Transcript Presentazione di PowerPoint - Istituto Nazionale di Fisica

2003 ICFA School
LABORATORY COURSE ON SILICON SENSORS
This course consists of five different exercises which illustrate the
main features of silicon detectors. You can use this presentation as an
introductory guide in the execution of the experiments.
Alan Rudge – [email protected]
Tutors:
Elisabetta Crescio – [email protected]
Marek Idzik – [email protected]
Danielle Moraes
1
General description of
silicon detectors
Silicon detectors are used in almost all High-Energy Physics experiments
built in the last 15 years, from large collider experiments to fixed-target
ones, and also in many specialized detectors like spectrometers for space
or detectors for medical diagnostics.
They offer these characteristics:
• speed of the order of 10 ns
• spatial resolution of the order of 10 m
• flexibility of design, with feature-size of the order of 10 m
• small amount of material (0.003 X0 for a typical 300 m thickness)
• excellent mechanical properties
• good resolution in the deposited energy (3.6 eV of deposited energy are
needed to create a pair of charges, vs. 30 eV in a gas detector)
General description of silicon detectors
2
Structure of the silicon sensor
diode
A silicon detector works like an ionization chamber: the impinging
ionizing particles generate electron-hole pairs, which 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
photon
Charged particle
P+-type implant
-V
n-type bulk
electron
hole
n+-type implant
General description of silicon detectors
+V
3
Why a reverse-biased diode?
The amount of charge deposited in the typical 300 m of thickness of a silicon
detector is very small (25 000 electrons is the average value for a relativistic,
singly-charged particle crossing the detector orthogonally to its surface), an
therefore it would be masked by the fluctuations of the current which the applied
field makes flow even in high resistivity, hyper-pure silicon.
If we reverse-bias the diode, we will have the necessary electric field and only a
very small current.
junction
To have full efficiency,
the polarization voltage
must be high enough to
deplete the full
detector thickness
(typically 300 m)
-V
Increasing the
polarization voltage,
it is possible to
extend the depletion
layer down to the
backplane.
depleted region
+V
General description of silicon detectors
4
Detector fabrication
Detectors can be fabricated both on p-type and n-type substrate, but the second
is the most widely used. The silicon must be of high purity and resistivity (of the
order of several kcm).
Based upon the different fabrication processes, silicon detectors can be
classified in three main types, namely the “diffused detector”, the “surfacebarrier” detector and the “ion-implanted” detector.
Diffusion was the first technique available for detector fabrication and it is
based on the diffusion of impurities through high temperature processing steps.
The high temperature, however, degrades the material resulting in higher
leakage currents.
Surface barriers detectors, on the contrary, are made in a low tempearture
process, evaporating in vacuum a thin Au layer directly on the n-type crystal
surface. In practice this metal-semiconductor interface is difficult to control
and the leakage current of these deviced is higher than that of the ones made
by implantation.
Ion-implantation nowadays is the most commonly used technique. The main steps
of the planar process are shown in next pages.
General description of silicon detectors
5
Planar process
N-type silicon
SiO2
n-type wafers are oxidized at 1030oC to have the whole
surface passivated.
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.
B
As
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 wondows in the
oxide. P+ strips are implanted with boron, while
phosphorous or arsenic are used for the n+ contacts.
B
P+
n+
An annealing process at 600oC allows partial recovery of
the lattice from the damage caused by irradiation.
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.
6
General description of silicon detectors
Silicon Strip Detectors
A silicon detector segmented in long, narrow elements is called
a micro-strip detector. It provides the measurement of one
coordinate of the particle’s crossing point with high precision
(down to 1 m).
The precision depends on the noise of the readout chain. If
digital readout is used (strip hit or not hit), the resolution is:
= pitch/(12)
where the pitch is the distance between strips.
Al
SiO2
Cross section of a DC
coupled strip detector
The detectors can be DC coupled, when the readout electronics
is connected directly to the strips.DC coupling can present some
difficulties, since the first stage of the preamplifier sinks the
leakage current, which can be large after a large radiation load,
and therefore changes working conditions depending on its value.
Often the readout goes through a
decoupling capacitor, which must be
much larger than the capacitance to
the neighbours to ensure good signal
collection (over 100 pF). A possible
solution consists in integrating a
capacitor directly on the strips, using
as plates the metal line and the
implant and a thin SiO2 layer as
dielectric.
General description of silicon detectors
Al
P+
n+
SiO2
P+
Cross section of a AC
coupled strip detector
7
Double-sided Strip Detectors
Since the micro-strip detector provides only one coordinate with good precision,
the segmentation of the backplane is a natural way to provide a second coordinate
and thus a space point without adding material on the trajectory of the particles.
The use of double-sided micro-strip detectors allows the correlation of signals
collected on the two sides, which apart from the readout electronics noise and
response is the same, thus reducing multi-hit ambiguities.
Fixed oxide charge
SiO2
Al
n+
SiO2
++++++
- - - - - -
n+ -
n+
n-type
Al
-
R few k
Al
Al
++ +
-
p+
n-type
backplane
R > few k
Al
++ +
-
-
n+
-
backplane
Subdividing simply the n+ contacts the presence
of positive charge at the Si-SiO2 interface
induces in the n-type substrate an accumulation
layer of electrons, resulting in a low resistance
between the strips. Therefore the signal spreads
over many electrodes, making the subdivision
ineffective.
A method used to solve this problem is to implant
a p+ blocking strip in between the n+ ones. The
blocking strips are left floating, since their
function is just to interrupt the conduction
channel.
8
General description of silicon detectors
Pixel and Pad Detectors
Producing a matrix of small diodes one can obtain in one detector true twodimensional onformation. In general this is called a silicon pad detector, and it
is connected to the readout electronics via a fanout circuit overlaid on the
silicon wafer and wire bonded to the individual pads. It is clear that silicon
pad detectors cannot have too many detecting elements, or the problem of
interconnections becomes unmanageable.
A way out is to design the readout electronics in form of a matrix, with each
channel occupying exactly the same surface as a detector element, and equip
each channel of electronics and every element of the detector matrix with a
bonding pad. Than a tiny (few tens of microns of diameter) ball of solder
(often an indium alloy) is deposited on the bonding pads, and the two chips are
put in contact face-to-face. This device is called a silicon pixel detector.
Advantages:
• unambiguous two-dimensional readout
Detector chip
• low diode capacitance, excellent signal-to-
Signal out
noise ratio at high speed
• very small leakage current per element,
radiation tolerance
electronics
chip
The price to pay is a very large number of
connections and of readout channels!
9
General description of silicon detectors
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.
Particle
n+
n+
n+
P+
P+
P+
n
P+
P+
P+
P+
P+
-+
+
- +-
P+
P+
x
y
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.
P+
10
General description of silicon detectors
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
Coordinate x (anode axis)
Detector obtained with a numerical simulation
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
11
General description of silicon detectors