Download Semiconductor detectors

Semiconductor detectors
An introduction to
semiconductor detector
physics as applied to
particle physics
PPE S/C detector lectures
Dr R. Bates
1
Contents
4 lectures – can’t cover much of a huge field




Introduction
Fundamentals of operation
The micro-strip detector
Radiation hardness issues
PPE S/C detector lectures
Dr R. Bates
2
Lecture 1 - Introduction



What do we want to do
Past, present and near future
Why use semiconductor detectors
PPE S/C detector lectures
Dr R. Bates
3
What we want to do - Just PPE


Track particles without disturbing them
Determined position of primary interaction vertex
and secondary decays

Superb position resolution


Large signal


Small amount of energy to crate signal quanta
Thin


Highly segmented  high resolution
Close to interaction point
Low mass

Minimise multiple scattering



PPE S/C detector lectures
Detector
Readout
Cooling / support
Dr R. Bates
4
Ages of silicon - the birth

J. Kemmer



*
Fixed target experiment with a planar
diode*
Later strip devices -1980
Larger devices with huge ancillary
components
J. Kemmer: “Fabrication of a low-noise silicon radiation detector
by the planar process”, NIM A169, pp499, 1980
PPE S/C detector lectures
Dr R. Bates
5
Ages of Silicon - vertex detectors

LEP and SLAC





ASIC’s at end of ladders
Minimise the mass inside tracking volume
Minimise the mass between interaction point
and detectors
Minimise the distance between interaction point
and the detectors
Enabled heavy flavour physics i.e. short
lived particles
PPE S/C detector lectures
Dr R. Bates
6
ALEPH
PPE S/C detector lectures
Dr R. Bates
7
ALPEH – VDET (the upgrade)




2 silicon layers, 40cm long, inner radius 6.3cm, outer radius 11cm
300mm Silicon wafers giving thickness of only 0.015X0
S/N rF = 28:1; z = 17:1
srf = 12mm; sz = 14mm
PPE S/C detector lectures
Dr R. Bates
8
Ages of silicon - tracking paradigm

CDF/D0 & LHC





Emphasis shifted to tracking + vertexing
Only possible as increased energy of particles
Cover large area with many silicon layers
Detector modules including ASIC’s and
services INSIDE the tracking volume
Module size limited by electronic noise due
to fast shaping time of electronics (bunch
crossing rate determined)
PPE S/C detector lectures
Dr R. Bates
9
ATLAS

PPE S/C detector lectures
Dr R. Bates
A monster !
10
ATLAS barrel


PPE S/C detector lectures
Dr R. Bates
2112 Barrel
modules mounted
on 4 carbon fibre
concentric Barrels,
12 in each row
1976 End-cap
modules mounted
on 9 disks at each
end of the barrel
region
11
What is measured


Measure space points
Deduce



Vertex location
Decay lengths
Impact parameters
PPE S/C detector lectures
Dr R. Bates
12
Signature of Heavy Flovours
Stable particles
t > 10-6 s
ct
n
2.66km
m
658m
Very long lived particles
t > 10-10 s
p, K±, KL0
2.6 x 10-8
7.8m
KS0, E±, D0
2.6 x 10-10
7.9cm
Long lived particles
t > 10-13 s
t±
0.3 x 10-12
91mm
Bd0, Bs0, Db
1.2 x 10-12
350mm
p0, h0
8.4 x 10-17
0.025mm
r,w
4 x 10-23
10-9mm!!
Short lived particles
PPE S/C detector lectures
Dr R. Bates
13
Decay lengths
E.g. B  J/Y Ks0
L
Secondary vertex
Primary vertex


L = p/m c t
By measuring the decay length, L, and the
momentum, p, the lifetime of the particle can
be determined
Need accuracy on both production and
decay point
PPE S/C detector lectures
Dr R. Bates
14
Impact parameter (b)
b = distance of closest
approach of a
reconstructed track
to the true interaction point
b
beam
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Dr R. Bates
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Impact parameter

Error in impact parameter for 2 precision
measurements at R1 and R2 measured in
two detector planes:
2
b
s b  a     c 2
 p
2



a=f(R1 & R2) and function of intrinsic
resolution of vertex detector
b due to multiple scattering in detector
c due to detector alignment and stability
PPE S/C detector lectures
Dr R. Bates
16
Impact parameter


sb = f( vertex layers, distance from main vertex, spatial
resolution of each detector, material before precision
measurement, alignment, stability )
Requirements for best measurement








