| AVS 64th International Symposium & Exhibition | |
| Plasma Science and Technology Division | Thursday Sessions |
| Session PS+VT-ThA |
| Session: | Plasma Diagnostics, Sensors and Control |
| Presenter: | Yunchang Jang, Seoul National University, Republic of Korea |
| Authors: | Y. Jang, Seoul National University, Republic of Korea H.-J. Roh, Seoul National University, Republic of Korea N.-K. Kim, Seoul National University, Republic of Korea S. Ryu, Seoul National University, Republic of Korea G.-H. Kim, Seoul National University, Republic of Korea |
| Correspondent: | Click to Email |
Ion response time to RF sheath voltage is important to control the energy spread of ion energy distribution (IED) in the dual frequency capacitively coupled argon plasma. IED is known as being governed by the dynamics of ion in RF sheath and the magnitude of RF voltage peak. In previous study, semi-analytic models to determine IED were derived from concept of ion response time (τi). Ion energy broadness (ΔEi) was represented in terms of the sheath voltage oscillation (Vpp) and τi/τrf. Ion response time was assumed as ion transit time across the sheath, τion by adopting correction factor without thorough understanding. In this study, we investigate the underlying physics of the correction factor, consequently defining the ion response time τi with RF sheath voltage oscillation. Experiment were performed in dual frequency CCP at 20 mTorr of argon gas which has the ratio of maximum sheath size to ion mean free path ~ 2. Various ranges of RF bias (from τion /τrf ~ 0.05 to τion /τrf ~10) were applied to bottom electrode to enhance the incident ion energy with very high frequency (VHF, τi/τrf ~10) applied on the top electrode (showerhead) to sustain plasma. A commercial retarding field analyzer (Impedans, Vertex V4.0.10) was employed to measure IED. Plasma density, electron temperature and plasma potential were measured by using RF compensated Langmuir probe. Experimental results of ΔEi to Vpp were compared with models under assumptions that ion response time is ion transit time across the sheath (τion) or one of ion plasma frequency (1/ωpi). Experiment results revealed that the time scale of ion response time is determined by 1/ωpi rather than τion in this high-density plasma. This result implies that ion response time is governed by the ion inertia at the sheath boundary to RF sheath oscillation. Ion inertia becomes the initial condition of ion acceleration and govern the ion energy arriving at surfaces.