Plasma Etching of SiO2 with High Selectivity Against Si3N4 and Si

Quadrupole mass-spectroscopy has reported significant neutral and ionic species in the unique C₅HF₇/O₂/Ar plasma, including C_xF_y (X>2), C_xHF_y, and C_xF_y (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.

Experimental

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 – C_xF_y or C_xHF_y and selective etching of SiO₂ films was a result of the selective formation of the C-rich fluorocarbon layer on SiN films.

Conclusion

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.

For more information on this source, please visit Hiden Analytical.