Silicon pad detectors for LCCAL: characterisation and first results

Antonio BulgheroniUniversity of Milan – Italy

on behalf of

LCCAL: Official INFN R&D project, official DESY R&D project PRC R&D 00/02Contributors (Como, LNF, Padova, Trieste): M. Alemi, M.Bettini, S. Bertolucci, E. Borsato, M. Caccia, P.Checchia, C. Fanin, G. Fedel, J. Marczewski, S. Miscetti , M. Nicoletto, M. Prest, R. Peghin, L. Ramina, E. Vallazza.

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Calorimeter Layout

Absorber

Scintillator Silicon pad detectors

25 x 25 x 0.3 cm3

25 Cells 5 x 5 cm2

3 layers725 Pads ~ 1 x 1 cm2

2, 6 and 12 X0

Total of 50 layers27 X0

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Sensor details

Main characteristics:– Sensor thickness: 300m– Resistivity: 4-6k– AC coupling

• Silicon dioxide thickness: 265 nm

– Bias grid and guard ring

• 3M bias resistors• Symmetric

structureGuard ring pad

Bias pad

7 cm

6 cm

~0.9

cm

~0.9 cm

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Hybridisation details Hybridisation through

conductive glue Analogue Readout Chip:

VA-HDR9c (IdeAs)– VA Viking family

– HDR High dynamic range

– 9c Four selectable gains

Gain*

[mv/fC]

Dynamic Range [mip]

3.3 ± 100

2.5 ± 140

1.7 ± 200

1.2 ± 300 *Measured value

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Motherboard design 6 sensors per motherboard with serial readout. Status of production:

– 16 sensors available– 2 motherboards fully and 1 partially equipped

Signal routing through Erni connectors

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Test beam: preliminary results*Two electrons with energy 750 MeV

X silicon chambers

Y silicon chambers

First layer

Second layer

Third layer

Cluster recognition

*Plots filled only with pads with SNR>10

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How we get there: details of the sensor characterisation

2 technological runs

First batch of 11 sensors

(spring ’02)

Second batch of 9 sensors

(summer ’02)

Next batch available in summer ‘03

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Measurement setup

KI 590 & 595HF & QS CV meters

SCS 4200With 2 SMU

Manual

prober

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I – V Measurement: the method

Total and bias current measured directly by an SMU Guard ring current calculated from the others two

A

ASMU2

SMU1

Total

Bias

Guard ring

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I – V measurements: the results

High guard-ring current (> 50A)

Dicing edge too close to the guard ring structure, but it works and the sensor

performances are NOT spoiled!

n

n+

Depletion region

metal

p+

Cutting edgeGuard ring

Solution: keep the cutting edge far away from the guard ring!

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Soft breakdown Bias current reasonable

(few A) Strange shape with a

“soft” breakdown n+ or metal shallow

impurities on the backplane

Metal

n

n+

Impurities

Depletion region

Solution: replace the implanted backside contact with a diffused one, but …

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I – V Characteristics

BATCH 12Total wafers produced 11

Rejected wafers 1

Mean depl. voltage 32.5 V

Mean bias current @ depl. 2.1 A

BATCH 13Total wafers produced 9

Rejected wafers 2

Mean depl. voltage 26.6 V

Mean bias current @ depl. 0.8 A

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C – V measurements

Useful technique to calculate the depletion voltage

Quite uniform behaviour of the depletion voltage

YIELD Batch 12 Batch 13

Not depleted

pads0/420 8/294

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“Leaky” pads: the discovery

In batch 12: the great part (90%) of the pads seems to be DC coupled

In batch 13: only few percents

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“Leaky” pads: the explanation

No pin holes in SiO2

Surface leakage residua of polysilicon after the etching of the polysilicon layer

Equivalent circuit with two opposite diodes.

Polysilicon residua

Readout metal

Al bridge

Resistor

Bias grid

R

C

D

D1

D2

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“Leaky” pads: the solution

External AC coupling SMD capacitors on the PCB

Next batch will be DC coupled with external capacitors

It will improve also the quality of the p-n junction reducing the thermal budget SMD capacitor

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Summary

Three motherboards equipped with 16 sensors

First test beam on going Yield satisfactory New batch with simpler technology has

been started yet promising a better yield