Introduction to Silicon Detectors

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Introduction to Silicon Detectors


G.Villani

STFC Rutherford Appleton Laboratory

Particle Physics Department

Outlook


Introduction to physics of Si and detection


Si electronic properties, transport mechanisms, detection


Examples of detectors


Strips, CMOS,CCD,MOS


Radiation damage


Conclusions


2

Introduction

The Si detection chain

3

Sensing/

Charge creation

Charge transport

and collection

Conversion

Signal

processing

Data TX

E

Si physical properties

Si device properties

Si device topologies properties

almost all the boxes of the detection chain process based upon Silicon

The

crystalline

structure

is

diamond

cubic

(FCC),

8

atoms/cell

with

lattice

spacing

of

5
.
43

A

~

5
x
10
22

cm
-
3


*

In

electronic

industry

all

crystallographic

forms

are

used

(Single

crystal,

Polysilicon,

α
-
Si)


The key to success of Si is related to its abundance and oxide SiO
2
, an
excellent

insulator (BV ~ 10
7
V/cm).


*

Micro

crystals

but

the

flexible

bond

angles

make

SiO
2

effectively

an

amorphous
:

its

conductivity

varies

considerably

(charge

transport

in

SiO
2

via

polaron

hopping

between

non
-
bonding

oxygen

2
p

orbitals)


Silicon properties

After Oxygen, Silicon is the 2
nd

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 1
st

Brillouin zone, INDIRECT GAP

CB

VB
-
H

VB
-
L

1
st

Brillouin zone of Diamond lattice

CB

VB


Silicon Band structure

The

electronic

band

structure

can

be

obtained

within

the

independent

electron

approximation

(normally

1

electron

SE

in

periodic

potential

neglecting

electron

interactions)

in

terms

of

Bloch

functions









k
E
e
r
u
r
E
U
T
n
r
jk
k
n
k
n






,
,



~ a wave associated with free motion of electrons modulated by the periodic solution u
n,k
. The energy
E is periodic in k so is specified just within the 1
st

unit WS cell of the reciprocal lattice (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

dispersion

relationship

E(k)

is

almost

parabolic
:





*
0
*
0
*
0
2
2
1
2
m
p
m
k
k
E
v
m
k
k
E
k













F
V
dt
p
d
dt
t
dk
r









*

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 ( averaged over different longitudinal

and transverse masses). The lower the mass, the higher
mobility




1/m*)



*

Similar

approach

used

to

calculate

the

E(k)

for

phonons
.







o
E
E
m
E
g
m
k
k
E




3
2
2
/
3
*
0
*
0
2
2
2
2
2



6

(
3
D)

Silicon electrical properties

The
carrier density

is calculated from:




The

density

of

states

g(E),

which

depends

on

dimension
;



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 g
D
(E) depends on the dimension







i
i
kT
Ev
Ec
V
C
D
D
p
n
e
N
dE
E
F
E
g
n







/
/











kT
E
E
E
F
F
exp
1
1
3

2

1

0

In 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



K
T
e
N
N
n
pn
kT
E
V
C
i
g
300
@
10
20
/
2





Conduction

of

Si

intrinsic

@

T

=

300
K
:



σ

=

q(
μ
n

+
μ
p
)

n
i

=

3
.
04
x
10
-
6
mho
-
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

odg

the

carrier

concentration
.


