Deuterium Ion Flux Absolute Quantification from an ECR Plasma Source

Surface modification, thin-film deposition and etching and other technical processes involve the use of plasma discharges. Quantitative characterization of plasma sources has become increasingly important as it provides a fundamental understanding of plasma sources and allows for the technological optimization of processes.

As hydrogen plasmas play a vital role in technical discharges, it is possible to accurately determine the absolute hydrogen fluxes to a plasma-treated surface. This is critical for knowing both the discharge mechanism and the plasma-induced processes at the surface.

Electron-Cyclotron-Resonance Plasma Source

The characterization of customized electron-cyclotron-resonance (ECR) plasma sources (Figure 1) with a biased sample holder was carried out corresponding to the deuterium fluxes impacting on a substrate holder.

A HIDEN plasma monitor (Model HIDEN EQP 300) was used to measure the mass distribution of the impinging ions. The flux approaching the usual sample position was measured upon inserting the plasma monitor into a mock-up sample holder with a 10µm aperture. The mock-up sample holder was made of soft iron to prevent the magnetic field required for the ECR discharge from affecting the plasma monitor.

Moreover, the ion optics of the plasma monitor was surrounded by an additional soft iron shielding cylinder. At the time of operation, the pressure was maintained at 10^-7 to 10^{- 6}Pa through differential pumping of the plasma monitor. A retarding field analyzer (RFA) was used for absolute quantification of the mass-integrated ion flux to the sample holder.

Figure 1. Schematic view of the experimental set-up. To quantify the particle flux to the substrate the substrate holder is replaced by a mock-up containing the EQP plasma monitor (not shown).

Figure 2 shows the resulting ion fluxes impacting on the substrate holder for a 144W microwave power as a function of operating pressure between 0.3 and 6Pa. D₃⁺ ions were the dominating ion species impinging on the sample surface at the lowest investigated pressure of 0.3Pa. An ion-molecule reaction in the bulk plasma was the main production pathway for these ions. A D₃⁺ and a D atom were produced as D₂⁺ ion strikes a D₂ molecule.

The probability for this reaction increases with increased pressure, so that the composition of relative ion species changes. With the continuous increase in pressure, D₃⁺ ions were produced along with the primary D₂⁺ ion generated in the bulk plasma through electron-induced ionization of D₂ molecules. The total target current was decreased at roughly 1.5 Pa pressure owing to the reduction in the plasma density while the relative contribution of D₃⁺ was kept increasing.

The contribution of D₂⁺ at the highest investigated pressure (6 Pa) was almost negligible. Unlike D₂⁺, no change was observed in the fraction of D⁺. D⁺ was produced by direct ionization of atomic D and by electron-induced ionization of D₂ molecules, yet the cross section for an ion molecule reaction of D⁺ is lower when compared to that of D₂⁺.

Figure 2. Ion-species-resolved deuteron flux as a function of the D₂ gas pressure, measured at a constant microwave input power of 144W and with the sample holder at floating potential.

The total deuteron flux in the ion form was estimated to be 5.6×10¹⁹Dm^-2 for the standard operating parameters of the plasma source, i.e. a D₂ plasma at p = 1.0Pa, PMW = 144W and the sample holder at floating potential.

This results in a total ion flux of 1.9×10¹⁹ions m^-2. From the total ions produced, 3% were D⁺ ions, 94% were D⁺³ and another 3% were D⁺². When the DC bias voltage was increased to -600V, the ion flux was monotonically increased by a factor of 2.

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

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