solid state detectors-2 · – f= m h dv /dt • in semiconductors electrical conduction takes...
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Solid StateDetectors-2
T. Bowcock
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Schedule
1 Position Sensors
2 Principles of Operation of Solid State Detectors
3 Techniques for High Performance Operation
4 Environmental Design
5 Measurement of time6 New Detector Technologies
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Why use a Solid StateDetector?
• 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
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B-physics
• Detecting vertices ...
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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
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Silicon
• Properties of Si– Crystal structure
– Group IV– 4 electrons in
valence shell
• 2D representation
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Ionisation and holes
• 1.1eV• Holes
– F=mhdv/dt
• In semiconductorselectrical conductiontakes place via twomodes of electron motion.Can be viewed as motionof e-’s with charge -q andeffective mass m*e andholes, +q, m*h
• Intrinsic semiconductors
+
e-
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Valence and ConductionBands
• In intrinsic semiconductors (noimpurities)
E
conduction band
valence bandE=Ev
E=Ec
Ef
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Thermal Properties
• In intrinsic semiconductor
• Note that in intrinsic semiconductorsn=p=intrinsic carrier density
• For Si (at room temp) ni=1.5*1010
)exp(
)exp(
kT
EENp
kT
EENn
vfv
fcc
−−=
−−= Nc=3*1019
Nv=1*1019
−==
kT
ENNnnp g
vci exp2
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Carriers properties
• Carrier mobility µ=v/E– 1300 cm2/Vs for e’s
– 500 cm2/Vs for holes
• Resistivity– reflects the doping level )(
1
pnq hn µµρ
+=
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Intrinsic Silicon cont’d
• At room temp in a 300 micron thickdetector with an area of 1cm2 totalfree carriers would be about 109
– cool (lower T)• can be done but cryogenics are bulky and
expensive
– reverse biased diode operation
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Impurities
• Group III (e.g. B)– acceptor type atoms
– majority carriers=holes– p-type
• Group V (e.g. P)– donor type atoms
– majority carrier=e’s– n-type
III
h
V
e
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Impurities
• Do not lead to net charge!– Donor concentration Nd
– Acceptor concentration Na
• simplifies if impurities >> ni
−++−=
−++−=
=−+−
22
22
)(42
1
)(42
1
0
adida
adiad
ad
NNnNNp
NNnNNn
NNnp
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Band Structure
• Doped silicon
E
conduction band
valence bandE=Ev
E=Ec
Ea
Ed
(p-type)
(n-type)
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pn-Junction
• Abrupt junction– diffusion
- + + -+ + + + + + + + + + + +
- - -
- - -- - -- - -
+ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + +
- - - - - - -- - - - - - -- - - - - - -- - - - - - -
+ - - +
- - - - - - -
+ - - ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + +
- - - - - - -- - - - - - -- - - - - - -- - - - - - -
- + + -+ + + + + + + + + + + +
- - -
- - -- - -- - -
+ - - +
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pn-junction
- - - - - - -
+ - - ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + +
- - - - - - -- - - - - - -- - - - - - -- - - - - - -
- + + -+ + + + + + + + + + + +
- - -
- - -- - -- - -
+ - - +
Space charge
Carrrier density
Field and potential
p-type n-type
acceptor
donor
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Diode Behaviour
• Built in potential
• Calculate depletion width (neutral)
• Use 1D Poisson Eq.
=
2ln
i
da
n
NNq
kTV
21 WNWN da =
εερ qN
dx
Vd=−=
2
2
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Depletion Depth
• Problem: Prove
• Conversely applying a potential increasesdepletion width
• Reverse biased diode• Note dependence on doping
( )
( )add
Bs
daa
B
NNqN
VW
NNqN
VW
/1
2
/1
21
+=
+=
ε
ε
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Depletion Region as aDetector
• Build a p+n diode– Na=1015
– Nd=1013
• At 50-100V bias voltage get 300micron depletion in the n-part (bulk)and < 1 micron in the heavily dopedpart.
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Depletion region
• In the depletion region continualthermal generation of eh pairs– leakage current
– depends on volume
• Ionisation will also create pairs thatwill also drift and be collected– signal/noise
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IV and CV of diodes
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Fabrication
• Key to use of Si is the processing• Photolithography
Organic Photoresist usually “spun” on
Photoresist forms a layer a fewmicrons thick in 30s
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Patterning
• Photoresist exposedusing a “mask”
• Mask contains thedesign and isproduced with e-beam lithography– feature size down to
0.25microns
• Negative or Positive
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Etching
• Negative Process• Chemical etch
– exposed partunaffected byetch 1
• Exposed patternmay be removedlater– second etch
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Example:
• Aluminium line
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Simple Strip Detector
• Oxide passivation
• Windows• Doping
– B– As
• Al Metallization• Al patterning
• Rear Contact
SiO2Al
n+
p+
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2D strips
Al
Si
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Wafer
Main detector
Test structures
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Pulse Height
Landau Distribution
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Signal Shape
• Simulation– charges follow e-
field– Ramo’s
Theorem– Finite element
• Matches data
0
100
200
300
400
500
600
700
800
900
0 8
16 24 32 40 48 56 64 72 80 88 96
250V Irrad
250v Unirrad
60V Unirrad
0
0.2
0.4
0.6
0.8
1
1.2
1 9 17 25 33 41 49 57 65 73 81 89 97
250V Irrad
250v Unirrad
60V Unirrad
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Strip Pitch
• Strip pitch is the dominant factor indetermining resolution
Typically 40-100 microns(why not smaller)
Resolution better than about pitch/4(why?)
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Charge Sharing
• Charged shared (see Ramo’sTheorem!) between strips
c
Pulse height
x
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Pulse on p-strip detector
Al
Si
-V
+-
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Electronics
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Electronics
• VLSI• ASIC
bonds
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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 100-1000 depending on speed– b varies from 20 to 100 depending on speed
– Sources from• leakage current• load resisitor
R
kTt
q
eENC
qIt
q
eENC
p
p
2
4
=
=
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Signal/Noise
• Electronics– speed
– intrinsic characteristics
• Thickness of detector and Vbias• MIPS• Capacitance of Strips
– resistors
• Desire about 10/1 S/N
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AC Coupled Devices
• Avoid drainingbulk current intoelectronics
• Usual detectorbuilt
• Higher cost
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Biasing Techniques
• FOXFET
• Reachthrough• Polysilicon resisitor
– 1-10MΩ
poly Al bias strip
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Double Sided Detectors
• Using the Ohmic side– divide up the “plane”
+++++++++- - - - - - - -
+ ++ +- -- -
p-stops
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Pixel Detectors
• Pixel detectors are identical inprincipal to strip detectors– shape of pads smaller
• few microns or 10’s of microns comparedwith strips of 6cm or so,
– more diffiucult to route out• expensive bump bonded electronics
– low capacitance(noise)– intrinsically 2D rather than 1D
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Charge Coupled Devices
• Very high precision (0.2microns)
• Moves charge in a potential well• 2D device• Slow
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CCD
• Use small low capacity elements andexchange information <10e noise
• Matrix of potential wells created
p-type
n-doping
“cell”
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Summary
• Have seen the basics of how astrip/pixel detector works– capable of adequate S/N
– only cost effective for last 10 years withadvent of second generation fabrication
– easily modifiable geometry
• Next: high performance operation