marcos fernandez garcia 1, christian gallrapp 2, michael moll 1, hannes neugebauer 1 1 ifca-csic,...

Marcos Fernandez Garcia1 , Christian Gallrapp2, Michael Moll1, Hannes Neugebauer1

1IFCA-CSIC, Universidad de Cantabria, Santander, Spain2CERN, Geneva, Switzerland

ESIPAP School – February 2015

Workshop on the characterization of irradiated silicon sensors

&Effects of Radiation on Solid State

Particle Detector Performance

OUTLINE:

• Particle Detectors: LHC and LHC Detectors• Semiconductors: pn junctions• Characterization techniques of silicon sensors • Radiation damage and radiation tolerant silicon detectors• Conclusions and Outlook

Slides available at http://tinyurl.com/ESIPAP-Silicon

http://tinyurl.com/ESIPAP-Silicon

SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015 -2-

How does an LHC detector look like?

First: how does the LHC look like?

SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

• Installation in existing LEP tunnel (27 Km)

• 4000 MCHF (machine+experiments)

• 1232 dipoles B=8.3T

• pp s = 14 TeV Ldesign = 1034 cm-2 s-1

• Heavy ions (e.g. Pb-Pb at s ~ 1000 TeV)

• First beam: Sept.2008• 2012: 2 x 4 TeV• 2015: 2 x 6.5 TeV

• LHC experiments located at 4 interaction points

p p

-3-

LHC Experiments


CMS

+

LHC-B+ LHCf

ATLAS

ALICE

LHC example: CMS inner tracker


5.4 m

2.4

m

• Inner Tracker Outer Barrel (TOB)Inner Barrel

(TIB) End Cap (TEC)Inner Disks

(TID)

• CMS – “Currently the Most Silicon” Micro Strip: ~ 214 m2 of silicon strip sensors, 11.4 million strips Pixel: Inner 3 layers: silicon pixels (~ 1m2) 66 million pixels (100x150mm) Precision: σ(rφ) ~ σ(z) ~ 15mm Most challenging operating environments (LHC)

• CMS

Pixel

93 cm

30 cm

• Pixel Detector

Total weight 12500 t

Diameter 15m

Length 21.6m

Magnetic field 4 T

Pitch ~ 50mm

Resolution ~ 5mm

p+ in n-

Reference: P.Allport, Sept.2010

Main application: detect the passage of ionizing radiation with high spatial

resolution and good efficiency.Segmentation → position

Micro-strip Silicon Detectors

Highly segmented silicon detectors have been used in Particle Physics experiments for nearly 30 years. They are

favourite choice for Tracker and Vertex detectors (high resolution, speed, low mass, relatively low cost)


Hybrid Pixel Detectors• Hybrid Active Pixel Sensors

• segment silicon to diode matrix with high granularity( true 2D, no reconstruction ambiguity)

• readout electronic with same geometry(every cell connected to its own processing electronics)

• connection by “bump bonding”• requires sophisticated readout architecture• Hybrid pixel detectors will be used in LHC experiments:

ATLAS, ALICE, CMS and LHCb


Flip-chip technique

Solder Bump: Pb-Sn

(VTT/Finland)

~ 15mm


Silicon Detectors

How does a silicon detector work?

-9-

How to obtain the signal?

In a pure intrinsic (undoped) semiconductor the electron density n and hole density p are equal.

inpn For Silicon: ni 1.451010 cm-3

300 m

1 cm

1 cm 4.5 108 free charge carriers in this volume,but only 3.2 104 e-h pairs produced by a M.I.P.

Reduce number of free charge carriers, i.e. deplete the detector

Most detectors make use of reverse biased p-n junctions

E f

E

valence band

conduction band

h

e


Si

Si SiSi

Si

Si

Si Si

Si Si

Si

Si

Si

Si

Si

Si

Si

Si Si

Si Si

Si

Si

Si

Si

Silicon atoms share valence electrons to form insulator-like bonds.

Covalent Bonding of Pure Silicon

Eg =1.1eV

Conduction Band (CB)

Valence Band (VB)

Energy

-11-

Thermal energy at RT: BT40 meV


Donor atoms provide excess electrons to form n-type silicon.

