Metal Layer Losses in Thin-Film Microstrip on LTCC

Thin-film microstrip transmission lines fabricated using a Ti adhesion layer followed by layered Cu, Pt, and Au films are measured to determine tradeoffs between manufacturability issues and microwave performance. Since Ti metal has approximately 25 times the resistance of Cu, and currents in a microstrip line flow mainly at the interface with the substrate where the Ti is located, there is the possibility of increased RF signal losses with this structure. It is found that Ti adhesion layers of ≤200-nm thickness cause minimal loss through 40 GHz on DuPont 9K7 low-temperature cofired ceramic substrates, so there is no significant electrical penalty for employing a metal stackup optimized for mechanical durability. These measurements, together with analysis and simulations suggest this will hold in general as long as the thinner, higher resistance adhesion metal is well below one skin depth in thickness at the operating frequency.