A.

Coating deposition

Most of the silver coatings and protected silver coatings presented in this article were prepared by magnetron sputtering in an inline-system equipped with 8 sputtering cathodes for the deposition of metal layers by DC sputtering and of dielectric layers by reactive MF sputtering in two separate vacuum chambers. The evaporated silver coatings were prepared by thermal evaporation in a Leybold Syrus Pro coating system.

ALD coatings were deposited by Plasma enhanced ALD in an Oxford Instruments OpAL open load reactor. For the SiO₂ coatings Tris[DiMethylAmino]Silane was used as Si precursor and O₂ plasma as the oxidant. The process parameters were optimized at various substrate temperatures from 100°C to 300°C. The pulse and purge times were reduced to the shortest values that still were sufficient to ensure a saturated ALD regime. With a total cycle time of 14.4 s and a growth rate of about 1Å/cycle the deposition rate amounts to approx. 25nm/h. The Al₂O₃ films have been deposited using alternating sequences of trimethylaluminum (TMA) and oxygen plasma at 120 C°. A 30 ms TMA dose and 5 s Plasma (O₂) with 3.5 s purge times between each precursor have been used. Working pressure was about 250 mTorr.

B.

Coating characterization and testing

Reflectance and transmittance measurements were conducted using a spectrophotometer Perkin Elmer Lambda 950 equipped with a VN-accessory for R- and T-measurements at 6° angle of incidence. For the analysis of defects a Zeiss scanning electron micrograph (SEM) Sigma, an Oxford Instruments Inca x-act energy dispersive X-ray detector and a Leica light microscope CTR 6000 were applied. For infrared measurements a Varian 3100 FTIR (Fourier transform infrared spectrometer) was used.

A.

Optical properties of silver films

The properties of silver films depend strongly on the deposition technology and the deposition parameters. Film properties like grain sizes, orientation of grains, density and impurities have a large impact upon the optical constants of the respective silver film. Silver surfaces are rough due to the crystalline structure of the silver film and tarnishing processes after exposition of a freshly deposited unprotected silver film to atmosphere. Figure 1 (left) shows measured reflectivity data of differently prepared silver films (S1, S2, S3, and E1) together with simulated reflectivities based on two different sets of optical constants found in the literature (Wilson [1] and Johnson and Christy [2]). The experimental data was measured at an evaporated silver film (E1) and three silver films deposited by magnetron sputtering (S1, S2, S3) using different sets of deposition parameters. It can be seen that the evaporated film shows a higher reflectivity than the sputtered films, and that there are also considerable differences between the three sputtered films due to the different deposition parameters.

Figure 1:

Left side: simulated reflectivity of silver films based on optical constants from Wilson and from Johnson and Christy and measured reflectivity of evaporated (E1) and sputtered (S1, S2, S3) silver films. Right side: Scanning electron microscopy images of the deposited films.

The scanning electron microscope images at the right side of Fig. 1 show that the reflectivity data of the differently deposited silver films can be correlated to the microstructure of the films. A larger grain size leads to higher conductivity and therewith higher reflectivity of the films. The reflectivity is influenced by the surface roughness, which leads to the formation of a more or less pronounced absorption feature in the UV reflectance spectrum, which is located around 340 nm. Additionally, the reflectivity of silver films changes during storage at ambient atmosphere due to the formation of corrosion products like Ag₂S at the surface. The combined effect of the interface roughness and the covering absorbing debris layer leads to a spectral shift of the plasmonic absorption feature. In combination with the appearance of a spectrally broad absorption tail mainly originating from the intrinsic absorption of silver sulfide [3], this mechanism can explain the observed differences between the films and also the loss in reflectance over the entire UV-VIS spectral region as a result of ageing. These influences upon the reflectance of silver films can be modelled [3] applying a model including an absorbing overlayer and a rough silver surface. With this model effective optical constants can be calculated even for aged silver films. In addition the model can be applied to predict the optical properties of protected silver surfaces.

B.

Investigation of a defect mechanism in protected silver coatings

As silver films are prone to corrosion, silver mirrors have to be protected by an additional dielectric protective layer. The protective layer prevents corrosion and the formation of defects. There are numerous approaches to developing protected silver coatings which combine an effective protection of the silver film with good optical properties. Several investigations of the durability of protected silver have already been published [4, 5, 7-10]. The most investigated coatings are based on the patented coating design from Wolf et al. [6]. Advanced telescopes, e.g. the Gemini twins or the Kepler Space Telescope [4, 5], are using mirrors with this kind of coating. However, even though protected silver coatings were extensively studied and optimized in the recent years, the appearance of defects after extensive exposure of protected silver mirrors to humid atmosphere remains still an important topic [7].

In order to prevent defects on protected silver or other metals, it is important to avoid defects in the protective layer caused for example by particles or substrate imperfections. One of the most promising recent approaches is to use protective layers deposited by conformal coating methods in order to avoid pinholes and seal defects. In many of these experiments ALD is used to deposit the protective layer [9, 11, 12]. The pinhole-free protective layer can prevent a permeation of corrosion products and thus a damage of the silver film. However, new investigations of defects at protected silver reveal, that the deposition of a defect-free protective layers is a necessary but not a sufficient condition for stability at ambient conditions. At ambient conditions hygroscopic air borne particles can be adsorbed on protected silver surfaces. In combination with humidity these particles can damage the protection and subsequently damage the silver [13]. This damage mechanism could even be observed in case of protective layers which are known to be nearly pinhole-free directly after the deposition of the coating.

