development of monolithic pixel sensors for alice experiment

139 Pages·2015·7.41 MB·English

IVAN RAVASENGA

DEVELOPMENT OF MONOLITHIC PIXEL SENSORS
FOR ALICE EXPERIMENT

Tesi magistrale in Fisica
Relatrice: prof.sa Stefania Beolè
S econdo Relatore: Dott. Yasser Corrales Morrales
Controrelatore: prof. Marco Costa
Università di Torino
Dipartimento di Fisica
Torino, 14 Ottobre 2015
Alla mia famiglia
e a tutte le persone che mi hanno sostenuto

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Contents
Introduction ....................................................................................................................................................... 7
Chapter 1: Heavy ion collisions physics ............................................................................................................. 9
1.1 The Quantum Chromodynamics ............................................................................................................... 9
1.2 Running coupling constant in QED and QCD ............................................................................................ 9
1.3 Big Bang model ....................................................................................................................................... 11
1.4 Transition phase diagram ....................................................................................................................... 13
1.5 QGP predictions ...................................................................................................................................... 14
1.6 Heavy ion collisions ................................................................................................................................ 14
1.6.1 Collision geometry ........................................................................................................................... 16
1.6.2 Collision details and space-time evolution ...................................................................................... 17
1.6.3 Particle multiplicity .......................................................................................................................... 20
1.6.4 Particle spectra and radial flow ....................................................................................................... 22
1.6.5 Anisotropic transverse flow ............................................................................................................. 24
1.6.6 Elliptic flow at LHC ........................................................................................................................... 27
1.6.7 Jet quenching and ℛ .................................................................................................................... 27
𝐴𝐴
1.6.8 Hints on charmonium suppression .................................................................................................. 32
Chapter 2: ALICE ITS Upgrade .......................................................................................................................... 34
2.1 Current ITS .............................................................................................................................................. 35
2.1.1 Silicon Pixel Detector ....................................................................................................................... 36
2.1.2 Silicon Drift Detector ....................................................................................................................... 37
2.1.3 Silicon Strip Detector ....................................................................................................................... 38
2.2 Physics motivation for the ITS upgrade .................................................................................................. 38
2.3 Current ITS limitations ............................................................................................................................ 39
2.4 ITS upgrade overview ............................................................................................................................. 41
2.4.1 Detector layout overview ................................................................................................................ 42
2.4.2 Experimental conditions and running environment ....................................................................... 44
2.5 Pixel Chip ................................................................................................................................................ 44
2.5.1 Choice of Pixel Chip technology ...................................................................................................... 45
2.5.2 Pixel Chip development ................................................................................................................... 45
2.5.3 Particle detection ............................................................................................................................ 46
2.5.4 General requirements on the Pixel Chip ......................................................................................... 47
2.6 Pixel architectures .................................................................................................................................. 48
2.6.1 MISTRAL: a modular design ............................................................................................................. 49
2.6.2 MISTRAL: readout mode ................................................................................................................. 49
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2.6.3 ALPIDE: general approach ............................................................................................................... 50
2.6.4 Hints on Priority Encoder ................................................................................................................ 51
2.6.5 ALPIDE: readout mode summary and parameters .......................................................................... 53
2.6.6 ALPIDE: beam test measurements .................................................................................................. 55
2.6.7 MISTRAL & ALPIDE: summary and comparison ............................................................................... 56
2.7 Modules and Staves in the OB and IB ..................................................................................................... 56
2.8 Flex Printed Circuit .................................................................................................................................. 57
2.8.1 FPC for Inner Barrel Stave ............................................................................................................... 58
2.8.2 FPC for Outer Barrel Stave............................................................................................................... 59
2.8.3 Pixel Chip to FPC connection ........................................................................................................... 60
2.9 Power Bus ............................................................................................................................................... 62
2.10 Stave configuration summary ............................................................................................................... 62
Chapter 3: Test of the FPC for the Outer Barrel Stave .................................................................................... 65
3.1 Test objectives ........................................................................................................................................ 65
3.2 Tested FPC .............................................................................................................................................. 65
3.3 Experiment steps .................................................................................................................................... 66
3.4 Eye diagram and BER measurements ..................................................................................................... 67
3.4.1 Bit Error Rate ................................................................................................................................... 67
3.4.2 Eye diagram ..................................................................................................................................... 67
3.4.3 Experimental setup ......................................................................................................................... 70
3.4.4 Measurement procedure ................................................................................................................ 71
3.4.5 Eye diagram and BER main results .................................................................................................. 72
3.5 Jitter measurements ............................................................................................................................... 77
