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Technische Hochschule N¨ urnberg - Georg Simon Ohm Bachelorarbeit im Fach

Angewandte Mathematik und Physik:

Investigation of the Impact of Surface Properties of a Gas Electron Multiplier on

its Performance

DESY Deutsches Elektronen-Synchrotron Ein Forschungszentrum der Helmholtz-Gemeinschaft

Notkestraße 85 22607 Hamburg

Vorgelegt von: Lisa Waldm¨ uller Matrikelnummer: 2547540

Abgabe: 28.07.2017 Betreuer : Ralf Diener

Erstgutachter: Prof. Dr. Norbert Koch

Zweitgutachter: Prof. Dr. Oliver Natt

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Abstract

A Time Projection Chamber (TPC) is foreseen as the main tracking detector in the International Large Detector (ILD) - one detector concept for the planned International Linear Collider (ILC). This TPC will use Micro Pattern Gaseous Detectors (MPGDs) for amplification, of which Gas Electron Multipliers (GEMs) are one possibility. This thesis is embedded in a series of studies on the long term high voltage stability of GEM foils and efforts to improve it. A possible candidate to avoid or lessen the negative effects of electrical discharges is the coating of the GEM copper surface with a thin layer of copper oxides. Such a coating presumably would offer a simple possibility to increase the GEM stability, since it can easily be applied by heat treating the foils in air. In this thesis, GEM foils were systematically heat treated and results are described. Firstly, the effect of different parameters as duration and temperature of the heat treatment on the oxide layer was studied. The GEMs foils were analyzed regarding their surface properties in terms of roughness, thickness, composition and resistivity. The results are described in detail and compared to the properties of untreated GEMs. Secondly, the high voltage stability and gas gain performance of untreated and heat treated GEMs are compared.

As part of the high voltage stability tests the nature of shorted GEMs is investigated as well. The hope to find a set of parameters for a simple baking procedure which is able to significantly improve the GEM performance in all aspects can not be confirmed within the work presented here.

Kurzfassung

Eine Zeitprojektionskammer (englisch: TPC) ist als zentraler Spurendetektor f¨ur den In- ternational Large Detektor (ILD) - ein Detektor Konzept, welches f¨ur den International Linear Collider (ILC) entwickelt wurde - vorgesehen. Diese TPC wird Micro Pattern Gaseous Detectors (MPGDs) als Verst¨arkungssystem verwenden, genauer gesagt Gas Electron Multiplier (GEMs), welche eine spezielle Form von MPGDs darstellen. Diese Arbeit versucht die Hochspannungsstabilit¨at von GEMs zu verbessern. Eine M¨oglichkeit, die negativen Effekte von elektrischen Entladungen zu verringern oder sogar zu ver- meiden, ist die Beschichtung der GEM Oberfl¨ache mit einer d¨unnen Schicht Kupfer- oxid. Eine derartige Schicht kann durch Hitzebehandlung der Folien in Luft relativ einfach und kosteng¨unstig aufgebracht werden. GEM Proben werden in einer system- atischen Testreihe thermisch behandelt. Es folgt eine Analyse der Effekte, die durch

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beschrieben und mit denen unbehandelter GEMs verglichen. Zus¨atzlich werden die Hochspannungsstabilit¨at und die effektive Gasverst¨arkung von hitzebehandelten und un- behandelten GEMs einander gegen¨ubergestellt. Im Rahmen der Hochspannungstests wer- den kurzgeschlossene GEMs optisch und elektrisch charakterisiert. Im Zuge dieser Arbeit konnte keine Kombination der Backparameter Temperatur und Zeit gefunden werden, die die GEM Performance verbessert.

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Contents

1 Introduction . . . 1

2 The Time Projection Chamber . . . 3

2.1 Ionization . . . 4

2.2 Electron Drift . . . 5

3 Gas Electron Multipliers . . . 8

3.1 Working Principle of GEMs . . . 8

3.2 GEM Stacks . . . 8

3.3 Charge Transfer and Amplification . . . 9

3.4 Ion Backdrift . . . 10

3.5 Discharge Probability . . . 11

4 Experimental Setup . . . 13

4.1 Geometrical Parameters of GEMs . . . 13

4.2 Capacity of a GEM . . . 14

4.3 GEM Modules . . . 14

4.4 High Voltage Conditioning . . . 15

4.5 Shortcut Resistance and GEM Curing . . . 16

5 Improvement of the High Voltage Stability of GEM foils by Heat Treatment 17 6 Surface Properties of Copper Coated Foils . . . 18

6.1 Heat Treatment of GEM and Dummy GEM Foils . . . 18

6.2 Color Changes of Oxidized Copper Layers . . . 18

6.3 Thickness of the Oxide Layer . . . 20

6.3.1 X-Ray Diffraction: Method . . . 20

6.3.2 X-Ray Diffraction: Results . . . 21

6.3.3 Ellipsometry: Working Principle . . . 22

6.3.4 Ellipsometry: Results . . . 24

6.4 Surface Roughness of Oxidized and Non-oxidized Dummy GEM Foils . . . 28

6.4.1 Perthometer Working Principle . . . 28

6.4.2 Perthometer Measurements: Results . . . 28

6.5 Optical Inspection and Composition Investigation of the Surface 30 6.5.1 Scanning Electron Microscope: Working Principle . . 30

6.5.2 Scanning Electron Microscope: Results . . . 30

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6.6.1 Working Principle and Measurement Setup of the

Van Der Pauw Method . . . 35

6.6.2 Results of the Resistance and Resistivity Measurements 36 6.7 Conclusion of the Investigation of the Surface Properties of Cop- per Coated Foils . . . 39

7 Comparison of Oxidized and Non-Oxidized GEMs concerning High Voltage Stability and Gas Gain . . . 40

7.1 Characteristics of analyzed GEM foils . . . 40

7.2 High Voltage Stability . . . 41

7.2.1 Experimental Setup and Data Analysis . . . 41

7.2.2 Results of the High Voltage Stability . . . 42

7.2.3 Summary and Conclusion of the High Voltage Stability Tests . . . 43

7.2.4 Protection Resistance . . . 44

7.2.5 Discussion of the Nature of Shortcuts . . . 45

7.3 Gain Measurements . . . 49

7.3.1 Experimental Setup of Gain Measurements . . . 49

7.3.2 Results of Gain Measurements . . . 53

7.3.3 Charge Up Effects . . . 56

7.4 Conclusion of the Comparison of Oxidized and Non-Oxidized GEMs . . . 56

8 Summary and Conclusion . . . 57

List of Figures . . . i

List of Tables . . . iv

References . . . v

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List of Abbreviations

CERN Conseil Europ´een pour la Recherche Nuclaire CFEL Center for Free-Electron Laser Science

CUMO Current Monitor

DESY Deutsches Elektronen Synchrotron EDX Energy Dispersive X-ray spectroscopy FLC Forschung mit Lepton Collidern GEM Gas Electron Multipliers

ILC International Linear Collider ILD International Large Detector LP Large Prototype

MPGD Micro Pattern Gaseous Detector NTC Negative Temperature Thermistor PCB Printed Circuit Board

PE Polyethylene

PTC Positive Temperature Thermistor PTFE Polytetrafluoroethyelene

SEM Scanning Electron Microscope SHV Safe High Voltage

TDR Tesla TDR Gas

TPC Time Projection Chamber

XPS X-Ray Photo Electron Spectroscopy XRD X-Ray Diffractometry

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1 Introduction

The International Linear Collider (ILC) is a possible future project in high energy physics planned as an electron positron collider with a center of mass energy of 500 to 1000 GeV in its upgraded version.

