High-density stretchable electronics: toward an integrated multilayer composite.

www.MaterialsViews.com C O M M High-Density Stretchable Electronics: Toward an Integrated Multilayer Composite U N IC By Liang Guo and Stephen P. DeWeerth * A IO N In recent years, interest has developed for implementing electronics using fl exible, foldable, and even stretchable materials; the potential applications of such technologies include consumer products ranging from fl exible/elastic displays to skinlike electronics. [ 1 ] Although it remains impractical to fabricate electronic components directly using soft materials, it is possible to connect an array of (rigid) component and subcircuit islands using fl exible/stretchable interconnects to achieve system-level fl exibility or conformability. [ 1 , 2 ] The components can be interconnected via a compliant substrate [ 1–5 ] or via a wiring mesh. [ 6–8 ] One of the primary challenges associated with the implementation of these systems is the limitation to integration density of the electronics when only a single layer of interconnects is used. To address this limitation, dual-layer interconnects have been fabricated (e.g., on fl exible parylene substrates). [ 9 ] Another major challenge is the achievement of robust electrical connections between the soft material and rigid components or substrate. This bonding issue has signifi cantly hindered the practical utility of fl exible/stretchable electronics. We have addressed these two challenges to create stretchable electronics by developing fabrication techniques that enable reliable multilayer interconnects both within an elastic polydimethylsiloxane (PDMS) substrate and between the PDMS and a rigid substrate. This interconnect technology addresses the challenge of integration density by providing multiple layers of wiring in the fl exible PDMS device. The PDMS–to– rigid-substrate interconnect facilitates robust, high-density electrical connections between the two materials. Taken together, these two integral parts facilitate a new high-density bonding method called “ via bonding ”—because an inclined via is used as part of the multilayer interconnect. This multilayer via-bonding technology facilitates high-density, module-level integration of electronic components in an integrated multilayer composite. The key to our via-bonding technology is the combination of a lift-off method, which requires an anisotropic metal-deposition process, with an innovative inclined-via technique. Figure 1 illustrates the method for fabricating multilayer interconnects within PDMS or between PDMS and another substrate. Such lift-off methods—because of their cleanness and process compatibility—are conventionally used in the patterning of thinfi lm metal features on PDMS. [ 10 , 11 ] In order to implement multilayer interconnects both within PDMS and between PDMS