contact etch stop layer (cesl)

**CESL** (Contact Etch Stop Layer) is a **thin nitride film deposited over the transistor gate** — that serves the dual purpose of providing an etch stop during contact hole formation AND applying uniaxial stress to the channel to enhance carrier mobility. **What Is CESL?** - **Material**: Silicon Nitride (SiN) deposited by PECVD. - **Stress Types**: - **Tensile CESL** (~1.5 GPa): Applied over NMOS to boost electron mobility. - **Compressive CESL** (~3 GPa): Applied over PMOS to boost hole mobility. - **Etch Stop**: During contact etch, the reactive ion etch (RIE) stops on the CESL, preventing over-etch into the gate. **Why It Matters** - **Dual Function**: One film does two jobs — strain engineering + process integration. - **Dual-Stress Liner (DSL)**: Using tensile CESL over NMOS and compressive CESL over PMOS on the same die. - **History**: Introduced at the 90nm node and used through 28nm before being partially replaced by other strain techniques. **CESL** is **the stress-and-shield layer** — a single thin film that both protects the transistor during manufacturing and permanently strains its channel for better performance.