One of the two proposed detector concepts for the ILC is the International Large Detec- tor (ILD). A time projection chamber (TPC) is foreseen as main tracking detector of the ILD [A+13]. The advantages of a TPC are a high number of tracking points with a good point resolution, which allows for an excellent pattern recognition. The end plates of the TPC include an amplification structure to produce a signal which is large enough to be detected. In the ’Forschung mit Lepton Collidern’ (FLC) group at Deutsches Elektronen Synchrotron (DESY), gas electron multipliers (GEMs) are the mainly investigated ampli- fication system.

While GEMs have been shown to work reliably, it is essential in the development of a new GEM-based readout structure to study and ensure its long term high voltage stability.

The present TPC GEM modules have been shown to work stable during a several week long period. The current step is to study and possibly improve their long term stability, since a damaged module with a broken GEM foil cannot be exchanged easily in a running experiment and would cause inefficiencies of the detector.

At DESY, GEM stability studies were performed under extreme conditions to provoke a high statistics of discharges. While most discharges do not cause relevant damage to the GEM foils, a very small number of them can cause a permanent damage by creating a shortcut between both sides of the foil. In these studies, it was also observed that darkened spots arise on the GEM surface after several discharges in the same area. This phenomenon is exemplified in Figure 1. These dark spots have be identified to be a thin layer of copper oxide. Even after a large number of discharges on these already oxidized areas, the GEM stays stable and is not destroyed by further discharges. Two Large Prototype (LP) GEMs [DBC+12] - one GEM with four working sectors and one GEM with 3 working sectors - were oxidized and exposed to several ten thousand discharges without being shorted, whilst normally GEMs have been destroyed already after a few hundred discharges. This observation arose the question, whether GEMs can be protected by such a copper oxide layer.

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(a) (b)

Figure 1: (a) Discharges on a GEM, visible as light spots. (b) Dark, oxidized spots at the region where many discharges happened [Fed17].

In the scope of this thesis, GEMs were systematically heat treated. The copper oxide layer on GEMs grown by this heat treatment is characterized and analyzed concerning its properties as color, roughness, thickness, composition and resistivity. The measurement methods are described and their results are discussed. Furthermore, the influence of the oxide layer on the performance of the GEMs regarding high voltage stability and gas gain is investigated and compared with untreated GEMs.

First, the working principles of TPCs (cf. section 2) and GEMs (cf. section 3) are explained.

This is followed by section 4, where the properties and the handling of the studied GEMs are introduced.

Section 5 includes an illustration of the theory behind the formed oxide layer.

In the following section 6, the copper oxide layer is characterized and the results of several measurement to quantify the properties of the oxide layer are reflected.

In section 7, the parameters of the oxidation of the GEM foils are listed and their high voltage stability and gas gain performance are discussed including a detailed description of the respective measurement methods.

The last section 8 summarizes the results and give a conclusion.

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2 The Time Projection Chamber

A Time Projection Chamber (TPC), as invented in the 1970s [Nyg74], is a tracking detector for charged particles based on ionization principles. TPCs consist of a gas filled volume with two electrodes as end caps, the cathode and the anode. The fundamental working principle of a TPC is illustrated in Figure 2.

Figure 2: Working principle of a time projection chamber [Sch05].

The cathode and the anode - in combination with the field cage - generate an homo- geneous electric field in the sensitive volume. An energetic charged particle passing the cylinder ionizes the gas along its path. The resulting electrons drift through the applied electric field towards the anode. Since a single electron cannot be detected due to the small amount of charge, the electron current have to be amplified. The signal of the amplified charge on a segmented readout plane (cf. Figure 2) results in the 2-dimensional projection of the particle trajectory. The working principle and setup of the amplification structure is explained in the next chapter. From the drift time tD and the drift veloc- ity vD of the electrons in the gas, the third dimension of the particle trajectory can be reconstructed:

z =vD·tD (1)

The drift time tD can be calculated by the arrival time of the electrons at the anode t1 and the time t0 when the particle crossed the sensitive volume. The time t0 cannot be measured by the TPC itself and has to be measured by an external detector. This detector has to have a sufficient time, and - in the case of experiments with multiple particles crossing the sensitive TPC volume during one measurement - space resolution,

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so the external information can be matched to the corresponding TPC measurement. In collider experiments, this external information comes from the inner vertex silicon detector with very good time and space resolution (cf. [ALI17a]). In prototype setups with low occupancies, a simple scintillator detector is mostly sufficient.

tD =t1−t0 (2)

Parallel to the electric field a magnetic field is applied. Due to the Lorentz force, the path of the charged particle crossing the sensitive volume is bent, depending on its momentum.

The measured curvature allows the reconstruction of the particle momentum. In order to obtain precise results and a reconstruction of the tracks, both the electric field has to be precisely adjusted and very homogeneous, and the magnetic field must be well known at high precision, respectively [DES17, Sau14].

2.1 Ionization

Fast, charged, heavy particles traversing matter produce inelastic hits with the shell electrons of the atoms of the crossed material. As a consequence, these atoms get excited or ionized. Thus, the particles sustain an energy loss, which depends on the velocity and charge of the particle and on the target material. The energy loss dE per unit of path length dx is approximately given by the relativistic Bethe-Bloch formula including two corrections [Leo94].

−dE

dx = 2πNar2emec2ρ· Z A · z2

β2 ·

ln

2meγ2v2Wmax I2

−2β2−δ−2C Z

(3) with

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E : energy of the particle x : path length

Na = 6.022 x 1023 mol−1 : Avogadro’s number re = e2

0mec2 = 2.817x10−15 m : classical electron radius me,0 : rest mass of the electron

c : speed of light

ρ : density of absorbing material

Z : atomic number of absorbing material A : atomic weight of absorbing material

z : charge number of the particle β : v/c of the incident particle γ : 1/p

1−β2

v : velocity of the particle

Wmax : maximum energy transfer in a single collision I : mean excitation energy of the material δ : density effect correction

C : shell correction

The formula is only valid under two assumptions: first, the ionizing particle has to be significantly heavier than an electron m me. Second, the gas molecules have to be at rest.

Figure 3 illustrates the energy loss versus momentum curves of different particles. The measurements were made in the ALICE TPC [ALI17b] at CERN (Conseil europ´een pour la recherche nuclaire). With the help of the reconstructed momentum and the measured specific energy loss, the particles - cataloged in Table 1 - can be identified, already by parameters only determined in the TPC.

2.2 Electron Drift

Due to the applied electric field, the free electrons are forced to move along the field lines towards the anode and the readout plane.

Due to scattering off the signal electrons off the gas molecules, the electron cloud gets larger with drift distance. Due to this diffusion, the arriving signal at the anode is smeared out. Due to the Lorentz force of the applied magnetic field, the diffusion is reduced in comparison to the field-free case, which results in a better spatial resolution.

