[PeterisP]

Yes, like other similar tamperproofing options (glitter, vacuum-sealed colored beads, etc) it's trivial, cheap and fast to get a random pattern, but absolutely impractical to control the pieces to get any specific pattern - perhaps someone like a microsurgeon could manipulate them properly given enough time, but that would take an absurd time (since there are many tiny pieces which each need to be manipulated within a gooey substance where each movement disturbs previous ones as well) and be absurdly expensive, and nobody has a "printing" technology to do it in a cost-efficient way.

Perhaps in future someone could develop an advanced combination of 3d printer and pick&place machines that could do it, but such future potential doesn't disqualify this tech from currently detecting counterfeiting of fancy shoes or something.

[kuschku]

Why would you need a 3D printer or pick&place machines? You can just do it photolithographically.

Coat a piece of glass with a thin layer of metal. Put a photoresist on top. Project the desired pattern onto it with UV light. Wash the unhardened photoresist away and etch the unnecessary metal.

Now you've got metal in exact the spots you'd like, of exactly the thickness you'd like. You can get the accuracy down to a few hundred micrometers for cheap today.

[gnramires]

That would work only for a planar distribution of material. A 3D distribution would require multiple layers (I guess it might quickly become infeasible if it requires thousands of layers).

In the case of 3D arrangements, I think some substrate materials (and also some properties of the particles) would be very difficult to get using photolithography (or some kind of micro 3D printing).

[kuschku]

In the case of 3D arrangements, you don't necessarily need to create all the layers photolithographically. You might be able to flatten N layers into 1 layer, then add a plastic coating equivalent to N-1 layers ontop, then repeat that. You'll have a very similar result to every layer being separate.

Imagine e.g. the "multiple layers of cardboard cutouts" scenery in theater vs it actually being 3D.

[PeterisP]

I don't know much about photolithography, but doesn't it rely on relatively expensive fixed masks prepared for each layer?

Assuming that doing the process you describe is sufficient, what's the ballpark of what "for cheap" means for you if you needed to print 1000 different fake tags, assuming many layers of "the desired pattern" to print the metal flakes?

[kuschku]

> doesn't it rely on relatively expensive fixed masks prepared for each layer

If you need perfectly sharp edges and high precision, sure. But I'm sure in this case that'd be unnecessary.

> Assuming that doing the process you describe is sufficient, what's the ballpark of what "for cheap" means for you if you needed to print 1000 different fake tags, assuming many layers of "the desired pattern" to print the metal flakes?

I described in another comment an additional way to quantize the layers to reduce the repetition steps, which would reduce costs further.

Regarding costs, you could fake a THzID chip for about 500€ per fake. Not cheap enough to do it for household items, but if you're faking designer bags, clothing, sneakers, or electronics, it'd be absolutely worth it.