silicon detectors in hep
DESCRIPTION
Silicon detectors in HEP. Enver Alagoz ACCTR Meeting 18 August 2012. Introduction to silicon. S emiconductor material Used for radiation detection/imaging in Particle and nuclear physics experiments Space and ground-based telescopes Medical applications Industrial applications … - PowerPoint PPT PresentationTRANSCRIPT
Silicon detectors in HEP
Enver AlagozACCTR Meeting 18 August 2012
2
Introduction to silicon• Semiconductor material
– Used for radiation detection/imaging in• Particle and nuclear physics experiments• Space and ground-based telescopes• Medical applications• Industrial applications• …
• Interacts with radiation– Radiation creates charge carriers – Electric field drifts carriers toward readout electronics– Signal integrated in frond-end electronics– Digitize integrated signal– Readout and store digital signal
3
Particle reconstruction in timePre-silicon era
Si + gaseous tracking
Si tracking+vertex+calorimetry
time
1970
s19
90s
2010
s
• Why detectors change – Interested in rare events
• in very high energies• with very small uncertainties• with very high background
• Need high precision– Very high granularity– low material budget
• High particle collision rates– High speed electronics– Good background tolerance – Long term reliability – Radiation hardness
4
Gaseous vs silicon detectors
Property Gas Silicon Importance
Density Low 2.33 g/cm3 Denser -> better spatial resolution
Charge electrons and ions electrons and holes
Ionization energy 30eV per e- ion pair 3.6 eV per e- hole pair Lower -> better energy resolution
Mobility 10 ns to 10 μs few ns to 20 ns Faster -> no dead time
Signal-to-noise Low High Reliable signal
• In silicon detector• No multiplication signal amplification needed• Optimum thickness to minimize multiple scattering
5
Vertex reconstructionOuter Si layer
Inner Si layer
R1R2
Outgoing particle
Primary vertexImpact parameter
b
If both layers have same spatial resolution (σ)
IP
6
CMS pixel detector
Forward Pixel Barrel Pixel
7
Silicon detectors in HEP• In HEP experiment
• Vertex reconstruction• Fast detection and position resolution
Experiments with silicon detector
E. Do Couto e Silva, Vertex 2000, Sept 10-15, National Lakeshore, MI, USA
8
Why silicon? 2nd most abundant element in Earth (28% by mass) Easy to process and purify to 0.001 ppb Natural SiO2 as insulation during fabrication Found in form of silicate minerals and SO2 (silica – sand) Discovered in 1824 by Swedish chemist J.J. Berzelius
Commercially utilized since 1824
Silicon powder
Silicon crystal
Spectral lines of Silicon
9
Silicon - 14Si28
10
Basic properties – Band gap
Conduction band
Valance band
Conduction band
Valance band
electrons
holes
Eg ~ 6 eV Eg ~ 1.12 eV
Conduction band
Vala
nce
band
overlap
Insulator Semiconductor Conductor
11
Basic properties
• Very pure material• Charge carriers are created by– thermal, optical, and other excitations or ionization
• Four valance electrons (covalent bonds)• Silicon (Si) and Germanium (Ge) are common• Doped with extrinsic semiconductors– N-type: excess electrons (e- donor), i.e. P, As etc.– P-type: excess holes (e- acceptor), i.e. Al, B, Ga, etc.
12
Silicon PN-junction• Silicon detector are in basic form of PN-junction
N-doped silicon P-doped silicon
N-type: PhosphorusP-type: BoronEg : Band gap (1.12 eV)EF
EC
EV
EF
Conduction band Conduction band
Valance bandValance band
Eg
-
SiSiSi
SiSi
SiSiSi
SiSi
+
Si
SiSi
Si SiSi
Si Si
Donor impurity Acceptor impurity
Excess electron Excess hole
13
Silicon pn-junction formation
Carrier concentration
Charge density
Electric field
Coulomb potential
Depletion zone
P-typeneutral
N-typeneutralNA ND
xNA ND
xeNA
eND
x
x
N,P
ρ(x)
E(x)
Φ(x)
NP
x=0
dP dn
NA: # of acceptor impuritiesND: # of donor impurities
14
PN-junction bias schemes
PN
E
WF
+_
VF
PN
E
WR
+ _
VR
PN
E
WE
V = 0
WF : depletion width at forward biasWR : depletion width at reverse biasWE : depletion width in equilibrium
WF < WE < WR
Reverse bias scheme is used for silicondetectors in HEP
15
A real life case• Larger active area provided by n(p)-type subtstrate
p-type substrate
p+
n+
E+
_
VR
depletion
depleted zone
undepleted zonet (th
ickn
ess)
Depletion with
Resistivity
Depletion voltage
Effective doping concentration
16
PN-junction current
Current
Voltage
Forward current
Reverse current(leakage)
Vbreakdown
Avalanche current IPN α T3/2 exp[-Eg/2kBT]
PN WR
+ _VR
PN
WF
+_VF
17
Leakage current
Current α T3/2 exp[-Eg/2kBT]
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Breakdown voltage• Extracted from capacitance measurements
0 50 100 150 200 250 300 350 400 450 5006.0E-24
2.0E-10
4.0E-10
6.0E-10
8.0E-10
1.0E-09
Voltage (VR)
Capa
cita
nce
(F)
0 50 100 150 200 250 300 350 400 450 5000.00E+00
2.00E+19
4.00E+19
6.00E+19
