• Journal of the Microelectronics and Packaging Society
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• v.25 no.4
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• pp.25-34
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• 2018

In this paper, we review the latest technical progress and commercialization of stretchable interconnectors, stretchable strain sensors, and stretchable substrates for stretchable electronics. The development of stretchable electronics can pave a way for new applications such as wearable devices, bio-integrated devices, healthcare and monitoring, and soft robotics. The essential components of stretchable electronic devices are stretchable interconnector and stretchable substrate. Stretchable interconnector should have high stretchability and high electrical conductivity as well as stability under severe mechanical deformation. Therefore several nanocomposite-based materials using CNT, graphene, nanowire, and metal flake have been developed. Geometric engineering such as wavy, serpentine, buckled and mesh structure has been well developed. Stretchable substrate should also pose high stretchability and compatibility with stretchable sensing or interconnecting material. We summarize the recent research results of new materials for stretchable interconnector and substrate as well as strain sensors. The Important challenges in development of the stretchable interconnector and substrate are also briefly discussed.

• Journal of the Microelectronics and Packaging Society
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• v.25 no.4
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• pp.129-135
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• 2018

In the stretchable electronic devices, the stretchable substrate is a very essential material which determines the stretchability, performances and durability of the stretchable electronic devices. In particular, the current stretchable materials have hysteresis making difficult to used as sensors and other electronic devices. In this study, we developed a PDMS-Ecoflex hybrid stretchable substrate mixed with PDMS and Ecoflex material in order to increase stretchability and improve hysteresis characteristics. Mechanical behavior of the hybrid substrate was evaluated using a tensile test, and optical transmittance of the hybrid substrate was also measured. As the content of Ecoflex increases, the PDMS-Ecoflex hybrid substrate becomes more flexible, and the elastic modulus decreases. In addition, the PDMS substrate failed a tensile strain of 270%, while the PDMS-Ecoflex hybrid substrate did not fail even at 500% strain indicating excellent stretchability. In the repeated tensile test, the hybrid substrate with 2:1 mixing ratio of PDMS and Ecoflex showed hysteresis. On the other hand, in the case of the hybrid substrate with the mixing ratio of 1:1, hysteresis did not occur at a strain of 50% and 100%. Hence, we developed a stretchable substrate with over 150% stretchability and no hysteresis characteristics. The optical transmittance of the Ecoflex substrate was 68.6%, whereas the transmittances of the hybrid substrate with mixing ratio of 2:1 and 1:1 were 78.6% and 75.4%, respectively. These results indicate that the PDMS-Ecoflex hybrid substrate is a potential candidate for a transparent stretchable substrate.

• Vacuum Magazine
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• v.4 no.2
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• pp.15-23
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• 2017

This article reviews technical trend in research of stretchable electrodes for wearable devices, bio-integrated devices, and stretchable electronics. Stretchable electronics is new emerging class of electronics following flexible electronics. One of the most difficult challenges in the development of stretchable electronic is to realize high performance stretchable electrodes with a low resistivity and high strain failure and stretchability against severe strain of the substrate. For this reason, there are many reports on the promising stretchable electrodes including CNT, graphene, Ag nanowire, and composite materials. We outline the recent research for stretchable substrate and stretchable electrode materials to realize highly stretchable electrodes.

• Journal of the Microelectronics and Packaging Society
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• v.22 no.4
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• pp.117-123
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• 2015

In order to develop stretchable substrate technology for stretchable devices, locally stiffness-variant stretchable substrates were processed with two polydimethylsiloxane elastomers of different stiffnesses and their deformation behavior was characterized. Low-stiffness substrate matrix and embedded high-stiffness island of the stretchable substrate were formed by using Dragon Skin 10 of the elastic modulus of 0.09 MPa and Sylgard 184 of the elastic modulus of 2.15 MPa, respectively. A stretchable substrate was fabricated to a configuration of 6.5 cm length, 0.4 cm thickness, and 2.5 cm width. The elastic modulus of a stretchable substrate was increased from 0.09 MPa to 0.13~0.33 MPa by embedding a Sylgard 184 island of 1 cm width and 1~6 cm length into the center part of the Dragon Skin 10 substrate matrix. The elastic modulus of a stretchable substrate was improved to 0.16~0.2 MPa by embedding a Sylgard 184 island of 4 cm length and 0.5~1.5 cm width and to 0.1421~0.154 MPa by embedding a Sylgard 184 island of 2 cm length and 0.5~1.5 cm width. With increasing the tensile strain of a stretchable substrate, deformation restriction of the locally stiffness-variant Sylgard 184 island was further enhanced due to substantial increase in the strength difference between Sylgard 184 and Dragon 10 at large strain.