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Figure 3: Specific energy loss (dE/dx) versus particle momentum. The measure- ment was performed in the ALICE TPC in pp collisions at a center of mass energy of √

s = 13 TeV and an applied magnetic flux density of B = 0.2 T. The lines show the parametrization of the expected mean energy loss [Col15].

Particle Charge Rest mass

e Electron 1 e = 1.6022·10−19 0.5 MeV

µ Muon 1 e 105.7 MeV

K Kaon 1 e 493.7 MeV

p Proton 1 e 938.3 MeV

d Down -1/3 e 4.7 MeV

t Top +2/3 e 173.7 GeV

π Pion 1 e 139.6 MeV

Table 1: Charge and rest mass of the particles detected in the TPC compared to

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As mentioned, the electrons experience the Lorentz force and scatter off the gas molecules.

These two effects are included in the Langevin equation [BRR08], which describes the drift velocity of the electrons:

med~v

dt =e ~E+e

~ v×B~

−K~v (4)

with

me : mass of the electron

e = 1.6022 · 10−19 As : elementary charge

~v : velocity of the electron E~ : applied electric field B~ : applied magnetic field

K : frictional force coefficient, caused by the interaction with the gas;

K = me

τ , whereτ denotes the mean time between collisions.

To calculate the drift velocity of the electrons, the electric field, the magnetic field and the used gas have to be known. Keeping these parameters constant stabilizes the drift velocity which is needed for a precise reconstruction.

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3 Gas Electron Multipliers

As mentioned before, the sole electron current produced by first order ionization is too small to be measured and therefore, it has to be amplified. Gas Electron Mul- tipliers (GEMs) were introduced as a new amplification structure in 1996 by Fabio Sauli [Sau97].

3.1 Working Principle of GEMs

In general, GEMs consist of an insulator foil coated with a conductor on both sides.

The foil is micro structured to form a grid of holes. Thus, GEMs can be described as a perforated capacitor. By applying a voltage over the two sides of a GEM foil, an electric field establishes. The field distribution and equipotential lines of a typical, double conical GEM hole is depicted in Figure 4. Simulations of the field distribution were performed by [Sob02] using the simulation tools Maxwell [Max17] and Garfield [Vee98].

The maximum of the electric field is located at the edges of the holes. Due to charge up effects caused by residual electrons accumulating on the insulator in the GEM holes, the electric field in the amplification region is further increased.

The electrons drift through the sensitive volume of the TPC (on top of the GEM) into the hole and after the amplification in the hole through the transfer field (bottom) towards the next GEM.

3.2 GEM Stacks

Usually, GEM amplification structures are build as stacks of more than one GEM foil.

Using several GEMs in a row, the same or higher effective gains can be reached by applying less voltage difference over the individual GEMs. Therefore, the discharge probability decreases and a GEM stack is more stable than a single GEM, achieving the same effective gain. Depending on the application and required gain, GEM stacks consisting of two to four GEMs are most commonly used.

The field between the cathode and the topmost GEM is called drift field. The electrical field between two GEMs is called transfer field, for illustration see Figure 4. The field between the last GEM (III) and the pad plane is called induction field.

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Figure 4: Electric field distribution (red) and equipotential lines (green) of two GEM holes with 1 kV/cm drift field strength, 6 kV/cm transfer field strength and 250 V applied over the GEM [Sob02].

3.3 Charge Transfer and Amplification

By applying a voltage difference of the order of several 100 V across the two copper surfaces, a strong electric field of several 10 kV/cm is generated inside the GEM holes.

This electrical field allows for proportional gas amplification inside the holes, and effective gains in the order of a few thousand can be achieved. The effective gain is described by the product of the charge transfer coefficients: Geff =C·G·X, with C, G and X being explained below, respectively.

Charge Transfer Coefficients:

To describe the charge transfer of electrons in GEMs, the following coefficients are needed [KLM+03]:

• The collection efficiencyCdescribes the fraction of electrons collected into the GEM holes:

C= Necollected into the hole

Nein the GEM volume

(5) The collection efficiency is illustrated in Figure 5 (a). Here, three out of four elec- trons get into the GEM hole, so C is 75%.

• The extraction efficiencyX denotes the fraction of charged particles extracted from

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Figure 5: Visualization of charge transfer coefficients factors. (a) collection effi- ciency C, (b) gain G, (c) extraction efficiencyX. The values used in this scheme are not related to physically relevant values, but are chosen for illustration. In this case, the effective gain was Geff = 1.5 [Vog08].

the GEM:

X = Neextracted from the GEM

Nein the GEM hole

(6) Figure 5 (c) exemplifies the extraction efficiency. Here, four out of six electrons do not end upon the GEM surface, hence X = 67%.

• The gainGof a GEM is the ratio of electrons getting into a GEM hole and electrons after the gas amplification:

G= Nein the hole, after the gas amplification

Necollected in the hole

(7) G is illustrated in Figure 5 (b). Here, one electron multiplies into three, soG is 3.

3.4 Ion Backdrift

Besides the electrons, each amplification system creates the same number of positive charged ions. Due to the applied electric field, these ions drift in the opposite direction as the electrons. As few ions as possible should reach the sensitive volume of the TPC, since the resulting field distortions must be avoided to guarantee a precise track reconstruc- tion. The ions would disturb the homogeneity of the electrical field and in addition can recombine with the primary signal electrons. In principle, the transfer coefficients defined in 3.3 are also valid for the ions. Due to their high mass, the ions follow preferably the electric field and are hardly effected by the magnetic field. Therefore, most of the ions end

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If the ion backdrift should be further decreased, so-called ion gates can be used. A recent development is the so-called gating GEM with a very high optical transparency [Aok17].

They are placed between GEM I and the drift volume of the TPC to catch as many ions drifting back towards the TPC volume as possible. Besides prevention of the ion backdrift, the gain also influences the choice of the best possible voltage settings of a GEM.

3.5 Discharge Probability

The criterion limiting the maximum gain of a GEM is called Raether limit: a self-sustained discharge in a gas is initiated, when the gain becomes larger than 108[Leo94]. Approaching this limit, a so-called streamer breakdown takes place. During this breakdown, an electron avalanche creates a narrow, bright plasma between the two copper electrodes of a GEM.

Since the plasma is conducting with a small resistivity, it causes a current flow between the two copper sides of a GEM and with it a discharge.

Such discharges should be avoided by usage of reasonable voltage settings for several reasons. On one side, the detector has a dead time after a discharge until the potentials on the GEMs are ramped up again to operational values. The worst however case is a destructive discharge, resulting in a stable conducting connection between the two metal sides of a GEM. A GEM which has experienced a destructive discharge becomes unusable as it cannot be charged any more.

The following reasons influence the discharge probability:

• The gas parameters in the detector chamber: gas purity, humidity level, temperature and pressure. In experiments, a so-called slow control system is implemented. It monitors and controls the detector parameters which show a change on large time scales and not in single measurements [Sch05].

• If the applied voltage over the both sides of a GEM is too high, the dielectric strength is exceeded and the electric field breaks down. Therefore, the voltages of the electrodes should not be set higher than 90 % of the dielectric strength.

