1. Charge collection studies with irradiated CMOS detectors

2. 15x6 array of
50x250 um2 pixels
Contacts:
single pixel
all other pixels
Structure B
IR laser beam
Two sets:
unthinned (700 um), no back plane, substrate bias over implant on top
thinned to 200 um, back plane processed, bias through BP

3. Passive test structure B, substrate resistitivity 300 kOhm cm

4. E-TCT, not thinned samples
Bonded:
outmost ring (substrate) – to GND
all n-wells except single
single n-well
 HV connected to n-wells
Charge profiles at different bias
Single n-well read out (others at HV)
Profile width vs. bias

5. E-TCT after irradiation
not thinned samples

6. Neff vs. Phi
Zoom to low
fluences
N_eff smaller than before irradiation not measured
 probably below 1e13, should measure there to get good estimate of acceptor removal constant
linear fit good enough, g_c larger than “usual” value g_c = 0.02 cm-1
similar as previous LFoundry

7. Measure also samples thinned to 200 um samples
full depletion clearly visible on thinned sample

8. 90Sr setup HV-CMOS: small signals, large noise  S/N very low
detector
ORTEC 142 charge sensitive amplifier + custom made shaper (25 ns)
+ digital oscilloscope
90Sr
PMT
ampl.
cooling jig
DUT
HV-CMOS: small signals, large noise  S/N very low
clean sample of events needed (no hits missing DUT)
require a large detector (trigger rate), good collimation, small scintillator
Measurement:
Calibration with a 300 µm thick Si pad detector
1) Record N ( = 2500) waveforms
2) Average over all waveforms and determine time of the signal peak
3) Sample waveforms at the peak
4) Fill spectrum

9. depleted depth from E-TCT ~ 280 um  expected MPV ~ 21000 el
Structure B is small and we can’t collimate so that we measure only events with tracks passing through the detector
but before irradiation and at low fluences signal and noise peak well separated and Landau could be fit
Before irradiation:
MPV = el
260 V
Thinned
Thinned:
Depleted depth ~ 190 um
 expected charge el
Not thinned:
depleted depth from E-TCT ~ 280 um
 expected MPV ~ el
Consistent!

10. After irradiation, Ф = 5e13 n/cm2
MPV = el
MPV = 9000 el
Thinned to ~ 200 um, substrate bias via back plane
depleted depth from E-TCT ~ 180 um (fully depleted)
 expected MPV: ~ el
 measured MPV ~ el
Not thinned, substrate bias from top:
depleted depth from E-TCT: ~ 260 um
 expected MPV ~ el
 measured MPV ~ el
 Large difference between samples with and without back plane after irradiation!
Diffusion neglected (from measurements with AMS structure we expect ~ 1000 el before irradiation (~ 0 el after irradiation))

11. Much smaller charge than expected from E-TCT in samples with top substrate bias
Samples with Back Plane contact as expected

12. Explanation: difference due to different weighting field in back or top bias after irradiation
No back plane, substrate biased via implant on top
Back plane (and thinned), substrate biased via back plane
Collecting electrode: Uw = 1
Collecting electrode: Uw = 1
Depleted region
Depleted region e d D d h
Undepleted bulk
Substrate bias: Uw = 0
Before irradiation:
Undepleted substrate before irradiation low
resistivity (Ohmic) weighting potential Uw = 0
carriers drift across whole weighting field: all charge collected
After irradiation:
substrate resistivity (Ohmic) high
weighting potential 0 at the back plane implant
 if fully depleted D = d full charge collection (except trapping)
 if not fully depleted carriers don’t cross whole weighting field
 charge collection reduced by a factor d/D
 depending on geometry and device thickness this factor
can be much better than in the case of top bias
Before irradiation:
Undepleted substrate before irradiation low resistivity (Ohmic)
weighting potential Uw = 0
carriers drift across whole weighting field: all charge collected
After irradiation:
substrate resistivity (Ohmic) high, weighting potential 0 at the
bias implant on top
 Carriers trapped in low field at the end of depleted depth, before
they reach the substrate bias electrode
 carriers don’t drift across all whole weighting field
 partial charge collection