Close as possible to interaction point
Maximum lever arm R2 – R1
Maximum number of space points
High spatial resolution
Smallest amount of material between interaction point and 1st
layer
Good stability and alignment – continuously measured and
correct for
100% detection efficiency
Fast readout to reduce pile up in high flux environments
PPE S/C detector lectures
Dr R. Bates
17
Impact parameter*
Blue = 5mm
Black = 1mm (baseline)
Effect of extra
mass and
distance from the
interaction point
Green = 0.5 mm
Red = 0.1 mm
Lower Pt
GR Width
Flux increase(%) to silicon
Improvement of the IPres. wrt 1mm(%)
-44
-38.10.9
0.5mm
+14.1
+5.8 0.7
0.1mm
+27.7
+10.0  0.7
5mm
*Guard Ring Width Impact on d0 Performances and Structure Simulations.
A Gouldwell, C Parkes, M Rahman, R Bates, M Wemyss, G Murphy, P Turner and S Biagi. LHCb Note, LHCb-2003-034
Why Silicon


Semiconductor with moderate bandgap (1.12eV)
Thermal energy = 1/40eV


Little cooling required
Energy to create e/h pair (signal quanta)= 3.6eV
c.f Argon gas = 15eV
High carrier yield
 better stats and lower Poisson stats noise
 Better energy resolution and high signal
 no gain stage required

PPE S/C detector lectures
Dr R. Bates
19
Why silicon

High density and atomic number



Higher specific energy loss
Thinner detectors
Reduced range of secondary particles


High carrier mobility  Fast!



Better spatial resolution
Less than 30ns to collect entire signal
Industrial fabrication techniques
Advanced simulation packages




Processing developments
Optimisation of geometry
Limiting high voltage breakdown
Understanding radiation damage
PPE S/C detector lectures
Dr R. Bates
20
Disadvantages

Cost  Area covered



Detector material could be cheap – standard Si
Most cost in readout channels
Material budget

Radiation length can be significant



Effects calorimeters
Tracking due to multiple scattering
Radiation damage

Replace often or design very well – see lecture 4
PPE S/C detector lectures
Dr R. Bates
21
Radiation length X0




High-energy electrons predominantly lose energy in
matter by bremsstrahlung
High-energy photons by e+e- pair production
The characteristic amount of matter traversed for
these related interactions is called the radiation
length X0, usually measured in g cm-2.
It is both:


the mean distance over which a high-energy electron
loses all but 1=e of its energy by bremsstrahlung
the mean free path for pair production by a high-energy
photon
1 
2

4N A Z Z  1re log 183Z 3 
1



X0
A
PPE S/C detector lectures
Dr R. Bates
22
Lecture 2 – lots of details




Simple diode theory
Fabrication
Energy deposition
Signal formation
PPE S/C detector lectures
Dr R. Bates
23
Detector = p-i-n diode



Near intrinsic bulk
Highly doped contacts
Apply bias (-ve on p+ contact)



Radiation creates carriers


Deplete bulk
High electric field
n+ contact ND=1018cm-3
ND~1012cm-3
signal quanta
Carriers swept out by field

Induce current in external circuit
 signal
PPE S/C detector lectures
Dr R. Bates
p+ contact NA=1018cm-3
24
Why a diode?


Signal from MIP = 23k e/h pairs for 300mm device
Intrinsic carrier concentration





ni = 1.5 x 1010cm-3
Si area = 1cm2, thickness=300mm  4.5x108 electrons
4 orders > signal
Need to deplete device of free carriers
Want large thickness (300mm) and low bias
But no current!


Use v.v. low doped material
p+ rectifying (blocking) contact
PPE S/C detector lectures
Dr R. Bates
25
p-n junction
(1)
p+
n
(5)
(2)
Carrier density
Electric field
(6)
(3)
Dopant
concentration
(4)
Space charge (7)
density
PPE S/C detector lectures
Dr R. Bates
Electric potential
26
p-n junction
1)
2)
take your samples – these are neutral but
doped samples: p+ and nbring together – free carriers move
two forces drift and diffusion
o
In stable state
Jdiffusion (concentration density) = Jdrift (e-field)
o
3)
p+ area has higher doping concentration (in
this case) than the n region
PPE S/C detector lectures
Dr R. Bates
27
p-n junction
Fixed charge region
Depleted of free carriers
4)
5)
o
o
o
o
Called space charge region or depletion region
Total charge in p side = charge in n side
Due to different doping levels physical depth of space
charge region larger in n side than p side
Use n- (near intrinsic)  very asymmetric junction
6)
Electric field due to fixed charge
7)
Potential difference across device
o
dV
r
E

dx

Constant in neutral regions.
PPE S/C detector lectures
Dr R. Bates
28
Resistivity and mobility

