<mkk_>
[Trevor Clarke, skywater-pdk] so, not sky130 relevant, but it does involve silicon wafers.
<mkk_>
Making Rugate optical filters with an acid bath and a programmable current source. An interesting note is at the very end where he points out that not only can you etch different filters in different layers but since the silicon can be used to make LEDs and photosensors there's a possibility to make some pretty interesting mono and dichromatic light sources and detectors.