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Studies on radiation hardness of new electroniccomponents in preamplifiers and summing

amplifier electronics for the ATLASHEC-upgrade Calorimeter.

Diploma thesis

Sava I. Potrebic

Faculty for physics at theLudwig Maximilians University, Munich

andthe Max Planck Institute for physics

Werner Heisenberg Institute, Munich

supervised by Prof. Dr. C. Kiesling

co-supervised by Prof. Dr. O. Biebel

Munich, 29.10.2008

AbstractRadiation hardness tests of transistors based on different technologies likeGaAs, Si and SiGe, have been performed. The devices have been exposedto a high fluence of neutrons. The performance has been studied for neutronfluence up to 5−7 ·1015 n/cm2. Gain and the linearity of the response were themain issues. Based on these results the technology to be used for the upgradeof the cold electronics of the ATLAS HEC calorimeter will be selected.

Contents

1 Introduction 21.1 The Standard Model . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1.1 Quarks, leptons and forces . . . . . . . . . . . . . . . . . 31.2 LHC physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2.1 Detectors at LHC and the pp collider ring . . . . . . . . 61.2.2 Luminosity . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.3 ATLAS calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . 81.3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.3.2 Energy loss by electromagnetic interaction . . . . . . . . 81.3.3 Energy loss by strong interaction . . . . . . . . . . . . . 91.3.4 Electromagnetic and hadronic shower cascades . . . . . . 101.3.5 Calorimeters . . . . . . . . . . . . . . . . . . . . . . . . . 111.3.6 The ATLAS calorimeter . . . . . . . . . . . . . . . . . . 131.3.7 The Hadronic Endcap Calorimeter (HEC) . . . . . . . . 16

1.4 Cold electronics, physical motivation . . . . . . . . . . . . . . . 18

2 The new cold electronics 202.1 Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.1.1 The electronic chain of the HEC . . . . . . . . . . . . . . 212.1.2 HEC Cold pre-amplification and summation . . . . . . . 222.1.3 Different technologies of electronic components . . . . . . 242.1.4 Radiation effects on electronic components . . . . . . . . 262.1.5 Radiation hardness and device stability . . . . . . . . . . 28

3 The first irradiation test 303.1 The setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.1.1 The NA methods . . . . . . . . . . . . . . . . . . . . . . 323.2 Neutron flux measurement . . . . . . . . . . . . . . . . . . . . . 34

3.2.1 Measured flux, Energies, INF . . . . . . . . . . . . . . . 353.3 Results, signatures for radiation damages . . . . . . . . . . . . . 37

3.3.1 NA-method, characteristics of the signal . . . . . . . . . 373.3.2 Gain over frequency . . . . . . . . . . . . . . . . . . . . 373.3.3 Linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.3.4 S-parameters over flux and the change of the linearity . . 40

ii

CONTENTS iii

3.3.5 Scope-method, HEC technology . . . . . . . . . . . . . . 423.4 The software for data aquisition . . . . . . . . . . . . . . . . . . 433.5 Conclusions after the first test . . . . . . . . . . . . . . . . . . . 43

4 The 2nd irradiation test 444.1 The setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444.2 Neutron flux measurement, energy distributions, errors . . . . . 454.3 Results, signatures for radiation damages . . . . . . . . . . . . . 48

4.3.1 Gain and reflection versus frequency . . . . . . . . . . . 484.3.2 Linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . 494.3.3 S-parameters over flux and changes in the linearity . . . 514.3.4 Current measurements . . . . . . . . . . . . . . . . . . . 53

5 Discussion, in general 555.1 Comparison of the two radiation test results, errors . . . . . . . 555.2 Conclusions & further investigation . . . . . . . . . . . . . . . . 56

6 Appendix 576.1 Conversion table for dB . . . . . . . . . . . . . . . . . . . . . . 576.2 Conversion table for dBm to mV . . . . . . . . . . . . . . . . . 57

iv CONTENTS

.

CONTENTS 1

Acknowlegments

In a great project like this it is a special pleasure to mention all the persons whohave any contribution to this work. From my first days at the institute, the projectleader P. Schacht was taking care about my progress giving me always good advicesin personal discussions or during long group meetings. I am also very thankfull tohim for the reading and the detailed corrections at the end. Many thanks to prof. C.Kiesling for his supervision and especially for taking time to help me improving myprograming knowlegde with consequences that the program was running perfectly atthe end. Prof. O. Biebel was the person who gave me those small but great advicesand showed me the best literature about radiation damage in electronics which hasplayed a crucial role in understanding some important parts of the complicatedmechanism.

The people from the institute technical department deserve special thanks forthe good preparation of our test boards, especially A. Fisher. On the Istitute forNuclear Physics in Rez, near to Prague, where we have made our irradiation tests Ithank P. Bem, M. Gotz and the rest of the crew for all the efforts they made. Notto forget their great hospitality in the institute during our stay and the care of M.Gotz for my security, always suppling me with the best digital dosimeters.

A great contribution to my work has also F. Bursgens from the LMU, depart-ment for Photonics, who gave me good advices for the best structuring of the DAQprogram with so many devices. A good cooperation in the scope-method investiga-tion I established with J. Ferencei from the Institute for Physics in Kosice, duringhis stay in Munich. V. Linhart has given his contribution with the RADMON dosismeasurement. Further, thanks for cooperation to the rest of our crew: H. Ober-lack, A. Hambarzumjan, O. Reiman, A. Rudert. Even not included directly in ourproject, discussions with J. Habring have always brought some new ideas, as well assome discussions during the last months with K. Ayadi. Special thanks for my greatC++ and ROOT programing improvement I owe G. Pospelov who has unselfishtaken time to teach me how to improve my writing of fast and efficient codes for ourdata analysing.

For the great patience and support in life and during this work I thank with allof my heart my father, my brother, my beloving wife and my mother who shortlypast away and unfortunately may not celebrate with us the finish of this work.

Chapter 1

Introduction

1.1 The Standard Model

The Standard Model is by now a well-tested physics theory. Precise exper-iments have repeatedly verified theoretically predicted effects. The questionwhy elementary particles have mass, and why they differ from each other, isthe most perplexing one. The answer within the Standard Model is given bythe Higgs mechanism [1]. The Higgs field has one new particle associated withit, the Higgs boson. If such a particle exists, the LHC should be able to discoverit.

In the standard model there are twelve basic building blocks, called funda-mental particles - fermions (particles with spin 1/2), governed by three fun-damental forces. Our best understanding of how these particles and forcesare related to each other is given in the Standard Model of particles andforces (fig. 1.1.) [3], [10], [12]. It has successfully explained many experimen-tal results and precisely predicted a wide variety of phenomena. Thus, theStandard Model has become established as a well-tested physics theory.

When the universe was young and much hotter than today, we supposethat all four forces behaved as one. Particle physicists hope to find a singletheoretical framework to prove this, and have already had some success. Twoforces, the electromagnetic force and the weak force were unified into a singletheory in the 1970s. This theory was experimentally verified in a Nobel prizewinning experiment at CERN a few years later. The weakest and the strongestforces, however, gravity and the strong force, remain apart. A very popularidea suggested by the unification of the forces including gravity is called su-persymmetry or SUSY for short. SUSY predicts that for each known particlethere is a supersymmetric partner. If SUSY is right, then supersymmetricparticles should be detected at the LHC. Antimatter hides another questionwhere the LHC will help us also to solve the riddle of it. It was once thoughtthat antimatter was a kind of perfect reflection of matter - that if you replacedmatter with antimatter and looked at the result in a mirror, you would not beable to tell the difference. We now know that the reflection is imperfect. The

2

1.1. THE STANDARD MODEL 3

LHC may be a very good kind of this mirror for antimatter, allowing us to putthe Standard Model through one of its most grueling tests yet.

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ELEMENTARY

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the "STANDARD THEORY"of elementary particles

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Figure 1.1: The Standard Model: particles and fields [6]

1.1.1 Quarks, leptons and forces

The basic particles in the standard model are quarks and leptons. Pairs ofleptons and quarks are grouped in three generations. The lightest and moststable particles make up the first generation, whereas the heavier and less stableparticles belong to the second and third generations. All of the stable matterin the universe is made from particles that belong to the first generation.

The six quarks are paired in these three generations – the up quark andthe down quark form the first generation , followed by the charm quark andstrange quark, then the top quark and bottom quark. The six leptonsare similarly arranged in three generations – the electron and the electron-neutrino, the muon and the muon-neutrino, and the tau and the tau-neutrino [10], [16]. The electron, the muon, the tau and the quarks all havean electric charge and mass, whereas the neutrinos are electrically neutral andhave a very small mass.

There are four fundamental forces: the strong force, the weak force, theelectromagnetic force and the gravitational force (fig. 1.2.). Gravity is theweakest but it has an infinite range. The electromagnetic force also has infiniterange but it is many times stronger than gravity. The weak and strong forcesare effective only over a very short range and dominate only at the level ofsubatomic particles. The weak force is stronger than gravity but it is indeedthe weakest of the other three. The strong force is the strongest among all

4 CHAPTER 1. INTRODUCTION

four fundamental interactions. The three fundamental forces result from theexchange of force carrier particles, the bosons between the fermions. Eachfundamental force has its own corresponding boson particle – the strong forceis carried by the gluon, the electromagnetic force is carried by the photon,and the W and Z bosons are responsible for the weak force. It stays to find

gravitationweak

interactionelectromagn.interaction

stronginteraction

radiactivity electricitynuclearforces

effects

magnitude

gravity

range

10-38

propagator "graviton" photon � gluon g

8 10-15

cm 10-13

cm

10-5 10

-21

8

W , W+ -

electroweak interaction,

prediction for the Z0

Grand Unified Theories ( )?prediction for the LEPTOQUARKS

GUT

Theorie Of Everything (TOE)? (Superstrings?)

Figure 1.2: The Standard Model: the fundamerntal forces [6]

the Higgs particle which would be a big step for particle physics, although itsdiscovery would not write the final ending to the story.

Quarks, Leptons

Of the twelve fermions, which are described by the Standard Model, only thequarks can strongly interact as they are the only fermions which carry colour-charge. Quarks exist inside hadrons and they are confined by the strong forcefields. Quarks have spin 1/2 and, by convention, positive parity. Antiquarkshave negative parity. Quarks have the additive baryon number 1/3, antiquarks-1/3 [10]. Quarks exist only confined in hadrons and were not separated ob-served yet.

Leptons are appearing in three known flavors: the electron, the muon,and the tau lepton. Each flavor is represented by a pair of particles called aweak doublet (fig. 1.1.). The first of them is a massive charged particle thatbears the same name as its flavor (like the electron). The second is a nearlymassless neutral particle called a neutrino (such as the electron neutrino). Allsix of these particles have corresponding antiparticles (such as the positron or

1.2. LHC PHYSICS 5

the electron antineutrino). All known charged leptons have a single unit ofnegative or positive electric charge (depending on whether they are particlesor antiparticles) and all of the neutrinos and antineutrinos have zero electriccharge. The charged leptons have two possible spin states, while only onehelicity is observed for the neutrinos (all the neutrinos are left-handed, and allthe antineutrinos are right-handed) [3].

When particles interact the lepton number is conserved. Generally thenumber of leptons of the same type (electrons and electron neutrinos, muonsand muon neutrinos, tau leptons and tau neutrinos) remains the same. Con-servation of the number of leptons of different flavors (for example, electronnumber or muon number) may sometimes be violated (as in neutrino oscilla-tion). The couplings of the leptons to gauge bosons are flavor-independent.This property is called lepton universality and has been tested in measurementse.g. of the tau and muon lifetimes and of Z-boson partial decay widths.

It is possible that this theories and experiments are only a part of a biggerpicture that includes new physics that has so far been hidden deep in thesubatomic world or maybe even in the dark recesses of the Universe. All thenew informations which we will get from the LHC should help us find more ofthese missing pieces.

