Why silicon detectors?Why silicon detectors?

Main characteristics of silicon detectors:

• Small band gap (Eg = 1.12 V)

good resolution in the deposited energy

3.6 eV of deposited energy needed to create a pair of charges, vs. 30 eV in a gas detector

•Excellent mechanical properties

•Detector production by means of microelectronic techniques

small dimensions

spatial resolution of the order of 10 m

speed of the order of 10 ns

small amount of material (0.003 X0 for a typical 300 m thickness)

Eg=1.12 V

Detecting charged particlesDetecting charged particles•The impinging charged particles generate electron-hole pairs

ionization

•Electron and holes drift to the electrodes under the effect of the electric field present in the detector volume.

•The electron-hole current in the detector induces a signal at the electrodes on the detector faces.

Metal contact

n+-type implant

n-type bulk

Charged particle-V

+V

electron

hole

P+-type implant

ReversebiasE

Charged particle detectionCharged particle detection• Energy loss mainly due to ionization

– Incident particle interacts with external electrons of Si atoms

• All charged particles ionize• Amount of ionization depends on:

– particle velocity – particle charge – medium density

K L

Minimum Ionizing Particle

Charged particle signal evaluationCharged particle signal evaluation• dE/dx values from Bethe-Block formula are average values• The ionization process is statistical fluctuations

–Thick absorber: many collisions with atoms Gaussian distribution

–Thin absorber: few collisions with atoms Landau distribution

• Minimum Ionizing ParticledE/dx most probable value in 300 m of Si = 84 keV3.63 eV to generate a e-h pair in Si≈ 25000 electron- hole pairs Q ≈ 4 fC small charge!!!!

Detecting photonsDetecting photons•Photons are not ionizing particles

•The impinging photons which interact in the detector volume create an electron (via Photoelectric, Compton or Pair Production)

•The electron ionizes the surrounding atoms generating electron-hole pairs

• Electron and holes drift to the electrodes under the effect of the electric field present in the detector volume.

•The electron-hole current in the detector induces a signal at the electrodes on the detector faces.Metal contact

n+-type implant

n-type bulk

-V

+V

electron

hole

P+-type implant

ReversebiasE

photon

photoelectron

Photon interactionsPhoton interactions

Photoelectirc effect

Compton scattering

e+e _

production

Mass attenuation coefficient (cm2/g)

Silicon

Photon signal evaluationPhoton signal evaluation• Not all photon interact and can be detected• Typical X photons in mammography

20 keV (mammography) Photoelectric effect

≈ 30% of incident photons do photons do photoelectric effect in 300 mm of Si ≈ all photon energy converted in electron energy

≈ 5000 electron-hole pairs Q ≈ 1 fC

• Small charges Just to compare… in a 1 cm x 1 cm x 300 mm pure Si volume at 25°C there

are 4.5108 free e-h pairs due to thermal excitation

need for reverse biased junction need for amplification

Silicon Microstrips detectorsSilicon Microstrips detectors• Micro-strip detector: silicon detector segmented in long, narrow elements.

•Each strip is an independent p-n reverse-biased junction

• Provides the measurement of one coordinate of the particle’s crossing point with high precision (down to 10 m).

N-type substrate

P+n+

Al

P+

SiO2

AC coupling to electronics

SiO2Al

DC coupling to electronics

DC vs. AC couplingDC vs. AC coupling

• DC coupling:

– the readout electronics is connected directly to the strips

– Problem: the first stage of the preamplifier sinks the leakage current

• Preamplifier working condition affected by leakage current fluctuations

• Problems due to radiation damage which make the leakage current increase

• AC coupling:

– the readout goes through a decoupling capacitor

• The decoupling capacitance which must be much larger than the capacitance to the neighbours to ensure good signal collection (over 100 pF).

• The capacitor is integrated directly on the strips, using as plates the metal line and the implant and a thin SiO2 layer as dielectric.

Al SiO2

P+n+

Al

P+

SiO2

Silicon microstrip detectorsSilicon microstrip detectors

Strip p+ connected to ground, high (40-100 V) positive voltage on backplane n+

Silicon microstrip detectorsSilicon microstrip detectors

Parameter Value

Depth 300 μm

Strip length 10 mm

Strip pitch 100 μm

Depletion voltage 20-23 V

Leakage curr. (22º C) 50-60 pA

Pad and pixel detectorsPad and pixel detectors• PAD detector: silicon detector segmented in both directions

– Matrix of small diodes true 2 dimensional information– Problem: difficult interconnection with electronics

• Solution: PIXEL detectors– Readout electronics designed in form of a matrix

• each channel has exactly the same surface as a detector element

– Bump bonding: small ball of solder between detector and electronics– Higher cost due to complex electronics and bump bonding

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How to treat the signal How to treat the signal

• The signal from the detector is a small (amplitude ≈ few A) and fast (τ ≈ 10-20 ns) current pulse– Signal too small to be transmitted over long distances

• Need to amplify the signal

Detector

AmplifierAmplifier

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• Signal increased by a factor of 10But it’s noisy…

• Need to put a second stage to decrease noise

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AMPLIFIER

Detector

The problem of noiseThe problem of noise• A signal X fluctuates in time around its average value X0

– The distribution of the signal value follows a normal distribution– The s of the gaussian is a measurement of the noise of the system

t

X

x0

Noise can be due to

External sources Fluctuations in the electronic devices

Can be screenedCannot be eliminated,

but it must be minimized

Low pass filteringLow pass filtering• Normal solution: put a filter (shaper) after the

amplifier– Filter = elaboration on the signal consisting in a selection on

its frequencies

ShaperShaper

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AMPLIFIER

SHAPER

• Changed signal shape (semi-gaussian pulse shaping)• Current-Voltage conversion• Different time scale

Readout architectureReadout architecture• The signal after the shaper is a continuous function

(analog)– Infinite number of “points”– Not good for computer storage and analysis

• Digital signals:– Discrete number of signals in time domain (sampling)

• Select a finite number of “points”

• Store the value of the signals in this discrete set of “points”

– Discrete signal amplitude (digitization)• Loss of information • The extent of this loss depends on the number of bits

used to represent the amplitude

• Technology:– Discrete components vs. integrated (VLSI) circuits

Analog readout architectureAnalog readout architecture

• Sampling:– At t=t0: C0 capacitor enabled integrate current beween t0 and t1

– At t=t1: C1 capacitor enabled integrate current beween t1 and t2

• Advantages:– No loss of information– Exact signal amplitude is read

• Disadvantages:– Huge amount of data– Transmission of analog data

C0 C1 C2

Binary readout architectureBinary readout architecture

• Discriminator:– Signal above threshold 1– Signal below threshold 0

• Advantages:– Simple and fast– Small amount of data (good for large detectors with many cahnnels)

• Disadvantages:– Reduced information– Threshold scans needed to access to analog quantities (gain,

noise…)

VTH

6.4 mm

ADC readout architectureADC readout architecture

• ADC = Analog to Digital Converter:– Signal above threshold 1– Signal below threshold 0

• Advantages:– Digitized information about amplitude– Robust

• Disadvantages:– Still large amount of data (especially in large systems)– Mix between digital and analog

ADC

1 cm

Complete systemComplete system

Chip RX64 → counts incident photons on each strip of the detector

4 cm

6.4 mm10 strip = 1 mm

micro-bondings

Silicon microstrip detectoreach strip is an independent detector which gives an electric signal when an X-ray photon crosses it and interacts with a silicon atom

Knowing from which strip the electric signal comes from,the position of the incoming X-ray phonton is reconstructed.