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2003 ICFA School
LABORATORY COURSE ON SILICON SENSORS
Elisabetta Crescio – [email protected]
Marek Idzik – [email protected]:
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]
Danielle Moraes
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General description of silicon detectors
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)
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Metal contactphoton
Charged particle
n+-type implant
n-type bulk
-V
+V
electron
hole
P+-type implant
General description of silicon detectors
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.
4General description of silicon detectors
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.
-V
+V
depleted region
Increasing the polarization voltage, it is possible to extend the depletion layer down to the backplane.
To have full efficiency, the polarization voltage must be high enough to deplete the full detector thickness (typically 300 m)
junction
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Detector fabrication
General description of silicon detectors
Based upon the different fabrication processes, silicon detectors can be classified in three main types, namely the “diffused detector”, the “surface-barrier” detector and the “ion-implanted” detector.
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 kcm).
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.
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Planar process
General description of silicon detectors
N-type silicon SiO
2
BB
As
n+
P+
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.
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. An annealing process at 600oC allows partial recovery of the lattice from the damage caused by irradiation.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.
Al
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.
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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.
SiO2
P+n+
Al
P+
Cross section of a AC coupled strip detector
Cross section of a DC coupled strip detector
Al SiO2
General description of silicon detectors
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.
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Double-sided Strip Detectors
General description of silicon 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.
+ + + + + +- - - - - -
SiO2 Al
n+
Al
n+
Fixed oxide charge R few k
n-type
R > few kSiO2
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.
backplane
backplane
Al Al Al
n+ n+p+n-type
+ + + + + + -
- - -
- -
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.
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Pixel and Pad Detectors
General description of silicon detectors
Producing a matrix of small diodes one can obtain in one detector true two-dimensional 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.
Detector chip
electronics chip
Signal out
Advantages:• unambiguous two-dimensional readout
• low diode capacitance, excellent signal-to-noise ratio at high speed
• very small leakage current per element, radiation toleranceThe price to pay is a very large number of connections and of readout channels!
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General description of silicon detectors
P+
P+P+
P+P+P+P+
P+P+P+
P+
n+
n+
n+
n+++- --
Particle
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.
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
7 cm
Silicon Drift Detectors (1)
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Silicon Drift Detectors (2)
General description of silicon detectors
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
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
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
Coordinate x (anode axis)
Coordinate y (drift axis)
