Estrategias para el cumplimiento de las propuestas

Cleaning experiments were performed on the porous and dense type of deposits. An example of porous Al/Aloxide film deposited on a Mo mirror, approx. 30 nm thick is shown in Figure 5.1

and exhibits a 50 % loss at 450 nm. The losses are composed of: (i) an increase of the diffuse reflectivity due to the roughening of the surface (approx. 20 % loss) and (ii) a decrease of the total reflectivity because the deposited film is not perfectly transparent as well as from the deuterium exposure of the Mo surface, leading to deuterium implentation as already shown in [73].

Figure 5.1: Specular reflectivity drop of 50 % at 450 nm of a Mo mirror (before coating; red line) after the deposition of porous Al/Aloxide film approx. 30 nm thick (after deposition; blue

5.1. Laboratory deposits

In addition, dense Al2O3 films were also cleaned. A 25 nm thick layer is displayed in Fig-

ure 5.2 with a typical nanocrystalline morphology. This kind of films exhibited low roughnesses and thus, diffuse reflectivities similar to the substrate ones. The total reflectivity is impacted by the deposited alumina layer and calculation using a Mo substrate and an Al2O3 film of 32 nm

thickness (using optical constants n,k from [85]) showed a perfect agreement with experimental values.

Figure 5.2: SEM image of 25 nm thick Al2O3 film deposited with facing magnetron on a

polycrystalline Mo mirror and total and diffuse reflectivity before and after coating. Calculated reflectivity (with n,k from Palik [85]) were added for comparison with a Mo substrate and 32 nm of Al2O3.

The plasma cleaning procedure mentioned in section 4.1.1 was employed for the subsequent experiments, using in situ reflectometry and XPS to quantify the cleaning advancement. SEM and ex situ reflectivity measurements were performed afterwards to characterize the cleaning efficiency. Experiments with Ar, Ar + D2 and Ne were conducted with a pressure of 0.5 Pa.

When the self-bias was swept from _{−150 to −260 V, the plasma potential was approx. 26 V} and the maximum ion energy is given by the difference between the plasma potential and the self-bias. Using RF plasma at 13.56 MHz, the deposits were almost always fully etched, followed by a recovery of the reflectivity. Once the deposits were removed (total removal, e.g. “T.R.” in Table 5.2), the cleaning was stopped. To avoid excess damage to the mirror, the cleaning was stopped for few experiments from the moment where Mo was measured by XPS even if the contaminants were not fully removed (a few % remaining on the surface, denoted partial removal, e.g. “P.R.” in Table 5.2). To validate this technique for ITER, cleaning were performed on porous as well as on compact films, presented in Table 5.2 and summarized as follow:

• A 200 nm thick porous Al/Aloxide film was removed using Ar at 286 eV;

• Porous W film (80 nm) was withdrawn from a mirror using Ar at 286 eV;

• Mixed porous Al/Aloxide/W films (25–350 nm) were removed using Ar, Ar + D2 or Ne

(ion energy varied from 176 to 286 eV);

• A dense Al2O3film deposited with magnetron (50 nm) was removed using Ar at 226 eV;

• A 50 nm thick Aloxidefilm deposited by ALD was partially withdrawn with Ar at 226 eV.

To assess the efficiency of the cleaning on the optical properties, the total and diffuse re- flectivity of each sample were measured before deposition of contaminants and after cleaning. The specular reflectivity change (in %) was calculated by subtracting the final to the initial reflectivity. An example of reflectivity measurements is shown in Figure 5.3.

Table 5.2: Cleaning results on various films deposited on Mo mirrors with RF discharge clean- ing. The reflectivity change (in %) represents the difference in specular reflectivity between the bare Mo mirror and the Mo mirror after plasma cleaning of the deposits. The estimated sputter rate was obtained with the estimated thickness of the deposits (obtained with QMB) divided by the cleaning time. N.A. stands for not measured.

