1. 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:

2. 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

3. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 3 The operation principle of MAPS Reset +3.3V Output SiO 2 N++ N+ P+ P- P+

4. 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 10 13 n eq /cm² MIMOSA IV Chips used for this work: MIMOSA-18(SB), MIMOSA-19(3T)

5. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 5 Operation principle of the 3T-pixel U1U1 Q ILIL

6. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 6 Random Telegraph Signal, a radiation damage of MAPS U1U1 Q ILIL U CDS [ADC] Time Output signal of two MAPS pixels Working hypothesis: I L (t) is modulated=> RTS in diode. Threshold Our concern: RTS may generate many fake hits First observation: >> 1% of all pixels generate fake hits How to improve? Irradiate sensor with ~ 1 MeV reactor neutrons (MedApp @ FRM II, Garching)

7. 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 No fake hits? * G.R. Hopkinson: “Radiation Effects in a CMOS Active Pixel Sensor”, IEEE-TNS Vol. 47, No. 6, P. 2480

8. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 8 RTS-pixels: How to identify t [a.u.] U CDS 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 threshold This work: t Segment >0.5 s  10 consecutive frames 4) If segment is found => RTS-pixel

9. 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

10. 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 cooling But: Cooling is not always possible. Other options? RTS pixel if signature with: Amplitude >150e ENC T min = ~ 0.5s T max = ~ 45 min Preliminary

11. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 11 Chips with intrinsic leakage current compensation (SB-pixel) ICIC ILIL U1U1 U2U2 vdd (+3.3V) ILIL t U CDS t I C, U 2 t RTS should only be visible in U CDS, if new equilibrium is being established Steps should be visible in U 2 (Output) Threshold

12. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 12 Raw data from SB-pixels with RTS Steps within dead time of the readout system Step within the lifetime of the readout system U CDS (t) [a.u.] U 1 (t) [shifted, a.u.] Time [a.u.] U vdd -U 2 U CDS Observation: SB-pixels suppress RTS in output signal.

13. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 13 Fake hit rates of irradiated MAPS detectors Fake hit rate of 3T-pixels seems inacceptable Cooled SB-pixels show acceptable fake hit rate for < 10 13 n eq /cm² Preliminary RTS dominates fake hit rate! Region of interest

14. 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 10 12 n eq /cm² should be SB-pixels cooled below 0°C.

15. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 15 MIMOSA V 1 MegaPixel, resolution < 2µm Thank you for your attention

16. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 16 RTS-pixels: Parameters in the identification algorithm U CDS 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!

17. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 17 RTS-pixels: Parameters in the identification algorithm t [a.u.] U CDS Unambiguous RTS-pixel RTS- pixels might be not recognized because of insufficient observation time. Observation time here: 4.5 min U CDS

18. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 18 Limitation in the detection efficiency for SB-RTS-pixels ILIL U2U2 ILIL t ICIC ILIL U1U1 U2U2 ILIL t Constant threshold: => Quantitative identification of RTS pixels is not reliable for SB-pixels => Expect drop of identification efficiency with increasing leakage current

19. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 19 Fraction of identified RTS-pixels (SB) RTS masked by increasing leakage current Preliminary

20. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 20 Distribution of RTS-pixels D. Doering, Bachelor Thesis Pixel X Pixel Y Normal Pixel RTS Pixel

21. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 21 Fake hits by RTS, a first impression Pixel number Fake Hits / 5000 frames >4000 fake hits per 5000 events 4000 35k 0 0 Typically 500 MPixel in vertex detector => ~ 5000 hits, up to ~10 5 – 10 6 fake hits / event => Inacceptable. How to improve? > 1% of all pixels affected, up to 80 % wrong indications No particle source

22. M. Deveaux, IEEE-NSS conference, 19 - 25 October 2008 Dresden, Germany 22 RTS in pixels with leakage current compensation Threshold