Charge collection studies on heavily irradiated spaghetti diodes
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Charge collection studies on heavily irradiated “spaghetti” diodes. G. Kramberge r , V. Cindro, I. Mandi ć, M. Mi ku ž Ϯ , M. Milovanovi ć, M . Zavrtanik Jo žef Stefan Institute, Ljubljana, Slovenia Ϯ also University of Ljubljana, Faculty of Physics and Mathematics.

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Charge collection studies on heavily irradiated spaghetti diodes

Charge collection studies on heavily irradiated “spaghetti” diodes

G. Kramberger, V. Cindro, I. Mandić, M. MikužϮ, M. Milovanović, M. Zavrtanik

Jožef Stefan Institute, Ljubljana, Slovenia

Ϯ also University of Ljubljana, Faculty of Physics and Mathematics

Many thanks to I. Tsurin, University of Liverpool, for mask design.


Motivation
Motivation

  • How does charge collection differ over the strip – homogeneity of response?

    • dependence of multiplication on hit position

    • annealing effects

RD50 had/has a “multiplication wafer” run with Micron, which included special devices/diodes for studying impact of various parameters on charge collection:

  • implant diffusion time

  • energy of implantation ions

  • detector thickness

    In addition it is interesting to explore:

  • role of implant geometry : spaghetti vs. strip detectors

  • long term annealing performance?

  • operation at Feq~1017 cm-2?

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento


Samples
Samples

  • Special diodes-pad detectors were designed on that wafers which are particularly suitable for studies of charge collection:

  • DC coupled, guard ring structure – high breakdown voltage

  • 80 mm pitch, 20 mm implant width (ATLAS geometry)

  • 4x4 mm2 , 300 and 150 mm thick

Type 3– fully metalized implant (CCE)

Type 1 – partially metalized implant (TCT)

  • All strips connected together at one side:

  • almost the same electric field as in a strip detector

  • much simpler handling (CCE, CV-IV, TCT measurements)

  • simpler analysis

  • weighting field has same shape as the electric field

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento


Irradiations and measurements

Tex2440

Amplifier

+shaper

pad detector

90Sr/241Am

thermal

isolation

cold plate

scintillator

Peltier cooler

water cooled heat sink

Irradiations and measurements

Samples from different wafers:

2935-2…9 – standard : 2e15 cm-2, 150 keV P, 5e15 cm-2, 80 keVB, 220 nm thermal oxide

2935-10 - double diffusion : 1000oC for 3h

2912-2,3 - double energy : 300 keV of P ions, doubly charged

2885-5– thin

  • Sample treatment:

  • Neutron irradiations: in steps up to 8∙1016 cm-2,80 min annealing at 60oC in between

  • Measurements done in the range [-20ºC, -25oC]

  • some samples irradiated to a fix fluence - single step

  • some standard p-type strip detectors from Micron were also used

Scanning-TCT Setup

(relative charge over strips)

CCE Setup (absolute charge)

HV

1064 nm

<100 ps

8 mm FWHM

Oscil.

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento


Cce and noise for non irradiated samples

CCE

Setup

CCE and noise for non-irradiated samples

noise without detector

variation of ~10% for charge at V>Vfd – several samples were re-measured and reproducibility was found to be better than that

good agreement of Vfddetermined from Q-V with that of C-V

noise performance in accordance with expectations – not optimized

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento


Cce comparison for all wafers

CCE

Setup

CCE comparison for all wafers

cm-2

cm-2

T=-23oC

T=-23oC

80 [email protected] - 2 irr. steps

80 [email protected]

  • almost no difference in charge collection efficiency for different implants (but only limited parameter region investigated)

  • superior performance of thin detectors (black squares) at lower voltages

    • very high CCE for thin detector (~10-11 ke for 3∙1015 cm-2).

    • up to 1000 V thin are at least as good as thick

    • only moderate increase of charge collection with high bias voltages for thin device – why don’t we see larger increase of multiplication?

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento


Cce of standard wafers for all fluences

CCE

Setup

CCE of “standard” wafers for all fluences

Even at 8∙1016 cm-2 a signal of ~1200 e can be expected i.e. few mip sensitivity with present electronics

An interesting observation – at very high fluences (2,4,8∙1016 cm-2) no problems with micro discharges – very stable operation!

