Robust Encapsulation of Printed Flexible Electronics via Cold Atmospheric Plasma Assisted Silicon Dioxide Coating

Flexible hybrid electronics (FHEs) are emerging devices that bridge the divide between conventional, rigid electronics, and fully printed devices. The FHEs have opened a series of unprecedented application possibilities with broad interest and potential for impact, especially in wearable and implantable healthcare applications. Many such devices are exposed to harsh environmental conditions (e.g., high humidity levels and mechanical stress/strain), requiring them to be encapsulated with insulating polymeric coatings. One important parameter that determines the lifetime and reliability of encapsulated devices is the adhesion between the polymer coating and the printed substrate. In high humidity conditions (e.g., underwater), improper bonding between layers will result in water condensation at the interface between the layers, leading to the failure of the device and electrical shortage. This issue can become more challenging for circuits with complex designs when encapsulant delamination happens in hidden areas that can hardly be noticeable during the initial stage of failure. The bonding between coatings and substrates can be improved by using adhesion-promoting intermediate layers such as SiO₂ deposited through atomic layer deposition (ALD) prior to encapsulating with the polymeric coating. In this process, the hydroxyl groups (-OH) on the glass surface will interact with the silanol groups in the polymer resin coating, resulting in strong Si – O – Si covalent bonds that form between the two materials. Although ALD technology provides precise control of layer thickness at the sub-nanometer scale and is successfully implemented in the packaging of OLED technologies, the process requires complex and time-consuming vacuum-assisted machinery with limitations on the substrate size or dimensions. To address this challenge, we will discuss the novel use of cold atmospheric plasma (CAP) as an alternative to ALD with great potential for scalability and implementation in additive and roll-to-roll manufacturing processes. The CAP deposition process can be used to deposit thin films in a continuous process and directly onto specific locations that are needed. In this process, a plasma head is used to chemically decompose volatile organosilicate precursors and as a source of active species involved in the SiO₂ film deposition.<br/>In this talk, we will review the CAP processing conditions for depositing SiO₂ coatings using Tetraethoxysilane precursor, and it's used as an intermediate barrier layer between the printed circuit and polymeric dielectric layer to improve moisture barrier properties. The processing conditions on the deposited coating’s chemical structure, morphology, and electrical properties have been carefully investigated under different CAP processing conditions. This investigation showed the intermediate SiO₂ coating provided a 1000-fold improvement in moisture-resistant barrier properties compared to samples without the intermediate layer. The FHEs prepared with the optimized packaging conditions also show excellent environmental and mechanical stability (even after 1,000 cycles of bending under water). As a proof of concept, the use of the technology was investigated as a reliable packaging approach for robust packaging of printed antennas in automotive applications. It is envisioned that the intermediate SiO₂ coating process can provide a new route in the scalable, more cost-effective, and robust packaging wide range of FHEs for emerging application areas in wearable healthcare, environmental, agricultural, and structural health monitoring applications.