1. Rint Simulations & Comparison with Measurements
Ranjeet , Ashutosh Bhardwaj, Kirti Ranjan
Center for Detector & Related Software Technology (CDRST)
Department of Physics and Astrophysics,
University of Delhi (DU), Delhi, INDIA
24 May 2013

2. Oxide charge density (Nox) for diff. doses
Nox increases with irradiation dose
then saturates
<100> crystal orientation have lower surface oxide charge density
– Annealing
5x1014cm-2 25MeV proton flux (~ phi_eq= 1e15cm-2) is equivalent to dose 1.4MGy which can produce Nox  1.5x1012cm-2 (M. Moll)
Doses ~ 0.1 MGy is capable of introducing Nox ~ 1x1012cm-2 .
J.Zhang et al., arXiv: (2012)

3. Strip isolation @ different doses - measurements
Loss of strip isolations can be estimated by measurements using normal strip sensors or Test structures (DC-CAP)
Measurements at normal strip sensors (strips connected to bias rings through ~ 1MOhm bias resistors) - Y. Unno et.al. (NIM A 579, 614–622)
- The isolation of the n+ strips was characterized by measuring the current between a pair of n-strips, the ‘‘inter-strip current’’, when a voltage of 5V was applied between the pair. When the isolation resistance is larger than the sum of the bias resistors of the pair, i.e., 3M, the inter-strip current levels off at 1.6 µA. The maximum current (1000µA) was limited with an external resistor of 5 k, showing no isolation.
1000µA current
-indicate no strip Isolation

4. Measurement Circuit for Rint for MSSD (Maria’s Thesis)
Measurements on special test structures DC-CAP
As a polysilicon resistance would distort the interstrip resistance measurement, the strips at CAP DC are isolated.
DC-Cap test structure were used to measure Rint. These test structures do not contain Polysilicon resistors and strips are isolated from bias ring. Small bias is given to Central DC Electrode while two neighboring Electrodes are shorted together. Reverse bias is provided from backside electrode while DC external resistance value is not known.

5. Rint Simulations - For unirradiated and Photon irradiated sensors

6. Simulation of Rint for MSSD with Double P-stops
For p-type of sensor, three strips structure was used for Rint simulations in which bias of 1V is given to Central DC Anode while two neighboring Anodes are shorted together. Reverse bias is provided from cathode (not shown), below while a very low DC external resistance of 1Ω is used to avoid scaling confusion.

7. Simulation of Rint – Effect of external bias
Rexternal = 2e6 ohm
Rexternal = 1e6 ohm
External bias resistor in simulation decide lowest possible Rint in case of No Strip Isolation condition.
But, How to decide the proper scaling for strip length ?
To avoid confusion simulations were performed for very low external bias resistance (1Ω)

8. Simulation of Rint – Effect of carrier life time
Tau0 = 1e-4 sec
Rint
(ohm-um)
-Log scale
Tau0 = 1e-5 sec
Tau0 = 1e-6 sec
Carrier life time strongly affect currents in un-irradiated sensors, so, Rint is also affected by change in Tau0
Experience with Diodes leakage current simulations – Very significant variations in current for different diodes (This imply large variation of carrier life times for different diode samples)
Can not hope to simulate all diode leakage current, with one Tau0
Similarly, we can not hope to exactly match all Rint measurements !
But Qualitative information about Rint is not affected by Tau0 variation !!

9. Rint Vs Vbias Curve for p-type – Qualitative information
(Maria’s Thesis)
Good strip isolation, which further improved just after few volt bias
- Indicate low Qf
DC external resistance
value is not known.
Good strip isolation, which further improved after hundred volt bias
- Indicate bit higher QF
Bad strip isolation, which does not improve even after hundreds of bias voltage
- Indicate higher QF
Different QF may be responsible for different features in Rint plots for p-type sensors
For n-type sensors, Very good Rint is expected for all QF and all bias points (unless breakdown occurs)

10. Simulation Parameters –
1.Substrate Doping Conc. (NB) = 3.4x1012 cm-3 2. Pitch =90 µm
3. Strip implant width = 18 µm (Doping depth = 2.2 µm)
4. Double Pstop (doping = 1e16 cm-3, 5e15 cm-3 (in HPK sensors)
Doping depth = 1.6 µm , width = 6 µm , Separation = 30 µm ) 5. Temp = 21 deg C corresponding to 294 K. 6. Backplane implant of 33 µm
7. Strip length = µm
8. External Bias resistor = 1 Ohm
9. Tau0 = 1e-4 sec
Phase 2 Sensors upgrade

