film stress

Film stress is the internal mechanical force (tensile or compressive) within a deposited thin film, arising from growth conditions and material mismatch. **Origin**: **Intrinsic stress**: From deposition conditions - atom arrangement, impurities, density. **Thermal stress**: From CTE (coefficient of thermal expansion) mismatch between film and substrate upon cooling. **Types**: Tensile - film wants to shrink, pulls edges. Compressive - film wants to expand, pushes outward. **Measurement**: Wafer bow measurement (Stoney equation). Laser-based curvature scanning before and after deposition. **Control knobs**: In PECVD - RF power, pressure, temperature, gas ratios. Higher bombardment generally gives compressive stress. **Magnitude**: Ranges from near zero to several GPa depending on material and process. **Impact**: Excessive stress causes cracking (tensile), delamination, or wafer warping. Affects lithography overlay. **Strain engineering**: Intentional stress used to enhance transistor mobility (tensile SiN for NMOS, compressive for PMOS). **Multilayer effects**: Stress accumulates through film stack. Total stress must be managed. **Stress migration**: Stress gradients can drive atomic diffusion in metal lines causing voiding over time.