1. Results from the first diode irradiation and status of bonding tests
P. Riedler, G. Stefanini
M. Fiorini, F. Petrucci
GGT meeting 4/7/2006 PR

2. 1. Irradiation Test
During the run the Si sensor will be subject to radiation damage causing an increase in leakage current and a change in depletion voltage (and also charge trapping).
Leakage current and depletion voltage will also define the time range of operation for the sensor.
Goal of the study:
Irradiate first IRST diodes to different fluences and anneal them measuring leakage currents and depletion voltages = simulate at shorter time scale the evolution of the radiation induced defects.
GGT meeting 4/7/2006 PR

3. Radiation Effects in Sensors Radiation damage in Si sensors
Bulk Damage
Displacement of an Si Atom and creation of a vacancy and an interstitial
Surface Damage
Creation of positive charges in the oxide and additional interface states
Electron accumulation layer
Point like defects (g, electrons)
Cluster Defects (hadrons, ions)
GGT meeting 4/7/2006 PR

4. Radiation damage in Si sensors
Bulk Damage=>Damage to the silicon crystal: Displacement of lattice atoms V I
Vacancy Interstitial
EK>25 eV
Distribution of vacancies created by a 50 keV Si-ion in silicon (typical recoil energy for 1 MeV neutrons):
Defects can be electrically active!
Conduction band
Valence band
+++
Donors
Acceptors
generation
recombination -
compensation
trapping I V
GGT meeting 4/7/2006 PR

5. Radiation Effects in Sensors- Macroscopic Effects
Radiation damage in Si sensors
Bulk Damage (NIEL)
Macroscopic changes:
Change of effective doping concentration
Increase of depletion voltage
influences operating voltage
Increase of leakage current
higher shot noise
thermal stability of the sensor
Increased charge trapping
loss of signal
Surface Damage (TID)
Increase of inter-strip
capacitance (strips!)
Pin-holes (strips!)
The defects also evolve after irradiation = ANNEALING, which is time and temperature dependent
GGT meeting 4/7/2006 PR

6. First sensor irradiation for P326
P-in-n sensors
6 Diodes from 2 IRST sensor wafers (200µm=nominal thickness)
Diodes A (7 mm x 7 mm, multi-guard)
Diodes B (3 mm x 3 mm, multi-guard)
A1: wafer 570-T
A2: wafer 566-B
B1: wafer 570-T, 4 multi
B2: wafer 570-T, 12 multi
B3: wafer 566-B, 4 multi
B4: wafer 566-B, 12 multi1
GGT meeting 4/7/2006 PR

7. Irradiated in Ljubljana TRIGA reactor
All diodes biased at 30V during irradiation
Immediately after irradiation stored in freezer (-20°C) to allow activity to decrease at very slow annealing rate
A1,B1 A2 B4 B3 B2
GGT meeting 4/7/2006 PR

8. Results: Fluence dependent measurements
Measured immediately after removing from freezer,
10kHz, Cp model
Pad current at 200V vs. fluence
Expected behaviour: A1 A1 A2 A2 B4 B4 B3 B2 B3 B2
DIvol=a feq
a = 4.08E-17 A/cm
In good agreement with literature!
GGT meeting 4/7/2006 PR

9. Results: Fluence dependent measurements
Full depletion voltage vs fluence
Expected behaviour: A1 A2 B4 B2 B3
Require more points (diodes) to get precise picture of inversion fluence.
General trend ok
Vfd after 2E14: 294V >> require ~Vfd V to achieve fast enough charge collection for electronics!
GGT meeting 4/7/2006 PR

10. Annealing measurements
Heat samples at 80°C (RD48 standard) for several minutes
>> measure I-V and C-V characteristics
>> repeat procedure several times
Simulates the radiation induced damage that will occur during the run (and also during the off-beam periods) at an accelerated speed.
Based on well-studied model (“hamburg-model”) employed in basically all LHC radiation damage studies on sensors.
Important: precise time and temperature log for each diode.
GGT meeting 4/7/2006 PR

11. Results: Annealing measurements
Leakage current vs. annealing time
Annealing at 80°C
Expected behaviour:
Current decreases and stabilizes
2 min. at 80°C ~ 10days at RT
At ~20°C:
105 µA/pad -> 214 µA/cm2 >> ~190 nA/pixel (300µ x 300 µ)
For a 24 mm x 52 mm sensor: 2.7 mA at RT !!! Cooling!!!
GGT meeting 4/7/2006 PR

12. Current measurement after annealing
A1 (2E14 1 MeV n/cm2)
Annealed for 152 min. at 80°C
Stable current up to 1000 V!
GGT meeting 4/7/2006 PR

13. Next steps:
Continue analysis of annealing data from April with special emphasis on the full depletion voltage.
Some puzzle still in guard and pad I-V behaviour at low voltages. To be discussed and investigated.
Prepare 6 diodes for irradiation at CERN. Due to PS delay and a magnet failure yesterday the irradiation period has been moved to end July/August.
The next diodes first went through a full process at VTT.
Irradiate the 6 diodes with 24 GeV/c protons, anneal them and compare with neutron results from Ljubljana.
GGT meeting 4/7/2006 PR

14. 2. Flip Chip Bonding Tests
Several wafers produced by IRST sent to VTT for processing (see talk in April)
Wafers reprocessed by IRST>> Bow is strongly reduced (see talk by Alberto in April)
Process 2 reprocessed wafers through full production chain using ALICE readout chips
Bump depositon
Bonding of 5 readout chips to 1 sensor
GGT meeting 4/7/2006 PR

15. Processing No problems in bump deposition or dicing.
Initial problems with photoresist track resolved.
Remaining issue:
Dirt observed on all reprocessed wafers during visual inspection (standard procedure) - result of reprocessing.
>> Causes problems during leveling in flip-chip process. Can affect bump yield.
Additional cleaning step by VTT not
successful.
Not expected to be a problem
for future wafers.
VTT/S.Savolainen-Pulli
GGT meeting 4/7/2006 PR

16. 3 ALICE type ladders produced from wafer No. 6 (reprocessed)
Tested according to ALICE production criteria
Results - Total leakage current
GGT meeting 4/7/2006 PR

17. Results - Bump bonding yield
Defects mainly located in corners<<effect of leveling problems due to dirt on sensor back side
Chip2/vtt226 not functional
ALICE limit: <82 defects/chip
Average production yield: 5 defects/chip
GGT meeting 4/7/2006 PR

18. Leveling problems due to dirt should not reappear.
Next steps:
Process new IRST wafers with improved bow process and check for bb yield.
Leveling problems due to dirt should not reappear.
New wafers currently being processed by IRST.
Processing by VTT foreseen for August/September.
GGT meeting 4/7/2006 PR

19. GGT meeting 4/7/2006 PR

20. GGT meeting 4/7/2006 PR