Experimental Project Report on Characterization of Single Sided Silicon Microstrip Detector Submitted by Saurabh Sandilya Under the guidance of Dr. Gagan Mohanty & Prof. Kajari Mazumdar Tata Institute of Fundamental Research Mumbai 400 005 [10 th Feb 2010]
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Experimental Project Report
on
Characterization of Single Sided Silicon
Microstrip Detector
Submitted by
Saurabh Sandilya
Under the guidance of
Dr. Gagan Mohanty
&
Prof. Kajari Mazumdar
Tata Institute of Fundamental Research
Mumbai 400 005
[10th Feb 2010]
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3
Characterization of Single Sided Silicon Microstrip
Detector
Abstract
This report describes the different tests carried out on Single sided Silicon microstrip Detectors. We
have characterised different prototypes of the silicon microstrip detectors, which are fabricated at
Bharat Electronics Ltd (BEL), India with the feasibility of future upgradation in the Silicon Tracker of
the CMS experiment of CERN and of the Belle experiment at KEK. We have done the study of current-
voltage, capacitance-voltage characteristics, quality control testing like pin-hole test and also tested
them with IR LASER to get a feel of the position resolution.
1. Motivation
Silicon microstrip detectors played a crucial role in the discovery of top quark (third generation of
quarks), it continues to play a key role in the CMS and LHC experiments and will be important in the
future discoveries of the “most wanted” particle in high-energy physics, the Higgs boson. [1]
The CMS tracker at CERN is the largest silicon tracker ever built. The inner tracking system of the
CMS is designed to give precise and efficient measurement of the trajectories of charged particles
resulting from the proton-proton collision at LHC, therefore the detector needed to be precise and
should have fast timing response. The flux of the particle is also so intense that it can cause severe
radiation damage to the tracker. In order to fulfil these multiple requirements of granularity (position
resolution), fast timing response and radiation hardness silicon microstrip detectors are preferred. [2]
2. Introduction
Semiconductor detectors, and in particular silicon detectors have several advantages over other types
of radiation detectors – better energy resolution, linear response over a broad energy range, fast timing
response, flexibility of design, tolerance to high energy doses etc.
In semiconductor detector, the electron hole pairs are produced by the interaction of an ionizing
radiation and are collected by the application of an electric field to provide a signal proportional to the
total quantity of ionization and therefore to the absorbed energy.
To achieve this, the detecting medium should have the following properties:-
(a) Low values of the average energy to produce an electron-hole pair is desirable so that the
fluctuation in the total number of electron-hole pair produced is small.
(b) The charge carrier must move readily through the detector material, such that their collection
time should be smaller than the carrier life time to complete the collection of free charges.
(c) Leakage current resulting from the application of a fairly large electric field (~1000V.cm-1
, in
order to collect charges), must be very small so that the tiny signal from the transient current
can be measured. Therefore, high resistivity semiconductors are used. [3]
Silicon detectors fit very well to criteria for a detecting medium we had discussed above. We usually
use silicon because it has a band gap of ~1.1eV and can be used as a detector at room temperature,
while germanium with a band gap of ~0.7eV is required to be cooled (using liquid nitrogen) to bring
down the leakage current to an acceptable level. On the other hand since germanium has higher
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atomic number, it is well suited for gamma-ray spectroscopy (as photoelectric cross-section varies as
Z5)
2.1 p-n junction as a semiconductor detector
In spite of the high degree of purity in which silicon or germanium is available, it cannot be used as a
detector, because the transient current of the signal will be completely masked by the noise current
(or, leakage current). So, it is essential to reduce the leakage current largely in the semiconductor
medium to use it as a detector. For this, it is required that the positive contact must not inject holes
and negative contact must not inject electrons into the bulk medium. [3]
In semiconductor detector it is achieved by providing “blocking contacts” to prevent free exchange of
holes and electrons between the semiconductors and the electrodes (to collect signal). A common
method of achieving this is to form p-n junction just beneath the surface of the detector and operated
with a reverse bias voltage, whereby the majority carriers cannot be injected into the detector material.
These detectors are produced by forming a heavily doped n-type layer (n+ layer) on the surface of p-
type material or a heavily doped p-type layer (p+ layer) on the surface of n-type material.
2.2 Silicon Microstrip detector
The position resolution of silicon detectors is achieved by dividing a large area p-n diode into many
small regions, pixel like or parallel strip like, and each pixel or strip like division act like an
independent electrode.
