Introduction to Silicon Detectors

E.G.VillaniSTFC Rutherford Appleton Laboratory

Particle Physics Department

1

Outlook

• Introduction to physics of Si and detection

• Examples of detectors• Radiation damage• Systems of detectors• Conclusions

2

Introduction

The Si detection chain

3

Sensing/Charge creation

Charge transportand collection

ConversionSignal

processingData TXE

Si physical properties

Si device properties Si device topologies properties

Detector physics:Silicon physical and electrical propertiesDetection principlesTransport mechanismsConversion

Detector examples:Strips MAPS

Detector system issues:Detection efficiencyPower

all the boxes of the detection chain process based upon Silicon

The crystalline structure is Diamond Cubic (FCC), with lattice spacing of 5.43 A

• Polysilicon consists of small Si crystals randomly oriented; in α-Si there is no long range order.

Apart from its abundance, the key to success of Si is related to its oxide SiO2, an excellent insulator (BV ~ 107 V/cm).

• Micro crystals but the flexible bond angles make SiO2 effectively an amorphous: its conductivity varies considerably (charge transport in SiO2 via polaron hopping between non-bonding oxygen 2p orbitals)

Silicon properties

After Oxygen, Silicon is the 2nd most abundant element in Earth’s crust (>25% in mass)

Si

1.48A

4

Silicon electrical properties

5

The appearance of Band Gap, separating CB and VB

The 6 CB minima are not located at the center of 1st Brillouin zone, Indirect Gap

CB VB-H VB-L

1st Brillouin zone of Diamond lattice

CB

VB

The electronic band structure can be obtained by solving 1 electron SE in periodic potential neglecting electron interactions : Bloch functions

kEerurEUT nrjk

knkn ,,~ a wave associated with free motion of electrons modulated by the periodic solution u n,k. The dispersion relationship E(k) is periodic in k so is specified just within the Brillouin zone.

Silicon electrical properties

The detailed band structure is complicated: usually quasi-equilibrium simplifications are sufficient to study the charge transport.Assuming that the carriers reside near an extremum, the E(k) is almost parabolic:

*0

*0

*0

22 1

2 m

p

m

kkEv

m

kkE k

FV

dt

pd

dt

tdkr

Under the assumptions of small variation of the electric field, the carrier dynamics resembles that one of a free particle, with appropriate simplifications.

The effective mass approximation takes into account the periodic potential of the crystal by introducing an effective carrier mass. The lower the mass, the higher mobility (µ 1/m*)

Similar approach used to calculate the E(k) for phonons.

oEE

mEg

m

kkE

32

2/3*0

*0

22

2

2

2

6

Silicon electrical properties The carrier density is calculated from:

• The density of states g(E)• The distribution function F(E);

Only partly filled bands can contribute to conduction: carrier density in CB and VB.

At equilibrium the carrier density is obtained by integrating the product:

The density of states gD(E) depends on the dimension

ii

kTEvEcVCDD pneNdEEFEgn /

/

kTEE

EFFexp1

1

3

2

1

0In intrinsic Si a creation of e in CB leaves behind a hole in VB, that can be treated as an e with positive charge and mobility of the band where it resides

CB

VB

*Fermi level: energy level @ 50% occupancy

7

KTcmeNNnpn

kTEVCi

g 300@10 320/2

Silicon electrical properties

From carrier density: Conduction of Si intrinsic @ T = 300K:

σ = q(μn +μp) ni = 3.04x10-6mho-cm = 329 kOhm-cm

By adding atoms of dopants, which require little energy to ionize( ~10’s mEV, so thermal energies @ ambient temp is enough) we can change by many orders the carrier concentration.

