A Silicon Microstrip System equipped with the RX64DTH ASIC for dual energy mammography

1
The most complex Silicon Detectors Silicon
Drift!
2
Silicon Drift Detectors (SDD)
E. Gatti, P. Rehak, Semiconductor Drift Chamber
- An Application of a Novel Charge Transport
Scheme, Nucl. Instr. and Meth. A 225, 1984, pp.
608-614.

• I rivelatori a deriva sono usati
• nell'ambito degli esperimenti con ioni pesanti
  ultrarelativistici (altissima desità di
  particelle, tracking con TPC, rivelatori lenti
  read-out time ms)
• per applicazioni di tipo medico / industriale
  (rivelazione raggi X) P.
  Lechner et al., Silicon drift detectors for high
  resolution room temperature X-ray spectroscopy,
  Nucl. Instr. and Meth. 1996 A 377, pp. 346-351.

Svantaggi / sfide - tempo di raccolta
lungo - elettronica complessa - alta tensione
(per i silici) - richiede calibrazione
accurata della temperatura - richiede
calibrazione accurata dell'uniformità del silicio
Vantaggi punti 2D senza ambiguità alta
risoluzione per entrambe le coordinate costo
minore rispetto ai pixel lettura analogica?
Particle ID
3
Rivelatori al silicio a deriva

• Il concetto
• 1. due giunzioni pn contrapposte
  ("back-to-back"), entrambe in polarizzazione
  inversa, svuotano il silicio dalle cariche libere
  e creano un campo elettrico con un minimo al
  centro

4
2. creando una catena di coppie di giunzioni
"back-to-back", ciascuna coppia con una tensione
decrescente rispetto a quella precedente, si
genera il campo di deriva per portare la carica
fino all'elettrodo n
5
Silicon Drift Detectors (1)
Silicon drift detectors are charged partcle
detectors capable of providing both
two-dimensional position information and
ionization measurements.
The operating principle is based on the
measurement of the time necessary for the
electrons produced by the ionization of the
crossing particle to drift from the generation
point to the collection anodes, by applying an
adequate electrostatic field.
7 cm
The transport of electrons, in a direction
parallel to the surface of the detector and along
distances of several centimetres, 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 impact point coordinate y. The charge
sharing beween anodes allows the determination of
the coordinate along the anode direction x.
x
y
6
Silicon Drift Detectors (2)
In practice, both the drift field and the
depletion bias are produced by p parallel strips
implanted on both faces of the detector. Each
strip is polarized with a negative voltage
proportional to its distance from the anodes, in
order to produce the drift field. In this way,
the p-n junctions are reverse polarized and can
assure the depletion of the detector through a n
ring placed at the periphery of the detector.
Potential energy in the Silicon Drift Detector
obtained with a numerical simulation
Coordinate x (anode axis)
The diffusion and Coulomb repulsion between
electrons play a significant role in the drift
detectors since the drift time is of the order of
a few ?m. In the thickness direction, they are
compensated by the parabolic potential, but
generate an increase of the electron cloud size
in both other directions. The electron cloud
reaches the collection zone with a size
increasing as a function of the total drift time.
Thus a charge may be collected by more than one
anode and the coordinate x is determined as the
centroid of the charge deposited on the touched
anodes. Typically, a 200 ?m pitch allows a
precision of 30 ?m.
ANODE
Coordinate y (drift axis)
The signal measured on each anode is amplified
and sampled with a typical frequency of a few
tens of MHz, depending on the drift velocity and
the peaking time of the electronics. The
coordinate y is measured by calculating the
elapsed time between an external trigger and the
arrival of the charge
7
Applicazioni mediche diagnostica con raggi X

• It consists of a volume of fully depleted
  high-resistivity silicon, in which an electric
  field with a strong component parallel to the
  surface drives electrons generated by the
  absorption of ionising radiation towards a small
  sized collecting anode.
• The electric field is generated by a number of
  increasingly reverse biased field strips covering
  one surface of the device
• The radiation entrance window on the opposite
  side is made up by a non-structured shallow
  implanted junction giving a homogeneous
  sensitivity over the whole detector area.

animation

• The unique property of this type of detector is
  the extremely small value of the anode
  capacitance, which is independent of the active
  area. This feature allows to gain higher energy
  resolution at shorter shaping times compared to
  conventional photo diodes and Si(Li) detectors,
  recommending the SDD for high count rate
  applications
• Due to the elaborated process technology used in
  the SDD fabrication the leakage current level is
  so low that the SDD can be operated with moderate
  cooling by means of a single stage Peltier
  element.
• The SDD's energy resolution (FWHM lt 145 eV _at_
  MnKa, -20oC) can be compared to that of a Si(Li)
  detector requiring no expensive and inconvenient
  liquid nitrogen cooling. It surpasses the quality
  of pin-diodes.

