. Image taken from Can somebody identify this 12" silicon wafer?
So this silicon wafer looks multicolored (and beautiful). But how does it get multicolored like a rainbow? What is the reason for this phenomenon?
If it's an old image or an image of any wafer made by a old process or with large node technology that does not have "dummy patterning" then the only thing that will make the scribe lanes show color is thin film interference.
However, starting at roughly 0.25 microns (early 1990's) dummy patterns were added everywhere (including the scribe lane) to wafers at topographical layers (as opposed to implants) to handle problems with processes that suffered from microloading and other pattern density-dependent processes. These included reactive ion etching and chemical-mechanical planarization (or polishing) as well as problems coating with the thinner photoresists necessary for 248 nm Deep UV photolithography which was introduced around the same timeframe. While optical proximity correction is a different think entirely, the adding of larger features like this is often folded in to the overall OPC corrections flow of mask pattern generation.
If that was the case for this wafer, then this could be either thin film or diffraction effects (a bit like DVDs do)
If it's thin film then the film has to be thick enough to produce different colors at slightly different angles for this photo, which often is the case after an oxide passivation covered with a nitride cap. The top, often silicon-rich nitride has an index of refraction of 2.0 or higher, and so Fresnel reflection is higher and the air-nitride interface acts more like a top "mirror" than an air-oxide (or spin-on glass) circa 1.5 index interface would.
Yes, once a feature is below a half-wavelength in air, you can not see colorful diffraction patterns for that wavelength looking at it at that wavelength, even edge-on with a flashlight.
But the dummy patterns don't always have to be minimum resolvable feature size, it depends on the details of the process and the step, but some can be much larger.