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ORIGIN OF THE CMN IN TOB/TEC MODULES

ORIGIN OF THE CMN IN TOB/TEC MODULES. A PLAUSIBLE EXPLANATION BASED ON MEASUREMENTS PERFORMED ON 1 (AND ONLY 1) MODULE. Guido Tonelli Laura Borrello Mariarosaria D’Alfonso Lino De Maria Suchandra Dutta Alberto Messineo Giusy Valvo B. Caltabiano C. Cerri, A. Profeti, P. Mammini.

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ORIGIN OF THE CMN IN TOB/TEC MODULES

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  1. ORIGIN OF THE CMN IN TOB/TEC MODULES • A PLAUSIBLE EXPLANATION BASED ON MEASUREMENTS PERFORMED ON 1 (AND ONLY 1) MODULE. • Guido Tonelli • Laura Borrello • Mariarosaria D’Alfonso • Lino De Maria • Suchandra Dutta • Alberto Messineo • Giusy Valvo • B. Caltabiano • C. Cerri, A. Profeti, P. Mammini

  2. Outline • Introduction. • Preliminary diagnostic analysis on TEC Module #30200020020516 • Electrical measurements. • Study of the dependence of CMN and leakage current from the potential difference DV between strip implant and metal. • Analysis of the time evolution of the CMN vs DV. • Simulation of the effect at the device level. • Consistency of the explanation with other observations. • Possible actions. • Conclusions.

  3. The Common Mode Noise • Module 5081(Sensors 23974205 23974314) • Channels: 20 at 300 V and 420 at One or more strips exhibiting a noise incompatible with the leakage current; so large that it affects the entire chip (sometimes in un-recoverable way). Strong correlation with increase in sensor leakage current with respect to sensor QTC data.

  4. Attempt to use sensors grading (W7B) 1.5 mA 1.5 mA AA A Kink in IV B (different scale) Grade AA : Itot < 1.5 µA (and no kink) Grade A: Itot< 1.5 µA (and kink) Grade B: Itot > 1.5 µA Contractual limit: Itot @450 V < 10mA 1.5 mA

  5. CMN modules and sensors grading. Over 150 modules 11(7%) exhibit CMN. Grade B sensors develop CMN in about 15% of the cases. Grade AA and A develop CMN in about 5% of the modules. No additional selection criteria found so far to further reduce this fraction.

  6. The Common Mode Noise The general quality of the first 300 TOB/TEC modules is good but the production yield is around 90-95% (to be compared with 95-97% of the TIB modules). In addition a part of the community attributes this effect to an intrinsic weaknessof the STM sensors and believes to have collected enough evidence that this effect will propagate with time to an important fraction of the tracker. Hence the actions to revise the STM contract (7000 sensors ordered to HPK); to slow down in using STM sensors to build final modules; to stop the STM production to initiate a new qualification procedure (new pre-series of 1000 sensors).

  7. Proposed explanations • intrinsic micro-discharge due to STM implant technology • effect of the leaky strips • in-appropriate handling • mechanical deformations (vacuum effect) • sensitivity to humidity • abnormal time structure of the leakage current • degradation of the leakage current with time? • Most/all explanations were concentrated on some intrinsic weakness of the STM sensors BUT NO CONCLUSIVE EVIDENCE HAS BEEN FOUND SO FAR.

  8. The CMN module: TEC 30200020020516 Built and tested in UCSB. Re-tested in Vienna. Received in Pisa on March 14-th. Investigations started on March the 22-nd.

  9. Preliminary diagnostics: initial tests I @ 450 Volt = 13,3 mA (1-2mA expected from QTC) Noise of strip #96 @ 400V = 60 ADC counts ! Ch 96 noisy already @ 30V; CMN switched on @80V ! Strip #360: normal leaky strip (no CMN). The results reproduce perfectly the data obtained at UCSB.

  10. General optical inspection Several small irrelevant mechanical damages spotted. The only relevant defect is a scratch on the bias ring (and on one strip-normal in terms of noise-). The scratch is mostly on the inner part of the bias ring. Not conclusive. No significant mechanical scratch on the back of the sensors.

