1. Semiconductor detectors
PPE S/C detector lectures
Semiconductor detectors
An introduction to semiconductor detector physics as applied to particle physics
PPE S/C detector lectures
Dr R. Bates

2. PPE S/C detector lectures
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

3. Lecture 1 - Introduction
PPE S/C detector lectures
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

4. What we want to do - Just PPE
PPE S/C detector lectures
What we want to do - Just PPE
Track particles without disturbing them
Determined position of primary interaction vertex and secondary decays
Superb position resolution
Highly segmented  high resolution
Large signal
Small amount of energy to crate signal quanta
Thin
Close to interaction point
Low mass
Minimise multiple scattering
Detector
Readout
Cooling / support
PPE S/C detector lectures
Dr R. Bates

5. Ages of silicon - the birth
PPE S/C detector lectures
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
Maybe add some nice stuff form the nim pages????
What to say about this slide:
First devices were very small single diodes. Soon become strip detectors but still only one layer was used.
HELP!!!
PPE S/C detector lectures
Dr R. Bates

6. Ages of Silicon - vertex detectors
PPE S/C detector lectures
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
Many layers of silicon now present
Allows one to have more the one space point.
Can start to use device to define a track
If you have two tracks from the same interaction can use the device to perform vertex location
Note that SLAC has a CCD vertex detector – a very beautiful device – very low noise (see later in lectures?)
PPE S/C detector lectures
Dr R. Bates

7. PPE S/C detector lectures
ALEPH
An experiment is like an onion – many layers
Want silicon detectors at the centre of the device (no mass in the way)
Close to beam pipe for less error on projected track origin
PPE S/C detector lectures
Dr R. Bates

8. ALPEH – VDET (the upgrade)
PPE S/C detector lectures
ALPEH – VDET (the upgrade)
Vdet is:
pitch 25um (distance between strips)
readout pitch 50um on r-phi side (strips run in beam direction, measure phi (polar coordinates))
100um on z-side (strips perpendicular to beam, measures distances along beam pipe)
1920 readout channels per module on z side only 1024 electronics channels
strip pairs are multiplexed
1024 r-phi channels and r/o strips
dimensions: 6 wafers per face, approx 6.5cm long each
inner radius 6.3cm
outer radius 11cm
length = approx 40cm
total thickness = 0.015Xo for tracks at 90deg incident angle
silicon wafers 300um thick
only 2 layers of silicon
results: S/N= r-phi 28:1, z = 17:1 (due to higher cap load)
point resolution: sigma (r-phi) = 12um, sigma (z) = 14um
Electrons are outside the tracking volume – no extra mass in the path of the particles
Detectors are double sided strips so that you can have 2-D (x,y) position resolution from the same device
Cooling (services etc) all at end of tracker – mass consideration
Several layers gives you 2 Space points for track reconstruction
Long silicon detectors allow you to have a large angular acceptance
Radiation length X0 =
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

9. Ages of silicon - tracking paradigm
PPE S/C detector lectures
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)
Only possible for higher energy particles as they can transverse all this extra mass
Tracking was preformed by gas devices – very low mass but slow. Now use silicon as fast
PPE S/C detector lectures
Dr R. Bates

10. ATLAS
A monster !
PPE S/C detector lectures
Dr R. Bates

11. ATLAS barrel
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
PPE S/C detector lectures
Dr R. Bates

12. What is measured Measure space points Deduce Vertex location
Decay lengths
Impact parameters
PPE S/C detector lectures
Dr R. Bates

13. Signature of Heavy Flovours
PPE S/C detector lectures
Signature of Heavy Flovours
Stable particles t > 10-6 s ct n
2.66km m
658m
Very long lived particles t > 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 > s t±
0.3 x 10-12
91mm
Bd0, Bs0, Db
1.2 x 10-12
350mm
Short lived particles
p0, h0
8.4 x 10-17
0.025mm
r,w
4 x 10-23
10-9mm!!
So from this table you see that not all decays take place over a small distance. The K-short K-long system was in fact very interesting for CP violation and had decay lengths in the cm to m range. You did not need to use silicon for the measurement of these.
As the mass gets higher the lifetime gets less and the distance they travel in turn falls.
The decays will now take place over distance that require you to get pretty close if you are to understand the system.
You can not place a detector between the production point (beam interaction) and the decay point of the B system
So you can only get the point that the system decayed at by looking at its tracks - it all happens inside the beam pipe.
You need to measure the production and decay points (both extrapolated) with sufficient accuracy that you can see differences of 10’s of micros.
PPE S/C detector lectures
Dr R. Bates