8.00E+19
1.00E+20
1.20E+20
1.40E+20
1.60E+20
1.80E+20
2.00E+20
Voltage (VR)
1/C2
VFD
W = t
19
Charge collection• Carriers are drifted under the electric field
μe- = 1500 cm2/Vs μh = 450 cm2/Vs
• Total drift time for a carrier created at depth xVB = applied voltageVFD = full depletion voltaged = detector thickness• Maximum drift time (VB >> VFD)
For d = 300 μm
20
Radiation detectionIonization energy loss:
• Ionization loss follows Landau statistics• Average energy loss is 390 eV/μm of Si• Average charge is 108 e-h/μm of Si• Most probable charge 80 e-h/μm of Si• 24 ke in 300 μm thick Si
Most ionizing particle = Landau MP
21
Signal-to-Noise (S/N)• Signal is Landau, noise is Gaussian
Landau+Gaussian fit
• Ionization energy loss follows Landau statistics • Random electronic noise centered at zero is Gaussian
Most probable signalNoise
=S/N
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Charge vs applied bias
• Collected charge increases until full depletion is reached
300 um thickSilicon detector p+
n+
E+
_
VR
depletion
depleted zone
undepleted zonet (th
ickn
ess)
23
Silicon wafer fabrication• Hyper-pure polysilicon chunks• Melt and add impurity to make n-type (P) or p-type (B)• Pour into a mold to make a polysilicon cylinder• Melt onto a mono-crystal silicon seed by means of RF power• Seed and melted polysilicon rotations are opposite to grow a round shape mono-crystal ingot• Employ a grindwheel to form the ingot into a desirable diameters• Cut ingot into wafers by means multi-wire-sawing• Lapping to flatten wafers• Chemical etching to smoothen wafers• Edge rounding for wafer robustness
24
Device processing
n-Si
SiO2
800-1200 °C
Photo-resist layer
UV light
etching SiO2
Boron implantation (p+)
Phosphor implantation (n+)
n+
p+p+n-Si
n+
p+p+n-Si
metal
metalmetal
600 °C
1D silicon strip detector
mask
25
Principle of operation
E
n+
p+
substrate
+++++++
++
- ---
---
- - 1 2 3 4
1. Preamplifier: amplifying small sensor signal2. Pulse shaper: improving signal-to-noise ratio by filtering signal and
attenuating electronic noise3. Analog-to-Digital Converter (ADC)4. Buffer: Store data Front-end electronics add noise to signal (smearing). Low noise
readout is essential
26
A silicon strip detector
Strip sensor Readout electronics
Wire bond (SEM image)
Wire bondsWire bond mechanism
27
Position resolution
p+
n+
E
particle trackpitch
p+
n+
E
particle trackpitch
n+
e-
Charge interpolation improves spatial resolution
Pad
sens
orSe
gmen
ted
sens
or
28
Position resolution
Binary spatial resolution
Charge interpolated spatial resolution
29
Microstrip detectors (1D)
30
Silicon drift detector (SSD)
ALICE SSD module
31
Double-side strip detector
32
Pixel detectorCMS BPIX module
CMS barrel pixel detector
33
Detector topology
Marco Battaglia
Marco Battaglia, EDIT 2012, Silicon Track, February 2012
34
Radiation damage
35
Radiation damageRecoil after 1 MeV neutron collision
Radiation induced damages• cause severe signal losses across sensor thicknessSignal loss can be covered partially• by increasing high voltageBut high voltage• degrades the position resolution
36
Radiation damage effects• Leakage current increases ΔI = α Φ V
– ΔI change in current in volume V– α is damage constant– Φ is time-integrated radiation flux
• Depletion voltage increases VFD = q|Neff|d2 /2εε0
– Neff = |Ndonor-Nacceptor|
– εε0 permittivity– d is sensor thickness
• Charge collection degrades• Position resolution degrades
37
Radiation damage – leakage current
38
Radiation damage - VFD
39
Radiation damage – charge collection
40
Radiation damage – S/N
41
Radiation damage – spatial resolution
42
Radiation harder approaches – 3D
ionizing particle
300µm
n+
p+
e
h
depletion
p-type
p+ n+
50µm
p-type
depletion
PLANAR: 3D:
• p+ and n+ electrodes are arrays of columns that penetrate into the bulk• Lateral depletion• Charge collection is sideways• Superior radiation hardness due to smaller electrode spacing: - smaller carrier drift distance - faster charge collection - less carrier trapping - lower depletion voltage• Higher noise• Complex, non-standard processing
SEM picture of 3D electrodes
Radiation harder approaches – 3D4E Configurationn+ (readout) p+ (bias)
100
μm
150 μm
2E Configuration
1E Configuration
SIN
TEF
3D
(200
μm
thic
k)CN
M 3
D (2
00 μ
m th
ick)
FBK
3D (2
00 μ
m th
ick)
Sing
le-s
ide
etch
ing
Doub
le-s
ide
etch
ing
Doub
le-s
ide
etch
ing
44
COMPLEXITY – CMS Tracker
45
References• Sze, Physics of semiconductor devices, 2nd Edition• Helmuth Spieler, Semiconductor Detector Systems• Olaf Steinkamp, Experimental Methods of Particle Physics, 2011• Enver Alagoz, Simulation and beam test measurements of the CMS pixel
detector, PhD Thesis, 2009• Daniela Bortoletto, An introduction to semiconductor detectors, Vienna
Conference, VCI 2004