Desarrollo del Equipo del Proyecto: Herramientas y Técnicas

A two target magnetron sputtering technique was used to obtain ranges of coating compositions. The idea behind this co-deposition method is described in detail in section 2.7.3.1. For each coating deposition run, two different targets (connected to different power supplies) were co-sputtered at the same time to produce coatings that consisted of a mixture of the elements from both targets.

All deposition runs were carried out in the machine called CVC1 (a customised dual source magnetron sputtering machine, shown in Figure 3.4). Figure 3.5 shows a schematic view of such a deposition chamber indicating the range of each target as well as a combination of their plasmas. It can be seen that samples located under Target 1 will be rich in the elements from Target 1, whereas samples located beneath Target 2 will be rich in the elements present in Target 2. Samples located between the targets will be a blend of elements from Target 1 and Target 2.

Figure 3.4 Deposition chamber with a two-target co-sputtering system

Figure 3.5 Side view of a deposition chamber with two targets [103]

Coatings produced by magnetron sputtering are homogenous. This is a result of the high energy present in the deposition process and the fact that the deposition takes place atom by atom [103].

Three sets of different coating compositions were produced using combinations of four different targets. The target compositions were:

 pure Cr (99.95% purity)

 Fe:Cr (50:50 wt%) with 99.9% purity

 Fe:Al (70:30 wt%) with 99.95% purity

 Fe:Al (80:20 wt%) with 99.99% purity

The targets were in the form of 3” diameter (76.2 mm) and 0.25” thick (6.35 mm) discs (Cr and 70Fe:30Al) or 6 mm thick (Fe:Cr and 80Fe:20Al). They were supplied by PI-KEM Ltd (Tamworth, England).

3.2.1 Calibration with the glass slides

Before a full coating deposition run, each target had to be calibrated separately in order to achieve the best deposition rates (i.e., to find the best experimental parameters such as power density, to obtain the desired deposition flux).

Interactions between the targets’ plasmas can also affect the deposition parameters; therefore these also had to be taken into consideration when calibrating the equipment [103]. For the calibration purposes, glass slides were used.

Prior to the coating deposition runs, some parts of the vacuum chamber were removed and cleaned using a grit blaster and then IPA. Glass slides were loaded into the chamber. After the slides had been placed into the chamber, it was pumped down to get a vacuum (recommended pressure is ~2×10^-6 Torr and below; 1 Torr ≈ 0.001 bar). To get the plasma and start the deposition, power was supplied to magnetrons and Ar gas was introduced to the chamber.

At first each target was sputtered separately and then both targets were sputtered at the same time.

For the separate target depositions, two glass slides were placed under each target as illustrated in Figure 3.6. The arrangement of the glass slides allowed the measurement of the thickness of the coating by forming a step (the difference between the heights of the coated and uncoated surfaces). Before being placed in the chamber, glass slides were cleaned with IPA (one run). To obtain a better cleanliness, the slides were cleaned by means of a plasma etcher (Polaron PT7160 RF Plasma Barrel Etcher, East Sussex, UK). The substrates were exposed to a 10% oxygen and 90% argon plasma mix for 3 minutes at a pressure of ~2 mbar and the FWD power of 19 W (RF power

control set to 150). This cleaning process was performed for the two other test runs.

Figure 3.6 Arrangement of glass slides during the sputtering of each target separately

For co-sputtering the glass slides were placed across the table below the targets (Figure 3.7). The arrangement was designed to be able to measure the thicknesses of the coatings as well as to analyse their composition across the holder (the dots in Figure 3.7 indicate where the appropriate EDX analysis were carried out). To obtain more accurate positioning, for some calibration runs, the slides were placed on a specially designed sample holder (also located between the targets) and a Kepton tape was applied to part of each sample to get a step. The composition of the coatings obtained was characterised with EDX.

Figure 3.7 Arrangement of glass slides used for co-sputtering

Once the deposition was finished, the thickness of the coatings was measured by means of Veeco Dektak 3ST Surface Profiler (Santa Barbara, California) which measures the step height on a sample.

Difficulties were observed during the sputtering of the Fe50Cr target, because of its strong ferromagnetic nature (the magnetic field formed around the target was not strong enough to eject atoms from the metal). Despite of applying various depositing conditions (for instance increasing the power density or decreasing the distance between the target and the samples) it was impossible to get a good quality coating with a decent thickness for this target. Therefore, the coatings located near the Fe50Cr target are very thin (~1 µm). It was also challenging to measure their thickness, thus it is given approximately. It also should be mentioned that after 5 hours 25 minutes of co-sputtering onto sapphire discs in one of the runs, the sputtering machine CVC1 broke down preventing further deposition. However, it was decided to use these coatings from Run 2 for short (20 and 50 hours) high temperature exposures.

3.2.2 Deposition onto sapphire discs

After the coating deposition parameters had been found, the next stage was coating deposition onto the sapphire discs. Three runs were undertaken (Table 3.3). Sapphire discs were chosen as substrates in order to avoid any interdiffusion between the coatings and their substrate, which could change the coating composition, as well as because of their thermal stability at high temperatures. The discs were placed in a specially designed sample holder (Figure 3.8, mentioned in section 3.2.1) which allows very precise location of the samples between two targets.

Table 3.3 Targets used for co-sputtering to deposit Fe-Cr-Al coatings

Run Target 1 Power density

Figure 3.8 Sample holder used for co-sputtering with sample location and labelling (modified from [103])

Before starting co-sputtering, the sample holder was cleaned with a grit blaster and then IPA. Discs were given a three stage cleaning process involving deionised water, acetone and IPA (the procedure has been described in section 3.1.1). For the last run, to obtain a better cleanliness, discs were cleaned in a plasma etcher (see section 3.2.1). Clean specimens were placed in the sample holder and then in the sputtering chamber. Several discs were covered by a silicon slide to check the thickness of the coatings using the Dektak.