Trakcing systems with Siliconwith special reference to ATLAS-SCT

• Some generalities about tracking• Special requirements in LHC environments• About silicon • About the ATLAS-SCT

Some general considerations

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Tracking measures particle 3-momenta

Lever arm, L Sa

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Precision of sagitta measurement: hits N 3

Interaction point

(N position measurements)

Requirements for good resolution

• Lever arm as long as possible (large plarge detector)

• Measurement of sagitta as precise as possible• Magnetic field as large as possible

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A ‘typical’ event Granularity is essential

Requirements to LHC tracker

• FAST (40MHz)• Excludes ‘standard’ Drift Chambers due to large drift times

• Spacial resolution a few tens of microns• High granularity• Radiation hard• Must minimize material

• Drift Chambers would be optimal from this point of view

• Silicon is a good choice

Does very precise tracking give very precise momentum estimates?

• Not necessarily due to Multiple (coulomb) scattering

• Direction change due to a concentrated scatterer:

msc

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x/X0 is the amount of material traversedin units of radiation lengths

Example: Atlas SCT, 3% X0 /layer, and pixel measurements inside (probably also about 3% X0

per layer).

Displacement of tracks in different tracker layers as function of momentum due to MSC.

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Excellent resolution never harms, but is sometimes useless….

The silicon strips of the ATLAS-SCT has a pitch of 80 µm

• …. So position resolution is 23 µm per hit….

• Standard deviation of a flat distribution =width/(√12)

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Basics of Silicon detectors

• ‘Simple’ p-n junctions• Reverse biased• Junctions can be segmented into strips

From L.G. Johansen, Thesis (Bergen)

~300um

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Charge density

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A model for the diode: Constant charge density in the depletion region of the n-bulk, and heavily doped p-side

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Nd is the donor concentration

Setting a reverse potential across the diode depletes it to a depth given by (about):

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How to measure the depletion depth?

• Charge stored in bulk: (charge density x volume (Al))

• Capacitance:

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The formula works!

20 detectors from Hamamatsu (L.G. Johansen, thesis)

Charge collection

• Typical detector thickness is 300 µm.• Bethe-Bloch equation + Landau fluctuations

gives a most probable energy loss of about 80keV

• To create a free electron-hole pair you need 3.6eV

Signal is about 22000 electrons ( 3.5 fC)

Energy loss distributions for 2 GeV electrons,pionsand protons, broader than Landau. (Bak et al , NPB288:681,1987)

The ATLAS SCT barrel detector

• Pitch 80 µm (resolution 23 µm)• 768 strips per detector• Thickness 285 µm• Size 6.36x6.40 cm2

• Should biassed to 350V• p strips on n material

Read-out electronics

• Must have low noise• Noise scales with capacitance Size limitations• microstrip detectors: inter-strip capacitances dominate.

• Must be compact ASICs• For ATLAS: Must operate at 25 Mhz• For ATLAS: Must be radiation tolerant

Signal from electrons from a Ru-106 source (mostly minimum ionisingparticles). Fluctuations are dominated by Landau fluctuations in the deposited energy. Spectrum collected with fast analog electronicsChip SCT128A (from B.Pommersche, thesis, Bergen)

N/bin

Signal/noise

Charge collection vs bias, 25 ns collection time

Signal/Noise as a function of Detector Voltage

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Detector nr 108oxygenated

Detector nr 7unoxygenated

From B. Pommeresche, Cand. Scient thesis, (U of Bergen)We must over-deplete the detectors to collect all the charge in time

Difference in signalcould be explained bydifferences in detectorthickness

What do we look for to assess the quality of a detector?

• Depletion voltage• Inter-strip capacitances

• Radiation hardnesswill require biassing to high volts

• Leakage currents must under control at high volts

The leakage current nightmare

• Current through the bulk: No problem• The Problem: Currents on detector surface, around

corners and who knows from where..….• High currents into the readout destroys the

electronics, to avoid it we take the following measures:• Capacitively (AC) coupled aluminium readout strips • Guard ring structures around active detector area (connect

to ground to suck out current)

The nightmare (part II)

• Large currents result in high power dissipation and heating of the detector system.

• Must be able to control the current, if not fully understood, it should at least be stable with time!

• Must test all detectors and detector modules for leakage current.

Careful detector design is required!

A detail of the detector for ATLAS-SCT

(picture from L.G. Johansen, thesis)

Leakage currents for some SCT detectors

(From L.G.Johansen, thesis (Bergen))

The detector modules must be radiation hard

• All components tested in a proton beam to a fluence of 3 x 1014 protons/cm2

• This is 50% more than expected for ten years of LHC operation

A few words on radiation damage

• The two most important effects are:1: Crystal defects are created in such a way that the effective

doping gets more p-like with fluence (dose). Vdep decreasesType inversionVdep increases

2: Leakage current increases increase in noise

3: Depletion from ‘below’n+ doping of back sidepreserves diode junction

Development of leakage current with time

L.G. Johansen, thesis

Signal/noise of an irradiated detector

Plot from L.G. Johansen (of course….)

Leakage current doubles for a temperature increase of 7 degrees

• ATLAS-SCT will be operated at about -10 degrees C

• Detector modules to be in thermal contact with cooling agent.

ATLAS-SCT readout electronics

• Digital readout (hit/no hit)• Pros and cons.of digital electronics

• Rad. Hard.• 128 readout channels per chip.

Schematic of the ABCD3T chip

The ALTAS SCT module

ATLAS-SCT barrel module

• 4 detectors• 1 baseboard (Patented TPG solution)

• Must be thermally conductive

• Hybrid with 12 chips, wraps around the sensor-baseboard.

• Strips are bonded together in pairs, to form 12 cm long strips.

• About 3000 wire bonds per module

A drawing of the module

Production of about 2000 modules at 4 university ‘clusters’ around the world

• Necessary for efficient use of small resources at each university.

• Production clusters:• Japan• US• UK• Scandinavia

Equipment needed or developed

• Cleanrooms• Tools for precision mounting (motorized jigs etc)• Microscopes• Metrology equipment (‘smart microscope’)• Bonding machines• Setups for electric tests

Module production

• Detector testing• Glueing of detectors to baseboard (5 um

precision)• Testing (IV), metrology• Hybrid testing and glueing • Bonding• Testing

To mount to 5 micron precision is not trivial!

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Noise occupancy must be under control!

Average noise/channel

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Some module IV curves (nightmare, part III)

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But, in the end, the project seems to have been successful

• Yield factor OK (above 85 %)• Module mounting on barrels in Oxford• Transfer to CERN OK• Cosmic tests OK• Now, the barrel is in the ATLAS pit to be cabled

and tested further……

Conclusions

• Silicon tracking is very attractive in HEP• But not at all trivial to make….

• Very cost and manpower intensive…