High-performance stacked-coil transformers with thick metal layers

Miniaturised stacked-coil radio-frequency (RF) transformers were fabricated with thick metal layers using the low-cost anodised aluminium substrate process. The transformer consists of a 34 μm-thick substrate-buried copper lower coil and a 30 μm-thick copper/gold upper coil with a 3 μm-thick benzocyclobutene (BCB) interlayer between the coils. Owing to extremely low losses and tight coupling, the fabricated transformers achieved very low insertion losses as low as 0.28 dB and high maximum available gains of up to 0.96 at GHz frequencies. The results verify that the stacked-coil transformer with thick metal layers can be a promising component for high-performance miniaturised RF circuits.