Doping concentration: 1012 to 1018 cm-3

In crystalline Si ~ 5*1022atoms cm-3

The relationship between carrier concentration and E is the same as in the intrinsic case:

332

17 1010:.. cmN

nppNpnNge

D

iDD

8

Charge transport

The charge transport description relies on semi-classical BTE (continuity equation in 6D phase space)

k

k

k

coll

krk

tkrfkEV

trW

tkrfkvV

qtrJ

tkrfV

trn

tkrSt

tkrftkrf

FtkrfkE

t

tkrf

,,1

,

,,,

,,1

,

,,,,

,,,,1,,

Q conservation

P conservation

E conservation

9

Higher moments of BTE correspond to more accurate transport descriptions

Charge transport

Under (many) simplifying assumptions the 1st moment of BTE gives the DD model(The semiconductor equations):

AD

pp

nn

ppp

nnn

NNnpV

UJqt

p

UJqt

n

pqDrEqpJ

nqDrEqnJ

1

1

DD expresses momentum conservation: it becomes invalid when sharp variation in energyof carriers occur (due to F for example: deep submicron devices, which require higher moments or alternative models)

Even in low injection regime, a small F renders the drift term >> diffusion term

Diffusion termDrift term

10

Detection principles

A: Ionization: by imparting energy to break a bond, electrons are lifted from VB to CB then made available to conduction. Most exploited concept ( ionization chambers, microstrip, hybrid pixels, CCD, MAPS…)

B: Excitation: Charge or lattice (acoustic or optical phonons) some IR detectors, bolometer

Bethe-Bloch formula for stopping power gives the rate of <energy loss>/unit length for charged particles through matter

Photon interaction zEoeIzI

α

MIP ~ = 3

11

Detection

In Si an average of 3.6 eV is required for pair creation (partly going into phonons) The MPV of e- generated by MIP is ~77/µm, vs. an average of 110/µm

The indirect BG of Si requires higher energy for charge excitation, because energy and momentum must be conserved (Phonon-assisted pair creation/recombination)

Ph: DQ~107 m-

1

DQ~1010 m-1

/p a

12

Detection

nmI

vR

cmRdx

dEn

i

110

10311 315

2

zEoeIzI

A MIP forms an ionization trail of radius R when traversing Si, creating ~ 77 e-/μm

MIP charge density

cmL

cmEm

h

7

15

1015.0

102

Low injection regime:The generated charge is too small to affect the internal electric field

Carrier dynamics does not need QM

mezeh

Pzn in

/106.5 6

Photoelectric charge density

An optical power of -60dBm (= 1nW) of 1keV photons generates ~ 6*106e-/μm

High injection regime:Plasma effects The internal electric field can be

affected by the generated charge

13

Detection

B: Excitation:

Dispersion relation for phonons in SiPhonon excitation energy ~ 10 meV : much lower threshold Can be used for sensitive bolometers

60meV

Ec

EF

EVEF

~10’s meV

Eigenvalues separation in quantized structures ~ 10’s meV

SiSiO2Poly Si

14

Half summary

Under conditions of • quasi equilibrium• non small feature size• low injection…The behaviour of detectors can be drastically simplified

Sensing/Charge creation

Charge transportand collection

ConversionE

E(k):

Free particle, effective mass

Charge transport:

Drift- diffusion model

Q generation:

<q>/L

15

Detectors examples

Strip / pixel detectors

HEP, Scientific applications

Monolithic Active Pixel Sensors (MAPS)

Consumer and scientific applications

16

Charge Coupled Devices (CCD)Imaging, scientific and consumer applications

PN junction

RAL PPD has (is) actively involved with all these detector technologies

Signal conversion: The pn junctionHomojunction: two pieces of same semiconductor materials with different doping levels:

• In equilibrium, the Fermi level equalizes throughout the structure

• thermal diffusion of charge across the junction leaves just a depleted region, with the ionized dopants : an electric potential Φ, and a field F, develops

• the charge concentration depends exponentially on Φ:

• by applying a small voltage to decrease the barrier, the charge increases exponentially

• by applying a voltage to increase the barrier, the depleted region

Increases

Unidirectionality of current characteristics

t

t

Vpp

Vnn

epppqDrEqp

ennnqDrEqn

0

0

0

0In equilibrium J = 00

17

F

Signal conversion: The pn junction

• when charge is generated in the depleted region is swept across by the electric field sustained by the ionized dopants and the biasing.

