Shaista Babar

We have determined the optical constants in the energy range 0.1–6 eV for bulk Cu, Ag, and Au using Kramers–Kronig analysis of previously unpublished reflectance data. The results are compared to those commonly used from the literature.

We describe an atomic layer etching (ALE) method for copper that involves cyclic exposure to an oxidant and hexafluoroacetylacetone (Hhfac) at 275°C. The process does not attack dielectrics such as SiO2 or SiNx, and the surface reactions are kinetically self-limiting to afford a precise etch depth that is spatially uniform. Exposure of a copper surface to molecular oxygen, O2, a weak oxidant, forms a ∼0.3 nm thick layer of Cu2O, which is removed in a subsequent step by exposure to Hhfac. The etch reaction involves disproportionation of Cu(hfac) intermediates, such that ∼0.09 nm copper is removed per cycle. Exposure of copper to ozone, a stronger oxidant, affords ∼15 nm of CuO; when this oxidized surface is exposed to Hhfac, 8.4 nm of copper is removed per cycle. The etch products, Cu(hfac)2 and H2O, are efficiently pumped away; H2O, a poor oxidant, does not attack the bare Cu surface. The roughness of the copper surface increases slowly over successive etch cycles. Thermochemical and bulk etching data indicate that this approach should work for a variety of other metals.

A systematic alteration in the optical properties of W:Al2O3 nanocomposite films is demonstrated by precisely varying the W cycle percentage (W%) from 0 to 100% in Al2O3 during atomic layer deposition. The direct and indirect band energies of the nanocomposite materials decrease from 5.2 to 4.2 eV and from 3.3 to 1.8 eV, respectively, by increasing the W% from 10 to 40. X-ray absorption spectroscopy reveals that, for W% < 50, W is present in both metallic and suboxide states, whereas, for W% ≥ 50, only metallic W is seen. This transition from dielectric to metallic character at W% ∼ 50 is accompanied by an increase in the electrical and thermal conductivity and the disappearance of a clear band gap in the absorption spectrum. The density of the films increases monotonically from 3.1 g/cm3 for pure Al2O3 to 17.1 g/cm3 for pure W, whereas the surface roughness is greatest for the W% = 50 films. The W:Al2O3 nanocomposite films are thermally stable and show little change in optical properties upon annealing in air at 500 °C. These W:Al2O3 nanocomposite films show promise as selective solar absorption coatings for concentrated solar power applications.

We report a simple process for the selective deposition of copper films on RuO2, while no Cu nucleation occurs on thermal SiO2 or porous carbon doped oxide (CDO). Using the precursor Cu(hfac)VTMS, selectivity is attained by adding a co-flow of excess VTMS to act as a growth inhibitor. With precursor alone, 52 nm of Cu grows on RuO2; on CDO or on thermal SiO2, nucleation is delayed such that 41 or 1.3 nm are deposited, respectively. Repeating the experiment with the co-flow of VTMS affords a 12 nm thick Cu film on RuO2 with roughness of 1.8 nm. But on CDO or thermal SiO2, the Cu deposition is only 0.10 or ∼0.04 nm, respectively. AFM scans of the CDO and SiO2 surfaces are identical to the bare substrates. The small quantity of Cu that is deposited must be finely distributed, presumably on defect sites; it can be etched to below the RBS detection limit using a co-flow of Hhfac and VTMS for few minutes at the end of the growth. The process window is wide: selective growth occurs for a range of VTMS pressures (0.5–2.0 mTorr), growth times (up to 90 min), and growth temperatures (up to 180°C).

During the chemical vapor deposition of thin films, a molecular inhibitor can be added to control the morphology during the nucleation stage and/or the conformal coating behavior during the growth stage. The authors use this control strategy to determine the separate influence of nucleation morphology and of conformal growth on the final surface roughness, evaluated through the power spectral density of AFM height data. The experimental system is HfB2 deposition from the precursor Hf(BH4)4 using NH3 as the inhibitor. For a nucleation layer consisting of mounds, the low frequency (long lateral range) roughness cannot be reduced by the overgrowth of film, even with the conformal growth conditions. Conversely, when the nucleation layer consists of a dense compact of islands, the low frequency roughness remains low throughout film growth, even when carried out in the nonconformal growth conditions. In all cases, the high frequency portion of the roughness decays in a similar manner, indicating that short-range smoothing mechanism is operative. The sensitivity of the final surface roughness to the morphology of the initial nucleation layer demonstrates that “shadowing” by peaks in the surface height is a strong kinetic driving force for roughening, consistent with previous theory. The use of an inhibitor molecule in CVD provides a means to obtain ultrasmooth films on relatively unreactive substrates, without the need for surface activation and without changing the film composition.