Carrier DRIFT velocity and E-field:
v  mE


mn = 1350cm2V-1s-1 : mp = 480cm2V-1s-1
Resistivity


1
r
qm n n  m p p 
p-type material
n-type material
PPE S/C detector lectures
r
1
r
qm p N D
1
qm N A
Dr R. Bates
29
Depletion width

Depletion Width depends upon Doping
Density:
2V  1
1 
W




q  ND N A 
For a given thickness, Full Depletion
Voltage is:
qN DW 2
V fd 

2
W = 300mm, ND  5x1012cm-3: Vfd = 100V
PPE S/C detector lectures
Dr R. Bates
30
Reverse Current

Diffusion current



From generation at edge of depletion region
Negligible for a fully depleted detector
j gen
Generation current


From generation in the depletion region
Reduced by using material pure and defect free


1 ni
 q W
2 t0
high lifetime
Must keep temperature low & controlled
 Eg 
n  NC NV exp   
 kT 
2
i
PPE S/C detector lectures
j gen  T
32
 1 
exp 

 2kT 
Dr R. Bates
j gen  2 for DT  8K
31
Capacitance



Capacitance is due to movement of
charge in the junction
Fully depleted detector capacitance
defined by geometric capacitance
Strip detector more complex
Inter-strip capacitance dominates
dQ
qN A N D
qN D


C




dV
2N A  N D V
2V
2mrV W

PPE S/C detector lectures
Dr R. Bates
32
Noise





Depends upon detector capacitance
and reverse current
Depends upon electronics design
Function of signal shaping time
Lower capacitance  lower noise
Faster electronics  noise contribution
from reverse current less significant
PPE S/C detector lectures
Dr R. Bates
33
Fabrication

Use very pure material

High resistivity

Low bias to deplete device


Low defect concentration



Easy of operation, away from breakdown, charge spreading for
better position resolution
No extra current sources
No trapping of charge carriers
Planar fabrication techniques

Make p-i-n diode

pattern of implants define type of detector (pixel/strip)
extra guard rings used to control surface leakage currents
metallisation structure effects E-field mag  limits max bias


PPE S/C detector lectures
Dr R. Bates
34
Fabrication stages

Stages



dopants to create p- & n-type regions
passivation to end surface dangling bonds and protect
semiconductor surface
metallisation to make electrical contact
n- Si

Starting material


Phosphorous
diffusion

PPE S/C detector lectures
Dr R. Bates
Usually n-
P doped poly n+ Si
35
Fabrication stages

Deposit SiO2

Grow thermal oxide
on top layer

Photolithography +
etching of SiO2

PPE S/C detector lectures
Dr R. Bates
Define eventual
electrode pattern
36
Fabrication stages

Form p+ implants




PPE S/C detector lectures
Boron doping
thermal
anneal/Activation
Removal of back SiO2
Al metallisation +
patterning to form
contacts
Dr R. Bates
37
Fabrication

Tricks for low leakage currents

low temperature processing



simple, cheap
marginal activation of implants, can’t use IC
tech
gettering


very effective at removal of contaminants
complex
PPE S/C detector lectures
Dr R. Bates
38
Energy Deposition

Charge particles
Bethe-Bloch
 Bragg Peak


Not covered
Neutrons
 Gamma Rays


Rayleigh scattering, Photo-electric effect,
Compton scattering, Pair production
PPE S/C detector lectures
Dr R. Bates
39
Charge particles
- concentrating on electrons


At   3 dE/dx minimum
independent of absorber
(mip)
Electrons


mip @ 1 MeV
E>50 MeV radiative energy
loss dominates
Momentum transferred to a free electron
at rest when a charged particle passes at its
closest distance, d. integrate over all possible values of d
PPE S/C detector lectures
Dr R. Bates
40
Well defined range



at end of range specific energy loss increases
particle slows down
deposit even more energy per unit distance
Bragg Peak
E = 5 MeV in Si:
(increasing charge) p

16O
PPE S/C detector lectures
R (mm)
220
25
4.3
Useful when estimating
properties of a device
Dr R. Bates
41
Energy Fluctuation


Electron range of individual
particle has large fluctuation
Energy loss can vary greatly Landau distribution