• The applied voltage to establish the induction field has to be set carefully as well. If this voltage is set too high, a discharge is propagated through the whole structure.

In argon based gases, which are mostly used, an induction field below 5 kV/cm has demonstrated to provide stable operation conditions [Hal10].

• In the case of GEMs produced by an etching process, not all of the holes are per- fectly aligned and double conical. Shifts of the holes can cause discharges as well.

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Since etching is a chemical process, it cannot be influenced without implementing a completely new method at the manufacturer (CERN Workshop).

• After a discharge, the gas is ionized. This ionization leads to even more discharges.

Thus the voltage is ramped down after a discharge, to prevent the above mentioned effect.

• Dust residues in the GEM holes facilitate the probability of a discharge. Due to this, a careful handling of GEMs in clean conditions is highly important.

Summarizing, the discharge probability can be reduced by careful handling of the GEMs, properly chosen voltage settings and controlling the gas parameters.

The more charge is transferred during a discharge, the higher the probability of a destruc- tive discharge. The transferred charge of a discharge is proportional to the charge stored in a GEM. This charge depends on the capacity of the GEM (cf. section 4.2), which can be limited by choosing a not too large size of the foil.

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Figure 6: Geometry of a standard CERN GEM: given are inner and outer hole diameter, pitch and row distance as well as the thickness of foil and substrate in units of µm [Web03].

4 Experimental Setup

The following describes the parameters of the GEMs used in this thesis and their sur- roundings.

4.1 Geometrical Parameters of GEMs

The used GEMs consist of a polyimide foil covered on both sides with copper. The given parameters describe a so called ’standard’ CERN GEM [Eur17]. One available standard size for small prototypes is 10 x 10 cm2. The polyimide substrate, mostly KaptonR developed by DuPont, is 50 µm thick and the coated copper layer on each side is 5 µm thick. Double conical holes with an outer diameter of 70 µm and an inner diameter of 50 µm are etched into the copper plated foil. The pitch - distance between the center of two neighboring holes - is 140 µm long. Figure 6 illustrates the geometry of a GEM.

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4.2 Capacity of a GEM

All relevant dimensions of a GEM to calculate its capacity are given in Figure 6. The area of one GEM hole is Ahole = 3.85·10−9 m2. In one GEM of 10 x 10 cm2, there are 826 rows with 714 holes in each row. This sums up to about 590000 holes in a standard CERN GEM.

The copper area of one GEM is:

Acopper = (0.1 m)2−(590000·3.85·10−9 m2) = 7.7·10−3 cm2

Using the copper surface size, the vacuum permittivity 0 = 8.854... · 10−12 As/Vm, the relative permittivity of the dielectric medium, in this case Kapton r = 3.4, and the thickness of the Kapton foil d = 50 µm, the capacity of a GEM is calculated as follows:

CGEM =0r·A

d = 4.7 nF (8)

The energy of a discharge increases with the energy stored in a GEM, which depends on the capacity of the GEM. If the GEMs get significantly larger than 10 x 10 cm2, they can be sectored on one copper surface. This reduces the capacity and thereby the stored energy.

4.3 GEM Modules

As mentioned, the GEMs typically are stacked. The stacks are mounted in so-called readout-modules. These modules are composed in addition of a readout printed circuit board (PCB) and an aluminum backframe. Between the GEMs, there are frames to support the GEMs. Detailed studies of the influence of the frames were made [Hal10].

The readout board is connected to the readout electronics of the TPC. The backframe including an O-ring is needed to ensure a gas tight connection to the TPC end plate. This setup, developed at DESY [M¨ul16] is sketched in Figure 7.

In this thesis, GEM I depicts the GEM facing the TPC volume. The middle GEM is named GEM II and GEM III is the GEM closest to the anode.

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Figure 7: Setup of the readout-modules of the TPC [M¨ul16].

Figure 8: Electric circuit for the so-called burn-in of GEMs

4.4 High Voltage Conditioning

Prior to GEM stack assembly, the individual GEM foils are tested. On one side, it is tested whether the GEM is actually working and functional, even at high voltages. On the other side, little impurities get burned away by the current during non-damaging discharges. This burn-in process is important to ensure a good quality of GEMs used in experiments.

For the test and conditioning, a GEM is mounted in a gas tight box, which is connected to a gas and a power supply. The power supply used in all the setups described in this thesis is a CAEN SY2527. Between each side of the GEM and the channels of the power supply a protective resistor is connected in series. Hence, the stored charge in the safe high voltage (SHV) cables due to their capacity is impended to flow trough the GEM and the current while a discharge is limited by

IDischarge= UGEM

2·Rprotective. (9)

This means a limitation of discharge current to 30 µA for a voltage of 600 V and a typical resistor of 10 MΩ. Figure 8 sketches the circuit for GEM-conditioning.

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Shortcut Resistance Curable / Not Curable

6 10Ω Not curable.

few kΩ Not curable.

Resistance increases to the MΩ region during an applied current and is curable.

> 1 MΩ Resistance decreases to the kΩ region during an applied current and is no more curable.

Curable.

Table 2: Overview which resistances of shorted GEMs are curable.

By increasing the voltage step by step, the GEMs get conditioned. The applied voltage starts at 0 V and is slowly raised up to 600 V. After a discharge, the voltage is ramped down and the conditioning is continued from the last stable point onwards.

4.5 Shortcut Resistance and GEM Curing

A permanent conductive path between the two GEM sides after a discharge can be cured in certain cases. The idea behind this healing process is to burn away the conductive path of the two sides of the GEM. Usually, currents between 3 µA and about 25 µA are chosen for either a slow or fast curing process. In practice, it could be seen that reversing the current a few times can help the process. The resistance between the two surfaces of the GEM is either measured directly or calculated from the applied voltage and resulting current according to Ohm’s law taking into account the other components of the experimental setup:

RGEM =Rtotal−2·R1, with Rtotal = ∆V

I . (10)

The GEM also acts like a capacitor. Since its capacitance, the capacitance of the SHV cables and the capacitance of the power supply is very low, it can be neglected here.

Table 2 gives an overview which shortcuts are curable. This table is based on empirical studies.

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Cu Cu2O CuO

conductor semiconductor semiconductor Color metallic yellow to red-brown black

Melting point 1083 C 1232 C 1326 C Resistivity (Ωcm) 1.68 ·10−6 103−108 0.01 - 1

Table 3: Physical properties of copper, Cu2O and CuO [Baa17, LWM+11].

5 Improvement of the High Voltage Stability of GEM foils by Heat Treatment

As mentioned before, there was an indication that oxidized GEMs might be more stable against destructive discharges. The aim of the presented work is to test this hypothesis.

During a discharge, a plasma channel is built with a certain self resistance. The current of a discharge is given by the voltage difference between the two sides of the GEM divided by the resistance of the plasma, assuming that the resistance of the copper is negligibly:

IGEM = UGEM

Rplasma (11)

Copper oxide has a higher resistivity than copper (cf. Table 3). By growing a copper oxide layer onto the copper surface of the GEM, the current of a discharge could be reduced:

Ioxidized GEM= Uoxidized GEM

Rplasma+Rcopper oxide

(12) One model for a destructive discharge is the formation of a carbon connective layer due to carbonized Kapton caused by discharge chemistry [KHL+12]. Less energy results in a reduction of the heat, which reduces the probability to carbonize Kapton. Hence, the danger of a conducting connection between the GEM surfaces is reduced. In conclusion, growing a copper oxide layer - acting like a resistor - on the copper surfaces of a GEM foil could increase the stability of GEMs regarding discharges.

