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

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CBM Silicon Tracking System Layout

Fluka Simulation

STS has 8 stations of DSSDs, Strip Pitch ~ 58 m, Strip Width ~ 20 m, 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.

1m

3

x set by strip pitchy set by stereo angle

3-Dimensional Grid

3-D TCAD simulation tools “SYNOPSYS” Sub packages

SentaurusInspectTecplotSPICE (Mixed Mode)

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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.

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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)

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Determination of full depletion voltage

c

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Determination of full depletion voltage

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Potential & Electric Field Distribution

0 V

50V

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Some Static Characteristics

CTotal = Cback+2*Cint

ENC α CTotal

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:

eqK

TtNNTtNTtNNN aeqYeqcaeqAaeqeffeffeff

0/1/1

))(,()())(,())(,(0,

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 Qf.

Year Fluence (assumed each year) x 1014 (neq/cm2)

Integrated Fluencex 1014 (neq/cm2)

Neff x 1012

(cm-3)

1 1 1 - 1.73

2 1 2 - 5.47

3 1 3 - 9.87

4 1 4 - 14.8

5 1 5 - 19.25

6 1 6 - 23.78

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Impact of Radiation Damage

Trap Model, University of Perugia

0.92.5*10-152.5*10-14CiOiEc+0.36Donor

0.95.0*10-145.0*10-15VVVEc-0.46Acceptor

1.6132.0*10-142.0*10-15VVEc-0.42Acceptor

η (cm-1)σh (cm2)σe (cm2)Trap

Energy (eV)Type VBD ↑ with fluence

Current ↑ by 3 orders Rint ↓ with fluence Detailed study needed

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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.

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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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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.

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Optimization of P-Spray Dose

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?

Mixed Mode Simulation (TCAD + SPICE)