CERN, Summer Student Workshop 26th to 28th July 2005
Characterization of silicon sensors
after irradiation with fast particles
Alison G. Bates and Michael Moll
CERN - Geneva - Switzerland
Workshop outline
• Presentation on silicon sensors:
- Operation of silicon detectors
- Introduction to radiation damage and annealing
• Exercise
- Calculate a C-V-curve
- Calculate the defect concentration in silicon
• Experiment
- Measure CV and IV curves
- Annealing experiment
• Data Analysis and
Conclusions for detector operation in the LHC
……Outlook: New detector concepts
CERN
DT2/SSD
Example from LHC: The CMS tracker
CMS
Inner Tracker
Outer Barrel
Inner Barrel
(TOB)
(TIB)
End Cap
Inner Disks
(TEC)
2.4 m
(TID)
CMS - Currently the Most Silicon








Pixel
Micro Strip:
~ 214 m2 of silicon strip sensors
Pixel Detector
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)
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Tracking detectors – Radiation levels
ATLAS - Inner Detector
1015
Pixel
3x1014cm-2
eq [cm-2]
1014
SCT - barrel
total eq
1013
neutrons eq
pions eq
1012
CMS Tracker
 200 m2 silicon sensors
Pixel
0

other charged
10
hadrons eq
SCT - barrel
20
Detectors and electronics will be harshly irradiated !
 ATLAS - Inner Detector:
eq up to 31014cm-2 per operational year
30 40
R [cm]
50
60
 What is the impact on silicon detectors ?
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
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
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Covalent Bonding of Pure Silicon
Energy
Si
Si
Si
Si
Si
Conduction Band (CB)
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Eg =1.1eV
Valence Band (VB)
Si
Si
Si
Si
Si
Silicon atoms share valence electrons
to form insulator-like bonds.
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Electrons in N-Type Silicon
with Phosphorus Dopant
Donor atoms provide excess electrons
to form n-type silicon.
Si
Si
Si
Si
Si
Si
Si
Si
P
Si
Si
P
Si
Si
Si
Si
Si
Si
P
Si
Si
Si
Si
Si
Si
Excess electron (-)
Conduction Band (CB)
Valence Band (VB)
Phosphorus atom
serves as n-type
dopant
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Conduction in n-Type Silicon
Positive terminal from
power supply
Negative terminal
from power supply
Free electrons flow toward
positive terminal.
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
Holes in p-Type Silicon with Boron Dopant
DT2/SSD
Acceptor atoms provide a deficiency
of electrons to form p-type silicon.
Si
Si
Si
Si
Si
Si
Si
Si
B
Si
Si
B
Si
Si
Si
Si
Si
Si
B
Si
Si
Si
Si
Si
Si
+ Hole
Conduction Band (CB)
Valence Band (VB)
Boron atom serves
as p-type dopant
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
“Hole Movement in Silicon”
Boron is neutral, but
nearby electron may
jump to fill bond site.
Hole moved from
2 to 3 to 4, and will
move to 5.
Boron is now
a negative ion.
Only thermal energy to
kick electrons
from atom to atom.
The empty silicon bond
sites (holes)
are thought of as
being positive,
since their presence
makes that
region positive.
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Conduction in p-Type Silicon
Positive terminal from
voltage supply
Negative terminal
from voltage supply
-Electrons flow toward
positive terminal
+Holes flow toward
negative terminal
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
p-n-junction
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
+-n junction
Reverse
biased
abrupt
p
DT2/SSD
Poisson’s equation
q
d2
 2   x   0  N eff
dx
0
Electrical
charge density
Electrical
field strength
Positive space charge, Neff =[P]
(ionized Phosphorus atoms)
depleted
zone
neutral bulk
(no electric field)
+VB<Vdep
+VB>Vdep
particle
(mip)
Full charge collection only for VB>Vdep !
depletion voltage
Electron
potential energy
Vdep
q0

 N eff  d 2
20
effective space charge density
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
+-n junction
Reverse
biased
abrupt
p
DT2/SSD
Poisson’s equation
q
d2
 2   x   0  N eff
dx
0
with

d
  x  w  0
dx
  x  w  0
q
d
 x   0  N eff  ( x  w)
dx
0
 x   
1 q0
 N eff  ( x  w) 2
2 0
depletion voltage
Vdep
q0

