High-aspect-ratio photoresist processing for fabrication of high resolution and thick micro-windings

DC winding losses remain a major roadblock in realizing high efficiency micro-magnetic components (inductors/transformers). This paper reports an optimized photoresist process using negative tone and acrylic based THB-151N (from JSR Micro), to achieve one of the highest aspect ratio (17:1) and resolution (∼5 µ m) resist patterns for fabrication of thick (∼80 µ m) micro-winding using UV lithography. The process was optimized to achieve photoresist widths from 5 µ m to 20 µ m with resist thickness of ∼85 µ m in a single spin step. Unlike SU-8, this resist can be readily removed and shows a near-vertical (∼91°) electroplated Cu side-wall profile. Moreover, the high resolution compared to available resist processes enables a further reduction in the footprint area and can potentially increase the number of winding thereby increasing the inductance density for micro-magnetic components. Resistance measurements of electroplated copper winding of air-core micro-inductors within the standard 0402 size (0.45 mm² footprint area) suggested a 42% decrease in resistance (273 mΩ–159 mΩ) with the increase in electroplated Cu thickness (from 50 µ m to 80 µ m). Reduction of the spacings (from 10 µ m to 5 µ m) enabled further miniaturisation of the device footprint area (from 0.60 mm² to 0.45 mm²) without significant increase in resistance. (paper)