Table 3 displays several properties of copper and of the two stable, further investigated copper oxides Cu2O and CuO. Due to the higher melting point of copper oxide com- pared to copper, the hope is to not harm the copper oxide by the heat produced in a discharge.

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6 Surface Properties of Copper Coated Foils

In order to study the copper oxide coating on GEM foils systematically and its effect on the GEM operation and discharge stability, several sample pieces were produced. The sample pieces are made of the same basic material the actual GEM foils are manufactured from. These so-called dummy GEM samples were baked in air atmosphere at different temperatures and for different durations. In a second step, actual GEM foils were treated the same way. The produced samples were studied using different techniques to determine their properties as color changes, roughness, constitution, thickness and resistance. In the case of the GEM foils, also their operational properties were studied. In the following the baking process will be introduced followed by introductions to the different measurement methods and results.

6.1 Heat Treatment of GEM and Dummy GEM Foils

The used oven in this thesis is a vacuum drying oven from BINDER, model VD115 [BIN16, BIN17]. The oven offers the option to heat in vacuum, air or an inert gas. The possible end vacuum is 10−2 mbar. The GEMs and dummy samples were placed in the oven lying on an aluminum plate which allows to oxidize several samples at the same time.

The GEMs were clamped into holders with crocodile clamps to avoid scratches.

6.2 Color Changes of Oxidized Copper Layers

Both, dummy GEM foil and GEM foil samples show prismatic colors due to heating at different temperatures and durations. Figure 9 illustrates these color changes, sorted by annealing temperature and time, which are listed in Table4.

The different colors indicate the change of the oxide layer thickness and are induced by thin-film interference [Con27]: the light gets reflected by the upper and lower boundaries of a thin film, and the reflected light waves interfere. In this case, the upper border is the boundary between copper oxide and air and the lower border is the boundary between copper oxide and copper. This reflection and interference mechanism is illustrated in Figure 10. The reflected light waves interfere with each other due to the superposition principle. Depending on the thickness of the copper oxide layer, the extinction appears

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Dummy GEM foil GEM foil

Time [h] Temperature [C] Time [h] Temperature [C]

0.5 160 0.5 180

1 170 1 190

1.5 180 1.5 200

2 190 2 210

2.5 200 2.5

3 210 3

220 3.5

4

Table 4: Temperatures and durations of dummy GEM and GEM sample oxidation.

(a) Dummy GEM foils (b) GEM foils

Figure 9: Samples annealed at different temperatures and durations.

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Figure 10: Interference phenomenon by the reflection of visible light on thin films [Fuj].

6.3 Thickness of the Oxide Layer

To determine the thickness of the copper oxide layers, ellipsometry and X-ray diffrac- tion were applied. Both methods use the reflection behavior of a surface to measure the thickness of thin films. Since the dummy GEM and GEM foils are not flat and stiff but rather flexible, it is not possible to measure these samples with either of these meth- ods. To allow nevertheless for such measurements, additional samples were produced by sputtering copper on glass microscope slides. The used sputter coater is type K550 from Emitech [Emi99] and the measurement was done at the group ’Forschungsgruppe Grenzfl¨achenphysik’ of the University of Hamburg [Gre17]. This resulted in stiff and flat substrates covered with a copper layer of about 250 nm. These samples with an area of about 1 x 1.2 cm2 were oxidized in the same way the foil samples have been to measure the copper oxide thickness. The two different spectroscopic methods and their results are presented in the following.

6.3.1 X-Ray Diffraction: Method

X-ray diffraction (XRD) is an established and non-destructive procedure to determine the thickness of thin films and investigate their crystal structure.

The working principle is based on the diffraction of the X-rays on crystal structures.

There, X-rays show the same diffraction appearance as all electromagnetic waves. Diffrac-

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Figure 11: In plane X-ray diffraction measurement of the oxidized copper on glass sample. The left big peak results from copper oxide and the right one from copper.

act like a three dimensional diffraction grating. The X-rays get diffracted at the atomic electron shells and interferes. The path difference depends on the distance between the atoms. Consequently, whether it comes to constructive or destructive interference under given angles correlates with the distance of the atoms. Since crystals consist of three dimensional, periodic structures constructive interference appears only at certain angles fulfilling Bragg’s equation:

nλ= 2dsin(θ) (13)

with n being an integer, λ is the wavelength of the monochromatic X-ray, d is the dis- tance of the atomic planes and θ is the angle between the atomic plane and the X-rays.

The whole principle and derivation are described in detail for example in [Gie17]. The measurement was performed at the ’DESY-Nanolab’ [nan17].

6.3.2 X-Ray Diffraction: Results

First, the untreated copper layer sputtered on the glass substrate was measured. The thickness of the sputtered layer amounts to about 260 nm ± 13 nm.

Then, the sample was oxidized for two hours at 200 C and measured again. The results can be seen in Figure 11.

Figure 11 shows the radiation intensity depending on 2θ where θ denotes the angle be- tween the atomic plane and the X-rays. Here, two characteristic peaks are visible. The first one indicates Cu2O and the second one depicts copper. This shows that the copper layer of 260 nm is not completely oxidized.

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Figure 12: Illustration of in-plane and out-of-plane measurement methods [IKU+13]

To determine the thickness of the oxide layer, an out-of-plane measurement as illustrated in Figure 12 was necessary. The measurement did not deliver any further results since there are no clear oscillations visible, see Figure 13. The assumption why the thick- ness cannot be determined with this measurement is that the oxide surface has too high roughness. Therefore, ellipsometry has been applied as alternative method.

6.3.3 Ellipsometry: Working Principle

Ellipsometry is a spectroscopic measurement method, by which the dielectric function of materials as well as the thickness of thin layers can be determined. The dielectric function describes the permeability of electric fields of a material. Nearly each sample can be investigated, if the following requirements are fulfilled:

• The investigated sample has to have a plane, relatively smooth surface

• The sample structure has to be well-known concerning the materials in each layer The used ellipsometer is a ES-850 Spektralellipsometer from the company SenTech Berlin [SEN17]. The measurements were performed with the help of the ’Photon Science’ group from Center for Free-Electron Laser Science (CFEL) at DESY [CFE17].

Ellipsometry determines the change of the polarization status of light during reflection.

Normally, linearly or circularly polarized light is used. Usually, the light is elliptically

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Figure 13: Orange: out-of-plane XRD-measurement, blue: in-plane XRD- measurement.

Figure 14: Working principle of an ellipsometer. A light source is found on the left side of the sketch. The light gets polarized and reflected on the sample surface.

The reflected light is detected by a detector [Neu17].

ellipsometry is the following:

rp rs

= tan(Ψ)·ei·∆ (14)

Ψ and ∆ are the so-called ellipsometry-parameters. ∆ denotes the phase difference be- tween the parallel and perpendicular components of the reflected radiation and tan(Ψ) ratio of their amplitudes. rp and rs are calculated by the Fresnel’s equation out of the reflection and diffraction angles. The derivation can be found in detail in [Neu17].

