Detectors for particles and radiation
Advanced course for Master students
Spring semester 2010
S7139
Tuesday 10:15 to 12:00 - Lectures
Tuesday 16:15 to 17:00 - Exercises
5 ECTS points
Detectors for particles and radiation
February 23
Kreslo, Gornea
Introduction, History of instrumentation
March 2
Kreslo, Gornea
Particle-matter electromagnetic interactions
March 9
Kreslo, Gornea
Gas detectors : counters
March 16
Kreslo, Gornea
Gas detectors : tracking
March 23
Kreslo, Gornea
Scintillating detectors :counters
March 30
Kreslo, Gornea
Scintillating detectors : tracking
April 13
Kreslo, Gornea
Semiconductor detectors : counters
April 20
Kreslo, Gornea
Semiconductor detectors : tracking
April 27
Kreslo, Gornea
Semiconductor detectors : tracking
April 27
Kreslo, Gornea
Cryogenic liquids : tracking
May 4
Bay, Gornea
Nuclear emulsions
May 11
Kreslo, Gornea
Calorimetry
May 18
Kreslo, Gornea
Particle Identification
May 25
Kreslo, Gornea
Momentum measurements
June 1
Kreslo, Gornea
Discussion + Lab demonstration
April 6
Particle-matter interactions: review
Particle-matter interactions: review
Particle-matter interactions: review
Semiconductor Solid State Detectors
- Why use Semiconductor Detectors ?
- How are Silicon Detectors made and how do they work ?
- Some types of practical design
- Radiation Damage in Silicon Detectors
- Outlook: Radiation tolerant detectors
- References
Why semiconductors?
Why semiconductors?
Semiconductor Detectors
Semiconductor Detectors
Choice of material
Semiconductors in periodic table
Energy levels in semiconductor
Conductivity
Semiconductors General
Semiconductors General
Semiconductors: Silicon
Semiconductor Detectors
Particle energy loss in Silicon
Signal formation in Silicon
Silicon doping
Doping and resistivity
e.g. Phosphorus
Si
Si
Si
Si
P
Si
Si
Si
Doping: n-type Silicon
Doping: p-type Silicon
- add elements from Vth group - add elements from IIIrd group
 donors (P, As,..)
 acceptors (B,..)
- electrons are majority carriers - holes are the majority carriers
E
E
E
CB
e
CB
f
Si
Ef
VB
Resistivity
- carrier concentrations n, p
- carrier mobility n, p
  1 q  n   p
0
n
p
detector
grade
doping
electronics
grade
 1012 cm-3  1017 cm-3
resistivity   5 k·cm
1 ·cm
h
VB
P-N junction
P-N junction
P-N junction in forward bias
P-N junction in reverse bias
Depth of the depletion region
Depth of the depletion region
Reverse biased abrupt p+ n junction
Poisson’s
equation
q
d2
 2  x   0  Neff
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
 0
 N eff  d 2
effective space charge density
Calculation of depletion voltage (diode)
Poisson’s
equation
q
d2
 2  x   0  Neff
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
w = depletion depth
d = detector thickness
U = voltage
Neff = effective doping concentration
dQ dQ  dw
C

