self-aligned contact process

**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.