Si

Si Si

Si

Si

Si

Si Si

Si

Si

Si

Si

Si

Si

Si

Si Si

Si

Si

Si

Si

Si

P

P

P

Electrons in N-Type Silicon with Phosphorus Dopant

Phosphorus atom serves as n-type dopant

Excess electron (-)


Valence Band (VB)

-12-

40-50 meV

In N-type Si, electrons are the majority charge carriers. They surpass the number of holes by orders of magnitude (ni~1010 cm-3, Nd=1012-1018

cm-3):𝑝𝑛=

𝑛𝑖❑2

𝑁 𝑑


Acceptor atoms provide a deficiency of electrons to form p-type silicon.

+ Hole Boron atom serves as p-type dopant

Si

Si Si

Si

Si

Si

Si Si

Si

Si

Si

Si

Si

Si

Si Si

B Si

SiSi

Si

Si

Si

B

B

Holes in p-Type Silicon with Boron Dopant


Valence Band (VB)

-13-

In P-type Si, holes are the majority charge carriers. They surpass the number of electrons by orders of magnitude.

SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, Summer 2013

“Hole Movement in Silicon”

Boron is neutral, butnearby electron mayjump to fill bond site.

Boron is now a negative ion.

Only thermal energy to kick electronsfrom atom to atom.

Hole moved from 2 to 3 to 4, and will move to 5.

The empty silicon bond sites (holes) are thought of as being positive, since their presence makes that region positive.

-14-

Sketchy view of a pn-junction (I)

Sketchy view of a pn-junction (II)Majority charge carriers in N-type and P-type have disappeared no current from majority charge carriers.

There is a small reverse current (leakage) coming from carriers injected by diffusion from the undepleted regions/surface (no E-field inside the undepleted bulk). Diffusion electrons (holes) enter from undepleted P (N) into depleted P SCR (N SCR). Leakage current due to minority carriers.

Leakage current increases with T, thickness and irradiation. It constitutes the “background noise” to the measurement.

In this workshop, we will measure the leakage current and capacitance characteristics of irradiated detectors

Detector = “reverse biased p-i-n diode”

Poisson’s equation

effNq

xdx

d

0

02

2

Electrical charge density

Electrical field strength

Electron potential energy

Positive space charge, Neff =[P](ionized Phosphorus atoms)

neutral bulk(no electric field)

p a rtic le (m ip )

+ V < VB d ep + V > VB d ep

• Full charge collection only for fully depleted detector (VB>Vdep)

• Depleted zone growth withincreasing voltage ( )BVw

2

0

0 dNq

V effdep

effective space charge density Neff

depletion voltage Vdepdetector thickness d



Silicon Detectors

Characterization techniques

-19-


Testing Structures - Simple Diodes

• Very simple structures in order to concentrate on the bulk features Typical thickness: 300m Typical active area: 0.5 0.5 cm2

• Openings in front and back contact optical experiments with lasers or LED

Example: Test structure from ITE

-20-

Characterization techniques for detectors

• Detectors need to be optimized for Minimum Ionizing Particles (MIPs) detection. Study charge collection efficiency with radioactive sources (Sr90, for instance) and finally test beams (bunched beams, real scale system test).

• Time resolved induced currents provide information on drift velocity, electric field configuration, trapping times.

• Best measurement conditions achieved with lasers:• Reproducibility: no fluctuations in deposited energy averaging possible -> S/N

improvement.• Selectable absorption length: by varying laser wavelength. Contribution from only one type

of carriers (red laser, top or bottom injection) or both types (IR laser).• Easy triggering

• TCT (Transient Current Techniques) use laser pulses to induce charge carriers in the detector. Time resolved induced currents are recorded and analysed.


Transient Current Technique• Time resolved induced current

can be calculated using Ramo theorem

where is the drift velocity, E is the electric field and the so-called weighting field


Electric field determines the charge trajectory and the velocity of the particle. It changes:

With bias voltageStrip geometryIrradiation of the detector

Electric field for typical detectors:Pad diode: linear electric field (~capacitor)Strip detector: peaked near the electrodes, linear in the center

Weighting field is the derivative of the weighting potential . This potential determines how charge couples to an electrode: (induced charge by a carrier moving from position 1 to 2 )

Weighting field for typical detectors:Pad diode: constantStrip detector: very asymmetric. Peaked near the collection electrode

ElectronsHolesTotal

TCT explained: diode


TCT example: diode


Top TCT, h-injection (p-bulk): • Induced current maximum at front junction• Longer collection time due to smaller drift velocity

Bottom TCT, e injection:• Drift velocity increase towards the front side• Shorter collection time, and higher amplitude of pulses, both due to higher drift velocity

TCT setup


Nicola Pacifico, PhD thesis, Bari University, 2012


Radiation Damage

What is radiation damage?

