Oxide charge density anomaly in Irradiated HPK MSSD sensors

1. Oxide charge density anomaly in Irradiated HPK MSSD sensors • Ranjeet, AshutoshBhardwaj, KirtiRanjan • Center for Detector & Related Software Technology (CDRST) • Department of Physics and Astrophysics, • University of Delhi (DU), Delhi, INDIA Sensor Simulation Meeting 27 January, 2013

2. Contents • Effects of Oxide charge density (QF) on various properties of p+n-n+ strip sensors • Observed trends for p+n-n+sensors • Effects of QF on properties of n+p-p+sensors • Observed trends for n+p-p+sensors • Measured doping densities of Pspray & Pstop isolations in HPK MSSDs • Trends of InterstripResistence (Rint) for unirradiated sensors • A possible explanation for low value of QF for hadron irradiated sensors

3. Effect of Oxide charge density (QF) for p+n-n+ sensors For N type wafers strips are that of p+ type - Strip isolation will be always ensured unless breakdown - e- accumulation layer due to QF will form a junction with p+ strips Since oxide charge density increases with irradiation, irradiation of sensors should lead to - Higher interstrip capacitance (Cint) - Lower breakdown values (VBD)

4. Observed trends for p+n-n+ HPK sensors ! What we are finding in HPK sensors : - No increment in Cint after irradiation (Cint for irraditaed sensors remain equal to or even lower to that of unirradiated sensors) See Marco Meschini talk , HPK workshop CERN , 21 January 2013 - Increase in Breakdown voltage with Irradiation These observation clearly indicate for a lowering of QF after irradiation Further, simulation of irradiated sensors of (N type wafer) also indicate a very low value of QF, even after irradiation. Cint vs. Vbias for two Qf Simulations only Qf = 1e12cm-2 Comparison of measurement and simulation with Qf = 1e11cm-2 at Flux = 5e14cm-2 Qf = 1e11cm-2

5. Effects of QF on P type of wafer (n+p-p+ sensors) • For P type wafers strips are that of n+ type • - To ensure strip isolation Pstops and Pspray doping are used • For Oxide charge ~2e12cm-2, just below Si/SIO2 interface, with a depth ~ 0.2µm, Accumulation e- density will be 1e17 cm-3 (approximately). To neutralize this Pspray/Pstop doping density should be higher then this. • For Pspray Isolation • For p+n- sensors (Claudio P. IEEE, 2006) have shown that a Pspray doping density ~ 8e16cm-3 is not sufficient to ensure strip isolation for QF = 2e12cm-2 (Similar simulation results were obtained by DU group also, though for n+n-p+sensor) • If Pspray is able to ensure strip isolation, Cint decrease with increase in Oxide charge density (as Pspray is progressively depleted by higher oxide charge density, reducing the effective doping density of Pspray). This conclusion is not valid for very low Pspray doping unable to ensure strip isolation. Further, Breakdown voltage will increase with increase in QF . • These conclusions are supported by measurements also (Sadrozinsky et all,Nuclear Instruments and Methods in Physics Research A 579 (2007) 769–774)

6. For P type of wafer, continued.. • For Pstop Isolations : • If Pstop is able to ensure strip isolation, the Cint increases with increase in QF (as accumulation electron layer act as extension of n+ strip and form wider junction with Pstop). Further, breakdown voltage decreases with increase in QF. • These conclusions have been verified by various simulation studies as well as in experimental studies. • Similarly, Pstop doping density should be greater than 1e17cm-3 to ensure the strip isolation.

7. Pspray/Pstop doping profile measurement No strip isolation is expected with so low value of Pstop and Pspray doping densities after sufficient high irradiation !

8. Measurement trends for n+p-p+ sensors • Measurements for irradiated n+p-p+ MSSDs : • For Pspray doping : Cint do not change much with irradiation • : Breakdown voltage increases with irradiation • For Pstop Doping : Cint do not change much with irradiation • : Breakdown voltage increases with irradiation • (Against the expectations of simulations and X-rays experiments !)

9. Trends of Rint for unirradiated sensors Oxide charge density is very important parameter for Rint…. • For N type wafers strips are that of p+ type • Strip isolation will be always ensured unless • breakdown • p+ strip will make junction with e- layer of • oxide charge Good strip isolation • But higher oxide charge density will lead to • lower breakdown value & higher Cint. • For P type wafers, strips are of n+ • Low Rint for these structure means • Pspray and Pstop doping densities are • not sufficient ! • (Even though the strip sensors were not • Irradiated and measured oxide charge • density is low ~ 5e10 cm-2 , strip isolation is • problem!) Rint values for different unirradiated sensor configurations at 20 C (Maria thesis)

10. Trends of Rint for pspray isolation Low oxide charge density … removed at lower reveres bias Intermediate oxide charge density … Removed ~110V reveres bias Higher oxide charge density … not removed even at higher reveres bias Conclusion : Pspray is very low ( Measured value = 1e15cm-3)…. It can not remove the electron layer (due to positive oxide charge density ~ 5e10cm-2) even for unirradiated sensors. One can not expect to provide strip isolation by this after irradiation .