Doping

concentration
:

10
12

to

10
18

cm
-
3

In

crystalline

Si

~

5
*
10
22
atomscm
-
3

In

equilibrium

and

for

non

degenerate

case

the

relationship

between

carrier

concentration

and

E

is

the

same

as

in

the

intrinsic

case
:




3
2
17
20
/
2
10
10
:
.
.
300
@
10











D
i
D
D
kT
E
V
C
i
N
n
p
pN
pn
N
g
e
K
T
e
N
N
n
pn
g
8

Silicon electrical properties

Charge transport

Charge transport:

The charge transport description in semiconductors relies on semi
-
classical
BTE

(continuity equation in 6D phase space)

















































k
k
k
coll
k
r
k
t
k
r
f
k
E
V
t
r
W
t
k
r
f
k
v
V
q
t
r
J
t
k
r
f
V
t
r
n
t
k
r
S
t
t
k
r
f
t
k
r
f
F
t
k
r
f
k
E
t
t
k
r
f
,
,
1
,
,
,
,
,
,
1
,
,
,
,
,
,
,
,
,
1
,
,


The distribution function f(r,k) can be approximated near equilibrium:


equilibrium

Near equilibrium

k

0



f
coll
f
f
t
t
k
r
f

0
,
,





Q conservation

P conservation

E conservation

9

Charge transport

Under (many) simplifying assumptions the 1
st

moment of BTE gives the DD model

(
The semiconductor equations
):




































A
D
p
p
n
n
p
p
p
n
n
n
N
N
n
p
V
U
J
q
t
p
U
J
q
t
n
p
qD
r
E
qp
J
n
qD
r
E
qn
J



1
1
Transport

of

charge

is

a

combination

of

drift

and

diffusion

mechanism


*

DD

expresses

momentum

conservation
:

it

becomes

invalid

when

sharp

variation

in

energy

of

carriers

occur

(due

to

F

for

example
:

deep

submicron

devices)

When

feature

size

is

0
.
’s
μ
m

the

DD

model

becomes

invalid
:

higher

momentum

required


Diffusion term

Drift 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

(

ionization

chambers,

microstrip,

hybrid

pixels,

CCD,

MAPS

)

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


Photon interaction





z
E
o
e
I
z
I



α

MIP

11

Detection

nm
I
v
R
cm
R
dx
dE
n
i
110
10
3
1
1
3
15
2

















z
E
o
e
I
z
I



A MIP forms an ionization trail of radius R

when traversing Si, creating
~ 80e
-
/
μ
m

Low injection regime:

the generated charge is
too small

to affect the
internal electric field



MIP charge density

cm
L
cm
E
m
h
7
15
10
1
5
.
0
10
2










The associated wavelength is
much smaller

than mean free path:

Each charge is independent from each other;

Carrier dynamics does not need QM







m
e
z
e
h
P
z
n
in







/
10
6
.
5
6






Photoelectric charge density

An optical power of
-
60dBm (= 1nW) of 1keV
photons generates
~ 6*10
6
e
-
/
μ
m


High injection regime:

Plasma effects

The internal electric field can be affected by
the generated charge


12

Detection

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

Dispersion relation for phonons in Si

Phonon excitation energy
~ 10 meV : much lower threshold

60meV

E
c

E
F

E
V

E
F

~10’s meV

Eigenvalues separation in quantized structures ~ 10’s meV

Si

SiO
2

Poly Si

13

Signal conversion: The pn junction

Homojunction
:

consider

two

pieces

of

same

semiconductor

materials


with

different

doping

levels
:

In

equilibrium,

the

Fermi

level

equalizes

throughout

the

structure

The

thermal

diffusion

of

charge

across

the

junction

leaves

just

the

ionized

dopants

:

an

electric

potential,

and

a

field

F,

develops


across

the

junction





t
t
V
p
p
V
n
n
e
p
p
p
qD
r
E
qp
e
n
n
n
qD
r
E
qn

















0
0
0
0
In equilibrium J = 0: using DD model

ASCE

(Abrupt

Space

Charge

Edge)

approximation
:

A

‘positive’

voltage

increases

(exponentially)

the

charge

concentration
:

high

direct

current
.

A

‘negative’

voltage

decreases

it

(down

to

leakage)
:

the

current

reduces

and

at

the

same

time

widens

the

depleted

region
.

Unidirectionality

of

current

characteristics


Near the interface, the carrier concentration exponentially
drops: a
depletion

region (empty of free charge) is formed.

