Compressive stress in a thin film means the film is being pushed inward by the substrate, causing it to want to expand outward. Mechanism: Film deposited with atoms packed tighter than equilibrium spacing. The film pushes against the substrate. Wafer bows convex (center rises). Causes: High ion bombardment during deposition (PECVD with high RF power) implants atoms into film, densifying it. Atoms frozen in non-equilibrium positions. Thermal contribution: If film CTE is less than substrate CTE, cooling from deposition temperature creates compressive stress in film. Measurement: Wafer curvature measurement. Convex bow on front side indicates compressive film stress. Magnitude: Can range from tens of MPa to several GPa. Failure modes: Buckling and delamination (film lifts from substrate). Hillocks in metal films from compressive stress relief. Beneficial uses: Compressive SiN capping on PMOS enhances hole mobility. Compressive stress in certain barrier layers. Control: Adjustable via deposition power, pressure, temperature. Lower bombardment energy reduces compressive stress. Stack management: Balance compressive layers with tensile layers to control total wafer bow. Reliability: Compressive films generally more resistant to cracking than tensile films.