ARCHIVES
MASSACHUSETTS INSTflUTE
OF TECHNOLOGY
JK. 252012
Analysis of 3D Silicon Pixel Vertex Detector Damage Effects due to
Radiation Levels Present in the LHC at CERN
By
Matthew R Chapa
SUBMITTED TO THE DEPARTMENT OF NUCLEAR SCIENCE AN ENGINEERING
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
BACHELOR OF SCIENCE IN NUCLEAR SCIENCE AND ENGINEERING
AT THE
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
JUNE 2012
Matthew R Chapa.
All Rights Reserved.
The author hereby grants to MIT permission to reproduce and to distribute publicly
Paper and electronic copies of this thesis document in whole or in part.
/
ze
Signature of Author:
7
Matthew R Chapa
Department of Nuclear Science and Engineering
May 11, 2012
Certified by:_
Dennis Whyte
Professor of Nuclear Science and Engineering
Thesis Supervisor
Accepted by:
Dennis Whyte
Professor of Nuclear Science and Engineering
Chairman, NSE Committee for Undergraduate Students
1
Analysis of 3D Silicon Pixel Vertex Detector Damage Effects due to
Radiation Levels Present in the LHC at CERN
By
Matthew R Chapa
Submitted to the Department of Nuclear Science and Engineering on May 11, 2012
In Partial Fulfillment of the Requirements for the Degree of
Bachelor of Science in Nuclear Science and Engineering
ABSTRACT
In high energy physics experiments, very high precision tracking of charged particles is
needed. Solid state detectors achieve the high precision necessary to provide track and vertex
reconstruction of the particles that traverse them, but tracking performance begins to deteriorate at
fluxes of radiation around 1014 - 1015 hadrons/cm 2. These radiation levels are congruent with those
experienced by the ATLAS pixel detector, the inner most part of the ATLAS tracking system,
which is vital to track and vertex reconstruction. During the planned shut-down of the Large
Hadron Collider (LHC) in 2013-2014, the energy and luminosity of the LHC will both be
increased. The current pixel detector has begun to suffer deterioration of performance, so the
ATLAS Collaboration has initiated an upgrade to take place during the scheduled shut-down
beginning in 2013, the Insertable B-Layer (IBL).
The IBL will be assembled and placed in between a reduced diameter beam pipe and the
current pixel detector, acting as the fourth layer of the ATLAS inner detector. The pixel sensors of
the IBL will have to sustain a radiation dose of 5 * 10" n,/cm2. Two sensor technologies are being
considered for the IBL upgrade: planar n-in-p silicon pixel sensors and 3D double sided n-in-p
pixel sensors. Research of both these technologies is being done by the IFAE in collaboration with
CNM-Barcelona. To cope with the increased data rate after the LHC upgrade, a new front-end chip
has also been produced, the FE-14 front-end chip. Test results and data analysis from five different
3D pixel sensor devices, all fabricated at CNM-Barcelona were done. Evaluation of these
technologies and the test results of irradiated 3D pixel sensor devices are carried out in this thesis.
Thesis Supervisor:
Dennis Whyte
Title:
Professor of Nuclear Science and Engineering
2
Table of Contents
Page
1
T itle P age ......................................................................................................
A bstract ..................................................................................................
. . 2
Table of C ontents..........................................................................................
3
L ist o f F igures................................................................................................4
L ist of T ables............................................................................................
1. Introduction .........................................................................................
2.
The LHC and ATLAS Detector..................................................................
2.1
3.
. 5
6
8
ATLAS Inner Detector..................................................................
12
2.1.1 TRT & SCT ......................................................................
13
2.1.2 Pixel Detector..................................................................
14
3D and Planar Pixel Sensors......................................................................15
3.1 Semi-Conductor Detectors.................................................................15
3.1.1 Semi-Conductor Properties........................................................15
3.1.2 Doped Semi-Conductors ......................................................
17
3.1.3 N oise.............................................................................21
3.2 Two Pixel Detector Technologies....................................................
22
3.2.1 P lanar............................................................................. 22
3 .2 .2 3D ....................................................................................
23
4
5
ATLAS Upgrade................................................................................25
4.1 Insertable B-Layer (IBL) .............................................................
4.2 3D Sensor Technology....................................................................26
25
4.3 Front-End Chip .........................................................................
4.3.1 FE-14 .............................................................................
27
27
M ethods .............................................................................................
28
5.1
3D Pixel Sensor Tests..................................................................28
5.1.1 Current vs. Voltage (I-V)......................................................28
5.1.2 Threshold and Noise...........................................................29
5.1.3 Noise vs. Voltage...............................................................30
31
5.1.4 Time over Threshold (ToT)..................................................
5.1.5 Charge Collection with Strontium-90 (Sr-90) Beta Source...............31
5.2 Test B eam ............................................................................... 33
6
Results ............................................................................................
. .. 35
6.1 3D Pixel Sensors Un-irradiated vs. Irradiated.........................................35
7
Su m m ary ................................................................................................ 42
8
C onclusion.........................................................................................
43
Acknowledgements......................................................................................
44
R eferences................................................................................................
45
3
List of Figures
Page
Figure 1. Schematic of LHC at CERN......................................................................9
Figure 2. Schematic of ATLAS Detector.................................................................10
Figure 3. Cross-section of ATLAS inner detector ..........................................................
11
Figure 4. Propagation of various particles in ATLAS detector........................................
12
Figure 5. Spacing of radial structure of inner detector..................................................13
Figure 6. Schematic of current ATLAS pixel detector..................................................14
Figure 7. Silicon material at 0 K and 300 K...............................................................17
Figure 8. A pn-junction and resulting depletion zone..................................................
20
Figure 9. Schematic of n-in-p planar pixel sensor........................................................23
Figure 10. Layout of 3D double-sided wafer sensor.....................................................23
Figure 11. Schematic of IBL insertion into ATLAS....................................................25
Figure 12. Photographs of an FE-14 USBpix system..................................................
27
Figure 13. Hits vs. Charge scan graphed with the ideal and real curves...............................30
Figure 14. Set-up of post-irradiation scintillator experiment with Sr-90 source....................32
Figure 15. DUTs and telescopes test beam set-up at CERN...........................................33
Figure 16. DOBOX sections which house the DUTs and dry ice....................................
34
Figure 17. I-V scan results for 4 irradiated devices from Table 2....................................
36
Figure 18. Noise vs. Threshold graph for device 34.......................................................37
Figure 19. Noise vs. Veff, # noisy pixels vs. Ve, and pixel map for Veff ................................
Figure 20. ToT MPV vs. Veff plot for 3 p-irradiated devices at different doses.....................39
Figure 21. Sr-90 charge collection of device 36 for 3 different Veff.....................................40
Figure 22. Sr-90 and occupancy scan results for device 34 and 36....................................41
4
38
List of Tables
Table 1. Classification of materials based on their energy band gap.....................................16
Table 2: List of 3D devices studied.......................................................................35
5
1. Introduction
Currently, there is no unifying theory of physics combining the four known forces of
physics acting on fundamental particles: strong force, weak force, electromagnetic force, and
gravity.[ 1,2] The Standard Model of Particle Physics presents the leading theory unifying all
fundamental forces except gravity, but even the Standard Model leaves questions such as the origin
of mass and why the weak force is still 1032 times stronger than gravity unexplained. The recently
confirmed mass of the neutrino does not fit into the Standard Model, which levels out the playing
field with the other two leading unifying theories of physics: supersymmetry (SUSY) and string
theory.[ 1,2] In order to investigate these questions, conditions comparable to the origin of the
universe need to be recreated. To study these conditions, particles need to be collided together at
very high energies.
As a result of the need to achieve high energy particles colliding, the LHC at CERN in
Geneva, Switzerland was built. The LHC is the largest and highest energy accelerator in the world.
It is designed to accelerate bunches of protons to 7 TeV. The accelerator accelerates bunches of
protons in opposite directions and are steered to collide at the four major detectors at CERN,
amounting to a combined energy of 14 TeV.[1] Currently, the combined energy of the proton
bunches is 8 TeV until the next upgrade is done during the planned shut-down of the LHC is done
in 2013-2014.
One of the two general purpose detectors at CERN is the ATLAS detector. The ATLAS
detector provides vertexing information about the particles that result from the high energy protonproton collisions as well as information about the particle momentum. There are many components
to the ATLAS detector, each designed specifically to detect different types of particles. Of
particular importance to this thesis is the pixel detector of the inner detector. The pixel detector is
the first line of detection after the protons collide and produce other particles. Accordingly, it
undergoes massive amounts of radiation as it is just centimeters away from the collision point.
