Improved polymer–glass adhesion through micro-mechanical interlocking

Mechanical interlocking provides a simple and effective means of improving adhesion between dissimilar materials in micro-electro-mechanical systems (MEMS). Following successful implementation in hybrid Si-polymer systems (Larsson, Syms and Wojcik 2005 J. Micromech. Microeng. 15 2074–82), it was established that maximum interface strengthening does not necessarily rely on the presence of overhang between interlocking lobes. Instead, careful design of the lobe profile is advised in order to balance the opposing actions of physical restraint and lobe pull-out and to obtain optimal interface strength. When an interlocked interface is immersed in aggressive liquid media, however, the situation is clearer: chemical bonds are degraded or completely destroyed and lobe overhang provides the only source of physical restraint. Generating overhanging features in Si substrates is possible through reactive ion etching (RIE), but in the case of glass, the situation is more problematic. A straightforward, robust process is now described that extends mechanical interlocking to generic MEMS substrates, avoiding the need for RIE. By using inexpensive and established processes such as electroplating and wet etching, interlocking features with an overhanging profile are generated in glass substrates. Peel tests on cured strips of SU-8 confirm an increase in average peel strength by a factor of 3.5, compared with strips peeled from smooth substrates. The method can readily be applied to a number of substrates, including Si, providing a low-cost route towards attaining mechanical interlocking.