How Stretchable Can We Make Thin Metal Films?
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How Stretchable Can We Make Thin Metal Films? Candice Tsay1, Stephanie P. Lacour1, Sigurd Wagner1, Teng Li2, Zhigang Suo2 1 Department of Electrical Engineering, Princeton University, Princeton, NJ, USA 2 Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA ABSTRACT Thin metal films deposited on elastomeric substrates can remain electrically conducting at tensile strains up to ~100%. We recently used finite-element simulation to explore the rupture process of a metal film on an elastomer. The simulation predicted the highest stretchability on stiff elastomeric substrates [1]. We now report experiments designed to verify this prediction. A ~15-µm thick silicone elastomer layer with Young’s modulus E ~ 160 MPa is deposited on a 1mm thick membrane of polydimethylsiloxane (PDMS), a silicone elastomer with E ~ 3 MPa. Metal stripes consisting of 25-nm thick gold (Au) film sandwiched between two 5-nm thick chromium (Cr) adhesion layers are fabricated either on top of the stiff layer spun onto the soft membrane substrate, or are encapsulated at the interface between the two elastomers. Encapsulated gold films remain electrically conducting beyond 40% strain. But conductors deposited on top of stiff elastomer lose conduction at strains of 3-8%. These results suggest that, in addition to the stiffness of the elastomeric substrate, the initial microstructure of the metal film plays a role in determining its stretchability. INTRODUCTION Conformable or skin-like electronics must withstand repeated and large deformations. For example, a stretchable sensor array wrapped around the elbow-joint of a prosthetic arm may experience tensile strains of 10% or more. However, thin film electronic materials fracture at lower strains than this. Free-standing gold films, for example, rupture at 1-2% strain [2]. We have demonstrated experimentally that a thin gold film bonded to an elastomeric substrate with Young’s modulus E ~ 3 MPa can stretch and remain conducting to 100% strain [3, 4]. This is a step toward fabricating elastic electronic circuits on elastomeric substrates using arrays of rigid device islands connected by stretchable metallization [5]. We used finite-element simulations to model the rupture process for these stretchable conductors. The results show that, while freestanding metal films rupture by deformation localization, an elastomeric substrate suppresses the localization and allows the metal film to elongate without immediate rupture [1, 6]. Furthermore, the simulation predicts greater stretchability for metal films on stiffer elastomeric substrates. This paper describes experiments designed to test the simulation prediction and also presents our observations on the effect of an encapsulating silicone layer on the conductor’s stretchability. Four types of samples are prepared, summarized in Figure 1a. Type A, in which the gold conductor is fabricated on top of the compliant silicone (E ~ 3 MPa) substrate, is the configuration tested many times before [3, 4], and is used here as a ba
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