Following the harsh radiation environment of the pixel detector at the LHC, the material is
damaged and degraded, leading to loss of performance. The ATLAS Collaboration has taken the
opportunity of the 2013 planned shut-down to insert a fourth layer in between the pixel detector
and beam pipe known as the Insertable B-Layer (IBL). [3]
The IBL will be even closer to the interaction point than the previous pixel detector. As
such, stringent requirements have been placed on the technologies being considered for the IBL:
6
3D silicon pixel detectors and silicon planar pixel detectors. Both of these technologies are solid
state detectors which provide high precision tracking, and silicon sensors are well suited for the
IBL due to their radiation damage resistance and high position resolution. The current pixel
detector is already experiencing performance degradation after just a few years of use. The
upgraded proton collision energy and hadron fluence of the LHC will require a new pixel detector
technology to be used in the IBL.[4] This thesis focuses on the 3D silicon pixel detector technology
in consideration for the IBL and how its performance is affected by the radiation levels it will
experience in the ATLAS detector.
An overview of the LHC at CERN and its mission is given in section 2. The ATLAS
detector, of particular interest to this thesis, is described in detail along with its purpose and future.
The layout of the inner detector in ATLAS, the pixel vertex detector, is given in order to
understand the purpose and necessity of the IBL upgrade described in section 4.
Section 3 gives an overview of semiconductor detectors and their general properties. A
technical description of the origin of pixel noise is done. With this information, a better insight may
be gained into the two competing technologies in consideration for the IBL upgrade described in
section 4 and which will provide better performance when subject to the harsh radiation
environments of the LHC.
Section 4 presents the need for an upgrade to the inner detector of ATLAS and describes
the 3D sensor technology studied in this thesis. The 3D pixel sensor chips are bump-bonded onto a
front-end chip, the FE-14, which is also an upgraded version of the previous FE-13 front-end chip.
The properties of the new FE-14 chip are also explored.
The various methods for the different tests involving the 3D sensors are described in
section 5. The test are ran on un-irradiated 3D sensors in order to characterize them before they are
sent to the test beam sight at CERN which will simulate many years' worth of radiation damage as
experienced in the LHC. The test beam setup is also described in section 5.
The results of the various tests described in section 5 are given in section 6. The tests that
were described in section 5 were done on the 3D sensors pre- and post-irradiation. The results and
discussion are presented in section 6. The conclusion of the results and this thesis are given in
section 7.
7
2. The LHC and ATLAS Detector
The European Organization for Nuclear Research (CERN) was founded in 1954 along the
Franco-Swiss border near Geneva, Switzerland. CERN was one of Europe's first multi-national
collaborations and now contains twenty member states. The mission of CERN from the outset in
1954 was simple, "...provide for collaboration among European States in nuclear research of a
pure scientific and fundamental character...." CERN also has no concern with work for militaries
and therefore publicly publishes the results of experimental and theoretical work done at CERN. [1]
The Large Hadron Collider (LHC) is the world's largest and most powerful accelerator and
the latest addition to CERN's accelerator complex. The LHC was built from 1998 to 2008 in the
hollowed out tunnel that previously housed the Large Electron-Positron collider (LEP) that
operated from 1989 to 2000. The LCH was the multi-national collaboration of over 10,000
scientists and engineering from over 100 countries along with hundreds of universities and
laboratories. The result is the current LHC ring, which has a circumference of 26.7 km and lies
between 50 and 175 m underground.[1] A schematic of the LHC can be seen in Figure 1.[5] The
goal of the LHC is to test predictions of various theories in fields of particle and high-energy
physics. In particular, discovery of the existence of the Higgs boson, in accordance with the
Standard Model, has been the root of many experiments and still remains one of the biggest
missing pieces of any complete physics theory. In order to achieve this, the synchrotron is designed
to collide beams of either protons or lead nuclei at energies up to 7 teraelectronvolts (TeV) per
nucleon or 574 TeV per nucleus, respectively. [3] The beams of particles are accelerated and steered
using superconducting magnets in opposite directions to collide at 4 points along the loop. Each of
these 4 collision points is the place of a detector, each designed to gain insight into specific areas of
physics.
The ATLAS (A Toroidal LHC AparatuS) detector and CMS (Compact Muon Solenoid)
detector are two complimentary detectors that serve as the LHC's two main general purpose
detectors. Having ATLAS and CMS are essential for cross-confirmation of any new discoveries in
high-energy physics. Both are used in the search and study for new physics such as the origin of the
electro-weak symmetry breaking, Supersymmetry, quark compositeness, leptoquarks, W' and Z',
and extra dimensions. They will also look for insight into the production processes of the Standard
Model such as W, Z, and jet production.[6] The aim is to precisely measure the W and top masses,
which are essential for consistency checks with Higgs studies.
8
LHC
SPS
Pb
Figure 1: Schematic overview of the LHC and its four main detectors
The LHCb (Large Hadron Collider beauty) is another one of the detectors. It specializes in
B-physics experiments, measuring the parameters of oscillation period and lifetime differences
between mass eigenstates of the B, mesons. It also studies the CP violation in the interactions of bhadrons, heavy particles containing a bottom quark, and rare decays, which could lead to new
physics.[1] The fourth main detector is ALICE (A Large Ion Collider Experiment), which is a
heavy-ion program that studies the collision of Pb-Pb (lead) nuclei at energies of 2.76 TeV per
nucleon. Post-collision temperatures and densities are expected to be enough to generate a quarkgluon plasma, a state where quarks and gluons are deconfined.[5]
For the purpose of this paper, the ATLAS detector will be explored in detail, particularly
pertaining to its inner detector, where the 3D pixel sensors explored in section 3 will be mounted.
The ATLAS detector has a diameter of 25 m and a width of 46 m. It weighs approximately 7000
tons and is located in an underground cavern approximately 100 m under the surface. There are
stringent requirements of the ATLAS detector. A schematic overview of the ATLAS detector is
seen in Figure 2.[5,6] The Insertable B-Layer (IBL) explained in section 4 will be inserted between
the current pixel detector, the innermost part of the ATLAS detector, and the beam pipe. The LHC
is designed to collide protons up to 14 TeV, which allows for the search of new particles up to 5
TeV at a design luminosity of
10
-
cm 2 s'. At full intensity, each proton beam will consist of 2808
bunches, and each bunch will hold about 1.15 * 10" protons.[6] If the bunches of protons are
considered at energies of 7 TeV each, the LHC stores beams of energy of about 360 MJ each.
9
Muon Detectors
h.J/
Tile Calorimeter
Liquid Argon Calorimeter
_
Toroid Magnets
Solenoid Magnet
SCT Tracker
Pixel Detector TRT Tracker
Figure 2: Schematic overview of the ATLAS detector and its various parts
After a proton-proton collision at the center of the ATLAS detector occurs, a large, and
therefore short-lived, particle of interest could result and decay into new particles that travel
through the detector. The particles that result from the decay carry with them information about the
original collision event and parent particle which produced them. In order for the ATLAS detector
to precisely measure the particle momentum and energy, the particle must be stopped within the
detector, but there are multiple types of particles that could result from a collision event.
Accordingly, there are four main sections of the ATLAS detector, each designed to detect and
obtain information from the various types of particles that could result from a collision
event.[1,3,6]
The outer most detector is the muon detector, which identifies and measures the momenta
of muons. It employs a muon detection system consisting of precision drift tubes, which measure
the curves of tracks, resistive plate chambers, for triggering and 2" coordinate measurement in the
central region, cathode strip chambers, which measure precision coordinates at ends of the detector,
and a thin gap chamber, for triggering and 2d coordinate measurement at the ends of the detector,
all inside a toroidal magnetic field of up to 4 T. [5]
The next two layers of the ATLAS detector, moving inward, are the calorimeter detectors.
There is the hadron calorimeter, a liquid argon (Ar) calorimeter, which measures the total energy of
hadrons and operates at -183 degrees C. The second of the two layers is an electromagnetic
10
calorimeter, a device that measures the total energy of "electromagnetic showers" produced by
electrons, positrons, and photons.[5]
The inner most detector is known as the pixel detector, designed to measure the momentum
of each charged particle. It is made from highly segmented silicon strips and contains three main
parts: the Transition Radiation Tracker (TRT), the Semi-Conductor Tracker (SCT), and the pixel
detector. For the purpose of this thesis, the inner detector will be explored in detail in section 2.1.
The pixel detector houses pixel sensors explored further in section 3. A figure of the inner detector
is seen in Figure 3.[5,6]
Barrel S
Forward SCT
TR T
Figure 3: A cross-section of the ATLAS inner detector
Each particle resulting from a collision event contains its own signature that it can be
identified by in the ATLAS detector. Neutral particles such as photons and neutrons are not
detectable in the tracking chamber at the innermost part of the detector. Instead, photons are
detected by the electromagnetic calorimeter, and evidence of neutrons is presented by the energy
they deposit in the hadron calorimeter. Charged particles such as electrons and protons are detected
in both the tracking chamber and the electromagnetic calorimeter. Muons can be detected in all
components of ATLAS while neutrinos are not detected by any components due to their rare
interaction with matter, but missing energies that match up with neutrino energies gives evidence
of them.[3] The propagation of the various types of particles resulting from a collision event and
their tracks through the ATLAS detector is seen in Figure 4.
