Silicon detectors in HEP

Enver AlagozACCTR Meeting 18 August 2012

2

Introduction to silicon• Semiconductor material

– Used for radiation detection/imaging in• Particle and nuclear physics experiments• Space and ground-based telescopes• Medical applications• Industrial applications• …

• Interacts with radiation– Radiation creates charge carriers – Electric field drifts carriers toward readout electronics– Signal integrated in frond-end electronics– Digitize integrated signal– Readout and store digital signal

3

Particle reconstruction in timePre-silicon era

Si + gaseous tracking

Si tracking+vertex+calorimetry

time

1970

s19

90s

2010

s

• Why detectors change – Interested in rare events

• in very high energies• with very small uncertainties• with very high background

• Need high precision– Very high granularity– low material budget

• High particle collision rates– High speed electronics– Good background tolerance – Long term reliability – Radiation hardness

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Gaseous vs silicon detectors

Property Gas Silicon Importance

Density Low 2.33 g/cm3 Denser -> better spatial resolution

Charge electrons and ions electrons and holes

Ionization energy 30eV per e- ion pair 3.6 eV per e- hole pair Lower -> better energy resolution

Mobility 10 ns to 10 μs few ns to 20 ns Faster -> no dead time

Signal-to-noise Low High Reliable signal

• In silicon detector• No multiplication signal amplification needed• Optimum thickness to minimize multiple scattering

5

Vertex reconstructionOuter Si layer

Inner Si layer

R1R2

Outgoing particle

Primary vertexImpact parameter

b

If both layers have same spatial resolution (σ)

IP

6

CMS pixel detector

Forward Pixel Barrel Pixel

7

Silicon detectors in HEP• In HEP experiment

• Vertex reconstruction• Fast detection and position resolution

Experiments with silicon detector

E. Do Couto e Silva, Vertex 2000, Sept 10-15, National Lakeshore, MI, USA

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Why silicon? 2nd most abundant element in Earth (28% by mass) Easy to process and purify to 0.001 ppb Natural SiO2 as insulation during fabrication Found in form of silicate minerals and SO2 (silica – sand) Discovered in 1824 by Swedish chemist J.J. Berzelius

Commercially utilized since 1824

Silicon powder

Silicon crystal

Spectral lines of Silicon

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Silicon - 14Si28

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Basic properties – Band gap

Conduction band

Valance band

Conduction band

Valance band

electrons

holes

Eg ~ 6 eV Eg ~ 1.12 eV

Conduction band

Vala

nce

band

overlap

Insulator Semiconductor Conductor

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Basic properties

• Very pure material• Charge carriers are created by– thermal, optical, and other excitations or ionization

• Four valance electrons (covalent bonds)• Silicon (Si) and Germanium (Ge) are common• Doped with extrinsic semiconductors– N-type: excess electrons (e- donor), i.e. P, As etc.– P-type: excess holes (e- acceptor), i.e. Al, B, Ga, etc.

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Silicon PN-junction• Silicon detector are in basic form of PN-junction

N-doped silicon P-doped silicon

N-type: PhosphorusP-type: BoronEg : Band gap (1.12 eV)EF

EC

EV

EF

Conduction band Conduction band

Valance bandValance band

Eg

-

SiSiSi

SiSi

SiSiSi

SiSi

+

Si

SiSi

Si SiSi

Si Si

Donor impurity Acceptor impurity

Excess electron Excess hole

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Silicon pn-junction formation

Carrier concentration

Charge density

Electric field

Coulomb potential

Depletion zone

P-typeneutral

N-typeneutralNA ND

xNA ND

xeNA

eND

x

x

N,P

ρ(x)

E(x)

Φ(x)

NP

x=0

dP dn

NA: # of acceptor impuritiesND: # of donor impurities

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PN-junction bias schemes

PN

E

WF

+_

VF

PN

E

WR

+ _

VR

PN

E

WE

V = 0

WF : depletion width at forward biasWR : depletion width at reverse biasWE : depletion width in equilibrium

WF < WE < WR

Reverse bias scheme is used for silicondetectors in HEP

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A real life case• Larger active area provided by n(p)-type subtstrate

p-type substrate

p+

n+

E+

_

VR

depletion

depleted zone

undepleted zonet (th

ickn

ess)

Depletion with

Resistivity

Depletion voltage

Effective doping concentration

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PN-junction current

Current

Voltage

Forward current

Reverse current(leakage)

Vbreakdown

Avalanche current IPN α T3/2 exp[-Eg/2kBT]

PN WR

+ _VR

PN

WF

+_VF

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Leakage current

Current α T3/2 exp[-Eg/2kBT]

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Breakdown voltage• Extracted from capacitance measurements

0 50 100 150 200 250 300 350 400 450 5006.0E-24

2.0E-10

4.0E-10

6.0E-10

8.0E-10

1.0E-09

Voltage (VR)

Capa

cita

nce

(F)

0 50 100 150 200 250 300 350 400 450 5000.00E+00

2.00E+19

4.00E+19

6.00E+19

8.00E+19

1.00E+20

1.20E+20

1.40E+20

1.60E+20

1.80E+20

2.00E+20

Voltage (VR)