14. PPE S/C detector lectures
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
One can see that the value of the decay length is a useful measurable to find important physical quantities about a given particle
Not usual to easure directly the two points but have to extrapolate to them.
PPE S/C detector lectures
Dr R. Bates

15. to the true interaction point
Impact parameter (b)
b = distance of closest
approach of a
reconstructed track
to the true interaction point b
beam
PPE S/C detector lectures
Dr R. Bates

16. PPE S/C detector lectures
Impact parameter
Error in impact parameter for 2 precision measurements at R1 and R2 measured in two detector planes:
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
Need to add some stuff on impact parameter resolution here
PPE S/C detector lectures
Dr R. Bates

17. 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

18. Impact parameter*
Blue = 5mm
Black = 1mm (baseline)
Green = 0.5 mm
Red = 0.1 mm
Effect of extra mass and distance from the interaction point
Lower Pt
GR Width
Flux increase(%) to silicon
Improvement of the IPres. wrt 1mm(%)
5mm
-44
-38.10.9
0.5mm
+14.1
+5.8 0.7
0.1mm
+27.7
+10.0  0.7
*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

19. 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

20. Why silicon High density and atomic number
Higher specific energy loss
Thinner detectors
Reduced range of secondary particles
Better spatial resolution
High carrier mobility  Fast!
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

21. Disadvantages Cost  Area covered Material budget Radiation damage
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

22. PPE S/C detector lectures
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
Alpha = fine structure constant
Na = Avogadro’s number
Z = atomic number of the traversed material
A = atomic weight of the traversed material
re = electron radius
High-energy electrons predominantly lose energy in matter
by bremsstrahlung, and 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 (a) the mean distance over which a high-energy
electron loses all but 1=e of its energy by bremsstrahlung, and (b) of the mean free
path for pair production by a high-energy photon
PPE S/C detector lectures
Dr R. Bates

23. Lecture 2 – lots of details
Simple diode theory
Fabrication
Energy deposition
Signal formation
PPE S/C detector lectures
Dr R. Bates

24. Detector = p-i-n diode Near intrinsic bulk Highly doped contacts
Apply bias (-ve on p+ contact)
Deplete bulk
High electric field
Radiation creates carriers
signal quanta
Carriers swept out by field
Induce current in external circuit  signal
n+ contact ND=1018cm-3
ND~1012cm-3
p+ contact NA=1018cm-3
PPE S/C detector lectures
Dr R. Bates

25. 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

26. PPE S/C detector lectures
p-n junction
Carrier density p+ n
(5)
(1)
(2)
Electric field
(6)
(3)
Dopant
concentration
Need to write signal generation slide (see 3d detectors b’ham talk – where is it?)
Electric potential
(4)
Space charge
density
(7)
PPE S/C detector lectures
Dr R. Bates

27. p-n junction
take your samples – these are neutral but doped samples: p+ and n-
bring together – free carriers move
two forces drift and diffusion
In stable state
Jdiffusion (concentration density) = Jdrift (e-field)
p+ area has higher doping concentration (in this case) than the n region
PPE S/C detector lectures
Dr R. Bates

28. p-n junction Fixed charge region Depleted of free carriers
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
Electric field due to fixed charge
Potential difference across device
Constant in neutral regions.
PPE S/C detector lectures
Dr R. Bates

29. Resistivity and mobility
Carrier DRIFT velocity and E-field:
mn = 1350cm2V-1s-1 : mp = 480cm2V-1s-1
Resistivity
p-type material
n-type material
PPE S/C detector lectures
Dr R. Bates

30. Depletion width Depletion Width depends upon Doping Density:
For a given thickness, Full Depletion Voltage is:
W = 300mm, ND  5x1012cm-3: Vfd = 100V
PPE S/C detector lectures
Dr R. Bates

