Techniques and mechanisms for providing physically unclonable function (PUF) circuitry at a substrate which supports coupling to an integrated circuit (IC) chip. In an embodiment, the substrate comprises an array of electrodes which extend in a level of metallization at a side of the insulator layer. A cap layer, disposed on the array, is in contact with the electrodes and with a portion of the insulator layer which is between the electrodes. A material of the cap layer has a different composition or microstructure than the metallization. Regions of the cap layer variously provide respective impedances each between a corresponding two electrodes. In other embodiments, the substrate includes (or couples to) integrated circuitry that is operable to determine security information based on the detection of one or more such impedances.