We investigated an electron cyclotron resonance plasma characteristics and neutral density depletion. In high density plasma such as electron cyclotron resonance plasma, neutral depletion, neutral heating, and nonlinear plasma-neutral coupling phenomena which were not shown in low density plasma are very important for stable and uniform process. To investigate the plasma properties, we used electric and optical diagnositic techniques. We measured the plasma density and electron temperature by using Langmuir probe, and the emission profile by using an newly developed optical probe.
In $SF_6/Ar$ plasma, we monitored the plasma density profile and plasma emission profile. To obtain the relative radical concentration profile, there are two method: one is the optical emission actinometry, the other is the optical emission spectroscopy with plasma parameter normalization. The results of above two methods have similar tendency. But there are some discrepancy. The reason was attributed to both the non-ideal optical probe system response function and non-uniform electron temperature profile.
The non-linear coupling of plasma and neutral in the plasma production chamber could generate the periodic oscillation of plasma properties without extra driving forces. The oscillation phenomena were interpreted as predator-prey model in ecology. The periodic oscillation was appeared at the critical points. And the predicted and measured frequencies were 1.7 kHz and 1.1 kHz, respectively.
In Ar ECR plasma, non-uniform neutral density profile or neutral heating could be generated through the plasma-neutral charge exchange collision and intense ionization. We observed the hollow emission profile in reaction chamber. We interpreted the emission profile as the neutral depletion caused by plasma-neutral collisional heating.
We measured the plasma production chamber wall temperature. Because the neutrals had a lot of wall collisions before the neutral were pumped out. There was temperature difference between two positions located in the plasma production chamber wall. This temperature difference indicates strong neutral density gradient. We investigated the emission profile with changing the external parameters such as magnetic field profile, pressure, and wall temperature. It was expected that there were two state transition with respect to pressure. The one state transition was caused by the propagation of microwave propagation into the reaction chamber. So, there was electron temperature nonuniformity along the radial direction when the microwave power propagates into the reaction chamber. And the other state transition was caused by neutral density depletion. The neutral density depletion in the plasma production chamber center reduced the plasma density at the center, and enhanced the plasma density at the chamber wall side. This plasma density profile could be sustained by the defocusing plasma effect of microwave power. Therefore, the hollow plasma density profile is related to the neutral density depletion in high density plasma.
