Dynamics of Pulsed-Power Chlorine Plasmas

Pulsed-power operation expands the boundaries of traditional plasma processing, and is a promising candidate for reducing plasma-induced damage and profile anomalies during plasma etching. Here, characteristics of chlorine, transformer-coupled pulsed plasmas are reported. Time dependencies of electron, positive ion, and negative ion densities as well as electron temperatures were measured with a Langmuir probe and microwave interferometry at two different average powers (240 and 500 W) and several pressures between 3 and 20 mTorr. During the off portion of the power modulation, the electron density decreases rapidly as Cl sup - is formed by dissociative attachment of Cl sub 2. The formation of Cl sup - is accelerated by conditions that favor high Cl sub 2 densities: high pressures and low powers. At pressures of 10 mTorr and higher, an ion-ion plasma forms near the end of the off portion of the cycle, the sheath collapses and C1 sup - reaches the wafer. Electron temperature decays even more rapidly in the off period and increases with a similar time constant at the beginning of the on cycle if electrons are present at a sufficiently high level. If the electron density is very low at the beginning of the on cycle, such as at high pressure (10 mTorr), the T sub 3 exhibits a spike at the beginning of the on period. In a comparison study, plasma induced damage is reduced when aluminum is etched under similar source power modulation conditions.