1.2 LHC physics

A new pp-storage ring, the Large Hadron Collider - LHC has been built atCERN/Geneva. It will provide two counter-rotating proton beams in a muchhigher energy and luminosity range than ever before. We might proof thetheoretical predictions in the Standard Model and maybe even more, open adoor to some new physics [5], [2]. Since the luminosity is about 1034 cm−2s−1

and the centre of mass energy 14 TeV, LHC will provide reasonable event ratesfor processes of interest. Therefore experiments at LHC will be a new challengein discovering new particles and processes, rather than proving the parametersof the Standard Model with higher precision.

There are some main research directions for the physicist at the LHC:

• To find the last missing particle predicted by the Standard Model - theHiggs boson [1]. This boson is playing the crucial role in breaking theelektroweak symmetry in the mass generation [7], [15]. If not existing inthe accesible mass range, it might bring us to some new physics beyondthe SM.

• Allready in the start up phase the LHC will produce enough top-quarksfor more precise measurements of their mass and couplings.

• Other theories are extending the SM like the Supersymmetry [7], GUTs[8] or Extra Dimensions[9]. Possible manifestations could be detectedgiving the sensitivity of LHC experiments to new physics.


• The high rate of central quark-quark collisions at higher energies aregoing to give very good possibilities for testing QCD and allow to searchfor quark sub-structure.

The discovery potential for the Higgs boson is one of the most importantgoals of LHC experiments. The mass range starts from the LEP2 limit ofmH = 114.4 GeV up to the theoretical limit of about 1 Tev. Depending on theHiggs mass there are many decays which offer good signatures for detection.Some of them [14] are in the next list (l may be either a muon or an electron):

• H → γγ for 90 < mH < 150 GeV,

• H → ZZ∗ → 4l± for 130 GeV < mH < 2mZ1

• H → ZZ → 4l±, 2l±2ν for mH > 2mZ ,

• H → WW, ZZ → l±ν,2 jets, 2l±2 jets for mH > 2mZ .

1.2.1 Detectors at LHC and the pp collider ring

The LHC consists of four detectors which are placed at four interaction pointsof LHC:

1. A Large Ion Collider Experiment - ALICE for studying heavy ion physics.

2. LHC-b is a spectrometer constructed specially for b-physics studies (CPviolation).

3. The multipurpose detector CMS (Central Muon Solenoid) - CMS.

4. The multipurpose detector ATLAS (A Toroidal LHC AparatuS).

Some of the requirements for the detector are:

• The trigger and the read-out have to cope with high event rates.

• To detect weakly interacting particles a high resolution in the missingtransverse energy reconstruction is needed. This requires a highly her-metic calorimeter with good acceptance and resolution up to large pseu-dorapidities2 η, where θ is the angle relative to the beam axis.

• For measuring electron and photon (e. g. in decays such H → γγ or H→ Z∗Z → e+e+e−e−) a very good electromagnetic calorimeter is needed.

1ZZ∗ means that the first Z boson exists on the mass shell and the other is a virtualone.

2In experimental particle physics, pseudorapidity, η = −ln tan(θ/2) is a commonly usedspatial coordinate describing the angle of a particle relative to the beam axis. As angleincreases from zero, pseudorapidity decreases from infinity. An angle of zero is usually alongthe beam axis.

1.2. LHC PHYSICS 7

Figure 1.3: The ATLAS [54]

• The possible hadronic decays of a Higgs with a large mass as well asof new supersymmetric particles sets high standards for the jet energyreconstruction in the hadronic calorimeter.

• In the same way the requirement to reconstruct leptons with high preci-sion sets high standards on the tracking detector.

• A good signature for the quark substructure is the change of the slopeof the transverse energy distribution of jets. Therefore, the linearity ofthe calorimeter up to very high energies is a key issue.

• Good muon momentum resolution is essential for detecting decays suchas H → Z∗Z → 4μ or Z’ → μμ over a large background.

1.2.2 Luminosity

Energy and the particle flux are key elements to increase the physics potential[3]. The particle flux is often referred to as the luminosity. It is the number ofparticles per unit area per unit time times the opacity of the target, expressedin units cm−2s−1. The integrated luminosity is the integral of the luminositywith respect to time. The achievable luminosity is an important parameter tocharacterize the performance of an accelerator.


The rate R of occurence for a process is the luminosity L times the crosssection σ of that process:

R = L · σ =dN

dt(1.1)

where N is the number of interactions.

Reasons for high luminosity

During the first few years of running, LHC will be operated at an instantaneousluminosity of 1033cm−2s−1. It is expected to be run about 107 seconds per yearcollecting a total integrated luminosity of 10 fb−1 per year. After this phaseit is foreseen to increase the instantaneous luminosity to 1034cm−2s−1 reachingan integrated luminosity of 100 fb−1 per year. The total integrated luminositysurpasses the highest integrated luminosity ever reached at a pp-collider byalmost two orders of magnitude. Due to this high luminosity there will bean average of 23 inelastic collisions per bunch-bunch crossing. This meansthat if an event is selected by the trigger, in average 23 pile-up events aresuperimposed.

1.3 ATLAS calorimetry

1.3.1 General

The methods to detect particles relays mostly on their interactions with somematter. In all Calorimeters exactly this sensitive materials are used. Of thefour fundamental interactions only the electromagnetic and the strong havesignificant relevance. In modern calorimeters for detecting particles there areused the important characteristic of better resolution with rising particle en-ergy and the reconstruction of missing transverse energy. Exactly the firstcharacteristic leads to great promisses for the future since the energies in thecolliders are rapidly growing. In the following only the energy loss by elec-tromagnetic and the strong interaction will be described thus they have thegreatest significance. A great discussion is reported in the ’Review of particlephysics’ [26], especially chapter 27. revised by [27].

1.3.2 Energy loss by electromagnetic interaction

If charged particles are incidenting matter at high energies, the Bethe-Blochequation is a very good description for such processes. Using E as the particleenergy and x as the distance which a particle achieve in an absorber, we maywrite the Bethe-Bloch formula in their full greatness:

−dE

dx∼= 4πr2

emec2NAz2ρ

Z

A

1

β2

[1

2ln

2mec2β2Tmax

I2(1 − β2)− β2 − δ

2

](1.2)

1.3. ATLAS CALORIMETRY 9

where by the index ’e’ the electron is ment. The symbol r and m stands forthe classical electron radius and mass, z and Z for the charge of the incomingparticle and the charge of the absorbers nucleus, NA and A for Avogadro’snumber and the relative atomic mass of the absorber, ρ and δ for the massdensity and the density effect correction to the ionisation energy loss, β = v/cand Tmax stands for the maximum kinetic energy, which can be transferred toa free electron in a single collision. Figure 1.3 is an example how a chargedparticle is loosing its energy for different kind of electromagnetic interactions.The dominating process of energy loss for electrons and positrons is ionisation.

Muon momentum

1

10

100

Sto

ppin

g po

wer

[M

eV c

m2 /

g]

Lin

dhar

d-S

char

ff

Bethe-Bloch Radiative

Radiativeeffects

reach 1%

μ+ on Cu

Without δ

Radiativelosses

βγ0.001 0.01 0.1 1 10 100 1000 104 105 106

[MeV/c] [GeV/c]

1001010.1 100101 100101

[TeV/c]

Anderson-Ziegler

Nuclearlosses

Minimumionization

Eμc

μ−

Figure 1.4: Energy loss of a muon in Cu [27]. There are shown influences to energyloss by some processes and the criticall energies.

The e+e− anihilation and Bhaba scattering are at low energies more unsignif-icant. Low-energy photons are loosing energy through the photo-effect, theCompton and Rayleigh scattering and the photo nuclear apsorption. As β isrising, the process of bremsstrahlung starts to dominate for charged particleswith lower mass. Since there is an inverse mass dependency, at energies below1 TeV it is not relevant exept for electrons. If we are dealing with high-energyphotons, we may expect their energy loss in the e+e−-pair production. Furtherdiscussion may be read in [8].

1.3.3 Energy loss by strong interaction

In describing all of the processes of the strong interaction the theory of QCDhas shown great succes, but there is no complete modell for describing theprocess of energy loss of hadrons incidenting matter. A good method is to


use the Monte Carlo simulations (MC) which is using the cross section mea-surements to simulate some reactions. The cross section for hadron-nucleuscollisions has a weak energy dependence above the nuclear resonances [8]. Wemay express the probability p(x) for a high energy hadron traveling throughmatter the distance x without interacting with absorbers nuclei, including thenuclear integration length λI for proton-nucleon interactions [26]:

p(x) = exp

(− x

λI

), λI

∼= 35A1/3

ρ

g

cm2(1.3)

where ρ is the density of the absorber.

1.3.4 Electromagnetic and hadronic shower cascades

When passing through matter, high-energy particles initiate a so called showercascade which is a result of different interactions of particles inside the material(the absorber of the calorimeter). The second generation of particles is causingnew interactions which again carry on this process. The cascade stops if theenergy of the last secondary is beyond the limit for particle production. Ithas to be distinguished between the electromagnetic and the hadronic shower.Because of more importance for the hadronic showers, the electromagneticshowers will be mentioned only shortly.

Electromagnetic shower cascades

Starting with high-energy photons which loose their energy in a e+e− pair pro-duction where the electrons radiate bremsstrahlung photons we get a showerwhich holds on so long until one of these cases appears: (a) the electron/positronenergies falls under a critical energy Ec where the energy loss by ionisation andexcitation starts to dominate over bremsstrahlung, or (b) the energy of thephotons falls under the pair production limit and the Compton effect starts todominate. A senseable parametrisation [31] is given which says that 98% ofthe incident particles energy is contained in the lenght of:

L(98%) = 2.5 · X0 ·[ln

(E

Ec

)+ C

](1.4)

where E and Ec are the incoming and critical energy (of the absorber), respec-tively. The factor C contains +0.5 for photons and -0.5 for electrons/positrons.Using the Molliere-Radius RM it is possible to express the transverse extensionof an electromagnetic shower [26]:

RM =mec

2√

4πα

Ec· X0 ≈ 21MeV · X0

Ec(1.5)


where X0 again is the radiation lenght of the absorber. In a calorimeter, 95%of the shower energy is deposited in a cylinder with radius 2RM around the axisof the showers incident particle. The expansion of the shower decreases withmaterials with high Z. It is useful to use such materials in electromagneticcalorimeters.

Hadronic shower cascades

In the same way as before, high-energy hadrons also produce shower cascadeswhich carries on until the energy is lower than the necessary one for newhadron production. Hadron shower does not exist in a pure hadron statebecause they also have an electromagnetic component. In each interactionwe expect to produce, on average, 2/3 charged pions and 1/3 neutral pions.We also expect the mean multiplicity to depend only logarithmically on theparent energy and we can, in lowest order, ignore this slow variation. Theπ0 production is an irreversible part of the shower because they immediatelydecay to photons. Photons then develop as an electromagnetic shower whichhas a characteristic length scale X0 which is typically shorter than λ0, sothe π0 are quickly droped out of the shower. The deeper energy transport isthus carried out by the charged pions. The electromagnetic component comesalso from weak decays of hadrons and the ionisation and excitation of theabsorbers material. The large fluctuations comes mainly from the fraction ofthis electromagnetic component. The mean path after which a hadron hasdeposited 95% of the initial energy is given as:

λ95%∼= [9.4 · ln(E/GeV) + 39] cm · λI

λI(Fe). (1.6)

The interaction length λI is typically larger than the radiation length X0 [33](a factor 6 for LAr and 10 for Cu). It comes out that a hadronic cascade isabout one order of magnitude higher than the electromagnetic one.

1.3.5 Calorimeters

Depending on the type, the read out signal in calorimeters is produced in theactive material either by ionisation, excitation or Cherenkov radiation of theshower process. Calorimeters are very important in high-energy physics dueto the characteristics:

• The energy resolution of an calorimeter improves with increasing energyof the incoming particle.

