Hydrofluorocarbon Ion Density of Argon- or Krypton-Diluted CH2F2 Plasmas: Generation of CH2F+ and CHF2+ by Dissociative Ionization in Charge Exchange Collisions

The analysis of rare gas (M) diluted CH₂F₂ plasma using quadrupole mass-spectroscopy (QMS) has shown specific formation of CHF₂⁺ ion for dilution of krypton (Kr) and CH₂F⁺ ion for dilution of argon (Ar). The ion densities of CHF₂⁺ and CH₂F⁺ were estimated using pathways of dissociative ionization, within charge exchange channels of collision in CH₂F₂ plasmas. In plasmas diluted with Ar, the predomination of CH₂F⁺ ions was due to the dissociative ionization between the appearance energy of C-F and Ar⁺ (Figure 1).

Figure 1. Cross section for dissociative ionization for a CH₂F₂ molecule.

For plasmas diluted with Kr the appearance energy of C-H was due to the predomination of CHF₂⁺ ions, with a similar charge exchange channel for collisions between CH₂F₂ and Kr+. According to the analytical results, adding Kr or Ar to CH₂F₂ plasmas provided some measure of control over the fraction densities of the CHF₂⁺ and CH₂F⁺ ions (Figure 2).

Figure 2. Individual CH₂F⁺ and CHF₂⁺ ion fraction on total CH_xF_y⁺ ion density at Kr- and Ar-diluted CH₂F₂ plasma.

Dissociative Ionization Pathways

Hydroflurocarbons have H atoms present, instead of the F atoms in flurorcarbon gases. Reactions of interest involving bonds between C-F and C-H are vital for density control of the reactive species. Dissociation of the C-F bond results in F atoms, which are a main etchant for Silicon (Si), whereas dissociation of the C-H bond produces H atoms, which allow polymers to be deposited on the substrate surface. To ensure accurate processing, species balance for deposition and etching is closely related and essential to the dissociation reactions in gas-phase.

A dual frequency capacitively coupled plasma (CCP) etching reactor was used in the experiment, and a quadrupole mass spectrometer (QMS) was installed on the wall of the chamber. A Kr or Ar gas mixture with CH₂F₂ gas was introduced into the reaction chamber, and the electrode was applied with very high frequency (VHF) power to preserve the plasma.

Mass Spectrometric Measurements of Positive Ions

The measurements obtained from positive ion mass spectrometry indicate that CHF₂⁺ and CH₂F⁺ are the dominant positive ions. Two channels are involved in the ionization pathway produced for the CHF₂⁺ and CH₂F⁺ ions. CH₂F⁺ via bond dissociation of C-H or C-F. The neutral charge counter fragments of F and H atoms were concurrently generated through mechanisms of dissociation. Collisions between electrons should be taken into account when there is a larger ion density for CH₂F⁺ in plasma diluted with Ar gas. The collisions of charge exchange between ions of rare gases and molecules of CH₂F₂ occurred due to the location of appearance energies in close proximity with Kr (14 eV) and Ar (16 eV).

Continuous studies are being carried out to elucidate reactions of dissociation in the plasma, through the diagnostics at gaseous phase used in mass-spectrometric measurements.

This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.

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