-26-

SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, Summer 2013

Radiation Damage – Microscopic Effects

particle

SiS Vacancy + Interstitial

point defects (V-O, C-O, .. )

point defects and clusters of defects

EK>25 eV

EK > 5 keV

V

I

·Neutrons (elastic scattering)· En > 185 eV for displacement· En > 35 keV for cluster

· 60Co-gammas· Compton Electrons

with max. E 1 MeV (no cluster production)

·Electrons·Ee > 255 keV for displacement·Ee > 8 MeV for cluster

Only point defects point defects & clusters Mainly clusters

10 MeV protons 24 GeV/c protons 1 MeV neutronsSimulation:

Initial distribution of vacancies in (1m)3

after 1014 particles/cm2

[Mika Huhtinen NIMA 491(2002) 194]


Impact of Defects on Detector properties

Shockley-Read-Hall statistics (standard theory)

Impact on detector properties can be calculated if all defect parameters are known:n,p : cross sections E : ionization energy Nt : concentration

Trapping (e and h) CCE

shallow defects do not contribute at room

temperature due to fast detrapping

charged defects Neff , Vdep

e.g. donors in upper and acceptors in lower

half of band gap

generation leakage current

Levels close to midgap most effective

-28-

Macroscopic Effects – I. Depletion Voltage

• Change of Depletion Voltage Vdep (Neff)


• “Type inversion”: Neff changes from positive to negative (Space Charge Sign Inversion)

10-1 100 101 102 103

eq [ 1012 cm-2 ]

1

510

50100

5001000

5000

Ude

p [V

] (

d =

300

m)

10-1

100

101

102

103

| Nef

f | [

1011

cm

-3 ]

600 V

1014cm-2

type inversion

n-type "p-type"

[M.Moll: Data: R. Wunstorf, PhD thesis 1992, Uni Hamburg]

• Short term: “Beneficial annealing” • Long term: “Reverse annealing” - time constant depends on temperature: ~ 500 years (-10°C) ~ 500 days( 20°C) ~ 21 hours ( 60°C) - Consequence: Detectors must be cooled even when the experiment is not running!

…. with time (annealing):

NC

NC0

gC eq

NYNA

1 10 100 1000 10000annealing time at 60oC [min]

0

2

4

6

8

10

N

eff [

1011

cm-3

]

[M.Moll, PhD thesis 1999, Uni Hamburg]

after inversion

before inversion n+ p+ n+p+

-29-

1011 1012 1013 1014 1015

eq [cm-2]

10-6

10-5

10-4

10-3

10-2

10-1

I /

V

[A/c

m3 ]

n-type FZ - 7 to 25 Kcmn-type FZ - 7 Kcmn-type FZ - 4 Kcmn-type FZ - 3 Kcm

n-type FZ - 780 cmn-type FZ - 410 cmn-type FZ - 130 cmn-type FZ - 110 cmn-type CZ - 140 cm

p-type EPI - 2 and 4 Kcm

p-type EPI - 380 cm

[M.Moll PhD Thesis][M.Moll PhD Thesis]

· Damage parameter (slope in figure)

Leakage current per unit volume and particle fluence

· is constant over several orders of fluenceand independent of impurity concentration in Si can be used for fluence measurement

Radiation Damage – II. Leakage Current


80 min 60C

· Strong temperature dependence

Consequence: Cool detectors during operation! Example: I(-10°C) ~1/16 I(20°C)

Tk

EI

B

effg

2exp ,eqV

I

α

• Change of Leakage Current (after hadron irradiation)

-30-


Radiation Hard Silicon Detectors

How to make silicon detectorsradiation harder?