dU dw  dU
2 0
w(V ) 
V
q0 N eff
q0

 N eff  d 2
2 0
effective space charge density
dQ  q0  N eff  A  dw
C (U )  A 
dw 
 0 q0 N eff
2U
 0
q0 N eff 2U
C ( w) 
 dU
 0 A
w
P-N junction: overview
Charge transport
Drift velocity in Silicon
Charge diffusion in Silicon
Energy resolution: Fano factor
Fano factor: derivation
Fano factor: derivation
Silicon detectors: manufacturing
Manufacturing of Si monocristals
Manufacturing of Si monocristals
How to make a Float Zone Silicon wafer?
Produce a polysilicon rod
 Melt very pure sand (SiO2)
together with coke (~1800°C)
SiO2  2C  Si  2CO
 Grind the “metallurgical grade
silicon” (98% Si) and expose it
to hydrochloric gas
Float Zone process
 Using a single Si crystal
seed, melt the vertically
oriented rod onto the seed
using RF power and “pull”
the monocrystalline ingot
Monocrystalline Ingot
 grind into round shape
 make the flat or a notch
Poly silicon rod
Si  3HCl ( gas)  SiHCl3  H2
Wafer production
 Slice the ingot into wafers of
 Trichlorsilane boils at 31.7°C
and can thus be distilled and
purified
300-500 m (diamond saw)
 lapping of wafers
 etching of wafers
 polishing of wafers
 Deposit silicon in a Chemical
Vapour Deposition process
SiHCl3  H2  Si  3HCl
 Cast silicon into a
polycrystalline silicon rod
Single crystal silicon
Manufacturing of Si monocristals
Silicon Sensor Production
A "simple“ production sequence (schematic)
 Polished n-type silicon wafer (typical  ~ 1-10 Kcm )
n-type silicon
SiO2 
Oxidation (800-1200°C)
 Photolithograpy (coat with photo resist; align mask,
expose to UV light, develop photoresist);
Etching of oxide
UV light
etch
 Doping with boron and phosphorus by implantation (or by diffusion)
Annealing to cure radiation damage and activate dopants
- p+ n junction on front side
- n n+ ohmic contact on back side
Boron
Phosphorus
Al
p+
 Aluminize surface (e.g. by evaporation)
p+
n+
 Pattern metal for diode contacts
p+
p+
n+
M.Moll
The Charge Signal
Collected Charge for a Minimum Ionizing
Particle (MIP)
Most probable charge ≈ 0.7 mean
 Mean energy loss
dE/dx (Si) = 3.88 MeV/cm
 116 keV for 300m thickness
 Most probable energy loss
≈ 0.7 mean
 81 keV
 3.6 eV to create an e-h pair
 72 e-h / m (mean)
 108 e-h / m (most probable)
 Most probable charge (300 m)
≈ 22500 e
≈ 3.6 fC
Mean charge
Charge Readout
Signal to noise ratio (S/N)
Landau distribution has a low energy tail
- becomes even lower by noise broadening
Noise sources: (ENC = Equivalent Noise Charge)
- Capacitance
Landau distribution
Noise
Landau distribution
with noise
ENC  Cd
- Leakage Current
ENC  I
[M.Moll, schematic figure!]
- Thermal Noise
(bias resistor)
ENC 
k BT
0
100
R
200
300
400
500
ADC channel (arb. units)
Noise
Good hits selected by requiring NADC > noise tail
If cut too high  efficiency loss
If cut too low  noise occupancy
Figure of Merit: Signal-to-Noise Ratio S/N
Typical values >10-15, people get nervous below 10.
Radiation damage severely degrades the S/N.
Signal
Cut (threshold)
Charge Collection time and diffusion
Charge Collection time
 Drift velocity of charge carriers v ≈ E, so drift time, td = d/v = d/E
Typical values: d=300 m, E= 2.5 kV/cm,
with e= 1350 cm2 / V·s and h= 450 cm2 / V·s
 td(e)= 9ns , td(h)= 27ns
Diffusion
 Diffusion of charge “cloud” caused by scattering of drifting charge carriers,
radius of distribution after time td:
  2 Dtd
D   kT
q
with diffusion constant
 Same radius for e and h since td  1/
Typical charge radius:  ≈ 6m, could exploit this
to get better position resolution due to charge sharing
between adjacent strips (using centroid finding), but
need to keep drift times long (low field).
MAPS – Monolithic Active Pixel Sensors
Monolithic detectors
 readout electronics directly within sensor material (same epi layer)
15m
 charge collected at n-well / p-epi
diode
 thermal diffusion of free charge
 reflection at potential barriers
between areas with different doping
concentration
no depletion voltage applied
 potential formed by different
doping
concentrations only
• no connections needed to electronics (e.g. no bumps)
• very small sizes achievable
DEPFET - DEP(leted)F(ield)E(ffect)T(ransistor)
 FET integrated on high resistivity bulk, bulk sideward depleted
 electrons collected in potential minimum at internal gate
- transistor current modulated by collected charge
- charge removed by reset mechanism (clear)
 switch on/off by (external) top gate to read out
source
p+
p-channel
internal gate
top gate
drain
bulk
p+
n+
n+
potential via axis
top-gate / rear contact
~1m
n
+
+
+
totally depleted
n--substrate
potential minimum
for electrons
~300 m
p+
rear contact
V
 amplification of charge at the position of collection  no transfer loss
