Random Telegraph Signal (RTS) in CMOS Monolithic Active Pixel Sensors (MAPS) for charged particle...

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