Close collisions (with bound
electrons)






rare
energy transfer large
ejected electron initiates
secondary ionisation
Delta rays - large spatial
extent beyond particle track
Enhanced cross-section for K-,
L- shells
Distance collisions


common
M shell electrons - free
electron gas
PPE S/C detector lectures
Dr R. Bates
42
e/h pair creation

Create electron density oscillation - plasmon



De-excite almost 100% to electron hole pair creation
Hot carriers





requires 17 eV in Si
thermal scattering
optical phonon scattering
ionisation scattering (if E > 3/4 eV)
Mean energy to create an e/h pair (W)
is 3.6 eV in Si (Eg = 1.12 eV  3 x Eg)
W depends on Eg therefore
temperature dependent
PPE S/C detector lectures
Dr R. Bates
43
Delta rays
PPE S/C detector lectures
Dr R. Bates
a) Proability of ejecting an electron
with E  T as a function of T
b) Range of electron as a
44
function of energy in silicon
Displacement from d-electrons

Estimate the error




Assume 20k e/h from track
50keV d-electron produced perpendicular to track
Range 16mm, produces 14k e/h
Assume ALL charge created locally 8mm from
particle’s track
D
PPE S/C detector lectures
20k  0mm  14k  8mm
 3.3mm
20k  14k
Dr R. Bates
45
Consequences of d-electrons
Centroid displacement
Resolution as function
of pulse height

70
6
60
5
Resolution (microns)
Probability (%)

50
40
30
20
10
0
4
3
2
1
0
0
2
4
6
8
10
Displacement (microns)
PPE S/C detector lectures
0
1
2
3
4
5
6
Pulse height (mip)
Dr R. Bates
46
Consequence of d-electrons
45º
45º
15 mm
E.g. CCD
300 mm
Most probable E loss = 3.6keV
10% proby of 5keV d
pulls track up by 4 mm
E.g. Microstrip
Most probable E loss = 72keV
10% proby of 100keV d
pulls track up by 87 mm
PPE S/C detector lectures
Dr R. Bates
47
Signal formation

Signal due to the motion of charge carriers inside
the detector volume & the carriers crossing the
electrode
 Displacement current due to change in
electrostatics (c.f. Maxwell’s equations)

Material polarised due to charge
introduction
Induced current due to motion of
the charge carriers
See a signal as soon as carriers
move


PPE S/C detector lectures
Dr R. Bates
48
Signal

Simple diode


Signal generated equally from movement through entire
thickness
Strip/pixel detector


Almost all signal due to carrier movement near the sense
electrode (strips/pixels)
Make sure device is depleted
under strips/pixels
If not:

Signal small

Spread over many strips
PPE S/C detector lectures
Dr R. Bates
49
Lecture 3 – Microstrip detector


Description of device
Carrier diffusion


Charge sharing





Why is it (sometimes) good
Cap coupling
Floating strips
Off line analysis
Performance in magnetic field
Details



AC coupling
Bias resistors
Double sides devices
PPE S/C detector lectures
Dr R. Bates
50
What is a microstrip detector?


p-i-n diode
Patterned implants as strips




One or both sides
Connect readout electronics to strips
Radiation induced signal on a strip due
to passage under/close to strip
Determine position from strip hit info
PPE S/C detector lectures
Dr R. Bates
51
What does it look like?


P+ contact on front of n- bulk
Implants covered with thin thermal
oxide (100nm)


Strips surrounded by a continuous
p+ ring


Forms capacitor ~ 10pF/cm

Al strip on oxide overlapping
implant



Wirebond to amplifier
The guard ring
Connected to ground
Shields against surface currents
Implants DC connected to bias rail


Use polysilicon resistors MW
Bias rail DC to ground
HV
PPE S/C detector lectures
Dr R. Bates
52
Resolution

Delta electrons



See lecture 2
Diffusion
Strip pitch



Capacitive coupling
Read all strips
Floating strips
PPE S/C detector lectures
Dr R. Bates
53
Carrier collection


Carriers created around track Φ  1mm
Drift under E-field




p+ strips on n- bulk
p+ -ve bias
Holes to p+ strips, electrons to n+ back-plane
Typical bias conditions



100V, W=300mm E=3.3kVcm-1
Drift velocity: e= 4.45x106cms-1 & h=1.6x106cm-1
Collection time: e=7ns, h=19ns
PPE S/C detector lectures
Dr R. Bates
54
Carrier diffusion

Diffuse due to conc. gradient dN/dx




Diffusion coefficient:
kT
D
m
q
RMS of the distribution: s  2Dt coll
Since D  m & tcoll  1/m


