Neutral dissociation in plasma after electron impact
This project investigates neutal dissociation after eectron impact in low temperature plasma. This process is important in microchip manufacturing as it largely determines chemical composition of plasma. In particular we focus on dissociation fluorine organic molecules used in plasma etching and plasma deposition. We assume that molecules in plasma dissociate after electron impact, which puts the molecule into lowest atates, usually triplet state
(1) e+M→M(T)+e.
Then dissociation into free radicals
(2) M(T)→A⋅ + ⋅B
occures after fast relaxation to the lowest triplet state. Molecular dynamics simulations can be performed similar to those of photochemistry as sketched on the fifure below
Electron impact sends the molecule to higher electronic state which leads to nonadiabatic transition and dissociation.
Despite the similarity between photodissociation and dissociation after electron impact, the latter has rarely been investigated and dissociation of triplet states created by electron impact is a novel direction of quantum nonadiabatic dynamics.
Free radicals then determine plasma deposition on the semiconductor surface. Free radical can also be ionised into stable singlet ions
(3) e+A⋅ →2e+A+
which participate in etching of the surface. Thus, both ion and radical composition of plasma should correlate with the rates of neutral dissociation after electron impact.
Based on the results of ab initio Molecular Dynamics simulations we have developed an Electrophore Model and simple rules of neutral dissociation after electron impact. Similarly to the chromophore in photochemistry electrophore is a small part of the molecule where triplet excitation is localised. For example, double bond or oxigen atom can serve as electrophore. The dissociation pattern of the triplet states produced by electron impact has several features that distinguish it from the dissociation of singlet states. Dissociation of triplet states is very fast as triplet states are more repulsive than the singlet states. The bonds containing the electrophore, such as double bond, dissociate readily, and the proximity to the electrophore also affects the dissociation yield of other chemical bonds.
The figure above shows localisation of triplet spin density around the double bond. Triplet spin character can be donated to the neighbouring bond making it repulsive and causing its dissociation.
More details can be found in the papers
- Dissociation of Hydrofluorocarbon Molecules after Electron Impact in Plasma J. Phys. Chem. Lett. 2024, 15, 3404−3411
- Rules of triplet state electron impact neutral dissociation in plasma from molecular dynamics simulations and an electrophore model J. Vac. Sci. Technol. A 43, 043003 (2025)
and in the theses
- Alfred Stonelake, Excited Triplet States during Organofluorine Molecular Dynamics upon Electron-Impact—A Study of Electronic Structure and Molecular Fragmentation
- Ryan Brook, Classical and quantum molecular dynamics simulations of dissociation after electron impact in plasma
A code for ab initio MD simulation of dissociation of triplet has been developed.