• Calorimeters allow to measure the energy of a neutral particle as well ascharged particles.

• If fine segmented, calorimeters provide good position measurements.


• If longitudinally segmented, calorimeters offer good particle identifica-tion.

The last characteristic is a very useful one which also may push down theexpences of the experiment. There are two types of calorimeters: homoge-neous and the sampling calorimeters. In the first case the active and thepassive material areidentical and the whole shower is contributing to the signalproduction. In the second case, the sampling calorimeter is built of alternat-ing active and passive materials and the read out of the active part results ina sampled signal of the deposited energy. The energy may be reconstructedfrom longitudinal summing. The longitudinal shower information offers someparticle identifications at reasonable efforts.

Energy reconstruction and missing energy

The number of the secondary particles produced in a cascade is directly propor-tional to the primary particle incident energy. Therefore, all the contributionsmay be added to get the total signal in the medium which is proportionalto the energy as E = α·signal, where the proportionality constant must bedetermined from the calibration. In the case of electromagnetic showers theconstant is energy independent, but for the hadronic showers the case is not soeasy to be solved. Two important effects has to be considered: (a) the energyof a hadron is not completely deposited in the calorimeter, and (b) a fractionof that energy is not converted to an electronic (read out) signal. The first caseis often called energy leakage. The second case is mainly due to particlesstopped in an insensitive absorber layer, energies from breaking nuclear bind-ings and the fraction of neutrons produced in the nuclear reactions which doesnot contribute to the signal. This effects added make the invisible energy .

Figure 1.5: The e/π ratio for the Hadronic Endcap Calorimeter, measured in astand alone beam test [51]

This phenomena leads to large fluctuations from event to event, with the con-


sequence that the energy response of a pure hadronic shower has a much widerdistribution than the energy response of the electromagnetic shower of thesame energy and that the response of these two showers are usually different:

e

hintr> 1 (1.7)

The ratio ehintr is the response of a calorimeter to EM and hadronic components

of a cascade and is energy independent. If the previous unequality holds it isreffered to a non-compenasating calorimeter.

The measured sampling ratio for electrons (e = Evis/Ebeam ≡ 1/αEM) isnot like the one for hadrons (π) which is energy dependent due to the energydependence of the electromagnetic component. For the special case of HECthe e/π is shown in fig. 1.5., [51].

Energy resolution

The number of secondary particles produced in a shower is proportional to thesignal and yields thus an energy resolution σ/E ≈ 1/

√E in first approxima-

tion, [32]. Including three most important terms in the parametrization of theresolution of an sampling calorimeter we get:

σ

E=

a√E

⊕ b ⊕ c

E(1.8)

The first term, named the sampling term, takes in account the intrinsic showerfluctuations. The second is a constant term which includes the leakage energyin the calorimeter, the inhomogenity of the calorimeter and the case e

hintr �= 1.The third term takes in account the noise of the electronics which should bethe same for all energies. Therefore the 1/E dependence.

1.3.6 The ATLAS calorimeter

The ATLAS calorimeter yields precision measurements of energy and direc-tions of electrons, photons, quarks and gluon-jets. The crucial role is in themissing transverse energy reconstruction which is indispensable in detectingweak interacting particles such as neutrinos or even new particles predicted bytheories wich may extend beyond the SM. This calorimeter offers a hermeticcoverage up to |η| = 4.9. Figure 1.5. shows the whole ATLAS calorimeter sys-tem which guarantees a minimum of 10 interaction lengths (λI) in total and 24radiation legths (X0) for the electromagnetic calorimeter with an acceptanceof |η| = 4.9. This system provides very fast signals as the input for the Level-1trigger, [34]. As mentioned, a fast particle identification is possible by recon-struction of shower expansion which are typical for every particle species. Allcalorimeters in ATLAS are of the sampling kind with the differencies in thematerial. The TILE calorimeter serves as hadronic calorimeter in the barrel


Figure 1.6: A schematic view of the ATLAS calorimeter system. The LAr calorime-ter consists of the barrel cryostat with the electromagnetic (EMC) calorimeter andtwo endcap cryostats with the electromagnetic (EMEC), hadronic (HEC) and for-ward (FCAL) calorimeters, [54].

region with |η| ≤ 1.7 [35]. It is using plastic scintillator as active materialand iron as passive material where the containment ranges from 8 to 14 λI .The signal, which is scintillation light is read out by photo multipliers so thatfast response time is achieved. For the calorimeter in the forward region theradiation is too high for a scintillator calorimeter. Therefore the liquid argon(LAr) technology has been chosen here.

Advances of the LAr calorimetry

LAr calorimetery is a mature technology as proven by other large successfulsystems such as H1 and D0. All calorimeters in ATLAS based on LAr technol-ogy are of the sampling type with LAr as the active material and with differentmaterials as absorber. This technology offers:

• good linearity of the signals in a wide energy range,

• low sensitivity to a high radiation dose,

• longterm stability with no strong temperature or aging dependence and

• high flexibility in design concerning the granularity.


Every incoming particle creates electron-ion pairs in the liquid argon. In anelectric field the electrons drift with the velocity vD in direction of a read outelectrode. The delivered current is:

I(t) = NevD

d·(

1 − t

tD

)= I0(Q0, vD) ·

(1 − t

tD

); Q0 = Ne (1.9)

where N is the number of generated electrons or ions, Q0 the generated chargeand tD the drift time for the whole gap of LAr. The induced current decreaseslinearly with time (fig. 1.6.a). A minimal ionising particle loses 2.1 MeV

Figure 1.7: Left: The ionisation signal before (a) and after (b) shaping (shapingtime here is 20 ns), [38]. Right: The shaping time dependence of the pile-up andelectronic noise for two different LHC luminosities.

per cm in pure LAr and in the gap of 2 mm induces a typical current of 48nA. In ALTAS, (for a normal setting of the high voltage) 23 minimum biasevents every 25 ns produce the so called pile-up noise. The total noise comesfrom the background interactions as well as from the electronics. The shapingtime might be used to optimise these contributions. Figure 1.6.b. shows thisrelations for two LHC luminosities. The signal will be finally sampled with40 MHz and the dots are showing the individual time samples used in thedigitisation.

In ATLAS five samples will be used to reconstruct the amplitude of thesignal using the optimal technique [39], [40]. The drift time tD in an ATLASLAr calorimeter gap of 2 mm is ≈ 400 ns. It is too long for collecting the totalcharge at LHC event rates. Instead the fast effective initial current is issuedto reconstruct the signal. A fast signal response time through the whole signalchain has to be maintained. The longitudinal summing of the signal is donevia passive cabling in conventional LAr calorimeters. Here some new methodshave been chosen:


• The absorber and the readout in the EMC has accordion structure sothat the summing is done already in one readout plane.

• The structure of the read out is conventionally realized in parallel elec-trodes, but the summing is done actively with preamplifying and sum-ming stages in direct proximity to the signal formation.

The LAr calorimeter in ATLAS is built in three cryostats with a diameter ofabout 5 m: The barrel cryostat houses the EMC for |η| ≤ 1.4 [36]. On each sideof the detector one cryostat holds the EMEC and the HEC (hadronic endcapcalorimeter) as well as the forward calorimeter. In the central EMC are twocylinders each with 16 modules with lead as absorber, sandwiched between 0.2mm stainless steel foils. The accordion shaped waves are running orthogonalto the beam axis. The readout electrodes are hold in position between theabsorber by a honeycomb spacer-mat. The range of the calorimeter is 24-33X0 depending on the |η| region. To distinguish π0 and γ there is a segmentedpresampler which acts as an active LAr layer (11 mm) and is also correctingthe energy losses from inactive material like cryostats, cables, etc. (≈ 1X0).The electromagnetic endcap calorimeter (EMEC) is constructed similar like theprevious but for the EMEC the accordion waves run parallel to the beam axis.The absorbers are mounted in an radial arrangement wheel. The inner andouter coaxial wheels consist each of eight modules and with increasing radiusthe LAr gap increases exactly like the accordions waves. Inactive materialin the order of 3 − 4X0 in front of the EMEC at 1.4 ≤ |η| ≤ 2.5 require apresampler as the first layer in the endcap cryostat. This is realised as a 5 mmLAr active layer. The forward calorimeter (FCAL) with its electromagnetic(FCAL1, copper as absorber) and hadronic (FCAL2, FCAL3, tungsten asabsorber) section is positioned in the region of 3.2 ≤ |η| ≤ 4.9 in a tube atthe centre of the endcap cryostat. In a matrix of the corresponding absorbermaterial the tubes holding the cylindrical electrodes are inserted and form soa cylindrical shell gap of LAr (FCAL1: 250, FCAL2: 375, FCAL3: 400 μm).The three active modules of the FCAL have a total containment of 212X0 or9.5λ.

1.3.7 The Hadronic Endcap Calorimeter (HEC)

The Hadronic Endcap Calorimeter is a conventional parallel-plate copper-LArsampling calorimeter [36]. The electrodes are made of carbon-loaded kaptonand are positioned in an 8.5 mm gap between the copper plates. The electrodesform a multi-gap electrode structure that provides redundancy in the case ofhigh-voltage (HV) faults. Every endcap cryostat contains a HEC A and a HECB wheel constructed from 32 azimuthal modules (Φ-wedges), fig.1.7. The HECwheels share the endcap cryostat with the EMEC and FCAL calorimeters.Copper plates of 25 mm (50 mm) thickness for the front HEC A (rear HEC B)wheel form the absorber structure [37]. In total there are 24 gaps for the HEC


Figure 1.8: The HEC wheel with 32 HEC modules (Φ-wedges) and the PSB’s oneach of them, [54].

A and 16 for the HEC B, constructed together with the read-out structurebased on the principle of an electrostatic transformer (EST) [41]. The ESTtechnique reduces the HV required per gap and offers redundancy in case ofHV problems. The EST in each gap is made of a central read out electrode(PAD) and two high voltage boards (EST board) (fig. 1.10.). The EST boardconsists of an isolating kapton layer which is sandwiched between two conduct-ing high resistive layers (HRL). The PAD is made of a segmented copper readout electrode which is defining the granularity covered on both sides with anisolating layer and a HRL. To keep the same distance between the differentboards of the EST over the whole area, a honeycomb spacer-mat is insertedin the four sub-gaps. The HEC uses the concept of active pads: the signalsfrom individual pads are fed into separate preamplifiers (based on high inte-grated GaAs electronics) and summed actively. The electronic pre-amplifyingand summing boards (PSB) are positioned directly at the periphery of themodule while the chips are operated in the LAr. Thus the input capacitancesare minimized to ensure a short-signal rise time. The use of cryogenic GaAs


Figure 1.9: Artist’s view on a HEC module. On the back side it is clear to see thePSBs and the zoomed part shows the read-out PADs at the position of a tie rod, [54].

preamplifiers and summing amplifiers provides the optimum signal-to-noise ra-tio and allows to detect single muons in the HEC. The requirements will bedescribed more in the section ’cold electronics’. The HEC covers the rapidityrange 1.5 ≤ |η| ≤ 3.2.

1.4 Cold electronics, physical motivation

Using electronic devices for scientifical purposes [19] in detectors, we mustprepare them for exposing to various types of radiation, including photons,electrons, protons, neutrons, and heavy ions. All the main effects of radiationon the semiconductor devices within the systems are ranged from basic gradualdegradation to total failure. In order to design and produce reliable systems itis necessary to understand the device-level effects of radiation and develop thesystem appropriate to suppress the vulnerability. The HEC electronics has tocope with high radiation. Being not accessable, radiation hardness and highreliability are the key requirements. In addition, low power consumption to

1.4. COLD ELECTRONICS, PHYSICAL MOTIVATION 19

Figure 1.10: The HEC LAr gap between the copper plates with the EST and threedifferent HV and read out borads. They are separated by a honeycomb spacer, [54].

prevent any LAr from boiling, safe operation with respect to HV dischargersand long term stability are important constraints.

