Species which can only exist in low-energy plasma have a significant impact on the plasma’s unique properties; this is one of the reasons why plasma processes need data on complete chemistry sets to be reliable.There is so much going on in a plasma that chemistries need to include both processes between feedstock gases and the products of their reactions with surfaces. So in order to have a complete chemistry set, you need information on electron collision processes as well as heavy particle collisions and surface reactions. That is all well and good, but actually calculating cross sections is not an easy task.
A team of researchers from University College London (UCL) and University of Michigan have used ab initio R-matrix calculations to calculate cross sections for electron collisions with NF3, NF2 and NF. These calculations were then used in a separate study which demonstrated the reliability of their calculations via excellent comparison with experimental measurements. The cross section data revealed includes elastic collisions, momentum transfer, electron impact dissociation, dissociative electron attachment, dissociative ionisation and ionisation. Once complete, the cross section set generated was tested in a global model simulation for low pressure, inductively coupled plasma (ICP) and the gas mixture used was Ar/NF3/O2.
The reaction mechanism for plasmas sustained in Ar/NF3/O2 developed by the UCL and University of Michigan team was compared to optical emission spectroscopy measurements of radical densities to validate results. The work was done in collaboration with researchers from Samsung Electronics and the paper ‘Insights to scaling remote plasma sources sustained in NF3 mixtures’ published. It demonstrated consistency between the model predictions, and the relative densities produced by experimental OES actinometry measurements. The only differences observed to the the axial distribution of F atoms which is thought to be due to ‘the spatial distribution of gas temperature and axial diffusion, neither of which are accounted for in the global model’.
A paper by Professor Klaus Bartschat and Professor Mark Kushner highlights the significance of using both experimental and theoretical/computational research in combination. They also explain that theoretical research has advanced to the point where it can be used to anticipate collision processes for even cases where experimental data is not or cannot be obtained. Results from the research teams working on the nitrogen trifluoride cross sections were used in the paper ‘Electron collisions with atoms, ions, molecules, and surfaces: Fundamental science empowering advances in technology’. As a result of this kind of research, plasma processes are becoming more efficient and industrial etching processes are optimised and less damaging. You can access the data which has been published in the Quantemol database (QDB) which includes full chemistry sets including heavy particle collision data for the publications above and many others.
Figure 1: Cross sections for electron impact reactions of (a) NF3 compiled by Lisovskiy et al.; and (b) NF2 and (c) NF calculated using the ab initio molecular R-matrix method.
Publications:
Calculated cross sections for electron collisions with NF3, NF2 and NF with applications to remote plasma sources
Link: http://iopscience.iop.org/article/10.1088/1361-6595/aa6bdf
Insights to scaling remote plasma sources sustained in NF3 mixtures
Link: http://avs.scitation.org/doi/full/10.1116/1.4978551
Electron collisions with atoms, ions, molecules, and surfaces: Fundamental science empowering advances in technology