1. Stephanie Majewski Stanford University
Silicon Detectors
Stephanie Majewski
Stanford University

2. Semiconductor Refresher
Si bandgap energy Eg = 1.12 eV
kBT = K
S. Majewski

3. Semiconductor Refresher
Si bandgap energy Eg = 1.12 eV
kBT = K
Doping:
n-type  dopant adds electrons to conduction band (e.g. P, As)
S. Majewski

4. Semiconductor Refresher
Si bandgap energy Eg = 1.12 eV
kBT = K
Doping:
n-type  dopant adds electrons to conduction band (e.g. P, As)
p-type  dopant adds holes to valence band (e.g. B)
S. Majewski

5. Reverse-Biased Diodes
sensitive detector region
Hamamatsu PIN Diode
depletion region
p-n junction
larger depletion region
p-i-n junction
(i = intrinsic)
S. Majewski

6. 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
S. Majewski

7. Ionization Energy Less material (1 m): More material (100 m):
Most probable
energy loss
Mean energy loss
Less material (1 m):
E dominated by counting statistics
More material (100 m):
Landau distribution with high-energy tail
Silicon:
Mean ionization energy = 3.6 eV
S. Majewski

8. Energy Loss (dE/dx) dE/dx = # e-h pairs  3.6 eV / (300 m  tan dip) p d t
BAD 1154
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
S. Majewski

9. Advantages of Silicon Low ionization energy: 3.6 eV (e-h creation)
Compare to gas ~30 eV
Many carriers / event
Long mean free path (~100 nm)
High charge collection efficiency
Large energy loss / distance traveled (3.8 MeV/cm for a minimum ionizing particle)
Large signals
High carrier mobility ( ) at room temp, even w/ doping
Rapid charge collection (~10ns)
Detector/electronics integration
Easy to fabricate
S. Majewski

10. Silicon Wafer Fabricationp i n
S. Majewski

11. Silicon Detector Geometries
Strip Detectors
BaBar, Belle, CDF, D0
Hybrid Pixel Detectors (at ATLAS, CMS)
Squares instead of strips; integrated electronics
Drift Detectors (used in Star at RHIC)
Electrons move through the Si bulk to an anode strip at the end
CCDs (used for SNAP, SLD)
3-D Silicon Detectors
Proposed in ’95 by S. Parker at U. Hawaii
Possible LHC detector upgrade
S. Majewski

12. Strip Detector Geometry
“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 /
Readout Pitch
Aluminum
Strip Pitch
n- Bulk
Silicon
dioxide
p+ Implant
S. Majewski

13. The BaBar Silicon Vertex Tracker

14. Silicon Wafers p+ strip side n+ strip side TOP VIEW Edge guard ring
P-stop
n+ Implant
55 mm
Polysilicon
bias resistor
p+ Implant
Bias ring Al
50 mm
Polysilicon
bias resistor
Edge
guard ring
p+ strip side
n+ strip side
TOP VIEW
S. Majewski

15. Si Sensor Schematic Bias Voltage ~ 40 V Leakage Current ~ 10 A
AC Coupling
SIDE VIEW
S. Majewski

16. Si Sensor Schematic +2 V Bias Voltage ~ 40 V Leakage Current ~ 10 A
Pre-Amp
Bias Voltage ~ 40 V
Leakage Current ~ 10 A
AC Coupling
+40 V
Pre-Amp
+40 V
S. Majewski
+42 V

17. 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)
S. Majewski

18. Readout Electronics AToM chip Radiation hard 128 channels
Injected Charge (fC)
Time Over Threshold
1 MIP
AToM chip
Radiation hard
128 channels
1-2 strips/channel
Minimum Ionizing Particle:
3.8fC  avg 7.5 counts
1-2 counts = noise
S. Majewski

19. Carbon/Kevlar Fiber Support Ribs
SVT Modules
Z Side
ATOM chip
 Side
Carbon/Kevlar Fiber Support Ribs
Si Wafers
S. Majewski

20. Position Resolution
Analog readout allows better resolution than pitch /
S. Majewski

21. Track Finding Efficiency
How Many Layers?
Number of Hits Found
Track Finding Efficiency %
BaBar TDR
5 layers
4 layers
Define a Helix
4 points confirm a helix in tracking
5 layers needed to compensate for gaps and dead modules
Inner 3 layers
angle/impact parameter
redundancy
Outer 2 layers
pattern recognition
low pT tracking
S. Majewski

22. SVT Data Transmission HDI Power Supplies Back MUX Link Si Wafers DAQ
HDI: High Density Interconnect. Mounting fixture
and cooling for readout ICs.
Kapton Tail: Flexible multi-layer circuit. Power,
clock, commands, and data.
Matching Card: Connects dissimilar cables.
Impedance matching.
HDI Link: Reference signals to HDI digital common.
DAQ Link: Multiplex control, demultiplex data.
Electrical -- optical conversion.
HDI
Matching
Card
Kapton
Tail
Front
Cables
Si Wafers
Back
Link
DAQ
Power
Supplies
MUX
Fiber Optic
to DAQ
S. Majewski

23. Radiation Damage Acute damage Bulk 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
S. Majewski

24. 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
S. Majewski

25. Radiation Protection See Adam’s talk next week!
Silicon is not radiation hard
BaBar SVT monitored by PIN diodes and diamond sensors
briefIntro.html
See Adam’s talk next week!
S. Majewski

26. The BaBar SVT Thin wafers (300 m) to limit multiple scattering
5 Layers
0.94 m2 of Si
~ readout channels
S. Majewski

27. Belle Silicon Vertex Detector
SVD 1
3 Layers
SVD 2
4 Layers
S. Majewski

28. ATLAS Semiconductor Tracker & Pixel Detectors
(SCT)
Pixel Detectors
S. Majewski

29. 3-D Silicon Detector Brunel Univ. (UK), Stanford, and Hawaii
Possible LHC detector upgrade
S. Majewski

30. The Rise of the Silicon Detector
S. Majewski

31. References General Silicon Detectors:
Lutz, G. Semiconductor Radiation Detectors: Device Physics. (Springer Verlag, Berlin, 1999).
Spieler, H. Lectures on Detector Techniques
Sadrozinski, H. “Applications of Si Detectors”, presented at IEEE
BaBar Silicon Vertex Tracker:
SVT Facts and Figures. Factoids.html
TDR
S. Majewski