Juhwan Lee

High-performance green flexible electronics based on biodegradable cellulose nanofibril paper

Todays consumer electronics, such as cell phones, tablets and other portable electronic devices, are typically made of non-renewable, non-biodegradable, and sometimes potentially toxic (for example, gallium arsenide) materials. These consumer electronics are frequently upgraded or discarded, leading to serious environmental contamination. Thus, electronic systems consisting of renewable and biodegradable materials and minimal amount of potentially toxic materials are desirable. Here we report high-performance flexible microwave and digital electronics that consume the smallest amount of potentially toxic materials on biobased, biodegradable and flexible cellulose nanofibril papers. Furthermore, we demonstrate gallium arsenide microwave devices, the consumer wireless workhorse, in a transferrable thin-film form. Successful fabrication of key electrical components on the flexible cellulose nanofibril paper with comparable performance to their rigid counterparts and clear demonstration of fungal biodegradation of the cellulose-nanofibril-based electronics suggest that it is feasible to fabricate high-performance flexible electronics using ecofriendly materials.

Radio-frequency flexible and stretchable electronics: the need, challenges and opportunities

Successful integration of ultrathin flexible or stretchable systems with new applications, such as medical devices and biodegradable electronics, have intrigued many researchers and industries around the globe to seek materials and processes to create high-performance, non-invasive and cost-effective electronics to match those of state-of-the-art devices. Nevertheless, the crucial concept of transmitting data or power wirelessly for such unconventional devices has been difficult to realize due to limitations of radio-frequency (RF) electronics in individual components that form a wireless circuitry, such as antenna, transmission line, active devices, passive devices etc. To overcome such challenges, these components must be developed in a step-by-step manner, as each component faces a number of different challenges in ultrathin formats. Here, we report on materials and design considerations for fabricating flexible and stretchable electronics systems that operate in the microwave level. High-speed flexible active devices, including cost effective Si-based strained MOSFETs, GaAs-based HBTs and GaN-based HEMTs, performing at multi-gigahertz frequencies are presented. Furthermore, flexible or stretchable passive devices, including capacitors, inductors and transmission lines that are vital parts of a microwave circuitry are also demonstrated. We also present unique applications using the presented flexible or stretchable RF components, including wearable RF electronics and biodegradable RF electronics, which were impossible to achieve using conventional rigid, wafer-based technology. Further opportunities like implantable systems exist utilizing such ultrathin RF components, which are discussed in this report as well.

Materials and design considerations for fast flexible and stretchable electronics

Flexible and stretchable high-speed devices that operate at microwave level may enable wireless functionalities for many applications. The rise of flexible or stretchable electronic systems using inorganic semiconducting materials allow integration of high-performance electronic devices on foreign substrates, such as plastic or rubber. Here, we report on materials and design considerations for fabricating flexible and stretchable electronics systems that operate in the microwave level. High-speed active devices, including cost effective Si-based transistors and high-power GaN-based transistors, performing at multi-gigahertz frequencies are fabricated on either plastic or rubber substrates. Furthermore, important passive components, such as the capacitors, inductors and transmission lines that can together complete a microwave integrated circuit, are also demonstrated. All of the components presented here have comparable high-performances to their rigid counterparts, which can be combined to form microwave integrated circuits that can be used for many applications, where wireless functionalities are desired.

Photolithography-Based Nanopatterning Using Re-entrant Photoresist Profile

Photolithography based on optical mask is widely used in academic research laboratories due to its low cost, simple mechanism, and ability to pattern in micron-sized features on a wafer-scale area. Because the resolution is bound by diffraction limits of the light source, nanoscale patterning using photolithography requires short-wavelength light source combined with sophisticated optical elements, adding complexity and cost. In this paper, a novel method of subwavelength patterning process using conventional i-line mercury lamp is introduced, without the use of such advanced optical tools. The method utilizes the re-entrant geometry of image reversal photoresist produced from the developing process, where a secondary mask is generated by isotropically depositing a metal layer to cover the re-entrant profile of the photoresist. Removing the photoresist by applying ultrasonic vibrations in acetone bath uniformly cracks the metal layer at the sidewalls of the re-entrant profile, exposing the substrate with a reduced feature size. The width of the initial mask pattern can be reduced by 400 nm in a controlled manner, regardless of the original width choice. As a result, the method is shown to achieve sub-100 nm scale linear patterns compatible for both subsequent deposition process and dry-etching process. Our approach is applicable to various shapes of the patterns and can be used in electronic device fabrication requiring nanoscale lithography patterning, such as the gate fabrication of AlGaN/GaN high-electron-mobility transistor.

Radio-frequency flexible and stretchable electronics (Key note)

Flexible and stretchable electronics have broad applications and will further change our life and improve our life quality in numerous ways in the near future. While low frequency flexible electronics and stretchable electronics have been well developed and have penetrated into many applications, the high-frequency (beyond GHz in the microwave frequency range) counterparts have faced great challenges. To realize high frequency applications, high quality semiconductor materials with high mobility/overshoot/saturation velocities and good mechanical bending properties are needed. Here, we review the development of flexible electronic devices that operate in microwave frequencies using a variety of materials, such as silicon, III-V compound semiconductors and 2D materials like MoS2 and graphene.