  11. Optical Inspection of the critical strip A suspicious defect on the poly resistor. Considered not relevant.

  12. Study of the mechanical deformations The z positions of the two sensors have been measured under a CMM with the module lying free on the granite table. The spread of the measurement points is several hundreds mm. No evidence of mechanical stress on the sensors similar to deformations due to vacuum.

  13. Behavior with humidity and time CMN CMN Measurements done using dry air in the test box (1% humidity APV ON) and high environment humidity (in the probe station APV OFF and in the test box APV ON). Some trend toward a reduction of the leakage current with time at high humidity but still a factor 2/3 higher and CMN still present in data. Basic independence of CMN from humidity and time.

  14. Study of different ground connections A direct connection to ground of the silicon sensors was used. Special bond to connect the sensors directly to ground. Removed the bonding bias ring-pitch adapter-ground through the hybrid Different shielding schemes used (clamshell/CF plate/support plate grounded or floating). CMN still present in data. Basic independence of CMN from shielding and grounding scheme.

  15. CMN module (@450V; 13,3µA) Residual system noise Zoomed View, red line depicts average system noise CMN due to the leakage current time structure? The same module was studied in Vienna to look for anomaly in the leakage current time structure. CMN present already in data taken at 80V!! Not conclusive.

  16. Study of the interaction sensors-APV25 We then decided to concentrate the attention on the interaction between sensors and read-out electronics at the module level (talk with Lino at CERN at the end of the CMS week). Investigation of the over-metal effect in CMS modules. The issue is particularly important for large pitch thick detectors (OB2). It was found to be critical already during the R&D phase when most of the work was done on HPK multi-geometry structures (see Lino’s talk to review the issue).

  17. + 0.75V 1.5MW 2.2kW R The idea. The input of the APV is slightly positive +0.75V; for normal strip leakage current Ileak<10nA the implant is practically at ground (a few tens of mV for a total Ileak= 10mA ). A metal over-hang at positive potential may induce breakdown.

  18. R Cross-check Measurement points + 0.75V 1.5MW 2.2kW We measured the potential difference of the two points during module operation using special micro-bonding to external wires.

  19. Measurement of the bias-ring voltage

  20. Measurement of the input voltage of the APV Several APV input channels measured during normal operations with different detector leakage current. Results consistent with expectation: DV=0.75V +- 40mV

  21. R The measurement set-up By using an external variable resistor to connect the sensors to ground we can exploit the total sensors leakage current to increase the potential of the bias ring. Resistors from 100kW to 7MW were used. The CMN should disappears when restoring the correct potential difference between metal strip and implant.

  22. CMN vs Vbias@ R= 0MW Strip noisy @ 30V and CMN switched on @ 80V

  23. CMN vs Vbias@ R= 2MW Strip noisy @ 150V and CMN switched on @ 350V

  24. CMN vs Vbias@ R= 5MW Strip noisy @ 350V and NO CMN UP to 500V

  25. Summary table

  26. CM subtracted noise vs Vbias and R

  27. Normal behavior of the entire module Strip 96 is now a normal “leaky strip”. The module is a grade A module.

  28. Normal response to Led & Pulse Shape Fiber spot

  29. IV vs R The most impressive evidence is the strong dependence of the total sensor current from the small potential applied between implant and metal. A factor 7 reduction in the leakage current which is now compatible with QTC data.

  30. Why a so small potential difference can affect the sensors ? • Because it affects the field distribution at the edge of the implants. • The metal over-hang at a positive potential with respect to the implants favors the breakdown.

  31. Metal over-hang Metal lines 4-8mm wider than the corresponding implant strips (metal over-hang) are adopted to increase the breakdown performance. We expect higher fields at large pitch & small w/p. Over-metal moves the high field region from Si to SiO2 Vbrk(Si) = 30 V/m Vbrk(SiO2) = 600 V/m BUT THE METAL SHOULD BE AT THE SAME OR LOWER POTENTIAL WITH RESPECT TO THE IMPLANT OTHERWISE IT COULD BE DANGEROUS

  32. I 10mA 10nA V Metal over-hang at positive V The high density of field lines at the implant edges yields to breakdown. The strip leakage current increases of orders of magnitude. A current of a few mA flowing through the poly resistor (1.5MW) increases the strip potential to values higher than the metal strip. We jump to the following case.

