Origami stacking of micro-fabricated coils for miniaturized eddy current sensors

Abstract

Eddy current sensors are non-destructive and highly precise, relying on high-frequency magnetic fields that induce circulating currents in nearby conductive materials. Such operation principle allows for non-contact measurement of distance, thickness, cracks, and vibration, which can be widely applied in the field of advanced manufacturing and device maintenance. However, conventional sensors based on wire wound inductors often suffer from size constraints due to the trade-off between inductance and physical volume, therefore restricted in application where sensing needs to be compact or requires high lateral resolution. Lithography can fabricate micro inductors but the number of turns is limited in 2D configuration. To overcome these limitations, we present miniaturized eddy current sensors fabricated by micro-fabrication techniques with enhanced sensitivity via origami-stacking. Two-dimensional inductor coils with 25 µm-width Cu lines were fabricated on a 25 µm-thick polyimide film via high-resolution photolithography and metallization. The inductors were then stacked into multi-layered 3D architecture using an origami-inspired folding method. Finite element simulations confirmed that inductance increases with the number of layers according to a power-law trend due to the enhanced mutual magnetic coupling. Experimental measurements at 3 MHz validated the simulation, demonstrating 125-fold enhancement in inductance when micro-fabricated copper 2D coils is stacked to 12 levels. The overall thickness of 12 layered origami-sensor architecture was under 1 mm. Compared to a commercial 6 mm-diameter coil inductor, the origami inductor demonstrated 2.46–5.77 times higher inductance at similar thickness. The proposed sensor also exhibited high sensitivity to both metal thickness and proximity, enabling its application in endpoint detection during semiconductor planarization and other precision sensing tasks.