28 years ago, I worked for Mietec, a company that had a fab that is now owned by OnSemi.
The mask preparation team told me the following:
One time they received the GDS2 file with the mask layers (that’s the golden file that gets created when you do a tape-out) from a customer, did their usual preparation and design rule checks, and sent it to the mask maker.
A few days later, they received an email stating that they couldn’t process the file due to “the presence of twitty birds and Christmas trees.” And, indeed, upon closer inspection, the design contained these elements.
However the metal vias to make those images were only on one layer, and not connected to others.
The resulting issue was one of “floating vias”. (That’s the exact wording that I remember all these years later, but Google doesn’t turn up anything.) For some reason, these unconnected vias couldn’t be held in place during some steps of either the mask masking process or the actual silicon production step, and that’s why the mask maker rejected it.
A new guideline was issued by my company that designs should not have these kind of graphic elements and logos to avoid wasting engineering resources on something frivolous.
What I don’t understand is why this issue wasn’t detected during design rule checking at the customer’s site or at our mask making preparation stage. But that’s a different story.
Either way: I’ve worked on a lot of chips and for a bunch of different companies, and I’m not aware of there being any graphical elements in them either. Masks were moderately expensive 28 years ago (something like $25k), they’re insanely expensive today (millions.) You don’t want to risk anything…