11
Tracking
Electromagnetic
Had ron
Muon
photons
muons
n
...Outermost Layer
Innermost Layer...
Figure 4: The propagation of various particles due to collision events in the ATLAS detector
2.1 ATLAS Inner Detector
The ATLAS inner detector was designed to give precise tracking of particles resulting
from colliding proton bunches every 25 ns (nanoseconds). A solenoid is used to measure the
momentum of charged particles, which makes the inner detector operate in a homogenous magnetic
field of 2 T. The inner detector is 7 m long with an outer radius of 1.15 m. The outer tracking
systems of the TRT, the microstrip detector of the SCT, and the pixel detector all contribute to the
reconstruction of the particle paths through the ATLAS detector. Two types of tracking systems are
combined for this task: a high-resolution tracking system at the closest point to the interaction and
a continuous tracking system located at the outer radius of 115 cm.[5] As seen in Figure 3, the
high-precision end-cap detectors are mounted in concentric cylinders around the beam axis. Figure
5 shows a more detailed layout structure of the inner detector and its spacing.
12
R =1082 mm
TRT
TRT
R =514 mmn
R =443 mm
SCT
R =371 mm
R =299 mI
{R = 122.5mm
Pixels R = 88.5 mm
iR = 50.5 mm
Pixels
R = 0mm
Figure 5: Detailed spacing of radial structure as a part of the inner detector to ATLAS
2.1.1 Transition Radiation Tracker (TRT) and Semi-Conducting Tracker (SCT)
Beginning at the outer radius of the inner detector, the TRT takes up about half the
volumetric area of the inner detector with 12 M3 . It is made up of 4mm diameter straw tubes with
30 pm gold-plated tungsten wire. There are 50,000 straw tubes in the barrel portion, each 144 cm
long, and there are 250,000 straw tubes in the end caps, each 39 cm long.[5] The purpose of the
TRT is to provide additional information about the particle that could not be seen in the pixel
detector of SCT such as whether the particle was an electron or pion. This is done by filling the
straw tubes up with Xenon gas that ionizes as charged particles pass through them and gives
information about them. [6]
The SCT constitutes the middle section of the inner detector. It contains 4 concentric
barrels with radii of 30.0 cm, 37.3 cm, 44.7 cm, and 52.0 cm, make up an area of 60 m2 over the 4
cylindrical barrel layers and
18
planar end caps. The layers are formed by p-in-n silicon microstrip
detectors unlike the smaller silicon pixel sensors used in the pixel detector, making the coverage of
a large area more practical.[5] The SCT is the one of the most important parts of the inner detector
due to its basic tracking in the plane perpendicular to the beam since it measures particles over a
much larger area than the pixel detector and contains more sampled points. [3,6]
13
2.1.2 Pixel Detector
The pixel detector is the innermost part of the inner detector that detects charged particles.
It encompasses 1.7 m2 and contains 1744 modules in total, each with area 10 cm 2, and 46,080
readout channels per module. Each module contains 16 pixel chips, each of area 50 x 400
resolution 14 x 115
sm2 and
sm2 .[5] These chip specs are required to provide high granularity since high
precision measurements of the momentum of charged particles close to the interaction point is
essential to understanding the physics of the collision event.[3] It is subdivided into three barrel
layers of radius 50.5 mm, 88.5 mm, 122.5 mm and three disks on either side of the forward
direction at lengths 49.5 cm, 58 cm, and 65 cm from the center of the detector. The total length of
the detector is 1.4 m and contains 1456 modules on the barrels and 244 modules on the discs.
These pixel specs can be seen in Figure 6.[5]
1442mmfl
430MMv~
\Barrel Layer 2
arre Layer I
//
1
Barrel Layer 0 (b-layer)
End-cap disk layers
Figure 6: Depiction of the current pixel detector in ATLAS
The pixel detector faces the highest amount of radiation of any component of the ATLAS
detector since it is the closest part of the detector to the interaction point. Because of this, there are
stringent requirements that the detector be radiation hard so that it can continue operating with little
efficiency loss under years of significant exposure. The pixel detector needs to retain the speed of
its read-out chips, so that all particle events can be recorded. Accordingly, silicon was chosen as
the material for the pixel detector due to its radiation hardness and fine granularity, which gives
excellent impact parameter resolution. The pixel detector specifics and its planned upgrade for
2013 are further explored in sections 3 and 4.
14
3. Planar and 3D Pixel Sensors
Two different pixel sensors are being investigated for inclusion into the 2013 ATLAS IBL
upgrade described in section 4. The two types of pixel sensors being tested are the planar and 3D
pixel sensors. Due to the extreme radiation environments of the pixel detector in the ATLAS inner
detector, stringent requirements have been placed on the pixel sensors that they be radiation hard.
They are required to sustain radiation environments up to 250 Mrad without seriously
compromising their efficiency or active area.[7,8] Before understanding how radiation levels
present at the LHC will damage the silicon pixel sensors and how that damage will affect their
performance, it is first important to understand how silicon semi-conducting detectors operate.
3.1 Semi-Conductor Detectors
In order to reconstruct the particles of the collision event in high energy physics and
identify the collision vertexes, track resolution is vital. The ATLAS tracking detector has a
resolution of about 10 pm.[3,7,8] This precision is achieved by a detector close to the interaction
point by having a high single hit detection resolution. There are some detectors that can meet these
characteristics, but it must also be radiation hard, which is achieved by semiconductor detectors.
3.1.1 Semi-Conductor Properties
The properties of semiconductor material are determined their energy band structure. The
energy band structures determine the energy states that electrons are allowed to occupy and largely
determine the electrical properties of the material, which is essential in electronics and detectors.
The valence band is the highest band in which electrons are present at absolute zero temperature (0
K). The conduction band is band of energies above the valence band, which are sufficient to free an
electron from binding with its individual atom and allow it to move freely within the lattice of the
material.[9] Energy band gaps constitute the regions within the electronic band structure of the
material in which there are no energy states available for electrons to occupy. Depending on the
band structure of the material and the magnitude of its band gap, it can fall into three different
categories: metals, semiconductors, insulators.[6,9]
Metals are materials in which the valence band overlaps with the conduction bands, and
electrons are allowed to move freely throughout the material. This is useful for sending electrical
current through a material with little resistance. Insulators are materials in which the energy band
gap is about 6 eV or above. This could be useful if no electrical conduction is desired. Of particular
15
interest to semiconductors are the semiconducting materials, classified as having a band gap of
about I eV to about 6 eV.[6,9] This information can be seen in Table 1.
Table 1: Classification of materials according to their band gaps
Material
Band Gap
Metal
Semiconductor
Insulator
0 eV (valence and conduction bands overlap)
~ 1eV - - 6 eV
> 6 eV
Semiconductor detectors provide excellent position resolution, high detection efficiency,
while operating in high radiation environments. The tracking detectors in the ATLAS experiment
are placed very close to the interaction point, around 5 cm currently. [5] The small distance from
the interaction point implies a high radiation dose received by the semiconducting material. It is
then important to consider many factors when determining which material to use for
semiconducting detectors. One of the most important factors for use in the pixel detector in the
ATLAS experiment is radiation hardness. Accordingly, many materials were explored to determine
the best possible material to use in the high radiation environments of the LHC. Among them were
pure materials and doped semiconductors.
Doped semiconductors are semiconducting materials that were doped with other elements
to improve the material properties such as material stability and providing energy levels where
there previously were none. Some common semiconducting materials used in semiconducting
detectors are: germanium, gallium arsenide, and silicon carbide.[9] Silicon is the material most
used in semiconductor detectors; it is the most understood and does not have to be cooled to
temperatures below 100 K such as germanium.
At 0 K, the lowest energy state of a semiconductor, all the electrons in the valence band are
covalently bonded between the lattice atoms. Silicon has four electrons in the valence band at
absolute zero temperature, but even at room temperatures, there is enough thermal energy to excite
an electron from the valence band to the conduction band. When this occurs, a hole will be created
in place of the excited electron that is now free in the lattice. A neighboring electron may jump
from its energy state to fill the hole, again creating a hole where that electron came from. Through
this process the hole effectively moves through the lattice. Since the hole is an absence of an
electron in a field of electron, it can be seen as a positively charged "particle." Therefore, the
semiconductor current comes from electrons and holes.[6] Figure 7 shows silicon at 0 K and room
temperature.
16
0 K(No electrons
300 K
in conduction band.)
........... 1.09 eV
Fermi.k"
Figure 7: Silicon material at 0 K (left) and 300 K (right) and its effective electron/hole pairs
It can be seen in Figure 7 that while at room temperatures, silicon can experience
electron/hole pairs being created. At the same time, there are a number of them that eventually
recombine, establishing an equilibrium between the two. The concentration of electrons (or holes),
ni, is then given by
ni =
NcNy exp(-
) = 2 u'
T exp
(3.1)
where Nc and Nv are the number of states of the conduction and valence bands, respectively, E. is
the energy gap at 0 K,
kB
is the Boltzmann constant, and T is the temperature. The electrons are
found to vary as T3 /2 by Fermi-Dirac statistics.[6]
If equation 3.1 is used in conjunction with the values given in Table 3.1, it can be seen that
the exponential factor is on the order of - 10
for insulators while only ~ 10-4 for semiconductors.