Chapter 2

The new cold electronics

2.1 Development

The HEC uses cold GaAs preamplifier and summing electronics, The readoutas well as trigger electronics is operated in the warm as for the other LAr sub-systems. The physical pulse from the LAr gap is subjected to bipolar shapingfor faster readout. The shaping time is optimized for running at a luminosityof 1034cm−2s−1 and the shaped signal is sampled every 25 ns (40 MHz), syn-chronised with the LHC clock. The samples are stored in an analog pipelinefor the readout after the Level-1 trigger processing. Only accepted events aredigitized and read out. The peak value of the shaped signal is constructedfrom five samples using the optimal filtering method (see above). The calibra-tion of the electronics is performed with the help of calibration pulser boardslocated in the Front End Crates (FEC) which hold the readout electronics. Aknown current pulse is injected near the electrodes. The position of the pulseinjection and the methods used for the electronic calibration differs for thedifferent sub-detectors. The requirements on the precision of the calibrationare very strong for the calorimeter in particular for the EM systems.

ATLAS sets new demands on detector electronics concerning speed as wellas reliability. The total LAr calorimeter (presampler, EM Calorimeter, HECand FCAL) has a total of about 190 000 read out channels [8]. The require-ments are specially set to a wide dynamic range, high speed and careful syn-chronisation.

The triangular-shaped current signal (fig. 1.7a.) coming from the detectorhas a fast rise time of 1-2 ns and decrease to zero after about 450 ns corre-sponding to the drift time of the ionisation electrons in the LAr gap. Thewidth of these gaps and therefore the drift times vary slightly between the dif-ferent sub detectors and even more the range of the signal from the differentsampling sections. The electromagnetic calibration constant for the HEC is0.32μA/GeV for the front sampling and 0.16μA/GeV for the back sampling.

The electronics must fulfill the following requirements:

20

2.1. DEVELOPMENT 21

• Signal sampling frequency of 40 MHz due to the time spacing of 25 nsbetween the bunch crossing.

• The measurement of the energy of jets and clusters requires summingover many cells of the coherrent noise to be less than 5% of the normalnoise.

• radiation hardness of the electronics.

• The large energy spectrum dynamic range from 3 TeV on the high endto 13 MeV for the minimal ionising particle on the low end. Thereforethe dynamic range has to be 16 bit.

• Due to the trigger latency of 2 μs data has to be stored in a pipeline forat least 2.5μs.

2.1.1 The electronic chain of the HEC

An overview of the different components and their basic elements forming theread-out system for the LAr Calorimeters is given in fig. 2.1. The signalprocesses the following steps:

• pre-amplifying

• shaping

• pipeline memory

• digitisation

• signal reconstruction via digital filtering.

The first preamplification and summation in HEC is done inside the cryostat inthe cold [42]. This solution allows to achieve an excellent signal-to-noise ratio,high enough to be able to detect muons. The Front End Crates (FEC) as anextension of the cryostat Faraday cage houses the Front End Boards (FEB).These crates hold also the calibration boards. The Tower Driver Boards (TDB)form different signals of the analogue summing for the cells of one trigger towerand drive the signals via cables to the trigger processing where the signal isprocessed by the Level-1 trigger. Monitor boards, which are not shown inthe figure 2.1., read out various monitor probes of the FEC. The high powerconsumption in the FEC and the temperature sensitivity of the surroundingsub-detectors requires a sophisticated cooling system. The system used isbasically the same leak-less water cooling which was developed for the L3experiment at CERN and is driven by an underpressure gradient. The part ofthe read-out system which is outside the detector is located in 50 m distancein a dedicated cavern. Besides the just mentioned Level-1 trigger system it

22 CHAPTER 2. THE NEW COLD ELECTRONICS

Figure 2.1: A schematic view of electronic chain in ATLAS. In cold: preamps onthe FEB (EMEC and FCAL, blue), preamplifiers and summing preamplifier in HEC(green), [8].

consists of the Read-Out-Driver system (ROD) which is connected by a fast1 Gbit/s optical link connected to the FEB’s. The ROD system is receivingthe raw data and is reconstructing the corresponding time, energy and dataquality tag before sending it to the data aquisitioning system.

2.1.2 HEC Cold pre-amplification and summation

The first amplifying and summing is done in the liquid argon starting from theRD33 project [44], but more details can be found in [18], [22], [43]. For reasonsof stable operation at cryogenic temperatures with excellent performance in thehigh frequency range, low noise and radiation hardness, the GaAs MESFET1μm technology was chosen for the pre-amplifier chip (the HEC chip). It

2.1. DEVELOPMENT 23

contains 8 identical pre-amplifier and two summing amplifier for summation.

Figure 2.2: An upper view of the HEC chip BB96. The chip is divided as shown intotwo regions for the two driver with 4 preamps each. This design was very helpfullduring the production of test boards which we used in our radiation tests in februaryand june 2008.

Requirements for the HEC chip are the stability in gain, linearity and lowlevel noise performance. The signal for the HEC electronics is produced inthe electrodes in the gaps. Following the concept of active pads the inputsignal of each pair of two consecutive pads is fed in a separate pre-amplifier.Groups of signals (2 to 16, depending on the region in the detector) from severalpreamplifiers (usual 4 preamps goes to one driver) are summed actively formingone output signal. With some auxiliary components like the driver feed backloop, summing resistor and the decoupling capacitors, the pre-amplifier chipsare mounted on so called pre-amplification and Summing Boards (PSB). Theseborads are positioned at the circumference of the HEC wheels. Eeach Φ-wedgeis read out by five PSB’s with a total of 67 chips. The 88 read out channelsper Φ-wedge are grouped in four longitudinal samplings, LS1-LS4. The front-end boards (FEB) are standard for all LAr calorimeters, with the exception ofpre-shapers mounted for the HEC at the signal input instead of pre-amplifiersas for the other sub-detectors. The analogue signal shapings weres done onthe FEB’s as well as the summing of the signals from individual cells to triggertowers where they are preparing the input signals for the Tower Driver Board(TDB). All signals are stored awaiting the decision by the Level-1 trigger whereselected signals will be digitised before being transmitted via optical links tothe off-detector electronics. In case of the HEC electronics chain the firstelement of warm electronics is the pre-shaper which has to compensate for thedifferent rise-time of the individual channels and to build an additional stageof integration to deliver the correct signal shape, amplitude range and polarity


for the following monolithic shaper. The next component on the FEBs lineis the shaping amplifier for optimising the signal-to-noise ratio, taking intoaccount the total noise given by the quadratic sum of the electronic noise(eq. 1.11) (decreasing with slower shaping) and the pile-up noise (increasingwith slower shaping). The large dynamic range of the input (16 bits) therebyis obtained using three different gains with a ratio of typically 100/101/102

before digitisation with a 12 bit ADC. After shaping the signals are sampledwith 40 MHz and stored to deal with the maximal trigger latency of 2.5 μs in a144 deep analogue pipeline, given by a Switched-Capacitor-Array (SCA) [45].Upon the Level-1 trigger reception the signals are digitised in a 12 bit ADC.Usually five samples are kept but for calibration and monitoring purposes upto 32 samples can be stored.

The LAr off detector electronics, the crates for digital signal processing(Read Out Crates) as well as the trigger system are located in a remote cavern.For reconstruction of the signal amplitude from the individual samples themethod of optimal filtering (earlier mentioned) is used. The ROD output dataare the time, reconstructed energy and tag quality.

2.1.3 Different technologies of electronic components

The technology and the choice of the transistor type plays a crucial role inthe design of the HEC electronics. In our investigations we have consideredtransistor technologies GaAs, SiGe and Si. Further transistor types has beenstudied: bipolar, CMOS n-type, CMOS p-type and MESFET. These types willbe shortly described (more detailes see [9]). Commonly are transistors classi-fied as unipolar and bipolar, depending if one or two charge carriers participatein the current flow. In consequence their suitability for specific applicationsdiffer very much. Bipolars are the right choice for high-speed applications andfor large currents. Unipolar yield more in moderate-speed low-noise applica-tions (JFET’s) and are mostly used in digital circuitry (MOSFETS).

A bipolar transistor consists of two back-to back p-n junctions with a thinbase. In normal operation the emitter-base junction is forward-biased while thebase-collector junction is reverse biased. To have good amplification properties,some requirements have to be fulfilled:

• The base must be very thin,

• the doping concentration of the emitter has to be large with respect tothe base and

• the doping concentration of the collector should be small.

Compared with MOS technologies, bipolar electronic has a higher technolog-ical complexity because of the necessity of building multilayer vertical struc-tures with a very thin base layer interleaved between emitter and collector

2.1. DEVELOPMENT 25

layers. Most of the space is used up for the connections and the proper tran-sistor region is the small region below the emitter (see fig. 2.4.). A proper

Figure 2.3: The cross section of the n-channel MOSFET. Dashed lines indicatethe space-charge region from the channel immediately below the gate and from theundepleted bulk. (after [9])

name for the MOS transistors would be Conductor-Insulator-Semiconductortransistors since in the silicon technology the metal gate has been replacedby polysilicon. It is the most important device in microprocessors, memoriesand high-density detector readout electronics. The electrical characteristics ofa Metal-Oxide-Semiconductor Field Effect Transistors (MOSFET) are simi-lar to those of JFET. The dashed lines show the charge drifting channel andthe depleted region. Fig.2.3. shows an n-channel MOSFET device with twoassymetrically doped n+-p junctions joined by a MOS structure. At zero ornegative gate voltage - using the convention that externally applied voltagesare taken with respect to the source - source and drain are insulated from eachother. An insulator interface will be formed at sufficient positive gate voltagefrom an inversion layer and the drain and source will be connected by a con-ductive layer. This conductivity can be controlled by the gate voltage and tolower level by the substrate voltage. The depletion depth may not exceed acertain value because an inverion layer will be formed by electron-hole pairsin the depletion region. Only if the inversion layer is connected to source anddrain and its voltage can be controlled, this restriction does not appear.

The older NMOS technology is based on n-channel enhancement and n-depletion transistors while the CMOS technology on n-channel and p-channelenhancement transistors. Beside the relative easy way of production there is alower power consumption in digital amplifications what the MOS technologygives an advance in comparement to others, especially to CMOS. The onlylimitation is, as mentioned in the speed where the bipolar technology is supe-rior. The CMOS is the newer one and for us more important, shown on fig.2.4. on the left side. Shematic shows both, the n-channel and the p-channelenhancement. The difference is obvious in the n-tube and the p+-implant by


the p-enhancement. Complementary Metal-Oxide-Semiconductor (CMOS) de-

Figure 2.4: Left: the CMOS technology, n- and p-channel enhancement transistors.Right: A simplified cross section of a bipolar n-p-n transistor. (after [9])

vices use in their simple form only n- and p-channel enhancement transistorsas active devices. They are insulated from each other by putting one of theminto a well with opposite doping to the original bulk material [9]. The n-dopedtube (well) has to be put at a defined potential (as it controls the transistortreshold voltage), usually to the positive supply voltage. Due to the differencein mobility of the electrons and the holes by a factor of three the two typesmay not be perfect complementary. Moreover the concentrations of the bulkand the well are different and the well potential can be chosen separately foreach p-type transistor. In case of irradiation (in the next sections) CMOS hasa latchup problem where this effect might be important. If taking enough largdistances between structures or implementing a guard structure, this effect canbe avoided.

2.1.4 Radiation effects on electronic components

There are four basic classes of radiation effects that might yield substantialdamage. These four effects are Neutron-(Non-Ionising Energy Loss, NIEL),Total Ionizing Dose-(TID), Transient Dose-(TDE), and Single Event (SEE)-Effects.

Neutron Effects: When neutrons strike a semiconductor chip, they displaceatoms within the crystal lattice structure. The minority carrier lifetime isreduced because of the increased recombination centers created. Silicon devicesbegin exhibiting changes in their electrical characteristics at levels of 1010 to1011 neutrons/cm2. Because bipolar components are minority carrier typedevices, neutron radiation affects them more than MOS devices. In bipolarintegrated circuits, the base transit time and width are the main physicalparameters affected. Therefore, neutron radiation significantly reduces gainin bipolar devices. MOS devices are usually not affected until levels of 1015

neutrons/cm2 are reached, as in our case.

