random telegraph signal (rts) in cmos monolithic active pixel sensors (maps) for charged particle...
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Random Telegraph Signal (RTS) in CMOS Monolithic Active Pixel Sensors (MAPS) for charged particle tracking.
Outline• Reminder: The operation principle of MAPS• RTS, a radiation damage in MAPS• RTS as function of temperature and radiation dose• Less RTS in pixels with leakage current compensation?• Summary and Conclusion
S. Amar-Youcef (1,3), A. Büdenbender(1), M. Deveaux(1), D. Doering(1), I. Fröhlich(1), C. Müntz(1), J. Stroth(1), F.M. Wagner(2)
(1) IKF, Goethe Universität, Frankfurt/M, Germany(2) Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Garching, Germany(3) Helmholtz Research School, H-QM
Supported by:
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 2
MAPS as detectors for charged particle tracking
Monolithic:
Read-out electronics and sensors are integrated on the same substrate.
Active Pixel :
An amplifier is integrated into each pixel
Sensor
MAPS were developed for visible light applications by industry.
MAPS are produced with cheap standard CMOS-processes.
Since 1999, they are modified for charged particle tracking by IPHC (former LEPSI/IReS) /Strasbourg
CMOS – Monolithic Active Pixel Sensors (MAPS)
Digital camera with 8.2 MPixel – CMOS MAPS
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 3
The operation principle of MAPS
Reset+3.3V+3.3V
Output
SiO2 SiO2 SiO2
N++ N++N+ P+
P-
P+
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 4
MAPS as vertex detectors in high energy physics
IPHC Strasbourg:
Minimum Ionizing Particle MOS Active Pixel Sensor
Features of the MIMOSA – detectors:
• Single point resolution 1.0 µm - 2.5µm
• Pixel – pitch 10 - 40 µm
• Thinning achieved to 50 µm
• Detection efficiency > 99%
• Radiation hardness: 1MRad ; 1 x 1013 neq/cm²
MIMOSA IV
Chips used for this work: MIMOSA-18(SB), MIMOSA-19(3T)
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 5
Operation principle of the 3T-pixel
U1
Q
IL
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 6
Random Telegraph Signal, a radiation damage of MAPS
U1
Q
IL
UC
DS [
AD
C]
Time
Output signal of two MAPS pixels
Working hypothesis: IL(t) is modulated=> RTS in diode.
Threshold
Our concern: RTS may generate many fake hitsFirst observation: >> 1% of all pixels generate fake hitsHow to improve?
Irradiate sensor with ~ 1 MeV reactor neutrons (MedApp @ FRM II, Garching)
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 7
Temperature effects on RTS
Hopkins et al. (2000): Frequency and amplitude of RTS increase with temperature
Output signal of a selected MAPS pixel showing RTS
Hypothesis: Cooling => RTS-Amplitude < Threshold => No fake hits?
* G.R
. Hopkinson: “R
adiation Effects in a C
MO
S A
ctive P
ixel Sensor”, IE
EE
-TN
S V
ol. 47, No. 6, P
. 2480
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 8
RTS-pixels: How to identify
t [a.u.]
UC
DS
Unambiguous RTS-pixel
1) Find the baseline of the pixel signal
2) Define a threshold: 150 e- still realistic for detector operation
3) Scan output for segments of data above this thresholdThis work: tSegment>0.5 s 10 consecutive frames
4) If segment is found => RTS-pixel
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 9
Detected RTS-pixels over time
Saturation value
Observation time: ~ 45 min
Data shown today
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 10
Identified RTS - pixels as function of temperature (3T-pixel)
Number of identified RTS - pixels is dramatically reduced by coolingBut: Cooling is not always possible. Other options?
RTS pixel if signature with:
•Amplitude >150e ENC •Tmin = ~ 0.5s•Tmax= ~ 45 min
Preliminary
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 11
Chips with intrinsic leakage current compensation (SB-pixel)
IC
IL U1
U2
vdd (+3.3V)
IL
t
UCDS
t
IC, U2
t
RTS should only be visible in UCDS, if new equilibrium is being establishedSteps should be visible in U2
(Output)
Threshold
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 12
Raw data from SB-pixels with RTS
Steps within dead timeof the readout system
Step within the lifetime of the readout system
UC
DS
(t)
[a.u
.]U
1(t)
[sh
ifte
d, a
.u.]
Time [a.u.]
Uvdd-U2
UCDS
Observation: SB-pixels suppress RTS in output signal.
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 13
Fake hit rates of irradiated MAPS detectors
0,0 5,0x1012 1,0x1013 1,5x1013 2,0x10131E-6
1E-5
1E-4
1E-3
Fake
hit
rate
[1]
Radiation dose [n/cm²]
0°C, SB-Pixel 20°C, SB-Pixel 0°C, 3T-Pixel 20°C, 3T-Pixel
Fake hit rate of 3T-pixels seems inacceptableCooled SB-pixels show acceptable fake hit rate for < 1013 neq /cm²
Preliminary
RTS dominates fake hit rate!
Region of interest
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 14
Summary and Conclusion
An exploratory study was performed to test, if RTS might generate unwanted fake hit rates in irradiated MAPS.
It was found that in classical 3T-pixels:• At high radiation doses and temperatures, few 10% of all pixels show RTS• RTS dominates the fake hit rates, which may reach 10-3
• Effective cooling reduces the number of affected pixels and fake hit rates
It was studied, if SB-pixels with intrinsic leakage current compensation might be less vulnerable to RTS. We observe:• Most SB-pixels show RTS.• The RTS signatures are suppressed by CDS• The fake hit rate of SB-pixels is more than one order below the one of 3T-pixels. It may be reduced to few 10-5 by cooling
To dim RTS, MAPS being operated above 1012 neq/cm² should be SB-pixels cooled below 0°C.
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 15
MIMOSA V
1 MegaPixel, resolution < 2µm
Thank you for your attention
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 16
RTS-pixels: Parameters in the identification algorithmU
CD
S
t [a.u.]This is not an RTS-pixel
Some pixels show too fast RTS for being detected.Minimum time of RTS signature plays a role.
Same data!
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 17
RTS-pixels: Parameters in the identification algorithm
t [a.u.]
UC
DS
Unambiguous RTS-pixel
RTS- pixels might be not recognized because of insufficientobservation time.
Observation time here: 4.5 min
UC
DS
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 18
Limitation in the detection efficiency for SB-RTS-pixels
10 20 30 40 50
1
2
3
4
IL
U2
I L
t
IC
ILU1
U2
I L
t
Constant threshold:=> Quantitative identification of RTS pixels is not reliable for SB-pixels=> Expect drop of identification efficiency with increasing leakage current
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 19
Fraction of identified RTS-pixels (SB)
RTS masked by increasing leakage currentPreliminary
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 20
Distribution of RTS-pixelsD
. Doe
ring
, Bac
helo
r T
hesi
s
Pixel X
Pixel Y Normal Pixel
RTS Pixel
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 21
Fake hits by RTS, a first impression
Pixel number
Fak
e H
its /
500
0 fr
ames
>4000 fake hits per 5000 events4000
35k0
0
Typically 500 MPixel in vertex detector => ~ 5000 hits, up to ~105 – 106 fake hits / event => Inacceptable.How to improve?
> 1% of all pixels affected, up to 80 % wrong indications
No particle source
M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 22
RTS in pixels with leakage current compensation
Threshold