PN junction as detector and signal converter: capacitor with a F across

• A device with a large depleted region can be used to efficiently collect radiation generated charge (Solid state ionization chamber)

da

dab

NN

NN

q

VW

2

W

To achieve large W high field region:• Low doping (high resistivity)• Large biasing voltages

18

Depletion width

Detectors examples

Array of long silicon diodes on a high resistivity silicon substrateA strong F in the high resistivity Si region helps collect charge efficiently, mostly by drift.

The high resistivity Si is not usually used in mainstream semiconductor industry:Hybrid solution: detector (high resistivity) connected (wire/bump-bonded) to the readout electronic (low resistivity)

(high res)

Vbias ~100’sV

F

ATLAS Strip detectors80μm

≈ 3

00

μm

19

Q transversal diffusion

Detectors examples

ATLAS SCT

4 single-sided p-on-n sensors

2x2 sensors daisy chained

Stereo angle 40 mrad

Strip pitch 80 µm

768 channels/side

Binary RO

Bias Voltage up to 500 V

Operating temperature -2 C

Space point resolution: rφ 17µm / z 50 µm

Power consumption: 5.6 W (initially ) to 10 W ( after 10y)

Rad-hard up to 2x10^14 1-MeV n eqv/cm2

20

768 Strip Sensors12 RO ASIC

ATLAS SCT RO ASIC (ABCD3TA)

128 channels/ASIC

RAD-HARD DMILL technology

ASICS glued to hybrid

40 MHz Ck

Detectors examples

ATLAS SCT

61 m2 of Si

6.3 * 10^ 6 channels

50 kW total power consumption

1 barrel- 4 layers- 4088 modules

2 end caps- 9 disks/each- 988 modules

2 T solenoidal field

21

Detectors examples

22

ATLAS Tracker overview

Pixel: (n+ on n) 1.8 m2, 80M channels

SCT: 6.3M 61 m2 channels

TRT: 0.4M channels

Detectors examples

23

Barrel insertion in the ATLAS Cavern

ATLAS SCT Red cables : power cables

Detectors examples

24

Detectors in operation

25

Detectors in operation

26

Detectors examples

RO electronic

3T ( 3MOS) MAPS structure

≈1’s m

RO

electronic

MAPS detectors

27

2 D array of pixels

Monolithic solution: detector and readout integrated onto the same substrate

Detectors examples

MAPS detectors

The charge generated in the thin active region moves by diffusion mainly:‘Long’ collection timeSmall signalLow radiation hardness

Complex circuit topologies allow DSP on pixels for low noise

N++ (low res)

P++ (low res)

P+ (low-med res)

Vbias ~V’s

Mechanical substrate100’s μm

Active region‘s μm

Electronics0.’s μm

28

Detectors examples

Charge collection time (s) in MAPS vs. perpendicular MIP hit

10-7

TPAC 1 pixel size 50x50 µm2

Chip size ~1cm2

Total pixels 28k>8Meg Transistors

n

collnn D

ltUnD

t

n 22

Example of MAPS detectors:

29

Detectors examplesExample of charge collection in MAPS: simulated MIP vs.1064nm laser 2x2um 5ns pulse

30

Detectors exampleProposed use of MAPS (50x50 μm2) sensors in SuperB Vertex Tracker

DNW MAPS proposed should improve collection efficiency and speed of response

31

Radiation damageIn HEP and space applications the detectors are exposed to high level of radiation: LHC: 10’s Mrad (100kGy) over 10years of operationN.B.: 1 rad/cm3 Si ~1013e/h pairs Lethal dose: 500 rad Total Body Irradiation

32

Radiation damageRadiation environment in LHC experiment

TID Fluence 1MeV n eq. [cm-2] @ 10 years

ATLAS Pixels 50 Mrad 1.5 x 1015

ATLAS Strips 7.9 Mrad 2 x 1014

CMS Pixels ~24Mrad ~6 x 1014 CMS Strips 7.5Mrad 1.6 x 1014

ALICE Pixel 250krad 3 x 1012

LHCb VELO - 1.3 x 1014/year

All values including safety factors.

