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The dependence of the electron density and the mean ion energy relative to the ground electrode on the driving frequency (13.56?156?MHz) is investigated in capacitively coupled argon plasmas. It is found that at constant rf power, the electron density increases with the driving frequency then starts to level off after reaching some transition frequency. When the driving frequency exceeds this transition frequency, almost all rf power is spent on electron heating and the electron density remains unchanged even when the frequency continues to increase. The measured data is compared with the results from an inhomogeneous plasma model. The variations of the measured electron density and mean ion energy are found to be consistent with the scaling between the electron heating and the ion acceleration power and their dependence on driving frequency, as obtained earlier by Godyak.

Boyle et al (2004 J. Phys. D: Appl. Phys. 37 1451) introduced a sheath model for dual-frequency capacitive discharges. The electron sheath edge position s and the dc potential Φdc in the sheath are obtained with the assumption α/β >> 1 (where α/β is the ratio of the low-frequency electric field to the high-frequency electric field). However, α/β ≤ 4 is usually found in processing applications. Under this condition, we show that the Boyle et al model gives multiple values for the dc potential Φdc(x) at any given position x in the sheath. For this reason, we introduce a model without the assumption α/β >> 1. By comparing the results from the two models, it is found that, as the ratio α/β approaches 1, the sheath thickness sm and the sheath dc voltage Vdc obtained with the Boyle et al model are significantly underestimated.

An analytical model is proposed for time-averaged ion energy distributions (IEDs) in collisional rf sheaths (?i?sm, where ?i is the ion mean free path and sm is the sheath thickness), in which charge transfer is the dominant ion-neutral collision mechanism. Our model is different from the model in Israel et al 2006 J. Appl. Phys. 99 093303 in two aspects. Firstly, to calculate the ion flux, we consider ions created in both the space charge region and the quasi-neutral region instead of only the quasi-neutral region. Secondly, to calculate the ion energy, we use an rf-modulated transit time instead of only the average transit time. Consequently, our model is valid over a wider pressure range (from a weakly collisional sheath to a strongly collisional sheath) compared with the model in Israel et al 2006 J. Appl. Phys. 99 093303. Our model shows that, in a collisional rf sheath (argon, 5?Pa, 27.12?MHz and 100?W), 65% of the ion flux in the IED comes from secondary ions created in the space charge region. Results of our model are compared with those obtained by measurement, direct integration method and particle-in-cell/Monte Carlo collision simulation.

The temporal evolution of electron density in a nanosecond pulsed argon microplasma is measured using a combination of Stark broadening and the optical emission line-ratio method. In the initial discharge period (0–100 ns), the electron density can reach as high as ∼1018 cm−3. It decreases to ∼1017–1016 cm−3 in the early afterglow period (100 ns–1 µs after the ignition) and ∼1016–1013 cm−3 in the late afterglow period (1–20 µs). It is demonstrated that the optical emission spectroscopy (OES) line-ratio method can obtain the electron density in the range 1013–1016 cm−3, while in the range 1016–1018 cm−3, the Stark broadening technique with argon 2p–1s lines (in Paschen's notation) is a better choice. These results are in good agreement with those from the Stark broadening technique with hydrogen Balmer lines. Finally, a possible mechanism for such a density evolution is briefly discussed.

A simple collisional–radiative model for argon and xenon is used, in conjunction with optical emission spectroscopy (line-ratio technique), to determine the electron temperature and electron density in low-pressure discharges containing argon and xenon. Satisfactory agreement is obtained between this method and the Langmuir probe for an inductively coupled plasma containing neon, argon and xenon. This method is applied for a capacitive discharge containing fluorocarbon, argon and xenon. The electron temperatures and electron densities obtained under various discharge conditions are compared with those reported in the literature by other techniques.

Gas temperature, electron density and electron temperature of a microwave excited microplasma are measured by optical emission spectroscopy. This microplasma is generated in the small gap of a microstrip split-ring resonator in argon at near atmospheric pressure. When less than 100 ppm of water is present in the plasma, the gas temperature can be obtained from the rotational temperature of the hydroxyl molecule (A 2Σ+, v = 0) and the electron density can be measured by the Stark broadening of the hydrogen Balmer β line. According to a collisional–radiative model, the electron temperature can be estimated from the measured excitation temperature of argon 4p and 5p levels. It is found that the values of these parameters (gas temperature, electron density and temperature) increase when the gap width of the resonator is reduced. However, when the microwave power increases, these parameters, especially the electron density, do not vary significantly. Discussions on this phenomenon, being very different from that in the low-pressure bounded discharges, are provided.