Silicon Pixel Detectors

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					     Silicon pixel detectors
with some info also on Strip and Drift detectors

• 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
   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


    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
   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
Advantages vs disadvantages

• Advantages:
•   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
   The hybrid pixel detector

Cell size: 50 x 425 microns

Thickness around 200 + 200
   The hybrid pixel detector

A matrix of 8192 (32 x 256) independent   1200 chips
cells is envisaged for each individual            ~ 10 M channels
pixel chip in ALICE
    Development of the readout electronics

Prototype     Cell size   No. of cells   Technology

Omega 2        75 x 500     16 x 63          3
Omega 3        50 x 500     16 x 127         1
ALICE1 test    50 x 420       2 x 65         0.5 
ALICE2 test    50 x 420       2 x 65         0.25 
ALICE1         50 x 425      32 x 256        0.25 
Bump-bonding techniques
    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)
                  1          pixel             130
                  2          pixel             40
                  3          drift             13
                  4          drift              5
                  5          strip               2

                  6          strip               1.5
   Irradiations of the readout electronics

List of irradiations tests carried out on the ALICE readout
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
From single modules to complex detectors

• Mass production organization
• Mechanics and assembly
• Cooling
• Beam tests
• Installation procedure
• ...
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
• ...
ALICE silicon pixel detector: a case study
  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
Wafer probing

                                                      Bridge to PC

                     Power Supply   VME-crate
                   Power Supply     with pilot and
                                    JTAG controller
   Probe Station
   with Probe

     Wafer probing: hardware
Semiautomatic Probe Station

Wafer probing: software

LabView-based programs

                          Analysis with
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.
   Readout CHIP classification

Chips classified as Class I (to be bump-bonded), Class II (minor
defects), Class III (major defects)

         CLASS    AM9VG4T     AB9VHXT      AZ9VETT      AV9VGWT

            I      46 (53%)    36 (42%)    64 (74.5%)   37 (43%)

           II      10 (12%)     8 (9%)       8 (9%)      5 (6%)
           III     30 (35%)    36 (42%)    14 (16.5%)   35 (41%)
  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

     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
     New applications of pixel detectors
     (Medicine, …) are being exploited
   Silicon Strip detectors: 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

    – 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?
  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.       h+ e-
     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).

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

                                                                The measured drift time gives
                                                                information on the particle
                 n+                                             impact point coordinate y. The
            n+                                                  charge sharing between anodes
       n+             P+   P+   P+    P+    P+              x   allows the determination of the
   n                                  -
                                     + +                        coordinate along the anode
                                     -+ -                   y   direction x.
    P+           P+        P+   P+   P+     P+
Principles of operation
   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

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