Geometry optimization of broadband, planar, spiral inductors via continuous trace width modulation

Variable-width planar inductors-commonly referred to as tapered inductors-have been shown to outperform their constant-width counterparts at specific target frequencies. This study extends that concept to the design of broadband inductors operating from DC to the low RF band (up to several hundred MHz), leveraging continuous trace width modulation to enhance performance. Circular spiral inductors with both constant and continuously varying trace widths were designed to have identical nominal inductance, and were subsequently simulated, fabricated, and experimentally characterized. DC performance was evaluated by measuring the magnetic flux density at the inductor center as a function of applied voltage and current, while RF performance was assessed through reflection coefficient measurements, from which frequency-dependent inductance and quality factor were extracted. Across the entire frequency range investigated, variable-width inductors consistently outperformed constant-width designs: they generated stronger magnetic fields at DC under equivalent bias conditions, and exhibited higher quality factors in the low RF range. These results demonstrate that continuous trace width modulation enables the design of high-performance inductors, optimized not only at specific frequencies, but also across broad spectral ranges.