Timo Peltola, The era of LHC-seminar, 2012

TCAD SIMULATIONS FOR SILICON DETECTOR UPGRADE AT LHC

T. Peltola1) 1)Helsinki Institute of Physics, CMS Tracker Project.

1

Outline: • Introduction/Motivation • CMS Tracker • TCAD simulations

o Physics of silicon detector o Device structures o Simulations of static characteristics o Simulations of particle detection

• Summary

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Si-detector signal evolution as a function of fluence [1].

LHC sLHC

sLHC LHC Strips

Pixels

Introduction

Upgrade: LHC → sLHC • L= 1035cm-2s-1 with an event rate of 40MHz

– ∫L = 3000fb-1 • Challenges for tracker:

– Higher radiation hardness – High occupancy → higher granularity – Reduce material budget

Currently used materials, p-in-n FZ for strip and n-in-n FZ for pixel sensors are not reliable at sLHC fluences.

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Approaches for radiation hardness upgrade • P-type material or more exotic materials like SiC or diamond → Only partial benefits for charged hadrons • Detectors in cryogenic temperatures → Difficult engineering • Special detector geometries (e.g. 3D) → High cost • Thin detectors → Small signal, difficult production, high cost • Forward-biased mode (Current Injected Detector [CID])

Evolution of collected charge as a function of fluence for CID and reverse biased detector [2].

CMS Tracker sensor upgrade:

Si Devices Simulation WG • Modeling of devices & study of static and dynamic properties • Comparison w/ experimental results RD50 Detector Simulation

group • Modeling of the dependence of CCE on the detector operational conditions (e.g. T, V, Ф)

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HCAL

Magnet

Tracker

Muon chambers

ECAL

CMS Tracker

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• Paths of charged particles are recorded by finding their positions at key points. • Position accuracy ~10 µm. • Design: Si microstrip detectors (MSSD) surrounding the core of Si pixels.

TOB

TIB

Pix

TID

TEC

300 cm

130 cm

|η| < 2.5

TOB

TIB

• 65M pixels

• 10M detector strips read by 80k microelectronic chips [3].

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Tracker Ecal Hcal. Muon chambers

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TCAD simulations

• Topics [4] – Modeling of devices

• Sensor and detector (Pixel, Strip) • 2D/3D

– Study of static and dynamic device properties • Doping conc., device configuration scans • Transient current scans

– Real devices • Comparison with Lab measurement results • Comparison between simulators

– Device design • Guidelines for pre-processing design optimization

– Study of • Charge Collection and S/N • Degradation of detector performance

• Technology Computer-Aided Design (TCAD): Using simulations to develop and optimize semiconductor processing technologies and devices. • Solves diffusion and transport equations for discretized geometries. • Deep physical approach → substitute resource-consuming test wafer runs by TCAD simulations. • Two main branches: process and device simulation

p+ implant

n-type substrate

Simulation framework

Synopsys TCAD E-2010.12:

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trap AD NNnpqP

Electrostatic potential from the solution of Poisson eq. [5]:

Continuity eqs. describing charge conservation:

t

nqqRJ netn

t

pqqRJ netp

Physics of a silicon detector

charged particle track

• P = ferroelectric polarization

• ρtrap= charge density of traps &

fixed charges

• Rnet= net recombination rate

• n, p from quasi-Fermi potentials

w/ Boltzmann statistics

Operating principle

of a silicon particle

detector:

- +

bNaN AD

0

2

2

dqNV

eff

fd

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Steps to create a Synopsys-TCAD simulation

1. Device geometry, doping levels, meshing

2. Tool flow, command files (Tcl)

4. Plotting & analysis

of data

3. Parameters, experiments, simulation run

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Device structures

X [um]

p-in-n MSSD 2D design

Backplane: n+

n-

Electrode (anode)

n-

p+

Front surface with two strips • 10/60µm width/pitch

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Mesh design

Al 0.5µm

Biasing electrode

Charge collecting electrode SiO2 0.3µm

n-

p+

p-n junction

Edge of the depletion region

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Pixel detector design with 18 guard rings (GR) • innermost GR 100µm • 16 GR’s 10/10 width/gap • outermost GR 100µm

12

Pixel n-in-n

Pixel and strip structures

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3D design of the MSSD with deep diffusion 120/320µm.

