Bias power is RF power applied to the wafer electrode to control ion energy and directionality in plasma etch. Purpose: Accelerate ions toward wafer surface. Higher bias = higher ion energy = more anisotropy. Separation from source: Modern tools separate plasma generation (source power) from ion control (bias power). Ion energy: Bias voltage determines sheath potential. Ions accelerated across sheath to wafer. Anisotropy mechanism: Energetic ions hitting surface vertically enable directional etching. Etch faster where ions hit directly. Low frequency bias: Lower frequency (e.g., 2 MHz) allows higher ion energy. Used for dielectric etch. High frequency bias: Higher frequency (e.g., 13.56 MHz) for lower ion energy, gentler process. Damage trade-off: Higher bias = better anisotropy but more substrate damage, lower selectivity. Process tuning: Balance source and bias power for optimal etch rate, selectivity, profile. Pulsed bias: Pulse bias for better profile control and reduced damage. Self-bias: In CCP, natural DC bias develops. Related to RF voltage and ion/electron mobility difference.