Engineering the Cracking Patterns in Stretchable Copper Films Using Acid-Oxidized Poly(dimethylsiloxane) Substrates

Stretchable conductors are a fundamental part of stretchable and wearable electronic devices. Although the high conductivity of metals makes them the materials of choice, metal films deposited on poly(dimethylsiloxane) (PDMS) elastomers develop destructive channel cracks with strain due to the mechanical mismatch between the metal and elastomer. Engineering how cracks form in metal films under strain is a promising way to control the resistance increase with strain and expand the utility of stretchable metal films to include strain sensors along with electrodes and interconnects. The topography of the PDMS surface dramatically influences the cracking and resistance response in the metal film. However, there is not yet a full understanding of the topography-cracking-resistance relationship to enable researchers to dial in a specific resistance response for a particular application. This study presents a fully solution-based approach to crack engineering to provide more insight into this relationship. Oxidizing the surface of a PDMS substrate in a mixture of sulfuric and nitric acids induces the formation of a hierarchical topography, which can be adjusted simply by changing the composition of the acid mixture. The topography influences crack formation with strain in an overlying copper film deposited by electroless metallization. This study reveals that the topography- cracking-resistance relationship is complex and microscale cracking patterns outperform nanoscale cracking patterns to retain the conductivity. The optimal topography is one that generates cracks with microscale interconnections that preserve the conductivity.

Automated summary

Stretchable conductors are essential for stretchable and wearable electronic devices. This study investigates the relationship between topography, cracking, and resistance in metal films deposited on PDMS elastomers. The findings reveal that microscale cracking patterns are more effective in retaining conductivity.