Plasma uniformity and power deposition in electron cyclotron resonance etch tools

We present measurements of the propagation and absorption of the 2.45 GHz waves which produce the plasma in an electron cyclotron resonance (ECR) etch tool. Measurements verify that the absorption occurs within the Doppler-broadened resonance location where ‖ω− ωce‖ ≲ k∥vte and B≲950±25 G. A ray tracing calculation is used to model power deposition in the ECR source region. Plasma radial uniformity improves when the microwaves are allowed to propagate several wavelengths (≳5) before absorption occurs. An explanation for this improved source uniformity is the fact that whistler waves refract toward regions of lower density which, when combined with ECR absorption at the low-density location, forces the profile toward radial uniformity. This effect explains previous observations of flat density profiles in configurations where the magnetic field at the microwave window is raised well above the resonant field. Etch rate uniformity is found to be closely correlated to the ion current density uniformity across the wafer surface in our system.