field configuration could be significant and would have to be studied in detail for such an application.
7.4 Conclusion of the Comparison of Oxidized and Non-Oxidized GEMs
The hope to raise the high voltage stability and to reach higher effective gains by oxidation of standard CERN GEMs can not be confirmed by the here presented measurements. But there is also no evidence found, that baking of GEMs harms their performance. Even if several oxidized GEMs were destroyed in the scope of this thesis, there is one all surviving oxidized GEM and no proof found why the other oxidized GEMs were destroyed already by the first shortcut. On top, one oxidized GEM sustained several curable shortcuts until a destructive one appeared. The slight trend to larger effective gains has to be
8 Summary and Conclusion
The motivation for the presented research was the improvement of the high voltage stabil-ity of GEM foils with the help of heat treatment. Therefore, investigations of the copper oxide layer on GEMs were performed to characterize, control and better understand the process of copper oxide formation. Furthermore, oxidized and non-oxidized GEMs were tested regarding their high voltage stability and gas gain performance.
The investigation of the surface properties of heat treated GEM and dummy GEM foils were one main subject of this thesis.
Studies regarding the color change of the copper surface were made. Several samples were baked with different parameters (durations and temperatures). Interference effects cause the appearance of different colors during the baking process which indicates differ-ent thicknesses of the oxide layer. These samples were further investigated as follows.
Two different methods were used to determine the thickness of the oxide layer: X-ray diffractometry and ellipsometry. Both measurements are based on the interaction of radi-ation with a thin surface layer. Unfortunately both of the methods did not deliver results regarding the actual thickness of the oxide layer. The surface has to be very smooth and on a stiff and flat substrate for these type of measurements, which is not the case for the characterized GEM foils. In addition, the composition of the different oxides (CuO, Cu2O and Cu3O2) have to be well known for ellipsometry since it is based on surface models.
Roughness measurements with a perthometer confirmed an increase of roughness for longer annealing times and higher temperatures.
The surface of GEM and dummy GEM samples were investigated regarding impurities and composition with a scanning electron microscope with an additional energy dispersive X-ray spectroscope. The optical surface investigation shows no difference between samples with various annealing parameters regarding the distribution and number of impurities.
Only the color changes due to the baking. The EDX measurements confirmed the as-sumption of growth of the oxide layer by a longer baking time or higher temperatures, respectively. The oxygen fraction rises by increasing one or both of these parameters.
The probably most important parameter in the scope of this thesis characterizing the copper oxide is the resistivity. The resistivity of an totally oxidized copper layer was measured by the Van der Pauw method. Due to several difficulties of establishing contact between the copper oxide and probes, only the magnitude for the resistivity can be given.
Nonetheless, it could be proven that the resistivity increases significantly by oxidizing copper.
Besides the surface properties of annealed GEMs and dummy GEMs, the difference in
performance of oxidized and non-oxidized standard CERN GEMs was investigated.
The improvement of the high voltage stability of GEMs due to oxidizing the copper surface could not be confirmed. Several oxidized and non-oxidized GEMs were shorted in the frame of this thesis. The nature of the shortcuts was characterized optically as well as electrically regarding their appearance and thermistor behavior. One oxidized GEM withstood several ten thousand discharges unharmed. But it could not be determined why this single GEM is that robust regarding discharges.
Moreover, an oxidized and a non-oxidized GEM were tested regarding their gas gain performance. No evidence was found that the copper oxide layer significantly changes the effective gain of GEM foils. In the end, the initial theory to avoid destructive discharges by oxidation could not be confirmed.
To give a clear statement on the effect of oxidization of GEMs, more detailed and extensive studies on stability and performance would be necessary.
