heavy ion irradiation on silicon strip sensors for glast & radiation hardening of silicon strip...
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Heavy ion irradiation onsilicon 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)
e+e-
Array of Silicon Strip Sensor
Detect gamma-raythrough e+e- conversion
will be launched in 2006
GLAST prototype sensorsingle-sided,n-bulk, p-stripAC coupling readout448 strips208 m 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
Al
p+strip
n+
SiO2
Si bulk
The difference comes from the nature of the SiO2/Si interface.
Si3N4
Irradiation (HIMAC@NIRS, Japan)
Fe ion500MeV/n
Absorberto slow down
Fe ions
Sensor(in the box)
150V bias
dE/dx=8GeV/g/cm2
<111>410m thick
<100>320m thick
Fe ion dose“8 krad” 111, 8 krad 100, 8 krad
Fe ion dose“22 krad” 111, 22krad 100, 22krad
Iradiated Sensors (4 sensors)
Expected dose for 5 years GLAST mission: 1 krad
Full Depletion Voltage
111 (410m)
↑depletion voltage: 100 V
100 (320m)
↑depletion voltage: 80 V
Leakage Current
111 (410m)
↑full depletion voltage
100 (320m)↑full depletionvoltage
Leakage Current (strip)
leakage current is very uniform (before and after)no dead or noisy channel (before and after)
after irradiation
before irradiation
after irradiation
before irradiation
111(8krad) 100(8krad)
Leakage Current vs Dose
11122krad
10022krad
1118krad
1008krad
leakage current : thickness×dose generated in bulkno difference between 111 and 10010nA/cm2/krad: typicallyexpected for ionizing damage
Coupling Capacitance
111(10krad) 100(10krad)
None of the coupling capacitors were broken. No differences between grounded strips
and floating strips.
Readout strip:grounded
after irradiation
before irradiation
Readout strip:grounded after irradiation
before irradiation
Al stripSiO2
Si bulk
n+
Al
40M
p+ strip
+150 V
Si3N4
Inter strip Capacitance
No differences between before and after the irradiation. No differences between grounded strips and floating strips.
111(8krad) 100(8krad)
Readout strip:grounded
Readout strip:groundedafter irradiation
before irradiation
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 sensorsWe 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 -ray (60Co) irradiation
+150 V
40Mbias resistor
0 ~ - 60 V
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.1 Strip No.384
The total of 25 readout Al strips were applied negative voltage. The rest of readout Al strips were floating
6% down 25% down
26% down11% down
@150 Vbias voltage
+150 V
40Mbias resistor
0 ~ - 60 V
strip leakage cyrrent : 0.1 nA (before irradiation)45nA (during -ray irradiation)
45nA×40M = 1.8 V
(+1.8 V)
←23% lower ←57% lower
←65% lower ←20% higher
+10 V (full depletion voltage is 60 V)
0 ~ - 60 V
depletionzone
Leakage current isgenerated at theinterface aroundp+ strip
Method II The electric field in the SiO2 layer points toward the surfaceThe generated holes in SiO2 layer are transported to the surface.We put conducting sheet on the surface of sensor to collect holes
conducting sheetantistatic mat2 mm thinksurface resistivity (108)
+ 100 V
Setup for the -ray irradiation (60Co)
conducting sheet
strip 9 - 219
Strip leakage current before and after the irradiation
covered area:strip 9 - 219
8 nA
24 nA
Summary
(1) The leakage current after -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+ stripfor 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 IIWe used the antistatic mat as the conducting sheet. (This is just first attempt)It should be thin coating on SiO2 layer. The material, thickness, resistivityis the future subject to study.