-31-

The RD50 Collaboration“Radiation hard semiconductor devices for very high luminosity colliders”

• RD50: 48 institutes and 261 members38 European and Asian institutes

Belarus (Minsk), Belgium (Louvain), Czech Republic (Prague (3x)), Finland (Helsinki, Lappeenranta ), Germany (Dortmund, Erfurt, Freiburg, Hamburg, Karlsruhe, Munich), Italy (Bari, Florence,

Padova, Perugia, Pisa, Trento), Lithuania (Vilnius), Netherlands (NIKHEF), Norway (Oslo)), Poland (Krakow, Warsaw(2x)), Romania

(Bucharest (2x)), Russia (Moscow, St.Petersburg), Slovenia (Ljubljana), Spain (Barcelona(2x), Santander, Valencia), Switzerland (CERN, PSI), Ukraine (Kiev), United Kingdom (Glasgow, Liverpool)

8 North-American institutesCanada (Montreal), USA (BNL, Fermilab, New Mexico,

Purdue, Santa Cruz, Syracuse)1 Middle East institute

Israel (Tel Aviv)1 Asian institute

India (Delhi)

Details: http://cern.ch/rd50


Approaches to develop radiation harder solid state tracking detectors

• Defect Engineering of SiliconDeliberate incorporation of impurities or defects into the silicon bulk to improve radiation tolerance of detectors

Needs: Profound understanding of radiation damage• microscopic defects, macroscopic parameters• dependence on particle type and energy• defect formation kinetics and annealing

Examples:• Oxygen rich Silicon (DOFZ, Cz, MCZ, EPI)• Oxygen dimer & hydrogen enriched Si• Pre-irradiated Si• Influence of processing technology

• New Materials Silicon Carbide (SiC), Gallium Nitride (GaN) Diamond (CERN RD42 Collaboration) Amorphous silicon, Gallium Arsenide

• Device Engineering (New Detector Designs) p-type silicon detectors (n-in-p) thin detectors, epitaxial detectors 3D detectors and Semi 3D detectors, Stripixels Cost effective detectors Monolithic devicesSSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015 -33-

Scientific strategies:

I. Material engineering

II. Device engineering

III. Change of detectoroperational conditions

CERN-RD39“Cryogenic Tracking Detectors”operation at 100-200K to reduce charge loss

3D detector concept


· “3D” electrodes: - narrow columns along detector thickness,

- diameter: 10mm, distance: 50 - 100mm

· Lateral depletion: - lower depletion voltage needed- thicker detectors possible- fast signal- radiation hard

n-columns p-columnswafer surface

n-type substrate

p+

---

++

++

-

-

+

30

0 m

m

n+

p+

50 mm

---

+ +++

--+

3D PLANARp+

Summary – Radiation Damage


· Radiation Damage in Silicon Detectors· Change of Depletion Voltage (internal electric field modifications, “type inversion”,

reverse annealing, loss of active volume, …) (can be influenced by defect engineering!)

· Increase of Leakage Current· Increase of Charge Trapping

Signal to Noise ratio is quantity to watch (material + geometry + electronics)

· Microscopic defects· Microscopic crystal defects are the origin to detector degradation.

· Approaches to obtain radiation tolerant devices:· Material Engineering: - explore and develop new silicon materials

(oxygenated Si)- use of other semiconductors (Diamond)

· Device Engineering: - look for other sensor geometries - 3D, thin sensors, n-in-p, n-in-n, …

Acknowledgements & References


· Most references to particular works given on the slides

· Books containing chapters about radiation damage in silicon sensors· Helmuth Spieler, “Semiconductor Detector Systems”, Oxford University Press 2005· Frank Hartmann, “Evolution of silicon sensor technology in particle physics”, Springer 2009· L.Rossi, P.Fischer, T.Rohe, N.Wermes “Pixel Detectors”, Springer, 2006· Gerhard Lutz, “Semiconductor radiation detectors”, Springer 1999

· Research collaborations and web sites· CERN RD50 collaboration (http://www.cern.ch/rd50 ) - Radiation Tolerant Silicon Sensors· CERN RD39 collaboration – Cryogenic operation of Silicon Sensors· CERN RD42 collaboration – Diamond detectors· Inter-Experiment Working Group on Radiation Damage in Silicon Detectors (CERN)· ATLAS IBL, ATLAS and CMS upgrade groups