3.5.1 A jitter definition and parametrization ........................................................................................... 77
3.5.2 Experimental setup and procedure ................................................................................................. 78
3.5.3 Experimental results ........................................................................................................................ 78
3.6 Conclusions ............................................................................................................................................. 82
Chapter 4: Study of the chip analog and digital parameters ......................................................................... 84
4.1 pALPIDEfs-v1 summary and pixel organization ...................................................................................... 84
4.2 In-pixel structure .................................................................................................................................... 86
4.2.1 Analog front-end section ................................................................................................................. 87
4.2.2 Digital front-end section .................................................................................................................. 87
4.3 Pixel indexing .......................................................................................................................................... 89
4.4 Pixel sector details .................................................................................................................................. 90
4.5 Test boards ............................................................................................................................................. 90
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4.6 Chip software .......................................................................................................................................... 92
4.7 Test objectives ........................................................................................................................................ 93
4.8 First test: Threshold and Noise Occupancy Scan at different voltages .................................................. 94
4.8.1 Test procedure ................................................................................................................................ 94
4.8.2 Experimental results on VDDD variation ......................................................................................... 98
4.8.3 Experimental results on VDDA variation ....................................................................................... 101
4.8.4 Experimental results on VDDD&VDDA variation ........................................................................... 104
4.8.5 General conclusions for all cases ................................................................................................... 107
4.9 Second test: Threshold and Noise Occupancy Scan without the decoupling capacitors on DVDD ..... 107
4.9.1 Test procedure .............................................................................................................................. 107
4.9.2 Experimental results ...................................................................................................................... 108
4.9.3 Second test conclusions ................................................................................................................ 110
4.10 Third test: Threshold and Noise Occupancy Scan without the decoupling capacitors on AVDD ....... 110
4.10.1 Test procedure ............................................................................................................................ 110
4.10.2 Experimental results .................................................................................................................... 111
4.10.3 Third test conclusions .................................................................................................................. 113
4.11 Fourth test: Threshold and Noise Occupancy Scan without the filter on VREF ................................ 113
4.11.1 Test procedure ............................................................................................................................ 113
4.11.2 Experimental results .................................................................................................................... 114
4.11.3 Fourth test conclusions ............................................................................................................... 116
Chapter 5: Study of the chip response as a function of noise injection....................................................... 117
5.1 Noise injection into pALPIDEfs-v1 ........................................................................................................ 117
5.1.1 Test objective ................................................................................................................................ 117
5.1.2 Experimental setup and test procedure ........................................................................................ 117
5.1.3 Experimental results: injection in the power planes (AVDD and DVDD) ...................................... 118
5.1.4 Experimental results: injection in the PWELL (back-bias) ............................................................. 121
5.2 Noise injection into pALPIDEfs-v2 ........................................................................................................ 126
5.2.1 Comparison between ALPIDE-v1 and -v2 ...................................................................................... 126
5.2.2 Experimental results: injection in the power planes (AVDD and DVDD) ...................................... 127
5.2.3 Experimental results: injection in the PWELL (back-bias) ............................................................. 130
Conclusions .................................................................................................................................................... 134
Future plans .................................................................................................................................................. 135
Ringraziamenti ............................................................................................................................................... 136
Bibliography ................................................................................................................................................... 137
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Introduction
ALICE (A Large Ion Collider Experiment) is designed to address the physics of strongly interacting
matter, and in particular the properties of the Quark-Gluon Plasma (QGP), using proton-proton,
proton-nucleus and nucleus-nucleus collisions at the CERN LHC. So the purpose is to study the
nuclear matter at high densities and temperatures.
One of the major goals of the ALICE physics program is the study of rare probes at low transverse
momentum. The reconstruction of the rare probes require a precise determination of the primary
and secondary vertices that is performed by the ALICE ITS (Inner Tracking System). The present
ITS is made of six layers of different types of silicon detectors and it allows, for example, to
reconstruct D mesons with the transverse momentum down to ~ 1 𝐺𝑒𝑉/𝑐.
The nature of the QGP as an almost-perfect liquid emerged from the experimental investigations at
CERN SPS and at BNL RHIC. ALICE has confirmed this basic picture, observing the formation of
hot hadronic matter at unprecedented values of temperatures, densities and volumes. These physics
results have been achieved by ALICE after only two years of Pb-Pb running and one p-Pb run,
demonstrating its excellent capabilities to measure high-energy nuclear collisions at the LHC.
Despite these successes there are several limitations for which the current experimental setup is not
yet fully optimized. ALICE is therefore preparing a major upgrade of its apparatus, panned for
installation during the Long Shutdown 2 (LS2) of LHC in the years 2018-2019.
The upgraded detector will have greatly improved features in terms of impact parameter resolution,
standalone tracking efficiency at low 𝑝 , momentum resolution and readout capabilities. So this
𝑇
upgrade will enhance ALICE physics capabilities, in particular for high precision measurements of
rare probes at low transverse momenta.
In this thesis, I will briefly describe the overall upgrade program and I will focus my attention on
the ITS upgrade.
In the first part I will present some important physics concepts regarding the heavy ion collisions
and then the new structures (circuitry, support structures, …) and detectors that will take place in
the new ITS will be presented. Finally, I will present the measurements regarding the new electrical
circuits of the ITS and the chip prototypes that will replace the silicon detectors of the current ITS.

