On-chip solidified ferrofluid-core self-rolled-up membrane microtube inductors

On-chip self-rolled-up membrane (SRuM) microtube inductors integrated with an in situ solidified ferrofluid core were demonstrated. Based on the silicon nitride (SiN X) SRuM nanotechnology, stress-engineered plasma-enhanced chemical vapor deposition provides large residual-stress difference in the SiN X bilayer, forming a hollow microtube architecture with small inner diameter, thereby enabling efficient capillary-driven infiltration of ferrofluid into the confined cavity. To stabilize the ferrofluid core and mitigate solvent-related redistribution and structural instability, two in situ solidification strategies—ultraviolet curing and thermal curing—are systematically investigated. Measurement data show a significant enhancement in inductance compared to hollow reference devices. At 100 MHz, the inductance increases from 7588 nH for the air-core inductor to 10 216 nH after incorporation of the ferrofluid core, corresponding to an improvement of approximately 1.3×. After ferrofluid-core solidification, the inductance achieves 8748 nH at 100 MHz within a compact footprint of 0.435 mm2, yielding an inductance density of 20 110 nH/mm2. Moreover, the solidified ferrofluid core exhibits excellent temporal stability, with inductance variation remaining below 5% over the 100–600 MHz range during an 80-day evaluation period.