1/C2

VFD

W = t

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Charge collection• Carriers are drifted under the electric field

μe- = 1500 cm2/Vs μh = 450 cm2/Vs

• Total drift time for a carrier created at depth xVB = applied voltageVFD = full depletion voltaged = detector thickness• Maximum drift time (VB >> VFD)

For d = 300 μm

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Radiation detectionIonization energy loss:

• Ionization loss follows Landau statistics• Average energy loss is 390 eV/μm of Si• Average charge is 108 e-h/μm of Si• Most probable charge 80 e-h/μm of Si• 24 ke in 300 μm thick Si

Most ionizing particle = Landau MP

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Signal-to-Noise (S/N)• Signal is Landau, noise is Gaussian

Landau+Gaussian fit

• Ionization energy loss follows Landau statistics • Random electronic noise centered at zero is Gaussian

Most probable signalNoise

=S/N

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Charge vs applied bias

• Collected charge increases until full depletion is reached

300 um thickSilicon detector p+

n+

E+

_

VR

depletion

depleted zone

undepleted zonet (th

ickn

ess)

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Silicon wafer fabrication• Hyper-pure polysilicon chunks• Melt and add impurity to make n-type (P) or p-type (B)• Pour into a mold to make a polysilicon cylinder• Melt onto a mono-crystal silicon seed by means of RF power• Seed and melted polysilicon rotations are opposite to grow a round shape mono-crystal ingot• Employ a grindwheel to form the ingot into a desirable diameters• Cut ingot into wafers by means multi-wire-sawing• Lapping to flatten wafers• Chemical etching to smoothen wafers• Edge rounding for wafer robustness

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Device processing

n-Si

SiO2

800-1200 °C

Photo-resist layer

UV light

etching SiO2

Boron implantation (p+)

Phosphor implantation (n+)

n+

p+p+n-Si

n+

p+p+n-Si

metal

metalmetal

600 °C

1D silicon strip detector

mask

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Principle of operation

E

n+

p+

substrate

+++++++

++

- ---

---

- - 1 2 3 4

1. Preamplifier: amplifying small sensor signal2. Pulse shaper: improving signal-to-noise ratio by filtering signal and

attenuating electronic noise3. Analog-to-Digital Converter (ADC)4. Buffer: Store data Front-end electronics add noise to signal (smearing). Low noise

readout is essential

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A silicon strip detector

Strip sensor Readout electronics

Wire bond (SEM image)

Wire bondsWire bond mechanism

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Position resolution

p+

n+

E

particle trackpitch

p+

n+

E

particle trackpitch

n+

e-

Charge interpolation improves spatial resolution

Pad

sens

orSe

gmen

ted

sens

or

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Position resolution

Binary spatial resolution

Charge interpolated spatial resolution

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Microstrip detectors (1D)

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Silicon drift detector (SSD)

ALICE SSD module

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Double-side strip detector

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Pixel detectorCMS BPIX module

CMS barrel pixel detector

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Detector topology

Marco Battaglia

Marco Battaglia, EDIT 2012, Silicon Track, February 2012

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Radiation damage

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Radiation damageRecoil after 1 MeV neutron collision

Radiation induced damages• cause severe signal losses across sensor thicknessSignal loss can be covered partially• by increasing high voltageBut high voltage• degrades the position resolution

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Radiation damage effects• Leakage current increases ΔI = α Φ V

– ΔI change in current in volume V– α is damage constant– Φ is time-integrated radiation flux

• Depletion voltage increases VFD = q|Neff|d2 /2εε0

– Neff = |Ndonor-Nacceptor|

– εε0 permittivity– d is sensor thickness

• Charge collection degrades• Position resolution degrades

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Radiation damage – leakage current

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Radiation damage - VFD

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Radiation damage – charge collection

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Radiation damage – S/N

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Radiation damage – spatial resolution

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Radiation harder approaches – 3D

ionizing particle

300µm

n+

p+

e

h

depletion

p-type

p+ n+

50µm

p-type

depletion

PLANAR: 3D:

• p+ and n+ electrodes are arrays of columns that penetrate into the bulk• Lateral depletion• Charge collection is sideways• Superior radiation hardness due to smaller electrode spacing: - smaller carrier drift distance - faster charge collection - less carrier trapping - lower depletion voltage• Higher noise• Complex, non-standard processing

SEM picture of 3D electrodes

Radiation harder approaches – 3D4E Configurationn+ (readout) p+ (bias)

100

μm

150 μm

2E Configuration

1E Configuration

SIN

TEF

3D

(200

μm

thic

k)CN

M 3

D (2

00 μ

m th

ick)

FBK

3D (2

00 μ

m th

ick)

Sing

le-s

ide

etch

ing

Doub

le-s

ide

etch

ing

Doub

le-s

ide

etch

ing

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COMPLEXITY – CMS Tracker

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References• Sze, Physics of semiconductor devices, 2nd Edition• Helmuth Spieler, Semiconductor Detector Systems• Olaf Steinkamp, Experimental Methods of Particle Physics, 2011• Enver Alagoz, Simulation and beam test measurements of the CMS pixel

detector, PhD Thesis, 2009• Daniela Bortoletto, An introduction to semiconductor detectors, Vienna

Conference, VCI 2004