Super-stretchable metallic interconnects on polymer with a linear strain of up to 100%

Abstract

Metal interconnects in flexible and wearable devices are heterogeneous metal-polymer systems that are expected to sustain large deformation without failure. The principal strategy to make strain tolerant interconnect lines on flexible substrates has comprised of creating serpentine structures of metal films with either in-plane or out-of-plane waves, using porous substrates, or using highly ductile materials such as gold. The wavy and helical serpentine patterns preclude high-density packing of interconnect lines on devices, while ductile materials such as Au are cost prohibitive for real world applications. Ductile copper films can be stretched if bonded to the substrate, but show high level of cracking beyond few tens of % strain. In this paper, we demonstrate a material system consisting of Indium metal film over an elastomer (PDMS) with a discontinuous Cr layer such that the metal interconnect can be stretched to extremely high linear strain (up to 100%) without any visible cracks. Such linear strain in metal interconnects exceeds that reported in literature and is obtained without the use of any geometrical manipulations or porous substrates. Systematic experimentation is carried out to explain the mechanisms that allow the Indium film to sustain the high strain level without failure. The islands forming the discontinuous Cr layer are shown to move apart from each other during stretching without delamination, providing strong adhesion to the Indium film while accommodating the large strain in the system. The Indium film is shown to form surface wrinkles upon release from the large strain, confirming its strong adhesion to PDMS. A model is proposed based upon the observations that can explain the high level of stretch-ability of the Indium metal film over the PDMS substrate. V C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4929605] The proliferation of 'smart' wearable devices is predicted to lead to the "Internet of Things" (IoT) revolution over the next two decades. The examples of such devices include flexible displays, 1-4 robotic skin, 5 stretchable circuits, 6 hemispherical electronic eye, 7 epidermal electronics, 8 cardiac sensors, and diagnostic contact lens. 9 Flexible electronic platforms typically require that its components be connected with each other on a flexible substrate using metallic interconnects. The design and manufacturing of such interconnects that have high spatial density and can be reliably stretched to large strains has become one of the most critical challenges for the IoT revolution. The essential requirements for the interconnect system include strain compatibility of components at the interfaces, minimal increase in resistivity under stretching, and recovery of the resistivity once the strain is released. In addition, any geometrical manipulation to accommodate deformation must be compatible with the interconnect density demanded by the industry. 10,11 The current methods to improve interconnect stretch-ability include creating serpentine structures of metal films 12-2...

On the deformation mechanisms and electrical behavior of highly stretchable metallic interconnects on elastomer substrates

“…To address these challenges, the authors have recently developed an interfacial engineering approach to incorporate some of the solution strategies above and created highly stretchable Indium interconnect films (thickness of $6 lm) on polydimethylsiloxane (PDMS) substrates, without geometric manipulations (e.g., creating helical or serpentine geometry). 36 The In interconnect film in this study sustained a linear strain of 80%-100% without failure during repeated cycling. 36 Note that our study included a 3-5 nm thick Cr adhesion interlayer, which had a cracked, discontinuous morphology.…”

“…36 The In interconnect film in this study sustained a linear strain of 80%-100% without failure during repeated cycling. 36 Note that our study included a 3-5 nm thick Cr adhesion interlayer, which had a cracked, discontinuous morphology. During stretching, the In film resistivity was observed to increase up to about 30% linear strain, followed by a plateau up to 100% linear strain.…”

“…During stretching, the In film resistivity was observed to increase up to about 30% linear strain, followed by a plateau up to 100% linear strain. 36 This suggested that either recovery or dynamic recrystallization limited the growth of dislocation density even as the film deformed plastically. A geometrical effect of out-of-plane wrinkling upon release from the first stretching cycle was also observed in this study and was attributed to the strain mismatch between the film and the elastomer.…”

“…A geometrical effect of out-of-plane wrinkling upon release from the first stretching cycle was also observed in this study and was attributed to the strain mismatch between the film and the elastomer. 36 In spite of the significant progress in understanding of the highly stretchable metal-elastomer systems, several questions need further clarification as follows:…”

Properties of Stretchable Graphite Intercalation Compound/Polydimethylsiloxane Composites after Cyclic Tensile Strain of 20%

“…7,9,11 Zheng 등은 GIC가 폴리메틸메타아크릴레이트 고분자의 전기전도도 및 유전성질에 미치는 영향을 연구하여 층간 구조를 가지는 그라파이트 필러가 복합재료의 전기전도도 상승에 효과적임 을 확인하였다. 12 Xu, Arafat 등은 실리콘 고분자 표면에 은 나노와이어를 함침시킨 후 반복 신축 실험을 진행한 후의 전 기적 성질 변화를 보고하였고, 13,14 Pan 등은 나일론 6에 팽창 GIC가 열전도도에 미치는 영향을 보고하였다. 15 (Figure 3(d))는 5 wt%를 포함하는 복합재료 (Figure 3(c) 참 고 문 헌…”