Advanced Control of Ion and Electron Energy Distributions and Investigation of in-situ Photo-Assisted Etching

Precise control of ion energy distribution (IED) is critical to achieve highly selective, low damage etching. A novel approach to control IED using pulsed plasma with synchronously pulsed dc bias on a boundary electrode in Ar gas is first presented. Synchronization of the dc bias applied during the afterglow of a pulsed plasma and the plasma rf power resulted in a double-peaked IED. The mean energies of the two peaks, as well as the peak separation, were controlled by adjusting the applied dc bias and the discharge pressure. Nearly mono-energetic IEDs can be extracted in the afterglow of a pulsed plasma. With precisely controlled IEDs, a new, important phenomenon is reported: photo-assisted etching of p-type Si in chlorine-containing plasmas. This mechanism was first discovered in mostly Ar plasmas with a few percent added Cl2. A substantial etching rate was observed, independent of ion energy, when the ion energy was below the ion-assisted etching threshold. Experiments were carried out with light and ions from the plasma either reaching the surface or being blocked, showing conclusively that the “sub-threshold” etching was due to photons. Sub-threshold etching rates scaled with the product of surface halogen coverage and Ar emission intensity. Etching rates measured under MgF2, quartz, and opaque windows showed that sub-threshold etching is due to photon-stimulated processes on the surface, with VUV photons being much more effective than longer wavelengths. In an effort to manipulate the electron energy distribution function (EEDF) and plasma density, a plasma reactor incorporating dual tandem plasma sources separated by a grid is presented. As feature sizes shrink to the nanometer scale, tuning the EEDF becomes increasingly important for both plasma etching and deposition. By pulsing the main plasma source, while maintaining the tandem source in continuous wave mode, a low electron temperature of ~1 eV at high plasma density (1011 cm-3) was realized. This was achieved by applying a dc bias to a boundary electrode in the tandem source. The electron temperature in the afterglow period could be controlled by changing that bias voltage.