Fabrication of high quality factor RF-solenoids using via structures

High quality factor inductors are key components in RF-circuits. Fabrication of on-chip coils is costly and suffers from low Q-factor. Design, simulation, and fabrication of novel off-chip solenoid inductors with high quality factor are reported. Minimum feature size of standard PCB fabrication has been used to make embedded solenoids in Duroid substrate with 2.2 relative permittivity. A solenoid turn included two imbedded copper via of 125 μm radius and two surface conductors of 250 μm wide to shape a rectangular coil. Different pitch sizes, conductor lengths, solenoid heights, and numbers of turns were simulated and fabricated. The maximum quality factor was 160 for a 500 μm pitch size and 1 mm conductor length in a substrate of 380 μm thick. In addition to the very high Q-factor, the inductor benefits from facilitated design and ease of fabrication. Analytical design has been investigated by a lumped circuit model and electromagnetic simulation. The result of S-parameter measurements was in close agreement with the designed circuit. The micro-solenoids had a higher quality factor compared to all other counterparts.