 full bulk sensitivity, bulk can be thinned down to 50 m if needed
 non structured entrance window (backside)
 very low imput capacitance  very low noise
Limitations of Silicon: radiation damage
Limitations of Silicon: radiation damage
Radiation Damage: Microscopic defects
Damage to the silicon crystal: Displacement of lattice atoms
EK>25 eV
SiS
particle
V
I
EK > 5 keV
80 nm
“point defects”, mobile in silicon,
can react with impurities (O,C,..)
point defects and clusters of defects
I
V
Vacancy
+
Interstitial
I
Distribution of vacancies
created by a 50 keV Siion in silicon (typical
recoil energy for 1 MeV
neutrons):
V
Schematic
[Van Lint 1980]
Simulation
[M.Huhtinen 2001]
Defects can be electrically active (levels in the band gap)
- capture and release electrons and holes from conduction and valence band
 can be charged - can be generation/recombination centers - can be trapping centers
Radiation Damage: Particle dependence
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
Compton Electrons
with max. E  MeV
(no cluster
production)
only point defects
Simulation:
Initial distribution of
vacancies in (1m)3
after 1014 particles/cm2
[Mika Huhtinen NIMA 491(2002) 194]
Neutrons (elastic scattering)
Electrons
Ee > 255 keV for displacement En > 185 eV for displacement
Ee > 8 MeV for cluster
En > 35 keV for cluster
point defects & clusters
10 MeV protons
neutrons
mainly clusters
24 GeV/c protons
1 MeV
Impact of defects on detector properties
Inter-center charge
transfer model
(inside clusters only)
Shockley-Read-Hall statistics
(standard textbook theory)
charged defects
 Neff , Vdep
trapping (e and h)
 CCE
generation
 leakage current
e.g. donors in upper
and acceptors in lower
half of band gap
shallow defects do not
contribute at room
temperature due to fast
detrapping
Levels close to midgap
are most effective
Impact on detector properties can be calculated
if all defect parameters are known:
n,p : cross sections
E : ionization energy
enhanced generation
 leakage current
 space charge
Nt : concentration
Radiation Damage in Silicon Sensors
Two general types of radiation damage:
 Bulk (Crystal) damage due to Non Ionizing Energy Loss (NIEL)
- Displacement Damage –
I.
Change of depletion voltage (higher operation voltage, underdepletion)
 constant cooling needed to avoid reverse annealing
II. Increase of leakage current (increase of shot noise, thermal runaway)
 needs cooling of sensors during operation
III. Decrease of charge collection efficiency
due to underdepletion and increased trapping
 Surface damage due to Ionizing Energy Loss (IEL)
- accumulation of positive in the oxide (SiO2) and the Si/SiO2 interface –
affects: interstrip capacitance (noise factor), breakdown behavior
and other structures depending on near-surface effects
Signal/noise ratio is the quantity to watch
 Sensors can fail from radiation damage !
Radiation Damage – I. Depletion Voltage
103
1000
500
102
 600 V
type inversion
100
50
10
5
1
10-1
101
1014cm-2
100
"p-type"
n-type
[M.Moll: Data: R. Wunstorf, PhD thesis 1992, Uni Hamburg]
10
0
10
1
10
2
10
eq [ 10 cm ]
12
3
10
10-1
-2
p+
n+
p+
n+
8
6
NY
NA
4
NC
gC eq
2
NC0
[M.Moll, PhD thesis 1999, Uni Hamburg]
0
1
10
100
1000 10000
annealing time at 60oC [min]
• Short term: “Beneficial annealing”
• Long term: “Reverse annealing”
- time constant depends on temperature:
~ 500 years (-10°C)
~ 500 days ( 20°C)
~ 21 hours ( 60°C)
- Consequence: Detectors must be cooled
even when the experiment is not running!
after inversion
• “Type inversion”: Neff changes from positive to
negative (Space Charge Sign Inversion)
before inversion
…. with time (annealing):
 Neff [1011cm-3]
5000
| Neff | [ 1011 cm-3 ]
Udep [V] (d = 300m)
Change of Depletion Voltage Vdep (Neff)
…. with particle fluence:
Radiation Damage – II. Leakage Current
Change of Leakage Current (after hadron irradiation)
…. with particle fluence:
…. with time (annealing):
10-2
10-3
6
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
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
-4
10
10-5
10-6 11
10
80 min 60C
1012
1013
1014
(t) [10-17 A/cm]
I / V [A/cm3]
10-1
I
α
V   eq
•
Leakage current
per unit volume
and particle fluence
 is constant over several orders of fluence
and independent of impurity concentration in Si
 can be used for fluence measurement
5
4
4
3
3
2
2
.
0
17
-3
oxygen enriched silicon [O] = 2 10 cm
parameterisation for standard silicon
[M.Moll PhD Thesis]
1
10
100
1000
o
10000
annealing time at 60 C [minutes]
[M.Moll PhD Thesis]
•
80 min 60C
5
1
1015
eq [cm-2]
Damage parameter  (slope in figure)
6
•
•
Leakage current decreasing in
time (depending on temperature)
Strong temperature dependence
 E