To determine the thickness of thin layers, an appropriate optical model reproducing the structure of the measured sample is needed. After choosing a start value for the layer thickness in the model, this model is fitted to the measured data by adjusting the thickness value. Since the maximum of steps for fitting the thickness is 100 steps, the start value have to be chosen cautiously and not too far away from the actual thickness.

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Figure 15: Measurement of the complex dielectric function of copper sputtered on glass (red) and a not treated dummy GEM foil (orange) compared to the dielectric function of copper in the ellipsometer database (blue). The dielectric function is recorded with a wavelength between 300 and 800 nm (visible light).

6.3.4 Ellipsometry: Results

As with the X-ray diffractometer (cf. section 6.3.1), the untreated copper sputtered on glass was measured first. Figure 15 shows the dielectric function of the copper on glass sample in comparison with a dummy GEM foil and the dielectric function of copper in the ellipsometer database. The functions of the sputtered copper on glass sample and the database copper are comparable, especially for the region of the visible light, whereas the dummy GEM foil shows large deviation, most probably due to the nature of the sample being a flexible foil.

Figure 16 shows the dielectric function of two different samples of untreated copper sput- tered on glass. Basically the same result for two different samples can be observed which confirms the reproducibility of the sputtering process.

Figure 17 illustrates the dielectric function of one sample at three different angles of incidence. This data confirm again the reliability of the measurements since the noise is

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Figure 16: Ellipsometer measurement of the complex dielectric function of two different samples where copper was sputtered on glass. The dielectric function is recorded with a wavelength between 300 and 780 nm (visible light).

Figure 17: Ellipsometer measurement of the complex dielectric function of copper sputtered on glass with three different angles of incidence. The dielectric function is recorded with a wavelength between 300 and 780 nm (visible light).

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smooth surface and with it a clear reflection pattern. The fit to the measured data was based on a ’Bruggemann’ model, which describes a layer consisting of an host and an inclusion material. Here, a layer of a CuO host with 50 % Cu2O inclusions is assumed on a copper substrate. Figure 18 gives an example for a fit of Ψ and ∆ of measurements at three different angles of incidence. The start value for the thickness of the oxide layer was 100 nm. The 100 nm are chosen due to the tabulated thickness in Figure 28. The calculated layer thickness of the fit amounts to 69.42 nm after 100 steps. By changing the start value for the thickness of the oxide layer, fluctuations of 100 % appeared. These fluctuations imply a wrong choice of the assumed model. Besides, large deviations between the model fits and the measured data are visible (Figure 18). To deduce a reliable result of the layer thickness, the curves of measurement and fit should be identical. Therefore, the investigation of the composition of the layer regarding the different copper oxides is necessary. Since Cu3O2 was not available in the ellipsometer database, a model based on this oxide could not be realized.

Summary

It is difficult to use the ellipsometry method to determine the thickness of the copper oxide layer since the GEM foil itself as well as the dummy GEM foil is not flat enough to be characterized by this method. Gluing the foil samples on flat and stiff substrates as ceramic plates, glass plates and polished sapphire crystal also did not yield any usable samples. The sputtered copper on glass had a plane surface and a flat substrate which could be measured. After oxidizing the samples a reflection was seen but it was not possible to deduce a compatible fit function for the measured curve. Due to the unknown copper oxide components, the model did not fit. To get convincing results for the thickness of the oxide layer, the single oxides in the oxide layer have to be well known, which was not the case here.

It was shown that it is possible to generate a reproducible and measurable copper layer on glass. The oxidized sample also provides a surface with good reflection. Before further measurements, the composition of the copper oxide layer has to be studied in detail. This was not possible in the scope of this thesis.

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Figure 18: Fit and measured data of Ψ and ∆ depending on the wavelength.

Three measurements at different angles of incidence are shown including one fit for each angle.

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6.4 Surface Roughness of Oxidized and Non-oxidized Dummy GEM Foils

Since the surface roughness of the foil samples has an impact on the application of other surface measurement methods as, i.e., ellipsometry, different samples have been tested.

The surface roughness has been measured with a perthometer. For these measurements, an uninterrupted path over the sample surface is needed which is much longer than the pitch of the GEM holes. Therefore, only sample pieces without perforation have been used.

This includes an untreated sample as reference and samples which have been oxidized with different parameters to determine the effect of the oxide layer. The measurement principle is described in the next section followed by the results of different samples.

6.4.1 Perthometer Working Principle

A perthometer is used to determine the roughness of surfaces by a mechanical procedure.

The profile method uses a thin diamond probe tip. The tip slides over the surface with a constant velocity. The vertical positional-shift of the tip provides information of the roughness. To describe the surface, standardized characteristic variables are build out of the measured profile. The shape of the probe tip influences the measured result. Scratches thinner than the probe tip can not be measured. Therefore, the individual choice of tip for each sample is important. The measured track depends on the roughness of the surface:

the more plane the surface, the smaller the working section. The used perthometer is a M1 type from Mahr [Per17] and was provided by the group ’ZMQS Qualit¨atssicherung’

at DESY [ZMQ17].

6.4.2 Perthometer Measurements: Results

Four different samples where measured. One untreated dummy GEM foil sample, one sample oxidized for one hour at 180 C, one sample oxidized for one hour at 200 C and the last sample oxidized for two hours at 200 C. Table 5 illustrates the results of the measurement.

It is clearly visible that the surface gets rougher with higher temperature and longer oxidation time, which correspondents to thicker copper oxide layers (see section 6.2).

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Temperature Time Working sequence Mean Deviation in z

not treated 1.75 mm 200 nm

180 C 1h 5.6 mm 300 nm

200 C 1h 5.6 mm 440 nm

200 C 2h 5.6 mm 550 nm

Table 5: Results of the roughness measurement of dummy GEM foils.

Figure 19: Grain structure due to annealing copper [FEG+08].

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6.5 Optical Inspection and Composition Investigation of the Surface

To investigate the surface and composition of the oxide layer, a scanning electron micro- scope (SEM) with additional energy dispersive X-ray spectroscopy (EDX) is used. The working principle is introduced and the results are discussed.

6.5.1 Scanning Electron Microscope: Working Principle

A scanning electron microscope means an electron microscope, where the electron beam is conducted across the investigated, augmented object in a particular pattern. The interaction of the electrons with the object is used to create an image. Therefore, the secondary electrons generated by the interaction of the electron beam with the atoms of the surface are detected by a special detector. Since the secondary electrons set free by the primary electrons stem from the upper few nanometers of a surface, the generated pictures image the topography of the object [Bro02, GNE+81].

The used SEM is of type SIGMA out of the GeminiSEM family from ZEISS [Car17]. It was provided of the group ’Forschungsgruppe Grenzfl¨achenphysik’ of the University of Hamburg [Gre17].

6.5.2 Scanning Electron Microscope: Results

The holes in the GEMs are clearly shown in the images recorded with the SEM (cf. Figure 20). At the borders of the black holes, the beginning of the Kapton at the edges of the holes is visible.

