A.V. Rogov1, Yu.V. Martynenko1,2, Yu.V. Kapustin1, N.E. Belova1 Fabrication of fine-dispersed coatings at deposition with simultaneous sputtering A.V. Rogov1, Yu.V. Martynenko1,2, Yu.V. Kapustin1, N.E. Belova1 1 NRC “Kurchatov institute” 2 Moscow Engineering Physics Institute ISI-2015, August 20-24, 2015

Outline The aim of the research General requirements Substrate features Troubles with columnar coatings Possible reasons for these troubles Experimental details Comparison of the coatings structure X-ray structure investigation Optical stability to sputtering Model of fine-structure coating formation Conclusions

The aim of the research The aim of the research is fabrication of nanostructured optical stable* coatings with decreased defectiveness deposited on polished polycrystalline substrate. * Here we consider that optical stability means preservation of the spectral dependence of mirror reflectance R(λ) on the wavelength of the radiation at uniform sputtering of the mirror surface. Hypothesis: Fabrication of coatings at magnetron sputter deposition with simultaneous sputtering in allows to eliminate the above-described coating defects and, thereby, improve the quality of the coating.

General requirements To provide optical stability of the coating 3 requirements should be performed: Homogeneity of the sputtering yield over the entire surface of the coating; The requirement to the maximum size of the structural element of the coating: d < (λmin / 4) (for H-alpha diagnostic λmin ≈ 450 nm → d ≲100nm); High adhesion between coating and substrate. b c Fig. 1. Film structure: a - structure zone diagram [doi:10.1016/j.tsf.2009.10.145]; SEM images of the columnar coating: top-view (b) and vertical structure (c)

Fig. 2. SEM images of polished surface (a) and sputtered surface (b) Substrate features Molybdenum polycrystalline mirrors were used as a substrate. All of them were fabricated and polished at Science Production Association “Luch” (Podolsk) and have identical structure. Sample sizes: Ø25x2 mm. Typical defects of the substrate surface: Scratches; Caverns. Thickness of the amorphous layer ~100 nm. a b Fig. 2. SEM images of polished surface (a) and sputtered surface (b)

Troubles with columnar coatings Disadvantages of coatings fabricated by magnetron sputtering on a cold substrate: Appearance of large (1-10 μm) crystallites on the coating, which have often druse-like shape; Dependence of the structure of individual sections of the coating on the orientation of the surface grains of the polycrystalline substrate; Preservation of typical defects of polished mirror surface (scratches, caverns). a b Fig. 3. SEM images of coating defects: a – druse, b – disorientation of the columns

Possible reasons for these troubles Presence of nuclei with a selected crystallographic orientation and an abnormally high rate of growth; Influence of grain mechanical properties of polycrystalline substrate on the dimension of the structural elements of amorphous layer formed during polishing; Coating preserves relief of the substrate surface.

Experimental details Fig. 4. Experimental scheme: 1 – sample holder; 2 – substrate; 3 – flux of deposited atoms; 4 – grid cylindrical hollow cathode; 5 – hollow cathode discharge area; 6 – embedded magnetron anode; 7 – magnetron target; 8 – magnetron body; G1, G2 – power supplies The joint work of magnetron and hollow cathode create combined discharge with shift of working parameters to the region of lower gas pressure and lower discharge voltage of hollow cathode. Typical regime: magnetron: PAr = 2·10-2 Torr; Im = 165 mA, Um = 303 V; Ihc = 60 mA, Uhc = 417 V. R = 8 nm/min

Comparison of the coatings structure Fig. 5. SEM images of coating surface morphology: a – magnetron sputter deposition on the cold substrate; b, c - magnetron sputter deposition assisted with simultaneous ion sputtering a b Fig. 6. SEM images of coating vertical structure: a – magnetron sputter deposition on the cold substrate; b, c - magnetron sputter deposition assisted with simultaneous ion sputtering

X-ray structure investigation Fig. 7. Diffraction patterns for coatings Fig. 8. Size distributions for crystallites oriented in directions [110] of coating (1) and substrate (2) The coating has fine-dispersed textured structure with basic crystallographic orientation [110].

Optical stability to sputtering Fig. 9. Mirror reflectivity changing during coating sputtering Fig. 10. Mirror reflectivity dependence on the thickness of sputtered layer (λ=656 nm) The coating has optical stability to sputtering for wavelengths down to 200 nm.

Model of fine-structure coating formation Growth rate of the coating q: q0 – atom deposition rate, j – ion current density, Y – sputtering yield, e - elementary charge Surface concentration C of adatoms: D – diffusion coefficient, YS – cluster sputtering yield, a – interatomic space, d - cluster diameter Effective diffusion coefficient Deff: A – distinctive surface area, A = a2(E/Ea)2/3; τ – cooling time of the area; s – sound velocity In our experiments at T < 400±50 K and the diffusion coefficient is independent of temperature.

Model of fine-structure coating formation Cluster concentration at the initial stage when N < C: At low concentrations of adatoms and clusters: Saturation of adatoms concentration will be reached at ts: At saturation of adatoms concentration the number of clusters grows until the distance between adatoms (C-1/2) becomes higher than the distance between clusters. At that time:

Model of fine-structure coating formation Average number of atoms in cluster n grows as Maximal atoms number in cluster nmax: Final cluster size d: Cluster size has minimum at Cluster size in the case of deposition without sputtering:

Conclusions New method of homogeneous Mo coating formation by magnetron Mo deposition on Mo polycrystalline substrate with simultaneous ion sputtering was developed; Deposition with simultaneous sputtering fabricates the coating without large crystalline defects and with nano sized homogeneous structure; Defects of substrate polishing such as scratch are smoothed; A theoretical model was developed for coating formation by proposed method.

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