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.