Biodegradable CMOS-based Wireless Power Transfer Device for Remote Sensing Applications

The growing demand for electronic devices in healthcare, environmental remote sensing, and IoT has raised critical concerns about sustainability, e-waste, and the safety of implantable systems. In precision agriculture (PA), achieving dense, real-time field monitoring is still limited by battery-powered nodes that require periodic maintenance and retrieval, by non-biodegradable hardware that accumulates in the soil as e-waste, and by the cost and complexity that cap sensor density and spatial resolution. To address these challenges, a promising direction for modern sensing platforms is to become (a) biodegradable to minimize long-term impact, (b) autonomous via wireless power transfer (WPT) to eliminate toxic rechargeable batteries, and (c) integrated with CMOS technology to ensure efficient energy conversion, robust signal processing, and multi-functionality. Despite significant progress, today's solutions address these aspects only partially, lacking a fully biodegradable system that combines WPT, sensing, and CMOS integrated circuits (IC) on a single substrate. This thesis tackles this gap by focusing on an intermediate yet crucial goal: the design and fabrication of a biodegradable WPT device for field deployment in PA, developed by photolithography-based processes as the starting point for future integration with CMOS and sensing components. The device realized here is an LC receiving coil antenna operating in Near-Field Communication (NFC) band, an inductive-coupling WPT method suited to short distances (~10 cm) and low-power (~ mW) sensing at standard frequencies of 13.56 MHz and 6.78 MHz. The Rx coil harvests energy from an external, electrically supplied Tx that can be carried by a drone, a wearable patch, or a rover (as in this project). To support later functions, the design includes two external output pads for interconnection and integration with a CMOS chip. To ensure biodegradability, polylactic acid (PLA) was used as both substrate and encapsulant, silicon oxide/nitride as insulators, and molybdenum (Mo) for the metal layers. While environmentally compatible, these choices introduce major challanges: (i) PLA’s glass-transition temperature (~65 °C) and melting point (~150 °C) impose a strict thermal budget on fabrication and complicate PLA/SiO₂/Mo interface engineering; (ii) biodegradable metals such as Mo exhibit higher resistivity than standard conductors (Cu, Au, Ag, Al), reducing the LC coil’s quality factor and thereby limiting WPT efficiency, here estimated at ~28% compared to >50% in conventional systems. A cleanroom-compatible process flow was developed at Delft University of Technology (Else Kooi Laboratory and Kavli NanoLab), exploiting advanced photolithography (direct laser writing) to pattern the LC coil on biodegradable substrates. The process was validated through systematic experiments that optimized Mo patterning on different substrates (PLA, Dextran+PLA, Dextran, and SiO₂). The results demonstrate that adapting biodegradable polymeric substrates to microfabrication is challenging but feasible, establishing a solid foundation for further development. This work provides the basis for biodegradable WPT devices and paves the way toward their integration with CMOS ICs and sensors. Future perspectives include using sacrificial layers (e.g. Dextran) in a different process sequence, to facilitate fabrication before final PLA deposition, and the extension of the developed process flow toward the fabrication of fully integrated biodegradable systems.