2.1

Evaporation

The evaporated samples were prepared in a Bühler Syrus Pro 1110 evaporation chamber. The deposition of the seed layer material and the gold was carried out from one electron source, while layer thickness and deposition rate were monitored by a quartz crystal system. The evaporation material was 99.999% pure gold and 99.5% titanium.

The samples were positioned in a chamber approximately at a distance of 600 mm right above the electron gun. For the sample preparation, the substrates were cleaned in the chamber by plasma etching for 300 s and a BIAS voltage of 125 V. At a starting pressure of < 2×10^-7 Pa, the adhesion layer was applied to the substrate with a rate of about 0.6 nm/s. Subsequently, a 200 nm gold layer was deposited on the seeded substrates with a rate of about 0.2 nm/s.

For the “measurement of reflectivity of unprotected Au after coating“ (see chapter 2.3) and the “exposure to increased temperatures“ (see chapter 2.4) the samples E1 and E2 were manufactured by evaporation.

2.2

Sputtering

Sputtering was carried out in inline sputtering system with top down arrangement. The substrate scans are vertical to the narrow side of the target. The layer thickness is controlled by determining the deposition rate in a preliminary test and adjusting the number of scans and scan velocity. Preliminary to the deposition of the first layer, substrates were treated with an Ar-plasma etching in the coating chamber. Due to this procedure water and carbon residues are removed from the surface.

For the “measurement of reflectivity of unprotected Au after coating“ (see chapter 2.3) and the “exposure to increased temperatures“ (see chapter 2.4) sputtering were performed with an industrial direct current (DC) magnetron sputtering system MRC (Kenotec SRL, Italy). This system has three sputtering targets of 120 x 380 mm² and a high-frequency etching station. During deposition, the cathodes were pulsed with a DC pulse frequency of 100 kHz. Argon (Ar) was used as process gas. Ti and Au were sputtered using pure Ti or Au targets. The TiO_X were sputtered reactively using O₂ as reactive gas. The background pressure was below 3 × 10^–6 mbar. Sample S1 and S2 were manufactured by sputtering with this MRC system.

For the “reflectivity and durability of protected Au” (see chapter 2.6) the deposition were carried out in an inline coating system¹¹ that has seven targets of 100 x 750 mm² and a high-frequency etching station (located in two deposition chambers separated by the gate valves). Ar was used as process gas. Au, Ti and TiO_X were deposited by DC magnetron sputtering.

Al₂O₃ and Si₃N₄ were deposited by reactive dual-magnetron sputtering with MF-sputtering (medium frequency). The oxides (Al₂O₃, Si₃N₄ and TiO_X) were sputtered reactively using O₂ as a reactive gas. The background pressure was below 3 × 10^–6 mbar.

2.3

Reflectivity of unprotected Au after coating

An overview of the abovementioned sputtered and evaporated coatings of unprotected Au is given in the Table 1. Theses coatings were deposited on quartz glass substrates with a diameter of 25.4 mm and an RMS surface roughness of σ_RMS ~ 0.7 nm (determined by AFM in an area of 10x10µm²).

Table 1.

List of unprotected Au-Coatings.

To obtain an optical performance in the wavelength range from 500 to 11000 nm a reflectivity of the samples was measured with spectrophotometers “Lambda 850” and “Frontier Optica FTIR” from Perkin Elmer. The performance of the coatings directly after deposition is presented in the Figure 1.

Figure 1.

Reflectivity of sputtered and evaporated Au directly after deposition.

2.4

Reflectivity of unprotected Au after exposure to increased temperatures

Reflectivity of unprotected Au after exposure to increased temperatures was determined for the samples, which are shown in table 1. Quartz glasses with a diameter of 25.4 mm have been used as substrates (RMS surface roughness of σ_RMS ~ 0.7 nm, determined by AFM in an area of 10x10µm²).

The exposure to increased temperatures was performed after evaluation of the reflectivity performance directly after deposition. Samples were exposed to 250°C under vacuum conditions in a SYSTEC-vacuum oven. The same sample was exposed to this test sequentially for 4 h, 24 h and 48 h. Therefore, summed up total exposure time for each sample is 76 hours. After every single exposure a measurement of the coating reflectivity took place in the wavelength region from 450 to 1050 nm. An optical performance of the samples before and after exposure to 250°C test-series is shown in the Figure 2.

Figure 2.

Decrease of reflectivity due to exposure to increased temperatures after exposure to 250°C.

2.5

Tape Test

An adhesion between Au and adhesion layer was tested via applying a tape test according to MIL-C-48497A. With the tape 3M 853 (high tear-resistant, transparent tape of polyester) one test per sample was performed. Peeling speed: slow. Subsequently, the tested sample and tape was examined by unaided eye.