For example defects were observed on silver coatings with Al₂O₃ protective layer after environmental testing or after storage at ambient conditions. An investigation of these defects revealed that in the centre of each defect, a particle or a contamination containing Cl and/or S appears as the origin [13]. Cl and/or S are commonly contained in hygroscopic airborne particles [14, 15]. The presence of such particles on the surface of mirrors and samples is very likely. These hygroscopic particles can attract water molecules from the ambient and form a droplet at the surface. Components of these particles can be solved in the droplet. Thus, a solution of Cl and/or S can be formed on the surface of mirrors and samples already at a humidity of 35-80 % RH [16].

These solutions affect the protective layer. At the investigated defects, the Al₂O₃ layer was damaged in the surrounding of the particles which appears as the origin of a defect. FTIR-measurements confirm that sputtered Al₂O₃ protective layers and also Al₂O₃ protective layer deposited by ALD become converted into hydrolysed products (AlO_x(OH)_y) due to exposition to water vapour. At 45 °C ± 5 °C, the conversion of a complete Al₂O₃ layer of 110 nm thickness takes 12h [13]. A similar transformation or dissolution is expected with other protective layer materials. The transformation or dissolution of the protective layer leads to a weakening of the protection and enables permeation of the solution. The permeated solution contains dissolved elements from the particles like Cl or S. These corrosive agents cause agglomeration of the silver grains and therewith a damage to the silver layer. The same kind of agglomeration can be observed at unprotected silver after exposure to NaCl-solution. This is a further indication of damage due to permeated Cl-solution. The described damage mechanism is schematically shown in Fig. 2.

Figure 2:

Schematic illustration (cross section) of the discovered damage mechanism at protected silver caused by hygroscopic airborne particles.

In order to further examine the described damage mechanism, protected silver samples have been artificially contaminated with small NaCl particles. Subsequent to the deposition of these particles on top of silver protected by an Al₂O₃ layer, the samples were stored at > 75 %RH at room temperature or exposed to 85 %RH at 45°C. After these environmental tests defects could be observed. The appearance of these defects was very similar to single defects which had been observed after environmental testing or after storage for long times at ambient conditions without artificial contamination. The characteristic agglomeration of the silver layer appears in the surrounding of the (NaCl) particle. This is another indication for damage due to permeated Cl-solution. Very similar defects occur also at silver films protected with Al₂O₃ deposited by ALD after the same period of time. As ALD-layers are known to be nearly pinhole free it can be concluded, that these defects can occur due to hygroscopic particles which damage the protective layer, subsequently permeate the protective layer and finally damage the silver. This finding shows the importance of clean and dry storage conditions. Furthermore, the test method of artificial contamination enables a direct comparison between different protective layers whereby a systematic improvement and optimization is possible [13].

C.

Enhanced Ag-coatings with improved stability

It is known that different coating materials have different resistances against different kinds of contamination. Hence, by the use of a multilayer as a protective system a high resistance against different contaminations can be achieved. Tests with combined contamination of NaCl and Na₂S and subsequently exposure to moisture have been performed. As a first result a protective coating with a multilayer consisting of Al₂O₃ and SiO₂ showed a considerably better stability than single protective layers consisting either of Al₂O₃ or SiO₂ alone. This approach of improving the durability of protected silver coatings by application of multilayer protective coatings consisting of different materials can be combined with an improvement of the optical performance.

Fig. 3 shows the optical performance of an UV-enhanced protected Ag-coating based on this multilayer approach in comparison to a conventional protected Ag-coating with single-layer protection. For the UV-enhanced coating a reflectance larger than 95% can be achieved for λ > 350 nm while for the conventional protected Ag the reflectance is larger than 95% for λ > 420 nm. Furthermore this multilayer UV-enhanced coating consisting of three different layer materials (SiO₂, Al₂O₃, Nb₂O₅) shows a considerably better stability than silver coatings with single layer protection of either of the three layer materials and identical overall thickness.

Figure 3:

Reflectance of UV-enhanced Ag-coating protected by multilayer coating (black curve) in comparison to conventional protected Ag with single protective layer (red curve) at 6° angle of incidence.

Table 1 shows the result of a comparative study of the stability of silver coatings with different protective layers. In order to allow a comparison of the different samples the protective layers have an identical thickness for all samples. The samples were artificially contaminated with NaCl or Na₂S and subsequently exposed to high humidity for several hours. After the testing all samples were carefully inspected. As can be seen in Tab. 1 only the silver film with the multilayer protective layer does not show any defects even after contamination with NaCl and Na₂S and subsequent exposure to high humidity. All protected silver coatings with single-layer protection show defects in the silver film after the testing with both contaminants.

Table 1:

Results of comparative testing of silver layers with different protective layers by artificial contamination + exposure to humidity. All protective layers have the same thickness.