I  exp  g

2
k
T
B


Consequence:
Cool detectors during operation!
Example: I(-10°C) ~1/16 I(20°C)
1
Radiation Damage – III. CCE
Deterioration of Charge Collection Efficiency (CCE)
2 mechanisms: - Trapping of electrons and holes
- Underdepletion (loss of active detector volume due to increase
of Vdep)
Trapping is characterized by an effective trapping time eff for electrons and
holes:


1
1
 N defects
Qe,h (t )  Q0 e,h exp 
 t
wher
 


eff e , h
 eff e,h 
e
0.5
0.4
data for electrons
data for holes
0.3
0.2
0.1
[M.Moll; Data: O.Krasel, PhD thesis 2004, Uni Dortmund]
0
0
2b/58
2.1014 4.1014 6.1014 8.1014
1015
particle fluence - eq [cm-2]
….. and change with time
(annealing):
Inverse trapping time 1/ [ns-1]
Inverse trapping time 1/ [ns-1]
Increase of inverse trapping time (1/) with
fluence 24 GeV/c proton irradiation
0.25
24 GeV/c proton irradiation
eq = 4.5.1014 cm-2
0.2
0.15
data for holes
data for electrons
0.1
[M.Moll; Data: O.Krasel, PhD thesis 2004, Uni Dortmund]
5 101
5 102
5 103
annealing time at 60oC [min]
R&D : Radiation tolerant tracking detectors
Defect Engineering of Silicon
Scientific Strategies:
I. Material engineering
II. Device engineering
III. Variation of detector
operational conditions
CERN R&D collaborations:
- RD50 “Radiation hard semiconductor
devices for very high luminosity colliders”
- RD42 “CVD Diamond Radiation Detectors”
- RD39 “Cryogenic Tracking Detectors”
Deliberate incorporation of impurities or defects into the silicon
bulk to improve radiation tolerance of detectors
– Needs: profound understanding of radiation
damage
(microscopic defects, macroscopic parameters,
dependence on particles type and energy,
defect formation kinetics and annealing processes)
– Examples: - Oxygen enriched silicon
- Hydrogen enriched silicon
- Pre-irradiated silicon
New Materials (other semiconductors than Si)
– Diamond, Silicon Carbide (SiC), …
New detector designs
– Examples: - p-type silicon detectors (n-in-p)
- thin detectors, epitaxial detectors
- 3D and Semi 3D detectors
Cryogenic operation of detectors
Operate detectors at 100-200K to reduce the
charge loss (“Lazarus effect”)
New Material: Oxygen enriched silicon – DOFZ
DOFZ (Diffusion Oxygenated Float Zone Silicon)
 1982 First oxygen diffusion tests on FZ
[Brotherton et al. J.Appl.Phys.,Vol.53, No.8.,5720]
 1995 First tests on detector grade silicon [Z.Li et al. IEEE TNS Vol.42,No.4,219]
 1999 Introduced to the HEP community by CERN - RD48 (ROSE-Collaboration
Strong improvement after charged
hadron irradiation observed
10
10
5
Cz as grown
1017
5
1016
5
1015
0
DOFZ 72h/1150oC
DOFZ 48h/1150oC
DOFZ 24h/1150oC
50
100
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
2
depth [m]
200
250
[G.Lindstroem et al.]
0
0
400
300
4
Oxygenated