8
Esperimenti di fisica nucleare delle alte
energie CERES - STAR

• CERES-NA45 _at_ CERN SpS
• Sistema di Drift circolari usate come rivelatore
  di vertice (1996-2000)
• Problemi legati al partitore esterno
• STAR _at_ RHIC Silicon Vertex Telescope
• 3 piani cilindrici di SDD rettangolari con
  partitore esterno
• smontate dopo 4 anni di presa dati
• Problemi legati al partitore esterno

9
SDD per ALICE RD

• inizio progetto 1992
• INFN DSI project in collaboration with CANBERRA
  Semiconductors. The aim of the project was the
  production of a large area SDD (5 inch wafers)
  with integrated high voltage divider
• studio del materiale
• Neutron Transmutation Doped 5-inch silicon wafers
  with a resistivity of 3kOcm and a thickness of
  300µm.
• simulazione del campo di deriva e raccolta
• definizione della geometria di catodi, anodi e
  metallizzazione
• definizione dei parametri di funzionamento V/gap
  tensione di polarizzazione
• simulazione delle zone di guardia
• raccolta corrente superficiale
• minimizzazione rischi di break-down tra catodi
  centrali e anello di guardia
• test prototipi e studi di radiation damage
• Primi prototipi 1993-94 ? rivelatori
  unidirezionali
• Primo prototipo con geometria quasi definitiva
  bidirezionale 1998

10
Simulazioni (I)

• We need an extensive numerical simulation of SDD
  electrical behaviour.
• The simulation of a complete large area detector
  cannot be performed because of the huge amount of
  memory required. Hence the calculation has to be
  done on limited portions of the device,
  introducing artificial boundary conditions.
• We use a 2D approximation ( 3D differential
  equations would in principle be required)
• This approximation can be done if we consider
  cross sections orthogonal to the cathodes in such
  a way that the partial derivative along the third
  dimension is negligible.
• We use ATLAS device simulation software produced
  by SILVACO. The simulations regard the following
  regions
• collection zone
• drift region
• guard region
• injectors

11
Simulazioni (II-drift region)

• The electric field needed to drift the electron
  cloud is imposed by a suitable bias of the drift
  cathodes
• In order to obtain a realistic result, the
  lateral boundaries must be kept far from the
  region of interest (the central region).

• For this reason we used a high number of cathodes
  (9), leading to a total length of about 1.5mm
  against a device depth of 0.3mm.
• Moreover, the first couple of cathodes on the
  left are extended in order to further remove the
  left edge.
• An anode is located at the right side in order to
  guarantee a contact for the bulk.

12
Simulazioni (III-drift region)

• The bottom of the potential gutter is perfectly
  linear The green line is the potential profile
  0.1µm under the Silicon oxide.
• It is visible the effect of the field-plate that
  lowers the potential variations (electric field)
  near the junctions
• It is worthwhile noting that the distance between
  the red and the green line, passing from one
  cathode to another, is constant, meaning that
  there is no influence of the boundary solution.

13
Simulazioni (IV-Collection zone)

• The collection zone is one of the critical
  regions in a SDD. Here the electron cloud is
  forced to move from the middle plane of the
  detector toward the anode array in the n-side.
  The forcing electric field is applied by properly
  biasing the last few cathodes in the proximity of
  the anodes.

• First we must avoid a trapping of the signal
  electrons under the oxide when approaching the
  surface.
• Second we have to minimize the non-linearity of
  the drift speed associated with the transversal
  movement towards the n-side of the detector.
• Third we have to guarantee a good potential
  separation between anodes and perimeter to avoid
  inefficiencies of the electron collection.

14
Simulazioni (IV-Collection region)

• .The picture shows an overlay of the potential
  map and the trajectory of eight electrons placed
  at various positions.
• It is worthwhile noting that the "pull-up" region
  is very short (200µm), minimising the systematic
  error on the drift time. Such a kind of
  collection minimizes also the risk of electron
  trapping under the oxide because the trajectories
  are kept far from the surface up to the anode
• The potential barrier between anodes and nbulk
  contact is about 6V.

