Silicon Strip Detectors - SLAC
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Transcript Silicon Strip Detectors - SLAC
Silicon Detectors
Stephanie Majewski
Stanford University
Semiconductor Refresher
S. Majewski
Si bandgap energy
Eg = 1.12 eV
kBT = 0.026 eV @ 300K
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Semiconductor Refresher
Si bandgap energy
Eg = 1.12 eV
kBT = 0.026 eV @ 300K
Doping:
n-type dopant adds
electrons to conduction
band (e.g. P, As)
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Semiconductor Refresher
Si bandgap energy
Eg = 1.12 eV
kBT = 0.026 eV @ 300K
Doping:
n-type dopant adds
electrons to conduction
band (e.g. P, As)
p-type dopant adds
holes to valence band
(e.g. B)
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Reverse-Biased Diodes
sensitive detector region
p-n junction
Hamamatsu PIN Diode
depletion region
larger depletion region
p-i-n junction
(i = intrinsic)
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Interaction of Charged Particles
A high-energy particle
produces uniform e-h
density along its path
The bias voltage attracts
the electrons/holes to
either contact
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Ionization Energy
Most probable
energy loss
Less material (1 m):
Mean energy loss
More material (100 m):
Landau distribution with
high-energy tail
Silicon:
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E dominated by counting
statistics
Mean ionization energy =
3.6 eV
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Energy Loss (dE/dx)
BAD 1154
d
p
t
Shape is Bethe-Bloch
(see M. Spitznagel’s
drift chamber talk)
dE/dx = # e-h pairs 3.6 eV / (300 m tan dip)
Limited ability to distinguish particles
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Advantages of Silicon
Low ionization energy: 3.6 eV (e-h creation)
Long mean free path (~100 nm)
Large signals
High carrier mobility
High charge collection efficiency
Large energy loss / distance traveled
(3.8 MeV/cm for a minimum ionizing particle)
Compare to gas ~30 eV
Many carriers / event
(v E )
at room temp, even w/ doping
Rapid charge collection (~10ns)
Detector/electronics integration
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Easy
to fabricate
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Silicon
Wafer
Fabrication
p
n
i
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Silicon Detector Geometries
Strip Detectors
Hybrid Pixel Detectors (at ATLAS, CMS)
Squares instead of strips; integrated electronics
Drift Detectors (used in Star at RHIC)
BaBar, Belle, CDF, D0
Electrons move through the Si bulk to an anode
strip at the end
CCDs (used for SNAP, SLD)
3-D Silicon Detectors
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Proposed in ’95 by S. Parker at U. Hawaii
Possible LHC detector upgrade
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Strip Detector Geometry
“Strip Pitch” (~50m) is the distance between strips,
whether they are connected to the electronics or not
“Readout Pitch” includes floating strips
Resolution ~ readout pitch / 12
Readout Pitch
Aluminum
Strip Pitch
Silicon
dioxide
p+ Implant
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n- Bulk
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The BaBar Silicon Vertex Tracker
Silicon Wafers
Edge
guard ring
Polysilicon
bias resistor
Bias ring
P-stop
55 m
n+ Implant
p+ Implant
Al
50 m
Polysilicon
bias resistor
p+ strip side
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TOP VIEW
Edge
guard ring
n+ strip side
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Si Sensor Schematic
Bias Voltage ~ 40 V
Leakage Current ~ 10 A
AC Coupling
SIDE VIEW
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Si Sensor Schematic
+2 V
Pre-Amp
Bias Voltage ~ 40 V
Leakage Current ~ 10 A
AC Coupling
+40 V
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Pre-Amp
+42 V
+40 V
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Readout
Strips are AC coupled to preamplifiers
Separates signal current from bias current
Guard rings to reduce noise and measure bulk bias
current
Charge sharing between strips
Analog readout of strips gives better resolution
Convert pulse height (charge) into long pulse in
time, then measure time over threshold (TOT)
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Readout Electronics
AToM chip
Radiation hard
128 channels
1-2 strips/channel
Minimum Ionizing Particle:
3.8fC avg 7.5 counts
1-2 counts = noise
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Time Over Threshold
Injected Charge (fC)
1 MIP
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SVT Modules
Z Side
ATOM chip
Side
Si Wafers
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Carbon/Kevlar Fiber
Support Ribs
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Position Resolution
Analog readout
allows better
resolution than
pitch / 12
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How Many Layers?
Define a Helix
Inner 3 layers
4 points confirm a helix in
tracking
5 layers needed to
compensate for gaps and
dead modules
angle/impact parameter
redundancy
Outer 2 layers
pattern recognition
low p tracking
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BaBar TDR
Track Finding Efficiency
5 layers
%
%
%
%
4 layers
Number of Hits Found
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SVT Data Transmission
•HDI: High Density Interconnect. Mounting fixture
and cooling for readout ICs.
•Kapton Tail: Flexible multi-layer circuit. Power,
clock, commands, and data.
Power
Supplies
•Matching Card: Connects dissimilar cables.
Impedance matching.
•HDI Link: Reference signals to HDI digital common.
•DAQ Link: Multiplex control, demultiplex data.
Electrical -- optical conversion.
Front
Cables
Si Wafers
Back
Cables
MUX
Power
HDI
Link
Matching
Card
HDI
Kapton
Tail
DAQ
Link
Fiber Optic
to DAQ
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Radiation Damage
Acute damage
pinhole – short in AC coupling capacitor
p-stop short – short between p-stop and metal contact
(DC)
Bulk damage
radiation displaces Si atoms
& creates defects
eventual type inversion in
bulk (n-type to p-type)
can see as change in voltage
needed to fully deplete
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Radiation Damage
Consequences of Defects
Recombination/generation centers increased
leakage current
Trapping centers introduce time delay reduced
signal
Charge density changes need increased bias
voltage
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Radiation Protection
Silicon is not radiation hard
BaBar SVT monitored by PIN diodes and
diamond sensors
http://www.slac.stanford.edu/BFROOT/www/Detector/SVT/Operations/SVTRAD/
briefIntro.html
See Adam’s talk next week!
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The BaBar SVT
Thin wafers (300 m) to limit multiple
scattering
5 Layers
0.94 m2 of Si
~150 000 readout channels
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Belle Silicon Vertex Detector
SVD 1
3 Layers
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SVD 2
4 Layers
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ATLAS Semiconductor Tracker
& Pixel Detectors
Semiconductor Tracker
(SCT)
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Pixel Detectors
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3-D Silicon Detector
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Brunel Univ. (UK), Stanford,
and Hawaii
Possible LHC detector
upgrade
http://www.pparc.ac.uk/frontiers/archive/update.asp?id=16U3&style=update
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The Rise of the Silicon Detector
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References
General Silicon Detectors:
Lutz, G. Semiconductor Radiation Detectors: Device
Physics. (Springer Verlag, Berlin, 1999).
Spieler, H. Lectures on Detector Techniques. 1998.
http://www-physics.lbl.gov/~spieler/SLAC_Lectures/index.html
Sadrozinski, H. “Applications of Si Detectors”, presented at
IEEE 2000. http://scipp.ucsc.edu/~hartmut/IEEE2000_embed.pdf
BaBar Silicon Vertex Tracker:
SVT Facts and Figures.
http://www.slac.stanford.edu/BFROOT/www/Detector/SVT/
Factoids.html
TDR
http://www.slac.stanford.edu/pubs/slacreports/slac-r-457.html31
S. Majewski