Trakcing%20systems%20with%20Silicon%20with%20special%20reference%20to%20ATLAS-SCT

1
Trakcing systems with Siliconwith special
reference to ATLAS-SCT

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

2
Some general considerations
Tracking measures particle 3-momenta
Particle track
Sagitta, s
Lever arm, L
r
Interaction point
Precision of sagitta measurement
(N position measurements)
3
Requirements for good resolution

• Lever arm as long as possible (large p?large
  detector)
• Measurement of sagitta as precise as possible
• Magnetic field as large as possible

4
A typical event Granularity is
essential
5
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

6
Does very precise tracking give very precise
momentum estimates?

• Not necessarily due to Multiple (coulomb)
  scattering
• Direction change due to a concentrated scatterer

x/X0 is the amount of material traversed in units
of radiation lengths
7
Example Atlas SCT, 3 X0 /layer, and pixel
measurements inside (probably also about 3
X0per layer).
8
Displacement of tracks in different tracker
layers as function of momentum due to MSC.
Excellent resolution never harms, but is
sometimes useless.
9
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/(v12)

10
Basics of Silicon detectors

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

300um thickness
From L.G. Johansen, Thesis (Bergen)
11
A model for the diode Constant charge density in
the depletion region of the n-bulk, and heavily
doped p-side
Nd is the donor concentration
12
Setting a reverse potential across the diode
depletes it to a depth given by (about)
13
How to measure the depletion depth?

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

14
The formula works!
20 detectors from Hamamatsu (L.G. Johansen,
thesis)
15
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)

16
Energy loss distributions for 2 GeV
electrons,pions and protons, broader than Landau.
(Bak et al , NPB288681,1987)
17
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

18
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

19
Signal from electrons from a Ru-106 source
(mostly minimum ionising particles). Fluctuations
are dominated by Landau fluctuations in the
deposited energy. Spectrum collected with fast
analog electronics Chip SCT128A (from
B.Pommersche, thesis, Bergen)
N/bin
Signal/noise
20
Charge collection vs bias, 25 ns collection time
Difference in signal could be explained
by differences in detector thickness
From B. Pommeresche, Cand. Scient thesis, (U of
Bergen) We must over-deplete the detectors to
collect all the charge in time
21
What do we look for to assess the quality of a
detector?

• Depletion voltage
• Inter-strip capacitances
• Radiation hardness
• ?will require biassing to high volts
• Leakage currents must under control at high volts

22
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)

23
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.

24
Careful detector design is required!
25
A detail of the detector for ATLAS-SCT
(picture from L.G. Johansen, thesis)
26
Leakage currents for some SCT detectors
(From L.G.Johansen, thesis (Bergen))
27
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

28
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 decreases
• Type inversion
• Vdep increases
• 2 Leakage current increases
• ? increase in noise
• 3 Depletion from below
• n doping of back side
• preserves diode junction

29
Development of leakage current with time
L.G. Johansen, thesis
30
Signal/noise of an irradiated detector
Plot from L.G. Johansen (of course.)
31
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.

32
ATLAS-SCT readout electronics

• Digital readout (hit/no hit)
• Pros and cons.of digital electronics
• Rad. Hard.
• 128 readout channels per chip.

33
Schematic of the ABCD3T chip
34
The ALTAS SCT module
35
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

36
A drawing of the module
37
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

38
Equipment needed or developed

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

39
Module production

• Detector testing
• Glueing of detectors to baseboard (5 um
  precision)
• Testing (IV), metrology
• Hybrid testing and glueing
• Bonding
• Testing

40
To mount to 5 micron precision is not trivial!
41
Noise occupancy must be under control!
42
Some module IV curves (nightmare, part III)
43
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

44
(No Transcript)
45
(No Transcript)
46
Conclusions

• Silicon tracking is very attractive in HEP
• But not at all trivial to make.
• Very cost and manpower intensive