Ken Shirriff Puts an Unusual 1990s Ferroelectric RAM (FRAM) Chip Under the Microscope
Complex and expensive to manufacture, FRAM excels in one key area: longevity, both in write-erase cycles and in memory retention.
Noted reverse engineer and computing historian Ken Shirriff has put a somewhat unusual device under the microscope for inspection: a ferroelectric random-access memory (FRAM) chip, manufactured in 1999 and holding just 8kB.
"The history of ferroelectric memory dates back to the early 1950s," Shirriff explains. "Many companies worked on FRAM from the 1950s to the 1970s, including Bell Labs, IBM, RCA, and Ford. Unfortunately, ferroelectric memory had many problems, limiting it to specialized applications, and development was mostly abandoned by the 1970s. Ferroelectric memory had a second chance, though. A major proponent of ferroelectric memory was George Rohrer, who started working on ferroelectric memory in 1968. He formed a memory company, Technovation, which was unsuccessful, and then co-founded Ramtron in 1984."
It's a Ramtron part, produced in 1999, that Shirrif has investigated: a 64-kilobit, eight kilobyte chip that offers considerably higher longevity in terms of both write-erase cycles and unpowered memory retention than even modern non-FRAM equivalents — one of the key reasons the technology hasn't been completely abandoned, despite complexities and expenses in its production.
Shirriff's analysis of the de-capped FRRAM chip identifies four blocks of memory components, surrounded by decoders and sensor amplifiers and separated by plate line drivers. Further investigation, and removal of some memory cells, unveils the unusual layout of a FRAM hardware: a three-dimensional structure that includes cubes of zirconate titanate, a material made up of lead, zirconium, titanium, and oxygen. These cubes react to electric fields, with the metals moving within a crystal lattice — flipping the bit in a way that remains stable even after decades without power.
"FRAM memory will eventually wear out," Shirriff admits, "but it can be written trillions of times, much more than flash or EEPROM [Electrically Erasable Programmable Read-Only Memory] memory. The memory is organized as 256 capacitors horizontally by 512 capacitors vertically, for a total of 64 kilobits (since each bit requires two capacitors). The memory is accessed as 8,192 bytes."
Shirriff's full write-up is available on his website, Righto.com.