1. Silicon pixel detectors with some info also on Strip and Drift detectors
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2. Summary What is a silicon pixel detector and how it works
Sensors and readout electronics
Tests and irradiation studies
From single modules to complex detectors
A study case: The ALICE silicon vertex detector

3. Main Features Two-dimensional segmentation detectors
Required for high quality vertexing capabilities (primary and secondary vertices, impact parameter,…)
Sizes of row and colums adapted to the needs
Row
Column

4. Main Features
Basic structure: a two-dimensional matrix (detector ladder) of reverse-biased silicon detector diodes (sizes: tens - hundreds of microns) flip-chip bonded to readout electronic chips
Each cell on the detector matrix is bonded to an equivalent cell on a CMOS chip, which contains most of the required electronics for that channel
Usually only binary information provided, with a selectable threshold

5. Main Features
Production of practical silicon pixel detectors made possible by the R&D progress in
component density achievable in CMOS electronic chips
development in flip-chip bonding techniques

6. Advantages vs disadvantages
True two-dimensional segmentation
Geometrical precision
Double-hit resolution
High signal-to-noise ratio
High speed
Disadavantages:
Only digital information
Large increase in number of connections and electronic channels
Fabrication techniques still in progress
Several phases of the process critical

7. The hybrid pixel detector
Cell size: 50 x 425 microns
Thickness around microns
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8. The hybrid pixel detector
1200 chips
A matrix of 8192 (32 x 256) independent cells is envisaged for each individual pixel chip in ALICE
~ 10 M channels

9. Electronic readout
Sensible progresses achieved in the number of channels, response uniformity, individual cell control and radiation tolerance

10. Evolution of the readout pixel chip families
Omega 3 (WA97/NA57)
Omega 2 (WA97/NA57)
ALICE2 Test
ALICE1 Test
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11. Development of the readout electronics
Prototype Cell size No. of cells Technology
Omega x x 
Omega x x 
ALICE1 test x x 
ALICE2 test x x 
ALICE x x 

12. Bump-bonding techniques

13. The expected radiation dose
Evaluation of the expected dose in the ALICE ITS by GEANT and FLUKA simulations taking into account 10 years operational period
Layer type Cumulated dose (krad)
pixel
pixel
drift
drift
strip
strip
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14. Irradiations of the readout electronics
List of irradiations tests carried out on the ALICE readout chips
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15. From single modules to complex detectors
To build a large detector with many channels several problems must be solved
ALICE: about 10 M channels
ATLAS: about 100 M channels

16. From single modules to complex detectors
Mass production organization
Mechanics and assembly
Cooling
Beam tests
Installation procedure
...

17. Mass production after R&D
After an extensive period of R&D, a mass production activity must be organized:
sensors and electronics chip tests
qualification criteria
time schedule
efficiency evaluation
...

18. ALICE silicon pixel detector: a case study

19. Wafer test procedures For ALICE each wafer has 86 readout chips
Tests to be carried out on each chip:
Current consumption (analog/digital)
JTAG functionality
Scan of all DACs
Determination of minimum threshold
Complete threshold scan of pixel matrix

20. Wafer probing CLEAN ROOM Bridge to PC MB-card VME-crate with pilot and
Power Supply
VME-crate
with pilot and
JTAG controller
Power Supply
Probe Station with Probe Card
CLEAN ROOM

21. Wafer probing: hardware
Semiautomatic Probe Station
Electronics

22. Wafer probing: software
LabView-based programs
Analysis with Root/C++

23. Typical results from a chip
Threshold distribution
A mean threshold of 17 mV corresponds to about 900 electrons.
Noise Distribution
A mean noise of 2.3 mV corresponds to about 120 electrons.

24. Readout CHIP classification
Chips classified as Class I (to be bump-bonded), Class II (minor defects), Class III (major defects)
35 (41%)
14 (16.5%)
36 (42%)
30 (35%)
III
5 (6%)
8 (9%)
10 (12%) II
37 (43%)
64 (74.5%)
46 (53%) I
AV9VGWT
AZ9VETT
AB9VHXT
AM9VG4T
CLASS

25. CHIP classification Minor Defects (CLASS II):
parts or whole columns are missing (no test-column effect!)
many noisy pixels (>1%)
inefficient pixels (>1%)
high threshold or noise (th> 30mV, noise>3mV)
Chips for Bump Bonding (CLASS III):
threshold<30mV (~2000 electrons rms)
no missing columns or parts of columns
no excess in current consumption
less than 1% of faulty pixels (noisy, inefficient)
test-column effect ignored

26. Beam tests at the CERN SPS
1 scintillator
2 scintillators
plane 1
plane 2
plane 3
x-y table
Bench
1 large scintillator
Layout of the experimental set-up for ALICE pixel detector beam tests

27. The ALICE pixel project
The ALICE pixel project requires joint efforts from physicists, engineers, technicians,…
A (not exhaustive) list of items under way or finalized:
Development of front-end chip
Bump bonding and assembly techniques
Carrier Bus development
Electronics for control and data transfer
Carbon fiber mechanics
Hardware and software alignment
Cooling system
Test of detectors under beam
Simulation tools for detector response
...