Gaussian
 x2 
dN
1
dx

exp  
N
4pDt
 4 Dt 
Width of distribution is the same for e & h
As charge created along a strip

Superposition of Gaussian
distribution
Dr R. Bates
PPE S/C detector lectures
55
Diffusion

Example for electrons:




Lower bias  wider distribution
For given readout pitch



tcoll = 7ns; T=20oC
s = 7mm
wider distribution  more events over >1 strip
Find centre of gravity of hits  better position
resolution
Want to fully deplete detector at low bias

High Resistivity silicon required
PPE S/C detector lectures
Dr R. Bates
56
Resolution as a f(V)
Spatial
resolution as
a function of
bias
Resolution (micros)
5
4
3
2
Vfd = 50V
1
0
0
20
40
60
80
100
Bias (V)

V<50V


charge created in undeleted region lost, higher noise
V>50V

reduced drift time and diffusion width less charge sharing
more single strips
PPE S/C detector lectures
Dr R. Bates
57
Resolution due to detector design

Strip pitch





BUT



Very dense
Share charge over many strips
Reconstruct shape of charge and find centre
Signal over too many strips  lost signal (low
S/N)
FWHM ~ 10mm
Limited to strip pitch 20mm
Signal on 1 or 2 strips
PPE S/C detector lectures
Dr R. Bates
58
Two strip events

Track between strips




Track mid way Q on both strips


Find position from signal on 2 strips
Use centre of gravity or
Algorithm takes into account shape of charge
cloud (eta, h)
best accuracy
Close to one strip

Small signal on far strip

Lost in noise
PPE S/C detector lectures
Dr R. Bates
59
Capacitive coupling
Strip detector is a C/R network
Cstrip to blackplace = 10x Cinterstrip
Csb || Cis  ignore Csb
Fraction of charge on B due to track at A:




K
 Ceff  Cis
Ceff
2Ceff  C AC
as C AC
B
Cis
QB
CB
K

QA  QB  QC C A  C B  CC
C C
CB  is AC  Ceff
Cis  C AC
C AC
Cis
 Cis  K  C
AC
Qs
C AC
Cis
C AC
A
C
 K is small
PPE S/C detector lectures
Dr R. Bates
60
Floating strips
20mm

strip pitch  s=2.2mm
Large Pitch (60mm) 1/3 tracks on both strips
60mm
20mm

Intermediate strip
20mm 20mm 20mm
PPE S/C detector lectures
Assume s = 2.2mm
2/3 on single strips
s = 40/12 = 11.5mm
Overall:
s = 1/3 x 2.2 + 2/3 x 11.5
= 8.4mm
Capacitive charge coupling
2/3 tracks on both strips
NO noise losses due to cap coupling
1/3 tracks on single strips
s = 2/3 x 2.2 + 1/3 x 20/12
= 3.4mm
Dr R. Bates
61
Off line analysis

Binary readout


No information on the signal size
Large pitch and high noise

Get a signal on one strip only
s x2 
<x> = 0
P(x)
12
2


x


x

P( x)dx

1 2
12

2
x
 P( x)dx
1 2
-½ pitch
½ pitch
PPE S/C detector lectures
s
Dr R. Bates
1
Pitch

12
12
62
Centre of Gravity


PHL
Have signal on each strip
Assume linear charge sharing between strips
PHR
X
 PH  x
 PH
i  strips
i  strips
i
i
i
Q
on 2 strips & x = 0 at left strip
PH R  P
X
PH L  PH R
e.g. PHL = 1/3PHR
P
x
X
PPE S/C detector lectures
Dr R. Bates
1 3 0  3 4  P 3
 P
1 33 4
4
63
Eta function

PHL
Non linear charge sharing due to
Gaussian charge cloud shape
PHR
P
More signal on RH strip
than predicted with
uniform charge cloud shape
Non-linear function to determine
track position from relative pulse
heights on strips
x
PPE S/C detector lectures
Dr R. Bates
64
Eta function

Measured tracks as
a function of
incident particle
flux
PPE S/C detector lectures

Dr R. Bates
Measured and
predicted particle
position
65
Lorentz force


Force on carriers due to magnetic force

  v 
F  q E   B 
c


Perturbation in drift direction




Charge cloud centre drifts from track position
Asymmetric charge cloud
No charge loss is observed
Can correct for if thickness & B-field known
vh
E
PPE S/C detector lectures
H
ve
qL
Dr R. Bates
66
Details

Modern detectors have integrated
capacitors




Thin 100nm oxide on top of implant
Metallise over this
Readout via second layer
Integrated resistors

Realise via polysilicon


Complex
Punch through biasing


Not radiation hard
Back to back diodes – depleted region has high R
PPE S/C detector lectures
Dr R. Bates
67
Details

Double sided detectors


Both p- and n-side pattern
Surface charge build up on n-side




Trapped +ve charge in SiO
Attracts electrons in silicon near surface
Shorts strips together
p+ spray to increase inter-strip resistance
PPE S/C detector lectures
Dr R. Bates
68
Lecture 4 – Radiation Damage

Effects of radiation




Microscopic
Macroscopic
Annealing
What can we do?