List of Figures
1 (a) Discharges on a GEM, visible as light spots. (b) Dark, oxidized spots
at the region where many discharges happened [Fed17]. . . 2
2 Working principle of a time projection chamber [Sch05]. . . 3
3 Specific energy loss (dE/dx) versus particle momentum. The measurement 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]. . 6
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]. . . 9
5 Visualization of charge transfer coefficients factors. (a) collection efficiency C, (b) gain G, (c) extraction efficiency X. 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]. . . 10
6 Geometry of a standard CERN GEM: given are inner and outer hole diam-eter, pitch and row distance as well as the thickness of foil and substrate in units of µm [Web03]. . . 13
7 Setup of the readout-modules of the TPC [M¨ul16]. . . 15
8 Electric circuit for the so-called burn-in of GEMs . . . 15
9 Samples annealed at different temperatures and durations. . . 19
10 Interference phenomenon by the reflection of visible light on thin films [Fuj]. 20 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. . . 21
12 Illustration of in-plane and out-of-plane measurement methods [IKU+13] . 22 13 Orange: out-of-plane XRD-measurement, blue: in-plane XRD-measurement. 23 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]. . . 23
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 di-electric function is recorded with a wavelength between 300 and 800 nm (visible light). . . 24
16 Ellipsometer measurement of the complex dielectric function of two differ-ent samples where copper was sputtered on glass. The dielectric function is recorded with a wavelength between 300 and 780 nm (visible light). . . . 25 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). . . 25 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. . . 27 19 Grain structure due to annealing copper [FEG+08]. . . 29 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. . . 31 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). . . 32 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. . . 33 23 EDX point measurement at the border of a GEM hole (Kapton). The
sample was annealed for 2.5 hours at 200 ◦C. . . 34 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 and the bright green line shows the oxygen fraction. . . 34 25 Setup for the resistance measurement. d is the thickness of the surface layer. 35 26 Measured voltage U [V] as a function of the applied current I [µA] including
systematic errors. . . 37 27 (a) Resistance as voltage divided by the current (cf. Figure 26) for all
measurements including the uncertainties. (b) Resistivity according to for-mula 16 including the uncertainties. . . 38 28 Copper oxide values regarding annealing temperature, thickness,
sheet-resistance and resistivity [FEG+08]. . . 38 29 Experimental setup to count discharges of one GEM. . . 41
30 (a) Overlay of 3000 discharges of GEM ’Virgin3’ (second shortcut). (b) Overlay of 270 discharges of GEM ’Baked3’. The blue frame illustrates the border of the GEM. All light spots outside the blue frame are just reflections on the glass box. . . 44 31 Behavior of a negative temperature coefficient thermistor (NTC) and
pos-itive temperature coefficient thermistor (PTC) compared to an ohmic one [H¨au17]. . . 46 32 Current-Voltage curves of the shorted GEM ’Baked3’. (a) Measurement to
calculate the resistance of the GEM right after the destructive discharge.
(b) Second measurement after a resistance drop. . . 47 33 Current-Voltage curve of the shorted GEM ’Baked6’. . . 47 34 Microscopic pictures of damages on GEM ’Baked5’. Picture (b) shows an
enlargement of the region of interest of (a). . . 48 35 Microscopic shot of damages on GEM ’Baked6’. Picture (b) shows an
enlargement of the region of interest of (a). . . 49 36 Sketch of the setup of the TPC prototype [Zen14]. . . 49 37 Sketch of distances and current denotations of the GEM stack in the
pro-totype [Zen14]. . . 50 38 Effective gain and mean value of three measurements of a non-oxidized
GEM at position III in a triple GEM stack. The red curve denotes a fit to the mean values. . . 54 39 Effective gain and mean value of three measurements of an oxidized GEM
at position III in a triple GEM stack. The red curve denotes a fit to the mean values. . . 55
List of Tables
1 Charge and rest mass of the particles detected in the TPC compared to an electron [Gro17]. . . 6 2 Overview which resistances of shorted GEMs are curable. . . 16 3 Physical properties of copper, Cu2O and CuO [Baa17, LWM+11]. . . 17 4 Temperatures and durations of dummy GEM and GEM sample oxidation. 19 5 Results of the roughness measurement of dummy GEM foils. . . 29 6 Baking parameters of the oxidizing process of the GEMs. . . 41 7 Results of the high voltage stability tests of non-oxidized GEMs. The
given parameters are the amount of discharges until a shortcut, shortcut resistance and the used protection resistor. All given values are rounded due to the precisions in the measurement method. . . 42 8 Results of the high voltage stability tests of oxidized GEMs. The given
parameters are the amount of discharges until a shortcut, shortcut resis-tance and the used protection resistor. All given values are rounded due to the precisions in the measurement method. The right arrow indicates a resistance drop during an applied current to burn away the conductive path. 43 9 Voltage settings for the gain measurements. . . 53
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