Frequency dependence of ion bombardment of grounded surfaces in rf argon glow discharges in a planar system

Abstract

The energy distribution of positive ions incident on a grounded surface in a low-pressure argon planar rf glow discharge system has been measured as a function of excitation frequency from 70 kHz to 13.56 MHz for both capacitive and direct coupling of the rf power to the excitation electrode. The results are interpreted by taking into consideration both the transit time for the ion to traverse the sheath relative to the period of the rf excitation voltage, and the resistive or capacitive characteristics of the sheaths. The importance of system geometry and of the dc potential of the excitation electrode (as determined by external circuitry) on the maximum energy of ions incident on grounded surfaces is shown.

Nanocrystalline diamond thin films deposited by 35 kHz Ar-rich plasmas

“…According to well-known concepts [15] on the plasma sheath ion dynamics and its dependence on the plasma excitation frequency, the maximum kinetic energy (neglecting excitation and ionisation processes in the sheath) of an ion reaching a surface depends on whether it moves across the sheath slowly (high frequency) or quickly (low frequency) relative to the oscillating plasma potential V p (t) [15]. The most important effect of changing the excitation frequency is that the voltages (at constant pressure and rf power) in the plasma increase as the frequency is decreased [16]. Therefore, ion bombardment energies are greater at low frequency.…”

“…If, on the contrary, the transit time is much shorter (low frequency) than the period of the rf excitation voltage the ion energies will reflect the temporal oscillations of V p (t) and their maximum energy will be e(V p ) max ; however, only some few ions will have such energy. Thus, the ion energy distribution at low plasma excitation frequencies is much broader (nonuniform) than at high frequencies (13.56 MHz) and it may reach much higher energies [16].…”

“…In general, at high frequencies the sheaths tend to act capacitively while they behave resistively in the low frequency range [16]. When the rf power is capacitively coupled to the excitation electrode and one works at high excitation frequencies (13.56 MHz), the capacitive sheath approximation [17] yields a time average plasma potential V p given by [18] …”

“…The resistive sheath approximation [16] provides another appropriate expression for low frequency excited capacitive coupled systems. On the other hand, measurements of dc self-bias voltage (V dc ) as a function of V rf in a capacitively coupled Ar plasma [16] excited at different frequencies show that, the higher the excitation frequency the closer V dc is to V rf at 13.56 MHz, V dc ' 0:95V rf , while at 105 kHz it goes down to V dc ' 0:80V rf and to even lower values at 35 kHz.…”

Influence of pressure and silane depletion on microcrystalline silicon material quality and solar cell performance

“…Also, in our case V dc is small compared to V p.p. ͑Ͻ3%͒ hence the capacitive sheath approximation 38,39 predicts the time average potential V as…”