Photoresist may be thinner than metal for a printed circuit, but above all, one photon affects several patterns of the polymer, which saves light. Metal deposition by light would just save the photoresist step but consume more light. Whether it's better depends on many aspects, like how fast, and so on.
You might try instead to produce locally an etchant by light, and start from a uniform copper instead of uniform insulating surface. At least, this doesn't need the clean deposition nor the adherence, which are both nontrivial. Maybe the chemists here can suggest molecules for it?
- Atomic oxygen from some sort of peroxide, hypobromide...?
- Singlet oxygen?
Etching copper has immediate uses, but just blackening a clear metal must have some applications.
I suspect - but haven't seen reports about it - that your closest competitor is a pulsed laser that evaporates the thin copper (35µm and 17µm are common) where desired. This process is the cleanest, has the least steps, needs the least skills to use a machine. Automatic milling machines exist for that exact purpose but their result may be less clean.
Interferences: they are independent of what process uses the photons, and do make patterns on photoresists too. Whether you achieve that with a liquid (or a paste!) must depend on how much the fluid migrates while you irradiate it. This includes diffusion, that is, Brownian motion.
For instance, the patterns on integrated circuits (chips) use interferences, in the sense that simple edges in the mask would produce diffraction fringes in the photoresist, but to combat it, the masks has carefully computed fringes whose interferences minimize the fringes at the photoresist.
As an other example, many simple patterns in photoresists, including zone lenses and sometimes diffusion gratings, are made as interference figures. Interferences really tell how many photons, how much energy arrives where, with all the consequences, be it on a screen, a photoresist, a metal deposition if you achieve it. Interferences are by the way not just fancy figures on special occasions, they are the deep nature of light and generally waves, and tell how a lens works for instance.
Control at will an interference pattern... Yes with limits, and presently it's not done in home labs.
- If the control elements change accurately the phase and amplitude, then nearly any interference pattern is possible, and the math is accessible to engineers. Aperture synthesis works this way in sonars, radars and radiocomms.
- The technology isn't mature enough for light. I've never seen a board of 10,000 lasers whose individual phase is controlled by data cables. What exists is one light source and a many-pixels reflector where the individual reflection amplitude is controlled, but the phase not properly to my knowledge.
- With such limited hardware, one may try to produce interefence patterns, but of poor quality, and with much stray light. Consider this as lab research, very far from making a printed circuit.