Restrictions of stress measurements using the curvature method by thermally induced plastic deformation of silicon substrates

• Curvature measurements of sputtered coatings on various silicon substrates
• Sophisticated temperature calibration of a high-temperature curvature device
• Plastic substrate deformation during deposition/measurement falsifies stress values.
• Approach to expand the temperature/stress measurement range

Within this study, the effect of plastic deformation of coated single-crystalline silicon substrates on stress measurement of hard coatings by the curvature method was illuminated. Silicon substrates with different thicknesses (380 and 500 μm) and orientations ((100) and (111)), coated with TiN and TiAlN, were examined and compared to each other with respect to their high-temperature plasticity. The curvature was measured during thermal cycling and isothermal annealing. Coated samples were additionally annealed in vacuum and subsequently characterized using scanning and transmission electron microscopy. The presence of plastic deformation of the silicon substrate was confirmed after thermal exposure by the detected high dislocation density. The resolved shear stress in the 500 μm thick substrate at 600 °C was calculated to 25 MPa, which is in the range of the critical value of silicon at this temperature. Plastic substrate deformation, corroborated by experimental and theoretical approaches, is thus a significant source of error in estimating residual stress from curvature measurements performed at elevated temperatures or applied to coatings with pronounced stresses grown at moderate/high substrate temperatures.