This can be surprising since the difference in the energy gap is only a few eV. As such, equation
3.1 has large implications for the effect of the energy gap in the electrical properties of a material.
Typically, the energy gaps of semiconductors are lower than the energy needed to create an
electron-hole pair.[6]
3.1.2 Doped Semi-Conductors
Pure semiconducting material such as silicon used in a detector cannot detect ionizing
particles since the free charge carriers present at room temperature (25 C) is almost four orders of
magnitude higher than the amount produced by a minimum ionizing particle (mip). To see this, a 1
cm x 1 cm x 300 pm slap of pure silicon is considered. An electron-hole pair created by a
minimum ionizing particle is dE
dx
.p MeVCM2.[9] According to equation 3.1, the concentration
f
of free electrons, ni, is about 108 . The concentration of electron-hole pairs produced by a mip, nh
17
mip,
is about 3.2 *
104 .
In order for a mip to produce any readable signal from the pure silicon slab,
the free carriers have to be removed from the material.
Pure semiconducting materials contain the same number of free electrons in the conduction
band and holes in the valence band. The amount of each can be changed by adding impurities that
integrate into the lattice which would effectively "steal" a free electron or hole from the conduction
band. This forms a doped semiconductor which can contain extra electrons (n-doped) or extra holes
(p-doped). It should be noted though that although the doped semiconductor may contain an extra
electron or hole, it is still electrically neutral since the seemingly negative or positive charges in the
lattice are compensated by the impurity. If a p-doped and an n-doped piece of silicon are joined
together, a pn-junction is formed, and a depleted zone with no free charges is formed by diffusion.
This can be seen in Figure 8. When this occurs, free electrons are joined with the free holes of the
doped silicon lattices, and an electric field is created by the excess charges in both lattices. The
charge density can be found from the concentration of donors, n-doped material, and acceptors, pdoped material
eND, 0 <x <Xn
X < 0
(
(32
P (X) = I-eNA, -- x <
where
ND
and
NA
are the concentrations of donors and acceptors, and xn and x, are the extensions
of the depleted zone into the n- and p-doped silicon materials.[6] If the material as a whole is
neutral, then
NAxp = NDxn
(3.3)
must also be true in accordance with charge conservation. According to Poisson's equation, the
voltage, V, is given by
2
d V
dX
2
p(x)
e
(3.4)
where E is the dielectric constant. If equation 3.4 is integrated once, the result is
E = -
dx
eND
-x
+ ce,
18
(3.5)
xP
<
X
< 0
where CN and CP are constants that are determined by the points in x. and -x, where E =0, which
gives rise to the full electric field equation
E--
dV
(X -x)
-
dx
,
O < x < x"
eNA
(x + x,),
9
x,
<x < 0
(3.6)
Equation 3.6 can be integrated one more time to obtain the electrical potential,
V =
-eND
x2
e C
2
) '+
A
_Xx
,
0<x<X
(3.7)
-x,
exx +C'
< x < 0
and the constants C and C' can be found by recognizing the potential is continuous across x
=
0.[6]
By setting V(x)= 0 and V(-x,) = Vo,
o= -(N
(3.8)
x+NAX)
is obtained. If equation 3.8 and equation 3.3 are used in conjunction, it is possible to obtain xn and
xp,
1
x-
'
e=
1
and x
=
eNA 1+N
\
ND)
eND 1+YD
(NA)!
(3.9)
When one of the concentrations of either NA or ND is much higher than the other (such as
NA >> ND), the depletion depth into the material of lower concentration takes the form of a Debye
length and is given by
d(xn+ x,
~e
(3.10)
By inspection of equation 3.10, it can be seen that it is possible to increase the depth of the
depletion zone by increasing Vo, bias voltage. Therefore, by increasing the bias voltage of a doped
semiconductor, the free electrons or holes will be removed and allow for the detection of electrons
generated by a charged particle.
19
depletion zone
000
600
000
S0
000
000
000
0oo
n
P
bias
metal contacts
Figure 8: Joining of n-doped and p-doped semiconductors to form a pn-junction and a resulting
depletion zone
Since charge electrons and holes are constantly being created, a volume current forms. The
volume current is proportional the concentration of electrons and holds which leads to
I=1
(3.11)
Ad
2r
where A is the area of the junction, ni is the concentration of electrons or holes from equation 3.1, d
is the thickness from equation 3.10, and Tis the electron/hole life-time.[6] If equation 3.10 and
equation 3.11 are combined, the volume current in a doped semiconductor sensor is then
vol =
(3.12)
e
Therefore, the volume current, also known as leakage current, increases proportionally with Vo" 2
if
Vo < Vbias. The pn-junction as discussed and seen in Figure 8 acts as a plane capacitor since the
depleted zone is an acting insulator and the un-depleted zones are conductors. The capacitance of a
planar capacitor is
A
A
C = e - oc e -r
d
g
(3.13)
where A is the area of the capacitor plates and d is the distance between them. [6] In the case of a
doped semiconductor, A is the area of the pn-junction and d is the depletion depth. Equation 3.13
states that the capacitance of a semiconductor detector decreases as the bias voltage increases.
20
3.1.3 Noise
The doped semiconducting material used in the pixel detector in ATLAS is obtained from
small signals of the charged particles that traverse it. Due to the very small signals, they must be
amplified, but this provides a problem, noise. There is a certain amount of noise that arises from
three different sources: thermal noise, shot noise, and I/f noise. If the signal to noise ratio is small,
then the noise will be amplified along with the signal, and large uncertainties in the data will result.
Thermal noise is a function of temperature, but the other two sources of noise arise from the
electronics. Various filters must be implemented in the electronics in order to reduce the amount of
noise and achieve the highest possible signal to noise ratio.[10,11,12]
The thermal noise arises from the temperature of the lattice. The only reasonable way to
reduce the thermal noise is to operate at a lower temperature, which is a common strategy when
using doped semiconductors. [61 Shot noise arises from the number fluctuation of quanta and is
temperature and frequency independent. It is connected to the way photons spatially arrive at the
pixel detector. Shot noise is the standard deviation (RMS) for the number of interactions per pixel
when charge quanta are emitted over a barrier, such as energy gap of the silicon.[ 11] A way to
reduce shot noise is to operating the detector in the dark such that external light does not ionize the
semiconducting detector and cause noise. The third type of noise is 1/f noise, where f is the
frequency of the electrical signal. The reason for this noise is neither well known nor understood,
but it believed to be caused by the preamplifier system. 1/f noise has also been thought to be caused
by interface traps at the silicon/silicon diode interface as a results of very high quality deposited
oxides.[10]
These three sources of noise combine together to form the equivalent noise charge (ENC),
which is given by
ENC = Qnoise = d1(Cd + C,)Q + d 2 (Cd + Cf)
2
+ d 3 IleakRC
(3.14)
where d1 , d2, and d 3 are constants, R and C are the resistor and capacitor values of the preamplification circuitry, and Cd and Cf are the capacitance of the semiconductor and a controlled
capacitor value in the pre-amplification circuitry, respectively. The first term of equation 3.14 is the
thermal noise; the second term comes from the 1/f noise, and the third term arises from the shot
noise. This is known as the ENC, which is the charge necessary to produce the same signal at the
end of the amplification system. Equation 3.14 shows the noise of the semiconductor detector is
21
directly dependent on the leakage current of the semiconducting sensor and the bias voltage, which
ultimately changes the value of Cd. Therefore, the ENC will raise as the leakage current rises as
well as if the bias voltage is lowered.[6]
3.2 Two Pixel Sensor Technologies
There are two main pixel sensor technologies: planar and 3D. The pixel sensors are at the
inner most part of the ATLAS detector, subject to the harshest radiation environments from the
LHC of any other ATLAS detector component. As such, the pixel sensors must be radiation hard,
suffer little loss to efficiency and material properties as it is continually irradiated over its years of
operation in the ATLAS detector. Other desirable sensor properties include little inactive area,
usually most prominent at the edges, minimum silicon chip thickness, below 250 pm, so multiple
scatterings do not result and cause uncertainties in the data, and continuing to offer excellent
position resolution after years of operation.[3,4] At the current radiation levels experienced at the
LHC, even the state-of-the-art silicon sensors fail to operate properly due to radiation damage to
the crystalline structure which alters the silicon sensor properties. The planar and 3D technologies
are being further developed and tested to try and negate these adverse effects of radiation damage
3.2.1 Planar
The planar sensors are a proven technology and what are currently being used as the
ATLAS pixel detector sensors. They are an n*-on-n pixel sensor diffused on an oxygenated floatzone silicon bulk. The opposite sides of the electrodes are in contact with a p* layer. This requires
that the current planar pixel sensors be fabricated on both sides of the substrate. The new planar
pixel sensors that are being developed are n-on-p pixel sensors. There are two pixel sensors
mounted on each modules of the pixel detector. The new substrate has a thickness of 200 pm,
which is a sizeable reduction from the current 256.pm thickness in the pixel detector, and only
requires single-sided wafer processing, which could lead to higher yields and reduction in
fabrication costs. [13] The planar sensors require a bias voltage between 700-1000 V to achieve a
high hit efficiency.[3] A schematic of the n-in-p planar pixel sensor is shown in Figure 9. Since the
purpose of this thesis is to explore the characteristics of 3D pixel sensors, this is the extent to which
the planar sensors will be discussed.