0


14

Signal conversion: The pn junction

The

electric

field

F

in

the

depletion

region

of

the

junction

is

sustained

by

the

ionized

dopants
.

When

charge

is

generated

is

swept

across

by

the

field


PN

junction

signal

converter
:

A

capacitor

with

a

strong

F

across


A device with a
large depleted region W

can be used to
efficiently collect radiation generated charge ( Solid state
ionization chamber)

d
a
d
a
b
N
N
N
N
q
V
W




2
W

To achieve large W high field region:



Low doping (high resistivity) Silicon is needed



Large voltages

Conversion: Q to V // Q to I

15

Detectors examples

Strip detectors


Scientific applications


Monolithic Active Pixel Sensors (MAPS)


Imaging, consumer applications



Charge Coupled Devices (CCD)


Imaging, scientific and consumer applications


MOS detector


scientific applications



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


16

Detectors examples

Use of Si Strip detectors

Almost all HEP experiments use Si detectors:

The high density track region usually covered by pixel detectors; by strip
at larger radius (cost reason)

17

Detectors examples

ATLAS SCT

4 barrel layers,2 x 9 forward disks


4088 double sided modules

Total Silicon surface 61.1m²

Total 6.3 M channels

Power consumption
~ 50kW

Events rate: 40MHz


Put stave pics of AUG!

768 Strip Sensors

RO

module

18

Detectors examples

Array

of

long

silicon

diodes

on

a

high

resistivity

silicon

substrate

A

strong

F

in

the

high

resistivity

Si

region

helps

collect

charge

efficiently

(drift)
.

The

transversal

diffusion

of

charge

implies

a

spread

of

signal

over

neighbouring

strips



The

high

resistivity

Si

is

not

usually

used

in

mainstream

semiconductor

industry
:


Hybrid

solution
:

detectors

connected

(wire/bumpbonded)

to

the

readout

electronic

(RO)

P
++

N
+
(high res)

V
bias

~100’sV

F

Strip detectors

768 Strip Sensors

RO electronic

Power supply

80
μ
m

300
μ
m

Wires

19

Detectors examples

High events rate require fast signal collection:

Estimate of charge collection time in strip detector:

















z
z
o
z
z
dz
F
W
z
F
dz
z
v
z
t
0
0
1
1
1
1
1

For a detector thickness of 300um and overdepleted V
b
= 50V and 10kohm resistivity


t
coll(e)

12ns


t
coll
(h)≈
35ns


The fast collection time helps the
radiation hardness:

The radiation damage to sensors is a crucial issue in modern HEP experiments

20

Detectors examples

RO electronic

3T ( 3MOS) MAPS structure

2D array of
~10
6

pixels

Monolithic solution:

Detector and readout integrated onto the same substrate

≈10’s

m

RO electronic

MAPS detectors

21

Detectors examples

MAPS detectors

The charge generated in the thin active region moves by diffusion mainly:

‘Long’ collection time

Small signal


Different implants arrangements for charge collection optimization

Circuit topologies for low noise

N
++
(low res)

P
++
(low res)

P
+
(low
-
med res)

V
bias
~V’s

Mechanical substrate

100’s
μ
m

Active region

‘s
μ
m

Electronics

0.’s
μ
m

22

Detectors examples

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

10
-
7

TPAC 1 pixel size 50x50um2

Chip size ~1cm
2

Total pixels 28k

>8Meg Transistors

n
coll
n
n
D
l
t
U
n
D
t
n
2
2







Example of MAPS detectors:

23

Detectors examples

Once the charge has been generated, it accumulates in the potential well, under the capacitor.

The control circuitry shifts the accumulated charge to the end of the row, to the input of a charge
amplifier. The sensor is fabricated in a optimized, dedicated process and the RO on a separate
chip. Superior imaging quality but less integration and speed.


