Metals have a high reflectivity in a broad spectral range . Of all metals, Ag has the highest reflectivity in the visible spectral range (VIS) and the short wavelength part of the IR range. Therefore, Ag coatings are indispensable for highly reflective components such as reflectors . Under ambient conditions, the high reflectivity of unprotected Ag coatings unfortunately decreases rapidly due to the formation of compounds like Ag2S, AgCl or Ag2O [2–6]. To avoid a rapid decrease of reflectivity Ag coatings with one or more sealing protection layers (protected Ag) are applied for reflectors. But even in the case of protected Ag, the appearance of defects which could decrease the reflectivity has to be considered [7–11]. This consideration and moreover the improvement of mirror durability due to the protection of Ag is an ongoing research topic . This article is a contribution to the improvement of protected silver coatings for reflectors.
For a coated reflector, the requirements concerning its durability differ strongly depending on the relevant environmental conditions for the application (e.g. between ground- and spaced based reflectors, or primary mirrors and reflectors located inside of a device). Leaving the different applications aside, the time span between the deposition of the highly reflecting coating and the actual application often exceeds months and sometimes years. In this period, transport, assembling and storage in several laboratories expose the coated reflectors to different environmental conditions [8, 12]. Therefore, reflectors for ground- and spaced-based applications have to be durable under these conditions. During the transport, assembling and storage, conditions like temperature, humidity and air pollution are only controllable with great effort. The existence of high humidity can lead to condensation on the reflectors surface. Due to hygroscopic airborne particles condensation on the coated reflector is also possible at a relative humidity of < 80 % . Beside the unintended condensation, a cleaning process of the coated mirror can require a sufficient resistance to liquid solutions.
Regarding the resistance of the coated reflectors to liquid solutions, a distinction between acidic and basic solutions is practicable. The material Si3N4 can offer a high resistance against acid  but shows significant reactions in a basic solution . SiO2 also has a high resistance to most of the acids. If SiO2 is exposed to a basic solution, a higher concentration of OH-ions goes along with a stronger reaction of SiO2 [16, 17]. The material Al2O3 also reacts with basic solutions . Therefore, when these materials are used to protect an Ag-layer, the durability to basic solution is of special interest. Because of this, investigations with an aqueous base were executed to determine a protective layer which is stable, even in a basic solution, and thus can contribute to an improved durability of the protected Ag-coating.
In addition to a high resistance to liquid solutions, the protective layer of an Ag coating should have a low mechanical stress. Tensile stresses can lead to cracks in the protective layer, and compressive stresses can lead to delamination of the protective layer [18–20]. Both, cracks or delamination, lead to a damage of the protected Ag-coating and result in a decrease of specular reflectivity.
Reactive sputtering is a suitable method for the deposition of Ag, Al2O3, SiO2 and Si3N4. With regards to the mentioned criteria for protective layers (high resistance to aqueous bases and a low mechanical stress) the materials Al2O3, SiO2 and Si3N4 have been investigated. The intrinsic material properties as well as the deposition-related layer-properties have been taken into account. For the generic consideration of the influence of deposition conditions, process curves were used for the sputtering of the oxides . SiNx was already successfully applied as a final layer of a protected Ag-coating for the Gemini Telescopes  and the Kepler-Space Telescope . Therefore, a detailed and comparative investigation of the potential of Si3N4, with regards to the critical properties of high resistance against aqueous solutions and a low mechanicals stress was undertaken.
The findings of the above mentioned research have been used to determine a final layer for a protected Ag-coating, which was applied for reflectors utilized in the CRIRES+-instrument. The reflectors were manufactured (by diamond turning and polishing) and coated at the Fraunhofer IOF in Jena . The CRIRES+-instrument is an instrument which will be installed at the Very Large Telescope (VLT). It is an infrared (0.92–5.2 μm) high-resolution cross-dispersed spectrograph which enables astrophysical activities like search for exoplanets and cosmology . In the period after the coating and before the application, the environmental conditions of these reflectors are comparable to the conditions existing for other hightech reflectors. Also for space based reflectors, a long period of transport, assembling and storage before launch and application have to be taken into account. Thus, for all these Ag-coated reflectors, a protective layer with a final layer which offers a high resistance to aqueous solutions and a low mechanical stress should be applied.
