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Heavy ion irradiation on
silicon strip sensors for GLAST
&
Radiation hardening of silicon strip sensors
S.Yoshida, K.Yamanaka, T.Ohsugi, H.Masuda
T.Mizuno, Y.Fukazawa (Hiroshima Univ.)
Y.Iwata, T.Murakami (NIRS)
H.Sadrozinski (SCIPP,UCSC)
K.Yamamura, K.Yamamoto, K.Sato (HPK)
GLAST (Gamma-ray Large Area Space Telescope)
will be launched in 2006
g
Array of Silicon
Strip Sensor
Detect gamma-ray
through e+econversion
e- e+
GLAST prototype sensor
single-sided,
n-bulk, p-strip
AC coupling readout
448 strips
208 mm strip pitch
9.5cm
9.5cm
↑
quarter
The aim of the heavy ion irradiation
(1) Investigate radiation damage due to high dE/dx particles.
 slowed-down Fe ions (8GeV/g/cm2 = 5000×MIP)
check items : full depletion voltage, leakage current,
coupling capacitance, interstrip capacitance
(2)Investigate the differece
between Crystal Orientations.
<111> and <100>
Al
Si3N4
SiO2
p+strip
Si bulk
The difference comes from the nature of
the SiO2/Si interface.
n+
Al
Irradiation (HIMAC@NIRS, Japan)
dE/dx=
8GeV/g/cm2
Sensor
(in the
box)
150V bias
Fe ion
500MeV/n
Absorber
to slow down
Fe ions
Iradiated Sensors (4 sensors)
<111>
410mm thick
<100>
320mm thick
Fe ion dose
“8 krad”
111, 8 krad
100, 8 krad
Fe ion dose
“22 krad”
111, 22krad
100, 22krad
Expected dose for 5 years GLAST mission: 1 krad
Full Depletion Voltage
111 (410mm)
↑depletion voltage: 100 V
100 (320mm)
↑depletion voltage: 80 V
Leakage Current
111 (410mm)
↑
full depletion voltage
↑
full depletion
voltage
100 (320mm)
Leakage Current (strip)
leakage current is very uniform (before and after)
no dead or noisy channel (before and after)
111(8krad)
after irradiation
before irradiation
100(8krad)
after irradiation
before irradiation
Leakage Current vs Dose
111
22krad
100
22krad
111
8krad
100
8krad
leakage current : thickness×dose
generated in bulk
no difference between 111 and 100
10nA/cm2/krad: typically
expected for ionizing damage
Al strip
p+ strip
40MW
Si3N4
Si bulk
n+
Al
+150 V
Coupling Capacitance
SiO2
None of the coupling capacitors were broken.
No differences between grounded strips
and floating strips.
111(10krad)
Readout strip:
grounded
after irradiation
before irradiation
100(10krad)
Readout strip:
grounded
after irradiation
before irradiation
Inter strip Capacitance
No differences between before and after the irradiation.
No differences between grounded strips and floating strips.
111(8krad)
Readout strip:
grounded
after irradiation
before irradiation
100(8krad)
Readout strip:
grounded
after irradiation
before irradiation
Conclusion
Full Depletion Voltage:
No significant differences between before and after the irradiation.
Leakage Current:
The increase after the irradiation is as expected from total dose.
The strip current are very uniform before and after the irradiation.
Coupling Capacitance:
None of strip were broken.
Inter Strip Capacitance:
No significant difference between before and after the irradiation.
None of the strips has become insensitive.
No significant differences between <111> and <100>.
No differences between grounded strips and floating strips.
Radiation hardening of silicon strip sensors
(preliminary results)
We focused on surface radiation damage of silicon strip sensors
We used leakage current as the probe for study
Microscopic reason of surface damage (increase of leakage current):
the generation of radiation induced interface traps
Interface trap formation:
Generated holes in SiO2 layer play a important role.
Transport of holes to SiO2/Si interface initiate the formation.
To prevent trasport of holes to SiO2/Si interface, we tried two methods
Method I : the leakage current after irradiation decreased by 26%
Method II: the leakage current after irradiation decreased by 67%
Method I
To collect the holes generated in SiO2 layer,
We applied negative voltage to the readout Al strips
during g-ray (60Co) irradiation
0 ～ - 60 V
40MW
bias resistor
+150 V
Strip No.1
0 0 0 0 0
(V)
0 –2 0
-1 -1
(V)
0 –6 0
-3 -3
(V)
0 –20 0
-10 -10
(V)
0 –60 0
-30 -30
(V)
Strip No.384
The total of 25 readout Al strips were applied negative voltage.
The rest of readout Al strips were floating
@150 V
bias voltage
6% down
25% down
26% down
11% down
strip leakage cyrrent : 0.1 nA (before irradiation)
45nA (during g-ray irradiation)
0 ～ - 60 V
40MW
bias resistor
(+1.8 V)
45nA×40MW = 1.8 V
+150 V
←23% lower
←65% lower
←57% lower
←20% higher
Leakage current is
generated at the
interface around
p+ strip
0 ～ - 60 V
depletion
zone
+10 V (full depletion voltage is 60 V)
Method II
The electric field in the SiO2 layer points toward the surface
The generated holes in SiO2 layer are transported to the surface.
We put conducting sheet on the surface of sensor to collect holes
antistatic mat
conducting sheet 2 mm think
surface resistivity (108W)
+ 100 V
Setup for the g-ray irradiation (60Co)
conducting sheet
strip 9 - 219
Strip leakage current before and after the irradiation
24 nA
covered area:
strip 9 - 219
8 nA
Summary
(1) The leakage current after g-ray irradiation can be reduced 26 % (Method I)
67 % (Method II)
Method I
(2) “-20 V” was the best among 5 trial bias voltage (0, -2, -6, -20, -60 V).
(3) In the case of “-20V”, the leakage current at 10 V bias voltage was 65 %
lower than floating strips.
 interface traps were reduced mainly around the p+ strip
for the sensors having smaller strip pitch, Method I may work effectively.
(4) In the case of “-60V”, the leakage current at 10 V bias voltage was 20%
higher than floating strips
 hole injection from Si bulk due to high electric field?
These results are consistent with the models that :
The main reason of surface radiation damage is due to the holes generated in SiO2
and the subsequent transport of the holes to the SiO2/Si interface.
Method II
We used the antistatic mat as the conducting sheet. (This is just first attempt)
It should be thin coating on SiO2 layer. The material, thickness, resistivity
is the future subject to study.