MSSD design with deep diffusion: Pitch=120µm, Implant width=16µm, Al width=29µm.

120/320μm MSSD p-in-n

200/320μm MSSD p-in-n

320μm MSSD p-in-n

Doping profiles at x=center of the implant

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120/320μm MSSD n-in-p

p-spray

Doping profiles: n+, p-spray

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Simulations of quasistationary characteristics

Finding the simulated full depletion voltage (Vfd):

Simulated current-voltage (IV) for p-in-n MSSD:

Experimental value of Vfd ~260V [6].

Vfd

MSSD 300μm p-in-n: 150V voltage ramps 0V 150V 300V

450V 600V

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MSSD 120/320μm p-in-n: 150V voltage ramps

Magnitude of E ~2 orders higher on the edges:

0V 50V 100V

150V

200V

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Two p-in-n MSSD detectors with equal active area size but different total thickness:

Difference in leakage current 10% at 300 V. Full depletion not as effective

in DF detector!

Thinned Deep diffusion (DF)

Simulated IV

Simulated capacitance-voltage (CV):

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Experimental results by Karlsruhe Institute of Technology (KIT), 2011.

Simulated CV/IV for deep diffusion strip detectors.

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0V 0V

0V

Pixel n-in-n

0V

Pixel n-in-n

Pixel n-in-n Pixel n-in-n

Behavior of depletion region and amplitude of E as a function of the position of grounded GR:

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0V

Pixel n-in-n

1 2 3 4 5

Assignement: How many GRs necessary around the active area?

Pixel n-in-n

16 floating GRs

0V 0V

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6e12

1e16 1e15

1e14 1e13

120/320μm n-in-p MSSD p-spray concentration scan : When p-spray concentration is increased 6e12 → 1e16 cm-3 , depletion region no longer extends to p+.

Expected behavior: Oxide charge in SiO2/Si interface needs to be defined as in real

detector.

I [A]

V [V]

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Simulations of particle detection

The generation rate caused by an alpha particle with energy E [5]:

2

2

121

2

2

222

2

1exp

2

1

2

1exp

2),,,(

ucec

w

wv

s

tt

s

atwvuG u

t

m

if , and by: 31 u 0),,,( twvuG ,if . 31 u • u = coordinate along the particle path • v, w = coordinates orthogonal to u • tm = time of the generation peak • α1 = Bragg peak max.

Transients produced by 5.5 MeV α-particle

injection.

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Charge injection scan of HPK 120/320μm p-in-n

I [A]

t [s]

MSSD 300μm p-in-n MSSD 300μm p-in-n backplane

Scans agree with experimental results of V. Eremin et al. [NIMA 500 (2003) 121-132]

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Summary

• Two simulation WGs founded 2011-12 at CMS • Structures: Strip (2D/3D)- and pixel detector 2D • Detector characteristics

o Doping concentration scans o Device configuration scans o Extraction of full depletion voltage from CV/IV

• Comparison with Lab measurement results o CV/IV properties agree mostly with experiment o Absolute values of CV/IV results need adjusting o Transient scans agree with experimental results

• Next step: o Device processing o Radiation induced degradation of detector performance

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References

[1] Karl-Heinz Hoffmann - CMS Tracker Collaboration (2010), Campaign to identify the future CMS sensor baseline. [2] J. Härkönen et al. (2008), Radiation hard silicon detectors for GSI/FAIR experiments [PPt slides]. [3] The Compact Muon Solenoid Experiment, http://cms.web.cern.ch/cms/Physics/Rewriting/index.html [4] A. Messineo – CERN (2011), Silicon Sensors Upgrade WG [5] Sentaurus Device User Guide, version F-2011.09, Synopsys