22
50 um
0
p
UBM
30 um 6um
guard ring
pixels
0
G
Figure 9: Cross-sectional schematic of the n-in-p planar pixel sensor
3.2.2 3D
The 3D pixel sensors are produced on 230 pm thick silicon wafer with a double sided
process. The 3D pixels wafers take advantage of new silicon technology advances that produce
column-like electrodes which penetrate the substrate instead of being implanted on the surface like
the planar technology. The wafer also exhibits n-in-p technology as columns of roughly 10 pm
diameter are etched into opposite sides of the wafer alternating n- and p-type in a process known as
deep reactive ion etching (DRIE).[4] A layout of the 3D double-sided pixel sensors can be seen in
Figure 10, where the columns are the pixel sensors etched into the silicon substrate.
BumP-
Si-P
4
7
7
Poly-id
P-stop
N-diffusi
285pm
Slicon
P-diffusion
250pm
holes
Figure 10: Layout of 3D double-sided wafer sensor, where the columns are the silicon pixels
23
The 3D sensor is intrinsically radiation hard because it decouples the electrode distance
from the bulk thickness and the small collection distance reduces the trapping probability,
governed by the separation of the columns. Due to this fact, 3D sensors can reduce the charge
collection path without having to suffer a reduction in sensor material the charged particles
traverse. This makes it possible for the 3D sensors to continue to fully deplete even after hadron
fluencies of 5 * 10"5 neq/cm 2 planned for the LHC upgrade in 2013. The 3D sensors only require
about 200 V to obtain the same high hit detection efficiencies that planar sensors achieve at 7001000 V.[3,4,6] This is important as it alleviates the stringent requirements of the cooling system for
the pixel detector. Due to the advanced design of the 3D pixel sensors, there is a low dependence
on the external magnetic field and little charge sharing between the pixels of the sensor. The pixel
sensors are mounted on the modules using only 1 chip compared to the 2 chip modules of the
planar pixel detector.
24
4. ATLAS Upgrade
During the scheduled 18 month shut-down of the LHC in 2013-2014, the ATLAS detector
will undergo some upgrades. During this time, the LHC luminosity will be upgraded as well to
more than double the expected increase from the onset. A total of 29 proposals for upgrades to
components in the ATLAS detector were reviewed, and 14 have been fully approved considering
the expected increase of luminosity to 103" fb-1 . This correlates to an expected hadron fluence
increase to 5 * 10" ne/cm 2 or 250 Mrad to the pixel detector, and the current pixel detector only
provides acceptable charge collection efficiency up to 2 * 10 ne/cm 2 .[4,14] Since the inner
detector of ATLAS is not scheduled to be fully replaced until 2020, one of the approved projects
for the 2013-2014 upgrades to ATLAS is the Insertable B-Layer (IBL) addition to the inner
detector.
4.1 Insertable B-Layer (IBL)
In order to deal with the current and expected degradation of the current pixel detector, a
fourth layer to the inner detector, the IBL, is being developed. It will be inserted in between the
current pixel detector and the beam pipe. The space is too small to house the IBL, so the beam pipe
will have a reduced radius in the ATLAS detector to appropriate room for the IBL. A schematic of
where the IBL will be located in ATLAS is seen in Figure 11.[7,8] The purpose is to achieve the
smallest possible radius for IBL in order to have better b-tagging performance, but due to the
decrease radius, it will sustain a higher radiation dose due to increased interacting particle density.
Figure 11: A schematic of where the IBL will be inserted in the inner detector of ATLAS
There are stringent requirements for the IBL; it must remain in the detector and function
without serious degradation until the 2020 upgrade when the entire inner detector will be replaced.
A high track resolution is essential of the IBL in order to distinguish the tracks of the various
25
charged particles in the high track density environment. As such, radiation hard sensors are
required to have a hit efficiency above 97% after irradiation to continue to collect charge in the
pixel sensors. The sensors need to have very little inactive edges as well to achieve the highest
possible active area ratio possible; this is vital since there is no long enough room to tilt the pixel
sensors to an optimum angle, but even so, the IBL will increase the parameter impact resolution
from the current pixel detectors. Along with these requirements comes a consciousness of the
material budget. It is desired to have the smallest amount of material on two different fronts. The
first reason is an obvious implied reduction in cost due to using less material. The second reason is
to avoid multiple scatterings off of the detector material which causes uncertainties in the
determination of the track and to avoid interfering with the other detector layers.[3,6]
4.2 3D Sensor Technology
The 3D sensors studied are produced from the Centro Nacional de Microelectronica Instituto de Microelectronica de Barcelona (CNM-IMB) at the Universitat Autonoma de Barcelona
(UAB) in Barcelona, Spain. At current, the plan for the IBL is to house 75% planar sensors and
25% 3D sensors. The main reason for this was that planar sensors are a well understood technology
that is currently being used in the pixel detector of ATLAS, and the 3D sensors have never before
been used on a large international experiment such as the ATLAS in the LHC. The specs of the 3D
sensors were explained in section 3, but it would do well to reiterate the advantages of 3D sensors
over the current pixel detector.
The current pixel detector sensors are degrading due to the radiation environment.
Accordingly, the bias voltage required for full depletion has increased along with the leakage
current. This requires an extensive cooling system. 3D sensors, even with equal radiation damage,
will give off less heat due to a lower required bias voltage for full depletion of the 3D sensor. With
the planned increase in luminosity, the LHC detectors could take 1000 fb 1 data which would
significantly extend the LHC's physics reach.[ 14] The 3D sensors can cope with this increased data
intake due to their faster charge collection over the current pixel detector. Because of the geometry,
there is a smaller charge trapping probability, and there is no charge shift from Lorentz angle.
Since the 3D sensors are more radiation hard than the current pixel detector, its leakage current will
also be less, again, requiring less cooling.
26
4.3 Front-End Chip
The signal from the pixel sensors is read out by a front-end chip attached to the sensor.
When a charged particle traverses the pixel sensor's depleted zone, it ionizes the sensor which
carries information about the particle with it. The front-end chip amplifies the collected charge and
sends a digital output signal from the pixel sensor to be evaluated. The current front-end chip used
in the current pixel detector is the FE-13, but an upgraded FE-14 is required by the IBL. The FE-13
does not have a high enough hit rate to keep up with the planned increase in luminosity, and its
active footprint is too small. The high luminosity would cause the FE-13 column drain architecture
to saturate.[4]
4.3.1 FE-14
The upgraded FE-14 front-end chip employs 130 nm technology and features an array of 80
x 336 pixels with a pixel size of 50 x 250 sm2 . Its larger chip size of 20.2 x 19.0 mm 2 results in a
larger active fraction than before, increasing it from 74% to 89%. Since the IBL radius is too small
to tilt the modules as done in the current pixel detector, it is vital to increase the active fraction
area.[4] The larger chip also reduces periphery and, therefore, the cost of manufacturing it. The FE14 front-end chip also has a primary output rate that is four times faster than that of the FE-13. A
picture of the FE-I4 front-end chip alongside a planar pixel sensor chip is shown in Figure 12. The
FE-14 was developed to improve on the FE-I3's resolution, performance, and radiation hardness.
The radiation tolerance of the FE-I4 exceeds the specified 300 Mrad. The FE-14 is bump-bonded to
the 3D pixel sensor chip using a tin-silver (Sn-Ag) solder. The process involves heating the
substrate and the chip to melt the bumps in order to establish an electric connection. [4]
Figure 12: Photographs of an FE-14 USBpix system: (left picture) A multi-IO board on the left side
and an FE-I4 adapter card on the right, (right picture) a FE-14 single chip card
27
5. Methods
The 3D pixel sensors will be mounted to an FE-14 chip as explained in section 4. Before
the pixel sensors can be used in the IBL, they must be tested and tuned; each pixel can differ
slightly and must be configured similarly with all the other pixels on the chip. This is achieved
through various tests first characterizing the sensors then tuning them based on their behavior.
5.1 3D Pixel Sensor Tests
Before any test is scan is done or any test administered to the chip, digital and analog scans
are done. These scans consist of injecting a certain charge several times through the front-end
pixels and receive a reading back. This makes sure all the pixels are operating and can receive
charge as well as are capable of sending signal. If equal numbers of events for the digital and
analog tests are recorded, the chip is operating properly, and further scans and tests described in
this section may be done.
