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Solid State Detectors-2

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Presentation on theme: "Solid State Detectors-2"— Presentation transcript:

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 Doping
B 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

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