1. Solid State Detectors-2
T. Bowcock

2. Schedule 1 Position Sensors 2 Principles of Operation of Solid State
Detectors
3 Techniques for High Performance Operation
4 Environmental Design
5 Measurement of time
6 New Detector Technologies

3. Why use a Solid State Detector?
Physics requires
high rate capability
rare processes imply huge event rates
high efficiency and low dead time
good signal./noise ratio
good resolution
electronics r/o
high speed

4. B-physics
Detecting vertices ...

5. Silicon Properties Electron-hole production at few eV
compare with 30eV in gas
Density reduces deltas
remember bubble chamber photos(!)
100 e-h pairs/micron
solid
easy to install close to interaction point

6. Silicon Properties of Si 2D representation Crystal structure Group IV
4 electrons in valence shell
2D representation

7. Ionisation and holes e- 1.1eV Holes
F=mhdv/dt
In semiconductors electrical conduction takes place via two modes of electron motion. Can be viewed as motion of e-’s with charge -q and effective mass m*e and holes, +q, m*h
Intrinsic semiconductors +

8. Valence and Conduction Bands
In intrinsic semiconductors (no impurities)
conduction band
E=Ec E Ef
E=Ev
valence band

9. Thermal Properties In intrinsic semiconductor
Note that in intrinsic semiconductors n=p=intrinsic carrier density
For Si (at room temp) ni=1.5*1010
Nc=3*1019
Nv=1*1019

10. Carriers properties Carrier mobility m=v/E Resistivity
1300 cm2/Vs for e’s
500 cm2/Vs for holes
Resistivity
reflects the doping level

11. Intrinsic Silicon cont’d
At room temp in a 300 micron thick detector with an area of 1cm2 total free carriers would be about 109
cool (lower T)
can be done but cryogenics are bulky and expensive
reverse biased diode operation

12. Impurities Group III (e.g. B) Group V (e.g. P) acceptor type atoms
majority carriers=holes
p-type
Group V (e.g. P)
donor type atoms
majority carrier=e’s
n-type h V e

13. Impurities Do not lead to net charge!
Donor concentration Nd
Acceptor concentration Na
simplifies if impurities >> ni

14. Band Structure Doped silicon conduction band E=Ec Ed (n-type) E Ea
(p-type)
E=Ev
valence band

15. pn-Junction Abrupt junction diffusion + - - + - + + - - - - - - - -

16. pn-junction donor p-type n-type - - - - - - - + - - + + + + + + + +
acceptor
Space charge
Carrrier density
Field and potential

17. Diode Behaviour Built in potential Calculate depletion width (neutral)
Use 1D Poisson Eq.

18. Depletion Depth Problem: Prove
Conversely applying a potential increases depletion width
Reverse biased diode
Note dependence on doping

19. Depletion Region as a Detector
Build a p+n diode
Na=1015
Nd=1013
At V bias voltage get 300 micron depletion in the n-part (bulk) and < 1 micron in the heavily doped part.

20. Depletion region
In the depletion region continual thermal generation of eh pairs
leakage current
depends on volume
Ionisation will also create pairs that will also drift and be collected
signal/noise

21. IV and CV of diodes

22. Fabrication Key to use of Si is the processing Photolithography
Photoresist forms a layer a few microns thick in 30s
Organic Photoresist usually “spun” on

23. Patterning Photoresist exposed using a “mask”
Mask contains the design and is produced with e-beam lithography
feature size down to 0.25microns
Negative or Positive

24. Etching Negative Process Chemical etch
exposed part unaffected by etch 1
Exposed pattern may be removed later
second etch

25. Example:
Aluminium line

26. Simple Strip Detector Oxide passivation Windows DopingB As
Al Metallization
Al patterning
Rear Contact Al
SiO2 p+ n+

27. 2D strips Al Si

28. Wafer
Main detector
Test structures

29. Pulse Height
Landau Distribution

30. Signal Shape Simulation Matches data charges follow e-field
Ramo’s Theorem
Finite element
Matches data

31. Strip Pitch
Strip pitch is the dominant factor in determining resolution
Typically microns
(why not smaller)
Resolution better than about pitch/4
(why?)

32. Charge Sharing Charged shared (see Ramo’s Theorem!) between strips
Pulse height c x

33. Pulse on p-strip detector-V Al + Si -

34. Electronics

35. Electronics
VLSI
ASIC
bonds

36. Electronic Noise Noise sources
coupling of strips to each other an back plane (extra load and signal loss)
intrinsic to amplifier
ENC = a +bC
d may vary from depending on speed
b varies from 20 to 100 depending on speed
Sources from
leakage current
load resisitor

37. Signal/Noise Electronics Thickness of detector and Vbias MIPS
speed
intrinsic characteristics
Thickness of detector and Vbias
MIPS
Capacitance of Strips
resistors
Desire about 10/1 S/N

38. AC Coupled Devices Avoid draining bulk current into electronics
Usual detector built
Higher cost

39. Biasing Techniques FOXFET Reachthrough Polysilicon resisitor 1-10M
Al bias strip

40. Double Sided Detectors
Using the Ohmic side
divide up the “plane”
+ +
- -
p-stops

41. Pixel Detectors
Pixel detectors are identical in principal to strip detectors
shape of pads smaller
few microns or 10’s of microns compared with strips of 6cm or so,
more diffiucult to route out
expensive bump bonded electronics
low capacitance(noise)
intrinsically 2D rather than 1D

42. Charge Coupled Devices
Very high precision (0.2microns)
Moves charge in a potential well
2D device
Slow

43. CCD
Use small low capacity elements and exchange information <10e noise
Matrix of potential wells created
“cell”
n-doping
p-type

44. Summary Have seen the basics of how a strip/pixel detector works
capable of adequate S/N
only cost effective for last 10 years with advent of second generation fabrication
easily modifiable geometry
Next: high performance operation