(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 DACs ( VSF, VCOMP , . . . , VANA ) Bandgap digital Reference DAC ( VDIG ) 1.2 V “digital” Digital analog Analog regulators Power regulator Power 2.0V/1.8V/ . . . ∼ 2.5 V ∼ 1.2 V ∼ 1.5 V ROC • 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 × 10 12 /cm 2 /fb − 1 → 0 . 1 Mrad/fb − 1 after 25 fb − 1 : 0 . 75 × 10 14 /cm 2 → 2 . 5 Mrad (25 kGy ) expect to see ( O )(10 mV ) 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 cm 2 (T.Rohe 2009) new proton irradiation results available soon (1e13 cm 2 , 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 = ∆ V bg • voltages follow the reference ∆ V V V bg • insensitive to transistor thresholds (open symbols ↔ 10 mV) W. Erdmann, PSI PSI46v2 currents Pixel General meeting 2012-12-06 6

currents • much steeper dependence (note the vertical scale), sensitivity varies • tranistor threshold shifts reduce currents • analog current and Isf most sensitive W. Erdmann, PSI PSI46v2 currents Pixel General meeting 2012-12-06 7

currents, absolute • expect some combination of transistor threshold and bandgap drift • Isf responsible for ∼ 50% of the digital current changes W. Erdmann, PSI PSI46v2 currents Pixel General meeting 2012-12-06 8

Analog current • I(V) curve very sensitive to bandgap drifts • ∆ I/I ∼ O (10) × ∆ V bg /V bg • 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 V SF − V ana (PSI46v2) • where (nominal): – V sf = 1000 mV + 1000 mV × VSF / 256 – V ana = 800 mV + 700 mV × VANA / 256 – I sf = f ( VSF − 0 . 7 × VANA ) W. Erdmann, PSI PSI46v2 currents Pixel General meeting 2012-12-06 10

PSI46v2 fit to measured I SF measurement looks like f ( VSF − 0 . 57 × VANA ) use an (unphysical) fit for modeling W. Erdmann, PSI PSI46v2 currents Pixel General meeting 2012-12-06 11

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 – ∆ V p = 0 . 28 mV/fb − 1 (measured) – ∆ V bg /V bg = 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 0 27.6 7 27.2 -4.5 -2 analog current grows with 2.9 .. 2.2e-4 mA fb 15 26.5 -5 -3.1 W. Erdmann, PSI PSI46v2 currents Pixel General meeting 2012-12-06 12

P5, linear drift (too) small slope change from I(Vana) non-linearity W. Erdmann, PSI PSI46v2 currents Pixel General meeting 2012-12-06 13

P5, saturated drift same initial ∆ V bg /V bg slope, but saturation at 10 % (as seen in highly irradiated ROCs) consistent with recent data ??? W. Erdmann, PSI PSI46v2 currents Pixel General meeting 2012-12-06 14

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 PSI46v2 currents Pixel General meeting 2012-12-06 15

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 PSI46v2 currents Pixel General meeting 2012-12-06 16

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 PSI46v2 currents Pixel General meeting 2012-12-06 18