 N eff  d 2
20
effective space charge density
w = depletion depth
d = detector thickness
U = voltage
Neff = effective doping concentration
dQ dQ  dw
C

dU dw  dU
dQ  q0  N eff  A  dw
20
w(V ) 
V
q0 N eff
C (U )  A 
dw 
0 q0 N eff
2U
0
q0 N eff 2U
C ( w) 
0 A
w
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
 dU
CERN
DT2/SSD
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Testing Structures - Simple Diodes
Example: Test structure from ITE
 Very simple structures in order to concentrate on the bulk features
 Typical thickness: 300mm
 Typical active area: 0.5  0.5 cm2
 Openings in front and back contact
 optical experiments with lasers or LED
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
The Charge signal
Collected Charge for a Minimum Ionizing Particle (MIP)
 Mean energy loss
dE/dx (Si) = 3.88 MeV/cm
 116 keV for 300mm thickness
Most probable charge ≈ 0.7 mean
Mean charge
 Most probable energy loss
≈ 0.7 mean
 81 keV
 3.6 eV to create an e-h pair
 72 e-h / mm (mean)
 108 e-h / mm (most probable)
 Most probable charge (300 mm)
≈ 22500 e
≈ 3.6 fC
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Radiation Damage – Microscopic Effects
 Spatial distribution of vacancies created by a 50 keV Si-ion in silicon.
(typical recoil energy for 1 MeV neutrons)
M.Huhtinen 2001
van Lint 1980
I
V
I
V
particle
SiS
EK>25 eV
V
Vacancy
+
I Interstitial
point defects
(V-O, C-O, .. )
EK > 5 keV point defects and clusters of defects
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
Radiation Damage – Microscopic Effects
DT2/SSD
particle
SiS
EK>25 eV
V
Vacancy
+
I Interstitial
point defects
(V-O, C-O, .. )
EK > 5 keV point defects and clusters of defects
60Co-gammas
Electrons
Neutrons (elastic scattering)
Compton Electrons
Ee > 255 keV for displacement  En > 185 eV for displacement
with max. E 1 MeV E > 8 MeV for cluster
 En > 35 keV for cluster
e
(no cluster production)
Only point defects
point defects & clusters
10 MeV protons
24 GeV/c protons
Mainly clusters
1 MeV neutrons
Simulation:
Initial distribution of
vacancies in (1mm)3
after 1014 particles/cm2
[Mika Huhtinen NIMA 491(2002) 194]
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Point Defects
 Intrinsic defects:
 The Vacancy (denoted V): an atom is removed.
 The Self-interstitial (denoted I ): a host atom sits in a normally
unoccupied site or interstice (various sites: bond centres,
tetrahedral sites, interstitial + displaced regular atom).
 Extrinsic defects: due to an impurity.
These can be:
 Substitutional, such as carbon substitutional (denoted Cs)
 Interstitial (such as the carbon interstitial (denoted Ci).
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Point Defects - Lattice strain
Ge, Sn
(b) A substitutional impurity in the crystal. The
impurity atom is larger than the host atom.
(a) A vacancy in the crystal.
Link: Vacancy - Hydrogen Defect
Cs
(c) A substitutional impurity in
the crystal. The impurity atom
is smaller than the host atom.
Ci, Oi
(d) An interstitial impurity in the crystal. It
occupies an empty space between host atoms.
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
Impact of Defects on Detector properties
DT2/SSD
Shockley-Read-Hall statistics
(standard theory)
charged defects
 Neff , Vdep
e.g. donors in upper
and acceptors in
lower half of band
gap
Trapping (e and h)
generation
 CCE
 leakage current
shallow defects do not
Levels close to
contribute at room
midgap
temperature due to fast
most effective
detrapping
Impact on detector properties can be calculated if all defect parameters are known:
n,p : cross sections
E : ionization energy
Nt : concentration
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Macroscopic Effects – I. Depletion Voltage
Annealing
103
1000
500
102
 600 V
type inversion
100
50
10
5
101
1014cm-2
0
10
n - type
1
10-1
"p - type"
10
0
10
1
10
2
eq [ 10 cm ]
12
-2
10
3
10
before inversion
4
NC
gC eq
NC0
0
1
10
100
1000
10000
o
annealing time at 60 C [min]
[Data from R. Wunstorf 92]
n+
p+
NY, = gY eq
Na = ga eq
6
2
10-1
 Type inversion:
SCSI – Space Charge Sign Inversion
n+
8
 Neff [1011cm-3]
5000
| Neff | [ 1011 cm-3 ]
Udep [V] (d = 300mm)
Change of Vdep (Neff)
 Short term: “Beneficial annealing”
 Long term: “Reverse annealing”
time constant :
~ 500 years (-10°C)
~ 500 days ( 20°C)
~ 21 hours ( 60°C)
after inversion
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
Macroscopic Effects – II. Leakage Current
DT2/SSD
Hadron irradiation
Annealing
6
10-2
-3
10
n-type FZ - 7 to 25 Kcm
n-type FZ - 7 Kcm
n-type FZ - 4 Kcm
n-type FZ - 3 Kcm
p-type EPI - 2 and 4 Kcm
10-4
10-5
10-6 11
10
80 min 60C
1012
1013
n-type FZ - 780 cm
n-type FZ - 410 cm
n-type FZ - 130 cm
n-type FZ - 110 cm
n-type CZ - 140 cm
p-type EPI - 380 cm
eq [cm ]
-2
1014
1015
[M.Moll PhD Thesis]
 Damage parameter  (slope)
I
α
V   eq
  independent of eq and impurities
 used for fluence calibration
(NIEL-Hypothesis)
(t) [10-17 A/cm]
I / V [A/cm3]
10-1
6
80 min 60C
5
5
4
4
3
3
2
2
1
0
1
oxygen enriched silicon [O] = 2.1017 cm-3
parameterisation for standard silicon
[M.Moll PhD Thesis]
10
100
1000
o
10000
annealing time at 60 C [minutes]
 Oxygen enriched and
standard silicon show
same annealing
 Same curve after proton
and neutron irradiation
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
1
CERN
DT2/SSD
Macroscopic effects - III. CCE
Deterioration of the Charge Collection Efficiency
 Two mechanisms reduce collectable charge:
 Trapping (electrons and holes)
 Underdepletion (detector design and geometry)
ATLAS microstrip + RO electronics
 Oxygenation has no influence
on trapping.
Q/Q0 [%]
100
80
60
40
20
0
0
Max collected charge (overdepletion) / Q0
 After 5·1014 p/cm2 (24GeV/c)
- 80% of charge collected (25ns)
- overdepletion needed !
standard
oxygenated
Data: [G.Casse, Liverpool]
1
4
3
2
p [1014 cm-2]
5
6
Data: Gianluigi Casse; 1st Workshop on Radiation Hard Semiconductor
Devices for High Luminosity Colliders; CERN; 28-30 November 2002
collection at Vdep:
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Radiation Damage
 I. Change of Depletion Voltage
 Under-depletion
 Type inversion
(segmented detector side not in the high field region any more)
 Reverse annealing: keep detectors cold even if experiment is not running
 II. Increase of Leakage Current
 Noise, power dissipation, thermal runaway
 Cooling of detectors during operation needed
 III. Degradation of Charge Collection Efficiency
 Loss of signal due to trapping
 Loss of signal due to under-depletion
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Our experiment
 Measure Capacitance-Voltage and Current-Voltage curves for:
a) non irradiated detectors
b) irradiated detectors (1e13 p/cm2 and 1e14 p/cm2)
 Perform an annealing experiment with the irradiated detectors:
Isochronal annealing
(iso-chronus = same time)
- annealing steps of 10 minutes at 50, 60, 70, 80, 90, 100, … °C
 Analysis of the results
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Damage Projection - ATLAS Pixel B-layer
 Radiation level:

