I took some pictures of a Microchip EPROM die from the late 80s / early 90s (I can't recall the exact part number). The wire bonding pads are surrounded by a comb-like structure. What is the purpose of this structure?
They are probably large p-MOS and n-MOS transistors that are used for ESD protection on the bonding pads. Here is a reference that shows various bonding pad designs in detail (in general this information is not easy to come by- IC manufacturers seem to treat ESD protection as a kind of trade secret). Image taken from the above pdf:
I don't recall Microchip ever making memory EPROMs. Is this part of an EPROM microcontroller?
Edit: Just looking at a Microchip PIC16C57, which is probably from a similar era. There are similar patterns on either side of most of the pins (which are I/O) but on only one side of the input-only pins such as T0CKI, /MCLR/Vpp, OSC1. So the structures appear to be drivers on one side and ESD protection circuitry of whatever kind on the other side.
At this writing, there are two "answers" that are total guesses - and wrong at that too.
These comb structures are as you might expect to see when you want to induce breakdown at a precise location and into controlled structures rather than else where in the chip. These are in the TOP metal layer, the combs are there to give lots of sharp edges to promote an excessively high ESD event to conduct at that location.
The diode and ESD clamping structures are by necessity in the Silicon.
These are very very far from being the transistor structures which are in the Si at least 3 - 7 metal layers down.
Look at lightning arrestors in the larger world. You will see these exact same things there.
Call it a belt and suspenders approach. Or rather a last chance, the actually ESD structures are rated for much lower voltage events.
Those structures are the large transistors required to drive the pins that are used as outputs.