Investigations about Al and Cu-Based Planar Spiral Inductors on Sapphire for GaN-Based RF Applications

Conventionally, Cu is preferred over Al to fabricate integrated inductors with higher quality factors on either silicon or sapphire substrates, profiting from its lower resistivity. However, after investigating and comparing these two kinds of metal multilayers in terms of fabrication process, electrical conductivity, in-depth profile analysis and performance of actual inductors, the Al-based metal multilayer exhibits competitive ability in fabricating thin-film inductors on sapphire compared to Cu-based multilayers. This is attributed to the degradation in electrical conductivity out of oxidation of Cu-based metal sublayers or forming alloys between them. Furthermore, in order to avoid complicated de-embedding procedures in the characterization of the on-chip inductors, a six-element equivalent physical model, which takes the parasitic effect of radio-frequency (RF) test structures into account, is proposed and validated by matching well with embedded measurement results.

Automated summary

Conventionally, copper is preferred over aluminum for fabricating integrated inductors on silicon or sapphire substrates due to its lower resistivity. However, upon investigation, aluminum-based metal multilayers show competitive ability in fabricating thin-film inductors on sapphire compared to copper-based counterparts, attributed to degradation in electrical conductivity. Additionally, a six-element equivalent physical model is proposed to simplify characterization of on-chip inductors and validated with measurement results.