Detector Design
Material Engineering
Cold Operation
Thin detectors/Electrode Structure – 3-D device
PPE S/C detector lectures
Dr R. Bates
69
Effects of Radiation

Long Term Ionisation Effects




Trapped charge (holes) in SiO2
interface states at SiO2 - Si interface
Can’t use CCD’s in high radiation environment
Displacement Damage in the Si bulk



4 stage process
Displacement of Silicon atoms from lattice
Formation of long lived point defects & clusters
PPE S/C detector lectures
Dr R. Bates
70
Displacement Damage

Incoming particle undergoes collision with
lattice


PKA moves through the lattice




produces vacancy interstitial pairs (Frenkel Pair)
PKA slows, reduces mean distance between
collisions
clusters formed
Thermal motion 98% lattice defects anneal


knocks out atom = Primary knock on atom
defect/impurity reactions
Stable defects influence device properties
PPE S/C detector lectures
Dr R. Bates
71
PKA




PPE S/C detector lectures
Clusters formed when
energy of PKA< 5keV
Strong mutual
interactions in clusters
Defects outside of
cluster diffuse + form
impurity related defects
(VO, VV, VP)
e &  don’t produce
clusters
Dr R. Bates
72
Effects of Defects
EC
e
EV
e
h
Generation
h
Recombination
Leakage Current
PPE S/C detector lectures
e
e
h
Trapping
Charge Collection
Dr R. Bates
Compensation
Effective Doping
Density
73
Reverse Current

-1
DI / V [A/cm3]
10
10-2
10-3
n-type FZ - 7 to 25 KWcm
n-type FZ - 7 KWcm
n-type FZ - 4 KWcm
n-type FZ - 3 KWcm
p-type EPI - 2 and 4 KWcm

n-type FZ - 780 Wcm
n-type FZ - 410 Wcm
n-type FZ - 130 Wcm
n-type FZ - 110 Wcm
n-type CZ - 140 Wcm
p-type EPI - 380 Wcm
10-4
10-5
10-6 11
10

1012
1013
Feq [cm ]
-2
1014

1015

 = 3.99  0.03 x 10-17Acm-1 
after 80minutes annealing at 60C
PPE S/C detector lectures
I = FVolume
Material independent
linked to defect
clusters
Annealing material
independent
Scales with NIEL
Temp dependence
Dr R. Bates
 E 
I T   T 2 exp   g 
 2kT 
74
Effective Doping Density
5
Neutron irradiation
3
300
Wacker
Polovodice
Wacker
Topsil
250
200
150
2
100
1
0

Vdep [V] (300mm)
|Neff| [1012 cm-3]
4
1.8 KWcm
2.6 KWcm
3.1 KWcm
4.2 KWcm
Donor removal and
acceptor generation


50
0.5
1
1.5
2

Feq [10 cm ]
14
-2
N eff F   N eff 0 exp  cF   F
type inversion: n  p
depletion width
grows from n+
contact
Increase in full
depletion voltage
V  Neff
 = 0.025cm-1 measured after beneficial anneal
PPE S/C detector lectures
Dr R. Bates

75
Effective Doping Density
Short-term beneficial
annealing
Long-term reverse
annealing

10
6
NY, = gY Feq
NA = ga Feq
4

NC
gC Feq
2

NC0
0
1
10
100
1000
1000
annealing time at 60 C [min]
RA

10000
o
BA
temperature dependent
stops below -10C
RA
BA
B
A
Neff = Zero
800
standard silicon
operation voltage: 600 V
600
600
400
400
200
200
oxygenated silicon
Increasing Radiation
0
PPE S/C detector lectures
1000
800
Vdep (250mm) [V]
D Neff [1011cm-3]
8
Dr R. Bates
1
2
3
4
5
6
time [years]
7
8
9
10
76
Signal speed from a detector