…


BACKUP

The LHC Upgrade Program• LHC luminosity upgrade (Phase II) (L=5x1034 cm-2s-1) in 2022


Challenge: Build detectors that operate after 3000 fb-1V.Boudry, CLIC Workshop, January 2015

The Charge signal


Mean charge

Most probable charge ≈ 0.7 mean

Collected Charge for a Minimum Ionizing Particle (MIP)

· Mean energy loss dE/dx (Si) = 3.88 MeV/cm 116 keV for 300m thickness

· Most probable energy loss ≈ 0.7 mean 81 keV

· 3.6 eV to create an e-h pair 72 e-h / m (mean) 108 e-h / m (most probable)

· Most probable charge (300 m)

≈ 22500 e ≈ 3.6 fC

-39-

Signal degradation for LHC Silicon Sensors


1013 5 1014 5 1015 5 1016

eq [cm-2]

5000

10000

15000

20000

25000

sign

al [

elec

tron

s]

n-in-n (FZ), 285m, 600V, 23 GeV p p-in-n (FZ), 300m, 500V, 23GeV pp-in-n (FZ), 300m, 500V, neutrons

p-in-n-FZ (500V)n-in-n FZ (600V)

M.Moll - 08/2008

References:

[1] p/n-FZ, 300m, (-30oC, 25ns), strip [Casse 2008][2] n/n-FZ, 285m, (-10oC, 40ns), pixel [Rohe et al. 2005]

FZ Silicon Strip and Pixel Sensors

strip sensorspixel sensors

Note: Measured partly under different conditions!

Lines to guide the eye (no modeling)!

Strip sensors: max. cumulated fluence for LHC

Pixel sensors: max. cumulated fluence for LHC

Situation in 2005

Signal degradation for LHC Silicon Sensors


1013 5 1014 5 1015 5 1016

eq [cm-2]

5000

10000

15000

20000

25000

sign

al [

elec

tron

s]

n-in-n (FZ), 285m, 600V, 23 GeV p p-in-n (FZ), 300m, 500V, 23GeV pp-in-n (FZ), 300m, 500V, neutrons

p-in-n-FZ (500V)n-in-n FZ (600V)

M.Moll - 08/2008

References:

[1] p/n-FZ, 300m, (-30oC, 25ns), strip [Casse 2008][2] n/n-FZ, 285m, (-10oC, 40ns), pixel [Rohe et al. 2005]

FZ Silicon Strip and Pixel Sensors

strip sensorspixel sensors



Strip sensors: max. cumulated fluence for LHC and LHC upgrade

Pixel sensors: max. cumulated fluence for LHC and LHC upgrade

LHC upgrade will need more radiation tolerant tracking detector concepts!

Boundary conditions & other challenges:Granularity, Powering, Cooling, Connectivity,

Triggering, Low mass, Low cost!


Poisson’s equation

Reverse biased abrupt p+-n junction

effNq

xdx

d

0

02

2

2

0

0

2dN

qV effdep

effective space charge density

depletion voltage

)(0

0 wxNq

xdx

deff

2

0

0 )(2

1wxN

qx eff

0

0

wx

wxdx

d

with

VNq

Vweff

0

02)(

dU

UNqdw

dwANqdQ

eff

eff

20

0

0

U

NqAUC

eff

2)(

00 w

AwC 0)(

dUdw

dwdQ

dU

dQC

w = depletion depthd = detector thicknessU = voltage

Neff = effective doping concentration

M.Moll, Bethe Forum on Particle Detectors, Bonn – April 2014

NIEL - Hypothesis• How to normalize radiation damage arising from different particles?

