Solid State Detectors- 3

1
Solid State Detectors- 3

• 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

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Techniques for High Performance Operation

• Strip Detectors
• Design and Fabrication Issues
• What to avoid!

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Review...

• In the p-strip in n-bulk (p-in-n) detectors
• Vdep100V
• Energy to create electron hole pair is
• 3.6eV ( not 1.1eV-why? )
• Average energy lost/mm
• 39keV (108eh/ mm)

-V
Al
Si

-
5
Drift

• Electric field in Depleted region linear
• 300mm detector
• at 100V E3.0keV/cm
• Diffusion/Drift by multiple collisions
• Takes 7ns for es, 20ns for holes

Higher diffusion at low temps!
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Ballistic Deficit
Charge lost is known as the ballistic deficit
Collection time
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Strip Pitch and Readout Pitch and resolution

• Select it

d
Single strip has d/?12
d/10
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Choosing the Pitch

• Why not make it infinitely small
• transverse diffusion
• 10-20 microns
• construction
• readout electronics!
• Readout pitch
• not necessarily the same as diode pitch (cost)

75mm readout (25mm diode)
9
Intermediate Strips

• Work by capacitive coupling
• induced current/charge is that seen by the
  electrons and holes (not a post-facto charge
  sharing!)
• Why no broader strips ?
• Interstrip capacitance lt1pF

Need field map!
10
Intermediate Strips?

• Loose signal
• An option if
• limited by resources
• little noise in electronics (slow es)
• Optimal choice is
• readout each strip
• pitch and width evaluated by FEA
• pitch between 20 microns and 100 microns

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Performance
50 mm with intermediate strip
25mm readout
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Resolution

• Test your resolution
• series of particles of known position
• testbeam telescope
• cosmic telescope
• longwavelength laser

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Checking Resolution
Optical fiber

• Tests
• laser
• problems?
• transparancy
• cosmics
• slower
• testbeam
• expensive
• labour intensive

Focus to 5 mm
1064nm Si transparent
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Two Track Resolution

• Reconstruction position as a function of
  proximity of one track to another

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Occupancy

• Best to reduce occupancy
• 1 considered the benchmark
• 10 too high
• Reduce the length of strips
• usually about 6cm
• reduce to 1cm for example

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AC Coupling Revisited

• e0.34pF/cm
• 200nm oxide
• 10pF/cm
• Greater than Interstrip capacitance
• Electronics at ground!

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Double Sided

• Needs AC coupling!
• Correlation of signals
• Strips can run opposite directions
• 2D style r/o

-V
- -
- -


0V
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Double Sided Detector

• Would like electronics at one end
• Can get correlated measurement (E) giving x/y
  measurement
• Reduces fakes
• Punchthrough

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Double Metal

• Add another routing layer
• more processing

via

• Expense can double
• Built in stresses in SiO2 can warp Si wafer
  badly

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Double Metal
Can also use to route on single sided detectors
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Strips
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Example of Double Metal Detectors

• LHCb prototypes

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Bond Pads

• Structure you will often see

Typically 80 by 200 microns
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n-strip detectors

• We can make single sided n-strip detectors (note
  depletion!)

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Field Plates

• MOS structures

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p-stops

• Individual p-stops

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Operating Voltage

• High (overvoltage is desirable)
• 250V
• reduced ballistic deficit
• BUT
• introduces very high field regions?
• Avalanche will set in if field exceeds 30V/m

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Analysis of structure
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Electric Field
Sample field map
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Guard Rings

• Reduce fields at edge

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Micro-Discharges

• Discharges may be seen as in increase in the
  noise with voltage

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Si Choices

• Resistivity
• n-type
• p-strips
• n-strips
• double sided
• p-type
• Crystal orientation

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Benchmark measures

• Charge Collection Efficiency
• Partial Depletion
• Ballistic Deficit

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Fabrication

• Control of all steps critical
• Of special interest
• resistor values
• implantation
• oxide quality for breakdown
• quality of lithography

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Quality Assurance

• Job of the physicist is to measure all the key
  parameters of the detectors
• IV and CV
• interstrip capacitance
• resistor values
• lightspot response

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Readout Chain
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F/E Electronics

• Binary vs Analog
• Amplifier Characteristics
• rise time and falltime
• undershoot
• Digital Performance
• pipeline logic
• Noise

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Hybrid Design
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Noise

• Hybrid is often a source of noise
• bad grounding for electronics
• bad grounding for supplies to detector
• sensor,analog and digital all connected
• The detector, f/e electronics and the hybrid
  should be regarded as one unit or MODULE

40
Module and Mounting
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Material Budget

• Ideally should be as low as possible
• avoid high mass materials
• gold
• Good detector about 1 of a radiation length

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Example DELPHI barrel
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Offline Analysis

• Can give improvement in resolution

w
L
R
Only true if charge uniform and if the width of
the cluster matches the strip width In general we
have a Gaussian distribution of width determined
by the diffusion coefficient (for normal
incidence)

d
x
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Offline

• Corrections for the angle of the track and the
  known (measured) charge sharing can give great
  improvement
• 20 to 30 in the case of 25 microns pitch
  detectors
• Good software must accompany good hardware
• Removal of deltas

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7 things to avoid

• Picking the wrong technology
• Picking the wrong manufacturer()
• Not enough Quality Control
• Bad design limiting operation
• Noise in system
• Treating sensor and hybrid separately
• Bad analysis

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Summary

• We have all the elements now to think about real
  detectors in real environments
• design issues
• noise problems
• See how we design a detector for LHCb