Improved adhesion in hybrid Si-polymer MEMS via micromechanical interlocking

A method for improving adhesion between polymer films on Si substrates is described involving mechanical interlocking. Isotropic reactive ion etching (RIE) is applied to form pits in the Si substrate which fill upon application of a polymer via spinning. After curing, interpenetrating polymer lobes form a robust bond with the Si substrate. Adhesion improvements over films on smooth substrates are demonstrated qualitatively through prolonged immersion in heated 40% potassium hydroxide (KOH) solution, and quantitatively through peel tests. Strips of SU-8 on smooth substrates separate completely within minutes of immersion in KOH, whereas mechanically interlocked strips remain attached throughout. Tests reveal significant improvements to peel resistance for strips with interpenetrating lobes, due to a combination of crack deflection and physical restraint at the bond interface. Surprisingly, lobes with a vertical profile offer better resistance to strip peeling compared with lobes having significant overhang. Stress concentrations at sharp bends in the latter raise local stresses to levels promoting failure in SU-8, limiting the nominal load-carrying capacity of the interpenetrating interface. For maximum peel resistance, the lobe profile needs to be optimized such that maximum pull-out stresses are close to the failure stress of SU-8.