Invited Paper PS1-ThA8
Process Control through Diagnostics and Understanding: Multi-frequency Discharges and Atmospheric Pressure Plasmas

Thursday, October 18, 2007, 4:20 pm, Room 606

Despite its technological importance, power coupling and ionisation mechanisms in radio-frequency (rf) discharges are not yet fully understood. Of particular interest are multi-frequency discharges and recently developed non-equilibrium rf discharges at ambient pressure. Insight into the complex dynamics requires close combination of advanced diagnostics and specifically adapted simulations. Phase resolved optical emission spectroscopy (PROES) in combination with particle-in-cell (PIC) simulations reveal details on the dynamics within the rf cycle. Multi-frequency discharges can provide additional process control for technological applications. The electron dynamics exhibits a complex spatio-temporal structure. Excitation and ionisation, and, therefore, plasma sustainment is dominated through directed energetic electrons created through the dynamics of the plasma boundary sheath. These electrons are predominantly produced during contraction of the low frequency sheath. This can be understood in the following picture. During the phase of low-frequency sheath expansion power dissipation is highest which determines plasma heating. This power is, however, deposited into a large number of electrons in the vicinity of maximum sheath expansion. The dissipated power during the collapse of the low-frequency sheath is deposited into a much smaller number of electrons, since the electron density close to the electrode surface is significantly lower. The power dissipation per electron can, therefore, be higher during the sheath collapse, which then creates energetic directed electrons. Recently developed rf discharges at ambient pressure bear enormous potential for future technological applications providing high reaction rates without the need of expensive vacuum systems. Fundamental discharge mechanisms are, however, only rudimentarily understood. The atmospheric pressure plasma jet (APPJ) is a homogeneous non-equilibrium discharge. A specially designed rf µ-APPJ provides excellent optical diagnostic access to the discharge volume and the interface to the effluent region. This allows investigations of the discharge dynamics and energy transport mechanisms from the discharge to the effluent. PROES measurements in the discharge volume show similar excitation and ionisation mechanisms as in capacitively coupled rf discharges at low pressure. An interesting phenomenon is the interaction between the two plasma boundary sheaths.