Session DT1: Plasma Surface Interaction I

Photoemission induced plasma breakdown

Laser-induced photoemission of electrons offers opportunities to trigger and control plasmas and discharges. However, the underlying mechanisms are not sufficiently characterized to be fully utilized. Photoemission is highly nonlinear, achieved through multiphoton absorption, above threshold ionization, photo-assisted tunneling, etc., where the dominant process depends on the work function of the material, photon energy and associated fields, surface heating, background fields, etc. To characterize these effects, Townsend breakdown experiments were performed and interpreted using a quantum model of photoemission. In the low-current regime considered, it is found that laser-induced photoemission is sufficiently de-coupled from space charge effects to be observable. The effect of laser heating of the electrode and the dominant photoemission mechanisms are characterized for different reduced electric fields and laser intensities and photon energies (<6.3 eV). Experiments were performed using a tunable picosecond laser that allowed the use of a two-temperature model for electrode heating.   

GEC Student Excellence Award Finalist Presentation - Dynamic surface surrogate model trained on atomistic data of AlN sputter depositions

Modeling plasma-surface interactions is an often encountered multi-scale and multi-physics problem. Stepwise solutions have been proposed by replacing the surface with machine learning surrogate models for non-reactive processes. However, their applicability is still limited due to missing time dependencies or computationally too demanding explorations of parameter spaces. These remedies are resolved in this work for the reactive sputter deposition of AlN by applying a novel combinatorial approach to establish an internal surface state, which may evolve in time. Surface processes are initially studied by means of hybrid reactive molecular dynamics / force-bias Monte Carlo simulations, utilizing a therefor derived charge transfer equilibration model and a revised COMB3 AlN potential. The results are used to train multiple ensembles of physics-constrained artificial neural networks, which form a dynamic surface surrogate model for a wide range of working conditions. This model can be readily coupled to plasma simulations and diagnostics to predict realistic wall interactions (with molecular dynamics fidelity) as well as the transient evolution of surfaces.   

Strategies to Enhance Etch Selectivity During Fluorocarbon Plasma-Assisted Atomic Layer Etching of Silicon-Based Dielectrics

Stringent processing windows are required for the fabrication of sub-7-nm semiconductor devices, which in turn place severe constraints on conventional plasma-assisted etching. Atomic layer etching (ALE) is a promising etching technique that can provide high etch fidelity, directionality, atomic-scale control, and selectivity to meet and even exceed the process constraints. In this presentation, I will primarily focus on two points: identification of the underlying surface phenomena during ALE of both SiO₂ and SiN_x with fluorocarbon gas (C₄F₈, C₄F₆) plasmas using in situ attenuated total reflection Fourier transform infrared spectroscopy combined with in situ four wavelength ellipsometry; and using selective gas-phase surface functionalization to enhance the overall etch selectivity of SiO₂ to SiN_x and vice versa. This approach of selective functionalization of surfaces is derived from area-selective atomic layer deposition and can be extended to other materials.

    

Secondary electron emission due to atomic and molecular iodine ion bombardment

A fundamental plasma-surface interaction is the emission of secondary electrons due to ion bombardment. While secondary electron emission (SEE) yields are available for many different target materials and projectiles, limited data exists for iodine ions. Iodine is an emerging alternative propellant for electric propulsion systems due to its lower cost and attractive physical properties. However, it is reactive with some materials and displays more complex plasma collisional reaction processes that can form both atomic (I⁺) and molecular ions (I₂⁺). Here we present an electrostatic probe technique and perform measurements of the SEE yield of different target materials (including Mo, W, Al, Ti, Cu, carbon-carbon, and steel) exposed to multi-species iodine ion beams with an energy between 600-1400 eV. Ion beams are generated using a gridded ion source and the beam composition is determined using time-of-flight spectrometry. While the SEE yield is found to depend only marginally on the beam composition for some target materials, the yields for both Cu and steel (which are reactive with iodine) are strongly affected. This suggests the formation of iodide layers leading to a modification in target surface properties.

    

Evidence of the dominant production mechanism of ammonia in a H2/N2 plasma

H₂/N₂ plasmas are used in a plethora of applications ranging from astrophysical comparisons to the generation of ammonia for industrial usage. Whilst the formation of ammonia is not quantitatively understood, it is widely accepted that this takes place via the progressive hydrogenation of nitrogen species at the surface through both the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. Absolute ground state atomic hydrogen densities were measured, by the utilization of two-photon absorption laser induced fluorescence, in a low-pressure electron cyclotron resonance plasma as a function of nitrogen admixtures. It was observed that a secondary population of atomic hydrogen exists due to the photolysis of ammonia that was further confirmed by mass spectrometry. This nascent contribution rapidly dominates the plasma produced atomic hydrogen, with nitrogen admixtures as low as 2500 ppm, and indicates an exceptionally high nitrogen dissociation degree: a factor of 500 larger than for hydrogen. Thermally loading the reactor showed a decrease in the ammonia density for increasing temperatures, this can be explained by considering the thermally dependent recombination coefficients of the ER and LH mechanisms, from which it can be stated that the LH mechanism is dominant.