NIEL – Non Ionizing Energy Loss scaling using hardness factor k of a radiation field (or monoenergetic particle) with respect to 1 MeV neutrons

E energy of particle D(E) displacement damage cross section for a certain particle at energy E D(1MeV neutrons) = 95 MeV·mb f(E) energy spectrum of the radiation field

The integrals are evaluated for the interval [EMIN,EMAX], being EMIN and EMAX the minimum and maximum cut-off energy values, respectively, and covering all particle types present in the radiation field

-43-

dEE

dEEED

neutronsMeVD )(

)()(

)1(

1

M.Moll, Bethe Forum on Particle Detectors, Bonn – April 2014

NIEL – Non Ionizing Energy Loss Displacement damage functions

• Hypothesis: Damage parameters scale with the NIEL Be careful, does not hold for all particles & damage parameters (see later)

-44-

10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104

particle energy [MeV]

10-5

10-4

10-3

10-2

10-1

100

101

102

103

104

D(E

) / (

95 M

eV m

b)

neutronsneutrons

pionspions

protonsprotons

electronselectrons

100 101 102 103 104

0.4

0.60.8

1

2

4

neutronsneutrons

pionspions

protonsprotons

1 MeV neutron equivalent

damage1

1 MeV

0.62 (24 GeV/c protons) 1.85 (26 MeV protons) 1.14 (300 MeV pions) 0.92 (reactor neutrons >100 keV)


Radiation Damage – Microscopic Effects

particle

SiS Vacancy + Interstitial

point defects (V-O, C-O, .. )

point defects and clusters of defects

EK>25 eV

EK > 5 keV

V

I

I

I

V

V

¨ Spatial distribution of vacancies created by a 50 keV Si-ion in silicon. (typical recoil energy for 1 MeV neutrons)

van Lint 1980

M.Huhtinen 2001

-45-

Radiation Damage – III. CCE (Trapping)

• Deterioration of Charge Collection Efficiency (CCE) by trapping


tQtQ

heeffhehe

,,0,

1exp)(

Trapping is characterized by an effective trapping time eff for electrons and holes:

where defectsheeff

N,

1

0 2.1014 4.1014 6.1014 8.1014 1015

particle fluence - eq [cm-2]

0

0.1

0.2

0.3

0.4

0.5

Inve

rse

trap

ping

tim

e 1

/ [n

s-1]

data for electronsdata for electronsdata for holesdata for holes

24 GeV/c proton irradiation24 GeV/c proton irradiation

[M.Moll; Data: O.Krasel, PhD thesis 2004, Uni Dortmund][M.Moll; Data: O.Krasel, PhD thesis 2004, Uni Dortmund]

Silicon Growth Processes

• Floating Zone Silicon (FZ)


· Chemical-Vapor Deposition (CVD) of Si· up to 150 mm thick layers produced· growth rate about 1mm/min

Single crystal silicon

Poly silicon

RF Heating coil

Float Zone Growth

· Czochralski Silicon (CZ)

Czochralski Growth

· Basically all silicon tracking detectors made out of FZ silicon [Oi ]< 5 1016 cm-3

· Some pixel sensors: Diffusion Oxygenated FZ (DOFZ)silicon [Oi ]~ 1-2 1017 cm-3

· Epitaxial Silicon (EPI)

· The growth method used by the IC industry.

· Difficult to producevery high resistivity

· [Oi ] ~ 5 1017 cm-3

-47-

0 2 4 6 8 10proton fluence [1014 cm-2]

0

200

400

600

800

Vde

p (3

00m

) [V

]0

2

4

6

8

10

12

|Nef

f| [1

012 c

m-3

]

FZ <111>FZ <111>DOFZ <111> (72 h 11500C)DOFZ <111> (72 h 11500C)MCZ <100>MCZ <100> CZ <100> (TD killed) CZ <100> (TD killed)

Standard FZ, DOFZ, MCz and Cz Silicon


24 GeV/c proton irradiation

· Standard FZ silicon• type inversion at ~ 21013 p/cm2

• strong Neff increase at high fluence

· Oxygenated FZ (DOFZ)• type inversion at ~ 21013 p/cm2

• reduced Neff increase at high fluence

· CZ silicon and MCZ silicon “no type inversion“ in the overall fluence range

(for experts: there is no “real” type inversion, a more clear understanding of the observed effects is obtained by investigating directly the internal electric field; look for: TCT, MCZ, double junction)

· Common to all materials (after hadron irradiation, not after irradiation): reverse current increase increase of trapping (electrons and holes) within ~ 20%

1014 5 1015 5 1016

eq [cm-2]

