No abstract

Measurements of ion energy distribution functions were made at the grounded electrode of a parallel plate radiofrequency discharge by means of a gridded energy analyser in discharges in Ar, He, and O2 at pressures of the order of 0.001-0.1 mbar. In this regime the transition from the collisionless space charge sheath (in front of the electrode) to a sheath dominated by collisions, mainly charge exchange collisions, appears. Although the Debye length is small compared to the mean free path for charge exchange, for high sheath voltages the sheath thickness increases and may exceed the mean free path. Measured distributions in Ar are compared to energy distribution functions calculated in a self-consistent sheath model, including charge exchange collisions. The agreement of theory and experiment is very good.

RF currents at frequencies well above the ion plasma frequency ω pi can be easily suppressed in the space charge sheath of a Langmuir probe by using a suitably chosen low capacitance series resistor R. Such a resistor was used to eliminate the influence of RF currents on the probe characteristics: for blocking harmonics in RF discharges with driving frequencies of the order of ω pi , such as the commonly used 13.56 MHz discharges and also for blocking the fundamental in discharges at higher frequencies (27 MHz and above). The influence of the resistor R on the probe characteristics was eliminated either numerically or automatically by including the series resistor into the negative feedback loop of a current-voltage converter circuit made from a low frequency operational amplifier. Its output was directly used as the probe current signal. RF blocking by a combination of a resistor and two resonant coils for 13.5 MHz and 9.5 MHz respectively, is shown to result in probe characteristics which are unperturbed even around the space potential.

The influence of chemo-ionization processes on the shape of the electron distribution function (EDF) is investigated both by solving the Boltzmann equation and by measurements in a dc low-pressure (0.3 Torr) low-current (i = 10-80 mA) helium discharge. Probes were used to measure EDFs, electron densities and the axial and radial electric field strengths. The concentrations of metastable helium atoms in triplet and singlet states were measured using an optical absorption method. For calculating distribution functions the kinetic equation was solved analytically in local and nonlocal approximations. Good agreement is achieved between experiment and the nonlocal approximation for electron energies above 5 eV. Chemo-ionization is found to influence the high-energy tail of the EDF and thus the rates of electron impact excitation and ionization.