eq(year) = 3.5  1014 cm-2 (full luminosity)
> 85% charged hadrons
 LHC-scenario:

1 year =
100 days beam (-7C)
30 days maintenance (20C)
235 days no beam (-7C)
RD48 standard
RD48 oxygenated (DOFZ)
CiS standard
CiS oxygenated (DOFZ)
40
RD48 standard
1000
30
CiS standard
20
500
RD48 oxygenated
10
0
Vdep (200mm) [V]
Neff (1012) [cm-3]
1500
CiS oxygenated
1
2
3
4
5
6
time [years]
7
8
9
 New:
•
Std. Silicon: rad.harder
than predicted by RD48
•
DOFZ:
reverse annealing
delayed and saturating
with high fluences
10
Aug. 2002 - simulation : M.Moll, CERN
- parameters : G.Lindstroem, Hamburg
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Annealing mechanisms
 Migration and complex formation
 Defects become mobile at certain temperature
and migrate through the silicon lattice
e.g. Vacancies (V) between 70 and 200 K
(depending on their charge state).
 Migrating defects are gettered at sinks, recombine
with their counterparts or form new defects
(complex) by association with identical or other
types of defects
e.g. V + Oi  VOi.
 Dissociation
 A complex dissociates into its components if the
lattice vibrational energy is sufficient to overcome
the binding energy. At least one of the
constituents migrates through the lattice until it
forms another defect or disappears into a sink
Em, EF, EB
e.g. at 350°C : VOi  V + Oi.
 All mechanisms need to overcome a
certain energetic barrier EA
= activation energies (EA)
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD Reverse Annealing - Temperature dependence
Measurement of NY(t) at different temperatures