Process curves and sample deposition
The generation of process curves and the layer deposition was carried out in an inline sputtering system with a top-down arrangement. The target geometry of this system is 750 mm × 100 mm. During the process, the substrate scans 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. Al2O3, SiO2 and Si3N4 were sputtered by reactive dual-magnetron sputtering with MF-voltage from an Al- or a Si-target, respectively. The sputtering of these three materials was done with a power of 4 kW. Ag was sputtered from an Ag target by DC-sputtering with a power of 5 kW. The background pressure was below 3 × 10-6 mbar. Ar was used as working gas and O2 or N2 were used as reactive gases. During the layer-deposition, Ar and N2 were delivered into the coating chamber with a constant flow, while the oxygen partial pressure in the chamber was controlled by a feedback control system. The actual value of the oxygen partial pressure was not measured directly. With a ZIROX XS22.3H vacuum probe a lambda probe value “L” was determined. This value is directly inverse in proportion to the oxygen partial pressure in the coating chamber. The pumping power of the inline sputtering system was kept constant during the experiments.
In the case of oxides (Al2O3, SiO2), S-shaped process curves have been generated by using the oxygen partial pressure as the control parameter. The lambda probe value L varied while the supply of the associated oxygen (gas flow into the coating-chamber) was recorded. The theoretical background of this kind of process curves is described in detail in the literature [21, 25, 26]. Three different values of Ar supply (gas flow into the chamber: 20 sccm, 90 sccm or 160 sccm) were used, whereby three different process curves could be generated. These curves are shown in figure 1. For each Ar-supply and thus for each process curve, an operation area has been determined (see figure 1). The sample deposition was done for every determined operation area. Thus, Al2O3 and SiO2 where deposited under three different conditions each.
Si3N4 has been deposited with four different sets of coating parameters. The deposition of the sample was done with an Ar-supply of 20 sccm in combination with a low supply of N2 (33 sccm) and a high supply of N2 (120 sccm). In addition, the sample deposition was done with an Ar supply of 80 sccm in combination with a low supply of N2 (47 sccm) and a high supply of N2 (120 sccm).
In each deposition run (3 x Al2O3, 3 x SiO2, 4 x Si3N4), a 3-inch Si-wafer and a smaller Si-substrate were placed, for the investigation of the film stress and the solubility behavior respectively. The layer thickness of the prepared samples is in the range of 280 nm ± 40 nm. The investigated samples and the dynamic deposition rates from the sample deposition are summarized in table 1.
Investigated layers (deposited on substrates), their sputtering conditions and the resulting deposition rate.
|Layer||Ar-flow [sccm]||operating point||N2-supply [sccm]||Dynamic deposition rate [nm*m/min]|
Solubility behavior of reactive sputtered layers
To determine the properties of the reactive sputtered layers (Al2O3, SiO2 and Si3N4) regarding their solubility, these layers were deposited on polished Si-substrates. These samples were exposed to a solution, which consisted of distilled water with a certain amount of Na2S. The exposure was interrupted frequently to determine the remaining layer thickness of the layers. With the help of this approach, it was possible to determine the removal rate of the layers in aqueous solution.
In a first test, all samples were exposed to a solution which consist of distilled water and 0.13 m% Na2S. This solution has a pH-value between 11 and 11.5. The frequent determination of layer thickness was carried out by IR absorption spectroscopy with a Varian 3100 FTIR. The characteristic peaks resulting from absorption due to Al2O3, SiO2 or Si3N4 have been analyzed. In this first test, only the Al2O3 layers and the layer “SiO2_160Ar” reveal a significant decrease of layer thickness. This decrease is shown in Figure 2. The negative slope represents the removal rate. The slope and the uncertainty of the measurement were determined by linear regression.