Deposited film Cleaning conditions Result Reflectivity change (%)

Estimated sputter rate 385 nm 2000 nm nm/min Porous W film (80 nm) 286 eV Ar T.R. − 1.9 − 2.5 0.21 Porous Al/Aloxide film (200 nm) 286 eV Ar P.R. − 1.6 − 2.0 0.56 Porous Al/Aloxide/W film (350 nm) 286 eV Ar P.R. − 1.5 + 1.0 0.38 Porous Al/Aloxide/W film (310 nm) 286 eV Ar/D2 T.R. − 1.5 + 0.4 0.31 Porous Al/Aloxide/W film (40 nm) 286 eV Ne T.R. − 1.1 − 1.9 0.07 Porous Al/Aloxide/W film (25 nm) 176 eV Ar T.R. + 0.8 + 1.0 0.19 Dense Aloxide film (mag- netron, 50 nm) 226 eV Ar T.R. N.A. N.A. 0.02 Dense Aloxide film (ALD, 50 nm) 226 eV Ar P.R. N.A. N.A. 0.40 500 1000 1500 2000 2500 0 1 2 60 70 80 90 100 R Spec Pristine mirror R Spec Cleaned mirror R Diff Pristine mirror R Diff Cleaned mirror R e f l e c t i v i t y ( % ) Wavelength (nm)

Figure 5.3: Specular and diffuse reflectivity of a Mo mirror before deposition of porous Al/Aloxide/W film (Pristine mirror; resp. red and black line) and after cleaning of the deposits

5.1. Laboratory deposits

Using the estimated sputter rate, the most efficient gas for sputtering appeared to be Ar (in comparison to Ar + D2 or Ne) and higher energies lead to faster cleaning. Considering the

film morphology, porous and not fully oxidized deposits were removed much faster than dense oxidized films. Finally, XPS measurements conducted before, in between and after cleaning of porous films indicated four trends:

• Similar to the results with the external plasma source, the ratio of Al metallic to oxidic Al decreased during the cleaning of porous Al/Aloxide and Al/Aloxide/W films (figure 5.4).

This phenomenon can be explained through selective sputtering and/or continuous oxida- tion of the film (target poisoning [40]);

• Selective sputtering was also observed during the cleaning of porous Al/Aloxide/W films

where the Al/W ratio was decreasing with ongoing cleaning (sputtering yields at 200 eV for Al and W are 0.5 and 0.2 ejected atoms per incoming ion, respectively);

• During the cleaning of porous Al/Aloxide and Al/Aloxide/W films, the binding energy of

the Aloxide peak was shifting towards lower values (figure 5.4);

• For W, either in porous W or Al/Aloxide/W films, no changes were observed for the XPS

measurements as displayed in figure 5.4.

80 78 76 74 72 70 40 38 36 34 32 30 28 Y X (c) (b) I nt e ns i t y ( a . u. ) Al 2p (a) (f) (e) (d) W 5p 5 /2 Z

Binding energy (eV) W 4f

Figure 5.4: Core level spectra of (a) Mo mirror after the deposition of a porous Al/Aloxidefilm

followed by respectively two RF cleanings with Ar (resp. (b) and (c)), and (d) Mo mirror after the deposition of a porous W film followed by respectively two RF cleanings with Ar (resp. (e) and (f )). The shown spectra are normalized for comparison. The open circles are the measured spectra and the black lines correspond to the sum curve of all components represented in colored lines. The vertical lines are given as eye guide for Aloxide(X), Almetal (Y) and Wmetal (Z).

To briefly summarize the obtained results, tokamak-like deposits containing Al and W as well as dense Al2O3films were deposited on Mo mirrors and removed with RF plasma at 13.56 MHz.

The RF discharge cleaning has demonstrated successful removal of deposits under a wide range of conditions (use of Ar, Ar + D2 or Ne with ion energies ranging from 176 to 286 eV) while

restoring the mirror’s optical properties. As laboratory experiments are only possible with Al used as a Be proxy, the cleaning process of mirrors deposited with Be should be validated. This is the task of the subsequent sections, where production and cleaning of Be, BeO and BeW films using RF plasma will be reported.