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Rint Simulations & Comparison with

Measurements

Ranjeet , Ashutosh Bhardwaj, Kirti RanjanCenter for Detector & Related Software Technology (CDRST)

Department of Physics and Astrophysics,University of Delhi (DU), Delhi, INDIA

24 May 2013

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Oxide charge density (Nox) for diff. doses

80o @10min – Annealing

-Nox increases with irradiation dose then saturates-<100> crystal orientation have lower surface oxide charge density

-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)

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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

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Measurement Circuit for Rint for MSSD (Maria’s Thesis)

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.

Measurements on special test structures DC-CAP-As a polysilicon resistance would distort the interstrip resistance measurement, the strips at CAP DC are isolated.

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Rint Simulations- For unirradiated and Photon irradiated

sensors

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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.

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Simulation of Rint – Effect of external bias

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)

Rexternal = 2e6 ohm

Rexternal = 1e6 ohm

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Simulation of Rint – Effect of carrier life time

Carrier life time strongly affect currents in un-irradiated sensors, so, R int 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(ohm-um)-Log scale

Tau0 = 1e-4 sec

Tau0 = 1e-5 sec

Tau0 = 1e-6 sec

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Rint Vs Vbias Curve for p-type – Qualitative information (Maria’s Thesis)

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)

Good strip isolation, which further improved just after few volt bias- Indicate low Qf

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

DC external resistance value is not known.

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1.Substrate Doping Conc. (NB) = 3.4x1012 cm-3

2. Pitch =90 µm3. 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 µm8. External Bias resistor = 1 Ohm9. Tau0 = 1e-4 sec

Simulation Parameters –

Phase 2 Sensors upgrade 16.05.2013

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Rint Vs Vbias Curve – Qf variation ( p-type)

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 biasesFor 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

Rint improves for QF=5e10cm-2 (~ 550V)

DC external resistance = 1 ohmP-stop doping = 5e15 cm-3 P-stop doping = 1e16 cm-3

Phase 2 Sensors upgrade 16.05.2013

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e- conc plot for QF = 5e10cm-2 , 500V and 600V

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.

High e- conc between pstops Very low e- conc ensuregood strip isolation

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e- conc plot for Qf = 6e11cm-2 at 400V & 600V

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- layer exist under Pstops400 V

600V

e- layer exist under Pstops also No e- layer under Pstops

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e- conc. plot for Qf = 6e11cm-2 at different reverse bias

Electron conc . is very low under P-stops leading to good Rint at 600V reverse bias.

High e- conc between pstops

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Rint comparison (Silvaco Vs Synopsys) ( p-type)

Similar, qualitative features for simulation plotsSlight 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)

DC external resistance = 1 ohm

Phase 2 Sensors upgrade 16.05.2013

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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.

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Typical Rint measurement (Robert Eber)

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One of the Rint measurement (Robert Eber)

Simulation indicate toward QF ~ 1.2e11 cm-2

Good measurements can be used to predict value of QF using simulations!

Measurement

Simulation

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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.

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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.

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backups

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Anode strip currents plot-1 for diferent QF

Rint variation can be seen for Qf=5e11 and 6e11 cm-2

High e- conc between pstops

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Anode strip currents plot-2 (Zoomed) for diff. QF

Rint improvements can be seen for lower values of QF

High e- conc between pstops

Rint improve forQF=5e10cm-2

at ~ 550V

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Rint comparison (Synopsis vs Sentarus) ( p-type)

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)

DC external resistance = 1 ohm

Qf =6e11cm-2

Qf =7e11cm-2

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Nox annealing

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Rint table

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