150
600
200
Vdep [V] (300 m)
18
|Neff| [1012cm-3]
O-concentration [cm-3]
Very long oxidation (e.g. 48h at 1150°C)
increases the oxygen content in silicon
)
100
[RD48-NIMA 465(2001) 60]
1
2
3
4
24 GeV/c proton [10 cm ]
14
-2
5
 2005: DOFZ silicon used for the ATLAS and CMS Pixel detectors
 2005: Other types of oxygen rich silicon under investigation: Czochralski Si, epitaxial Si
n-in-n vs p-in-n
n-type silicon after type inversion:
p+on-n
p-on-n silicon, under-depleted:
n+on-n
n-on-n silicon, under-depleted:
• Charge spread – degraded resolution •Limited loss in CCE
• Charge loss – reduced CCE
•Less degradation with underdepletion
•Collect electrons (fast)
References and Acknowledgements
Besides references given on the transparencies, the following sources have
been used:
Books
 Gerhard Lutz, “Semiconductor Radiation Detectors”, Springer, ISBN 3-540-64859-3
 S.M.Sze, “Physics of Semiconductor Devices”, John Wiley & Sons, ISBN 0-471-05661-8
Articles
 Anna Peisert, “Silicon microstrip detectors”, Instrumentation in High Energy Physics, World Scientific, 1992
 Michael Moll, “Radiation Damage in Silicon Particle Detectors”, PhD thesis, DESY, December 1999
 Geoffrey Hall, “Semiconductor particle tracking detectors”, Rep.Prog.Phys. 57 (1994) 481-531
Lectures and Presentations




Georg Steinbrück, Lecture for summer students, Hamburg University, August 15, 2008
Alan Honma, “Silicon Detectors”, Nato Advanced Study Institute, Virgin Islands, 06/2002, http://cern.ch/honma/
Christian Joram, “Particle Detectors”, CERN, Summer Student Lectures June 2003
Paula Collins, “Recent Detector R&D and operational experience”, IWORID07, Riga, September 2003








Gerhard Lutz, “Semiconductor Radiation Detectors”, Louvain, Seminar, June 2002
Marcello Mannelli, “Tracking at the LHC: The CMS example”, CERN Academic Training, March 2005
Pierre Jarron, “Microelectronics, Nanoelectronics, Monolithic Pixel Detectors” CERN Academic Training, Jan. 2004
Hans Dijkstra, “Overview of Silicon Detectors”, Vienna conference, VCI 2001
Volker Adler, “The TESLA Vertex Detector” ZEUS Student Seminar, Jan.2004
Daniela Bortoletto, “An introduction to semiconductor detectors”, Vienna conference, VCI 2004
A list of conferences about Solid State Detectors and Radiation Damage: http://cern.ch/mmoll/links/conferences.htm
Vertex 2004 conference: http://sucimaweb.dipscfm.uninsubria.it/vertex04/
In the next lecture:
Semiconductor particle detectors
Strip tracking detectors
Pixel tracking detectors
Exercises:
1. Calculate full depletion voltage for the PN junction with the
following parameters:
2. Calculate detected charge from 1GeV muon passing at a normal
angle through such a detector.
3. Calculate energy resolution of such detector for 1MeV proton.