Cracking effects in squashable and stretchable thin metal films on PDMS for flexible microsystems and electronics

Abstract

Here, we study cracking of nanometre and sub-nanometre-thick metal lines (titanium, nickel, chromium, and gold) evaporated onto commercial polydimethylsiloxane (PDMS) substrates. Mechanical and electromechanical testing reveals potentially technologically useful effects by harnessing cracking. When the thin film metal lines are subjected to uniaxial longitudinal stretching, strain-induced cracks develop in the film. The regularity of the cracking is seen to depend on the applied longitudinal strain and film thickness—the findings suggest ordering and the possibility of creating metal mesas on flexible substrates without the necessity of lithography and etching. When the metal lines are aligned transversally to the direction of the applied strain, a Poisson effect-induced electrical ‘self-healing’ can be observed in the films. The Poisson effect causes process-induced cracks to short circuit, resulting in the lines being electrically conducting up to very high strains (~40%). Finally, cracking results in the observation of an enhanced transversal gauge factor which is ~50 times larger than the geometric gauge factor for continuous metal films—suggesting the possibility of high-sensitivity thin-film metal strain gauge flexible technology working up to high strains.

A rapid technique for the direct metallization of PDMS substrates for flexible and stretchable electronics applications

“…Copper film thickness and PDMS surface roughness were measured by a Dektak 3 Stylus optical profilometer and optical images of the film surface were obtained using LEICA CTR 6500 instrument.…”

“…Stretchable and flexible electronics have received great interest for multiple applications ranging from wearable electronics, soft robotics, displays to bioelectronics [1], [2]. The increasing demand for flexible microelectronic systems requires the development of flexible circuits with interconnections that can withstand mechanical stresses under high strains [3]. Various nanocomposite materials and elastic polymers such as polyimide, polyethylene naphthalate and polyethylene terephthalate are currently being investigated as soft substrates for fabrication of flexible electronic devices with stretchable interconnections linking electronic parts [4].…”

Design and fabrication of PDMS‐based electrostatically actuated MEMS cantilever beam

“…Another type of cracking is associated with the post-process strain developed due to peeling of the metal deposited PDMS layer. This can be termed as the strain-induced cracking (SIC) [38]. One of our fabrication requirements is to peel-off the metal deposited PDMS layer to obtain a pattern.…”

“…When the metal is sputter deposited on the PDMS substrate, thermal stresses are developed during and after metal deposition, which leads to the formation of the cracks thus limiting its conduction [36, 37]. This can be termed as process‐induced cracking (PIC) [38], which occurs due to metallization. These cracks are mainly formed due to tensile stresses developed due to the melting point of the metal, substrate temperature, metal deposition rate etc.…”

The Use of a Water Soluble Flexible Substrate to Embed Electronics in Additively Manufactured Objects: From Tattoo to Water Transfer Printed Electronics

“…Another promising approach consists of the fabrication of electronics on flexible or stretchable planar substrates before the transfer step onto the 3D object [ 16 , 17 , 18 , 19 , 20 , 21 ]. This approach benefiting from the advantages of subtractive cleanroom processing that allows the use of a micrometric accuracy and a wide range of compatible materials (insulators, conductors, and semiconductors).…”