Evolution of electron energy distribution function in capacitively coupled argon plasma driven by very high frequency

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Capacitively coupled plasma driven by a very high frequency power has attracted much attention due to its rather independent control of ion flux and energy. In this paper, Langmuir probe diagnostic technique is used to observe the evolution of plasma properties such as electron energy distribution function, electron temperature and density, etc. Our experiment is performed in capacitively coupled argon plasma driven by a 40.68 MHz frequency. Experimental results show that the electron energy probability function changes from bi-Maxwellian type to single-Maxwellian type and then to Druyvesteyn type with the increase of the discharge pressure. At a low gas pressure, the electron collisionless heating in bulk plasma leads to bi-Maxwellian type in electron energy possibility function (EEPF), which has a double temperatures structure in EEPF. As the gas pressure increases, the electrons with low energy are able to collide with the neutral species more frequently, thus they gain energies through collisional heating. Therefore, these electrons can overcome the dc ambipolar potential and the collisional heating becomes a main electron heating mechanism. Increasing the input power enhances the electron population with low energy. From the discharge center to the edge, electron population with low energy decreases clearly due to the dc ambipolar potential, and they are unable to reach an oscillating sheath where collisionless heating occurs. However, electron population with high energy is slightly increased. The result indicates that more uniform plasma can be achieved at a high gas pressure. Additionally, EEPFs are measured for different discharge gaps between electrodes. The change of electrode gap for the plasma leads to a transition of electron heating mode along the axial direction. In order to characterize the electron behavior further, we introduce the ratio of the cold electron density to hot electron density (α) and the ratio of cold electron temperature to hot electron temperature (β). The ratios also show the proportional distributions of the cold and hot electron populations. The electrode gap has a great influences on α while little influence on β. When the discharge gap between electrodes varies from 20 to 40 mm, α changes from 0.2 to 0.5 while β has the same trend. Spatial distributions of electron density and temperature with low and high energy are also discussed.