5.1.1 Current vs. Voltage (I-V)
The current vs. voltage scan done is classified as a bare sensor property. The scan is done
directly to the sensor, and the how the leakage current depends on the bias voltage is a property of
the sensors. At a certain voltage, known as the breakdown voltage, the charge carriers resulting
from the bias voltage, or charged particles when in use in the IBL, have enough energy to ionize
the semiconductor which produces more electron-hole pairs, which in turn leads to more
ionizations. This kind of avalanching cascade saturates the pixel and detector. The goal is to
operate below the voltage breakdown to avoid damaging the pixel and electronics while
maximizing the bias voltage and minimizing the leakage current. Both of the goals work to lower
the noise as shown in equation 3.14.
The leakage current has three major contributing components: volume current generation,
surface generated current, and avalanche breakdown current. The volume current generation is
produced by the bias voltage as it generates charge flow. From equation 3.12, it can be seen that the
volume current increases as the square root of the bias voltage, JV7ias The surface generated
current arises due to large radiation doses that are to be experienced when in use in the IBL.
Specifically, the large radiation doses sustained by the IBL induce SiO 2 -Si interface states that
produce a surface current. The last contribution to the leakage current is the avalanche breakdown
current which occurs when the charge carriers produced by charged particles from the LHC then
28
produce electron-hole pairs that have enough energy to create further electron-hole pairs. The
situation is compounded by this process and the pixel sensors saturate.
When characterizing the pixel sensors in the lab, an upper current limit of 200 pA
(microamperes) is set, and the leakage current is measured against the bias voltage as it increases
from 0 V up to the breakdown voltage. This scan serves two purposes. One is so that the
breakdown voltage of the sensor is known, and it can then be operated below this level. The second
purpose is to check for any damages that might have incurred during the various stages of
assembly. If a pixel is damaged, it will be immediately apparent through this scan.
5.1.2 Threshold and Noise
Threshold and noise scans are done to measure pixel threshold for triggering and
associated noise. These scans are classified as sensor behaviors as opposed to properties. The
threshold scan injects specific charges in increasing amounts in each individual pixel to see when
the pixel sensor triggers. Each individual pixel is slightly different and needs to be configured such
that all pixels will trigger at a given threshold. Ideally, once configured, no pixel will trigger for
hits below the threshold which would produce a step function in a hits vs. charge graph as seen in
Figure 13. These ideal characteristic are not observed in reality. Since noise is present, the ideal
step function is smoothed by an s-curve. If a hit produced a charge slightly below the threshold, the
pixel sensor still might trigger due to the addition of charges from noise.[3,6]
29
0
100
2000
3060
4000
600
6000
7000
Charge (e')
Figure 13: Result of a Hits vs. Charge scan graphed with the ideal (dotted) and real (red) data
There becomes a problem of where to set the threshold at. If the threshold is set too high,
many signals produced by charged particles would not trigger. Conversely, if the threshold value is
set too low, false hits will be recorded due to the fluctuating noise levels. The sources of noise are
explained in section 3.1.3 and section 5.1.1. A normal value for the threshold in 3D pixel sensors is
3200 electrons. This corresponds to a charge below what a mip will produce but higher than the
expected noise in normal operating conditions. Ideally, if there were no noise, then the threshold
could be set to 0 electrons for triggering and any signal would only results from charged particle
detection. It is essential to achieve uniformity throughout the entire device by setting the threshold
level in each individual pixel.
5.1.3 Noise vs. Voltage
The lowest possible noise is desired in pixel sensors in order to decrease uncertainty and
allow for more sensitive charged particle detection. Section 3.1.3 showed that both thermal and
shot noise are dependent on the bias voltage, but the 1/f noise is not. [10,11,12] Semiconductor
detectors require a depleted zone in order to remove the charged carriers generated thermally. As
shown in section 3, the depletion region grows from the pn-junction as seen in Figure 8. As the bias
30
voltage increases, the depletion zone also increases as seen in equation 3.10, decreasing the thermal
noise. In order to measure how the noise varies with the bias voltage, a threshold scan must be
done to each individual pixel at varying bias voltages from 0 V up to the characteristic sensor
breakdown voltage.
5.1.4 Time over Threshold (ToT)
The ToT indirectly measures the charge deposited by a particle traversing the pixel sensor.
This is also classified as a behavior of the sensor. Specifically, the ToT is the distribution of the
deposited charge by the charged particles. The result is the number of read-out events as a function
of the ToT's characteristic of the clusters, which follows a landau distribution. The time the
preamplifier's output remains over the threshold is proportional to the pulse height. This time
depends on the charge of the particle, the threshold of the discriminator, and the feedback current
of the preamplifier. Each pixel sensor must be tuned such that they all have the same ToT for a
given charge. After this tuning is done, a ToT verification scan that injects a defined reference
charge and returns the corresponding ToT information of the pixel chip is run to verify that the
pixel chip has uniform ToT for each pixel.
5.1.5 Charge Collection via Strontium-90 (Sr-90) Beta Source
Once all the tests and scans are done and the device is tuned properly, a charge collection
via a collimated Sr-90 beta source used to simulate mip tracks is used to further analyze the pixel
sensors. Since the rate and energy of the beta particles are known and all pixels under the
collimated beam have an equal probability of being hit, any single pixel with no charge collection
or abnormally high charge collection can be easily spotted. The main objective is to test the
functionality of the pixel sensors. Before irradiation of the pixel sensors, a scintillator set-up for
coincidence checking is not necessary. Instead this task is performed by triggering of the hitbus
signal. Post-irradiation, a scintillator is necessary because the pixel chip becomes activated through
massive doses of proton radiation. The deposited charge is seen in the level 1 (LVLl) trigger
distribution.
The LVL1 trigger distribution reveals the number of readout events as a function of the
time after the scintillator, or hitbus, trigger initializes the readout. The synchronization begins
detecting signal at around 5 LVL1 values of approximately 125 ns (5 * 25 ns) when the trigger
from the scintillator arrives. The noise is apparent in the LVL I distribution as well. For the FE-14
front-end chips, the result of the LVL1 trigger is a tall column of deposited charge. If there is
31
background noise, it is shown in LVL 1 distribution by a lower flat level of continuous charge
collection representing the noise.
The scintillator set-up used to test the irradiated devices can be seen in Figure 14. The
hardware components used for the external trigger are: scintillator, NIM crate, HV source
(C.A.E.N. Mod.N 470), discriminator (Le Croy Mod.N 821), level adaptor (Le Croy Mod.N
688AL), oscilloscope, and Sr-90 beta source. The FE-I4 device is inserted into the climate chamber
with the collimated source set on top of the pixel sensor and the scintillator symmetrically below it.
The scintillator is connected to the HV source at a voltage of -900 V. The scintillator is then
connected to the input of the discriminator so the analog signal can be digitized and shown on the
oscilloscope which is connected to the output of the discriminator and terminated. Another output
of the discriminator is connected to the input level of the adaptor in order to change the signal type
so that it can be sent to the USBpix controller. With the low activity Sr-90 source used at CNM, a
15-20 Hz hit rate is expected.
Crmte Chamber
industrietechnikpie
vM 400
NIM Crmte
Figure 14: Set-up of post-irradiation scintillator experiment with Sr-90 source
32
5.2 Test Beam
In order to properly study the radiation damage effects of the 3D pixel sensors, the devices
must be characterized before and after irradiation at levels that the IBL will have to sustain. The 3D
pixel sensors bump-bonded to the FE-I4 front-end chip that are fabricated and characterized at the
UAB are sent to the test beam at CERN to simulate a full lifetime of radiation in the IBL. Once the
devices are irradiated, they are sent back to the UAB to be characterized by the same tests on the
same equipment, to determine how the 3D pixel performance was affected.
At CERN, the test beam accelerates protons to 400 GeV and hits targets that convert the
protons into mostly pions, the type of charged particles that interact with the IBL pixel detector.
The beam is split in order to supply several beam lines. Splitting the beam lines also allows for
modification of the beam by magnets and collimators as desired depending on the experimental
requirements. At the test beam sight, a 3D pixel device under testing is referred to as a device
under test (DUT). The DUTs are mounted at various angles to simulate time in the IBL. A
schematic of the DUT set-up with respect to the test beam along with possible angle shifts is shown
in Figure 15.[3,4]
0
Telescopes
3
4
5
Telescopes
1
2
Beam
Figure 15: DUTs located in between information telescopes on either side of them and beam
propagation direction
In order to operate the DUTs in the absence of light, which causes unnecessary noise, and
at low temperatures, a Dortmund Box (DOBOX) was built to encase the DUTs. The DOBOX is
split into two major sections. The first section is where the DUTs are mounted. In this first section,
the carrier card (SCC) is attached to an L-mount. In order to avoid short circuiting the electronics
due to condensation on the DUTs, the DOBOX is flushed with nitrogen. The second section is
33
filled with dry ice, and copper tape connects the dry ice with the 3D pixel chips on the DUTs such
that the two are always in thermal contact. This acts to reduce the temperature while the nitrogen
avoids getting condensation on the circuits. Each dry ice filling only lasts about 8 hours in the test
beam before the temperature begins to rise and it must be replaced. The major advantage of the
DOBOX as shown in Figure 16, is that the dry ice can be refilled without disturbing the mounted
samples.[4]
Figure 16: Schematic of the DOBOX sections that house the DUTs and the dry ice
34
6. Results
The results of the scans and tests of the 3D pixel sensors before and after radiation in the
test beam at CERN are presented in this section. Some devices studied were neutron irradiated and
others proton irradiated. The results were different, so the focus will be on the proton irradiated 3D
pixel devices for 2 *
1015
neg/cm 2 and 5 * 10"5 neq/cm 2 since this is the type of irradiation the pixel
sensors will undergo as a part of the IBL.