2.1. DEVELOPMENT 27

Figure 2.5: A positive charge layer trapped in the gate oxide of an nMOS transistor.(after [48])

Total Ionizing Dose Effects: Total Ionizing Dose is the accumulation of ion-izing radiation over time, typically measured in rads. The total dose createsa number of electron-hole pairs in the silicon dioxide layers of MOS devices.As these begin to recombine, they create photocurrents and changes in thethreshold voltage that make n-channel devices easier to turn on and p-channeldevices more difficult to turn on. Even though some self-recovery takes placein the device, the change is essentially permanent. Some holes created dur-ing ionizing pulses are trapped at defect centers near the silicon/silicon oxideinterface. Charges induced in the device create a field across the gate oxide suf-ficiently high to cause the gate oxide to fail, or sufficient carriers are generatedin the gate oxide itself to cause failure.

Transient Dose Effects: A Transient Dose is a high-level pulse of radiation,typical in a nuclear burst, which generates photocurrents in all semiconductorregions. This pulse creates sudden, immediate effects such as changes in logicstates, corruption of a memory cell’s content, or circuit ringing. If the pulseis large enough even permanent damage may occur. Transient doses can alsocause junction breakdown or trigger latchup, destroying the device.

Single Event Effect (SEE): They typically affect only digital devices signif-icantly, but SEE’s are of primary concern in our today’s digital age. A SEEoccurs when a single high-energy particle strikes a device, leaving behind anionized track. The ionization along the path of the striking particle collects ata circuit node. If the charge is high enough, it can create a Single Event Upset(SEU), such as a bit flip, a change in state that causes a momentary failure inthe device output, or corruption of the data in a storage element. A SEE canpossibly trigger a device latchup and burnout. Latchup occurs when sufficientcurrent is induced in a part of the device that causes the device to latch into afixed state regardless of circuit input. Burnout occurs when the radiation in-duces sufficient power dissipation to cause catastrophic device failure. Burnoutsometimes occurs as a result of latchup.


Single Event Upsets (SEU): These are also known as soft errors that occurdue to either the deposition of depletion of charge by a single ion at a circuitnode, causing a change of state in a memory cell. In very sensitive devices, asingle ion hit can also cause multiple-bit upsets (MBU’s) in adjacent memorycells. This type of event causes no permanent damage and the device can bereprogrammed for correct function after such an event has occurred.

Single Event Latchup (SEL): This can occur in any semiconductor devicewhich has a parasitic n-p-n-p path. A single heavy ion or high energy pro-ton passing through either the base emitter junction of the parasitic n-p-n-pthiristor, or the emitter-base junction of the p-n-p transistor can initiate re-generative action. This leads to excessive power supply current and loss ofdevice functionality. The device can burnout unless the current is limited orthe power to the device is reset. SEL is the most concern in bulk CMOSdevices.

Single Event Snapback(SES): This is also a regenerative current mechanismsimilar to SEL, but a device does not need to have a p-n-p structure. Itcan be triggered in a n-channel MOS transistor with large currents, such asIC (integrated circuit) output driver devices, by a single event hit-inducedavalanche multiplication near the drain junction of the device.

Single Event-Induced Burnout (SEB): This event may occur in power MOS-FETs when the passage of a single heavy ion forward biases the thin bodyregion under the source of the device. If the drain-to-source voltage of thedevice exceeds the local breakdown voltage of the parasitic bipolar, the devicecan burn out due to large currents and high local power dissipation.

Single Event Gate Rupture (SEGR): This has been observed due to heavyion hits in power MOSFETs when a large bias is applied to the gate, leadingto thermal breakdown and destruction of the gate oxide. It can also occur innonvolatile memories such as EEPROM during write or erase operations, thetime when high voltage is applied to the gate.

2.1.5 Radiation hardness and device stability

Although we are using neutrons in the energy range of 2-32MeV (in the case ofthe cyclotron in Rez), there always appear some ionising and other mentionedeffects. It is important to test a device to all posible radiation types which canappear in the detector.

A critical part of the detector is the surface region where at least a smallregion is terminated by an insulator (SiO2). Already without irradiation thesurface region is irregular over a depth of few lattice spacing. It is exactly at theboundaries between strongly doped and only-insulator-covered regions wherethe highest field strengths are present. These high fields from the positivecharges are present at the oxide-semiconductor boundary (fig. 2.5.).

The principal difference between the insulator and the semiconductor is thewidth of the band gap and the different material structure. The oxide and the

2.1. DEVELOPMENT 29

surface region are, in difference to the bulk, very irregular and are sensitive tothe ionising irradiation such as photons, X-rays and charged particles. As inthe case of semiconductors the change in the structure comes from the electronand hole capture process. If the traps are deep enough, the emission of capturedcarriers into the conduction and valence bands is virtually impossible (bandgap of insulator is 8.8 eV for SiO2 while it is 1.12 eV for Si). The radiation-generated electrons are of three orders faster diffunding and escape very fastfrom the insulator so the primary capture effects are on the holes. As soonthe traps are filled, it comes to a saturation of the positive charges and noincrease of the oxide charge is possible [47]. A positive voltage shift appears.For the positive bias of a MOS structures, a stronger flat-band voltage shift isobserved than for the opposite biasing.

In the case of irradiating a silicon device with neutrons, the whole interiorof the detector was inverted, but not the surface region. A possible naturalexplanation for no inversion in the surface region comes from the considerationof the defect formation. The primary defects (Si- interstitials and vacancies)are mobile at room temperature and without any other influence they wouldjust diffuse out of the surface and the cristal would come to its perfect state.During their movement they can affect other primary defects forming in thisway some stable defects in the surface region that alter the electrical behaviorof the semiconductor.

While the signal properties of a MOS transistor (in the case of ionisingirradiation) are more affected by the oxide-semiconductor interface, the noiseproperties are affected by both oxide and bulk damage.

Radiation hard detector is in principle easy to build if using only de-vices which are enough radiation hard for our purposes. Alternatively, thereis possible to design devices which tolerate the changes in material proper-ties. In CMOS electronics with the use of quality gate oxides the magnitudeof radiation-induced threshold voltage shifts were reduced. Properly dopedguard rings around an individual transistors prevent the appearence of par-asitic shorts between neighboring devices due to radiation induced positivecharges in the wide field oxide.

In general, there is no ’magic formula’ which can be used to design a ra-diation hard detector. It depends of the purpose of the detector, the usedmaterial for the bulk, the material used for the oxid layer, the thickness of thebulk, the thickness of the field oxide, the type of doping, the insulators, thedimensions, the duration and type of irradiation it will be exposed to and alot of other small parameters.

Chapter 3

The first irradiation test

The irradiation tests of transistors using different technologies have been car-ried out at the Institute for Nuklear Physics in Rez near Prague. Before start-ing with our first irradiation test in february 2008., we have made a pre-testfew months earlier to make sure that the noise level of our setup was accept-able. I will call this pre-test the zero-test. At that time we have used a networkanalyser measuring the performance of the present HEC chip. Similar studieshave been done using the cabling system as antenna. The frequency studiedwas from 100kHz to 100MHz over 1600 points. No significant disturbancieshave been seen in this frequency range which could affect our future measure-ments. The cyclotron and the neutron production will be shortly described inone of the next sections. The requirement for radiation hardness for the nextgeneration of cold electronics which may be applied in the HEC upgrade inthe SLHC (Super Linear Hadron Collider) is 1.2 · 1015 neutrons/cm2 (safetyfactor of 10 is included) and 1.2 · 1012 protons/cm2. Due to the differencein required fluence and the radiation damage, neutrons will be applied. Thenext generation of cold electronics must be 10 times radiation harder than theactual electronics.

3.1 The setup

The task of the first irradiation test (also named as first run) was to test prelim-inary the first few new technologies and compare them with the old technologywhich is already used in ATLAS HEC. Figure 3.1. presents the principle of thenew measurement setup with some parts based on the setup used in [22]. Forour measurements we have used 2 testboards (TB) with the HEC chips (onone HEC TB were 3 pre-amplifiers (PA) separately and one summing system(one preamplifier and one summing amplifier (SUMAMP) connected in a serialline)), 4 TB each with 4 IHP transistors (2n + 2p-types) and one TB withthree Triquint CFH800 transistors. The voltage box is supplied with +15Vand -15V (grounded) and is regulating the voltages for a specified device. The

30

3.1. THE SETUP 31

DSW3 is switching through all channels so that the digital multimeter (DMM)may measure the actual voltages. Both, the DSW and the DMM for clos-ing/opening channels and the readout are connected to the PC over the GPIBbus. The HEC chips are supplied with power directly from the low-voltage boxwhile the other devices are supplied over a BIAS TEE box mixed directly withthe signal from the NA4 since they have no separated inputs for the power sup-ply. The function of the ASW5 is to leave signals passing through only for theacctual measured device while keeping the pass for the rest closed. The ASWis also used to switch between two measuring methods which will be describedin the further text. The ASW was able to measure directly the temperaturevia a PT100 extension (a ceramic 4-wire thermometer loaded with 100 Ohm).An additional ethernet connection was needed because of the slow informationflow through the GPIB interface during the data saving from the NA to thePC. A local network connection between the main PC and the PC integratedinside the NA was established. The second purpose of the ethernet connectionwas the connection to the second PC where the online-monitoring is runing.Using the HEC chip (GaAs MESFET6 transitors inside) on one board we have

Figure 3.1: The setup for the first irradiation test in february 2008.

3DSW: shortcut for ’digital switch’4NA: shortcut for ’network analyser ’5ASW: shortcut for ’analogue switch’6MESFET stands for MEtal Semiconductor Field Effect Transistor. It is quite similar

to a JFET in construction and terminology. The difference is that instead of using a p-n

32 CHAPTER 3. THE FIRST IRRADIATION TEST

activated 4 preamplifiers (out of 8) and one summing amplifier (out of 2). Oneof the preamplifier is directly connected with the driver. Therefore there were4 devices on one HEC board. Irradiation tests with this chip were made inprevious studies [22] where the ’scope’ method [4] was used. All the compo-nents of the new cold-electronics are required to be at least 10 times radiationharder than the previous HEC technology (previous section). This means thatthe new technology has to withstand a fluence of 1015 n/cm2. The design andtest results for the HEC BB96 chip can be found in [28], (For a descriptionof the HEC cold electronic chain for ATLAS see [29]). The first part of thesetup is the scope7 method. In this method triangle signals are generated witha pulse generator with different output amplitudes. This signals were sentsplitted to the input of the device (HEC system8) and directly to the scope.The generator is directly connected to the scope and all the parameters like:rising time, amplitude or pulse width are measured with the oscilloscope andvia GPIB are read out on the PC. The original signal will be therefore dividedinto two equal signals over a power splitter where we may read out the firstpart directly with the scope. The other part of the signal is travelling over theASW to the input of the HEC system where the signal will be inverted by thechip itself and amplified, it will be measured.

