Source Electrode Assembly

Source, upper, and lower electrodes, along with the cathode and anode assemblies that often coincide with them, are the physical metal surfaces that actually couple RF power into a capacitively coupled plasma — their exact geometry, spacing, and which one carries the bias determine how the plasma forms and how ions strike the wafer.

Which electrode carries the wafer and which carries the bias power is the single design choice that determines how directional and energetic the ion bombardment at the wafer actually is. In most etch-focused CCP configurations, the wafer sits on the lower electrode and that's also where bias power is applied, so ions accelerate directly toward the wafer across the sheath above it; the upper electrode, often grounded or carrying source power instead, completes the circuit and frequently doubles as the gas showerhead, delivering process gas and RF coupling from the same physical structure.

Electrode spacing is itself a tunable process parameter, not a fixed mechanical dimension set once at tool design. The gap between upper and lower electrodes directly affects plasma density, uniformity, and sheath characteristics, and some reactor designs allow that spacing to be adjusted between recipes or even dynamically during a process step — treating electrode gap as an active process knob alongside pressure, power, and gas flow rather than a static piece of hardware geometry.

Source, upper, lower, cathode, and anode electrode assemblies together define the basic electrical geometry every capacitively coupled plasma process depends on, and the specific roles assigned to each — which one holds the wafer, which one carries bias, how far apart they sit — are foundational process decisions baked into the chamber's hardware design.