Nobel Prize 2009 for Physics to

inventors Boyle and Smith

CCD detectors

24

Detectors examples

In
-
situ Storage Image Sensor:
ISIS

CCD in CMOS process 0.18
μ
m

Charge collection under a PG then
stored under a 20 pixels storage
CCD


RG RD OD RSEL

Column
transistor

On
-
chip logic

On
-
chip switches

Global Photogate and Transfer gate

ROW 1: CCD clocks

ROW 2: CCD clocks

ROW 3: CCD clocks

ROW 1: RSEL

Global RG, RD, OD

Imaging pixel

5

m

㠰8

m


55
Fe


s潵牣e


Mn(K




Mn(K
b


25

Signal conversion: The unipolar MOS device

By applying a voltage to the G with respect to the Substrate

an electric field develops across the SiO
2
: a charge channel

is formed between Source and Drain.

The Ids characteristics depends on the Vgs applied.


The CMOS process refers to the minimum feature size

achievable i.e. the channel length)

Currently 45nm: the modelling of the characteristics of the

device of this size is non
-
trivial:


Quantization effects at the boundary;


QM tunnelling across the gate;


Hot carriers near the D/S junction;





Metal Oxide Semiconductor device are unipolar
devices based on voltage modulation of charge.

The control gate is physically separated by the
active region where the charge moves by a thin
(nm) layer of SiO
2
.

P
++

N
++

SiO
2

NMOS

26

LET in SiO
2

for different particles
















0
0
1
)
(
)
(
1
!
!
)
,
,
(
m
n
n
m
l
l
m
K
A
o
ox
l
K
m
A
e
e
K
r
T
F
Y
kT
q
r
r
r
A
SiO
c
o
c
2
4
,
2



kT
r
qF
K
o
ox

Generation rate in SiO2 vs. electric field

The SiO
2

is a very good insulator: a strong electric
field can be applied to it and the charge

generated in SiO
2

by ionizing radiation efficiently
collected

However SiO
2

is a polar material: the recombination

processes are stronger than in Si. Furthermore, hole

Transport is non Gaussian (low ‘mobility’) and traps
form near Si interface.

27

Signal conversion: The unipolar MOS device

Addition of a Floating Gate (FG): the electrical characteristics of the device are controlled by the charge

stored in the FG. The electric field in the SiO
2

due to the FG drifts charge towards/away from it.

The discharge of the FG alters the device electrical characteristics

FG

Si
O
2

FG

Si
O
2

Floating Gate

Control Gate

Conversion: Q to I

Pre
-
rad

Post
-
rad

Reprog

I
ds
(A)

Chip #1

100Gy

<∆V
th

>

0.6152

Std dev

0.00598

Radiation sensitivity

The MOS structure easily allows

excitation based radiation detection


28

SiO
2

Signal conversion: The unipolar MOS device

Radiation damage

NIEL/cm
2
/yr

TID/Gy/yr

ATLAS

Total Ionizing Dose (rad = 0.01Gy)

Non Ionizing energy Loss (1MeV neutrons/cm2
fluence)

In HEP and space applications the detectors are exposed to high level of radiation:

LHC: 10’s Mrad (100kGy) over 10years of operation


N.B.: 1 rad/cm
3
Si
~10
13
e/h pairs

29

Radiation environment in LHC experiment




TID


Fluence






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


ATLAS Pixels


50 Mrad

1.5 x 10
15


ATLAS Strips


7.9 Mrad


2 x 10
14

CMS Pixels


~24Mrad


~6 x 10
14


CMS Strips


7.5 Mrad

1.6 x 10
14

ALICE Pixel


250 krad


3 x 10
12

LHCb VELO



-

1.3 x 10
14
/year


All values including safety factors.

30

Radiation damage

Microscopic effects:

Bulk damage to Silicon :
Displacement of lattice atoms (
~ Kinetic Energy Released)

Atoms scattered by incoming particles leave behind vacancies or atoms in
interstitial positions (Frenkel pairs).

