radiation damage in the star environment and performance of maps sensors
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IPHC-LBNL meeting
3-5 April 2008
Radiation damage in the STAR environment and performance of MAPS
sensors
Compilation of different test results mostly from Michael’s thesis
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Outline
What can we expect at STAR? Figure of merit – S/N Ionizing radiation effects Non-ionizing radiation effects Simple noise model Some solutions and issues
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Based on H.W. estimates (http://rnc.lbl.gov/~wieman/radiation dose straus oct 2007 HW.ppt)
For the radius of 2.5 cm:– Ionizing radiation:
Total dose: 155 kRad TLD projection: 300 kRad
– Non-ionizing radiation average pion count for 1 Yr: 3x1012 cm-2
TLD projection (pion assumption): 12x1012 cm-2
What can we expect at STAR?
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Figure of merit for high detection efficiency is S/N
Test results of MimoSTAR2 show that
– Measured with cuts on clusters– Similar performance can be expected from single threshold algorithm
for S/N>12 detection efficiency >99.6%
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Ionizing radiation effects
Increased leakage current– Increase of shot noise– Faster discharge time of the self-biased structure that
in extreme cases could lead to signal losses
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Time constant for discharge of the self-biased node
Dose (rad) τ
0 30 ms 11ms (~40 °C? ) measured at LBL
20 k 25 ms
500 k 7 ms
1 M 4 ms
from Michael’s thesis for the radiation tolerant diode layout and the operating temperature of 40 °C
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Discharge time
At LBL =130 ms at 30 °C and about 11 ms at the unknown (uncontrolled “ambient”) temperature for a non-irradiated sensor
– the charge collection efficiency (CCE) at the level of 77%, the sensor integration time was 1.7 ms
– at IPHC for the integration time of 4 ms CCE of 75%
short discharge time constant on the order of 10 ms (6 x integration time) did not affect signal collection
Degradation of the discharge time after integrated ionizing dose of a few hundred krad should not be a problem.
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Non-ionizing radiation
Charge losses due to bulk damage Increased leakage current (not negligible)
– Increase of shot noise
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Charge losses
-20 °C – minimized contribution from shot noise (negligible for fluences up to 2x1012 neq/cm2)
Mimosa-9 -20 °C Tint 0.7 ms
Mimosa-15 -20 °CTint 0.7 ms pixel pitch 20 μm.
Half of signal
0.7 signal
irradiation inherently added ionizing background that is approximately 100 krad for the fluence of 1x1013 neq/cm2. This background can be expected to scale linearly with the dose.
Mimosa-9 -20 °C Tint 0.7 ms
Half of signal
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Charge collection efficiency (CCE)
Beam Test Calibration of the Mimosa-15 prototype (S. Amar-Youcef, M. Deveaux, M. Goffe – 05.2006) At -20 °C and +20 °C:
– 20 μm pitch and 2.1x1012 neq/cm2 the CCE decrease to about 0.5– 30 μm pitch (MimoSTAR-like) CCE decreases to
0.66 (4.7 x 1011 neq/cm2) 0.39 (2.1 x 1012 neq/cm2)
20 °C
50 100 150 200 250 300 350ADC
0.2
0.4
0.6
0.8
1Entries
black chipRef, blue chip4, purple chip6, red chip10, green chip1120 °C
50 100 150 200 250 300 350ADC
0.2
0.4
0.6
0.8
1Entries
black chipRef, blue chip4, purple chip6, red chip10, green chip11
20 °C
50 100 150 200 250 300 350ADC
0.2
0.4
0.6
0.8
1Entries
black chipRef, blue chip4, purple chip6, red chip10, green chip1120 °C
50 100 150 200 250 300 350ADC
0.2
0.4
0.6
0.8
1Entries
black chipRef, blue chip4, purple chip6, red chip10, green chip11
Black: RefBlue: 4.7x1011
Purple: 2.1x1012
Red: 5.8x1012
Green: 1.1x1013
MimoSTAR2 pixel type
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Consequences
The charge losses after 2.1 x 1012 neq/cm2 will lead to the S/N reduction:– 28 to 14 for the 20 μm pitch– 20 to 8 for the 30 μm pitch
This results in an unacceptable degradation of S/N for the MimoSTAR2-like pixel.
MIP detection efficiency of irradiated prototypes was measured only at 0 °C due to the limited duration of the beam tests.
This limits the estimation of the detection efficiency achievable at STAR to theoretical considerations of the evolution of S/N.
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Leakage current (noise) from bulk damage
0
5
10
15
20
25
30
35
40
-20 0 20 40
T °C
no
ise
chip 3
chip 4
chip 5
chip 6
chip 7
chip 8
chip 9
chip 10
chip 11
after 2x1012 neq/cm2 the noise degrades from 15 to 18 e-
Chip 3 refChip 4 4.7x1011
Chip 6 2.1x1012
Chip 10 5.8x1012
Chip 11 1.1x1013 Chip 5 refChip 7 23 krad γChip 8 20 krad XChip 9 1 Mrad X
3T pitch 20 μm SB pitch 30 μm.
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Ni(I,Tint) = A × √I × √Tint N=√(N02+Ni2) Temperature
– Rule of thumb 2 x I every 10°C Ionizng radiation
– At 40 °C shot noise contribution is 0 krad => 10 e- 500 krad => 20 e- 1000 krad => 30 e-
– linear fit: (0.002 x Dose(krad)+1)– After tuning: (0.003 x Dose(krad)+1)
– Model fits data well (within 10%) Non-ionizing radiation
– At 20 °C shot noise contributions is 0 => 11 e- (incompatible with results for ionizing radiation) 2.1 x 1012 => 15 e- 5.8 x 1012 => 21 e- 1.1 x 1013 => 29 e-
– Linear fit: (0.14 x Dose(1012)+1)– Model fits data well but noise incompatible with the measurements on Mimosa-11
(leakage current needs to be x4 to fit the results)
Simple model for noise performanceMimosa-11(rad-hard)
N=√(N0^2+ (A × √I × √Tint × (0.003 x Dose(krad)+1) × (0.14 x Dose(1012)+1))^2)
A × √I = 10 at 40 °C
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MimoSTAR2 pixel
Single detector replacement – not sufficient Full elimination of shot noise – not sufficient Annealing ?
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Solutions
Decrease pixel pitch to 20 μm– # columns x 1.5 => power dissipation x 1.5– column length x 1.5 => integration time unchanged
SUZE can not run faster than designed (160 ns) Cluster size?
– Signal decrease to 0.5 from non-ionizing radiation damage (2.1 x 1012 neq/cm2)
– And noise increase 1.15 @ 0.2 ms integration time (assumption for 155 krad @ 40 °C)
– S/N goes down to about 0.44 (28 =>12) it looks OK but what is the accuracy of these estimations? (detector replacement S/N~16)
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Other solutions
Smaller pixel pitch (15, 18 μm) (power dissipation?) 30 μm pitch with multiple charge collecting diodes?
Graded substrate (!) Deep P implants
Latch up in SUZE and Mimosa22 needs to be investigated Hot pixels after non-ionizing irradiation (Function of dose?
Annealing?)– How to mask them for SUZE?
Irradiations with pions?
Other issues
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