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Chapter 1: Heavy ion collisions physics
1.1 The Quantum Chromodynamics
The Quantum Chromo-Dynamics (QCD) is the gauge field theory which describes the features of
the interaction between the quarks and gluons found in hadrons in the Standard Model.
This gauge theory is based on the symmetry group SU(3), that is a non-abelian group.
In the lagrangian of this theory, we can find a term of interaction quarks-gluon and gluon-gluon (3
or 4); fig. 1 depicts the correspondent vertexes:

Figure 1: QCD interaction vertexes.
the last two vertexes are typical of a non-abelian theory. In the figure above ℊ is the coupling
𝑠
constant of the QCD. But, it is more common to find 𝛼 = ℊ2/4𝜋.
𝑠 𝑠
1.2 Running coupling constant in QED and QCD
In Quantum Electro-Dynamics (QED) the coupling constant is called 𝛼 = ℊ2/4𝜋. In this case, the
vacuum is a sort of a dielectric: virtual pairs 𝑒+𝑒− (vacuum polarization) screen the charge of an
electron, hence the charge depends on the distance. In fig.2 a test particle sees different charges of
the electron in the middle depending on the distance R (screening).
Figure 2: Vacuum polarization in QED
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So, considering that 𝛼 depends on the charge of the electron (~𝑒2), the previous consideration
brings to what we call “running coupling constant”. In fact in fig. 3 we note that 𝛼 decreases with
the distance.

Figure 3: From vacuum polarization to running coupling constant
Considering the Heisenberg indetermination principle, high energies correspond to low distance and
vice versa. Hence, the coupling constant 𝛼 increases with the energy or 𝑄2 that is the transferred
momentum.
Also in QCD we have the vacuum polarization, but there is an important difference: the gluons are
colored and can interact with each other (photons can’t do this). Then, if we consider for instance a
red charge, it will be rounded by other red charges and a probe entering in this region sees less red
charge approaching to the red charge in the middle. Fig. 4 depicts this phenomenon. This effect is
called anti-screening.

Figure 4: Vacuum polarization in QCD
Considering that 𝛼 depends on the color charge, we have an opposite trend with respect to 𝛼 in
𝑠
QED (fig. 5).
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