  33. I 10mA 10nA V Metal over-hang at negative V The density of field lines decreases and the strip exits from breakdown. 10 nA flowing through the poly resistor (1.5MW) are not able to maintain the implant potential positive with respect to to the metal. We step back to the previous situation.

  34. Huge CMN: intermittent breakdown The previously described mechanism can explain why this “leaky strip” is so different. A “normal” breakdown (10mA through a single strip) is not sufficient to account for these effects (60 ADC rms noise and CMN on the chip). It is not a normal breakdown it is an intermittent phenomenon. To study the time evolution of the process we analyzed the data taken on the module in different conditions..

  35. Data taking in multiframe-mode Huge variations (oscillations) in the noise behavior. Silent periods followed by explosions.

  36. Time evolution of the CMN strip #96 #95 and #97 #50 #106 Neighboring strip (95/97) and far away strips (50/106) plotted for comparison

  37. Comparison with a normal TIB leaky strip TIB TIB Noise amplitude distribution. TEC 0MW TIB

  38. Area vs Dt and number of flips TEC@0MW TIB TEC@0MW TIB

  39. TEC plots @ R=1MW Amplitude Area vs Dt Un-distinguishable from TIB in all variables. Number of flips

  40. Simulation To understand better the mechanism in our devices, we have asked STM to perform a detailed simulation for OB2 devices (need of using the appropriate doping profiles). The exercise was done for two different oxide charge density (roughly corresponding to Flat Band Voltage of 1V-recent productions- and 5V-old sensors-). Details in Lino’s presentation.

  41. Electric field (detailed view) Qox=1.5·1011 charge/cm2 Case D Case C Aluminum Oxide

  42. Electric field across the implant Case D Case C Case B Case A The combined effect of oxide charge density and positive voltage on the metal may account for more than a factor 2 increase of the peak. A few percent increase may account for a small probability effect.

  43. Consistency with other observations • The effect escaped our QTC measurements because in the standard set-up the metal is floating (see Alberto’s talk). • The appearance of CMN is more likely in old (high flat band) and lower quality (B-C grade) sensors (the sensor grading could be already an indication that some strips are close to breakdown). • In OB2 should be more likely than in OB1. • Observations by F. Hartmann (effect reduced after irradiation). • Observations by T. Affolder (effect disappeared if the AC metal is connected to ground). • Observations by M. Poettgens (increase of the sensors leakage current only when connected to the electronics). • Remember that we are dealing with a small probability effect: • 5% of the modules exhibiting CMN means 2.5% of the sensors, or 2-3 strips over 51.200 (less than 10-4).

  44. Actions (1) • I would require the technical endorsement of the sensor group to recommend a list of actions to be approved by the TSC/TIB. • Review and repeat our measurements on more CMN modules; increase the statistical significance of our tests; add further tests. • Implement/optimize a new procedure of testing with the metal fixed at some positive potential (+5V ?) to identify potentially weak strips (Alberto’s talk) and operate a screening. • Discuss with STM the possibility to adopt the same procedure for the 1000 sensors to be delivered as new pre-series before using them to build modules.

  45. Actions (2) • 4) Use ourselves the same screening procedure on the sensors already delivered that could be probably used without any risk for CMS. • 5) Perform a stress test on the several hundreds of good TOB-TEC modules already built. The test could be done by biasing the APV with respect to a virtual ground brought at a positive potential of a few volts with respect to the sensor ground. • Cure the CMN modules by simply removing the connection to the APV of the noisy strip and bonding it to ground. • Study feasibility (and drawbacks) to use this feature to increase in general the breakdown performance (or the lifetime) of the tracker by means of a NEGATIVE BIAS APPLIED TO THE OVERMETAL (APV INPUTS). We have discovered a new way of operating AC coupled silicon detectors for extended breakdown performance

  46. Conclusions As usual the work never ends!

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