Looking inside a 1970s PROM chip that stores data in microscopic fuses

The MMI 5300 was a memory chip from the early 1970s, storing 1024 bits in tiny fuses.1 Unlike regular RAM chips, this was a PROM (Programmable Read-Only Memory); you programmed it once by blowing fuses and then it held that data permanently. The chip I examined originally cost $70 and was built by MMI (Monolithic Memories Incorporated), a leading PROM manufacturer at the time. The highly magnified photo below shows the chip's silicon die. The metal layer on top of the silicon is most visible ...

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