GLAST (Gamma-ray Large Area Space Telescope)

1. 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)

2. GLAST (Gamma-ray Large Area Space Telescope) will be launched in 2006 g Array of Silicon Strip Sensor Detect gamma-raythrough e+e-conversion e- e+

3. GLAST prototype sensor single-sided,n-bulk, p-stripAC coupling readout448 strips208 mm strip pitch 9.5cm ↑quarter 9.5cm

4. 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 betweenCrystal Orientations. <111> and <100> Si3N4 Al SiO2 p+strip Si bulk The difference comes from the nature of the SiO2/Si interface. n+ Al

5. Irradiation (HIMAC@NIRS, Japan) Fe ion500MeV/n dE/dx=8GeV/g/cm2 Sensor(in the box)150V bias Absorberto slow downFe ions

6. Iradiated Sensors (4 sensors) Expected dose for 5 years GLAST mission: 1 krad

7. Full Depletion Voltage 111 (410mm) ↑depletion voltage: 100 V 100 (320mm) ↑depletion voltage: 80 V

8. Leakage Current 111 (410mm) ↑full depletion voltage ↑full depletionvoltage 100 (320mm)

9. Leakage Current (strip) leakage current is very uniform (before and after)no dead or noisy channel (before and after) 111(8krad) 100(8krad) after irradiation after irradiation before irradiation before irradiation

10. Leakage Current vs Dose 11122krad 10022krad 1118krad leakage current : thickness×dosegenerated in bulkno difference between 111 and 10010nA/cm2/krad: typicallyexpected for ionizing damage 1008krad

11. Si3N4 Coupling Capacitance Al strip SiO2 Si bulk p+ strip None of the coupling capacitors were broken. No differences between grounded strips and floating strips. 40MW n+ Al +150 V 111(10krad) 100(10krad) Readout strip:grounded Readout strip:grounded after irradiation after irradiation before irradiation before irradiation

12. 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:grounded after irradiation after irradiation before irradiation before irradiation

13. 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.

14. 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%

15. 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 40MWbias resistor +150 V

16. 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

17. @150 V bias voltage 6% down 25% down 26% down 11% down

18. strip leakage cyrrent : 0.1 nA (before irradiation) 45nA (during g-ray irradiation) 0 ～- 60 V 40MWbias resistor (+1.8 V) 45nA×40MW = 1.8 V +150 V

19. ←23% lower ←57% lower ←20% higher ←65% lower

20. Leakage current is generated at theinterface around p+ strip 0 ～- 60 V depletionzone +10 V (full depletion voltage is 60 V)

21. 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 antistatic mat 2 mm think surface resistivity (108W) conducting sheet + 100 V

22. Setup for the g-ray irradiation (60Co) conducting sheet strip 9 - 219

23. Strip leakage current before and after the irradiation 24 nA covered area: strip 9 - 219 8 nA

24. 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.