2.6

Reflectivity and durability of protected Au

The reflectivity and durability of protected Au is shown for substrates that are composed of an Al-alloy, with a polished NiP-layer.

Based on the findings from “reflectivity of unprotected Au after coating” and “after exposure to increased temperatures”, a TiO_X adhesion layer was chosen, to obtain a good adhesion between substrate and the Au coating. A Al₂O₃-Si₃N₄-laminate was deposited as a final layer. The laminate comprises 4 pairs. Each pair consist of ca.30 nm Al₂O₃ and ca. 30 nm Si₃N₄. The shown reflectivity of this coating (Fig. 3) was measured on witness samples that have been coated together with the mirrors M1, M2 or a tilted mirror respectively.

Figure 3.

Reflectivity of the protected Au-coating, containing a sputtered TiO_X adhesion layer and an Al₂O₃-Si₃N₄-laminate as final layer.

Beside the investigation of a reflectivity, a qualification has been performed on sample level. Beside other properties, the adhesion, cleanability and radiation hardness have been successfully tested.

3.1

Reflectivity of unprotected Au after coating

The reflectivity of the sputtered and evaporated unprotected Au is shown in Figure 1. The reflectivity in the IR is comparable for all samples. Comparison of reflectivity between sputtered and evaporated Au-layers shows a slightly higher reflectivity for evaporated Au. This difference could originate from a differing in Au-particle transport during the deposition (before arriving the substrate). In case of evaporation, the arrival of Au clusters on the substrate can lead to a beneficial layer structure¹². In case of sputtering, the predominantly arrival of single atoms can be assumed.

3.2

Reflectivity of unprotected Au after exposure to increased temperatures

Figure 2 shows the reflectivity of the unprotected Au (see Table 1) after coating and after exposure to 250°C in vacuum. The reflectivity was measured in the wavelength region from 450 nm to 1050 nm, since the changes of the thin film structure are more noticeable in a shorter wavelength region.

In case of Ti underneath the Au-layer, the decrease of reflectivity is significant. At a wavelength of 750 nm, the decrease of reflectivity is ~ 3 % for the sputtered samples S1 and ~ 0,75 % for the evaporated sample E1. In case of TiO_X adhesion layer, a decrease of reflectivity is not notable at all for the sputtered sample S2. The evaporated sample E2 shows a slight decrease of reflectivity (~ 0,5 % @ 750nm). By comparing S1 and E2, it should be noted that E2 has a Ti-layer underneath the TiO_X (see Tab. 1). Under the assumption of a diffusion driven decrease of reflectivity, the coatings with TiO_X adhesion layer are less prone to increased temperatures. In a case of TiO_X, a variety of binding energies must be considered¹³. However, Ti is used as a getter material for oxygen. To apply a stable compound as an adhesion layer could lead to coatings that are more stable toward (inter-) diffusion.

3.3

Tape Test

In case of the sample E2, coating was delaminated through the test. All other samples (S1, S2, E1) didn’t show any degradation of the coating which can occur through this test. Since sample S2 passed the tape test, one can suggest that sputtered TiO_X can be a suitable material for an Au-coatings-adhesion layer.

3.4

Reflectivity and durability of protected Au

The reflectivity of evaporated Au is slightly higher than the reflectivity of sputtered Au (see Figure 1). However, the sputtered coating S2 (TiO_X underneath Au) is not prone to decrease of reflectivity due to the interdiffusion (see Figure 2). As one of the main origins for interdiffusion can be radiation, especially particles of lower energy can provide high levels of dose in coatings⁷, the sample S2 is probably more stable concerning this radiation. With regards to the adhesion, S2 passed the tape test (in contrast to E2). In addition, sputtering is suitable for the deposition of a dense protective layer. In case of sputtering, several materials can be deposited inline (without interruption of vacuum), which is certainly a further advantage for the deposition of protected and enhanced Au coatings. Therefore, sputtering technology was used for the manufacturing of a protected Au coating. An applied protective Al2O3-Si3N4-laminate (see chapter 2.2), enables the cleaning of the coating. In the framework of coating qualification, an abrasion test and a cleanability test were passed. With regards to radiation hardness concerning high-energy particles, a radiation test with electrons was successfully passed.

plication wavelength of 1064 nm is much larger than the single layer in a nanolaminate (30 nm). Thus, the laminates can be modeled as a single optical material with an effective refractive index. Taken this refractive index into account, the overall thickness of this laminate was tailored to achieve a max. reflectivity at 1064 nm. The reflectivity at 1064 nm (wavelength of the GALA-application) is shown in the Figure 3. The reflectivity was measured on witness samples that have been coated together with the mirrors M1, M2 or the tilted mirror in separated batches. The very comparable curve progression (see Fig. 3) demonstrates a good reproducibility of the coating.