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 nontreated 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.

(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.

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 200^◦C.

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

(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 200^◦C. 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.

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

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

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

R12,34 = U₃₄

I₁₂ (15)

with R_12,34 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) ·R_12,34= 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 (DIGISTANT^R 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.