• Journal of the Microelectronics and Packaging Society
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• v.26 no.4
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• pp.47-53
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• 2019

Stiffness-gradient stretchable electronic packages of the soft PDMS/hard PDMS/PTFE structure were processed using the polydimethylsiloxane (PDMS) as the base substrate and the more stiff polytetrafluoroethylene (PTFE) as the island substrate, and their stretchable deformation-resistance characteristics were characterized. The flip-chip joints, formed by bonding the chip bumps of 50 ㎛-diameter onto the PDMS/PTFE substrate pads, exhibited an average contact resistance of 96 mΩ. When the stretchable package of the soft PDMS/hard PDMS/PTFE structure was deformed to 30% elongation, the strain on the PTFE was restrained to 1%, resulting in a negligible resistance increase of 1% in the daisy-chain circuit formed on the PTFE island substrate. The circuit resistance increased for 1.7% after 2,500 cycles of 0~30% stretchable deformation.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.389.2-389.2
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• 2014

Polydimethylsiloxane (PDMS) based electronic devices are widely used for various applications in large area electronics, biomedical wearable interfaces and implantable circuitry where flexibility and/or stretchability are required. A few fabrication methods of electronic devices directly on PDMS substrate have been reported. However, it is well known that micro-cracks appear in the metal layer and in the lithography pattern on a PDMS substrate. To solve the above problems, a few studies for fabrication of stiff platform on PDMS substrate have been reported. Thin-film islands of a stiff region are fabricated on an elastomeric substrate, and electronic devices are fabricated on these stiff islands. When the substrate is stretched, the deformation is mainly accommodated by the substrate, and the stiff islands and electronic devices experience relatively small strains. Here, we report a new method to achieve stiff islands structures on an elastomeric substrate at a various thickness, as the platform for stretchable electronic devices. The stiff islands were defined by conventional photolithography on a stress-free elastomeric substrate. This technique can provide a practical strategy for realizing large-area stretchable electronic circuits, for various applications such as stretchable display or wearable electronic systems.

• Applied Science and Convergence Technology
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• v.27 no.1
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• pp.5-8
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• 2018

In this paper, plasma treatment effects on a ploy(dimethyl siloxane) substrate were analyzed for the applications of stretchable silver nanowire (Ag NWs) electrodes. The oxygen plasma treated sample shows the best performance compared to nitrogen treated and untreated samples. The lowest sheet resistance and reasonable stretching capability was achieved up to 20% strain condition without open circuit fail for the oxygen plasma treated sample.

• Journal of the Microelectronics and Packaging Society
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• v.22 no.4
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• pp.91-98
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• 2015

In order to apply to stretchable electronics packaging, locally stiffness-variant stretchable substrates consisting of island structure were fabricated by combining two polydimethylsiloxane elastomers of different stiffnesses and their elastic moduli were characterized as a function of the width of the high-stiffness island. The low-stiffness substrate matrix and the embedded high-stiffness island of the stretchable substrate were formed by using Dragon Skin 10 of the elastic modulus of 0.09 MPa and Sylgard 184 of the elastic modulus of 2.15 MPa, respectively. A stretchable substrate was fabricated to be a configuration of 6.5-cm length, 0.4-cm thickness, and 2.5-cm width, in which a high-stiffness Sylgard 184 island, of 4-cm length, 0.2-cm thickness, and 0.5~1.5-cm width, was embedded. The elastic modulus of a stretchable substrate was increased from 0.09 MPa to 0.16 MPa by incorporating the Sylgard 184 island of 0.5-cm width to Dragon Skin 10 substrate matrix. The elastic modulus was further improved to 0.18 MPa and 0.2 MPa with increasing the Sylgard 184 island width to 1.0 cm and 1.5 cm, which were in good agreement with values estimated by combining the Voigt structure of isostrain and the Reuss structure of isostress.

• Journal of the Microelectronics and Packaging Society
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• v.24 no.3
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• pp.27-34
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• 2017

We investigated the feasibility of parylene F usage as an intermediate layer between a polydimethylsiloxane (PDMS) substrate and an Au thin-film interconnect as well as the swelling effect of PDMS substrate on the stretchable deformability of an Au thin film. The 150-nm-thick Au film, which was sputtered on a PDMS substrate without a parylene F layer, exhibited an initial resistance of $11.7{\Omega}$ and an overflow of its resistance at a tensile strain of 12.5%. On the other hand, the Au film, which was formed with a 150-nm-thick parylene F layer, revealed an much improved resistance characteristics: $1.21{\Omega}$ as its initial resistance and $246{\Omega}$ at its 30% elongation state. With swelling of PDMS substrate, the resistance of an Au film substantially decreased to $14.4{\Omega}$ at 30% tensile strain.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.351.1-351.1
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• 2016

While conventional electronic devices have been fabricated on the rigid and brittle Si based wafer as a semiconducting substrate, future devices are increasingly finding applications where flexibility and stretchability are further integrated to enable emerging and wearable devices. To achieve high flexibility and stretchability, various approaches are investigated such as polymer based conducting composite, thin metal films on the polymer substrate, and structural modifications for stretchable electronics. In spite of many efforts, it is still a challenge to identify a solution that offers both high stretchability and superior electrical properties. In this paper, we introduce a highly stretchable entangled-mesh graphene showing a potential to address such requirements as stretchability and good electrical performance. Entangle-mesh graphene was synthesized by CVD graphene on the Cu foil. To analyze the mechanical properties of entangled-mesh graphene, endurance and stretching tester have been used.