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Concentrations of metal ions from a substrate were found in coatings adjacent to the substrate by studying the coatings using X-ray photoelectron spectroscopy before and after annealing. Small metal ions easily diffused into the coating from the substrate, whereas larger metal ions had more difficulty doing so because of their large atomic radii. A higher annealing temperature and a lower packing density induced a faster diffusion rate and a higher concentration of metal ions in the coating. Smaller metal ions passed through a SiO2 layer and preferentially accumulated in the Ta2O5 layer due to the migration of oxygen vacancies. These results are relevant for selecting the coating temperature, annealing temperature, and the substrate.
© 2016 Optical Society of America
1. Introduction
When coating a substrate with an additional layer, the substrate can be heated during coating to improve the adhesion and the density of the coating [1–3].The stress [4] and stoichiometric ratio of the coating can be improved by post-treatment processing using annealing. The diffusion of metal ions into glasses (as induced by heating) has been studied previously [5], but the diffusion of metal ions from a substrate into adjacent coatings has not yet been extensively studied. In this study, the diffusion of metal ions into different kinds of coatings were tested during annealing, to allow for the informed selection of the substrate, coating temperature, and annealing temperature when designing or working.
2. Experimental
K9 (BK7) and JGS1 substrates used for both reference surfaces and coating supports were cleaned by ultrasonication. The coatings were deposited on these two substrates via magnetron sputtering (MS), electron beam evaporation (EBE) or ion beam sputtering (IBS), producing the coatings described in Table 1. In order to avoid diffusion due to baking during vacuum deposition, these coating procedures did not utilize baking.
Table 1. Details of the coatings prepared by three different coating techniques
Each sample was placed in a high purity alumina crucible to avoid contamination, which was then placed into a muffle furnace (L9/12/P330, Nabertherm GmbH) at 250 °C or 500 °C to thermally anneal the samples. The heating rate was 5 °C/min, and the annealing time began after reaching the target annealing temperature. The heating device automatically stopped after reaching the final annealing time.
Component analysis was performed using X-ray photoelectron spectroscopy (XPS; K-Alpha, Thermo Scientific), using a monochromatic Al Kα source, with a characteristic emission line of 1486.6 eV. The depth profile was cut using Ar+ ions at 1 keV ion energy; the reference etching rate of the Ta2O5 was 0.13 nm/s, and the raster area was 4 mm × 2 mm. Analysis of the XPS spectra was carried out after a Shirley background subtraction.
Phase identification was performed using X-ray diffraction (XRD; Empyream, PANalytical) operating in a continuous scanning mode using Cu Kα1 radiation (λ = 0.15406 nm) at 40 kV and 40 mA. The working pattern was generated using grazing incidence XRD (GIXRD). The minimum step size was 0.0001°, and the repeatability of the entire machine was 0.001°.
The variation in the coating thickness before and after annealing was measured using transmission electron microscopy (TEM; Tecnai G2 F20 S-Twin, FEI).The point resolution was 0.24nm.
3. Results and discussion
3.1 Metal ion contents in the substrates
Some K9 and JGS1 substrates were broken before annealing in order to obtain pristine surfaces from within the substrates; these were labeled as fracture surfaces. The fracture surface results were compared to the results from the polished substrate surfaces. The main components of K9 and JGS1 were O and Si [Figs. 1(a) and 1(b)], and the K9 substrate also contained small quantities of Na, K and Ba [Figs. 1(c)–1(e)]. The ion contents between the fracture surface and the polished surface were consistent.
Fig. 1 Percentage of atoms in the fracture surface and the polished surface of the two types of substrates at room temperature: a) Si, b) O, c) Na, d) K and e) Ba.
