The quality of the thin oxide films deposited on a variety of materials during exposure to plasmas containing oxygen depends strongly on the identity and energies of the negative oxygen ions arriving at the growing films. There have, however, been few experimental studies in which these ion energies have been measured and the available data is markedly less abundant than for the corresponding positive ions. We discuss reasons for the lack of data and suggest suitable techniques for obtaining energy distributions for mass identified ions for a variety of plasmas, including both steady-state and pulsed DC and RF plasmas. For asymmetric RF plasmas the distributions depend on the relative dimensions of the sheath regions in front of the discharge electrodes, whereas for DC magnetron systems a dominant parameter is the voltage applied to the magnetron cathode, particularly when this is pulsed. Sample data for O , O 2 ,and O 3 ions are shown for mixtures of oxygen and argon for a number of systems including a DC plasma system in which the ions were sampled through an orifice in the anode electrode, and a small magnetron device for which the ions arriving at a grounded substrate were observed.

Electrical plasmas operating at atmospheric pressure are in widespread use for materials processing and in related areas such as the characterisation of surfaces. The plasma sources are of many types. improvement of their design and optimisation of their operation require high quality diagnostic data. It is very helpful, for example, to have available information on the nature and energies of the active species produced in a source and impacting on a surface. The extension of diagnostic techniques first developed for work with low pressure plasmas to a wide variety of atmospheric plasmas is discussed, together with examples of the data obtainable.

It is known that negative ions are important in the plasma oxidation of silicon and silicon nitride surfaces and there is interest in the use of nitrous oxide instead of oxygen as the plasma gas. The present paper describes an investigation into the distribution of energies with which the O- ions produced in a capacitive rf plasma impinge on the grounded discharge electrode and compares these energies with those of the positive N2O+ ions produced under the same plasma conditions. It is shown that the maximum energy of the O- ions is largely determined by the d.c. bias of the driven electrode and is independent of changes in the plasma potential. It is, therefore, possible in such systems to select, independently: the energies of the positive and negative ions. The energy distribution of the O- ions shows interesting structure, which for a given d.c. bias, is a function of the input rf signal. The investigation confirms that, for strongly asymmetric geometries, negative ions produced in the sheath at the driven electrode of a capacitive rf plasma system travel through the plasma and impact on the counter electrode with a range of energies.

We measured energy distributions of negative ions at the grounded electrode of an oxygen parallel-plate rf discharge. Negative ions are generated in the plasma sheath in front of the rf electrode, are accelerated away from the driven electrode, and can be detected at the grounded electrode. The maximum energy of the negative ions corresponds to the negative self-bias voltage of the rf electrode. Structures in the energy distribution reflect sheath properties and characteristics of the ion generation processes.