We present a cyclic process for selective and anisotropic atomic layer etching of copper: an oxygen plasma modulates the depth and directionality of the oxidized layer, while formic acid vapor selectively removes the copper oxide scale from the metallic copper. Via density functional theory, with finite temperature and pressure free energy corrections, we evaluate the feasibility of formation of gas-phase Cu­(II) and Cu­(I) complexes with formate, water, formic acid, and combinations thereof as ligands. These complexes result from the neutralization reaction between copper oxide (CuO and Cu2O) and formic acid, with and without water. We identified and evaluated the formation free energies of formato, formic acid, aquahydroxo, and aquaformato complexes of Cu­(II) and Cu­(I). Under relevant experimental pressures, we find the water-free dimeric tetra­(μ-formato)­dicopper­(II) “paddlewheel” complex (Cu2(HCOO)4) to be the most favorable etching product, with its formation reaching equilibrium conditions from CuO. The most likely precursor for the dimer is the diformatodi­(formic acid)­copper­(II) monomer, which favorably dimerizes under the same water-lean condition at which the dimer persists. Stabilization of gas-phase Cu (oxide) derivatives thus can be achieved through complexation, enabling gas-phase etching of Cu. This work provides complementary experimental and theoretical studies that illuminate the nature of highly controlled etching with formic acid of nanoscopic CuO(s) layers covering Cu nanoarchitectures, which is relevant for the fabrication of next-generation integrated circuits.

Atomic layer etching (ALE) has received much attention in recent years as a viable state-of-the-art patterning technique for the fabrication of future generations of nanoelectronics. Thermal excitation or plasma activation, coupled with chemical reactions have been shown as different approaches to enable ALE. In this review, the importance of surface oxidation state is considered as a viable parameter to tailor the chemical contrast that is needed in realizing ALE. With the help of thermodynamic assessment of viable reaction pathways, an alternative approach that combines both plasma and thermal ALE concepts is proposed: the physical effect from energetic ions results in not only chemical conversion but also directionality, while the chemical effect dictates the selective removal of the converted material. This hybrid plasma-thermal ALE approach allows for a simultaneous control of selectivity and anisotropy and a wider ALE window. This approach is tested on a number of model systems and could be extended to more complex materials systems that are needed in future integrated circuits.

Schenkmann, and John Wiley & Sons Ltd. on behalf of the International Federation for Cell Biology. The withdrawal has been agreed because the authors could not be reached for fi nal approval of the publication of the article.

The stringent requirement for patterning highly absorbing metal thin films as a mask for the next-generation extreme ultra-violet lithography system dictates the development of an atomic layer etching process to tailor the etch rate and the etch profile. A “plasma-thermal” atomic layer etching process is developed where an oxygen plasma is used to convert the metallic Ni layer into NiO, followed by formic acid vapor reacting with NiO to form nickel formate [Ni(COOH)2], thereby removing nickel. The directionality of the oxygen ions is used to create a directional chemical conversion of Ni into NiO, resulting in an anisotropic etch profile. Using the SiO2 patterned Ni thin film, a high etching selectivity to the mask (virtually no etching of SiO2) and a high etching anisotropy (a sidewall angle up to 87°) are achieved.

In this work, a special focus is given to atomic layer etching (ALE) of metals, since this is a relatively new field but is expected to grow rapidly given the major advancements potentially enabled via metal incorporation throughout the manufacturing process of integrated circuits. The impact of John Coburn’s work on the development of ALE processes is analyzed with a focus on ion energy and the neutral-to-ion ratio. To realize atomic precision in removing etch-resistant materials with complex compositions or structures, the surface reactivity would replace etch rate as the parameter of interest to control the chemical contrast needed for selectivity. The desirable etching anisotropy dictates the usage of directional ions. John Coburn’s work on ion-enhanced etching of Si serves as an example that a fine control of ion energy and the neutral-to-ion ratio could be the gateway of reactivity control, which is demonstrated with recent progress on thermal-plasma ALE of Ni. The effect of surface reactivity is studied from first-principle atomistic calculations and confirms the experimental findings.