Figure 20a shows a non-treated GEM foil. On the second picture, a GEM sample - oxidized for two hours at 200 C - is shown. Both pictures are recorded with an electron voltage of 6 kV and the same magnitude to be comparable. The only difference is the working distance with 6.7 mm for the non-treated sample and 6.9 mm for the oxidized GEM foil. This difference is mirrored in the resolution of the pictures. The gray scale deviation is due to a difference in the contrast and additionally the oxidized GEM is a little darker than the non-oxidized. Nevertheless, not much difference can be seen, except in the distribution of impurities.

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(a) (b)

Figure 20: SEM measurement of two different samples. While (a) is a not treated GEM foil, (b) represents a annealed GEM foil for two hours at 200 C.

Information about the composition of the surface can be gained by the use of the additional EDX unit of the SEM.

6.5.3 Energy Dispersive X-Ray Spectroscopy: Working Principle

The energy dispersive X-ray spectroscopy (EDX) is a measurement device for material analysis. An electron beam with known energy excites the atoms in the sample. These atoms emit X-rays with a specific energy for each element, the so-called characteristic X-rays. This radiation gives information about the elementary constitution of the inves- tigated sample [GNE+81]. There are three different measurement methods: point scan, area scan and line scan. A point measurement means a measurement method, where just one single point on the surface is measured contrary to area measurements where a certain area is scanned. In a line scan, the sample is investigated along one straight line.

The used EDX also is from ZEISS and is attached to the used SEM.

6.5.4 Energy Dispersive X-Ray Spectroscopy: Results

In order to determine the composition of several samples, the energy of the primary elec- trons was set to 6 keV for all measurements. This energy of the electron maximizes the abscissa - the energy of the characteristic X-rays. For each investigated sample there was a copper-peak (double-peak), an oxygen-peak and a carbon-peak seen in the spec- trum.

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Figure 21: EDX area scan measurement of two different samples. While (a) is a not treated dummy GEM foil, (b) represents an annealed dummy GEM foil (two hours at 200 C).

Figure 21 illustrates the EDX point measurement of two dummy GEM samples. Figure 21 (a) shows the measurement of a not treated dummy GEM foil. As expected, the largest fraction is copper, represented by the highest peak. The two other peaks depict oxygen (middle) and carbon (left). The oxygen and carbon peak are very small and found in each sample. Even right after sputtering copper on glass, a little oxygen and a little carbon peak were found.

After the baking process, the oxygen content rises as shown in Figure 21 (b), as expected due to the formation of copper oxide. The carbon peak remains at the same height.

Besides the dummy GEM samples, GEM foil samples were studied with point and line scans. Figure 22 (a) shows the measurement result GEM foil oxidized for 15 minutes, Figure 22 (b) shows a sample baked for two hours. Both samples were oxidized at 200C.

For the GEM samples, the same peaks are observed as for the dummy GEMs. For the GEM foil annealed for 15 minutes, the carbon and the oxygen peak are very small. As for the dummy GEMs, the carbon peak stays the same relative height despite the oxidation.

The oxygen peak rises significantly with oxidizing duration, as expected. The relevant

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(a) (b)

Figure 22: EDX point measurement of two different GEM foil samples. Both samples were oxidized at 200 C. (a) shows a GEM foil annealed for 15 minutes and (b) represents a GEM foil baked for two hours.

nitrogen and oxygen, these are the expected elements. Figure 23 shows the measurement of the Kapton and confirms the expected result.

The line scans in Figure 24 display a scan along one line for each sample. The shown GEM- sample in Figure 24 (a) is oxidized for 1.5 hours at 200C. The second one (Figure 24 (b)) is oxidized for 2 hours at 200 C. The scan starts at the left blue point and follows the green line over the GEM hole to the right blue point. Every 2 µm a measurement point was taken and the element composition was measured. There are three different elements visible: carbon, copper and oxygen. While the copper line (leaf green) is relatively high and constant for the area, where the copper surface of the GEM is measured, it is zero at the GEM hole. The carbon part (violet) is low over the copper surface, increases at the border of the hole, is zero for the actual hole and rises back to the initial value when the copper surface is measured again. Since the border of the hole is Kapton, with carbon as main part, the rise is expected. The oxygen (bright green) is relatively constant during the measurement of the copper surface, as expected for an oxidized sample. At the border of the hole (Kapton) the oxygen part increases a little and falls to zero in the hole. All lines show the expected behavior. The oxygen fraction increases for the 2 hours of baking compared to the 1.5 hours. This fact reflects the increasing growth of copper oxide for longer annealing times and supports the assumption of a linear dependence of baking time and oxide layer thickness.

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Figure 23: EDX point measurement at the border of a GEM hole (Kapton). The sample was annealed for 2.5 hours at 200 C.

(a) (b)

Figure 24: EDX line scan between red dots in blue circles over an oxidized GEM hole and parts of the GEM surface. The oxidizing parameters were 1.5 hours at 200 C. The violet line shows the carbon, the upper, leaf green line shows copper

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Figure 25: Setup for the resistance measurement. dis the thickness of the surface layer.

6.6 Resistance and Resistivity of Oxide Layer

An important characteristic of the copper oxide layer is its resistivity. It quantifies how strongly a given material opposes the flow of electric current. A significant resistivity could explain a protection effect against destructive discharges. Further, using literature values, the resistivity value could be used to estimate the thickness of the copper oxide layer. One established method to measure the resistivity of a material is the Van der Pauw method, first published in the 1950s [vdP59].

6.6.1 Working Principle and Measurement Setup of the Van Der Pauw Method

The Van der Pauw method is a four point contact resistivity measurement. A known current is applied over two contacts while the voltage drop is measured over the two others. The setup is sketched in Figure 25.

The requirements for the Van der Pauw method are the following:

• The thicknessdof the sample has to be small compared to the distance of the probes

• The probes are located at the border of the sample

• The sample has to be simply connected in the mathematical sense. Meaning there are no holes or islands in the surface layer

• The size of the probes has to be small compared to the area of the sample so their resistance can be neglected

The calculation principle of the resistance is based on the electrical field generated by the applied current. The resistance and resistivity is calculated out of the four measurement

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points. The formulas are simplified in the case of a symmetric sample and can be written as:

R12,34 = U34

I12 (15)

with R12,34 being the resistance between points 1 and 2 or 3 and 4, respectively, U34 the voltage over the probes 3 and 4 andI12the current applied over the probes 1 and 2. Then the resistivity ρ is given by:

ρ= πd

ln(2) ·R12,34= πd ln(2) · U34

I12 (16)

To measure the resistance of copper oxide, a 50 nm ± 0.5 nm copper layer was sputtered on glass. This layer is completely oxidized by annealing the sample for 4 hours at 200 C.

Four wires - arranged in a square - are glued with conducting silver epoxy to the copper oxide surface. Two of the wires are connected to the used current supply (DIGISTANTR type 6426 [Ohm17]). The two other wires are linked to the utilized voltage meter (FLUKE 8846A [FLU17]) as shown in Figure 25.

A sample made on glass substrate was used, since the copper thickness of a dummy GEM foil amounts to 5µm which did not completely oxidize. So it cannot be used as sample for the resistivity measurement. If there is still some pure copper beneath the oxide layer, the measurement principle of the Van der Pauw method does not work. The current follows the way of the smallest resistance, through the oxide layer, along the copper and through the oxide layer again.