6.1 Un-Irradiated vs. Irradiated 3D Pixel Sensors
The four devices that are studied pre- and post-irradiation can be found in Table 2. The
nomenclature used to name the devices consists of information about its origins. The first three
characters denotes where the devices were bump-bonded to the FE-14 front-end chip. The second
set of letters denotes the production site. The third set of characters denotes the type sensor, 3D,
and the last set of characters assigns the devices a series number for identification. "BON" means
that the devices were bump-bonded in a lab at Bonn University in Germany, and "GEN" means
they were bump-bonded at the lab in Genoa, Italy. The "CNM" means the devices were produced
at CNM, and the next type of sensor and series number are just a product of the time the 3D sensors
were fabricated.
Table 2. The five 3D pixel devices that are studied
Name
BONCNM_3D_34
BON CNM_3D_36
BONCNM_3D_97
BON CNM_3D_100
GENCNM_3D_55
Type of irradiation
Proton
Proton
Proton
Proton
-
Level of irradiation
5 * 10" neq/cm 2
6 * 10" neq/cm 2
5 * 10"'neg/cm 2
2 * 10'5 neg/cm 2
Un-Irradiated
The results of the pre- and post-irradiation I-V scans of the devices are shown in Figure 17.
The I-V scans were done at 20 C before irradiation. After the samples were sent to the test beam at
CERN to be proton irradiated and returned, the I-V scans were re-done for the devices in a climate
chamber at -20 C. The devices were cooled to recreate the operating temperatures of the ATLAS
detector. The lower temperature also reduces the thermal noise. The results show a significant
increase in the leakage current after irradiation, but the curves still follow the characteristic rise
with the bias voltage until the breakdown voltage is reached.
35
-+-CNM36, p-irr 6E15neq/cm2 (-20 C) --- CNM 36,before irradiation (20C)
-+-CNM 34, p-irr 5E15neq/crn2 (-20 C) -- CNM34, before irradiation (20C)
450
300
400
250
350
200
S300
250
150
200U
100
50
50
0
150
100
50
200
0
250
50
200
150
100
Bias Voltage (V)
Bias Voltage (V)
(b)
(a)
-+-CNM 100,p-irr 2E15neq/cm2 (-20 C) -- CNM 100, before irradiation (20C)
-+-CNM 97, p-irr 5E15neq/crn2 (-20 C) -+-CNM97, before irradiation (20C)
400
120
350
100
300
r
250
80
C200
60
150
40
100
20
0
50
100
150
50
200
150
100
200
250
Bias Voltage (V)
Bias Voltage (V)
(d)
(c)
Figure 17: I-V scan results for the 4 devices from Table 2
It can be seen from Figure 17 that the I-V curves before irradiation follow a pattern. At low
bias voltages, the leakage current raises proportional to j
. As the bias voltage continues to
raise, the curve somewhat plateaus. Then, when the bias voltage reaches the breakdown voltage,
there is a linear increase in leakage current with bias voltage. After irradiation, the devices can be
seen to have roughly the same characteristics but with a much higher initial increase in leakage
current with bias voltage.
Threshold and noise scans were done to determine their relationship with the bias voltage.
These were done by injecting a known charge in the pixel sensors. The expected result is known,
and it can be compared against what the sensor actually read out. As long as the charge was above
the threshold, any additional charge seen above what was injected was the result of noise. The
threshold is the charge point above which the pixel sensor will trigger, resulting in a signal; below
the threshold level, the trigger is not fired, and therefore, no signal is transmitted or recorded. A
36
noise vs. threshold plot is seen in Figure 18. The noise decreases with threshold. This occurs since
at lower thresholds, the noise of the pixel, that is dependent on temperature and bias voltage, causes
the pixel to trigger falsely. At higher thresholds, this is avoided since only high charges generated
by ionized particles trigger the pixel.
a Noise BONCNM_3D_34
SE15 neq/cm2
180
175
___
170
165
.i
U
z 160
-
----
155
150
145
0
500
1000
1500
2000
2500
3000
Threshold (e)
Figure 18: Noise vs. Threshold plot for device 34
The results of the noise scans can be seen in Figure 19. The noise scans were done for three
proton irradiated devices at -20 C with increasing bias voltage. The noise rose with bias voltage as
expected according to equation 3.14 and 3.12. A dramatic increase in the number of noisy pixels on
the chip began after the bias voltage began to exceed 160 Veff. This was also expected given their
breakdown voltage as seen in Figure 17. Figure 19 also shows the propagation of noise pixels on
the chip. The y axes for Figure 19c are the pixel position on the left side, going from 0 to 350, and
the normalized level of noise where 100, yellow, represents a saturated signal as a results of an
avalanche cascade. The x axis is again the position of the pixel, going from 0 to 80. As seen in
Figure 19c, the noise begins at the lower corners or the chip and spreads to the edges of the chip.
37
Noise Vs. HV for P-irr Devices
2. 280
DON
a
o
260
B
C_3D_34
0C
O20
BONCNM13D_97
BON CW_3D36
200.
180
160
10
120
140
130
150
170
160
180
190
200
210
Effective Bias Voltage(V)
(a)
solN_CNM_30_34
_CUM
- .. -...
.
SL34
Nc"0143036
9000so.
9 5 0 00
.
-
A.......
200
...
...
.6
l ,0
130
140
1W0
150
170
(b)
*
200
210 220
190
10
EIfTec tive Bias VoltagetV)
IN
Up
Ws
.380Vh@ap
170 Veff
iBOCAM
1a
m~
CA
(c)
Figure 19: Noise vs. Veff (a), # of noisy pixels vs. Veff (b), and pixel map for 3 Vff (c) where left y
axis and x axis represent the position, and the right y axis represents the level of noise
38
The next sensor behavior considered is in regards to the ToT. For the purpose of the
conditions in the IiBL, the ToT for the devices was tuned to 8 ToT for 20 k electrons. The reason
for this is that in a silicon chip of 250 pm thickness, a mip is expected to create a signal of 20,000
electrons. The distribution gives an idea about the time synchronization between the telescope and
the DUT planes during the test beam. After irradiation, the amount of collected charge decreases as
shown in Figure 20, which decreases the signal of the pixel. For this reason, the device should be
operated at the highest possible Veff below the breakdown voltage. The most probable value (MVP)
of the ToT is plotted vs. the Vff for 3 devices, two of which are proton irradiated (p-irradiated) at
different levels and the other is un-irradiated.
0
CNM_97 p-irr 5E15
a CNM_55 un-irradiated
CNM_100 p-irr 2E15
8
7
M
6
I0-
3
A
-- --- -- - -
----
2
1
0
50
100
150
200
Effective Bias Voltage (V)
Figure 20: ToT MPV vs. Veff for 3 devices: 2 p-irradiated at different levels and 1 un-irradiated
A charge collection study on the devices using a Sr-90 beta source is the last characterizing
test of the devices studied in this thesis. Section 5.1.5 explains how this test is done before and after
irradiation. Sr-90 charge collection prior to irradiation is used to check for dead pixels and mask
out noisy pixels, the results of which are not particularly interesting but necessary to characterize
the device. The charge collection tests after irradiation give insight to the important characteristics
of the device, and these tests are the ones shown in Figure 21 and Figure 22. Figure 21 shows the
charge collection of device 36 for three different voltages. In Figure 21 a, it can be seen that the
bottom edge is beginning to become noise at the voltage of 125
Veff.
In Figure 2 1b, the bottom
pixels are masked to avoid noisy pixels in the pixel map at 150 Veff, but in Figure 21 c, the edges
show to be very noisy at 200 Veff. This this expected from Figure 16b, which shows the breakdown
39
voltage of device 36 begins around 150 Veff. The left y axis and x axis represent the pixel position
on the chip, and the right y axis represents the number of beta particle hits in a certain pixel.
