Dual frequency DBD: influence of the amplitude and the frequency of applied voltages on glow, Townsend and radiofrequency DBDs

To study dual frequency Townsend glow and radio frequency DBDs in an Ar/NH3 Penning mixture, different voltages are applied on each electrode of a plane/plane DBD. The one which has the higher amplitude and frequency, VH, determines the discharge regime. The other one, which has a lower amplitude and frequency, VL, is more similar to a bias voltage. VH frequency ranges from 50 kHz to 13.56 MHz and VL frequency from 1 kHz to 2 MHz. The amplitude of VL is always kept lower than the breakdown voltage. The discharge behavior is characterized by space and phase resolved optical emission spectroscopy and Fourier transform of the plasma induced light intensity. When the frequencies of two voltages are close to each other, VL higher than 150 V increases the discharge light intensity. In glow and Townsend discharges, this increase is attributed to the enhancement of the plasma density, due to the ions drift from the plasma bulk to the cathode, which produces a higher secondary electron emission. For the radiofrequency discharge, the enhancement of the light intensity is attributed to an enhancement of the positive space charge due to the higher voltage amplitude.When the frequencies of two voltages are very different, e.g. for VL between 1 and 100 kHz and a 5.5 MHz α-RF discharge, the behavior largely depends on VL amplitude. Above some low frequency (LF), the discharge tends to extinguish when VL amplitude is at its maximum. This is explained by a diminution of the plasma density resulting from the ion drift to the cathode due to VL. When VL is very high, it enhances the discharge intensity. This discharge amplification is associated with a transition from α to γ RF mode, which only occurs when the two voltages have the same polarity. Whatever the RF discharge mode, γ or α, the discharge is diffuse. To determine the interest of LF-RF dual frequency DBD for the PECVD, SiOx coatings, made with a single and a dual LF-RF frequency DBDs, are compared. It is concluded that the ɣ dual frequency removes OH functions from the material lattice and densifies the layer.