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

Oxide charge density (Nox) for diff. doses Nox increases with irradiation dose then saturates <100> crystal orientation have lower surface oxide charge density 80o @10min – 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:1210.0427(2012)

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

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.

Rint Simulations - For unirradiated and Photon irradiated sensors

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.

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)

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 !!

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)

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 = 10000 µm 8. External Bias resistor = 1 Ohm 9. Tau0 = 1e-4 sec Phase 2 Sensors upgrade 16.05.2013

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 16.05.2013

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.

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

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.

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 16.05.2013

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.

Typical Rint measurement (Robert Eber)

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!

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.

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.

backups

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

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

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)

Nox annealing

Rint table