Today the non-volatile memory (NVM) foundation is the eFuse. It is typically available for free from the foundry and is the default choice because, like Mount Everest, it is there. However, like Mount Everest it is big. It is also power hungry and slow. eFuse solutions blow the silicide on the poly line creating a change in resistance. There are other technologies, such as embedded flash, but these require additional process steps and cost. Others, like ROM, are only really suitable when every die contains the same code (such as fonts in a printer).
Modern eFuse is built on polysilicon with Cobalt or Nickel silicide on top. The fuse is programmed by a well-known reliability mechanism called electromigration in which electron momentum pushes the silicide atoms out of the conductor link. Still, most fuses can only be programmed at wafer and have stringent power requirements for programming. It makes programming in packaged parts difficult. The bitcell is the largest of the standard CMOS NVM technologies. For higher bit density memory applications, e.g. greater than 4Kb, the size of the fuse quickly begins to take up the area of the SOC. eFuse is usually custom-designed and provided by the foundry as macros. As a result, it cannot be legally ported to another foundry without the consent of the foundry.