31. Reverse Current Diffusion current Generation current
From generation at edge of depletion region
Negligible for a fully depleted detector
Generation current
From generation in the depletion region
Reduced by using material pure and defect free
high lifetime
Must keep temperature low & controlled
PPE S/C detector lectures
Dr R. Bates

32. 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
PPE S/C detector lectures
Dr R. Bates

33. 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

34. Fabrication Use very pure material Planar fabrication techniques
High resistivity
Low bias to deplete device
Easy of operation, away from breakdown, charge spreading for better position resolution
Low defect concentration
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

35. Fabrication stages Starting material Phosphorous diffusion n- Si
dopants to create p- & n-type regions
passivation to end surface dangling bonds and protect semiconductor surface
metallisation to make electrical contact
Starting material
Usually n-
Phosphorous diffusion
P doped poly n+ Si
n- Si
PPE S/C detector lectures
Dr R. Bates

36. Fabrication stages Deposit SiO2 Grow thermal oxide on top layer
Photolithography + etching of SiO2
Define eventual electrode pattern
PPE S/C detector lectures
Dr R. Bates

37. Fabrication stages Form p+ implants Removal of back SiO2
Boron doping
thermal anneal/Activation
Removal of back SiO2
Al metallisation + patterning to form contacts
PPE S/C detector lectures
Dr R. Bates

38. PPE S/C detector lectures
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
Gettering can be used to remove contaminants from the sensitive
regions by providing capture sites for contaminants. This requires
that the critical contaminants are sufficiently mobile so that they will
diffuse to the gettering sites and be captured.
Fortuitously, the most common contaminants that introduce mid-gap
states are fast diffusers!
Disordered materials tend to be efficient getters (e.g. polysilicon).
Gettering can be promoted by chemical affinity (Phosporus)
Both can be combined, e.g. P-doped polysilicon
PPE S/C detector lectures
Dr R. Bates

39. Energy Deposition Charge particles Not covered Bethe-Bloch Bragg Peak
Neutrons
Gamma Rays
Rayleigh scattering, Photo-electric effect, Compton scattering, Pair production
PPE S/C detector lectures
Dr R. Bates

40. Charge particles - concentrating on electrons
At   3 dE/dx minimum independent of absorber (mip)
Electrons
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

41. 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)
R (m)
p 220
 25
16O 4.3
Useful when estimating
properties of a device
PPE S/C detector lectures
Dr R. Bates

42. 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

43. e/h pair creation Create electron density oscillation - plasmon
requires 17 eV in Si
De-excite almost 100% to electron hole pair creation
Hot carriers
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

44. Delta rays Proability of ejecting an electron
with E  T as a function of T
b) Range of electron as a
function of energy in silicon
PPE S/C detector lectures
Dr R. Bates

45. 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
PPE S/C detector lectures
Dr R. Bates

46. Consequences of d-electrons
Centroid displacement
Resolution as function of pulse height
PPE S/C detector lectures
Dr R. Bates

47. Consequence of d-electrons
45º
45º
15 m
E.g. CCD
300 m
Most probable E loss = 3.6keV
10% proby of 5keV 
pulls track up by 4 m
E.g. Microstrip
Most probable E loss = 72keV
10% proby of 100keV 
pulls track up by 87 m
PPE S/C detector lectures
Dr R. Bates

48. 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

49. Signal Simple diode Strip/pixel detector
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

50. Lecture 3 – Microstrip detector
Description of device
Carrier diffusion
Why is it (sometimes) good
Charge sharing
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

51. 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

52. What does it look like? HV P+ contact on front of n- bulk
Implants covered with thin thermal oxide (100nm)
Forms capacitor ~ 10pF/cm
Al strip on oxide overlapping implant
Wirebond to amplifier
Strips surrounded by a continuous p+ ring
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

53. Resolution Delta electrons Diffusion Strip pitch See lecture 2
Capacitive coupling
Read all strips
Floating strips
PPE S/C detector lectures
Dr R. Bates

54. 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

55. PPE S/C detector lectures
Carrier diffusion
Diffuse due to conc. gradient dN/dx
Gaussian
Diffusion coefficient:
RMS of the distribution:
Since D  m & tcoll  1/m
Width of distribution is the same for e & h
As charge created along a strip
Superposition of Gaussian distribution
dN/N is the fraction of charge carriers (or collected charge) in dx at a distance x from the origin after a time t
PPE S/C detector lectures
Dr R. Bates

