1. Radiation hardness of Monolithic Active Pixel Sensors (MAPS)
D.Berst, J.Bol, W. de Boer M. Caccia, G.Claus, C.Colledani, G.Deptuch,
M. Deveaux, W.Dulinski, G.Gaycken, D. Grandjean, L.Jungermann,
J.L. Riester, M.Winter
Radiation hardness of Monolithic Active Pixel Sensors (MAPS)
Outline:
Operation principle of MAPS
Features of MAPS
Radiation hardness tests on MAPS

2. Readout-electronics and sensors are integrated on the same substrate.
What stands MAPS for?
Monolithic:
Readout-electronics and sensors are integrated on the same substrate.
Active Pixel :
Signal processing microcircuits are integrated in each pixel.
Sensor
MAPS were developed for visible light applications by industry.
MAPS are produced with standard CMOS-processes.
R&D for International Linear Collider VD since IReS/LEPSI
CBM collaboration meeting, GSI Darmstadt, 6-8 Oct 2004, Michael Deveaux

3. The operation principle of MAPS
Sensor design:
P-Well
Diode (N-Well)
A MIP creates ~80e/h pairs per µm in Si
The Active volume (Epitaxial layer) is not depleted.
Charge gets collected via thermal diffusion.
Epitaxial Layer
Substrate
Diode
1-2µm
4-14µm
CBM collaboration meeting, GSI Darmstadt, 6-8 Oct 2004, Michael Deveaux

4. The operation principle of MAPS
Particle trajectory
Preamplifier (one per pixel)
Diffusing free electrons
~20-40µm
CBM collaboration meeting, GSI Darmstadt, 6-8 Oct 2004, Michael Deveaux

5. Classical MAPS-design Leakage currents get compensated
Some simple preamplifiers
3 Transistor Pixel
Self Bias Pixel
Amplifier
(Source Follower)
High resistivity diode
Amplifier
(Source Follower)
Classical MAPS-design
leakage current => Pedestals after CDS
Regular RESET is required
Leakage currents get compensated
No RESET is required
CBM collaboration meeting, GSI Darmstadt, 6-8 Oct 2004, Michael Deveaux

6. The MIMOSA - Technology
Minimum Ionizing Particle MOS Active Pixel Sensor
Features of the MIMOSA (I – VI) – detectors:
Single point resolution 1.5µm - 2.5µm
Typical Pixel – pitch ~20µm
Thinning achieved to 120µm (1 Wafer to 50µm...)
S/N for MIPs 20 – 40
Detection efficiency > 99%
1MPixel sensor „serial“ readout in ~10ms
Produced in various commercial CMOS-Processes
Radiation hardness: several 1011 up to 1012neq/cm²
MIMOSA IV
CBM collaboration meeting, GSI Darmstadt, 6-8 Oct 2004, Michael Deveaux

7. Studies on radiation hardness
Key parameter:
Charge collection
MAPS were calibrated with 5,9 keV (55Fe) Photons.
Produce ~1640 free electrons (same order of magnitude as MIPs)
Very local interaction
Hit in the depleted area.
100% collection efficiency.
Hit in the epitaxial layer. Collection efficiency?
Hit in the substrate close to epitaxial layer.
Sensitive with limited efficiency.
Build a „charge collection spectrum“

8. First studies Mimosa 2 Conclusion on Mimosa 2 after ~ 400kRad:
Peak from epitaxial layer.
Substantial drop in charge collection after irradiation
~ 400kRad
Peak from depleted area.
No change => Readout electro-nics ok.
Underground from substrate
Conclusion on Mimosa 2 after ~ 400kRad:
Leakage currents increases by a factor ~5.
Noise increases by some percent.
Readout electronics OK.
Charge collection drops by ~50% (kills the chip)

9. Two preamplifiers, two different results
Mimosa 2 (MIETEC 0.35)
Mimosa 4 (AMS 0.35)
Amplifier
(Source Follower)
High resistivity diode
Amplifier
(Source Follower)
3 transistor pixel
Self bias pixel
Conclusion:
Charge loss observed
leakage current can be measured
Indication:
No Charge loss
Leakage current cannot be measured
CBM collaboration meeting, GSI Darmstadt, 6-8 Oct 2004, Michael Deveaux

10. A working hypothesis:
Main difference: Presence of an enclosed reset transistor
SiO2 non irrad.
SiO2 irradiated
Reset transistor + positive charge create locally high fields.
P-Well barrier gets depleted.
Parasitic charge collection by the Reset-Transistor.
CBM collaboration meeting, GSI Darmstadt, 6-8 Oct 2004, Michael Deveaux

11. Successor1, a chip to study radiation hardness
SUCCESSOR = Sucima Cmos ChargE SenSOR
Designed within the SUCIMA project (FP5).
Tests: IReS, SUCIMA and GSI.
Process: AMIS 0.35 (Should be similar to MIETEC 0.35)
Design goal: Radiation hard prototype for a medical dosimeter with high spatial resolution.
8 different pixel designs, only 1 discussed here:
33 x 32 pixels
3.2 x 3.2 µm² diode size.
Source and drain of reset transistor swapped
Non uniform pixel pitch (25 and 35 µm steps)
Irradiated: Up to 1MRad X-Rays
Silicon ultra fast cameras for electron and gamma sources in medical applications
CBM collaboration meeting, GSI Darmstadt, 6-8 Oct 2004, Michael Deveaux

12. Successor1 leakage current before and after irradiationy x
Observation:
Leakage current increases by a factor
Leakage currents are very different for identical pixels depending on their position on the chip.
=> Induced heat due to nearby output buffer?
CBM collaboration meeting, GSI Darmstadt, 6-8 Oct 2004, Michael Deveaux

13. Successor1 noise Noise increase depends on the running conditions:
(Lines to guide the eye)
(Lines to guide the eye)
Noise increase depends on the running conditions:
More than factor 2 for +20°C and 2.5MHz sampling (~1ms integration time)
Less than 20% for –15°C and 10MHz sampling (~0.2ms integration time)
Even lower integration time is required for most tracking applications
CBM collaboration meeting, GSI Darmstadt, 6-8 Oct 2004, Michael Deveaux

14. Successor1 charge collection
Successor1 before and after 1MRad X-Rays
Observations (Priliminary):
No significant influence on gain
T = °C
t = ~ 200µs
Charge collected in 1 pixel [ADC]
No significant charge loss
Charge collected in 9 pixels [ADC]
CBM collaboration meeting, GSI Darmstadt, 6-8 Oct 2004, Michael Deveaux

15. Charge loss, being the main problem so far, seems stopped.
Conclusion:
Charge loss, being the main problem so far, seems stopped.
Noise increase can probably be handled.
Evidence that a 1MRad resistant MAPS-detector can be build.
Outlook:
Analysis on Successor1-data has to be refined and completed
Beamtests with similar tracking chip were done.
Results underway.
Long term: Find a way to reduce leakage currents
CBM collaboration meeting, GSI Darmstadt, 6-8 Oct 2004, Michael Deveaux