2250
50
-25
1501
ismo
sm
100-
SO
,0
010
20 30
100
~~i
40
(a)
50
o
. 000
sO0
6070
Colunn
0
100p
10
20
30
40
50
60
.0
'-
0
020
30
40506070
Column
Colunn
(C)
(b)
Figure 21: Sr-90 charge collection of device 36 for 125 Veff (a), 150 Veff (b), and 200 Veff (c) where
left y axis and x axis represent the position, and the right y axis represents the number of beta hits
Figure 22 shows the results of Sr-90 scans and occupancy scans as a function of effective
bias voltage for devices 34 and 36, both p-irradiated at 5 *
10"5 ne,/cm
2
. An optimum voltage of
around 160 V was used. Figure 22a is the result for device 34, and Figure 22b is the results for
device 36. On both graphs, the blue curve represents the effective bias voltage vs. percent noisy
pixels. Noisy pixels are those that exhibit much higher charge collection than was expected in the
initial signal of known charge. The red dots on the graph represent the effective bias voltage vs. the
time over threshold, which shows that the charge collection increases as Veff is increased.
40
* CNM_34 p-irr 5E15 Sr90
-*-CNM_34 p-irr SE15 Noise
6
20
-20C
18
0
,0
M
16
14
S4
0
12
3
10
8
e2
E
6
0
0
1
0
110
130
150
170
2
-0
210
190
Effective Bias Voltage (V)
(a)
* CNM_36 p-irr 6E15 Sr90
.
-.- CNM_36 p-irr 6E15 Noise
6
20
-20C
18
5
16
14
0
4
12
.C
S
e
10
3
2
8
0
6
z
0
4
1
2
0
60
80
100
120
140
160
180
+---~
200
0
Effective Bias Voltage (V)
(b)
Figure 22: Sr-90 and occupancy scan results for device 34 (a) and device 36 (b) where the blue
curve represents Veff vs. % Noisy Pixels, and the red dots represent Veff vs. ToT
41
7. Summary
In summary, the CNM 3D doubled sided pixel devices performed up to the standards that
the ATLAS Collaboration set for the IBL upgrade which will be the fourth layer of the inner
detector of ATLAS. The I-V scans showed that even after proton irradiation, a significant decrease
in the optimum operating bias voltage compared to the current n-in-n planar pixels devices was
seen. The optimum operating bias voltage is below the breakdown voltage and such that it creates a
sufficiently large enough depletion zone that it can collect all charge created by the charged
particles that traverse it. This depletion zone becomes smaller as the crystal structure of the silicon
is compromised under harsh radiation environments, but since the 3D pixel sensor technology
reduces the distance between the pixel sensors as shown in Figure 10, it may be operated at a lower
optimum bias voltage than the planar devices. The lower operating bias voltage compared to planar
devices also dissipates less heat since the leakage current is reduced with the bias voltage; this
eases the cooling requirement as well.
The 3D pixel sensors are intrinsically more radiation hard than the current pixel detector
technology. The 3D silicon pixel structure is less damaged and, therefore, has a low leakage
current. With a reduced leakage current, comes reduced noise, and the threshold can be set lower
and raise detection efficiency of charged particles. This raises the overall efficiency of the pixel
detector given by
he
ef
=
number of detected hits
of expected hits
(3.15)
-number
where hff is the hit efficiency.[3,6] The efficiency requirement set by the ATLAS Collaboration is
efficiency greater than 97%. The number of expected hits from the Sr-90 source used to produce
the results of Figure 21 is known through calculating the source strength, and the USBpix program
counted the recorded number of triggered hits from the pixel sensors. Using these numbers, it was
determined that device 36 had a hit efficiency over 99.5%.
Overall the tests and results proved that the 3D double sided pixel devices met and
exceeded the requirements to be included in the IBL project.
42
8. Conclusion
During the 2013-2014 planned shutdown of the LHC, the ATLAS Collaboration will
install the IBL directly mounted on a new reduced radius beam pipe at an average radius of 3.3 cm.
After extensive analysis of the characterization of planar and 3D technologies, both have been
considered for inclusion in the IBL. At present, the plan is to have the IBL composition 75% planar
pixel sensors and 25% 3D pixel sensors. This thesis present the various advantages of 3D pixel
sensors over planar when subject to radiation levels present in the future upgraded LHC at CERN.
Post-test beam studies of CNM 3D pixel sensors show the devices meet the stringent
criteria of the IBL in terms of hit reconstruction efficiency. The efficiency requirement set by the
ATLAS Collaboration is efficiency greater than 97%. This efficiency criterion must hold even after
p-irradiation at a level of 5 * 10' nq/cm 2 , which was done at the CERN test beam site for the CNM
3D pixel sensors studied in this thesis. The CNM 3D pixel sensors have exhibited het greater than
99.5% even after test beam irradiation.
The characterization of CNM 3D n-in-p pixel sensors bump-bonded to FE-14 front-end
chips were done at the UAB. According to the I-V scans, the 3D sensors meet the IBL design
requirements of low leakage currents after p-irradiation of 5 * 1015 n,/cm 2 . Noise levels were kept
sufficiently low, even for thresholds of 1500 electrons. The constraints of the 3D pixel sensors with
regard to the high voltage and temperatures are less restrictive than the planar pixel sensors.
It is worth noting that there was another 3D pixel sensor technology designed at
Fondazione Bruno Kessler (FBK) that was developed simultaneously, but this technology showed a
marginal efficiency drop after irradiation that was not present in the CNM 3D pixel sensors.
Another note is that initially there were three technologies considered for the IBL upgrade: planar,
3D, and diamond sensors. The diamond sensors are no longer being considered for the IBL upgrade
since there was not enough research or conclusions able to be drawn at the time of the decision.
Diamond sensors may still be a viable option to future upgrades to the inner detector beyond the
IBL.
43
Acknowledgements
I would like to take this opportunity to thanks those entities and people who helped me not
only with this thesis but also with my time here at MIT. I would first like to thanks Alicia
Goldstein and the MISTI-Spain program, without which I would have never gotten the opportunity
to work with the IFAE at the UAB in Spain over the summer of 2011. At my time there, I
generated a great interest in my work, and I formed the results into this thesis. Specifically, I must
thank Sebastian Grinstein for taking me in as a student researcher and providing me with the proper
background and experience that led to the work shown here. I worked alongside Shota
"Constantine" Tsiskaridze and Ali Harb in the lab and with data analysis, and they were a great
help as a resource for my work as well.
I would also like to thank the Nuclear Science and Engineering department at MIT for
providing engaging professors and useful lab experience in the fields of electronics and radiation
detectors. I have thoroughly enjoyed my time here, and I felt that I have grown as a student, a
person, and a future mentor. I would also like to thank my thesis advisor, Professor Dennis Whyte,
for taking the time to read my thesis and provide useful feedback.
Finally, I would like to thank my family and friends for their continued support in my time
at MIT. Whether it be for academics, athletics, or life in general, I have never found myself alone
or unable to find a helping hand when I have needed it the most.
44
References
[1]
Halpern, Paul. "Collider: the search for the world's smallest particles." John Wiley & Sons
Inc. Hoboken, NJ. 2010.
[2]
Griffiths, David. "Introduction to Elementary Particles."
2nd
ed. Wiley-VCH. Weinham.
2008.
[3]
Tsiskaridze, Shota. "Beam Test Performance of 3D Pixel Detectors for the IBL Upgrade."
Universitat Autonoma De Barcelona. Institut de Fisica d'Atles Energies. Jan 2012.
[4]
Jentzsch, Jennifer. "Characterization of Planar n+ -in-n ATLAS FE-14 Single Chip
Assemblies in Laboratory and Testbeam Measurements." TU Dortmund - Fakultat Physik.
Sep 2011.
[5]
"ATLAS Fact Sheet." The ATLAS Experiment. CERN. Geneva, Switzerland. 2011.
[6]
Armadans, Roger. "New pixel sensor technologies for the ATLAS upgrade." Universitat
Autonoma de Barcelona. Institut de Fisica d'Atles Energies. Jan 2012.
[7]
Grinstein, Sebastian. "Overview of Silicon Pixel Sensor Development for the ATLAS
Insertable B-Layer (IBL)." Institut de Fisica d'Altes Energies (IFAE). Barcelona, Spain.
Dec 2011.
[8]
Grinstein, Sebastian. "Overview of the ATLAS Insertable B-Layer (IBL) Project." Institut
de Fisica d'Altes Energies (IFAE). Barcelona, Spain. Jan 2012.
[9]
Knoll, Glenn F. "Radiation Detection and Measurement." 4"' ed. John Wiley & Sons Inc.
Ann Arbor, MI. 2010.
[10]
Leijtens, Johan et al. "Active pixel sensors: the sensor of choice for future space
applications." TNO Science and Industry. Delft University of Technology. Netherlands.
2007.
[11]
Janesick, James R. "Photon Transfer." SPIE. Bellingham, WA. 2007.
[12]
Wermes, N. "Pixel Vertex Detectors." Bonn University. Bonn, Germany. 2006.
[13]
Alam, M. S. et al. "The ATLAS silicon pixel sensors." CERN-EP-99-152. CERN. Geneva,
Switzerland. Oct 1999.
[14]
La Rosa, Alessandro. "ATLAS IBL Pixel Upgrade." ATLAS IBL Collaboration. CERN.
Geneve 23, Switzerland. Jul 2010.
45