Another difficulty was to establish a proper contact between the probe and the copper oxide. Several tested probes scratched the copper oxide surface. It was also not possible to solder a contact on the oxide since the soldering tin does not attach properly.

Finally, it was only possible to establish a stable connection using a conducting silver epoxy glue.

6.6.2 Results of the Resistance and Resistivity Measurements

The applied current range reaches from 1 µA up to 6 µA. Applying more current was not possible due to exceeding the voltage range of the used current supply. The result is shown in Figure 26.

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Figure 26: Measured voltage U [V] as a function of the applied current I [µA]

including systematic errors.

∆R =|δR

δU| ·∆U +|δR

δI | ·∆I =|1

I| ·∆U +|−U

I2 | ·∆I (17) The resulting resistance of the measurement data amounts to 0.446 MΩ±0.03 MΩ.

The thicknessdof the oxide layer is 50 nm±0.5 nm. The resulting systematic uncertainty of the resistivity can be calculated as follows:

∆ρ=|δρ

δd| ·∆d+|δρ

δR| ·∆R =| π ln(2)

U

I| ·∆d+| πd

ln(2)| ·∆R (18) The resistivity of the measured oxide layer - according to formula 16 - is ρ = 10.1 Ωcm ± 1 Ωcm. The calculated resistance and resistivity including their uncertainties are shown in Figure 27. The deviations between the different measurement points are in the range of the uncertainties.

There are differences seen to the values given in literature [FEG+08, LWM+11] but the deviation is in the range of the deviations in the paper itself. Figure 28 tabulates the results of resistance and resistivity measurements of copper oxidized at different tempera- tures. The here calculated resistivity ofρ= 10 Ωcm of copper oxide - annealed at 200C - is considerably less than the literature value of ρ = 100 Ωcm. The source could be the differences in the thickness of the annealed copper which results in a difference of the copper oxide thickness and the formation of different oxides with different resistivities (cf.

Table 3).

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(a) (b)

Figure 27: (a) Resistance as voltage divided by the current (cf. Figure 26) for all measurements including the uncertainties. (b) Resistivity according to formula 16 including the uncertainties.

Figure 28: Copper oxide values regarding annealing temperature, thickness, sheet- resistance and resistivity [FEG+08].

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6.7 Conclusion of the Investigation of the Surface Properties of Copper Coated Foils

Several samples of systematically heat treated GEM foils, dummy foils and on glass sput- tered copper samples were measured by various methods and for the first time in the frame of this thesis.

Color changes were cataloged regarding different baking durations and temperatures. The appearance of different colors is caused by the interference of reflected light on the bound- aries of the copper oxide layer. The change of the color indicates a change of the thickness of the oxide layer. Therefore, these samples were further investigated.

The thickness of oxide layers were probed by X-ray diffraction and ellipsometry measure- ments. Both measurements are based on reflection, diffraction and interference. It can be stated, that XRD as well as ellipsometry did not yield conclusive results. To get precise results, the copper oxide layer would have to be smoother and its composition better known regarding CuO, Cu2O and Cu3O2 parts. One possible method to find out these components would be X-ray photo electron spectroscopy (XPS). Such a measurement de- vice was unfortunately not available in the scope of this thesis.

Perthometer measurements showed that the roughness of the copper oxide surface in- creases with higher annealing time and temperature.

The usage of scanning electron microscope and energy dispersive X-ray spectroscopy de- vices supported the theory of oxidizing due to the baking. The composition was measured with the help of the characteristic X-rays of elements emerged due to electron irradiation.

The longer the heat treatment the more oxygen was detected in the copper surface, which shows a higher copper oxide fraction.

With the Van der Pauw method it was also possible to measure resistance and resistivity of the formed copper oxide. By applying linear increasing currents the voltage also in- creases linearly which results in a stable value for the resistivity of ρ = 10 Ωcm.

Even if more statistics of all measurements would be desirable, the assumptions of oxide growing due to higher baking temperatures and longer annealing durations as well as a higher resistivity of copper oxide compared to copper were confirmed.

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7 Comparison of Oxidized and Non-Oxidized GEMs concerning High Voltage Stability and Gas

Gain

In order to determine the performance difference between oxidized and non-oxidized GEMs, tests regarding the high voltage stability and the effective gain were made.

As mentioned, the idea of the copper oxide layer is minimizing the probability of de- struction of GEMs by discharges. To test this idea, a setup which counts the amount of discharges is used and the results are presented in the following as well as effective gain measurements. Since the effective gain of a GEM is one of its key parameters, the gain of an oxidized GEM has been studied and compared to the performance of an untreated GEM.

7.1 Characteristics of analyzed GEM foils

Different GEMs - oxidized and non-oxidized - were tested. First their high voltage stability, than the effective gas gain of the individual GEMs was determined.

Three non-oxidized GEMs were tested. All of them are standard CERN GEMs as ex- plained in section 4.1, produced at CERN. The brand-new ”virgin” GEMs were condi- tioned as described in section 4.4. In the following, the three GEMs are named ’Virgin1’,

’Virgin2’ and ’Virgin3’.

In addition, another four brand-new GEMs were oxidized in the scope of this thesis, after testing their functionality. They were - one after another - placed in an oven as described in section 6.1. The oxidizing process took place in normal air. Two additional GEMs, called ’Baked1’ and ’Baked2’, were treated before the start of this thesis and the baking parameters were not fully documented. One of the four GEMs, ’Baked6’, was placed in the oven already in the heating-up phase in addition to the usual baking time. This treatment was chosen since ’Baked1’ and ’Baked5’ were treated like this.

The baking parameters were individual for each GEM as listed in Table 6. In the following, these GEM foils are called ’Baked1’ to ’Baked6’.

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Name Temperature Time

’Baked1’ unknown unknown

’Baked2’ unknown unknown

’Baked3’ 180 C 3.5 hours

’Baked4’ 200 C 2 hours

’Baked5’ 200 C 3.5 hours

’Baked6’ 0 C to 200 C 2.5 hours 200 C 3.5 hours

Table 6: Baking parameters of the oxidizing process of the GEMs.

Figure 29: Experimental setup to count discharges of one GEM.

7.2 High Voltage Stability

In this section, an experiment is described, which tests how many discharges a GEM can withstand until it suffers a destructive one and a permanent shortcut is formed.

7.2.1 Experimental Setup and Data Analysis

The discharges are monitored optically. The GEM is put in a gas tight box with a high voltage system installed. The box is connected to a gas system and flushed with nitrogen (N2). The high voltage is connected via protection resistors to decamp the capacitance of the SHV cables from the GEM. A camera is mounted above the box and connected to a readout computer next to the setup. Each discharge generates a light spot on the GEM, which is detected by the camera. The whole setup is covered by a black blanket to shield it from outside light. A sketch of the setup can be seen in Figure 29.

A python program evaluates the data of the camera. Each picture is represented by three matrices, one for the colors red, green and blue (rgb) each. If all the three values at the same point of the matrices equals zero, this picture pixel is black. For a stable working GEM, where no discharges happen, all values of the three matrices are around zero due to

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