Paper PS1-FrM9
Ignition Delay in Electronegative Pulsed Dual Source Tandem Plasmas

Friday, November 14, 2014, 11:00 am, Room 305

Control of the ion energy distribution (IED) is of utmost importance in semiconductor manufacturing. Pulsed plasmas can produce IEDs with small energy spread, necessary to enhance etching selectivity and minimize damage. However, the IED is broadened during the power-on period by capacitive-coupling, imposing an RF potential on the DC plasma potential. This can be eliminated with a Faraday shield but, with electronegative gases, it is not possible to ignite pulsed ICPs, since electron density rapidly decays when power is off, and re-ignition requires large electric fields produced by high-voltage capacitive coupling. Motivated by this problem, we have explored a “tandem” plasma system, where a continuous (auxiliary) ICP is injected through a grid into a pulsed (main) ICP. Using such a system, an ignition delay was observed in pulsed (period 1 ms) Cl₂ plasmas with a duty ratio (DR) ~ 60%. The ignition delay monotonically increased with DR to reach a maximum of 500 ms at DR ~99%. It was also observed that, for a given DR, the ignition delay increased by increasing the main ICP power or by decreasing the auxiliary ICP power. The ignition delay may be attributed to the low electron density in the main ICP, which decays to a value less than the density when the auxiliary ICP were operating alone. At low electron densities, power transfer efficiency is poor. The flux of seed electrons from the auxiliary ICP then acts to restore the electron density, thereby improving the power transfer in the main ICP and allowing plasma re-ignition. Similar results were also observed using other electronegative gases such as SF₆ and CF₄/O₂ mixtures.