Development of radiation hard microstrip detectors for the CBM Experiment

1. Development of radiation hard microstrip detectors for the CBM Experiment Sudeep Chatterji GSI Helmholtz Centre for Heavy Ion Research DPG Bonn 16 March, 2010

2. CBM Silicon Tracking System Layout Fluka Simulation 1m • STS has 8 stations of DSSDs, Strip Pitch ~ 58 mm, Strip Width ~ 20 mm, stereo angle ~ 7.5 0 • Expected Radiation Damage ~ 1 x 1014 neq cm-2 year-1 • Need detailed simulations to optimize device parameters to maximize VBD and minimize Noise. • Also simulation needed to understand the impact of radiation damage.

3. 3-Dimensional Grid • 3-D TCAD simulation tools “SYNOPSYS” • Sub packages • Sentaurus • Inspect • Tecplot • SPICE (Mixed Mode) x set by strip pitch y set by stereo angle

4. Stereo Angle in Strips X-Y plane of the 3D grid. One can see there is a stereo angle on either side of 7.50.

5. Understanding the Geometry • Using SYNOPSYS TCAD 3-D simulation package • Silicon volume ~ Cuboid (Six Rectangular faces). • Dopant Implants, P-Stop and Contacts ~ Parallelogram • Oxide ~ Cuboid, Enough space needed at the corners for junction curvature (0.8*Junction Depth)

6. Determination of full depletion voltage c

7. Determination of full depletion voltage

8. Potential & Electric Field Distribution 0 V 50V

9. Some Static Characteristics • CTotal = Cback+2*Cint • ENC αCTotal • Optimization needed to maximize breakdown voltage & minimize ENC

10. Radiation Damage in Silicon • The major effect expected from bulk damage is the change in the effective • carrier concentration (Neff) leading to Type Inversion. • The change in Neff is parameterized using Hamburg model: • The change in Minority carrier lifetime is parameterized using Kraners model: • For high quality oxide, the value of surface oxide charge (Qf) is expected to be • 3e11cm-2 (for non-irradiated detector) while after irradiation Qf increases and • saturates at about 1e12 cm-2. Flatband Voltage gives an approx. of Qf.

11. Type Energy (eV) Trap σe (cm2) σh (cm2) η (cm-1) Acceptor Ec-0.42 VV 2.0*10-15 2.0*10-14 1.613 Acceptor Ec-0.46 VVV 5.0*10-15 5.0*10-14 0.9 Donor Ec+0.36 CiOi 2.5*10-14 2.5*10-15 0.9 Impact of Radiation Damage • VBD↑ with fluence • Current ↑ by 3 orders • Rint ↓ with fluence • Detailed study needed Trap Model, University of Perugia

12. Transient Simulations • Can simulate the passage of Heavy Ion, α-particle and Laser. • Can include angle in the passage of MIPs. • Plan to do complete scan of interstrip region and compare with test beam data.

13. Summary • We need radiation hard Double Sided Silicon Strip detectors. • TCAD simulation package, SYNOPSYS has been installed on • CBM batch farm and running. • We are doing Mixed Mode simulation using SPICE models • available in Sentaurus. • Have procured the Probe Station. • Plan to carry out measurements before and after irradiation both • with proton and with neutrons. • Carry out systematic annealing studies. • Work closely with CiS, Erfurt on microstrip detector R&D.

14. Backup Slides

15. Optimization of W/P & P-Spray Width • When W/P is too small, there is inappreciable impact of ↑ the strip width on VBD • A narrower strip width and a wide P-Spray width can reduce the ENC.

16. Optimization of P-Spray Dose Mixed Mode Simulation (TCAD + SPICE) • With ↑ in P-Spray dose, the VBD ↓ • Using P-Spray rather than P-Stop seems better for detector performance • What happens at high radiation damage?