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

Tiffany Baëtens,Emiliano Pallecchi,Steve Arscott,Vincent Thomy

doi:10.1038/s41598-018-27798-z

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.

Highlights

Understanding the behaviour of materials under high mechanical stresses and strains is the key to being able to optimise processes for the manufacture of original and robust flexible[1], stretchable[2], and squashable electronic systems—this approach will no doubt have a major impact in a multitude of applications including inter alia wearable electronics[3,4,5], displays[6,7] soft robotics[8,9], and biomedical fields[10,11,12]
The increase of transversally-measured electrical resistance with strain is apparent in Fig. 10a which shows the resistance of 3 lines plotted as a function of applied strain—it should be noted that these results are for the first strain sweep using previously unstrained samples
Two types of cracking are observed in such systems—process-induced cracking (PIC) and strain-induced cracking (SIC)

Summary

Introduction

Understanding the behaviour of materials under high mechanical stresses and strains is the key to being able to optimise processes for the manufacture of original and robust flexible[1], stretchable[2], and squashable electronic systems—this approach will no doubt have a major impact in a multitude of applications including inter alia wearable electronics[3,4,5], displays[6,7] soft robotics[8,9], and biomedical fields[10,11,12]. An important part of any electronic system is that of reliable, robust conducting interconnections which link the different devices and components on the integrated chip This is all the more critical for flexible, stretchable and squashable systems—where parts, including interconnections, can be exposed to high mechanical strains causing stresses to be generated. It is important to note that it is becoming apparent that the specific type of mechanical solicitation determines the observed electromechanical behaviour[37] In this context, the present article addresses some of the issues concerned in the quest for robust, thin film interconnections for squashable and stretchable electronics using thin metal films. The work here can be set into a more general context since cracking is a universal phenomenon which occurs in materials on the macroscopic scale[38,39], the micrometre scale[40,41,42,43], and the nanometre scale[44,45]

Methods

Results

Conclusion