15
ALICE DB-2
Deriva
Deriva
SDD Alice 7.02 ? 7.53 cm ? 300mm Diviso in due
volumi attivi dal catodo centrale polarizzato a
-2370V Ciascun volume ha 256 anodi di raccolta
(passo di 294 mm). 292 catodi (passo di 120mm)
permettono di impostare il campo di deriva
tramite un partitore di resistenze impiantate.
16
Caratteristiche principali

• partitore integrato sul rivelatore
• doppia catena di resistenze in polisilicio
  (R160kO)
• una per catodi di campo
• una per catodi di guardia

anodi
griglia isolante
17
Guard Zone

• At each side of the drift cathode array, p
  implants (guard strips) grade the potential from
  the highest negative voltages to grounded outer
  n implant ring. Since this region should be as
  small as possible the electric field needs a
  careful evaluation.
• Furthermore, as the voltage difference between
  two consecutive guard-zone strips is 16V (see
  detector description), the punch-through
  phenomenon should be carefully evaluated

18
Caratterizzazione dei rivelatori

• Produzione iniziata settembre 2004 (Canberra
  Semiconductors)
• Test sui rivelatori nudi svolti presso INFN
  Trieste
• Calculating on a yield of 50, about 500
  detectors had been characterized at best
• First a visual inspection has to be done to check
  for interruptions or shorts in the metal.
• The second step is to check both the current at
  the anodes
• the probe card connects together the anodes in
  groups of eight
• and the linearity of the potentail on the
  divider
• checking the voltage drop every ten drift
  cathodes

Connection made by use of two probe cards able to
contact the detector safely from both faces
(probe pad coordinates) to choose only the
well-performing ones with minimal risk of damage
to the detector during this operation.
19
Risultati tipici
uniformità del partitore corrente agli anodi
20
Detector selection criteria

• Linearity of the potential distribution on the
  integrated divider.
• Due to local defect generating high current or to
  a punch-through current among the cathodes.
• Non-linearity of the potential distribution
  generates a systematic error on the position
  resolution along the drift direction. Furthermore
  when the distortion on one side
• If the difference on the voltage drop on the
  resistor connecting two consecutive cathodes is
  of the order of 0.1V ? the electron cloud is
  shifted dangerously to one of the surfaces
• anode leakage current
• this determines the noise - and therefore
  efficiency - of the detector readout (the anode
  capacitance can be ignored since it will always
  be small compared to the fixed contribution
  introduced by the readout microcables).
• LIMIT for Anode current 100 nA

21
Tracking ALICE major challenge
Nch(-0.5lt?lt0.5)8000
22
SDD electronic chain in beam test

• Test of final detectors electronics with beam _at_
  PS

Noise levels
One MIP
Signal vs. drift distance
23
Esempio di acquisizione di segnali
24
Esempio di un evento dall'esperimento STAR
Ampiezza
Tempo
deriva
Anodo
25
Problemi delle SDD

• Sensibilità alla temperatura
• l'integrazione di strutture di calibrazione
• Sensibilità all'uniformita del silicio di base
• la mappatura dei sensori

26
Temperature stability dependence
La velocità di deriva dipende dal campo, ma anche
dalla temperatura (per non dover correggere per
l'influenza della temperatura sulla misura,
ci vorrebbe una stabilità di 0.1 gradi
Celsius) gt per calibrare, strutture chiamate
"iniettori" sono integrate sul rivelatore permette
ndo di generare elettricamente segnali da
posizioni conosciute
27
Iniettori
iniettori

• 3 injector lines are inserted between consecutive
  drift cathodes at distances of 3 mm, 17.6 mm and
  34 mm from the anodes.
• Each line consists of a metal strip deposited
  over the oxide. Beneath the strip, separated by a
  100 nm thick oxide, it runs a p implant
  interrupted in 33 points that constitute the
  injection locations.
• In these points, 100 mm wide, there is an
  accumulation of electrons due to the positive
  oxide charge. Applying a negative pulse to the
  metal line we push a certain number of these
  electrons in the silicon bulk.
• At the anodes we obtain three sets the drifted
  images of the 33 injectors.

