Quadrupole mass-spectroscopy has reported significant neutral and ionic species in the unique C5HF7/O2/Ar plasma, including CxFy (X>2), CxHFy, and CxFy (Y/X<2). Using C5HF7/O2/Ar plasmas, plasma etching of SiO2 films with high selectivity against SiN films was achieved in comparison with traditional C5F8/O2/Ar plasmas.
The results of the analysis showed that the mechanism involves impinging the C-rich hydrofluorocarbon species in the novel plasma to create a thick C-rich fluorocarbon film on SiN.
Through electro-impact ionization at 70eV, C5F8 and C5HF7 are predominantly fragmented into C3F3 and C3HF2, respectively. H-containing species were detected by the fragmentation pattern for C5HF7 by substituting one F atom with one H atom, i.e. CF2→CHF, C2F2→C2HF, CF3→CHF2 and so on. Figure 1 shows the schematic of experimental apparatus.
Figure 1. Schematic of experimental apparatus
The dissociation, ionization and attachment in real plasma for dielectric etching are triggered by collisions with electrons with a Maxwellian energy of a few eV. This created a huge difference in gas chemistries between neutral species with and without H atoms for the actual C5F8/O2/Ar and C5HF7/O2/Ar plasmas.
When combined with quantum chemical calculations, the results (Figure 2) revealed C5F8→CF2+C4F6 as the major pathway of dissociation of the cyclic C5F8 molecules. Further, multiple dissociations resulted in smaller fragmentation after the reactions; CF2→CF+F, or C4F6→C3F3+ CF3. This makes it possible to detect large fractions of CF3, CF2, CF, C3F3 and each related H-substituted species, including CHF3, CHF, CH, and C3HF2.
Figure 2. Quadrupole mass analytical results of neutrals in C5F8/O2/Ar or C5HF7/O2/Ar plasmas
As the entrance of the quadrupole mass spectrometer was present at the chamber wall of the commercialized reactor, only information about the composition of positive ions at the closed chamber wall excluding the sample surface could be obtained.
Besides Ar+, ions like CF3+, CF2+, CF+ are the other major positive ionic species that were detected. The coincidence between the nature of large molecule ions – CxFy or CxHFy and selective etching of SiO2 films was a result of the selective formation of the C-rich fluorocarbon layer on SiN films.
This research is mainly focused on the characterization of gas molecules to improve etching. The key aspects that improve the etching performances include etched profile control, material’s selectivity, etch rates are surface reactions that are in accordance with plasma chemical properties.
This makes feedstock gases a major concern. In the etching of SiO2, the main gases of selection have changed over time, for example, CF4, C2F6, C4F8, C4F6, and C5F8. In addition, H- and O-containing species such as CHF3, CH2F2, C3F6O, C5F10O, etc. can be used as controls, taking into account the amount of F atoms used in the plasma chemistry.
Researchers have highlighted the significance of the relationship between the etching properties and the chemistry of novel gases for scientific and industrial applications. Hence, they are performing continuous research to interpret the etching process via the determination of the gas phase and surface analysis.
This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.
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