Those samples which show no significant decrease in the first test were exposed to a solution consisting of distilled water and 0.63 m% Na2S. This solution has a pH-value between 12,5 and 13. The determination of the remaining layer thickness was not based on absorption spectroscopy, like in the first test, but on optical characterization. For this determination, the measurement of reflectivity in a spectral range from 400 nm to 1100 nm by a spectrophotometer (Lambda 850/900/950 from Perkin Elmer) and a subsequent characterization of the measurement by the software OptiChar  was realized (a dispersion model according to the Cauchy equation was applied). The reason for the change of the determination of the layer thicknesses is a lower measurement uncertainty for the optical characterization. The results of this second test are shown in Figure 3. The negative slope represents the removal rate. The slope and the uncertainty of the measurement were determined by linear regression.
Mechanical stress in reactive sputtered layers
A Tencor FLX-2320 was used to determine the deflections of polished Si-wafers (3″) before and after the coating. Taking the layer geometry, the wafer geometry, the intrinsic material properties of the substrate and the change of deflection into account, the stress of the coated layers can be determined by applying the Stoney equation . With this method, the stress of the coated layers was determined frequently (over a period > 3500 h after coating). The repeatability of measurements was < 5 MPA and the measurement uncertainty for the absolute value is <30 MPa. During the period of frequent measurements, the samples were stored under ambient conditions of 45% ± 20% humidity and a temperature of 23 ° C ± 4 ° C. The determined stress is compressive, which is indicated by the minus sign.
Process curves and sample deposition for reactive sputtering
For Al2O3 and SiO2, the S-shaped process curves for reactive sputtering with O2 could be generated by using the partial pressure of oxygen as a control parameter (figure 1). As described in the literature [21, 25, 26], these curves reveal areas, where sputtering takes place in the metallic mode, in the transition mode and in the compound mode:
Sputtering in the metallic mode takes place if the operating point is in the part of the process curve that stresses from the lowest oxygen partial pressure to the first inflection point. In this mode, the target is predominantly not poisoned due to reactions with a reactive gas. Thus, mainly the pure target material is sputtered. Sputtering in this mode often results in a deposition of layers which are highly absorbing in the visual spectral range. Therefore, this mode is not suitable for the deposition of protective layers for Ag-coatings.
Sputtering in the transition mode takes place if the operating point is between the first and the second inflection point. In this mode, the surface of the target is partly occupied by the reaction products (Al2O3 or SiO2).
Sputtering in the compound mode takes place, if the operating point is in the part of the process curve that stresses from the second inflection point in the direction of higher oxygen partial pressure. In this mode, the surface of the target is predominantly poisoned with Al2O3 or SiO2 respectively. The deposition rate for sputtering in the compound mode is expected to be lower than for sputtering in the transition mode and in the metallic mode.
Like shown in figure 1 and table 1, the sample deposition has been done with operating points in the transition mode. Thereby the sputtering of transparent layers with a relatively high deposition rate (compared to the sputtering in the compound mode) was carried out. The only exception is the SiO2 layer which was sputtered with an Ar supply of 20 sccm and an operating point in the compound mode. The deposited rate of this SiO2 layer is lower than the deposition rate of the other SiO2 layers (table 1).
The chosen parameters for the Si3N4 have an influence on the deposition rate. A higher supply of N2 leads to a lower deposition rate. This is true for a supply of working gas (Ar) of 20 sccm and of 80 sccm (table 1).
Solubility behavior of reactively sputtered layers
The exposure of the samples to an aqueous (basic) solution reveals that the removal rates of the three different Al2O3 layers are higher than the removal rates of all other tested layers (three different SiO2 layer and 4 different Si3N4 layer). Even in an aqueous solution with a relative low pH-value between 11.0 and 11.5, these layers dissolve within a few hours (Figure 2 a). Furthermore, the SiO2 layer “SiO2_160Ar” shows a removal rate in the aqueous solution with a pH-value between 11.0 and 11.5 (Figure 2 b), even though this removal rate is significantly lower than the one of the Al2O3 layers. The SiO2 layers sputtered with a lower supply of Ar and the Si3N4 do not show a significant decrease in layer thicknesses due to exposure to the aqueous solution with a pH-value between 11.0 and 11.5.