28
simulazionee risultati dei test di laboratorio
Injector event in SDD module mounted on the ladder
29
Uniformita del drogaggio (resistivita)

• Se la resistività del silicio del rivelatore non
  è abbastanza uniforme si creano dei campi
  elettrici parassiti che spostano la carica dalla
  traiettoria ideale,
• quindi si trova un errore sistematico fra la
  posizione misurata e la posizione dove la
  particella è realmente passata

30
Mappe degli errori sistematici per metà di un
rivelatore(beam test data)
?xgt0
?x XSDD-XREF
?xlt0
?ygt0
?y vdrifttSDD-YREF
?ylt0
31
Scelta del materiale

• studi accurati su silicio Floating zone e
  Neutron Transmutation Doped NTD 1992-1994
• Floating zone fluttuazioni di resistività fino a
  30
• NTD lt10 (Silicio Wacker)
• Produzione ALICE
• iniziata su Wacker -gt Fluttuazioni viste in slide
  precedente ? necessità di mappare ogni singolo
  rivelatore per correggere off-line i dati dagli
  effetti sistematici (stabili nel tempo)
• continuata su TOPSIL ? fluttuazioni pressoche
  inesistenti
• stazione di mappatura presso lab tecnologico INFN
  Torino usata come stazione di test su moduli
  completi (DETECTOR FEE CAVI ALIMENTAZIONE E
  DATI schede ausiliarie)

32
SDD front-end

• FEE amplify-memory-ADC (PASCAL)
• event buffer chip (AMBRA)

33
SDD module p-side with ladder cables and
end-ladder boards
LV Board
Transition Cable
HV Board
LV Board
Heat exchanger at the back of Hybrid
34
LInner Tracking System di ALICE
35
The ALICE Inner Tracking System
SSD
SDD
SPD
Lout97.6 cm
Rout43.6 cm

• 6 Layers, three technologies (keep occupancy
  constant 2)
• Silicon Pixels (0.2 m2, 9.8 Mchannels)
• Silicon Drift (1.3 m2, 133 kchannels)
• Double-sided Strip Strip (4.9 m2, 2.6 Mchannels)

36
ITS Mechanical assembly
Positioning rings
CF support cones
37
ITS cabling
Strip connectors
Pixel connectors
Drift connectors

• connectors are for both the liquid cooling and
  the electrical power
• every pixel box represents the connectors for one
  SPD sector, distributed along the Z axis
• every strip or drift box represents the
  connectors for two sectors, distributed along the
  Z axis

38
ITS/FWD cabling maquette
Temporary support ring
Patch panels
Cables
V0 detector
FMD Si disks
Beam pipe support
T0 detector
Front Absorber
T0
SPD barrel
Cooling pipes
SPD support
Air ducts
Cooling pipes
Bellows
39
SILICON DETECTORS

• PART I
• Characteristics on semiconductors

40
Reverse biased p-n junction (I)
V
d
NAgtgtND
-xp
xn
x
x
41
Reverse biased p-n junction (II)
Depletion voltage voltage necessary to deplete
all the junction thickness
How to know the depletion voltage of a diode?
Measurement of the capacitance
42
Leakage current
The main sources of leakage current in a silicon
sensor are
1) Diffusion of charge carriers from undepleted
regions of the detector to the depleted
region. Generally well controlled, small
contribution few nA/cm2
2) Thermal generation of electron-hole pairs in
the depleted regions. Temperature dependent,
contribution ?A/cm2
3) Surface currents depending on contamination,
surface defects from processing.. It may be the
dominant contribution, but it can be reduced
processing guard rings
43
p-n junction as detector
P
Energy necessary for a m.i.p. to produce a pair
of electron-hole in Si 3.6 eV
A m.i.p. produces 25000e- 4fC
Energy lost by a m.i.p. in 1 mm of silicon is
300 KeV. The typical thickness of detectors is
300?m.
44
Fabrication
n-type wafers are oxidized at 1030oC to have the
whole surface passivated.
SiO2
Using photolithographic and etching techniques,
windows are created in the oxide to enable ion
implantation. Different geometries of pads and
strips can be achieved using appropriate masks.
The next step is the doping of silicon by ion
implantation. Dopant ions are produced from a
gaseous source by ionisation using high
voltage.The ions are accelerated in an alectric
field to energy in the range of 10 keV-100 keV
and then the ion beam is directed to the windows
in the oxide. P strips are implanted with boron,
while phosphorous or arsenic are used for the n
contacts.
As
P
An annealing process at 600oC allows partial
recovery of the lattice from the damage caused by
irradiation.
n
Al
The next step is the metallisation with
aluminium, required to make electrical contact to
the silicon. The desired pattern can be achieved
using appropriate masks.
The last step before cutting is the passivation,
which helps to maintain low leakage currents and
protects the junction region from mechanical and
ambient degradation.
45
Detecting 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
-V
P-type implant
Reverse bias
n-type bulk
electron
hole
V
n-type implant
46
Charged 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