In order to verify whether or not the metal ions in the coating originated from the substrate, a Ta2O5 monolayer coating was deposited on K9 and JGS1 substrates by MS. Figure 2 shows the content of six elements in the Ta2O5 thin film at various conditions. After annealing, the content of Ta showed marked variation in Ta2O5@K9 [Fig. 2(a)], the contents of Si and O had slight variation [Figs. 2(b) and 2(c)]. Na and K ions were found in Ta2O5@K9 [Figs. 2(d) and 2(e)], and the concentration of Na ions in the Ta2O5 coating was higher than that in the substrate. Na and K ions were not found in Ta2O5@JGS1, indicating that the Na and K ions originated from the K9 substrate.
Fig. 2 Atomic percentages in Ta2O5 coating before and after annealing: a) Ta, b) Si, c) O, d) Na, e) K and f) Ba. The black line, blue line and purple line represent the sample on the K9 substrate. The red line, green line and brown green line represents the sample on the JGS1 substrate.
As the annealing temperature increased, the atomic activity gradually increased, and the diffusion coefficients of Na and K ions increased [6]. Ba did not diffuse into the Ta2O5 coating when annealed at 250 °C, but did concentrate near the interface between the coating and the substrate when annealed at 500 °C [Fig. 2(f)]. This result indicates that metal ions with larger atomic radii have smaller diffusion coefficients. In addition, such ions have difficulty occupying the ionic positions of elements having smaller atomic radii [7–9].
3.2 Diffusion of metal ions from the K9 substrate into different coatings
In order to compare the diffusion of metal ions in different coatings, XPS was used to detect the content of metal ions in the Ta2O5/SiO2/Ta2O5@K9 and SiO2/Ta2O5/SiO2@K9 stacks before and after annealing. The content changes of six different elements in these stacks after different annealing treatments are illustrated in Fig. 3 and Fig. 4. The annealing process caused a significant decrease of the Ta content [Fig. 3(a), Fig. 4(a)], a slight content variation of Si and O [Figs. 3(b) and 3(c), Figs. 4(b) and 4(c)]. Ba concentrated into the Ta coating near substrate when annealed at 500 °C [Fig. 3(f), Fig. 4(f)]. The annealing process caused Na and K ions to accumulate in higher concentrations within the Ta2O5 layer [Figs. 3(d) and 3(e), Figs. 4(d) and 4(e)], passing through the SiO2 layer in both types of stacks. The concentration of diffused Na and K ions depended on the annealing temperature and the annealing time. This demonstrates that the SiO2 coating cannot effectively prevent the diffusion of Na and K ions [10], and that the Na and K ions preferred to reside in the Ta2O5 layer compared to the SiO2 layer.
Fig. 3 Changes in the elemental contents of the Ta2O5/SiO2/Ta2O5@K9 stack before and after annealing: a) Ta, b) Si, c) O, d) Na, e) K and f) Ba.
Fig. 4 Changes in the elemental contents of the SiO2/Ta2O5/SiO2@K9 stack before and after annealing: a) Ta, b) Si, c) O, d) Na, e) K and f) Ba.
The reason for this preference was that a small number of oxygen vacancies were generated in the Ta2O5 layer during the coating process. Although oxygen vacancies were energetically unfavorable in both materials, they were more stable in SiO2 than in Ta2O5 [11]. This implies that, if an oxygen-deficient Ta2O5 layer was near a SiO2 layer, the oxygen vacancies moved across the Ta2O5-SiO2 interface, entering the SiO2 layer to provide a stable neutral charge state [12, 13]. The oxygen vacancies in the Ta2O5 were sometimes healed by oxygen moving from the SiO2 layer to the Ta2O5. The formation of oxygen vacancies was accelerated by annealing in the SiO2 layer, relatively increasing the O content in the Ta2O5 layer [14]. Due to the existence of oxygen vacancies in the SiO2 layer, ions near the vacancies could easily move into the vacant positions, forming a diffusion channel that carried the Na and K ions to the Ta2O5 layer. Due to the increased oxygen contents in the Ta2O5 layer, K and Na ions were more stabilized in the Ta2O5 layer.