3.1.1 The NA methods

In the first run we made three different measurements of the gain for eachelectronic device. As mentioned, the first measurement was the scope methodwhich has used the triangle pulse input signal. The second and third mea-surement was the NA method which has used a sine input signal. In the firstmethod we get the gain from the slope of the linearity curve9 by fitting pointsup to 250μm. A similar procedure has been applied in the linearity measure-ment with the NA using specific frequencies (20 MHz, 40 MHz and 80 MHz).The gain from the NA method is about half of the gain obtained with thescope method possible due to the difference in the shape of the input signal.The dynamic range of the HEC chip depends on the polarity of the signal. Themeasurement of the NA is symmetric with respect to the polarity of the signal.The useful signal for the HEC chip is only the negative part. Therefore, onlyhalf of the gain is expected in this case. In the section Results these expec-tations are confirmed. Thus the link between this two methods is establishedand the results may be compaired with [4]. The third measurement method

junction for a gate, a Schottky (metal-semiconductor) junction is used.7As mentioned, the ’scope method’ means the usage of the oscilloscope, which does not

appear in the ’NA’ method8HEC system means that the signal is first passing one of the preamplifiers of HEC and

then the HEC summing amplifier. The outcoming signal is the signal of that system.9Although linearity for a line means the straightness of it, it is common to say linearity

curve for the transconduction function U/I

3.1. THE SETUP 33

is based on the use of the s-parameters (s-parameters will be closer describedin the next subsection). In the NA method the gain and reflection may bedetermined in the frequency range from 10kHz to 8GHz for a certain inputpower. This allows us to observe the change in the gain in a certain frequencyrange under irradiation and to compare these results with the gain from thelinearity measurement. The two different types of measurements of the gainshould agree.

S-parameters, Linearity

Linear networks (or nonlinear networks operating with signals small enoughto response) may be completely characterised by parameters measured witha two-port terminal without regarding details of the content of the network.After determining once the parameters of a network, it is possible to predict thebehaviour of the network in an external environment. Two-port or many-porttheories simplify the response of a device into a black box described by a setof four linear parameters, [30], [53]. Lumped models use the Y-parameters(conductances), Z-parameters (resistances) and h-parameters (mixture ofconductances and resistances). Distributed models use scatter-parameters, theso called s-parameters (coefficients of reflection and transmission). The s-parameters are given by four parameters:

two-port

network~

ZS

ZL

VS

a1

a2

b1

b2

s21

s11

s22

s12

a1

a2b

1

b2

�S

�L

bS

Figure 3.2: The principle of the Two-port network with incident waves (a1, a2) andreflected waves (left) with the belonging flow graph (right).

• s11 = b1a1|a2=0 = Input reflection coefficient with the output port termi-

nated by a matched load (ZL = Z010 sets a2 = 0)

• s22 = b2a2|a1=0 = Output reflection coefficient with the input port termi-

nated by a matched load (ZS = Z0 sets VS = 0)

• s21 = b2a1|a2=0 = Forward transmission (insertion) gain with the output

port terminated by a matched load

• s12 = b1a2|a1=0 = Reverse transmission (insertion) gain with the input

port terminated by a matched load

10It is convinient to assume that the reference impedance Zi is positive and real so thatall variables and parameters will be referenced to a single positive and real impedance Z0.


The variables ai and bi are normalised complex amplitudes incident on andreflected from the i-th port of the network, defined in terms of the terminalVoltage Vi, the terminal current Ii and an arbitrary reference impedance Zi

(figure 3.2). The s-parameters obey the following relation:

b1 = s11a1 + s12a2 (3.1)

b2 = s21a1 + s22a2 (3.2)

From previous relations the s11 parameter may be expressed in terms of thematched load Z0 and the input impedance Z1 at port 1:

s11 =b1

a1=

V1

I1− Z0

V1

I1+ Z0

=Z1 − Z0

Z1 + Z0(3.3)

with

Z1 = Z0 · 1 + s11

1 − s11and Z1 =

V1

I1(3.4)

where V1 and V2 are the input and output voltage in the case of the port1-to-port2 direction.

As mentioned, the four s-parameter and their phases over all frequencieshave been measured in the frequency range of interest. It is possible to savethis data in a nine-coloumn TOUCHSTONE format. We set the power of theinput signal to be -35 dBm and read off directly from the plot on the screengain or reflection at a given frequency, in relative logarithmic units dB (see inappendix, section 6.1).

To obtain the gain from the linearity measurement some additional stepsare required. At a fixed frequency must the input power be runed from -40 to+10 dBm. The output power is to be measured. To obtain the informationon the input current (in μA) and output voltage (in mV), it is necessary totransform from the units dBm to A and V (see section 6.2.). It can be donewith help of the s11 parameter (as shown above) because the reflection is nowa dynamic variable and is changing for every value of the input power. Toobtain the output voltage V2 must be considered instead of V1

3.2 Neutron flux measurement

We have irradiated our test boards in the Institute of Nuclear Physics in Reznear Prague with a ’p(37MeV)+D2O’ neutron source [49]. The neutronswere produced in the reaction D2O(p,xn) hitting a heavy water target withprotons (up to 37 MeV). The diameter of the target is about 16 mm circularlyshaped. The beam, is operating at 150 Hz (6.67 ms) repetition rate, the spilllength is 1.667 ms. The micro structure is a bandwidth of 3 ns at 25.87 MHzrepetition rate. Some small noise level has been observed at this frequency (in

3.2. NEUTRON FLUX MEASUREMENT 35

the zero run). The heavy water is circulating bubble free in a laminar flowyielding always the same target density. Thus a constant neutron density isguaranteed. The neutron flux density (NFD) is directly proportional to theproton current beam of the cyclotron. The integrated neutron flux density(INFD) is the integration of the NFD over all energies in the range of 2-35MeV. This range contains 86% of the total flux. To obtain the integratedneutron flux or shortly neutron fluence (INF) a correction factor of 1.163 hasto be applied to correct for the range of 0-2 MeV.

The neutron spectra has been investigated using the activation of dosimetry-foils in the space specified by a cylinder of diameter d = 16 mm. Activationfoils (Al) were put in the 5th slot and the 10th slot of the setup. The reac-tion used for was: 27Al (n, α) 24Na (lifetime 14.96 hrs.). The uncertainties of3-20% at different energy intervals of spectra were determined experimentallyfor spectral data above 7 MeV. In the energy range below 7 MeV the spec-trum is yet determined by the MCNPX11 calculation only [52]. At a distanceof x = 20mm we have expected a flux of about 3 · 1010 n/cm2s for energiesabove 2 MeV. Together with the correction factor of 1.163 the reached INFwas about 5 · 1015 n/cm2.

3.2.1 Measured flux, Energies, INF

To calculate the INF for all slot positions, first the proton current has beenintegrated over the irradiation time. A total charge of 1.2575 C for the irradi-

Figure 3.3: The NFD vs. energy.

ation time of 39.02 hrs has been obtained. Integrating NFD over all energieswe obtained the INFD normalised to charge for every slot of the setup. Mul-tiplying INFD with the summed charge we get the INF. Investigating fig. 3.3.

11MCNPX is a general-purpose Monte Carlo radiation transport code for modeling the in-teraction of radiation with anything. MCNPX stands for Monte Carlo N-Particle eXtended.


and fig. 3.4. it is obvious that the INF is not falling linearly with respect todistance, but more in a quadratic way. Further analysis has shown that it is apolynom of nearly the 5th order [46]. The table 3.1. shows the results for all

Figure 3.4: The INFD vs. energy.

Figure 3.5: First run: neutron fluence vs. time

3.3. RESULTS, SIGNATURES FOR RADIATION DAMAGES 37

Slot No. INFD INF∗ INF[109 n

cm2μC] [1015 n

cm2 ] [1015 ncm2 ]

1 3.77 4.74 5.512 1.63 2.05 2.383 0.86 1.08 1.264 0.53 0.67 0.786 0.26 0.33 0.387 0.20 0.25 0.298 0.16 0.20 0.23

Table 3.1: INF (neutron fluence) depending on the slot position (∗ shows the uncor-rected fluence for the neutrons in the 0-2 MeV range).

slots. The minimal systematic error of the setup is calculated after the secondrun where we have compared two methods for the dosis measurement. Theresults are presented in section 5.1.

3.3 Results, signatures for radiation damages

3.3.1 NA-method, characteristics of the signal

Using a network analyser allows us to obtain the gain in the input directiondirectly as the s21- and the input reflection as the s11- parameter. Alternativelyone may use the linearity measurements and using the ratio of the output/inputof the signal and the slope of this curve. The NA is sending sine signals. Beforeany further measurements it is required to calibrate each channel. Thus anyloss in the signal cabling system is corrected for. In this setup twice 35 mcable of the type RG174 have been used (a single loss of 20 dB/100 m). Theresidual losses on the board between the SMA connectors and the device arenegligible.

The following parameter set was used: input power -35 dBm, frequencyrange 10 kHz to 100 MHz, number of points 1600, scaning bandwith 100 kHz.The gain and reflection on the input and the output (4 s parameters) havebeen measured in this frequency range. A TOUCHSTONE 12 file have beenautomatically created and saved to the NA.

3.3.2 Gain over frequency

In this subsection, as an introduction to our devices, the gain in the inputdirection as a function over frequency will be presented. This functionality isshown in fig. 3.6. for the GaAs technologies HEC PA and system, and the

12A file format which includes in this case 9 coloumns containing frequency, the four sparameters and their four phase angles.


Si technology of IHP p- and n-type CMOS transistors, already described inthe previous sections. Triquint CFH-800 transistor [25] is shown separately infig. 3.7. The units on the y-axis are given in dB. A useful conversion table

Figure 3.6: s21 (gain) over frequency: In order of appereance (from left to right)thefollowing technologies are presented: HEC PA(GaAs), HEC system (GaAs), IHPp-type (Si) and IHP n-type (Si).

is given in the appendix, chapter 6. The requirements are stability in gain upto 80 MHz before irradiation starts. During irradiation this limit is decreasingdepending on the total fluence (see section 2.1.2). The tolerance in the changeof the gain when switching to high frequencies under irradiation is up to 3 dB.In the case of the HEC PA this holds up to 85 MHz. After irradiation thislimit is moving to lower frequencies and tells us if the technology stays withinthis 3 dB and if there are possibilities to be used for he next generation ofdetectors in SLHC.

As can be seen all of the three GaAs devices fulfills this requirement. Thedifference in the initial gain depends on the design of the device and mayalso differ depending on the production process. The general quality controlrequirements for the HEC chip ask for all preamplifiers in one chip to havethe gain identical within ±2%. The negative gain of the HEC preamplifier isexplained in [4]. The change of the s21 parameter for four different kind of


Figure 3.7: s21 (gain) over frequency: Triquint CFH800 (GaAs).

devices and four different moments of irradiation is shown in fig. 3.6. It iscomparable with fig. 3.9b. where the s21 is plotted over the total flux. It maybe estimated which technology is radiation hard even at fluences over 1015.The triquint is shown as a separate figure (fig. 3.7.) due to the characteristicsof the signal under neutron fluence. This device has shown the lowest radiationhardness. The reasons will be discussed more in the next sections and in theconclusion in section 5.2.

3.3.3 Linearity

For the linearity masurement we have compared the output power to the inputpower in the range from -40 dBm to +10 dBm for three frequencies (see fig.3.8.): 20 MHz, 40 MHz and 80 MHz. In the upper line of fig. 3.8. the fullmeasured range is presented and below frames in the range of the input currentup to 250 μA.

The left column shows the linearities of a device before irradiation andthe right one after 39 hrs. irradiation (1st run) corresponding to 5.5 · 1015

n/cm2. The same colour shows a given device but at different frequencies. TheIHP technology seems to be more radiation hard. The HEC and the Triquinttechnologies (both GaAs and MESFET) are irresistant to higher irradiationlevels. The Triquint is breaking down from the begining (pointing to a problemin the design or a sensitive part of the transistor).

Depending on the saturation in the gain for different devices a range forlinear fit is chosen and the fit was applied. The slope of the linear fit wastaken for every event and plotted over flux (fig. 3.10.). For reason of bettercomparement to the direct s-parameter measurement over flux (fig. 3.9.) thisfigure is shown in subsection 3.3.4. To convert dB to a multiplication factorsee table 6.1. (an example is also shown in the next section).


Figure 3.8: The input-output stability (linearity in certain regions).

3.3.4 S-parameters over flux and the change of the lin-

earity

Fig. 3.9. shows the full irradiation range at 20 Mhz and -35 dBm input signal.The gain of the HEC system decreased about 3 dB at 1 ·1015 n/cm2. The gainof the HEC pre-amplifier is decreasing only about 1 dB for the same irradiationlevel. This difference is pointing to a certain part in the summing amplifier ofthe HEC integrated circuit which is not enough radiation hard.

