bias power

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.