(towards) Understanding currents of the ROCs W. Erdmann, PSI Pixel - - PowerPoint PPT Presentation

(towards) Understanding currents of the ROCs

• W. Erdmann, PSI

Pixel General Meeting, 2012-12-06

• Introduction
• Radiation effects
• ROC currents overview
• Modelling P5
• Conclusion
• W. Erdmann, PSI

PSI46v2 currents Pixel General meeting 2012-12-06 1

Introduction: PSI46v2 Bandgap Reference 1.2 V digital DAC(VDIG) DACs (VSF, VCOMP, . . . , VANA ) “digital” regulators 2.0V/1.8V/. . . analog regulator ∼1.2 V ROC Digital Power ∼ 2.5 V Analog Power ∼1.5 V

• internal supply and control voltages, contolled by DAC or DAC + regulator

– independent of the external supply voltages – but depend directly on the reference

• different parts of the ROC have different (nonlinear) I(V) curves

some currents depend on more than one voltage, VDIG indirectly affects everything

• W. Erdmann, PSI

PSI46v2 currents Pixel General meeting 2012-12-06 2

(known) radiation effects: transistor thresholds

• L. Hoppenau, 2012

TID effect L1 : 3 × 1012/cm2/fb−1 → 0.1Mrad/fb−1 after 25 fb−1 : 0.75 × 1014/cm2 → 2.5Mrad(25kGy) expect to see (O)(10mV ) shifts Faccio et. al

• W. Erdmann, PSI

PSI46v2 currents Pixel General meeting 2012-12-06 3

bandgap reference drift small effects expected from γ irradation tests (P. Vogler et al 2008) recently re-measured (on Burlington devices, L. Hoppenau 2012) proton irradiated ROCs suggest saturation @10%, before 3e14 cm2 (T.Rohe 2009) new proton irradiation results available soon (1e13 cm2, 230 MeV)

• W. Erdmann, PSI

PSI46v2 currents Pixel General meeting 2012-12-06 4

simulation of selected parts of PSI46v2 (dc-currents, T=10C) I (mA) S(Vbg) (mA/%) S(Vth) (mA/mV) iana 20.660 2.641

• 0.184

Vana idig 20.820 0.666

• 0.065

ic 3.369 0.175

• 0.020

Vcomp, Vcthr, Vana isf 2.235 0.280

• 0.038

Vsf, Vana idacs 2.706 0.032

• 0.001

bus-ana 2.161 0.025

• 0.000

VIbiasBus ibus-dig 1.575 0.021

• 0.000

VIbiasBus ilvds 1.056 0.025

• 0.002

idcol-readout 0.622 0.008

• 0.000

VIBiasOp ireadout 5.275 0.078

• 0.003

VIBiasRO

• just indicative: some ROC parts missing, no clocked current (5 mA), no TBM
• working point dependent (incl. VDIG, T (→ backup))
• iana, isf show largest sensitivity to radiation effects
• W. Erdmann, PSI

PSI46v2 currents Pixel General meeting 2012-12-06 5

voltages

• voltages follow the reference ∆V

V

= ∆Vbg

Vbg

• insensitive to transistor thresholds (open symbols ↔ 10 mV)
• W. Erdmann, PSI

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currents

• much steeper dependence (note the vertical scale), sensitivity varies
• tranistor threshold shifts reduce currents
• analog current and Isf most sensitive
• W. Erdmann, PSI

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currents, absolute

• expect some combination of transistor threshold and bandgap drift
• Isf responsible for ∼ 50% of the digital current changes
• W. Erdmann, PSI

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Analog current

• I(V) curve very sensitive to bandgap drifts
• ∆I/I ∼ O (10) × ∆Vbg/Vbg
• varies ROC-to-ROC (2 examples shown)
• depends on VANA (right figure)
• next slides:

– use iana to fix the bandgap drift (P5)→ predict other effects

• W. Erdmann, PSI

PSI46v2 currents Pixel General meeting 2012-12-06 9

“Source Follower” current, Isf

• suspected contribute significantly to the digital current drift
• buffer for sample/hold circuit in each pixel (4160 per ROC)

several mA contribution to the digital current

• affects pulse-height transfer characteristic (non-linearity)
• first order: current depends on VSF − Vana (PSI46v2)
• where (nominal):

– Vsf = 1000mV + 1000mV × VSF/256 – Vana = 800mV + 700mV × VANA/256 – Isf = f(VSF − 0.7 × VANA)

• W. Erdmann, PSI

PSI46v2 currents Pixel General meeting 2012-12-06 10

PSI46v2 fit to measured ISF measurement looks like f(VSF − 0.57 × VANA) use an (unphysical) fit for modeling

• W. Erdmann, PSI

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Modeling P5 Bandgap drift after 2.4 Mrad γ irradiation too small to explain anything Assuming real drifts are larger, can we explain observations?

• L1

– ∆Vp = 0.28mV/fb−1 (measured) – ∆Vbg/Vbg = 2.2 %/fb−1 (chosen to reproduce 2.9 mA fb)

• L2, L3 ∝ Lr−1.3
• I(V) using fits to measurements
• analog currents readjusted twice

L(fb−1) I (mA) L12 L3 27.6 7 27.2

• 4.5
• 2

15 26.5

• 5
• 3.1

analog current grows with 2.9 .. 2.2e-4 mA fb

• W. Erdmann, PSI

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P5, linear drift (too) small slope change from I(Vana) non-linearity

• W. Erdmann, PSI

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P5, saturated drift same initial ∆Vbg/Vbg slope, but saturation at 10 % (as seen in highly irradiated ROCs) consistent with recent data ???

• W. Erdmann, PSI

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VSF VSF -10 restores initial current in L1 (black circles → red circles) consistent with evolution of the “linearity “ parameter ⇒ Isf changes and Iana changes are consistent with the same bandgap reference drift

• W. Erdmann, PSI

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Conclusion

• ROC currents have several contributions with different responses to radiation
• analog and “source follower” currents are most sensitive
• we don’t really know yet how the reference voltage changes in P5
• the relative sizes of effects appear compatible with being caused by the same reference

voltage shifts

• working towards parametrizing/modeling irradiation induced changes
• W. Erdmann, PSI

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• W. Erdmann, PSI

PSI46v2 currents Pixel General meeting 2012-12-06 17

simulation of selected parts of PSI46v2 (dc-currents, T=25C) I (mA) S(Vbg) (mA/%) S(Vth) (mA/mV) iana 25.180 2.947

• 0.210

Vana idig 21.290 0.693

• 0.064

ic 3.455 0.173

• 0.020

Vcomp, Vcthr, Vana isf 2.654 0.311

• 0.043

Vsf, Vana idacs 2.714 0.033 0.004 ibus-ana 2.158 0.025

• 0.000

VIbiasBus ibus-dig 1.577 0.021

• 0.000

VIbiasBus ilvds 1.036 0.024

• 0.001

idcol-readout 0.626 0.008

• 0.000

VIBiasOp ireadout 5.235 0.077

• 0.003

VIBiasRO

• W. Erdmann, PSI

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