The best stability in gain and reflection shows the IHP, especially the p-type. As the same devices were installed in different slots of the setup, theresponse to irradiation could be directly compared. Since the signal behaviorof the n-type is the same as of the p-type under the same irradiation level, itmight be expected that the n-type will also be radiation hard even at 5.5 ·1015

n/cm2.

The reflections shown in these figures for the Triquint and IHP can notbe distinguished on this scale since they are all close to zero, but remainedstable under irradiation. More datailed plots can be found in [46]. A linearfit has been done to the linearity curves (fig. 3.8.) in the range of 0 - 250μA, except the for the HEC, where the fit range has been restricted to 50 μA


Figure 3.9: Left: reflection of all 5 technologies under the neutron fluence (INF -integrated neutron flux). Right: forward gain of all 5 technologies under the neutronfluence.

Figure 3.10: Slope of the linearity under neutron fluence: HEC syst., Triquint, IHPp- and n-type.

- 70 μA (because of the sensitivity of the HEC chip to signal polarity). Asmentioned earlier, the reason also may be the shape of the signal or the designof the chip). The IHP technology has shown best stability. Next we maycompare two type of gain measurements for the HEC chip: the slope of theoutput/input function (fig. 3.10.) with the s21 parameter (fig. 3.9.). Whentaking the proportion 180 mV/120 mV we get 1.5 as a relative gain factor.This is about 3.5 dB obtained in the s-parameter measurement. Therefore thetwo methods are in cinsistance.


3.3.5 Scope-method, HEC technology

The scope method was described in section 3.1., the results are shown in fig.3.11. and fig. 3.12.

Figure 3.11: The linearity of the HEC system measured with the scope-method.

Figure 3.12: Dependence of the slope on the neutron flux.

For comparison, there are shown the results of the linearity measurementsfor the HEC system with the NA method as well. The signal is again a factorof about two than in the scope method. The black curve labeled ’Dubna’ is themeasurement from 2001 in the reactor of Dubna [4], in the cold and after an

3.4. THE SOFTWARE FOR DATA AQUISITION 43

irradiation level of about 1 · 1015 n/cm2. Some differences between the Dubnameasurements and ours appear probably due to the difference in temperature(Dubna was measured in the cold) and the difference in the cable type (RG51).

Fig. 3.12. shows the dependence of the slope on the integrated neutronflux of both investigated HEC systems in our setup. HEC syst. labeled as 2was mounted in the 8th slot where the irradiation level has reached only about2.4 · 1014 n/cm2. The difference in the slope offset is due to a difference in theproduction but the behavior remains the same.

3.4 The software for data aquisition

The DAQ program was written in Visual Basic 6.0. Every device controlledvia GPIB is a separate class module where all commands are defined for acertain device. The on-line monitoring program was written in Visual C++13.Online plots were available for s-parameters, linearity, temperature, currents,etc. The figures in chapter 3 and chapter 4 has been made with ROOT andVisual C++ 2005 express (for Windows). Some of them were redesigned inCorelDraw 12. Origin 8.0 was used for figure 3.11. The design of the testboards is made with PADS logic.

3.5 Conclusions after the first test

We have measured the gain and gain stability of different technologies of am-plifiers as function of the neutron fluence. Measurements with the NA methodas well as with the oscilloscope method was carried out. Detailed analysis hasshown that our measurements are in consistence with the previous measure-ments of the HEC chip. The HEC chip may be operated up to about 1 · 1015

n/cm2. Triquint CFH800 (TQP) technology seems to be very sensitive to neu-tron fluence for the set operating points and started to show low radiationhardness from the begining of irradiation, even at 1 · 1014 n/cm2. Further in-vestigation on Triquint has to be made since there might be a problem withthe coating or the operating points. For the next test it is planed to includean other GaAs technology named Triquint die (TGF1350-SCC) which has aceramic coating and differs slightly in the design [25] from the CFH800.

The IHP, both p- and n-type have shown excellent radiation hardness withrespect to gain, reflection and linearity. Even after 5.5 · 1015 n/cm2 thesetransistors had a deviation of less than 1 dB in gain and less than ±10%deviation in the slope during the whole irradiation time. Further tests in thecold must be performed.

13written by G. Pospelov

Chapter 4

The 2nd irradiation test

The second test is rather similar to the first test except that we have not usedthe scope method. As has been shown that the measurements with the NAare consistent with the scope method, only the NA method has been used inthe second irradiation test. One measurement cycle using the scope methodfor one device has taken about 120 seconds and using the NA only 20 seconds.In the first run have been tested 27 devices and in the second 37 devices butwith the same duration of one cycle due to the difference of about 100 secondswe ’won’ from the scope method.

The HEC board is still kept in the setup to have a reliable monitor of theneutron fluence and to understand better the systematic error related to thesetup.

4.1 The setup

In the second run these 37 devices, mounted on 8 test boards. Two additionaltest boards were not in the beam and were used for monitoring purposes.Two new technologies have been studied: AMS (AustriaMicroSystems) pre-amplifier (SiGe, Bipolar, two transistors in each pre-amplifier) and Triquintdie TGF1350-SCC (GaAs, MESFET, ceramic coated) transistor. In additionas in the previous run the Triquint CFH800, the HEC and the IHP were used.Three AMS versions have been studied, differing in the length of the activezone of the emitter: 2.4 μm (SBC3), 24 μm (SBC30) and 96 μm (SBC300). Asimpod of radiation on the performance of the HEC, IHP and Triquint plastic(TQP) which is known, the AMS’s and the Triquint dies (TQD) have beenmoved closer to the target (slot positions 1 and 2). The TQP’s and the HEC’swere in the third and fourth slot position. In the last four slots the boardswith the AMS, the IHP (both types on one TB), the HEC and again the IHPhave been placed.

The setup (very similar to the first setup) is shematically shown in fig. 4.1.Concerning the measurements with the NA, all parameters were as before.

44

4.2. NEUTRON FLUX MEASUREMENT, ENERGY DISTRIBUTIONS, ERRORS45

Figure 4.1: The setup for the second irradiation test in june 2008.

An additional current measurement is included to have a further check of theworking properties of the devices under irradiation. Together with the currentmeasurements, a complete cycle (all 37 device) took about 14 minutes. Othertechnical details about the setup are allready explained in the section 3.1.

4.2 Neutron flux measurement, energy distri-

butions, errors

The neutron beam was as in the previous test (see 3.1.1.).The foil-activation flux measurement was done this time inside the head of

the target in front of the the slot 1. No additional activation foils were used.But two radiation monitors (RADMON’s) with two Si diodes on each boardhave been inserted in slot 5 and slot 10. The evaluation of the neutron fluxbased on the Si diodes response was made by [50]. The Si diodes yield the 1MeV equivalent neutron flux.

The energy dependence of the neutron flux density was slightly differentfrom the previous test. The results are given in table 4.1. The average protoncurrent was 11.29 μA, integrated over the total irradiation time of 41.59 h.This yields a total current integral of 1.690 C.

The integrated flux in the first slot was 7.19 · 1015 n/cm2. Having thistime two different methods of flux measurement, a preliminary assessment of

46 CHAPTER 4. THE 2ND IRRADIATION TEST

Figure 4.2: NFD vs. E

Figure 4.3: INFD vs. E

the systematic error may be obtained. The neutron dosis measured with theRADMON for slot 5 is: Φ = 4.76 · 1014 n/cm2 and Φ = 4.83 · 1014 n/cm2 andfor slot 10: Φ = 1.40 · 1014 n/cm2 and Φ = 1.83 · 1014 n/cm2. The average forslot 5 is: Φ = 4.795 · 1014 n/cm2 and for slot 10: Φ = 1.615 · 1014 n/cm2 (seetable 4.2.).

Factor f[1MeV] to obtain the 1 MeV equivalent flux is NF RADM/INF. Withtwo diodes per slot position the variation of the average with the respect toslot position might be used to estimate the systematic error in f[1MeV] [21]. Oneobtains

f[1MeV] = 00.726 ± 0.043 .

The relative error of 0.769/0.726 = ±5.9% (for the 10th slot) includes inprincipal the other uncertainties (partially), but we may call it our systematicerror for the 1 MeV equivalent neutron flux factor. The systematic error inthe RADMON measurement may be reached including the annealing effects

4.2. NEUTRON FLUX MEASUREMENT, ENERGY DISTRIBUTIONS, ERRORS47

Slot No. INFD INF∗ INF[109 n

cm2μC] [1015 n

cm2 ] [1015 ncm2 ]

1 3.66 6.19 7.192 1.60 2.70 3.143 0.85 1.44 1.674 0.52 0.88 1.025 0.36 0.60 0.706 0.26 0.44 0.517 0.20 0.34 0.398 0.16 0.27 0.319 0.13 0.22 0.2610 0.11 0.19 0.22

Table 4.1: INF (neutron fluence) depending on the slot position (∗ stands for theuncorrected range of 0-2 MeV).

up to 5%, after [50]. The result of 5.9% is therefore in good consistence withthis obtaining.

Further investigation and measurement are planed to estimate the system-atic error in a more precise way (see also section 5.1.).

Slot No. NFRADM NF RADM INF f[1MeV]

[1014 ncm2 ] [1014 n

cm2 ] [1014 ncm2 ]

5 4.765 4.835 4.795 7.02 0.68310 1.4010 1.8310 1.615 2.10 0.769

Table 4.2: Comparison of the direct flux measurement: the RADMON method (1MeV equivalent neutron flux, NFRADM ) and the current method (INF ).


Figure 4.4: Second run: neutron fluence vs. time

4.3 Results, signatures for radiation damages

The characteristics of the NA measurements are similar to the presented above.The same type of cables were used and the calibration was done in the sameway.

4.3.1 Gain and reflection versus frequency

In this setup we have included some new technologies: AMS300, AMS30 andAMS3 transistors (SiGe), already described in 4.1. and the Triquint dielectric-coated (TQdie, GaAs). We inserted the TQdie to prove if the problem of theTriquint CFH800 (plastic coated) could be in the coating, suspecting that theneutrons may have pushed carbon particles into the device causing failure bychanging the doped structure. The frequency dependence of the signal of theAMS300, AMS30 and AMS3 preamplifiers were similar, only shifted in theoffset. The results from the other technologies were as shown in chapter 3 (thefirst setup). The significant difference in the AMS signal to other technologiesis the unstability in the gain as a function of frequency (fig. 4.5. right). Also,there are no decrease in gain even for frequency over 100 MHz, which couldpoint to a design for operation in higher frequency range.


Figure 4.5: s21 (gain) versus frequency - before irradiation: The left figure showsthe AMS3 (SiGe) measurement and the right figure the Triquint die (GaAs) mea-surement.

4.3.2 Linearity

The Linearity of the of the response for devices investigated in the first test isas expected. The Triquint transistors and the IHP preamplifier are showing aperfect linearity even at higher input currents (fig. 4.6). The linear range ofthe AMS preamplifiers depends on the AMS type. In fig. 4.9. the linearity at20 MHz for all AMS types are shown.

Figure 4.6: The linearity of the input-output : The Triquint plastic (CFH800) coatedand the Triquint ceramic coated (TGF1350-SCC) transistors.

The AMS preamplifiers are showing similar linearity curves as the HECsystem (fig. 4.7.), exept for lower amplification. The fit is made in range 0-100μA. The problem appears to be the saturation at 100 μA. (A device should bestable up to 250 μA of input current.) Changes in the design might solve theproblem.

Further investigation should be made with the triangle shaped pulse ofATLAS to obtain the behavior under that conditions.

By taking a look at the IHP technology, it is still obvious that it is leadingin radiation hardness and the output linearity. The change of the output/input


Figure 4.7: The linearity of the HEC system.