28. Conclusions
Silicon pixel detectors are now widely used in LHC experiments
Only recently the process of building a large detector is entirely reliable
There is room for further progresses in the field
New applications of pixel detectors (Medicine, …) are being exploited

29. Silicon strip detectors
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30. Principles of operation
Basic motivation: charged particle position measurement
Use ionization signal (dE/dx) left behind by charged particle passage _ + _ + _ + _ +
Use the drift chamber analogy: ionization produces electron-ion pairs, use an electric field to drift the electrons and ions to the oppositely charged electrodes.
In a solid semiconductor, ionization produces electrons-hole pairs. For Si need 3.6 eV to produce one e-h pair. In pure Si, e-h pairs quickly recombine  need to drift the charges to electrodes … but how?

31. Principles of operation
Charge collection
Need to isolate strips from each other and collect/measure charge on each strip  high impedance bias connection (resistor or equivalent)
Usually want to AC couple input amplifier to avoid large DC input currents
Both of these structures are often integrated directly on the silicon sensor. Bias resistors via deposition of doped polysilicon, and capacitors via metal readout lines over the implants but separated by an insulating dielectric layer (SiO2 , Si3N4). – h+ e- +

32. Principles of operation
Magnitude of collected charge
Usually specified in terms of minimum ionizing deposition:
Most probable charge ≈ 0.7 mean
Mean charge
(dE/dx)Si = 3.88 MeV/cm, for 300m thickness  116 keV
This is mean loss, for silicon detectors use most probable loss (0.7 mean)  81 keV
3.6 eV needed to make e-h pair
Max collected charge  e
(=3.6fC)

33. Principles of operation
Charge collection time, diffusion
Drift velocity of charge carriers v=E, so drift time, td = d/v = d/E
Typical values: d=300 m, E= 2.5kV/cm, e= 1350 cm2 / V·s, h= 450 cm2 / V·s, so td(e)= 9ns , td(h)= 27ns
Diffusion of charge “cloud” caused by scattering of drifting charge carriers, radius of distribution after time td:
 = 2Dtd , where D is the diffusion constant, D=kT/q
Same radius for e and h since td  1/
Typical charge radius:  ≈ 6m, could exploit this to get better position resolution due to charge sharing between adjacent strips (using centroid finding), but need to keep drift times long (low field).

34. Principles of operation
Double-sided detectors p n
Obvious question: why not get a 2nd coordinate by measuring the position of the (electron) charge collected on the opposite face?
This is possible and is often done but is not as simple as it might seem.
Problem: unlike the face with the p-strips, nothing prevents charge to spread horizontally on the back face.

35. Performance Position resolution: strip pitch and read-out pitch
If one treats the detected charge in a binary way (threshold discrimination), the resolution is simply:
 = p/ 12
As mentioned earlier, if the charge distribution is shared between adjacent strips, can use centroid finding to improve this resolution. However, since typical charge distribution sizes are of order 5-10 m this implies quite fine strip pitch. In fact it is not practical to make sensors of pitch less than 20m and most are greater.
Test devices have been made that have achieved  < 3.0m (using read-out pitch of 25 m), this is near the limit on precision determined by diffusion and statistical fluctuations of the ionizing energy deposition.
Read-out electronics pitch limit is 50m and time/cost constraints often argue for even larger read-out pitches. Fortunately there is a trick to preserve resolution with larger read-out pitch...

36. Performance Position resolution: capacitive charge division
If one reads out only every nth strip but preserves the signal magnitude, the charge gets shared such that the centroid resolution is nearly that obtained by reading out every strip. The limitation is that some signal is lost (capacitive coupling to the backplane) and noise is a bit higher (more input capacitance) and one loses two track separation capability.
This is clearly an economic solution in the case of low occupancy and has been used extensively. It does require a good signal/noise ratio, our next topic.

37. Performance Signal to noise ratio (S/N) Why is it important?
noise distribution
Signal to noise ratio (S/N)
Why is it important?
Landau distribution
with noise
Landau distribution has significant low energy tail which becomes even lower with noise broadening.
One usually has low occupancy in silicon sensors  most channels have no signal. Don’t want noise to produce fake hits so need to cut high above noise tail to define good hits. But if too high you lose efficiency for real signals. The centroid determination is also degraded by poor signal to noise.
Signal
Basic signal produced is ≈ e
Typical losses of 5-10% depending on the nature of the chosen electrical network (AC coupling capacitor, stray capacitances and resistances) and front-end electronics.

38. Summary Silicon strip detectors
- Built on simple p-n junction diode principle, now a “mature” technology
- Widespread use and cost drop thanks to microelectronics industry
- Many options and design possibilities
- Replaces wire chambers in high radiation

39. Silicon drift detectors
F.Riggi

40. Principles of operation
The transport of electrons, in a direction parallel to the surface of the detector and along distances of several cm, is achieved by creating a drift channel in the middle of the depleted bulk of a silicon wafer. At the edge of the detector, the electrons are collected by an array of small size anodes. P+ n+ n + -
Particle
The measured drift time gives information on the particle impact point coordinate y. The charge sharing between anodes allows the determination of the coordinate along the anode direction x. x y

41. Principles of operation

42. Large scale applications
Large scale applications of such detectors require low cost, high quality and high production yields
Silicon Drift Detectors are now in operation or planned as part of LHC experiments

43. SDD in ALICE Average spatial precision r  /z 38 /28 m
Cell size x 300 m
Detector area x 75.3 mm
Total number of readout channels K
Total number of cells M