Carrier gas is an inert gas used to transport precursor vapors from their source to the deposition chamber in CVD and ALD processes. Common gases: Nitrogen (N2) and argon (Ar) most common. Helium occasionally for thermal properties. Mechanism: Carrier gas flows through or over heated liquid/solid precursor source, picks up precursor vapor, and delivers it to chamber. Bubbler delivery: Carrier gas bubbles through liquid precursor. Precursor concentration depends on bubbler temperature, pressure, and carrier flow rate. Flow control: Mass flow controllers (MFCs) precisely meter carrier gas flow. Precursor delivery rate controlled indirectly through carrier flow and source temperature. Inertness: Carrier gas must not react with precursor or deposit on surfaces. Must be ultra-high purity. Dilution effect: Carrier gas dilutes precursor concentration in chamber. Affects deposition rate and uniformity. Purge function: In ALD, carrier gas also serves as purge gas between precursor pulses, removing unreacted species. Chamber pressure: Carrier gas contributes to total chamber pressure. Process pressure set by combination of gas flows and pumping speed. Thermal role: Carrier gas can assist heat transfer in some chamber configurations. Purity requirements: Parts-per-billion purity. Inline purifiers remove residual moisture and oxygen.