@User: The devices I referred to are in sky130_fd_pr and are called "rf_test_coil1" through "rf_test_coil3". They have a weird pizza-slice pattern drawn on top in the LVS text layer which I don't know the reason for. Otherwise, they look like typical octagonal coils. Probably it would be best to give up on those, and make a parameterized cell that automatically generates coil designs, then fill a test chip with them, and figure out how to map the coil parameters to the measured inductance.

Thank you for the reply. I don't think there is enough time for me to start/finish a PCELL. However, I can do a manual layout for square spiral inductor by using online calculator(http://smirc.stanford.edu/spiralcalc.html). So should I use metal5 for that or redistribution layer? Why there will be bump bonds on whole layout? For flip chip?

@Affan Abbasi: There will be no opportunity to do anything on the redistribution layer other than the bump bonds for this shuttle run. After some suggested changes by SkyWater, there is now more area on the chip and I would like to redesign the bump pattern with space in the middle for inductor experiments. But that requires some significant redesign and will have to wait for another shuttle run. The existing bump bond pattern will cover the surface with a 6x10 array at a 500um pitch. Meanwhile, the metal5 layer is pretty thick and should give you some decent inductors; while not as thick as the redistribution layer, I assume it is much better controlled.

@Affan Abbasi Have a look at https://github.com/patrickschulz/openPCells. It's a PCell generator that's set up for SKY130. It has a square and octagonal spiral inductor generators.

@diadatp Many thanks.

I considered putting similar structures on a user die but the RDL routing above and requirement that every de-embedding structure and DUT "seeing" the same environment around it makes that route seem unlikely.

@diadatp Hmmmm......so I can have probe cuts in the center and can remove bonds from 100 or 50 samples that I'll receive after tapeout? I have the capability to do my own wirebonding.

@Affan Abbasi: Top metal of the SkyWater part of the process (metal 5) is aluminum. The redistribution layer (post-processed by a 3rd party supplier) is copper.

@Tim Edwards Thank You

@diadatp: SkyWater reserves the 42nd spot for their own uses. We agreed on 40 slots, so the 41st is technically an "extra". I could envision using the space for experiments in the redistribution layer (either added or subtracted). The only real restriction is that the padframe has to be the same size as Caravel, because of the space reserved in the reticle frame (the easiest thing to do there would be to use Caravel, but wirebond it). The more important part would be convincing @Tim 'mithro' Ansell that we should have one slot for an "almost anything goes" purpose, as there is simplicity (read: low budget) in having every single die treated exactly the same. But it's easy enough to generate a pick map for the wafer.