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento


Dependence of cce on voltage
Dependence of CCE on voltage

500 V:Q0=12600

800 V: Q0=20200

1000 V: Q0=26000

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento

Power law dependence of QMVP on fluence (“an empirical formula”) surprisingly works over two orders of magnitude.

The relative difference in collected charge at given bias voltages is independent on fluence (as if combination of trapping and multiplication would perfectly compensate).


Leakage current performance
Leakage current performance

expected generation current from entire bulk

all scaled to -23oC

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento

Leakage current does not increase linearly with fluence, but exhibits saturation.

  • it falls below the expected generation current (the field is present in the entire volume)

  • Possible carrier recombination (minority carrier lifetime decreases) limits the current

    • How much does it influence the CCE (trapping times << recombination times)?

    • A very rough estimation of lower limit gives recombination lifetime of <1ns (8∙1016cm-2).


Noise performance at high fluences
Noise performance at high fluences

color corresponds to figure on left at the highest

voltage shown

standard Micron

Process samples

Main observations:

  • there is no apparent increase of noise with leakage current

  • noise is Gaussian

Conclusions:

  • Decrease of M with fluence?

  • M increases with voltage

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento


Annealing of wafers with different implants
Annealing of wafers with different implants

1e16 cm-2 , annealed at 60oC

There is no large difference between different implants in terms of CCE during long term annealing (double diffusion seem to show less increase in multiplication at 1280 min)

Thin detector performs best also during long term annealing

The slope of the charge rise with voltage increases with annealing – seen already several times before (CERN, JSI, Glasgow…)

CCE>1 for the thin device already at 600 V after 5120 min @ 60oC

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento


Annealing of wafers with different implants1
Annealing of wafers with different implants

2912-3

2912-3

2912-3

The Q-V plot shows rapid rise of charge after 1280 min annealing.

At low bias voltages the charge is smaller for longer annealing times (<400 V).

Increase of noise is related to increase of charge, but a detector with smaller electrodes would have larger S/N (series noise should dominate over shot noise).

I-V has a similar shape as Q-V with a difference because of generation current annealing (initial drop of current).

Similar behavior seen for all investigated materials.

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento


Difference between spaghetti and strips
Difference between spaghetti and strips

Trapping induced charge sharing

for a track through the center of the strip!

signal

n+ strips

diode

p+ strips

usual threshold

n+ - higher signal in hit electrode p+ -wider clusters

Two standard FZ-p strip detectors of same strip geometry from Micron were used for comparison:

  • Electric field is almost the same, hence multiplication effects are the same.

  • The difference is therefore due to effects of the weighting field “trapping induced charge sharing” – residual charges on the neighbors.

  • The QMPV dependence on bias voltage has the same shape.

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento


Uniformity of response over the strip

Does charge collection depend on position of the e-h pair generation ?

  • electrons may drift to different locations in the implant

  • different path lengths result in different amount of trapping

    This can be probed by TCT by scanning the inter-strip region.

Guardring

no metal

Non-irradiated detector

Bias = 100 V

metal

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento


TCT

Setup

Uniformity of response over the strip

direction of the scan

Samples annealed in the setup

Φeq= 5·1015 n/cm2;

Bias = 600 V;

implant

charge collection profile

more non-uniform with annealing

  • enhanced multiplication at the edge of implants for both bias voltages

  • The increase of collected charge after annealing is smaller than measured by 90Sr electrons?

implant

Φeq= 5·1015 n/cm2;

Bias = 1000 V;

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento


Uniformity of response over the strip

2e15, annealed 5120 min

5e15, annealed 5120 min

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento


Conclusions
Conclusions

  • Within the parameter space investigated in RD50 Micron Multiplication run:

    • the “double energy” of implantation ions and

    • the “double diffusion time”

      processed diodes perform equally after irradiation and also during long annealing to spaghetti diodes processed in a standard way.

  • Thin diodes perform better than standard ones for both fluences for bias <=1000 V.

  • Strong increase of charge during long term annealing, but also noise and leakage current.

  • As expected the spaghetti diodes perform slightly worse than strip detectors (due to trapping induced charge sharing) at given fluence.

  • Spaghetti diodes are still “alive” at 8∙1016 cm-2– the charge of 1250 e can be expected (probably more for strip detectors) at 1000 V .

  • The uniformity of the signal over the strip grows with multiplication – highest signal at implant edges.

G. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento


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