Figure 3 shows the decrease of layer thickness for an aqueous solution (pH-value between 12.5 and 13). The SiO2-layer sputtered with an Ar supply of 35 sccm in the transition mode is less stable than the SiO2 sputtered with an Ar supply of 20 sccm in the compound mode. However, the least stable layer of SiO2 is still more stable than the best Al2O3-layer. Furthermore, the least stable of the Si3N4 layers (Si3N4_3) is still more stable than the best SiO2-layer (SiO2_20Ar).
Mechanical stress in reactive sputtered layers
The layer stress (compressive stress) of the three investigated Al2O3 layers is between -145 MPa and -210 MPa (Figure 3 a). The layers did not show a significant change of layer stress (above 10 MPa) in a time period of more than 3500 h.
The SiO2 layers show a significant change of layer stress (figure 3 b). The layer “SiO2_35Ar” shows a crucial increase of compressive stress of more than 120 MPa. The sample “SiO2_160Ar” shows a variation of the compressive stress of ~ 40 MPa and thus also a significant change of layer stress. The sample “SiO2_20Ar” has the most stable SiO2-layer. However, a small increase in the compressive stress of ~ 10 MPa over a period of > 3500 h should be considered.
The Si3N4 layers show huge differences with regard to the mechanical stress. The Si3N4 layers sputtered with an Ar supply of 20 sccm have a high compressive stress of approximately -1490 MPa (Si3N4_1) and approximately -670 MPa (Si3N4_2). In contrast, the Si3N4 layers sputtered with an Ar supply of 80 sccm (Si3N4_3 and Si3N4_4) have a low compressive stress between -20 and -90 MPa. In case of the Si3N4, the layer Si3N4_4 shows a significant change of layer stress (figure 4 c and d).
Process curves and sample deposition
S-shaped process curves for reactive sputtering can be generated as described in the literature [21, 25, 26], whereby an interpretation of the influence of the sputter parameter is possible. The lower deposition rate for oxides (SiO2), sputtered with an operating point in the compound mode instead of in the transition mode stands in accordance with the literature . The reason for this behavior is that the sputter yield of the compound material (e.g. SiO2) is often lower than the sputter yield of the pure (target-) material (e.g. Si) . A lower sputter yield for the compound material as for the pure (target-) material could also explain why Si3N4 sputtered with a higher supply of N2 has a lower deposition rate than Si3N4 sputtered with a low supply of N2.
Solubility behavior of reactive sputtered layers
With regard to the solubility behavior of the sputtered layers, an influence of the intrinsic material properties and of the deposition parameters are visible.
Since the most stable Al2O3 layer (lowest removal rate) is less stable than the most unstable SiO2 layer, and the most stable SiO2 layer is less stable than the most unstable Si3N4 layer, the influence of the intrinsic material properties is evident. These significant differences can be explained by the different bonds of the materials. Due to the application of “Fanjans rules” , it can be estimated that the fraction of covalent bonds increases from Al2O3 to SiO2 to Si3N4. These rules consider how far the cation polarizes the anion and thereby increases the fraction of covalent bonds which leads to a higher resistance in polar solutions . A small radius (rcation) and a high charge (zcation) of cations, as well as a large radius (ranion) and high charges (zanion) of anions lead to an increase of the covalent fraction. To estimate and compare the potential of the building of covalent bonds, calculations are possible [30, 31]. The potential of the building of covalent bonds has been calculated for the three tested materials (Al2O3, SiO2 and Si3N4) with the following equation:
Assuming the charges and ionic radii given in table 2, the potential for the building of covalent bonds is as follows: Al2O3 = 13, SiO2 = 21 and Si3N4 = 34. Thus, the influence of the intrinsic material properties with regards to solubility behavior can be explained.
