Download Silicon Strip Detectors - SLAC

Silicon Detectors
Stephanie Majewski
Stanford University
Semiconductor Refresher
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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)
S. Majewski
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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)

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
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)
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Long mean free path (~100 nm)

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Large signals
High carrier mobility

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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
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Strip Detectors
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Hybrid Pixel Detectors (at ATLAS, CMS)
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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


S. Majewski
Proposed in ’95 by S. Parker at U. Hawaii
Possible LHC detector upgrade
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Strip Detector Geometry
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“Strip Pitch” (~50m) 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
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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
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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
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Inner 3 layers

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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
S. Majewski T

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
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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
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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!
S. Majewski
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The BaBar SVT
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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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S. Majewski
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