Figures 5(a) and 5(b) depicted the TEM images,the results showed that annealing led to a slight decrease in the coating thickness.
Fig. 5 Thickness of the SiO2/Ta2O5/SiO2@K9 stack: a) before annealing b) after 24 h annealing at 500 °C.
3.3 Effects of packing density and crystallinity on the diffusion of metal ions
The packing density of the coatings prepared by IBS was close to the density of bulk materials. By contrast, the coatings prepared by EBE were loose with pillar structures. Ta2O5, HfO2 and SiO2 coatings with thicknesses of 500nm were prepared by IBS and EBE on a K9 substrate. Na, K and Ba distributions were measured by XPS before and after annealing. Etching started from the surface of the coatings. Figures 6–8 show that annealing for 24 h at 500 °C could cause the Na and K ions to diffuse to the surface of these different coatings, even at thicknesses of 500nm. Previous studies reported that the self-diffusion coefficient of Na ions in a 31.8Li2O-5.8A12O3-62.4SiO2 (mol%) glass was 1.30 × 10−9cm2/s [9], or 0.13 μm2/s. This was conducted under annealing conditions of 400 °C baking for 2 h; Na was measured by the radioisotope method, and the diffusion depth per hour was as much as 20 μm. The diffusion coefficient of K ions in an aluminosilicate glass was 1.80 × 10−3 μm2/s [15], under annealing condition of 430 °C baking for 12 h; the diffusion depth per hour was nearly 3μm. These studies showed that Na and K could diffuse through the entire film even though the thickness was on the order of 101 microns when the film component had experienced a high temperature baking process for enough time.
Fig. 6 Atomic contents of metal ions in HfO2@K9 coatings before and after annealing: a) Na, b) K and c) Ba.
Fig. 7 Atomic contents of metal ions in SiO2@K9 coatings before and after annealing: a) Na, b) K and c) Ba.
Fig. 8 Atomic contents of metal ions in Ta2O5@K9 coatings before and after annealing: a) Na, b) K and c) Ba.
The concentrations of Na and K in the coatings prepared using IBS were much lower than those prepared by EBE [Figs. 6(a) and 6(b), Figs. 7(a) and 7(b), Figs. 8(a) and 8(b)], the content of Ba had no obvious change [Fig. 6(c), Fig. 7(c), Fig. 8(c)], indicating that Na and K ions diffused more easily in coatings with either a columnar structure or a low density [13, 16]. The HfO2 coating prepared by EBE was crystalline [Fig. 9(a)], but was amorphous when prepared by IBS. Annealing at 500 °C caused the HfO2 coating prepared by IBS to crystallize, as shown in Fig. 9(b). Crystallization had no significant impact on the diffusion behavior.
4. Summary
The diffusion of metal ions from a substrate into an adjacent coating was related to the type of coating, annealing temperature and packing density employed. Oxygen vacancies from a Ta2O5 layer crossed over a Ta2O5-SiO2 interface, entering the SiO2 layer to show a stable neutral charge state. The O contents in the Ta2O5 layer relatively increased as a result. The oxygen vacancies in the SiO2 layer provided a channel for the diffusion of metal ions. The metal ions passed through the SiO2 layer and accumulated in the Ta2O5 layer due to the increased O content of this layer. A higher annealing temperature and a lower packing density induced faster diffusion and higher final metal ion contents. Metal ions with large atomic radii diffused less readily than those with smaller atomic radii. Crystallization had no significant impact on diffusion behavior.
These results showed that metal ions from a substrate could diffuse into coatings during heating or annealing. Metal ions in these coatings can form micro defects, which could lead to decrease dintrinsic damage thresholds [14, 17]. Therefore, pure substrates should be chosen for the coating process, and the baking temperature and time should be kept to a minimum, not only during coating but also during post-treatment processing, in order to reduce metal ion diffusion from the substrate to the coating.
Funding
National Natural Science Foundation of China (NSFC) (No.61405225).
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