The detectors (single sided) that we have used are fabricated using n-type silicon wafers as bulk
material. The p+
strips are used to deplete the n-type bulk material and act as electrodes. When an
incident ionizing particle interacts with the detector medium (depleted region), it creates electron-hole
pairs, the holes drift towards the p+ (negative electrode) and the electrons drift towards the n
+ (positive
electrode) due to electric field in the depletion region. The collected charges produce a current pulse
which is read out. Hence, the strip which produces the signal gives the position of the ionizing
particle. Now the magnitude of the signal measured on a given strip depends on its position relative to
the site of charge formation.
Figure 1: Operational principle of a single-sided p
+ n Silicon detector. Figure shows reversed biased strip
diodes, operated at voltage sufficient to fully deplete the n-type bulk material.
Silicon microstrip detectors are generally fabricated by passivated planar techniques, which combine
the technique of ion implantation and photolithography.
In this process at first n-type silicon is taken as a substrate, it is then chemically cleaned and oxidised
by heating it in O2 atmosphere at around 1000oC to have whole surface passivated. Next, selected
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areas of oxides are removed by using photolithography techniques. The opened areas now acts as a
window, enables doping of silicon in desired areas (by ion implantation techniques). Radiation
damage caused by ion implantation technique is removed by annealing. Finally surface of wafer is
metalized by Al and by proper masking desired pattern is generated. The oxide can be left to make a
capacitor (integrated coupling capacitors).
3. Specification of the detector
The detector that I have tested was single sided silicon microstrip detector (Detector # 2 and 8004-7),
whose mask designing was done at TIFR and was processed at BEL, India. Detector # 2 was having
11 sets of 32 strips (two of them set numbers 5 and 10 was having 30 strips) with different strip width
and pitch and detector # 8004-7 was having 1024 strips with fixed strip width and pitch.
Table 1: Specification of detector # 2
Wafer n type Silicon, 4inch Diameter, 300 micron
thickness
Resistivity
5 KΩ-cm
Number of independent
set of detectors
11
Type of implantation
for strips
p+
Number of strips
Per set
32
Table 2: specification of each set of detector # 2
Set number Strip length (μm) Width
(μm)
Pitch
(μm)
Number of strips
1 74734 12 65 32
2 74734 48 73 32
3 74734 12 80 32
4 74734 20 80 32
5 74734 35 80 30
6 74734 25 100 32
7 74734 35 100 32
8 74734 25 120 32
9 74734 35 120 32
10 74734 48 120 30
11 74734 25 135 32
Table 3: Specification of detector # 8004-7
Wafer n type Silicon, 4inch Diameter, 300 micron
thickness
Resistivity
2-4 KΩ-cm
Type of implantation
for strips
p+
Number of strips
Per set
1024
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The actual area of the detector # 8004-7 is 79600μm x 28400μm (and the effective area of the detector
is 76800μm x 25600 μm). Strip width is 50 μm and pitch is 75 μm and length of each strip is 26500
μm.
Figure 2(a) Figure 2(b)
In the above figure 2(a) and 2(b), detector # 8004-7 and detector # 2 has been shown respectively.
Figure 3: Photograph of a corner of single sided silicon microstrip detector
The individual strips (p+ strips) are connected to “bias line” through polysilicon resistance such that
we can apply a same fixed potential in all strips. The resistance for bias resistance are kept high
enough to separate the strip from others.
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The detector strips are surrounded by guard ring which prevents charge accumulations in the silicon
edges, and thus leakage current is prevented from being absorbed by the edge strips. It also degrades
the potential towards cut edge of silicon.
4. Reverse current-Voltage (I-V) Characterization
For measuring I-V characteristics of the detector, we applied reversed bias voltage and measured the
leakage current (or, reverse current). For a given bias voltage, reverse current depends to a certain
extent on environmental factors. So, we had carried out the experiment in “class 10000” room, where
temperature, humidity, dust and other environmental factors were controlled.
We used mechanically controlled biasing probe for giving negative potential to the bias pad by
looking in to the microscope. Positive potential was applied to the “chuck” on which detector rests
and fixed by using vacuum. The detector was placed in a box for minimising the effect of external
light. The voltage applied and the reverse current was measured by “Keithley 237”, and we have used
LabVIEW as interface.
The measurements were performed at room temperature, typically about 21oC and at a relative
humidity around 64%.
Figure 4: Schematic diagram of the experimental setup for measuring I-V characteristics.
We increased the reversed biased voltage in the step of 10V, (using Keithley 237) maximum up to
350 V or up to breakdown if observed before. We did this experiment for all the 11 sets of detector #
2 and for detector # 8004-7 and we obtained I-V characteristics as shown in figure 5 and figure 6
respectively.
Figure 5 shows block diagram of the connection made for obtaining I-V characterstics.
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Figure 5: Reverse current as a function of biased voltage of detector # 8004-7. It shows the breakdown
voltage to be at 310V
Figure 6: Reverse current as a function of biased voltage of different sets of detector # 2