Time resolved electron densities, temperatures and energy probability functions (EEPFs) of modulated-power glow discharges through argon and helium in the Gaseous Electronics Conference reference reactor have been measured using an RF compensated Langmuir probe and a microwave interferometer. RF power was capacitively coupled to the glow and square wave amplitude modulated with a 50% duty cycle and 100% modulation depth. We found that a metastable-metastable ionization reaction can produce energetic electrons during the afterglow. This reaction can also cause the electron density to increase during the afterglow despite the RF excitation being off. The electron density as a function of time can be modelled and the result is an estimated metastable atom density as a function of time. The hot electrons in the EEPF can also be modelled, but the modelling result does not fit the experimental EEPF until the smoothing of the EEPF caused by the experimental method is taken into account. This smoothing of the EEPF can be accounted for using the Druyvesteyn method formula and indicates that accurate measurements of the EEPF in very low electron temperature plasmas can become difficult. In effect, one should have some knowledge of the shape of the EEPF before the experiment in order to obtain an accurate measurement. The electron density and EEPF results become self-consistent once the smoothing is taken into account.By moving the Langmuir probe along the diameter of the chamber it was determined that the electron density decreases more quickly between the electrodes than outside the electrode edges. This causes the plasma density profile in argon to becomes doughnut shaped during the afterglow and causes the glow to re-ignite from the edges into the centre. The electron temperature at re-ignition in helium discharges can become larger than that at steady state in the active glow. It quickly relaxes to the steady state value. This last effect is not nearly as pronounced in argon.
Time-resolved measurements of pulsed discharges can provide information on how negative ions can be used for surface processing. Negative ions are ordinarily trapped inside the plasma volume, but pulsed plasmas allow for efficient negative ion extraction during the afterglow period because the negative ion to electron concentration ratio can increase dramatically. In addition, high-density sources can facilitate negative ion extraction because of their thin sheaths and remote position with respect to the processing wafer. In either case, the first negative ions to reach a processing surface are likely to have crossed the bulk of the sheath region as electrons and attached near the surface.
A Langmuir probe biased above or below a floating potential draws net electron or ion current. As a result, a different electron current must flow through the grounded electrode sheath to maintain the plasma's time-averaged quasi-neutrality. The direct current components of the sheath, plasma and floating potentials must change from their values at zero probe current in order to change the electron current to the grounded electrode. The rate at which the floating potential changes with probe (and sheath) current is called the sheath resistance. Analysis of probe data could give incorrect values for the plasma potential, the electron temperature and the electron density (n e ), and give a distorted electron energy probability function if sheath resistance is ignored. The sheath resistance was measured using a Langmuir probe and a microwave interferometer in inductively coupled plasmas in argon, nitrogen and an argon(95%)/chlorine(5%) mixture in the Gaseous Electronics Conference reactor. The sheath resistance was found to be small for these measurements, consequently if it had been ignored, the electron density would have been underestimated by up to 42% and the electron temperature overestimated by up to 31%. The sheath resistance was found to have an n −1 e dependence in argon as expected, but an n −1.4 e dependence in nitrogen and an n −0.6 e dependence in the argon/chlorine mixture.
The electron and ion densities, electron temperatures, and electron energy probability functions (EEPF) of modulated-power glow discharges through argon and helium in the Gaseous Electronics Conference reference reactor have been measured using an RF compensated Langmuir probe and microwave interferometer. RF power was capacitively coupled to the glow and square wave amplitude modulated with a 50% duty cycle and 100% modulation depth. The time resolution of the probe was approximately 10 µs and of the interferometer better than 1µ s. We found that a metastable-metastable ionization reaction can produce hot electrons in the EEPF. In addition this reaction can cause the electron density to increase in the afterglow rather than decrease as one might first expect. By moving the Langmuir probe along the diameter of the chamber it was determined that the electron density decreases more quickly between the electrodes by diffusion. This gives rise to a plasma density in argon which becomes somewhat donut shaped during the afterglow and causes the glow to re-ignite from the edges into the center. The electron temperature at re-ignition in helium discharges can become larger than that at steady state in the active glow. It quickly relaxes to the steady state value. This effect is not nearly as pronounced in argon. MOTIVATION AND BACKGROUNDModulated power discharges (also called pulsed discharges) have exhibited advantages over continuous glows in a wide variety of circumstances. Researchers have found that the discharge chemistry can be altered, [1][2][3][4] the properties of deposited films can be altered, 5 , 6 the processing rate can be maintained despite the lower average power, 7 -9 dust particle formation can be reduced 6 , 1 0 and trenching notching and charging damage can also be reduced.1 1 -1 4 At least some of these advantages are derived from how the plasma turns on and off.
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