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) e+e+ e-e-  Array of Silicon Strip Sensor Detect gamma-ray through e + e - conversion will be launched in 2006

GLAST prototype sensor single-sided, n-bulk, p-strip AC coupling readout 448 strips 208  m strip pitch 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/cm 2 = 5000×MIP) check items : full depletion voltage, leakage current, coupling capacitance, interstrip capacitance (2)Investigate the differece between Crystal Orientations. and Al p + strip n+n+ SiO 2 Si bulk The difference comes from the nature of the SiO 2 /Si interface. Si 3 N 4

Irradiation Japan) Fe ion 500MeV/n Absorber to slow down Fe ions Sensor (in the box) 150V bias dE/dx= 8GeV/g/cm 2

410  m thick 320  m thick Fe ion dose “8 krad” 111, 8 krad100, 8 krad Fe ion dose “22 krad” 111, 22krad100, 22krad Iradiated Sensors (4 sensors) Expected dose for 5 years GLAST mission: 1 krad

Full Depletion Voltage 111 (410  m) ↑depletion voltage: 100 V 100 (320  m) ↑depletion voltage: 80 V

Leakage Current 111 (410  m) ↑ full depletion voltage 100 (320  m) ↑ full depletion voltage

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 krad krad 111 8krad 100 8krad leakage current : thickness×dose  generated in bulk no difference between 111 and nA/cm 2 /krad: typically expected 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 strip SiO 2 Si bulk n+n+ Al 40M  p + strip +150 V Si 3 N 4

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 after 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 and. 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 SiO 2 layer play a important role. Transport of holes to SiO 2 /Si interface initiate the formation. To prevent trasport of holes to SiO 2 /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 SiO 2 layer, We applied negative voltage to the readout Al strips during  -ray ( 60 Co) irradiation +150 V 40M  bias resistor 0 ～ - 60 V

(V) 0 –2 0 (V) 0 – (V) 0 – (V) 0 – (V) Strip No.1Strip No.384 The total of 25 readout Al strips were applied negative voltage. The rest of readout Al strips were floating

6% down25% down 26% down 11% V bias voltage

+150 V 40M  bias 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 depletion zone Leakage current is generated at the interface around p + strip

Method II The electric field in the SiO 2 layer points toward the surface The generated holes in SiO 2 layer are transported to the surface. We put conducting sheet on the surface of sensor to collect holes conducting sheet antistatic mat 2 mm think surface resistivity (10 8  ) V

Setup for the  -ray irradiation ( 60 Co) conducting sheet strip

Strip leakage current before and after the irradiation covered area: strip 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 + 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 SiO 2 and the subsequent transport of the holes to the SiO 2 /Si interface. Method II We used the antistatic mat as the conducting sheet. (This is just first attempt) It should be thin coating on SiO 2 layer. The material, thickness, resistivity is the future subject to study.