Rüdiger Reuter

Calibration of a miniaturized retarding field analyzer for low-temperature plasmas: geometrical transparency and collisional effects

Retarding field analyzers (RFAs) are important diagnostics to measure fluxes and energies of ions impinging onto the wall of a plasma reactor. Any quantitative use of the data requires a proper calibration, which is here performed for a miniaturized RFA. The calibration accounts for the transparencies of the RFA grids as well as for collisions inside the RFA. An analytical model is derived which covers both geometrical and collisional effects. The model is calibrated and experimentally verified using a Langmuir probe. We find that the transparency of an RFA is a random variable which depends on the individual alignment of the RFA grids. Collisions inside the RFA limit the ion current transfer through the RFA at higher pressures. A simple method is presented which allows one to remove these artefacts from the RFA data and to obtain quantitative ion velocity distributions.

Fundamental aspects of substrate biasing: ion velocity distributions and nonlinear effects

Ion bombardment of the substrate is a significant parameter in plasma processing such as dry etching or thin film deposition. The ion bombardment is described by ion velocity distribution functions (IVDFs), which were here measured quantitatively at a sinusoidally and non-sinusoidally biased electrode. The electrode voltage was monitored and controlled in the frequency domain using fast Fourier transformation. IVDF measurements were performed by a floating retarding field analyzer. A full modulation of the IVDF by arbitrary bias waveforms is only achieved if sufficiently high sheath voltages are used. If the applied sheath voltages become too low, the IVDFs are only partly determined by the RF bias waveforms and the system response becomes nonlinear. An analytical sheath model is derived from the experimental data, which accounts for arbitrary bias waveforms as well as for collisional and nonlinear effects in the sheath. It is shown that a combined DC and RF biasing of the electrode is required to gain full control over the ion bombardment of the substrate.

Mass spectrometry of positive ions and neutral species in the effluent of an atmospheric pressure plasma with hexamethyldisiloxane and oxygen

The effluent of a non-equilibrium atmospheric pressure plasma jet in He with admixture of hexamethyldisiloxane (HMDSO) and O2 has been investigated by means of molecular beam mass spectrometry. Positive ions and neutral plasma chemistry products have been detected and their possible role in the deposition of good-quality SiO2 films is discussed. Positive ion spectra reveal the presence of protonated water clusters and H+ : HMDSO and H3O+ : HMDSO ions. These ions are most probably produced by photoionization. This is corroborated by optical emission spectroscopy data obtained in the wavelength range of 50–300 nm, where helium excimer continuum emission centred around 84 nm has been observed. No ion driven polymerization products of HMDSO have been detected. Measurements of neutral species have allowed the quantification of the HMDSO depletion and absolute densities of trimethylsilanol and pentamethyldisiloxane. Two neutral polymerization products have been observed as well. The results indicate that the Si–O bond of HMDSO is preferentially broken. Additionally, the mass balance of plasma chemistry products is discussed.