Sir @Tim Edwards do you know any documentation regarding the design of analog pad cells and filler cells? I've searched trough sky130's documentation, but I just found specs of the pad cells provided by SkyWater. I'd like to design a new pad cell but I haven't found documentation of how to design those cells.

Analog pad design is very complicated, and there generally are never more than vague guidelines. Most often pads are developed over a number of iterations and tested for ESD performance. I created my own set of pads from the basic pad structure from the SkyWater I/O (metal5 pad with a ring of metal4 underneath), large diodes to power and ground, etc.; they worked fine for a project but I've never had them characterized.

I understand. Do you know the file route where I can find this basic pad structure from SkyWater I/O?

Any work along those lines would certainly be appreciated.