Low pressure (<5 mTorr) electron cyclotron resonance (ECR) sources are being developed for downstream etching and deposition, and production of radicals for surface treatment. Industrial use of such sources for large area processing is increasing. LAM currently uses a 200 mm line, soon to be a 300 mm. While Hitachi has recently begun work on a 300 mm reactor. The spatial coupling of microwave radiation to the plasma in these systems is a concern due to issues related to the uniformity of dissociation, electron heating, and ultimately process uniformity. Studies suggest thta ion flux to the substrate surface depends highly on the profile and location of the power deposition within the reactor. Furthermore, it has been shown that certain waveguide electromagnetic mode fields tend to provide better uniformity over a larger area.
To investigate these issues, we have developed a finite-difference-time-domain (FDTD) simulation for microwave injection and propagation. The FDTD simulation has been incorporated as a module in the 2-dimensional Hybrid Plasma Equipment Model (HPEM). Plasma dynamics are coupled to the electromagnetic fields through a tensor form of Ohms law. During each iteration through the model, the FDTD simulation uses a leap-frog scheme for time integration, with time steps that are 30% of the Courant limit, until reaching the steady state. Power deposition calculated in the FDTD module is then used in solving the electron energy equation. The system of interest uses circular TE(0,n) microwave mode fields injected along the axis of a cylindrically symmetric downstream reactor.
Figure 1 Electromagnetic Tranverse Electric (TE) Mode Fields.
Figure 2 ECR Device.
Power profiles tend to follows incident electromagnetics field profiles. For the TE(0,1) mode, there occurs significant power deposition and absorption of the electromagnetic field upstream. This occurs because at the lower modes there is a greater efficiency in coupling of the microwave fields to the plasma. This off-axis peak in the power profiles can be seen in Figure 3.
Figure 3 Power deposition for varying TE(0,n) modes.
Flux profiles of ions to the substrate maintain their off-axis production rate distributions. Results suggest that ion flux profiles are highly dependent on ionization distributions, and less dependent on heating mechanism. Figure 4 shows radial ion flux profiles for TE(0,1) and TE(0,2) modes.
Figure 4 Ion flux profiles for varying TE(0,n) modes.