Duration of signal = carrier collection
time
Speed  mobility & field
Speed  1/device thickness
PROBLEMS

Post irradiation mobility & lifetime reduced


mt lower  longer signals and lower Qs
Thick devices have longer signals
PPE S/C detector lectures
Dr R. Bates
77
Signal with low lifetime material

Lifetime, t, packet of charge Q0 decays
Q(t )  Qo e



t t
In E field charge drifts
Time required to drift distance x:
x x
t 
v mE
Remaining charge:
Drift length, L  mt
mt is a figure of merit.
Q( x)  Q0e
 x mEt
Q 0 e
x L

PPE S/C detector lectures
Dr R. Bates
78
Induced charge

Parallel plate detector:
d
d
1
1
Qs   Q( x)dx   Q0e  x L dx
d0
d0

L
Qs  Q0 1  e  d L
d

Qs L
L  d :

Q0 d
In high quality silicon detectors:
 t  10ms, me = 1350cm2V-1s-1, E = 104Vcm-1
 L  104cm (d ~ 10-2cm)




Amorphous silicon, L  10mm (short lifetime, low mobility)
Diamond, L  100-200mm (despite high mobility)
CdZnTe, at 1kVcm-1, L  3cm for electrons, 0.1cm for holes
PPE S/C detector lectures
Dr R. Bates
79
What can we do?




Detector Design
Material Engineering
Cold Operation
Electrode Structure – 3-D device
PPE S/C detector lectures
Dr R. Bates
80
Detector Design

n-type readout strips on n-type substrate






post type inversion  substrate p type 
depletion now from strip side
high spatial resolution even if not fully depleted
Single Sided
Polysilicon resistors
W<300mm thick  limit max depletion V
Max strip length 12cm  lower cap. noise
PPE S/C detector lectures
Dr R. Bates
81
Multiguard rings


Poly
Guard
rings
Enhance high
voltage operation
Smoothly decrease
electric field at
detectors edge
back plane
strip bias
bias
V
PPE S/C detector lectures
Dr R. Bates
82
Substrate Choice


Minimise interface states
Substrate orientation <100> not <111>
Lower capacitive load
 Independent of ionising radiation


<100> has less dangling surface bonds
PPE S/C detector lectures
Dr R. Bates
83
Metal Overhang
Used to avoid breakdown performance
deterioration after irradiation
2
SiO2
p+
(1)
(2)
n
n+
4mm
p+
0.6mm
Breakdown Voltage (V)

1
Strip Width/Pitch
<111> after 4 x 1014 p/cm2
PPE S/C detector lectures
Dr R. Bates
84
Material Engineering



Do impurities influence characteristics?
Leakage current independent of impurities
Neff depends upon [O2] and [C]
1E+13
500
8E+12
Standard (P51)
O-diffusion 24 hours (P52)
O-diffusion 48 hours (P54)
O-diffusion 72 hours (P56)
Carbon-enriched (P503)
-3
|Neff| [cm ]
7E+12
6E+12
5E+12
400
St = 0.0154
300
4E+12
[O] = 0.0044 0.0053
3E+12
2E+12
200
100
VFD for 300 mm thick detector [V]
[C] = 0.0437
9E+12
1E+12
0
0
1E+14
2E+14
3E+14
4E+14
0
5E+14
Proton fluence (24 GeV/c ) [cm-2]
PPE S/C detector lectures
Dr R. Bates
85
O2 works for charged hadrons
Neff unaffected by O2 content for
neutrons
Believed that charge particle irradiation
produces more isolated V and I


7
standard FZ
400
neutrons
pions
protons
5
oxygen rich FZ
neutrons
pions
protons
4
3
300
200
2
Vdep [V] (300mm)
|Neff| [1012 cm-3]
6
100
1
0
0.5
1
1.5
2
2.5
3
3.5
V + O  VO
V + VO  V2O
V2O  reverse annealing
High [O] suppresses V2O
formation
Feq [10 cm ]
14
PPE S/C detector lectures
-2
Dr R. Bates
86
Charge collection efficiency

Oxygenated Si enhanced due to lower
depletion voltage
CCI ~ 5% at 300V
after 3x1014 p/cm2
CCE of MICRON ATLAS prototype
strip detectors irradiated with 3 1014 p/cm2
PPE S/C detector lectures
Dr R. Bates
87
ATLAS operation
Vdep (250mm) [V]
Damage for ATLAS barrel layer 1
1000
1000
800
800
standard silicon
operation voltage: 600 V
600
600
400
400
200
200
oxygenated silicon
0
1
2
3
4
5
6
7
8
9
Use lower resistivity Si to
increase lifetime in neutron field
Use oxygenated Si to increase
lifetime in charge hadron field
10
time [years]
PPE S/C detector lectures
Dr R. Bates
88
Cold Operation


