Research Laboratory(SERL) 22
Thinning the substrateThinning the substrate using our customized grinding machine Initial thickness 1000µmFinal thickness 200µm
Superconductor Electronics Research Laboratory(SERL) 23
Thermal diffusion length
Depends on modulation frequency and substrate materialsDecreases at high value of modulation frequencyDetermines the knee frequency of device
Superconductor Electronics Research Laboratory(SERL)
Thermal diffusion length of common substrate materials versus modulation frequency
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05
The
rmal
Dif
fusi
on le
ngt
h(µ
m)
Frequency(Hz)
Lf(LaAlO3)(µm)
Lf(SrTiO3)(µm)
Lf(MgO)(µm)
Thinning effect on the responsivity and noise
No major effect on the device noise at high frequenciesIncreasing device response in the frequency range lower than the knee frequencySlight increase of 1/f noiseSlight destructive effect on the film quality
Superconductor Electronics Research Laboratory(SERL)
Measured values of optical response and voltage noise of sample A before and after thinning the substrate.
Knee frequencies
f -1 slope
f -0.5 slope
Absorber effect
Backside absorber no destructive effect on film quality Wider range of absorber materialsOnly enhances responsivity in the frequency range in which Lf>Ls
Superconductor Electronics Research Laboratory(SERL)
Measured values of optical response and voltage noise of sample “A_200” before and after applying absorber layer on the back side of device.
Increasing surface absorption of device increases device response Applying thin layer of black paint as an absorber on the backside of device
26
Detectivity calculationAbout one order of magnitude gain in device detectivity by thinning and using absorber
Calculated detectivity versus modulation frequency of sample “B”
Superconductor Electronics Research Laboratory(SERL)
Thinner substrate thickness Higher device detectivity
SrTiO3 is brittle at low thicknesses
Using silicon substrate as a support
27
Results conclusion
Samplecode
Amm2
Ls
µmR
T=300K(kΩ)
Ronset
(kΩ)ΔT(K)
GmW/K
dR/dTmax
kΩ/K
D*max
cmHz^1/2/W
A_1000 8.97 1000
7 1.4 1.2 14.076 1.4 2×108
B_1000 1.257 1000
5 1.2 0.8 9.16 1.8 4.3×108
A_200 8.97 200 7 1.4 1.4 5.2 1.2 4.35×108
B_200 1.257 200 5 1.2 0.87 2.22 1.75 9.5×108
A_200_Absorber
8.97 200 7 1.4 1.4 5.2 1.2 7.73×108
B_200_Absorber
1.257 200 5 1.2 0.87 2.22 1.75 3.6×109
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New device structure
Structural optimization of bolometer
Silicon Substrate
Buffer Layer
YBCO thin film
Gold Layer
Film Substrate
Absorber Layer
29
Extrapolation of Responsivity
20 times increase in device responsivityExpected value of detectivity is
7.2×1010
Superconductor Electronics Research Laboratory(SERL)
Extrapolated value of responsivity for 10µm substrate thickness
30
Planned response 10µm thickness
SummaryFormulation of Detectivity Optimization
Thinning substrateReducing thermal conductanceUsing absorber layer
One order of magnitude gain in detectivity by model based device engineeringA bolometer with detectivity of 3.6×109 Obtained Successful use of Absorber on the backside
No destructive effect on film qualityPossibility of wider range of absorber materials
Target: Wide Band D* over 1010
31