Low energy particle ~ point defects

High energy particles ~ cluster defects

V

I

V
acancy


+

I
nterstitial

E
K
>25 eV

31

Radiation damage

Energy

deposition

Atoms

displacement

altered

Lattice

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

@

same

T)



Reduced

recombination

time

(

quicker

charge

loss

,

reduced

signal)



Charge

trapping



Scattering



Type

conversion

Altered

Electrical

characteristics

Conduction band

Valence band

Band gap

+++

Donor levels

Acceptor levels

generation

recombination

-

compensation

trapping

32

Radiation damage

Detrimental

Macroscopic

effects
:



Noise

increases

because

of

increase

leakage

current



Charge

Collection

Efficiency

(CCE)

is

reduced

by

trapping



Depletion

voltage

increases

because

of

type

inversion

10
15

1
MeV

n
-
eq
.












t
Q
t
Q
h
e
eff
h
e
h
e
,
,
0
,
1
exp
)
(

defects
h
e
eff
N

,
1

33

Radiation damage

To

increase

the

Radiation

Hardness

of

Sensors
:



Operating

conditions

(cooler



lower

leakage)



Material

engineering

(

OFZ

-

Diamond

detectors)



Device

engineering

(n

in

n



3
D

detectors)


Electrodes

in

the

bulk



lateral

collection

The

device

achieve

full

depletion



Low

depletion

voltage



short

collection

time



claim

reduction

in

signal

33
%

after

8
.
8
X
10
15

1
Mevn



difficult

to

manufacture



3
D

DDTC

similar

to

3
D

but

easier

to

manufacture
;

also

better

mechanical

strength
.


*

Radiation

damage

affects

also

the

RO

electronics,


but

modern

process

can

address

the

problem

efficiently

(

guard

rings,

sub

micron

devices)



34

Radiation damage

Addendum
-

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

Challenging engineering issues

35

ALICE

ATLAS

CMS

LHCb

Strips

4.9m
2

64m
2

210m
2

14.3m
2

Drift

1.3m
2

Pixels

0.2m
2

2m
2

1m
2

0.02m
2

Number of Channels

Strips

2.6 x
10
6

6.3 x
10
6

9.6 x
10
6

1 x 10
6

Drift

1.3 x
10
5

Pixels

9.8 x
10
6

80 x
10
6

33 x
10
6

1 x 10
6

Addendum
-

Detector systems

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

SP

DC2DC

Alternative powering schemes:

ATLAS SCT Barrel 3 at CERN. Half of the
384 cables are visible; the rest enters the
other end of the detector.

36

The field of semiconductor detectors encompasses different scientific and technology


fields: solid state physics, nuclear and particle physics, electrical engineering, …


Some of the issues relevant to radiation detectors:




Radiation hardness



Topologies optimization (power reduction, noise reduction)



Development of new detection techniques based on novel and well established

semiconductor material: ( phonon
-
based detectors, compounds, low dimensional)



Integration with electronics (monolithic solution to achieve more compactness


and reduce cost),3D structures



Conclusions

37

Backup
-

Detector systems

=

At pixel level, power consumption could be optimized by
using a non linear approach:

The positive feedback structure is biased near threshold
(variable)

A small signal triggers the structure

Power reduction at detector level

I

Backup
-

Detection


Intrinsic resolution of Si and Ge based detectors

The variance in signal charge
σ
i

associated to the ionization process is related to the phonon excitation











1
i
i
i
pn
i
o
i
E
E
E
E




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


Fano factor
~0.1 in Si

II

Backup
-
Detection

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


In Si an average of 3.6 eV is required for pair creation

Put values of photon momentum typ.

Ph
: 
Q
~10
7

m
-
1


Q
~10
10

m
-
1

/
a

III

Backup

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

Si
-
poly

Si
-
sub

SiO
2

Q
-
effects

IV

Backup
-

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)

V