Know as the
“Lazarus effect”
Recovery of heavily
irradiated silicon
detectors operated
at cryogenic temps

observed for both
diodes and microstrip
detectors
PPE S/C detector lectures
Dr R. Bates
89
The Lazarus Effect

For an undepleted heavily irradiated detector:
2
 t drift 
d

CCE    exp  


D
t
 
 trap 
where
d2 
d
active region
1
N eff
T = 300 K
e
undepleted region
D
T = 77 K
e
conduction band
conduction band
electron trapping
electron de-trapping
e
trap filled
trap filled
h
hole trapping
hole de-trapping
valence band
h

h
valence band
Traps are filled  traps are neutralized
Neff compensation (confirmed by experiment)
B. Dezillie et al., IEEE Transactions on Nuclear Science, 46 (1999) 221
PPE S/C detector lectures
Dr R. Bates
90
Reverse Bias
Measured at 130K - maximum CCE
CCE falls with time to a stable value
PPE S/C detector lectures
Dr R. Bates
91
Cryogenic Results

CCE recovery at cryogenic temperatures



CCE is max at T ~ 130 K for all samples
CCE decreases with time till it reaches a stable value
Reverse Bias operation


MPV ~5’000 electrons for 300 mm thick
standard silicon detectors irradiated with
21014 n/cm2 at 250 V reverse bias and T~77 K
very low noise

Forward bias is possible at cryogenic temperatures

No time degradation of CCE in operation with forward bias or in
presence of short wavelength light

same conditions: MPV ~13’000 electrons
PPE S/C detector lectures
Dr R. Bates
92
Electrode Structure

Increasing fluence


Reducing carrier lifetime
Increasing Neff



Higher bias voltage
Operation with detector under-depleted
Reduce electrode separation


Thinner detector  Reduced signal/noise ratio
Close packed electrodes through wafer
PPE S/C detector lectures
Dr R. Bates
93
The 3-D device

Co-axial detector


Micron scale


Dr R. Bates
Readout each p+ column
Strip device

PPE S/C detector lectures
USE Latest MEM
techniques
Pixel device


Arrayed together
Connect columns
together
94
Operation
SiO2
+ve
-ve
-ve
+ve
-ve
-ve
p+
h+
h+
Bulk n
e-
W3D
n+
Equal detectors
thickness
W2D>>W3D
E
Carriers swept horizontally
Travers short distance between electrodes
PPE S/C detector lectures
W2D
e-
E
Dr R. Bates
+ve
Carriers drift total
thickness of material
95
Proposed by S.Parker, Nucl. Instr. And Meth. A 395 pp. 328-343(1997).
Advantages

If electrodes are close
Low full depletion bias
 Low collection distances
 Thickness NOT related to collection
distance
 No charge spreading
 Fast charge sweep out

PPE S/C detector lectures
Dr R. Bates
96
A 3-D device



Form an array of holes
Fill them with poly-silicon
Add contacts


Can make pixel or strip
devices
Bias up and collect charge
PPE S/C detector lectures
Dr R. Bates
97
Real spectra
At 15V
Plateau in Q collection
Fully active
Very good energy
resolution
PPE S/C detector lectures
Dr R. Bates
98
3-D Vfd in ATLAS
2000
s ta n d a rd s ilic o n
1500
1500
6 000 e fo r B -lay er
1000
1000
dep
( 2 00 m m ) [ V ]
2000
V
opera tio n voltag e: 60 0 V
500
500
o x y g e n a te d s ilic o n
0
1
2
3
4
5
6
tim e [y e a rs ]
7
8
9
•
3D detector!
10
Damage projection for the ATLAS B-layer
(3rd RD48 STATUS REPORT CERN LHCC 2000-009, LEB Status Report/RD48, 31 December 1999).
PPE S/C detector lectures
Dr R. Bates
99
Summary


Tackle reverse current
 Cold operation, -20C
 Substrate orientation
 Multiguard rings
Overcome limited carrier lifetime and
increasing effective doping density
 Change material
 Increase carrier lifetime
 Reduce electrode spacing
PPE S/C detector lectures
Dr R. Bates
100
Final Slide





Why?
Where?
How?
A major type
A major worry
PPE S/C detector lectures
Dr R. Bates
101