11. Trends of Rint for Pstop isolation Conclusion : Pstop is also very low (Measured Doping 5e15cm-3)… Do not ensure strip isolations (i.e. is not able to neutralize e- layer due to Oxide Charge Density) even for unirradiated sensors Qf ~ 5e10cm-2)

12. Rint is very high for N-type of wafers…as expected

13. Oxide charges simulations and measurements • Measurement of Oxide charges is always carried out by MOS test structures where no leakage current is flowing. • This do not represent actual operating condition for Si sensors irradiated by hadrons where large leakage current is flowing. But this may be nearly good for x-ray and gamma irradiated sensors as no significant leakage current increment is seen for them • For very high irradiation level, very high oxide charge density (~2-3e12cm-2) in the interface of SiO2/Si will be produced (and have been measured by many groups say Jiaguo Zhang, 18th RD-50 conference) • For TID ~ 200Krad, Sadrozinsky et all (Nuclear Instruments and Methods in Physics Research A 579 (2007) 769–774), measured (using Co-60 gamma irradiation) Qf ~ 2e12cm-2. While for sLHC TID is expected to ~ 100Mrad • So, we expect : • Irradiated sensors in HPK campaign should have very large oxide charge density . • And it should increase with irradiation flux !

14. Pspray and Pstops with doping densities < 5e15 cm-3 (which are not adequate for strip isolations even for unirradiated sensors with QF ~ 5e10cm-2) will be simply useless ! • What we found instead, that for HPK sensors, • Strip isolation was not a problem even at very high irradiation flux 2e15cm-2 ! • Interstrip capacitance simulations & its comparison with measurements (for irradiated N-type of strip sensors) also, clearly, indicate a very low oxide charge density for irradiated sensors ! • Breakdown voltage increases with irradiation • Somehow, Oxide charge density is suppressed for hadron irradiations (and not suppressed in X-ray and gamma irradiations) • Or no surface damage is created by hadron irradiation (unlikely case! Irradiated MOS Measurements, see backup )

15. What is going inside irradiated sensors ! • Irradiation of Si (by hadrons) sensors creates • High leakage current • High leakage current leads to very high • E-fields near strips (or near pixel) • Can we see this….? • Yes by eTCT

16. Measurement of E-field in a irradiated Si strip sensorG. Kramberger et all , 2009, IEEE conference E field profile for a non-irradiated sensors <8000V/cm for reverse bias = 200V (Remember, Higher reverse bias leads to higher electric fields which leads to better removal of e- from the accumulation layer due to QF) E field profile for a irradiated sensors (flux=5e14cm-2) Can be as high as 80000V/cm, near the strips for reverse bias = 200V ! which will force e- due to +ve oxide charge density to do the neutralization or/& removal of it! Very high E field generated due to traps helps in suppression of Qf ! Because of this we never had much problem of strip isolations in hadron irradiation expt!

17. Conclusion : Two type of irradiation • Irradiation with x-rays (as done by many peoples for XFEL) and γ-ray (Co60 is favorite, as done by Sadrozensky et all, M. Moll thesis etc) - Only surface damage is significant, resulting in very high QF (see backup slide) - Leakage current is very low, α is at least three orders of lower and no effect of annealing (Point to very low bulk damage, M.Moll thesis) For this type irradiation : No High electric field near strips : Oxide charge density ~ 2-3e12 cm-2 after some irradiation (in MOS as well as in strips and pixel sensors), leading to very serious problems for isolation , breakdown Cint • Irradiations with p,n or pions (as done in CMS, ATLAS, CDF, D0 and HPK etc) - Significant bulk damage very high leakage current - This leads to very high Electric field near the strips - QF is very low (in actual sensors )….. No problems for strip isolation, Cint and breakdown - QF measured by MOS will be very high as it is measured with MOS structures with no leakage current, leading to no high E field near Si/SiO2 junction (& hence no suppression of QF).

18. How can we test this idea! • Irradiate some HPK sensors with different flux of gamma or X-rays • Measure the different characteristics for them like Breakdown voltage, Cint & Gint etc • This will provide the indications for the oxide charge density variation for different flux • Then, • - Irradiate the same sensors with different flux of hadrons (1e14cm-2 to 1e15cm-2) • Change in different properties will give indication of possible QF suppression. • Suggestions !

19. Backup

20. Effect of irradiataion type on Leakage currentM. Moll, Thesis

21. For European XFEL

24. From Maria thesis, MOS measuremet