Silicon-Carbon (Si:C) Source/Drain is a strain engineering technique for NMOS transistors — where carbon atoms are incorporated into the source/drain silicon lattice, which has a smaller lattice constant than pure Si, inducing tensile stress in the channel.

How Does Si:C Work?

• Principle: Carbon atoms are smaller than silicon atoms. Substitutional C in the Si lattice contracts the S/D region, pulling the channel into tensile strain.
• Carbon Content: Typically 1-2% C (higher %C is difficult to incorporate substitutionally).
• Challenge: Carbon easily migrates to interstitial sites during thermal processing, losing its strain effectiveness.
• Growth: Selective epitaxial growth in etched S/D cavities (similar to eSiGe process flow).

Why It Matters

• NMOS Complement: Provides tensile stress for NMOS, complementing the compressive eSiGe for PMOS.
• Limited Adoption: The strain levels achievable (~1% C) are lower than eSiGe (~30% Ge), making the mobility boost more modest.
• Alternatives: CESL tensile liners and SMT often provide comparable or better NMOS strain with simpler processing.

Si:C Source/Drain is the tensile counterpart to SiGe — using the smaller carbon atom to stretch the silicon channel and boost NMOS electron mobility.