56. Diffusion Example for electrons: Lower bias  wider distribution
tcoll = 7ns; T=20oC
s = 7mm
Lower bias  wider distribution
For given readout pitch
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

57. Resolution as a f(V) Spatial resolution as a function of bias
Vfd = 50V
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

58. Resolution due to detector design
Strip pitch
Very dense
Share charge over many strips
Reconstruct shape of charge and find centre
Signal over too many strips  lost signal (low S/N)
BUT
FWHM ~ 10mm
Limited to strip pitch 20mm
Signal on 1 or 2 strips
PPE S/C detector lectures
Dr R. Bates

59. 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, )
Track mid way Q on both strips
best accuracy
Close to one strip
Small signal on far strip
Lost in noise
PPE S/C detector lectures
Dr R. Bates

60. 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: B A C
PPE S/C detector lectures
Dr R. Bates

61. Floating strips Large Pitch (60mm) Intermediate strip
20mm strip pitch  s=2.2mm
Large Pitch (60mm)
Intermediate strip
1/3 tracks on both strips
Assume s = 2.2mm
2/3 on single strips
s = 40/12 = 11.5mm
Overall:
s = 1/3 x /3 x 11.5
= 8.4mm
60mm
20mm
20mm
20mm
20mm
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 /3 x 20/12
= 3.4mm
PPE S/C detector lectures
Dr R. Bates

62. Off line analysis Binary readout No information on the signal size
Large pitch and high noise
Get a signal on one strip only
<x> = 0
P(x)
-½ pitch ½ pitch
PPE S/C detector lectures
Dr R. Bates

63. Centre of Gravity Have signal on each strip
Assume linear charge sharing between strips
PHL
PHR
Q on 2 strips & x = 0 at left strip P x
e.g. PHL = 1/3PHR
PPE S/C detector lectures
Dr R. Bates

64. Eta function
Non linear charge sharing due to Gaussian charge cloud shape
PHL
PHR
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 P x
PPE S/C detector lectures
Dr R. Bates

65. Eta function Measured tracks as a function of incident particle flux
Measured and predicted particle position
PPE S/C detector lectures
Dr R. Bates

66. Lorentz force Force on carriers due to magnetic force
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 H qL ve
PPE S/C detector lectures
Dr R. Bates

67. 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

68. Details Double sided detectors Surface charge build up on n-side
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

69. 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

70. 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

71. Displacement Damage Incoming particle undergoes collision with lattice
knocks out atom = Primary knock on atom
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
defect/impurity reactions
Stable defects influence device properties
PPE S/C detector lectures
Dr R. Bates

72. PKA 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
PPE S/C detector lectures
Dr R. Bates

73. Effects of Defects EC e e e e h h h EV Leakage Current
Generation
Recombination
Trapping
Compensation
Leakage Current
Effective Doping
Density
Charge Collection
PPE S/C detector lectures
Dr R. Bates

74. after 80minutes annealing at 60C
Reverse Current
I = Volume
Material independent
linked to defect clusters
Annealing material independent
Scales with NIEL
Temp dependence
 = 3.99  0.03 x 10-17Acm-1
after 80minutes annealing at 60C
PPE S/C detector lectures
Dr R. Bates

75. Effective Doping Density
Donor removal and acceptor generation
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

76. Effective Doping Density
Short-term beneficial annealing
Long-term reverse annealing
temperature dependent
stops below -10C
PPE S/C detector lectures
Dr R. Bates

77. Signal speed from a detector
Duration of signal = carrier collection time
Speed  mobility & field
Speed  1/device thickness
PROBLEMS
Post irradiation mobility & lifetime reduced
 lower  longer signals and lower Qs
Thick devices have longer signals
PPE S/C detector lectures
Dr R. Bates

78. Signal with low lifetime material
Lifetime, , packet of charge Q0 decays
In E field charge drifts
Time required to drift distance x:
Remaining charge:
Drift length, L  mt
mt is a figure of merit.
PPE S/C detector lectures
Dr R. Bates