Binding partner and corresponding ionic radius .
|ionic radii [nm]||0,060||0,047||0,125||0,132|
In addition to the intrinsic material properties the deposition parameters have a significant influence on the solubility behaviour of the investigated layers. For Al2O3 and for SiO2, the most unstable layer is the one sputtered with the highest supply of Ar (160 sccm). Since the pumping of the coating-chambers was done with a constant rate in all deposition runs, a higher supply of Ar leads to a higher pressure in the chamber. The higher pressure leads to a higher probability of inelastic scattering between Ar and sputtered adatoms, whereby the kinetic energy of the adatoms is reduced. A reduced kinetic energy of the adatoms can lead to an porose layer structure with an higher specific surface [32-34], whereby a higher removal rate of layer-thickness in an aqueous solution can be explained.
Mechanical stress in reactive sputtered layers
In contrast to these Al2O3 layers, the SiO2 layers show a significant change of mechanical stress due to storage at ambient conditions. The change of stress can result from an interaction between water from the environment and the SiO2-layers. The penetration of water into a layer can lead to water-induced stresses  which can be tensile stresses  or compressive stresses [19, 36]. For (vapor-deposited) SiO2 layers in particular, the occurrence of water-induced stresses has been published [19, 36–39]. Water-induced stress could also be the reason for the unstable stress of the layer Si3N4_4.
The high compressive stress for sputtered Si3N4 layers (circa -1,5 GPa) is in accordance with results found in the literature . In both cases, our findings and the findings descripted in the literature, the high compressive stress results from sputtering with a low working pressure or with a low Ar-supply, respectively . Like discussed above, a higher working pressure leads to a higher probability of inelastic scattering between Ar and sputtered adatoms, whereby the kinetic energy of the adatoms is reduced and the microstructure of the layer becomes more porous [32-34]. A more porous microstructure can lead to lower compressive stress or higher tensile stress. The correlation between a low working pressure during sputtering and a compact and dense microstructure, which often goes together with compressive stress, is already described for Si3N4 . Thus, the low stress for the Si3N4 layers sputtered with an Ar-supply of 80 sccm (Si3N4_3 and Si3N4_4) can be explained.
For ground- and spaced based applications, Ag coated reflectors are indispensable because of the high reflectivity of Ag. A long period of transport, assembling and storage of these reflectors take place, before they are used for the actual applications. To endure this period without a decrease of reflectivity, protective layers with a final layer which offers a high resistance to aqueous solutions and a low mechanical stress should be used.
Typical protective layers like Al2O3 and SiO2 can be deposited by reactive sputtering. S-shaped process curves for reactive sputtering with O2 are useful for the interpretation of the deposition conditions and an optimization of the layer properties. E.g., sputtering SiO2 with a more poisoned target results in a lower deposition rate but can increase the resistance of the sputtered layer to aqueous solutions.
With regards to the resistance of the investigated Al2O3, SiO2 and Si3N4 to aqueous solutions, the influence of the intrinsic material properties is higher than the influence of the deposition conditions. All Si3N4 layers are more stable than SiO2, and all SiO2 layers are more stable than Al2O3. Although the sputtered Si3N4-layers offer by far the best sustainability in aqueous (basic) solutions, not all of these layers are suitable as a final layer of the protective coating for Ag. In two cases (Si3N4_1 and Si3N4_2) the compressive stress is too high, because it harbors the danger of delamination. In case of Si3N4_4, the stress is shifted and thus the layer is not stable. Fortunately, the layer Si3N4_3 fulfilled both conditions, a high resistance to aqueous solutions and a low mechanical stress. Therefore, the parameters which have been used for the deposition of the layer Si3N4_3 were also used for the deposition of the final layer for the protected Ag-coating, which was applied for reflectors utilized in the CRIRES+-instrument (used at VLT). This protective coating, with the Si3N4 layer of high durability and low mechanical stress as a final layer, was designed and fabricated following a patented approach . The optical performance for the protected Ag-coating for the CRIRES+-instrument (a spectrograph for the IR-range) is shown in figure 5.