Advanced Pulsed Plasma Techniques

A method for controlling ion energies on insulating surfaces using pulsed plasmas is presented. DC pulses are periodically applied to the chuck holding the substrate in the afterglow of a pulsed plasma to attract an electron swarm to the sample surface. Surface potential measurements validated the proposed method and helped investigate the effect of changing pulse width, amplitude, and frequency of the chuck bias on the resulting surface potential waveform. Retarding field energy analyzer measurements were performed and corrected for the non-uniform charge distribution that prevailed at an applied RF frequency less than the ion sheath transit frequency. Etching of quartz discs and 1000nm-thick SiO2 films on Si wafers was also performed. An etching threshold was found at 100 V chuck bias for both types of substrates, beyond which the etching rate increased proportionally with the square root of chuck bias. No clear effect of the boundary bias on etching rate was seen.

Time-resolved UV absorption of pulsed electronegative fluorocarbon and chlorine inductively-coupled plasmas is presented. The time-dependent variation of Si-byproducts were verified against previous publications as well as the limits of the current apparatus. The lack of decay of CF2 in 100 Hz and 500 Hz pulsed C4F8 plasmas was also seen.

Time-dependent studies of power-modulated chlorine inductively-coupled plasmas are presented. Power at 13.56 MHz applied to the plasma was modulated between high and low power states. Time-resolved optical emission, power delivery, and Langmuir probe measurements revealed at least two periodic steady-state conditions upon switching from high to low power: a “normal” mode in which electron temperature (Te) remains constant, while electron and ion number densities (ne and n+) and optical emission spectroscopic (OES) intensities smoothly drop to a level roughly equal to the fractional drop in power, and an “abnormal” mode in which ne, n+ and OES intensities plummet before rising to values commensurate with the drop in power. Whether the plasma operates in the normal or abnormal mode is sensitive to settings on the matching network and is also a function of pressure and pulsing parameters.