1. Static Surface Charges on Differently Passivated Silicon Strip Sensors Axel König, HEPHY11 th Trento Workshop, LPNHE Paris

2. 1Axel König23.02.2015 Content Introduction Sensor prototyping with Infineon The defective area of strips First occurrence Affected sensor parameters Connection to static charges Presumable mechanism Reproducing the defective area of strips Quantization of the threshold potential Mapping of surface charges Differently passivated sensors of Infineon Sensors of different vendors Summary and Conclusion

3. 2Axel König23.02.2015 Introduction Since 2009, HEPHY and Infineon are collaborating on the development of silicon strip sensors Started with p-in-n sensors on 6’’ wafers Continued with n-in-p sensors on 8’’ wafers (currently under investigation) Main Goal: Prototype sensors suitable for todays and future LHC experiments Basic sensor specifications AC-coupled strips Biasing via polysilicon resistors 8’’ n-in-p wafer of Infineon 6’’ p-in-n wafer of Infineon

4. 3Axel König23.02.2015 The defective area of strips Nearly all sensors of the 6’’ run were electrically characterized Global parameters: IV and CV Single strip parameters Single strip current I strip Polysilicon resistance R poly Coupling capacitance C ac Current through the dielectric layer I diel Area of defective strips was observed Very narrow for first 6’’ batch Much broader for second 6’’ batch So far no defective area observed for the 8’’ run (n-in-p with p-stop or p-spray) Defective area observed in batch 1 Defective area observed in batch 2 1 st 6’’ batch 2 nd 6’’ batch

5. 4 Axel König 23.02.2015 Affected sensor parameters The defective area expresses as follows: Increase in I strip Decrease in R poly Decrease in C ac I diel nearly unaffected Decrease in interstrip resistance R int ➔ shorted strips in a confined region Interstrip resistance measurements only partly realized (very time consuming) Single strip parameters of a sensor showing a defective area of strips. Bottom: Interstrip resistances R int with basic schematics R poly R int

6. 5Axel König23.02.2015 Conncection to static surface charges Extensive investigations at HEPHY and Infineon were conducted Results No defective area for sensors of wafers which are not sawed No defective area for sensors sawed with CO 2 enriched water Bathing in water removes defective area Application of ionizing blower removes defective area All observations are linked to static surface charges responsible for the defective area Sensor sawed with deionized water as lubricant (red) and sensor sawed with CO 2 enriched water as lubricant (green)

7. 6Axel König23.02.2015 Presumable mechanism Structural similarities between silicon strip sensors and field effect transistors (FET) FET: Current flow between source and drain is controlled via a potential applied at the gate Inversion layer “p-channel” (for n bulk) serves as a conductive connection Same mechanism might be responsible for the observed defective area Potential evoked by static charges located on top of the passivation “p-channel” inversion layer might be formed in case of negative surface charges ➔ Low ohmic interconnection of strips resulting in a defective area Schematic cross section of a silicon strip sensor Schematic Working principle of a FET

8. 7Axel König23.02.2015 Reproducing the defective area - equipment Idea: Reproduce the defective area by applying static charges onto a sensors surface Charge application achieved by self made corona discharge device High voltage cascade  up to 30 kV between a needle tip and a grounded chuck Corona discharge happening at the needle tip Application of charges in a confined region possible Further confinement possible by using masks made out of conductive rubber Setup used for charge application Close up of corona discharge needle tip Potential distribution of charged up sample Measured cone of the corona discharge

9. 8Axel König23.02.2015 Reproducing the defective area - results Single strip parameters before and after charge application

10. 9Axel König23.02.2015 Quantization of threshold potential causing the DA Single strip current vs. gate voltage Specially prepared sample with applied “gate” pads

11. 10Axel König23.02.2015 Mapping surface potentials Monroe ESVM model 279L Mapped surface potentials using the ESVM

12. 11Axel König23.02.2015 Removing static surface charges Mapped surface potentials for differently lasting times of inflow. From left to right: initial, after 1 min of ionflow, after 6 min of ionflow, after 21 min of ionflow Ionizing blower (left) and sample (right)

13. 12Axel König23.02.2015 Persistance of static charges – setup and procedure Sample set Initial condition after application of ionizing blower

14. 13Axel König23.02.2015 Persistance of static charges - results Discharge dissolves horizontally along the strips Only one sample exhibits a complete discharge after 5 days ➔ Sample with passivation v2 v6 v5 v4 v3 v2v1

15. 14Axel König23.02.2015 Comparing charge persistance to other vendors (1) Initial condition after application of ionizing blower

16. 15Axel König23.02.2015 Again discharge dissolves horizontally along the strips for all sensors Micron sensors exhibit very a fast charge dissipation Slow charge dissipation for sensors of Hamamatsu and ITE Sensors of Infineon exhibit the same behavior as seen before (passivation v2) Comparing charge persistance to other vendors (2) Mapped surface potentials of sensors of different vendors Micron IFX HPK ITE initial 1 day2 days 3 days

17. 16Axel König23.02.2015 Static surface charges are able to induce a defective area of strips ➔ Importance of ESD safe handling and storage ➔ Not an issue for sensors of Infineon only ➔ Partly performed charge up tests of sensors of other vendors show a similar behavior Surface charges are highly persistent depending on the material used for passivation ➔ No short-term effect ➔ Right choice of passivation becomes more important Application of an ionizing blower speeds up charge dissipation Threshold potential can be determined by specially prepared samples Ongoing and upcoming investigations Influence of static surface charges on n-on-p sensors (p-stop or p-spray strip separation) Determination of threshold potentials for differently passivated sensors Investigate long-term charge up effects due to charge trapping in the passivation Find the optimal passivation regarding the protection against environmental influences and charge dissipation behavior Summary and outlook

18. Thank you for your attention! 23.02.2015Axel König17

19. Backup 23.02.2015Axel König18

20. 19Axel König23.02.2015 Charge up test Infineon n-on-p

21. 20Axel König23.02.2015 Charge up test Hamamatsu n-on-p