1
NY (t )  1 
 1 t
Y








Extraction of Y(T)

Arrhenius plot

E
ln(  Y )   ln( k0Y )  AY
 Y (T )  1 k  exp( E AY k T )
k BT
0Y
B
activation energy :
EAa = (1.33  0.03) eV
frequency factor :
k0Y=1.51015{4341014} s-1
 interpretation: decay of defects
(k0 close to most abundant phonon frequency)
100oC 80oC
 prediction:
60oC
40oC
20oC
EA = (1.33 ±0.03) eV
15
ln( Y [s] )

10
[Feick 93]
[Moll 99]
5
time constants for other temperatures
2.6
2.8
3
-3
3.2
-1
3.4
1/T [ 10 K ]
T
Y
accel
-10°C
516 y
1/396
0°C
61 y
1/47
10°C
8y
1/6
20°C
475d
1
40°C
17d
29
60°C
1260 min
544
80°C
92 min
7430
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Rate of Reaction - Example I
 Defect dissociation (e.g. at 350°C : VOi  V + Oi )
 Simple description as 1st order process (like radioactive decay)
d NX

 k NX
dt
NX = defect concentration
k = rate constant
 Rate constant k is given by the Arrhenius relation
 E A  k = frequency factor
 0
k  k0 exp  
 k B  T  EA = activation energy
kB = Boltzmann constant (8.6 x 10-5 eV/K)
 Frequency factor k0 lies in the order of the most abundant phonon frequency
 kBT/ h = 2.1·1010 x T[K] s-1
Ref.: [Corbett 1966]
 1013 s-1 (at 300K)
“ attempt-to-escape frequency”
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Rate of Reaction - Example II
 Diffusion limited processes
 Diffusion limited reaction of two defects X and Y
NX = concentration defect X
Ny = concentration defect Y
d NX
d NY
D = Diffusivity


 4 R D N X NY
dt
dt
R = capture radius
 Diffusion constant D0 is given by the Arrhenius relation
D0 = diffusion constant
 EA = activation energy
 kB = Boltzmann constant (8.6 x 10-5 eV/K)
 E
D  D0 exp   A
 kB  T 
 Special case:
NX << NY
e.g. V + Oi  VOi with [V]<<[Oi]
 similar kinetics as for simple 1st order process (Example I)
d NX

 4 R D N Y  N X
dt
4 R D0 NY



d NX
 k NX
dt
k0
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Outlook
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Influence of Carbon and Oxygen concentration
24 GeV/c proton irradiation
8
Carbon-enriched (P503)
Standard (P51)
O-diffusion 24 hours (P52)
O-diffusion 48 hours (P54)
O-diffusion 72 hours (P56)
Carbonated
500
6
Standard
400
300
4
Oxygenated
2
0
0
600
200
Vdep [V] (300 mm)
|Neff| [1012cm-3]
10
100
1
2
3
4
24 GeV/c proton [10 cm ]
14
-2
5
Compared to standard silicon:
 High Carbon
 less radiation tolerant
 High Oxygen
 more radiation tolerant
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Device Engineering: 3D detectors
proposed by Sherwood Parker
 Electrodes:
 narrow columns along detector thickness-“3D”
 diameter: 10mm distance: 50 - 100mm
 Lateral depletion:
 lower depletion voltage needed
 thicker detectors possible
 fast signal
n
n
p
n
Present size
up to ~1cm2
n
p
n
n
Michael Moll and Alison G. Bates – Summer Student Workshop 2005
CERN
DT2/SSD
Device Engineering: 3D detectors
proposed by Sherwood Parker
 Electrodes:
 narrow columns along detector thickness-“3D”
Present size
 diameter: 10mm distance: 50 - 100mm
up to ~1cm2
 Lateral depletion:
 lower depletion voltage needed
 thicker detectors possible
 fast signal
 Hole processing :
 Dry etching, Laser drilling, Photo Electro Chemical
 Present aspect ratio (RD50) 13:1, Target: 30:1
 Electrode material
 Doped Polysilicon (Si)
 Schottky (GaAs)
n
n
p
n
n
p
n
n
Michael Moll and Alison G. Bates – Summer Student Workshop 2005