Gecko-Inspired Adhesive Conductive Film for Flexible Electronics

A large number of flexible electronic applications are envisioned among them, wearable electronics and bio-electronics applications. One of the main challenges in this field is to integrate semiconductor devices on unconventional substrates subject to mechanical stress (bending and stretching). Interconnect technologies must be implemented at low pressure and low temperature to avoid damaging the substrate. They must also be able to withstand slight deformation, like a medical patch in contact with the skin. Currently, the solutions for interconnecting electronic components with their active face facing on substrates are mainly based on metallic soldering. These solutions are not ideal for flexible substrates as the soldering is rigid and irreversible. The used of a thin stretchable anisotropic conductive adhesive that can be used as a conductive film between the silicon device and the substrate could solve these issues.<br/>In this work, we have developed gecko-inspired adhesive based on an arrays of microscale pillars made of polydimethylsiloxane (PDMS), a biocompatible and low-cost polymer. The pillars have mushroom-shaped tips known that are known as the most efficient form of contact. First, mushroom-shaped molds with varying geometries were fabricated in a silicon-on-insulator wafers (SOI) with typically a 2 µm-thick buried oxide layer and a thick Si top layer ranging from 15 to 25 µm. The mushroom molds were realized by standard photolithography and etching process and the surface of the molds was then covered with a hydrophobic thin film using a vapor phase process. Then, gecko-inspired films were performed using a solution of PDMS poured into the molds and spread out by spin-coating. The spin-coating parameters were adjusted to obtain films with a backing layer thickness of approximately 190 µm. Finally, the gecko-inspired films were cured at 100°C and peeled from the SOI molds. This method allow to produce microstructured films with very few defects. To make these films locally conductive, composites based on high aspect ratio multi-walls carbon nanotubes (1 wt%, mean aspect ratio of ~830) and PDMS were used. The localization of the conductive composite was performed by screen-printing. CNT-PDMS composite was deposited through a stencil composed of holes of 300 µm diameter directly on the SOI molds. After screen-printing, the CNT-PDMS was thermally cured and pristine PDMS was deposited by spin coating, cured and demolded.<br/>The adhesion performances of the different mushroom-shaped microstructured films were characterized by compression and pull-off tests on glass and on silicon. For both surfaces, adhesion forces higher than 6 N.cm^-2 can be obtained depending on the geometry. However, the shape of the mushrooms needs to be carefully designed, as some configurations (particularly when aspect ratios are too high) are not leading to high adhesion forces. Preliminary results show also that a slight degradation of the adhesion (by a factor ~3) is observed with conductive mushrooms. This results could be related to the difference of elastic modulus induced by the presence of CNT fillers in the composite and by a difference of the surface state of the films. The electrical conductivity obtained for microstructured films is evaluated at 10^-2 S.m^-1 which is lower than the bulk conductivity of the composite (by one order of magnitude). The surface state of the conductive film may also be at the origin of this result, inducing a high contact resistance.