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Charge collection studies on heavily irradiated “spaghetti” diodes

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### Charge collection studies on heavily irradiated “spaghetti” diodes

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

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

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

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

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

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

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

- 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

- 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

Amplifier

+shaper

pad detector

90Sr/241Am

thermal

isolation

cold plate

scintillator

Peltier cooler

water cooled heat sink

Irradiations and measurementsSamples 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

Setup

CCE and noise for non-irradiated samplesnoise 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

Setup

CCE comparison for all waferscm-2

cm-2

T=-23oC

T=-23oC

80 [email protected] - 2 irr. steps

- 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?

Setup

CCE of “standard” wafers for all fluencesEven 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!

Dependence of CCE on voltageG. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento

500 V:Q0=12600

800 V: Q0=20200

1000 V: Q0=26000

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

expected generation current from entire bulk

all scaled to -23oC

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

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

Annealing of wafers with different implantsG. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento

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

Annealing of wafers with different implantsG. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento

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.

Difference between spaghetti and stripsG. Kramberger, Charge collection studies on heavily irradiated "spaghetti" diodes, 8th Trento Workshop, 2013, Trento

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.

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

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;

Uniformity of response over the strip

2e15, annealed 5120 min

5e15, annealed 5120 min

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

- 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.

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