33

Radiation damageMicroscopic effects: Bulk damage to Silicon : Displacement of lattice atoms

Atoms scattered by incoming particles leave behind vacancies or atoms in interstitial positions (Frenkel pairs).Low energy particle ~ point defectsHigh energy particles ~ cluster defects

V

I

Vacancy + InterstitialEK>25 eV

34

Radiation damage

Energydeposition

Atoms displacement

alteredLattice

periodicity

Band gap Spurious

states

The appearance of spurious band gap states affects the electro/optical characteristics of the device:

• Thermal generation of carriers (increased leakage current)• Reduced recombination time ( quicker charge loss , reduced signal)• Charge trapping• Scattering• Type conversion

AlteredElectrical

characteristics

Conduction band

Valence band

Band gap

+++Donor levels

Acceptor levels

generation recombination

-compensation

trapping

35

Radiation damage

Macroscopic effects: Charge Collection Efficiency (CCE) is reduced by trapping Noise increases because of increase leakage current Depletion voltage increases because of type inversion

1015 1MeV n-eq.

tQtQ

heeffhehe

,,0,

1exp)(

defectsheeff

N,

1

36

Radiation damage To increase the Radiation Hardness of Sensors:• Operating conditions (cooler – lower leakage)• Material engineering ( OFZ - Diamond detectors)• Device engineering (n in n/p – 3D detectors)

• Electrodes in the bulk – lateral collection reduces the drift distance

• Lower depletion voltage – less power consumption• difficult to manufacture

• 3D DDTC similar to 3D but easier to manufacture; alsoBetter mechanical strength.

n+

p+F

37

Detector systems

The ATLAS SCT (semiconductor tracker) detector. The thick red cables on show feed the detector with half of its power – adding more will take up even more space

HEP experiments: large detector systems

A serial powering or DC2DC approach can increase efficiency in power distribution compared to a parallel approach

Alternative powering schemes:

SP

DC2DC

38

Detector systems

Low power solutions are crucial for future HEP experiments:

RO consists of several block, each consumes power:

10 – 1000 μW continuousEnergy deposited by a particle: 0.1 – 10’s fJNoise occupancy 1% : hit pixel fires/100secRequired energy/deposited energy >> 1010 !!!Extremely huge energy inefficiency

HEP presents challenging engineering and technical issues: their solution may have important consequences in fields outside PP (biomedical, telecommunications…)

39

The field of semiconductor radiation detectors encompasses a number of scientific and technology fields:solid state physics, nuclear and particle physics, electrical engineering, …

Some of the specific issues relevant to radiation detectors:

Development of new detection techniques, based on novel or common semiconductor material: ( phonon-based detectors, quantum detectors, compounds, low dimensional)

Integration with electronics (monolithic solution to achieve more compactness and reduce cost) and/or 3D structures

Topologies optimization (power reduction, noise reduction)

Radiation hardness

Conclusions

40

Backup slides

Quantization effects due to band bending in Si-SiO2 interface: excitation based detection

Si-polySi-sub

SiO2

Q-effects

I

The bipolar transistor device

A bipolar transistor can be thought of as a two diode system, connected in anti series;• One is forward biased;• The other is reverse biased

The bipolar transistor can be (and it is) used as a high gain detector

Main limitations arising from speed: the minority carriers diffuse through the base ( relatively low speed)

II

Detection

Intrinsic resolution of Si and Ge based detectors

The variance in signal charge σi associated to the ionization process is related to the phonon excitation

1

i

i

i

pn

i

oi EE

EE

High resolution requires smaller band gap (εi ), direct or small phonon excitation energy

Fano factor ~0.1 in Si

III