Figure 4.8: The linearity of the IHP’s.

Figure 4.9: The linearity of the AMS preamplifiers.


response e.g. linearity under irradiation will be shown in the next section asslope versus INF.

4.3.3 S-parameters over flux and changes in the linear-

ity

The figure 4.10. shows the s11 and s21 parameter in dependence of the neutronfluence for all technologies. The characteristic curves for the HEC and TriquintCFH800 are similar to those in the first test. As expected, the gain of HECwas decreasing by 3dB at about 1015 n/cm2, while the Triquint CFH800 isdecreasing constantly from the start of irradiation.

Figure 4.10: Reflection and gain under irradiation, all technologies together.

Since the IHP transistors have proven to be radiation hard up to 5 · 1015

n/cm2, we used in the second test the slots 7 and 8 for that technology (see infig. 4.10.). The gain as expected was stable under irradiation. The gain of theTriquint die transistors drops rather fast with the neutron fluence. Showingsimilar response as the Triquint transistor with plastic coating, one might arguethat the coating is not affecting the performance. A look at the drain current(see next section) could bring some light to this problem.

The AMS technology has some interesting points: small preamplifiers showbetter stability under irradiation (fig. 4.10. and 4.12.). The input reflection


Figure 4.11: The slope under irradiation (lin. fit in the range 50 - 250 μA), TriquintCFH800.

Figure 4.12: The slope under irradiation (linear fit in the range 50 - 100 μA), AMS,at 20 MHz.

Figure 4.13: The slope under irradiation (linear fit in the range 50 - 250 μA), IHP.


is changing for all three types opposite to the gain. The AMS3 preamplifieris almost perfect stable with respect to reflection and gain. Fig. 4.12. showsthat even at a fluence of 7.2 · 1015 n/cm2 this device was operating withoutproblems. Further investigations could be made to reach the upper limit forthis technology.

Figure 4.14: The slope of the linearity : The HEC syst. and the Triquint ceramiccoated (TGF1350-SCC) transistor.

4.3.4 Current measurements

The measurement of the currents might reveal some signs of damage due toradiation. The current measurements have been applied (as mentioned) to allinputs and outputs of a device, separately. For an example, if the drain cur-rent is decreasing under irradiation, there are some signs that the conductionchannel may be destroyed. Such a device is not able to operate under neutronfluence. In the next figures the currents as a function of time are shown (fig.4.15. - 4.17.). The first 18 hours (27.06.2008, 16:00) no irradiation is applied.

Figure 4.15: Currents vs. time, IHP

A very good example for sensitivity to irradiation is the Triquint die tech-nology, where the gain curve is exactly following the drain current curve (com-pare fig. 4.16 left and 4.10. but note the difference in scales). Similar behaviour


can be observed for the HEC and the Triquint plast. devices. The explana-tion is more complicated for the HEC system as it contains the preamplifierand the summing preamplifier in the same device. As discussed in chapter 2,

Figure 4.16: Currents vs. time, Triquint

typical effects of radiation damage starts to get visible. The consequence ofthe change in the source-drain currents is the change in the gain or the inputreflection. The IHP transistors and the AMS preamplifier had stable currentsand therefore stable gain and linearity.

Figure 4.17: Currents vs. time, HEC and AMS (Vsup is the set current, the in andout currents are induced by the transistor.

Chapter 5

Discussion, in general

5.1 Comparison of the two radiation test re-

sults, errors

The values of some relevant parameters of the neutron fluence are shown in thetable 5.1. The cyclotron current, which is directly connected to the neutron

Run ir. time I INFD1 Q INF∗ INF Δ[h] [μA] [109 n

cm2μC] [C] [1015 n

cm2 ] [1015 ncm2 ] [%]

feb.’08 39.02 8.95 3.77 1.257 4.73 5.51 -

jun.’08 41.59 11.29 3.66 1.690 6.19 7.19 6.11

Table 5.1: Comparison of the two radiation tests

flux density (NFD), was 26,15% higher than in the first run. The irradiationtime of the second run was 6,59% (9252 s) longer. It is to be expected thatour fluence at the first slot for the 2nd run is 32,74% higher than in the 1strun. This would yield a flux of 7, 31 · 1015n/cm2. We have got slightly less,7, 19 ·1015n/cm2. The difference comes from the different distance between thedevices in the first slot and the target of the beam. The distance in the firstrun was < 0, 5 mm, in the second ≈ 3 mm. If we rise 5, 51 · 1015n/cm2 for30,5% we get our measured flux of 7, 19 · 1015n/cm2. The difference of 2,24%which is 0, 16 · 1015n/cm2 of the measured flux is in good contribution withthe simulation I made for the distribution of the INF over the slot positions((0, 20 ± 0, 05) · 1015n/cm2). The result is 3, 0 ± 0, 5 mm [46].

55

56 CHAPTER 5. DISCUSSION, IN GENERAL

Δ in table 5.1. is only the error for 1 MeV flux. The error of the slot positiondependence (based on the RADMON Si diodes measurement) is ±7% [50]. Thesystematic errorin the first approximation with respect to these errors may beexpressed as a quadratic sum:

σ =√

0.072 + 0.0612 = 0.0929 = 9.29%

Therefore the estimated neutron fluence in the second test is:

INF = (7.19 ± 0.67) · 1015 n/cm2

5.2 Conclusions & further investigation

Following the investigation of few technologies for radiation hardness and out-put stability, some conclusions can be drawn. The Triquint technology, al-though used in the actual HEC seems not to be as radiation hard as requiredfor the HEC upgrade for SLHC. Neither the CFH800 plastic version nor the ce-ramic version had stable gain after radiated with neutrons. The reason mightbe in the design that prevents the currents flow through the conducting chan-nel or (as mentioned) a problem in the operating points of the transistors. It ispossible that for some operating points these kind of transistors behaves as aShottky diode where the metal-semiconductor region is behaving as a resistor.Further tests will aproove or reject this assumption.

In the near future more technologies will be studied for radiation hardness(IBM, SIRENZA, etc.). In our tests we have shown that Si and SiGe transistorsshow good radiation hardness up to 7 · 1015 n/cm2 (the range studied so far).Examples like IHP and AMS3 are rather radiation hard. Further points haveto be considered:

• signal noise,

• tolerance in production

Finally the radiation test have to be carried out at liquid Ar temperatures.Only then a technology may be selected for the new generation of the HECintegrated circuits.

Chapter 6

Appendix

6.1 Conversion table for dB

A useful table for direct conversions from dB to Relative voltage level (RVL)for some often appearing values. The RVL shows the multiplikation factor tobe applied on the voltage for a certain value expressed in dB.

dB 0 0.5 1 3 6 10 20 30 40RVL 1.00 1.06 1.12 1.41 2,00 3.16 10.0 31.6 100

Table 6.1: Conversion table for dB

Conversion formula:

Gain(dB) = 20 · log10

(Vout

Vin

)

The input and output impedance in the voltage calculation has to be con-sidered (usual 50 Ω (as in our case) or 600 Ω).

6.2 Conversion table for dBm to mV

The unit dBm is a Relative power level based on 1 mW . The voltages in mVin the table are effective values.

dBm 10 5 1 0 -1 -5 -10 -20 -35mV 707 398 251 224 199 126 70.7 22.4 3.98

Table 6.2: Conversion table for dBm to mV

The recalculation to A for dynamic input impedance using the s11 param-eter for a z0 = 50 Ω system:

Uin = 0.224 V

57

58 CHAPTER 6. APPENDIX

Uout = 0.223 V · 10X(dBm)

20

ZD = Z0 ·∣∣∣∣1 + s11

1 − s11

∣∣∣∣The resulting current is:

I =Uout

ZD

For every measured point (in our case 1600 points) of the input power expressedin dBm, a value for the s11 parameter has to be implemented.

List of Figures

1.1 The Standard Model . . . . . . . . . . . . . . . . . . . . . . . . 31.2 The Standard Model: the fundamerntal forces . . . . . . . . . . 41.3 The ATLAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.4 Energy loss of a muon in Cu. . . . . . . . . . . . . . . . . . . . 91.5 The e/π ratio for the HEC . . . . . . . . . . . . . . . . . . . . . 121.6 A schematic view of the ATLAS calorimeter system. . . . . . . . 141.7 The ionisation signal in ATLAS . . . . . . . . . . . . . . . . . . 151.8 The HEC wheel . . . . . . . . . . . . . . . . . . . . . . . . . . . 171.9 A HEC module . . . . . . . . . . . . . . . . . . . . . . . . . . . 181.10 The HEC LAr gap . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.1 A schematic view of electronic chain in ATLAS. . . . . . . . . . 222.2 An upper view of the HEC chip BB96. . . . . . . . . . . . . . . 232.3 The cross section of the n-channel MOSFET. . . . . . . . . . . . 252.4 CMOS and bipolar transistors . . . . . . . . . . . . . . . . . . . 262.5 A positive charge layer trapped in the gate oxide of an nMOS

transistor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.1 The setup for the first irradiation test . . . . . . . . . . . . . . . 313.2 The principle of the Two-port network . . . . . . . . . . . . . . 333.3 The NFD vs. energy. . . . . . . . . . . . . . . . . . . . . . . . . 353.4 The INFD vs. energy. . . . . . . . . . . . . . . . . . . . . . . . 363.5 First run: neutron fluence vs. time . . . . . . . . . . . . . . . . 363.6 Gain over frequency, HEC and IHP . . . . . . . . . . . . . . . . 383.7 Gain over frequency, Triquint . . . . . . . . . . . . . . . . . . . 393.8 The linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.9 Reflection and gain under neutron fluence . . . . . . . . . . . . 413.10 Linearity slope . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.11 Linearity of the HEC system . . . . . . . . . . . . . . . . . . . . 423.12 Slope of the HEC system . . . . . . . . . . . . . . . . . . . . . . 42

4.1 The setup for the second irradiation test . . . . . . . . . . . . . 454.2 NFD vs. E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464.3 INFD vs. E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464.4 Neutron fluence vs. time . . . . . . . . . . . . . . . . . . . . . . 48

59

60 LIST OF FIGURES

4.5 Gain over frequency. . . . . . . . . . . . . . . . . . . . . . . . . 494.6 The linearity of Triquints . . . . . . . . . . . . . . . . . . . . . . 494.7 The linearity of the HEC system. . . . . . . . . . . . . . . . . . 504.8 The linearity of the IHP’s. . . . . . . . . . . . . . . . . . . . . . 504.9 The linearity of the AMS preamplifiers. . . . . . . . . . . . . . . 504.10 Reflection and gain under irradiation, all technologies together. . 514.11 The slope under irradiation: Triquint CFH800 . . . . . . . . . . 524.12 The slope under irradiation: AMS . . . . . . . . . . . . . . . . . 524.13 The slope under irradiation: IHP . . . . . . . . . . . . . . . . . 524.14 The slope under irradiation: HEC and Triquint die . . . . . . . 534.15 Currents vs. time, IHP . . . . . . . . . . . . . . . . . . . . . . . 534.16 Currents vs. time, Triquint . . . . . . . . . . . . . . . . . . . . . 544.17 Currents vs. time, HEC and AMS. . . . . . . . . . . . . . . . . 54

List of Tables

3.1 INF (neutron fluence) depending on the slot position . . . . . . 37

4.1 INF (neutron fluence) depending on the slot position. . . . . . . 474.2 Current-flux- and RADMON- measurements . . . . . . . . . . . 47

5.1 Comparison of the two radiation tests . . . . . . . . . . . . . . . 55

6.1 Conversion table for dB . . . . . . . . . . . . . . . . . . . . . . 576.2 Conversion table for dBm to mV . . . . . . . . . . . . . . . . . 57

61

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.

Erklarung

Hiermit erklare ich, dass ich die vorliegende Arbeit selbststandig verfasst undkeine anderen als die angegebenen Hilfsmittel und Quellen verwendet habe.

Munchen, am 29. Oktober 2008.

66

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