A Novel Interconnect Design with High Stretchability and Fine Pitch Capability for Applications in Stretchable Electronics
Authors: Hsu Yung-Yu , Gonzalez Mario , Bossuyt Frederick , Axisa Fabrice , Vanfleteren Jan , De Wolf Ingrid 
Published: November 2009

Large area deformable macroelectronics, such as flexible display, smart bandage, electronic textile, have to withstand various modes of deformation (e.g., bending, twisting and stretching). Such electronic systems usually are composed of inorganic parts with limited deformability, and organic parts which can sustain large deformations. Because of the elastic nature of elastomers, these materials are often used as substrates for the specific applications mentioned above. In order to fulfill the demand of deformability, many concepts have been developed. One of these concepts consists of small rigid islands with active devices or individual thin chips which are interconnected by thin metal conductor lines. All rigid components are placed on the small islands to ensure that the strains acting on these brittle components are small when the structure is subjected to a large deformation. Since the thin conductor lines have to withstand all these deformations, a proper structural design is necessary to avoid losing structural integrity and electrical functionality during this deformation.
Several technologies have been proposed in recent years, such as in-plane patterned metal conductors, out-of-plane wrinkling metal films, and conductive polymers or liquid alloys. It has been reported that by depositing a thin metal strip with a thickness of few nanometers on an elastomeric substrate, the elongation of the metal can go up to 50% while the strip remains conductive. Upon large strain, the elastic deformation of the elastomeric substrate causes local debondings of the metal film coevolving with strain localizations. Thanks to this coevolved process, even a �straight� line, deposited on a polymer substrate, is stretchable. However, a drawback of this unique characteristic is that the resistance of the thin metal film changes with elongation, which might be a disadvantage for certain applications. Another approach for having a stable resistance is to use bulk metal conductors. Compared to a freestanding bulk metal straight line which ruptures at strains of 1%˜2%, an in-plane patterned horseshoe metal conductor can be stretched up to 100% with a stable resistance before electrical failure.

This paper demonstrates a novel in-plane patterned zigzag structure, with resistance independent of elongation before metal rupture, which can be stretched beyond 40%. The advantage compared to the horseshoe is that due to the geometrical design, the in-plane zigzag structural interconnects can be applied in a fine pitch microelectronic device. The experimental observations by both scanning electron microscopy micrographs and resistance measurements show that there is no significant local necking in either the transverse or the thickness direction at the metal breakdown area. Micrographs and simulation results show that a debonding occurs due to the local twisting of a metal interconnect, out-of-plane peeling and strain localized at the crest of a zigzag structure.