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 SiO 2 or SiN x , 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, O 2 , a weak oxidant, forms a ∼0.3 nm thick layer of Cu 2 O, 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 H 2 O, are efficiently pumped away; H 2 O, 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.
We report a simple process for the selective deposition of copper films on RuO 2 , while no Cu nucleation occurs on thermal SiO 2 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 RuO 2 ; on CDO or on thermal SiO 2 , 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 RuO 2 with roughness of 1.8 nm. But on CDO or thermal SiO 2 , the Cu deposition is only 0.10 or ∼0.04 nm, respectively. AFM scans of the CDO and SiO 2 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).
Thin films of manganese nitride MnxNy are grown by chemical vapor deposition (CVD) using the new precursor bis(2,2,6,6-tetramethylpiperidido)manganese(II), Mn(tmp)2 = Mn(NC9H18)2, with ammonia as a coreactant. This precursor can be prepared in high synthetic yield and has good thermal stability at room temperature; it is one example of a new class of precursors that have the potential to deposit late transition metal nitrides. Under low-pressure CVD conditions, the precursor reacts with ammonia to afford MnxNy thin films in the temperature range of 50–350 °C. The stoichiometric ratio x/y is 2.3–2.5 for all growth conditions used, with oxygen and carbon impurities less than 7 at.% and 1 at.% in the bulk, respectively, as analyzed by x-ray photoelectron spectroscopy. The MnxNy films are x-ray amorphous and are characterized by low root-mean-square surface roughness, 0.4–0.7 nm. Film thickness profiles on trench substrates indicate that growth contains species of both high and low sticking probabilities. The proposed mechanism of film growth is a combination of gas phase and surface transamination between the precursor and ammonia to afford reactive intermediates responsible for film growth.
Superconducting thin films of vanadium nitride have been grown by low temperature (250–300 °C) chemical vapor deposition from tetrakis(dimethylamido)vanadium (TDMAV) and ammonia. For example, films grown from TDMAV (1 sccm Ar as carrier gas) and 7 mTorr ammonia at 300 °C are nanocrystalline (cubic δ-phase) with an average crystal size of 20 nm, have relatively low room temperature resistivities of 250 μΩ cm, and are superconducting with critical temperatures as high as 7.6 K (versus a bulk value of 9 K). The films have a V:N ratio of 1:1, with a carbon content of <5 at. % and an oxygen content of <3 at. % (as determined by high resolution XPS). The V 2p3/2 and N 1 s XPS binding energies of 513.5 and 397.3 eV, respectively, are consistent with the presence of a nitride phase. In contrast, films grown at lower temperatures <200 °C show carbon incorporation, have a much higher resistivity of ∼3000 μΩ cm, and are not superconducting. The results suggest that, at low temperatures, the thermally activated transamination reaction with ammonia becomes too slow to remove dimethylamido groups from the surface, resulting in carbon-rich films (10–15 at. % carbon). The conformal step coverage of the VN films depends on the growth conditions. For thermal growth of nonsuperconducting films at 150 °C, the step coverage is >95% in trenches of an aspect ratio of 4:1; for superconducting films grown at 250 °C, the step coverage is 65% for an aspect ratio of 3:1. At 150 °C, near-stoichiometric films with <2 at. % carbon and <3 at. % oxygen can be deposited if the gaseous ammonia is precracked by a remote plasma source; the resulting films have low resistivities of 320 μΩ cm but are not superconducting down to 4 K.
The activation of inert C─H bonds by transition metals is of considerable industrial and academic interest, but important gaps remain in our understanding of this reaction. We report the first experimental determination of the structure of the simplest hydrocarbon, methane, when bound as a ligand to a homogenous transition metal species. We find that methane binds to the metal center in this system through a single M···H-C bridge; changes in the 1 J CH coupling constants indicate clearly that the structure of the methane ligand is significantly perturbed relative to the free molecule. These results are relevant to the development of better C─H functionalization catalysts.
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