The  joint research team of assistant professor Donghee Son at Sungkyunkwan University (SKKU), professor Dae-Hyeong Kim at Seoul National University (SNU), and professor Sung Gap Im at Korea Advanced Institute of Science and Technology (KAIST) developed a soft, stretchable, nanometer-thick polymer dielectric film with exceptional thermal/chemical resistance through a initiated chemical vapor deposition (iCVD) process. The professor Son et al. applied the vacuum-deposited stretchable polymer film to intrinsically stretchable transistors and various logic gates in the 4-inch wafer scale. (Figure 1).

Recently, various approaches for adopting soft materials have been developed for intrinsically stretchable electronics which does not need any specific structural designs owing to their deformability. However, such devices employed solution-processed dielectric materials and thereby encounter critical challenges in achieving high electrical performances. Specifically, solution-processed dielectric materials exhibit micrometer-scale thicknesses, low insulating performances, chemical instability, low uniformity, and incompatibility with conventional microfabrication processes. Such features result in low gate controllability, high operation voltages, and limited scalability to large-scale circuits. In this regard, the development of an ultrathin, stretchable, and high-performance dielectric material has remained a predominant goal in the field of intrinsically stretchable electronics.

▲Figure 1. A vacuum-deposited nanoscale ultrathin polymer dielectric for large-area stretchable electronics

In the current study, we present a new approach to the design of dielectric materials to resolve the aforementioned challenges in intrinsically stretchable electronic devices. Our large-scale vacuum-deposited stretchable dielectric enables the scalable fabrication of intrinsically stretchable devices with electrical performances comparable to those fabricated using the non-stretchable inorganic and stretchable organic dielectric materials (e.g., Al₂O₃ deposited via atomic layer deposition & spin-coated viscoelastic layer). Such high performance allows for the fabrication of intrinsically stretchable transistors and logic circuits that operate with the lowest reported power consumption. We consider that the observations of our study would transform the conventional paradigm of soft electronics.

To realize the vacuum-deposited polymer dielectric, its fabrication started with copolymerizing two different monomers, isononyl acrylate (INA) and 1,3,5-trimethyl-1,3,5-tryvinyl cyclotrisiloxane (V3D3) through an initiated chemical vapor deposition (iCVD). The INA acts as a soft segment that provides stretchability and V3D3 serves as a cross-linkable hard segment, giving the polymer film robust insulating properties. The mixing ratio of the monomers (INA and V3D3) was optimized to achieve both insulating and stretching performance of the device. In this study, the world’s 1^st vacuum-deposited polymer film with both elasticity and insulation properties even at a ultra-thin thickness of approximately 100 nanometers enabled by optimal combination of soft monomer and crosslinker was developed (Figure 2).

▲Figure 2. A vacuum-deposited nanoscale ultra-thin stretchable polymer dielectric film with large-area uniformity, thermal/chemical stability enabled by copolymerization in iCVD process

The joint team demonstrated the wafer-scale fabrication/integration process using iCVD film. As-fabricated stretchable transistors consisting of a network-structured semiconducting carbon nanotubes (CNTs), microcrack-based stretchable metal electrodes (e.g. gate, source, and drain) featured field-effect mobility of 14.05 cm²/Vs at 10 μm channel length, subthreshold swing (SS) of 265 mV/dec, threshold voltage (V_th) of 2.47 V, and log (Ion/Ioff) of 4.63 while showing operational uniformity and stretching stability.

▲Figure 3. Wafer-scale integration of stretchable electronics using an iCVD polymer dielectric

The iCVD stretchable dielectric broke the existing technical barrier of conventional organic dielectric that could not combine stable insulating performance and stretchability with submicron thickness, achieving the highest output drive current in the same channel area, which was enabled by nanoscale ultra-thin thickness-derived high capacitance and low-voltage operational property (Fig. 4).

Professor Donghee Son said "We think achieving the energy-efficient performance of the stretchable electronic devices is the most important issue in the long-term reliable wearables. In this regard, we innovatively overcame the limitation of conventional flexible organic dielectric materials, which were major bottlenecks in the field of stretchable electronics technology, by demonstrating large-area integration of low-power stretchable electronic devices using vacuum-deposited nanoscale-thick ultra-thin polymer dielectric with thermal/chemical stability."

SKKU-SNU-KAIST joint research team published on the paper on Feburary 3rd(Fri) on the electronic engineering department of international journal called (Nature Electronics, IF: 33.255, JCR 0.18%)

This study was allowed with the support from Ministry of Science and ICT-Directorate for Basic Research in Science & Engineering, Institute for Basic Science(IBS-R006-A1 and IBS-R015-D1), National Research Foundation of Korea Basic Research in Science & Engineering(2021R1I1A1A01060389), Samsung Science & Technology Foundation(SRFC-IT2102-04).

※ Title: A vacuum-deposited polymer dielectric for wafer-scale stretchable electronics

※ DOI: https://doi.org/10.1038/s41928-023-00918-y