1. Guido_Tonelli / CMS_TSC / 5 February 2004 1 Time stability of ST sensors The problem The sensors re-measuring campaign Failure analysis Conclusions

2. Guido_Tonelli / CMS_TSC / 5 February 2004 2 The problem: anomalous Common Mode Noise in an important fraction of the first modules 25 ADC Module #: 869 881 1010 1011 1013 1014 1015 1016 1030 1031 1038 1042 6.5 ADC Chips with CMN (micro-discharge problem) Common mode subtracted noise in blue For majority of modules with problems, the CM subtraction is imperfect. 7 of 12 have >2.0 ADC noise 3 of 12 have more than twice the usual noise Off scale  Joe’s talk @ the Tracker Week, 23 October 2003

3. Guido_Tonelli / CMS_TSC / 5 February 2004 3 CMN Summary The CMN problem appears to be a sensor problem Not created during module assembly or bonding No visible damage or indication of defects from sensor QTC Pre-screening sensors (measuring IV) in US appears to improve situation greatly but: We do not know how the problem will evolve Rate of appearance vs. time, power cycles, etc. Changes with radiation damage How problem parts will act with in sub-structures with final electronics and power supplies Do not currently have provisions to pre-screen in US production centers

4. Guido_Tonelli / CMS_TSC / 5 February 2004 4 Common Mode increase observed in US Gigi’s talk @ the CMS Steering Committee, November 2003

5. Guido_Tonelli / CMS_TSC / 5 February 2004 5 IV Correlation with CM noise increase

6. Guido_Tonelli / CMS_TSC / 5 February 2004 6 Correlation I leak increase with CM noise. 69 Modules produced 52/53 good modules exhibit a  wrt Data Base data <5  A 13/16 CMN modules exhibit a  I wrt Data Base data >10  A OUTCOME OF THE CMS DECEMBER WORSHOP BIG WORRY FOR A POSSIBLE TIME INSTABILITY OF STM SENSORS REQUEST TO STM TO SCALE DOWN THE DELIVERY RATE FROM 1000 TO 300/MONTH. START-UP A REPROBING CAMPAIGN TO QUANTIFY AND UNDERSTAND THIS INCREASE IN LEAKAGE CURRENT

7. Guido_Tonelli / CMS_TSC / 5 February 2004 7 Re-probing campaign 1085 sensors have been re-measured. Among them 58 (5.3 %) sensors exhibit an increase of leakage current failing the acceptance criteria of 10 µA. 11 sensors from 2001 (6.6%) INFN CONTRACT 43 sensors from 2002 (6.5%) INFN CONTRACT 4 sensors from 2003 (1.6%) CERN CONTRACT After the meeting with STM on January the 12-th a common task force STM- CMS was set-up to understand the problem and a list of actions was established. The first decision was to collect all detectors failing outside specifications to investigate in detail their characteristics with all possible diagnostic tools. 35 have been shipped to Pisa to be re-measured. Here we present the preliminary results on 29 of these sensors.

8. Guido_Tonelli / CMS_TSC / 5 February 2004 8 List of sensors under scrutiny The I-V measurements have been repeated under severely controlled conditions (automatic- semiautomatic probe stations). Small or no mechanical deformations. A careful visual inspection was performed particularly on the edges to identify all possible mechanical damages. All sensors were kept under bias at high voltage (I-t for ~1 hour) to study the time evolution of the leakage current. In a fraction of sensors a complete strip scan was done.

9. Guido_Tonelli / CMS_TSC / 5 February 2004 9 I-V behaviour of the failing sensors DATA BASE DATA RE-PROBING DATA

10. Guido_Tonelli / CMS_TSC / 5 February 2004 10 First outcome 19/29 sensors have been found GOOD (WELL WITHIN SPEC)

11. Guido_Tonelli / CMS_TSC / 5 February 2004 11 18/19 passed the “I-t test” (1h@400V) ??

12. Guido_Tonelli / CMS_TSC / 5 February 2004 12 Effect of the mechanical stresses (vacuum) on the outcome of the re-probing ORIGINAL RE-PROBING RE-PROBING with and without strong mechanical stresses WE WERE ABLE TO TRANSFORM A GOOD DETECTOR IN A BAD SENSOR BY SIMPLY SWITCHING ON 5” VACUUM. THIS IS A WELL KNOW FEATURE DEEPLY STUDIED IN CMS. THE MECHANICAL STRESSES ON LARGE SILICON SENSORS MAY CHANGE SIGNIFICANTLY THE I-V CHARACTERISTICS. IT IS DIFFICULT TO HAVE COMPLETELY UNIFORM PROCEDURES IN AS MANY AS 10 LABORATORIES INVOLVED IN SENSORS CHARACTERIZATION IN CMS. (MAIN EXPLANATION FOR THE 19 SENSORS TAGGED AS BAD AND FOUND GOOD) DATA BASE RE-PROBING XX PISA_NO_STRESS PISA _STRESS

13. Guido_Tonelli / CMS_TSC / 5 February 2004 13 10/29 SENSORS CONFIRMED TO BE BAD

14. Guido_Tonelli / CMS_TSC / 5 February 2004 14 10/29 SENSORS CONFIRMED TO BE BAD 30210421958307 No effect of vacuum on the quality of the bad sensors

15. Guido_Tonelli / CMS_TSC / 5 February 2004 15 30210414739915 Visual Inspection on bad sensors Found very strong correlation 9/10 with mechanical damage and heavy scratches in critical regions (guard ring & edges)

16. Guido_Tonelli / CMS_TSC / 5 February 2004 16 30210420275724 Evidence of mechanical damage introduced by our “Quality Control” procedures Very suspicious the high (8/35) fraction of sensors passed through one of our I-t centers; heavy signs of mis-handling around the micro-bonding.

17. Guido_Tonelli / CMS_TSC / 5 February 2004 17 30210415060501 Evidence of mechanical damage introduced by our “Quality Control” procedures 30210421741606 These bonds should have been done on the bias ring (bad chuck?)

18. Guido_Tonelli / CMS_TSC / 5 February 2004 18 “I-t” test of BAD sensors Most of the BAD sensors somehow “recover” (another behavior typical of sensors mechanically damaged) General behavior unpredictable over time.

19. Guido_Tonelli / CMS_TSC / 5 February 2004 19 Conclusions There is NO INDICATION SO FAR OF A TIME EVOLUTION of STM sensors as such. Detectors produced since more than two years and measured several times give NO EVIDENCE OF AN INTRINSIC DETECTOR DETERIORATION WITH TIME. On the other hand we have found evidence that our handling, packing and testing procedures within the complex CMS organization HAVE INTRODUCED MECHANICAL DAMAGE in a not negligible fraction of sensors. A careful review of these operations is recommended to avoid wasting a significant fraction of the modules. A well documented and uniform protocol is also recommended to be followed in sensor handling during the module assembly and testing.

20. Guido_Tonelli / CMS_TSC / 5 February 2004 20 Conclusions There is also evidence that, to be able to spot phenomena at per cent level, our testing procedures should be made more uniform throughout the collaboration. The STM sensors so far accepted within the CMS specifications are fully qualified to build production modules. Based on these conclusions I would recommend the following actions: 1. complete the study as outlined by the sensor task force led by M. Krammer. 2. start the TOB and TEC production at full speed 3. Resume the STM full production capability as soon as possible 4. Recover the fraction of production missing so far (25-30%) putting in place a second producer (HPK) as STM subcontractor starting by the end of February.