We experienced that the most delicate step during the micro-cavity fabrication is the plasma surface oxidation of the spin-coated elastomer sample (see Step 6 in Procedure). The oxidation is required to increase the surface energy of the pristine silicone elastomer layer and to prevent formation of gold nano-islands when subsequently depositing the top metal layer onto the elastomer surface. This improves the reflectance of the gold film and thus ensures sufficient interference contrast to measure the local thickness of the micro-cavity reliably (see Supplementary Fig. 1 of [Kronenberg et al.] for details). At the same time, excessive surface oxidation of the elastomer will significantly increases the apparent stiffness, E*, of the stack (see Supplementary Fig. 1 of [Kronenberg et al.] for details), and non-appropriate elastomer oxidation can result in surface buckling or wrinkling, or lead to the formation of cracks in the micro-cavity.
We therefore suggest to systematically vary the plasma power, duration, process pressure, gas mixture, gas flow, plasma-sample distance in a range around the parameters used in this protocol, and to investigate how variations affect the micro-cavity stiffness (e.g., via AFM indentation measurements as described in the Online Methods of [Kronenberg et al.]) and the top gold mirror reflectance. This is best done with test samples that contain no bottom mirror stack. For these test samples a reflectance of ~12% at 650 nm should be reached to guarantee sufficient interference contrast in the full micro-cavity stack.