New for December 2010! It's about time we had a page on thin-film networks, often abbreviated TFNs.
Thin-film networks are circuits fabricated on hard substrates (usually alumina), with metalization deposited by sputtering, evaporation plating or some combination thereof. The metal is "thin" as it is on the order of just a few microns (or hundreds of micro-inches).
Thin film networks can be simple, planar structures of just one type of metal, or they can be much more complex, with etched and filled vias, thin-film resistors, and even airbridges and capacitors. However, the most complicated thin-film ideas have been career killers in the past. In the future, any time you are looking at a potential thin-film project, ask yourself first, "would this make more sense on silicon, or perhaps on a PWB?"
There will always be a place for TFNs, so the more you learn about them, the better off your career will go.
Substrate materials
Metalization
Jokes about gold-plated toilet seats for the military have been around for decades. There is some truth to this when you are discussing thin-film networks, as the preferred conductor metalization is gold. Gold has pretty good conductivity (about the same as aluminum but less than copper or silver) but it does not oxidize or react with other materials in harmful ways. Gold provides a great surface for wirebonding.
Because of the gold in TFNs (and their cousins, co-fired ceramics), electronics junk has some value, but usually not much. The low value of metal makes an attractive business case to unscrupulous recyclers who ship e-junk to the third world and the metals are extracted in the worst possible ways (burning, acids, mercury). For these reasons, in the future an alternative to gold metalization should be adopted, even by microwave engineers. What is that alternative? Perhaps copper or aluminum, with a very thin over-coating of gold.
Adhesion layers
Patterning and etching
Resistors
Stabilization bake
Resistor materials such as tantalum nitride are slightly unstable in oxygen atmosphere.
More to come..