Backside gas (typically helium) is flowed between the wafer backside and the chuck surface to improve thermal contact and temperature uniformity. Purpose: Wafer sits on chuck but microscopic surface roughness creates gaps. Without backside gas, thermal contact is poor and non-uniform. Gas choice: Helium preferred for high thermal conductivity (5-6x better than N2 or Ar). Light molecule penetrates small gaps effectively. Pressure: Typically 5-20 Torr. Must be below electrostatic clamping force to prevent wafer pop-off (de-chucking). Zones: Often two zones - center and edge - with independent pressure control for temperature uniformity tuning. Thermal mechanism: He molecules in the gap conduct heat between wafer and chuck via gas-phase conduction. Temperature impact: Without backside He, wafer temperature can be 50-100 C higher than chuck setpoint during plasma processing. With He, wafer temperature closely tracks chuck temperature. Leak monitoring: He leak rate monitored as indicator of chuck condition and wafer clamping quality. Excessive leak = poor clamping or chuck damage. ESC interaction: Backside gas pressure must balance with electrostatic clamping force. Higher pressure needs stronger clamping. Process effects: Backside He pressure affects wafer temperature, which affects deposition rate, film properties, and etch rate. Critical process parameter.