Printing of flexible and stretchable carbon-based electrical components by femtosecond laser-direct-writing

• Main Author: Lim, Joel Chin Huat
• Other Authors: Murukeshan Vadakke Matham
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Materials::Microelectronics and semiconductor materials::Nanoelectronics and interconnects; Engineering::Electrical and electronic engineering::Electronic circuits
• Online Access: https://hdl.handle.net/10356/155022
• Institution: Nanyang Technological University

Traditional silicon electronics industry is undergoing a rapid transformation to fulfil the future expectations of the modern era. In the electronics industry, there has been a paradigm change from the "components in a box" approach to the forefront of advancement of flexible and stretchable electronics. Since then, advancements in flexible and stretchable electronics have opened new methodologies and interacting mechanisms for consumer electronics such as wearable electronics, electronics skin, and soft robotics among other significant applications. The use of printed electronics technology is embraced as the one of the frontline changers, which currently acts as the complementary technology for flexible and stretchable electronics. However, this technology is still in its infancy. There are a few challenges that must be addressed in order to improve the level of technology readiness for mass implementation. Finding the right material is one of the most difficult challenge in the fabrication of flexible and stretchable electronic devices. Traditional materials such as silicon and fiberglass commonly used in printed circuit boards (PCBs), are not suitable for stretchable electronics due to their intrinsic brittleness. Novel carbon-based materials such as graphene could be potential candidates owing to their unique combination of properties such as excellent electrical conductivity, high transparency, and flexibility. It should be noted that the full complexity of traditional silicon circuitry may not be possible to achieve in a fully printed electronics circuit due to the limitations of current technology. Device elements such as sensors and energy sources still relay on conventional components. Such quasi-flexible devices containing flexible and non-flexible elements are often referred to as flexible hybrid electronics (FHE). However, the dream of being able to fabricate low cost, fully printed stretchable devices is still being explored. New electronics printing technologies are needed for the adaption of complex form factors and un-conventional materials. In this thesis, a novel electronics printing system using femtosecond laser direct writing (FsLDW) for printing electrical interconnects and other electronic components is proposed and demonstrated. The laser-based photoreduction system eliminates most of the disadvantages associated with the conventional chemical and thermal reduction schemes. In addition, the FsLDW system presents advantages such as chemical and mask-free patterning, tunable photoreduction, low-cost, and environmental friendliness. The research work presented in this thesis involves the research and development of an FsLDW based electronics printing system and investigation into the light-matter interaction of ultrafast pulses with un-conventional materials for electronics purposes. The electronics printing system developed includes all the software and hardware components integrated into the system, which allows the users to easily digitize their designs and control various laser and writing parameters. Investigation into the methodology for flexible and stretchable component fabrication and the electrical characterisation of the interconnect traces, resistors and energy storage devices fabricated using the electronics printing system have been carried out. Such printed electrical traces and resistors with tunable electrical properties are demonstrated for real-life applications such as repairing and modifying flexible printed circuit boards. In addition, components such as supercapacitors have been investigated for the use in FHE and stretchable electronics. After careful review and feasibility measurements, the photoreduction of graphene oxide (GO) to reduced graphene oxide (rGO) was selected to be the strategy for the printed electronics. This allowed unique advantages such as achieving various reduction levels, and high-resolution mask-free patterning with tunable electrical properties and physical dimensions. The GO film was prepared by the drop-casting of GO solution and has been proven to be repeatable with a coefficient of variation of 3.25%. Photoreduction and ablation thresholds were evaluated to be 64.5 J/m2 and 94.0 J/m2. The photoreduction and ablation regimes were explored to understand the morphology and electrical property changes. Three different photoreduction regimes were investigated based on the surface morphology and chemical composition, namely the onset of photoreduction (I), photoreduction and ablation (II), and ablation (III). Ramen spectrometry and scanning electron microscopy were used to analyse the physical changes due to photoreduction. The concept of laser direct writing of conductive traces for repair and modification of PCBs is proposed and demonstrated as the next step. The trace resistance of conductive traces is dependent on the resistivity and thickness of the traces. The effect of critical laser parameters such as the pulse energy, repetition rate, and scanning speed on the spatial resolution, sheet resistance, and thickness of the printed rGO structure were investigated. The linewidths observed were 28.4 ± 0.7 µm at 40 nJ pulse energy, and 69.9 ± 1.8 µm at 220 nJ pulse energy. The line width of rGO trace is estimated to be 0.237 µm per nJ. The smallest line width obtained was 20.6 ± 0.6 µm at 1000 mm/s scanning speed using 200 nJ pulse energy. The rGO thickness was found to be tunable from 0.6 µm to 4.4 µm. Different substrate materials such as FR-4, polyimide (PI), and polyethylene terephthalate (PET) were investigated for their influence on the sheet resistance of the printed rGO sheets. The lowest sheet resistance achieved was 100 /sq. The stretchable aspect of the rGO trace is also investigated. In general, carbon-based materials have limited stretchability. A method to improve the stretchability by the addition of stretchable filler materials such as PDMS was investigated. It is noted that the addition of the filler material also affects the electrical properties of rGO; the rGO/PDMS composite film showed an average increase in resistance of about 10% when compared to the original rGO film. Both the flexibility and stretchability aspects of the fabricated rGO/PDMS composite film were investigated. The laser scanning direction was found to be a crucial factor in determining the gauge factor of the printed rGO structure. The change in resistance (at 20% strain) of rGO films photoreduced by single directional vertical laser scanning was found to be around 700% while it was about 350% for rGO films fabricated by both horizontal and bi-directional laser scanning. Another aspect to be considered for the realisation of fully stretchable device applications is the integration of a stretchable power source. The concept of a flexible and stretchable microsupercapacitor (MSC) was explored to achieve a fully stretchable energy source for stretchable electronics applications. Interdigitated patterns of rGO were fabricated using the developed electronics printing platform, which acted as the electrodes for the MSCs. PVA/H2SO4 gel was used as the electrolyte, which was placed over the active area of the MSCs. For improving the areal capacitance, the FsLDW-MSC was coated with polydopamine layers using 2 mg/mL dopamine solution to form a micropseudocapacitor (MPC). The MPCs showed 5 times higher areal capacitance in comparison to the MSCs. The fabricated MPC had a volumetric energy density of 1.08 mWhcm−3, which is similar to the performance of a conventional thin film lithium-ion battery. The volumetric power density of the MPC was 83.5 mW/cm3, which is about 13 times higher than the volumetric power density of conventional lithium-ion thin-film batteries. An array of 6 × 2 MPCs embedded in PDMS film was adhered to a safety goggles and could successfully power a light emitting diode (LED). The array's total capacitance remained at 97 percent of its original value, even after 200 cycles of attachment and detachment, indicating good sturdiness. The MPCs thus would be an excellent choice as stretchable power supplies for fully stretchable electronic devices. In conclusion, this thesis investigated the direct printing of flexible and stretchable electronic components using an in-house developed electronics printing system. Parametric studies were carried out to understand the critical patterning parameters. Some key components such as interconnects, resistors, and energy sources for realising fully stretchable devices were fabricated using the developed electronics printing system and were investigated. The direct printing of conductive traces, for the repair and modification of printed circuit boards, was demonstrated in real time as a novel application of this technique. It is envisaged that the key findings from this thesis will advance the use of printed electronics technology, especially in fully stretchable electronics and wearable electronics areas.