Outline : Introduction ISE simulation of non-irradiated and irradiated devices

VERTEX 2002 – Hawaii, 3-7 Nov. 2002

Department of Physics

Outline:

•Introduction

•ISE simulation of non-irradiated and irradiated devices

•Non-homogeneous irradiation of large area microstrip detectors

•Study of the non-homogenously irradiated detector - CCE(V) and charge sharing -

•Signal/noise as a function of the irradiation

•Conclusions

Study of charge collection properties of silicon microstrip detectors with different read out geometries

after high doses of proton irradiation

G. Casse, P.P. Allport, S. F. Biagi, T.J.V. Bowcock, A. Greenall, A. Smith, P. Turner



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Bias [V]

Q/Q

M

Q (10 ns) Q (25 ns)

Q (40 ns) Q (80ns)

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Q/Q

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Q (10 ns) Q (25 ns)

Q (40 ns) Q (80 ns)

(a) (b)

Vfd

CCE in silicon diodes before and after irradiation (4 1014 cm-2)

The radiation damage introduces charge trapping and changes in VFD, electric field profile, dielectric properties of non-depleted bulk



We use ISE-TCAD to simulate non-irradiated and irradiated silicon detectors.

The radiation effects have been introduced by electron and hole traps in the silicon band-gap. The trap density below corresponds to a fluence of 1x1015 1MeV neutron equivalent cm-2.

Trap typeTrap

density[cm-3]

Energy from mid

band gap [V]

El. capture cross section [cm-2]

Hole capture cross section [cm-2]

*Electron 1.50 1015 0.39 1.00 10-14 5.50 10-13

*Electron2.20 1015 0.13 2.00 10-15 1.20 10-14

Electron 3.60 1014 0.035 1.20 10-15 1.20 10-14

Hole 3.24 1014 -0.045 1.20 10-14 1.20 10-15

*Hole 1.50 1015 -0.20 1.50 10-14 2.00 10-15

* Hallen et al. J. Appl. Phys. 79(1996) 3906



ISE simulation of the electric field profile in a n-bulk silicon diode before and after irradiation (4 1014 p cm-2)

Note the presence of an electric field in the ‘non-depleted’ bulk at low biases and the ‘double-junction’

p+-implantn+-implant



ISE simulation of the majority carrier concentration in a silicon diode before and after irradiation (4 1014 p cm-2)

p+-implant n+-implant



1

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34 5

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8

Strip #128

Strip #256

Strip #384

Strip #512 Strip

#640

Strip #768

Strip #896

Inner radius 8mm

Outer radius42mm

1 2 3 4 5 6 7 6 5 4 3 2 1

Fluence contours x1014 p/cm2

Non-homogenous irradiation of large area LHCb VELO phi-type prototype detectors



Beam profile

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Position from Ref [mm]

Mea

sure

d flu

ence

[E13

p c

m-2

]

Back AlFront Al

Irradiated devices : 200 m n-in-n 200 m p-in-n 300 m p-in-n

Irradiated together, maximum fluence ~ 7 1014 p cm-2

Maximum fluence ~ 4.6 1014 p cm-2



Tools for studying the non-homogeneously irradiated detector: comparison between CCE with infrared (1060 nm) laser and 106Ru ß–source. All measurements with SCT128-VG (LHC speed electronics)

CCE(V) for irradiated, 200m thick, detector with laser data (normalised to value at 400V) superimposed



From fits to the CCE(V), the depletion voltages for the different regions of the detector can be extracted.The Vfd (Neff) profile corresponds to the irradiation profile and allows to study the properties of the detector with a steep gradient of Vfd(Neff).

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Strip Number

V fd [

cm-3

]

0.0E+00

5.0E+11

1.0E+12

1.5E+12

2.0E+12

2.5E+12

3.0E+12

3.5E+12

4.0E+12

4.5E+12

Nef

f [cm

-3]

h

ee

h

Gradient of Neff can introduces a ‘transverse’ component of the electric field and a distortion in the reconstructed cluster position. Distortions are expected to have opposite sign for opposite sign of the gradient of Neff.



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Strip 517-518 Vfd=29 V


= QR/(QR+QL)

N-in-n 200 m detector



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Local position [µm]

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ht c

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200 V

Strip 582-583 Vfd=90V

Strip 612-613 Vfd=105V

Strp 632-633 Vfd=110V


= QR/(QR+QL)N-in-n 200 m detector



= QR/(QR+QL)P-in-n 300 m detector

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Low radiation region Vfd=75V

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Irradiated region with positive gradient of |Neff| as a function of the strip number (Vfd 230 V)

Irradiated region with negative gradient of |Neff| as a function of the strip number (Vfd 230 V)

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No evidence of distortion (spread observed () is approximately 2µm) in the reconstructed cluster position due to the high gradient of Neff in the detector. The experimental results are also supported by ISE simulations

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Righ

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#244 -245 (@ 200 Volt)

#634 - 635 (@ 400 Volt)

#760 - 761 (@ 400 Volt)



Signal (106Ru ß–source) degradation as a function of fluence in the non-homogeneous irradiated detector (n-in-n).



Noise as a function of the applied bias: dose varying from 2. 1014 to 7. 1014 p cm-2

The noise doesn’t change with irradiation and bias (when the total reverse current is kept low, below 1mA)

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0.E+00 2.E+14 4.E+14 6.E+14 8.E+14

Fluence (p cm-2)

Nois

e (A

DC c

ount

s)

80 V

100 V

200 V

300 V



The signal/noise measured with this 200 m thick detector with about 7.5 pF input capacitance is about 16 and 12.5 in the non-irradiated and in the most irradiated areas respectively, as measured with the SCT128-VG analogue electronics

Signal of fast electrons from 106Ru source (non-irr. area)

Cluster significance (non-irr. area)

Cluster significance (irr. area)



Laser (1060 nm) CCE(V) in the highest irradiated areas for a n-in-n (7. 1014 p cm-2) and p-in-n (6. 1014 p cm-2) 200 m thick microstrip detectors

For simple one dimensional structures eg large area diodes little difference is expected between the signals seen on the n-side or the p-side.

Direct comparisons of n-side and p-side detectors with the same masks fabricated on the same material confirm the superiority of n-side read-out after irradiation.


Department of PhysicsConclusions:

• ISE simulations describe well the device properties also after irradiation and successfully predict charge collection properties and are being used for updating designs.

• The effect of non-uniform irradiations (with resulting high gradient - 2.6 1012 cm-4 - of Neff across the detector and perpendicular to the strip) has been studied and small limit to the distortion of the reconstructed cluster position have been placed.

• The charge collected at a given voltage is reduced both by the trapping and by the changes to the effective doping concentration.

• The former is addressed by n-side read-out while the latter can be helped by using an oxygen enhanced substrate.

• Combining the techniques of n-side read-out (to reduce the influence of trapping) and enhanced interstitial oxygen should yield tracking detectors good to 1015p/cm2 at least.

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Outline : Introduction ISE simulation of non-irradiated and irradiated devices

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