CBC2: X-ray Irradiation Results Davide Braga, Mark Raymond 6 November 214
HL-LHC dose for CBC2 < 3e+5 Gy = 3 Mrad With x2 safety margin expect to be radhard to >6Mrad NB: calculated for 3 fb -1 but with present Tracker https://twiki.cern.ch/twiki/bin/view/cmspublic/ BRILRadiationSimulation Davide Braga, IC IOP 214 Meeting 2
Total Ionizing Dose test at Diamond X-ray facility Molybdenum tube Wire-bonded CBC2 ( face-up ) CBC2 always biased and operational during irradiation and annealing CBC2 Xray tube Bias/readout Accelerated annealing Irradiation cabinet 3
1st room-temp irradiation 1 chip irradiated to ~12 Mrads no significant change to behaviour apart from: current increase during early stages of irradiation ~ 1 Mrad decays away if stop irradiation effect disappears at higher dose levels so not necessarily a problem, but ma 6 5 4 3 2 1 exponential time constant ~ 2 mins total chip current nominal analogue biases total chip current analogue biases set to zero chips in HL-LHC will be cold - does that affect the behaviour? (the rate current decays away) not clear what is going on and in what part of the chip (no separation between analogue/digital supplies) 3 krad/hr. 2 1 2 4 6 8 12 Mrad 8 4 2 4 time [mins.] 6 8 *https://indico.cern.ch/event/312869/session/1/contribution/7/material/slides/.pdf 4
Cooled setup results 6 ma VDDD and VDDA now supplied separately 5 VDDD current (analog biased) VDDD current (analog off) current increase clearly confined to the digital rail (VDDD current) but magnitude depends on whether analog stages biased or not 4 3 2 VDDA (analog biased) 1 5 VDDA (analog off) 1 15 2 25 current saturates at around 8 krads stopped irradiation at this point irradiation period @ 85 rads/min. time [mins.] current still decays away with time constant ~1 minutes, even at this cold temperature (c.f. ~2 minutes at ~3 o ) NB: 85 rads/min is ~ 85x the worst case doserate at HL-LHC More details in: https://indico.cern.ch/event/332678/session/1/contribution/4/material/slides/.pdf 5
dose-rate [ rads/min.] Cooled setup results 3 ma 25 try a low dose-rate irradiation (new chip) VDDA (analog biased) couldn t get continuous dose-rate low enough as that at HL-LHC* but current plateaus at ~ 4 rads / min. 2 15 1 VDDD current (analog biased) VDDD current (analog off) 5 VDDA (analog off) 2 4 6 8 1 x1 3 4 15 krads in 4 mins. 8 3 ~ 4 rads / min. 6 2 4 1 2 2 4 6 8 1 x1 3 time [mins.] 6 *3 Mrads / (1 years x 2 days x 24 hours x 6 mins) = ~1 rads/min integrated dose [krads]
Summary so far Cooled irradiation setup shows: current increase confined to digital supply rail (VDDD) but magnitude affected by values in analog bias registers (nominal or zero) current is not frozen in by operation at low temperature continuous annealing even at T= -15 C should not be significant at HL-LHC dose-rates Masked Irradiation Demonstrated that increase in digital current is due to static leakage in SRAM memory at high-dose M.Raymond 7
Summary Excess static current due to drain-to-source leakage in pull-down nmos of SRAM cell (non- ELT). Effect of analogue biasing on magnitude of the current explained by the biasing conditions during irradiation Origin of excess leakage and failure mechanisms understood Effect should only be a problem at high-rate: should not cause problems for outer tracker (<3 Mrads over 1 years) but could cause problems at lower radii if CBC were to be used there Currently considering improved and alternative SRAM designs anyway
Higher dose rates 35 ma 3 have now taken 1 chip to ~4 Mrads 25 ~3 krad/hour 2 room temperature irradiation 15 power ~ static after initial digital transient 1 small shift in bandgap ~2 mv 5 VDDD current (analog biased) VDDD current (analog off) VDDA (analog biased) bandgap VDDA (analog off) 7 mv 6 5 4 3 2 1 4 dose [Mrads] 3 2 4 6 8 1 12 x1 3 2 1 2 4 6 8 time [mins.] 1 12 x1 3 9
Analogue bias currents 8 ma 6 5 4 5 5 4 pre-rad 1 Mrads 2 Mrads ~3 Mrads 41 Mrads IPRE1 IPRE2 IPSF IPA IPAOS ICOMP 4 5 4 5 4 3 3 3 3 3 4 2 2 2 2 2 2 1 1 1 1 1 25 25 25 25 25 25 I2C value 1
Analogue bias voltages pre-rad 1 Mrads 2 Mrads ~3 Mrads 41 Mrads mv 9 8 9 9 VPAFB VPC VPLUS 8 8 7 7 7 6 6 6 5 5 5 4 4 4 3 3 3 2 5 1 15 2 2 25 5 1 15 2 2 25 5 1 15 2 25 I2C value 11
VCTH units rms VCTH units S-curves pre-rad 1 Mrads 2 Mrads ~3 Mrads 41 Mrads 3. 2.5 2. 1.5 1. pedestals noise.5 13 s-curves mid-points. 5 1 15 2 25 12 pedestals 3. 2.5 test pulse noise 11 2. test pulse 1.5 1 1..5 9 5 1 15 2 25. 5 1 15 2 25 channel number channel number 12
Conclusions CBC2 shows expected radiation insensitivity to ~ 4 Mrads (4 Mrad limit dictated by availability of source) Irradiation to 4Mrad showed only a short-term increase in current which recovers completely even at T=-15 C annealing after a few hours Modules instrumented with CBC2s can be safely irradiated, at low dose it might be necessary to wait for a few hours before running the chips, at high-dose it should make no difference. 13