5

10

15

20

25

Col

lect

ed C

harg

e [1

03 ele

ctro

ns]

n-in-p (FZ), 300m, 500V, 23GeV p [1]n-in-p (FZ), 300m, 500V, neutrons [1,2]n-in-p (FZ), 300m, 500V, 26MeV p [1]n-in-p (FZ), 300m, 800V, 23GeV p [1]n-in-p (FZ), 300m, 800V, neutrons [1,2]n-in-p (FZ), 300m, 800V, 26MeV p [1]

n-in-p-Fz (500V)

n-in-p-Fz (800V)

n-in-p-Fz (1700V)

n-in-p (FZ), 300m, 1700V, neutrons [2]p-in-n (FZ), 300m, 500V, 23GeV p [1]p-in-n (FZ), 300m, 500V, neutrons [1]

p-in-n-FZ (500V)

M.Moll - 09/2009

References:

[1] G.Casse, VERTEX 2008 (p/n-FZ, 300m, (-30oC, 25ns)

[2] I.Mandic et al., NIMA 603 (2009) 263 (p-FZ, 300m, -20oC to -40oC, 25ns)

[3] n/n-FZ, 285m, (-10oC, 40ns), pixel [Rohe et al. 2005][1] 3D, double sided, 250m columns, 300m substrate [Pennicard 2007][2] Diamond [RD42 Collaboration][3] p/n-FZ, 300m, (-30oC, 25ns), strip [Casse 2008]

FZ Silicon Strip Sensors

Silicon materials for Tracking Sensors• Signal comparison for p-type silicon sensors




highest fluence for strip detectors in LHC: The used

p-in-n technology is sufficient

n-in-p technology should be sufficient for Super-LHC at radii presently (LHC) occupied by strip sensors

LHC SLHC

Solid State Detectors – Why Silicon?• Some characteristics of Silicon crystals

Small band gap Eg = 1.12 eV E(e-h pair) = 3.6 eV ( 30 eV for gas detectors)

High specific density 2.33 g/cm3 ; dE/dx (M.I.P.) 3.8 MeV/cm 106 e-h/m (average)

High carrier mobility e =1450 cm2/Vs, h = 450 cm2/Vs fast charge collection (<10 ns)

Very pure < 1ppm impurities and < 0.1ppb electrical active impurities

Rigidity of silicon allows thin self supporting structures

Detector production by microelectronic techniques

well known industrial technology, relatively low price, small structures easily possible

• Alternative semiconductors

Diamond

Gallium arsenide (GaAs)

Silicon Carbide (SiC)

Germanium (Ge)

Diamond SiC (4H) GaAs Si Ge Atomic number Z 6 14/6 31/33 14 32 Bandgap Eg [eV] 5.5 3.3 1.42 1.12 0.66 E(e-h pair) [eV] 13 7.6-8.4 4.3 3.6 2.9 density [g/cm3] 3.515 3.22 5.32 2.33 5.32

e-mobilitye [cm2/Vs] 1800 800 8500 1450 3900 h-mobilityh [cm2/Vs] 1200 115 400 450 1900


Principle of operation

• Goal: precise charged particle position measurement• Use ionization signal (dE/dx) from charged particle passage

(In a semiconductor, ionization produces electron hole (e-h) pairs

· Problems: - In pure intrinsic (undoped) silicon there are more free charge carriers than those produced by a charged particle- electron – hole pairs quickly re-combine

· Solution:- Deplete the free charge carriers and collect electrons or holes quickly by exploiting the properties of a p-n junction (diode)- electric field is used to drift electrons and holes to oppositely charged electrodes

-51-


Conduction in n-Type Silicon

Free electrons flow toward positive terminal.

Positive terminal from power supply

Electron Flow

Electron Flow

Negative terminal from power supply


Conduction in p-Type Silicon

Electron Flow

Electron Flow

Hole FlowHole Flow

Positive terminal from voltage supply

Negative terminal from voltage supply

+Holes flow toward negative terminal

-Electrons flow toward positive terminal

Doping, resistivity and p-n junction• Doping: n-type silicon add elements from Vth groupdonors (P, As,..)

electrons are majority carriers

• Doping: p-type silicon– add elements from IIIrd groupacceptors (B,..)

