Profiling contact ridge of soft substrates with metallic thin films using a novel interference technique

Hypothesis As technology moves toward flexibility, embedded systems, and miniaturization, which often have metallic elements integrated with the functional substrates, understanding the contact deformation of such soft substrates coated with thin metallic films is crucial. However, probing nanoscale deformations optically, typically in a few hundred nanometers, poses challenges. Dual Wavelength - Reflectance Interference Contrast Microscopy (DW-RICM) holds promise in unveiling nanoscale deformations. However, when paired with conventional glass probes to measure deformation of underlying soft substrates, DW-RICM faces limitations due to light reflection at the glass-air interface, impeding accurate determination of the contact zone (e.g., contact area, contact ridge). Here we hypothesize that using alternative probes to eliminate unwanted reflections can enhance understanding of soft substrate deformation with thin metallic films (bilayer substrates). Experiments In this work, we investigate the deformation profile of viscoelastic soft substrates with sputter-coated gold thin films (referred to herein as bilayer substrates) using DW-RICM. We employ millimeter-sized black ink-coated glass spheres as probes, absorbing most light under DW-RICM. We focus our laser beams associated with DW-RICM at the contact zone between the probe and the underlying soft substrates. Findings Our new technique of using black ink-coated glass probes in contact with soft bilayer substrates enables us to accurately extract the deformation profile, particularly the contact ridge. With the increase in the gold sputter coating time, the contact area and contact ridge of gold-polymer bilayer substrates decrease. Further, long-term contact analysis of gold-coated soft substrates reveals a steady increase in the contact ridge height over time, unlike the gradual decrease observed in their bare counterparts until reaching a steady state.