The Contact Etch Stop Layer (CESL) serves a dual purpose in modern semiconductor device fabrication: it protects the underlying source/drain regions during the deep Pre-Metal Dielectric (PMD) oxide etch, and it intentionally induces mechanical strain in the transistor channel to enhance carrier mobility P1.Following the sequential PMD 1, 2, and 3 etch steps, the thick SiO2 layer has been removed, stopping accurately on the SiN CESL to prevent damage to the active areas P3.The "CESL 2 - Etch" ste