SAC (Self-Aligned Contact) is a process integration technique where the source/drain contact is defined by the gate spacers rather than by a separate lithography step — enabling the contact to be placed immediately adjacent to (or even overlapping) the gate without risk of gate-to-contact shorts.

How SAC Works

• SAC Cap: A dielectric cap (SiN) is formed on top of the metal gate.
• Contact Etch: The contact etch removes ILD material but stops on the SiN cap and spacers — the contact opening is self-aligned.
• Etch Selectivity: Requires excellent etch selectivity between ILD (SiO$_2$) and SAC cap (SiN).
• Fill: The contact is filled with a metal (W, Co, Ru) that connects to the S/D.

Why It Matters

• Overlay Tolerance: Eliminates the need for tight overlay between contact and gate lithography layers.
• Device Scaling: Allows contact-to-gate spacing below what lithography overlay can guarantee.
• Standard: SAC has been standard since the 14/10nm node — essential for all advanced devices.

SAC is letting the spacer guide the contact — self-alignment replaces lithographic precision for gate-to-contact spacing.