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  STS has 8 stations of DSSDs, Strip Pitch ~ 58  m, Strip Width ~ 20  m, stereo angle ~  Expected Radiation Damage ~ 1 x n eq cm -2 year -1  Need detailed simulations to optimize device parameters to maximize V BD and minimize Noise.  Also simulation needed to understand the impact of radiation damage. 1m

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

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

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

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8 Potential & Electric Field Distribution 0 V 50V

9 Some Static Characteristics  C Total = C back +2*C int  ENC α C Total  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 (N eff ) leading to Type Inversion.  The change in N eff 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 (Q f ) is expected to be 3e11cm -2 (for non-irradiated detector) while after irradiation Q f increases and saturates at about 1e12 cm -2. Flatband Voltage gives an approx. of Q f. YearFluence (assumed each year) x (n eq /cm 2 ) Integrated Fluencex (n eq /cm 2 ) N eff x (cm -3 )

11 Impact of Radiation Damage Trap Model, University of Perugia * * CiOiEc+0.36Donor * * VVVEc-0.46Acceptor * * VVEc-0.42Acceptor η (cm -1 )σ h (cm 2 )σ e (cm 2 )Trap Energy (eV)Type  V BD ↑ with fluence  Current ↑ by 3 orders  R int ↓ with fluence  Detailed study needed

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.

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

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