11. Rint Vs Vbias Curve – Qf variation ( p-type)
DC external resistance = 1 ohm
P-stop doping = 1e16 cm-3
P-stop doping = 5e15 cm-3
Rint improves for QF=5e10cm-2 (~ 550V)
Similar, qualitative features for simulated plots
For low values of QF , good strip isolation, which improve at progressively higher reverse bias
For intermediate values of QF , strip isolation is very poor for low biases but improve at higher biases
For higher values of QF , Rint remain very low even at higher reverse bias
Loss of strip isolation, for lower Pstop doping results for lower QF
Phase 2 Sensors upgrade

12. e- conc plot for QF = 5e10cm-2 , 500V and 600V
High e- conc between pstops
Very low e- conc ensure
good strip isolation
High electron conc . between P-stops for voltages 500V or lower than that, may lower Rint between n+ strips.
At higher voltages (>550V) electron conc . between P-stops is significantly lowered which further improve Rint between n+ strips.

13. e- conc plot for Qf = 6e11cm-2 at 400V & 600V
e- layer exist under Pstops also
No e- layer under Pstops
e- layer exist under Pstops
600V
400 V
High electron conc . exist even under P-stops leading to very poor Rint at 400V reverse bias.
Electrons are progressively removed by higher leading to good strip isolation at 600V

14. e- conc. plot for Qf = 6e11cm-2 at different reverse bias
High e- conc between pstops
Electron conc . is very low under P-stops leading to good Rint at 600V reverse bias.

15. Rint comparison (Silvaco Vs Synopsys) ( p-type)
DC external resistance = 1 ohm
Similar, qualitative features for simulation plots
Slight difference for intermediate values of QF
(For Silvaco, QF = 6e11 cm-2, transition from no-isolation to Isolation at ~ 500 V but for Synopsis, QF= 7e11cm-2 transition at ~ 400V)
Phase 2 Sensors upgrade

16. Rint Vs Bias voltage (for n – type)- Log scale
Similar, qualitative features for measurements
Good strip isolation for all values of QF and all biases.

17. Typical Rint measurement (Robert Eber)

18. One of the Rint measurement (Robert Eber)
Simulation
Simulation indicate toward QF ~ 1.2e11 cm-2
Good measurements can be used to predict value of QF using simulations!

19. Rint simulations for hadron irradiated sensors
Simultaneous use of surface damage + Bulk damage model
Preliminary simulation (in process) indicate strong suppression of accumulation layer
Strip isolations was possible at 600V with flux = 5e14cm-2 & QF = 1e12cm-2 !
Further simulations are in progress.

20. Summary / Future Outlook
Simulation study for Rint for p and n-type MSSD’s with different amount of surface damages have been performed.
For p-type MSSD’s Rint simulations for different values of QF for MSSD with double P-stops has been carried out.
Qualitative features in Rint measurements can be reproduced
Simulations can lead to better understanding of Rint
Good Rint plots can be used to predict surface oxide charge density values.
For n-type MSSD’s strip isolation is not a problem.
Initial simulation with simultaneous surface + bulk damage indicate good strip isolations even with higher Nox.
Further simulations are in progress.

21. backups

22. Anode strip currents plot-1 for diferent QF
High e- conc between pstops
Rint variation can be seen for Qf=5e11 and 6e11 cm-2

23. Anode strip currents plot-2 (Zoomed) for diff. QF
High e- conc between pstops
Rint improve for
QF=5e10cm-2
at ~ 550V
Rint improvements can be seen for lower values of QF

24. Rint comparison (Synopsis vs Sentarus) ( p-type)
DC external resistance = 1 ohm
Qf =6e11cm-2
Qf =7e11cm-2
Similar, qualitative features for simulation plots
Good strip isolations for lower values of QF
Slight difference for intermediate values of QF (For Silvaco, QF = 6e11 cm-2, achieve transition from no-isolation to Isolation at ~ 550 V but for Synopsis, Qf = 7e11cm-2 go for this transition at ~ 500V)

25. Nox annealing

26. Rint table