1. Characterization of prototype BTeV silicon pixel sensors before and after irradiation
Maria Rita Coluccia
Simon Kwan
Fermi National Accelerator Laboratory
2001 IEEE Nuclear Science Symposium, San Diego 3-10 Nov
Friday, Nov 8, 2001

2. Maria R. Coluccia - Fermilab
Outline
BTeV SINTEF pixel sensor prototypes
Proton Irradiation at IUCF
Characteristics before and after irradiation
Conclusions
Maria R. Coluccia - Fermilab

3. BTeV SINTEF silicon pixel sensor prototypes tested
n+/n/p configuration that allows them to operate
partially depleted
Sensor thickness: 270 um
Low resistivity material: KOhmxcm
P-stop electrode isolation technique
Oxygenated and non-oxygenated wafers
Various guard ring configurations
Maria R. Coluccia - Fermilab

4. Maria R. Coluccia - Fermilab
P-stop sensor
Common p-stop
Individual p-stop
p implant
n implant
n implant
p implant
gap between n and p
gap between n and p
gap between adjacent p
bump pad
Maria R. Coluccia - Fermilab

5. Summary of the implant widths and gaps
We tested two different pixel arrays:Test cell sensors (12x92 cells) and FPIX1 sensors (18x160 cells)
Maria R. Coluccia - Fermilab

6. P-side guard ring designs
Three different designs.
For the test cell sensors:
10 guard rings
18 guard rings
For the FPIX1 sensors:
11 guard rings
metal
active area
p-implant
10 GR
11-18 GR
Maria R. Coluccia - Fermilab

7. I-V curves before irradiation for standard SINTEF test cell sensors
7 wafers tested
A few sensors had bad performance (high leakage current, low
breakdown voltage) but this doesn’t depend on the p-stop layout
Maria R. Coluccia - Fermilab

8. I-V curves before irradiation for oxygenated SINTEF test cell sensors
Maria R. Coluccia - Fermilab

9. I-V curves before irradiation for standard SINTEF FPIX1 sensors
For all these sensors we have a Vbreak of 300 V.
This is due to the different p-implant width:
For FPIX1_SIP (single individual p-stop) the gap between 2 adjacent p-stop rings is 3 um compared to 5 um in the test cell sensors
For FPIX1_SCP (single common p-stop) the p-implant width is 3um compared to 9um in the test cell sensors
Maria R. Coluccia - Fermilab

10. Breakdown Voltage Distribution
Very high values (700 V) without significant differences
between individual and common p-stop sensors and
between oxygenated and standard sensors
Very high yield for the SINTEF wafers
Maria R. Coluccia - Fermilab

11. Maria R. Coluccia - Fermilab
Irradiation test at IUCF (Indiana University Cyclotron Facility) with a 200 MeV proton beam
2 test cell standard sensors (individual and common
p-stop) with 8 x 1013 p/cm2
4 FPIX1 sensors with 2 x 1014 p/cm2 : 2 oxygenated
(individual and common p-stop) and 2 standard (individual
and common p-stop)
4 test cells sensors with 4 x 1014 p/cm2: 2 oxygenated
Irradiation was done in air at room temperature.
After irradiation the tested devices have been kept at –15 oC
Maria R. Coluccia - Fermilab

12. Leakage current: temperature dependence
Fluence: 8 x 1013 p/cm2
After irradiation Ileak significantly increases, but the problem associated with the large current can be minimized by operating at reduced temperature.
Maria R. Coluccia - Fermilab

13. Leakage current: fluence dependence
Maria R. Coluccia - Fermilab

14. I-V curves after irradiation with various fluences
standard sensors
oxygenated sensors
We see no breakdown Voltage below 500 V
for the test cell sensors.
Maria R. Coluccia - Fermilab

15. Capacitance: temperature and frequency dependence after irradiation
Individual p-stop sensor (10 GR)
fluence: 4 x 1014 p/cm2
23 oC
40 KHz
A logarithmic change in frequency gives the same pattern of C-V’s as a linear change in temperature.
Maria R. Coluccia - Fermilab

16. Maria R. Coluccia - Fermilab
Depletion Voltage
No difference between oxygenated and standard sensors observed!
Maria R. Coluccia - Fermilab

17. Guard Rings: Voltage Distribution Before and After Irradiation
FPIX1_SCP oxygenated (11 GR)
before
after
Innermost guard ring
Innermost guard ring
29 V
Measurements performed with the innermost guard ring floating.
We have a potential drop across the device edges after type inversion.
Maria R. Coluccia - Fermilab

18. Maria R. Coluccia - Fermilab
Conclusions
Excellent results for the SINTEF sensors
Very high yield
No significant difference between common and
individual p-stop layout
Important effects introduced by different p-
implant widths
No difference between oxygenated and standard
sensors before and after irradiation for SINTEF
sensors
More investigations needed for the guard ring
structures
Next step: to study performance of the sensors bonded
to ROC and charge collection efficiency before and after
irradiation
Maria R. Coluccia - Fermilab

19. Maria R. Coluccia - Fermilab
SINTEF wafer layout
We tested two different
pixel arrays:
1)Test cell sensors (12x92 cells)
2) FPIX1 sensors (18x160 cells)
Maria R. Coluccia - Fermilab

20. Maria R. Coluccia - Fermilab
After Dicing
We diced several wafer:
Some sensors present different
result after dicing (high Ileak, low Vbreak)
Cleaning carefully the surface and
the edges of the sensors we can
restore the performances that we had
before
All the sensors with three guard rings
present performances degradation
after dicing
Maria R. Coluccia - Fermilab