– holes are majority carriers

Si Si Si

Si

Si

Si Si

Si P

e.g. Phosphorus

Ef

E

VB

CBe

Ef

E

VB

CB

h

Ef

E

VB

CB

p n

e.V

• p-n junction There must be a single Fermi level!band structure deformationpotential differencedepleted zone

detector grade

electronics grade

doping 1012 cm-3 1017 cm-3

resistivity 5 k·cm 1 ·cm

• resistivity r– carrier concentration n, p– carrier mobility mn, mp

pnq pn 0

1

Single Sided Strip Detector• Segmentation of the p+ layer into strips (Diode Strip Detector) and connection of strips to

individual read-out channels gives spatial information

typical thickness: 300mm (150m - 500m used)

pitch

• using n-type silicon with a resistivity of

= 2 KWcm (ND ~2.2.1012cm-3)

results in a depletion voltage ~ 150 V

• Resolution depends on the pitch p (distance from strip to strip)

- e.g. detection of charge in binary way (threshold discrimination) and using center of strip as measured coordinate results in

typical pitch values are 20 mm– 150 mm 50 mm pitch results in 14.4 mm resolution

12

p

Signal to noise ratio• Landau distribution has a low energy tail becomes even lower by noise broadening

Noise sources: (ENC = Equivalent Noise Charge)

Capacitance

Leakage Current

Thermal Noise (bias resistor)

• Good hits selected by requiring NADC > noise tail If cut too high efficiency loss If cut too low noise occupancy

• Figure of Merit: Signal-to-Noise Ratio S/N

• Typical values >10-15, people get nervous below 10 Radiation damage severely degrades the S/N !

0 100 200 300 400 500

ADC channel (arb. units)

Landau distribution

Noise

Landau distribution with noise

[M.Moll, schematic figure!]

NoiseSignal

Cut (threshold)

dCENC

IENC

RTkENC B

Detector Module• Detector Modules “Basic building block of silicon based tracking detectors”

Silicon Sensors Mechanical support (cooling) Front end electronics and signal routing (connectivity)

• Example: ATLAS SCT Barrel Module SCT = SemiConductor Tracker ASICS = Application Specific Integrated CircuitSTPG = Thermal Pyrolytic Graphite

Hybrid (x1)- flexible 4 layer copper/kapton hybrid

- mounted directly over two of the four silicon sensors- carrying front end electronics, pitch adapter, signal routing, connector

Silicon sensors (x4)- 64 x 64 mm2

- p-in-n, single sided- AC-coupled- 768 strips- 80m pitch/12mm width

128 mm

ASICS (x12) - ABCD chip (binary readout) - DMILL technology - 128 channels

Mechanical support- TPG baseboard- BeO facings

Wire bonds (~3500) - 25 mm Al wires

• ATLAS – SCT- 15.552 microstrip sensors

- 2.112 barrel modules - 1.976 forward modules

- 61 m2 silicon, 6.3.106strips

s(rf) ~ 16 mm, s(z) ~ 850mm [NIMA538 (2005) 384]

Pixel Detectors• HAPS – Hybrid Active Pixel Sensors

segment silicon to diode matrix with high granularity( true 2D, no reconstruction ambiguity)

readout electronic with same geometry(every cell connected to its own processing electronics)

connection by “bump bonding” requires sophisticated readout architecture Hybrid pixel detectors are used in all LHC experiments:

ATLAS, ALICE, CMS and LHCb

Flip-chip technique

Solder Bump: Pb-Sn ~ 15mm

Example: The CMS Silicon Tracker• CMS

5.4 m

2.4

m

93 cm

30 cm

Outer Barrel (TOB 6-layer)Inner Barrel

(TIB – 4 layer) End Cap (TEC)Inner Disks

(TID)

Pixel

• Inner Tracker

• Pixel Detector

• CMS – Compact Muon SolenoidMicro Strip:· ~ 214 m2 of silicon strip sensors · 11.4 million stripsPixel:· Inner 3 layers: silicon pixels (~ 1m2) · 66 million pixels (100x150mm)· Precision: σ(rφ) ~ σ(z) ~ 15mm· Most challenging operating environments (LHC)

TCT example: diode


G. Kramberger,PhD thesis, Un. Ljubljana, 2001

marcos fernandez garcia 1, christian gallrapp 2, michael moll 1, hannes neugebauer 1 1 ifca-csic,...

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