Residual stress within nanoscale metallic multilayer systems during thermal cycling

Abstract

Projected applications for nanoscale metallic multilayers will include wide temperature ranges. Since film residual stress has been known to alter system reliability, stress development within new film structures with high interfacial densities should be characterized to identify potential long-term performance barriers. To understand factors contributing to thermal stress evolution within nanoscale metallic multilayers, stress in Cu/Nb systems adhered to Si substrates was calculated from curvature measurements collected during cycling between 25°C and 400°C. Additionally, stress within each type of component layers was calculated from shifts in the primary peak position from in-situ heated X-ray diffraction. The effects of both film architecture (layer thickness) and layer order in metallic multilayers were tracked and compared with monolithic Cu and Nb films (1µm total thickness). Analysis indicated that the thermoelastic slope of nanoscale metallic multilayer films (with 20nm and 100nm individual layer thicknesses) depends on thermal expansion mismatch, elastic modulus of the components, and also interfacial density. The layer thickness (i.e. interfacial density) affected thermoelastic slope magnitude (−1.23±0.09MPa/°C for 20nmCu/Nb vs. −0.89±0.03MPa/°C for 100nmCu/Nb) while layer order had minimal impact on stress responses after the initial thermal cycle (−0.82±0.07MPa/°C for 100nm Cu/Nb). When comparing stress responses of monolithic Cu and Nb films to those of the Cu/Nb systems, the nanoscale metallic multilayers show a similar increase in stress above 200°C to the Nb monolithic films, indicating that Nb components play a larger role in stress development than Cu. Phase specific stress calculations (Cu vs. Nb) from X-ray diffraction peak shifts in 20nm Cu/Nb collected during heating reveal that the component layers within a multilayer film respond similarly to their monolithic counterparts.