Absorption cross-section spectra for gold nanoparticles were calculated using fully quantum Stochastic Density Functional Theory and a classical Finite-Difference Time Domain (FDTD) Maxwell solver. Spectral shifts were monitored as a function of size (1.3-3.1 nm) and shape (octahedron, cubeoctahedron, and truncated cube). Even though the classical approach is forced to fit the quantum TDDFT at 3.1nm, at smaller sizes there is a significant deviation as the classical theory is unable to account for peak splitting and spectral blue shifts even after quantum spectral corrections. We attribute the failure of classical methods at predicting these features to quantum effects and low density of states in small nanoparticles. Classically, plasmon resonances are modeled as collective conduction electron excitations, but at small nanoparticle size these excitations transition to few or even individual conductive electron excitations, as indicated by our results. * These authors contributed equally to this work. arXiv:1805.00538v1 [physics.atm-clus] 1 May 2018

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.

The formation of thin oxide films is of significant scientific and practical interest. In particular, the semiconductor industry is interested in developing a plasma atomic layer etching process to pattern copper, replacing the dual Damascene process. Using a nonthermal oxygen plasma to convert the metallic copper into copper oxide, followed by a formic acid organometallic reaction to etch the copper oxide, this process has shown great promise. However, the current process is not optimal because the plasma oxidation step is not self-limiting, hampering the degree of thickness control. In the present study, a neural network potential for the binary interaction between copper and oxygen is developed and validated against first-principles calculations. This potential covers the entire range of potential energy surfaces of metallic copper, copper oxides, atomic oxygen, and molecular oxygen. The usable kinetic energy ranges from 0 to 20 eV. Using this potential, the plasma oxidation of copper surfaces was studied with large-scale molecular dynamics at atomic resolution, with an accuracy approaching that of the first principle calculations. An amorphous layer of CuO is formed on Cu, with thicknesses reaching 2.5 nm. Plasma is found to create an intense local heating effect that rapidly dissipates across the thickness of the film. The range of this heating effect depends on the kinetic energy of the ions. A higher ion energy leads to a longer range, which sustains faster-than-thermal rates for longer periods of time for the oxide growth. Beyond the range of this agitation, the growth is expected to be limited to the thermally activated rate. High-frequency, repeated ion impacts result in a microannealing effect that leads to a quasicrystalline oxide beneath the amorphized layer. The crystalline layer slows down oxide growth. Growth rate is fitted to the temperature gradient due to ioninduced thermal agitations, to obtain an apparent activation energy of 1.0 eV. A strategy of lowering the substrate temperature and increasing plasma power is proposed as being favorable for more self-limited oxidation.

Plasma-thermal atomic layer etching is a promising technique to enable selective and directional etching on metals. It involves a plasma activation step and a thermal step where etchant molecules remove a fraction of the surface. To accelerate process development, a computational model for the thermodynamics of the thermal removal step is highly desirable. An energy expression is developed here to calculate the removal step energy for an activated slab structure. The approach samples the configurations of the activated metal surfaces and determines the thermodynamic balance of the removal step for each obtained configuration. The heterogeneity of the surface terminations is treated by the equilibrium crystal shape method. The models are put to test with combinations of two modifiers (O & N), two substrates (Cu & Ni) and two etchants (formic acid and formamidine). It is found that higher coverages of modifiers leads to more favorable etching. In addition, our results show that removal step energies vary 1 among different terminations, with differences on the order of 0.5 eV. This suggests etching can preferentially occur over certain crystal terminations. Qualitative agreement with the experiment on the Ni/O/formic acid system is obtained with the layer model at high coverages of oxygen atoms.

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