Electron heating, mode transitions, and asymmetry effects in dusty single- and dual-frequency capacitive discharges

摘要

Many gas mixtures used in plasma processing applications, such as plasma enhanced chemical vapor deposition (PECVD), exhibit a high concentration of negatively charged ions and/or dust particles. For instance, the agglomeration of dust in hydrogen diluted silane discharges significantly affects the deposition of silicon thin films. Therefore, the role of the dust in plasma processes needs to be understood. Here, a capacitively coupled radio frequency discharge in H/SiH is investigated experimentally under typical PECVD process conditions using Phase Resolved Optical Emission Spectroscopy, two-dimensional laser light scattering on the dust particles as well as current and voltage measurements. The results show that the presence of dust particles strongly affects the electron heating, which is strongest in the spatial region, where the major amount of large dust particles is located. In particular, a mode transition will occur from the traditional α-mode to a bulk drift field mode (Ω-mode), if the amount of dust is increased. The electron dynamics in the Ωmode can be explained using an analytical model. External forces such as the thermophoretic force (due to gas temperature gradients) or gravity cause an asymmetry in the dust particle density profile. This, in turn, induces an asymmetry in the electron heating and, thereby, in the ion density profile of a single frequency parallel plate discharge. Subsequently, a DC self-bias develops in a geometrically symmetric capacitive discharge.In dual-frequency discharges driven by a fundamental frequency and its second harmonic, the discharge asymmetry can be controlled by adjusting the phase angle between the two frequencies. According to the Electrical Asymmetry Effect (EAE), the DC self-bias is an almost linear function of the phase angle. The results demonstrate that the EAE works in discharges operated in both the α-mode and the Ω-mode: The DC self-bias val- e is shifted by an asymmetric dust density profile, whereas the width of the control interval via the EAE is almost independent of the (stationary) dust distribution.