79. Induced charge Parallel plate detector:
In high quality silicon detectors:
  10ms, e = 1350cm2V-1s-1, E = 104Vcm-1
 L  104cm (d ~ 10-2cm)
Amorphous silicon, L  10m (short lifetime, low mobility)
Diamond, L  m (despite high mobility)
CdZnTe, at 1kVcm-1, L  3cm for electrons, 0.1cm for holes
PPE S/C detector lectures
Dr R. Bates

80. What can we do? Detector Design Material Engineering Cold Operation
Electrode Structure – 3-D device
PPE S/C detector lectures
Dr R. Bates

81. Detector Design n-type readout strips on n-type substrate Single Sided
post type inversion  substrate p type  depletion now from strip side
high spatial resolution even if not fully depleted
Single Sided
Polysilicon resistors
W<300m thick  limit max depletion V
Max strip length 12cm  lower cap. noise
PPE S/C detector lectures
Dr R. Bates

82. Multiguard rings Enhance high voltage operation
Smoothly decrease electric field at detectors edge
back plane
bias
Poly
strip bias
Guard
rings V
PPE S/C detector lectures
Dr R. Bates

83. 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

84. Metal Overhang
Used to avoid breakdown performance deterioration after irradiation 2
SiO2 p+
(1)
(2) n 1 n+
Breakdown Voltage (V)
4m
0.6m p+
Strip Width/Pitch
<111> after 4 x 1014 p/cm2
PPE S/C detector lectures
Dr R. Bates

85. Material Engineering Do impurities influence characteristics?
Leakage current independent of impurities
Neff depends upon [O2] and [C]
PPE S/C detector lectures
Dr R. Bates

86. O2 works for charged hadrons
Neff unaffected by O2 content for neutrons
Believed that charge particle irradiation produces more isolated V and I
V + O  VO
V + VO  V2O
V2O  reverse annealing
High [O] suppresses V2O
formation
PPE S/C detector lectures
Dr R. Bates

87. 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 p/cm2
PPE S/C detector lectures
Dr R. Bates

88. PPE S/C detector lectures
ATLAS operation
Damage for ATLAS barrel layer 1
Use lower resistivity Si to
increase lifetime in neutron field
Use oxygenated Si to increase
lifetime in charge hadron field
PPE S/C detector lectures
Dr R. Bates

89. 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

90. The Lazarus Effect For an undepleted heavily irradiated detector:
Traps are filled  traps are neutralized Neff compensation (confirmed by experiment) B. Dezillie et al., IEEE Transactions on Nuclear Science, 46 (1999) 221 d D
undepleted region
active region
where
PPE S/C detector lectures
Dr R. Bates

91. Reverse Bias Measured at 130K - maximum CCE
CCE falls with time to a stable value
PPE S/C detector lectures
Dr R. Bates

92. 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

93. Electrode Structure Increasing fluence Reduce electrode separation
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

94. The 3-D device Co-axial detector Micron scale Pixel device
Arrayed together
Micron scale
USE Latest MEM techniques
Pixel device
Readout each p+ column
Strip device
Connect columns together
PPE S/C detector lectures
Dr R. Bates

95. Operation -ve -ve -ve +ve -ve +ve E W2D W3D +ve E Carriers drift total
SiO 2 p + h + h +
Bulk n E
W2D e - e - n +
W3D
Equal detectors thickness
W2D>>W3D
+ve E
Carriers drift total
thickness of material
Carriers swept horizontally
Travers short distance between electrodes
PPE S/C detector lectures
Dr R. Bates
Proposed by S.Parker, Nucl. Instr. And Meth. A 395 pp (1997).

96. 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

97. 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

98. Real spectra At 15V Very good energy resolution
Plateau in Q collection
Fully active
Very good energy resolution
PPE S/C detector lectures
Dr R. Bates

99. 3-D Vfd in ATLAS Damage projection for the ATLAS B-layer1 2 3 4 5 6 7 8 9 t i m e [ y a r s ] V d p ( ) n l c o x g v : f B -
Damage projection for the ATLAS B-layer
(3rd RD48 STATUS REPORT CERN LHCC , LEB Status Report/RD48, 31 December 1999).
3D detector!
PPE S/C detector lectures
Dr R. Bates

100. 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

101. Final Slide Why? Where? How? A major type A major worry
PPE S/C detector lectures
Dr R. Bates