F-RAM or Fe-RAM stands for Ferroelectric Random Access Memory. When one hears that term, one may tend to think about the memory of the early computers. Memories that were made up by a grid of wires, where each intersection point has a little ferrite bead to store the bit. F-RAMs are however a little different story, but there are similarities. It is a non-volatile memory that stores data in an internal crystal structure. The centre atom of the crystal will become oriented to the applied current. Recently, one of the leading developers of F-RAMs, Rampton, announced that new parts are being released.
F-RAM is one of many types of non-volatile memory available today. In spite of not being well known among today’s developers, it can be used as a drop-in replacement of EEPROM or Flash memory modules having the same size. When using a Flash memory, one has to choose between NOR- or NAND-flash. This is basically choosing between slow write- /fast read access or the opposite. Also, the Flash memory often requires a complex state machine for performing operations such as erase- or write (where the flash must be polled for completion). Here the F-RAM has the advantage providing faster and simpler write cycles. You don’t need to care about any special erase cycles, just write your new data to the memory address. Looking at the data sheet of F-RAM devices from one of the leading providers, the write cycle time is about 100 ns and the access time about 60 ns.
Let’s have a short look at the F-RAM’s power performance. Again looking at the data sheets for available devices, at normal operation it consumes about 7 mA and it can be as low as 5 ?A in sleep mode. Another example shows an operating current of about 15 mA. Some articles claim that the operating current can be as low as 150 ?A, at 100 kHz. The data retention time is normally 10 years and the number of write cycles is between 1010-1016 times. Comparing this to the EEPROM and Flash technology, the number of write cycles is doubled. Some even say virtually unlimited.
Looking at these properties, the F-RAM is well suited for a system that requires a fast access non-volatile storage system. I’m thinking about distributed data acquisition systems, or industrial systems where using a flash memory either limits the speed or there is a risk of losing data due to the write latency. Other system uses log-buffers to buffer debug data, error-logs for later failure analysis or storing the state before being shut down. Since a Flash memory sometimes is too limited in these applications, today a capacitor–backup or battery-backup RAM is used.
The process of manufacturing F-RAM is similar to that of DRAM. In fact, they can be manufactured using almost the same process. In the DRAM case, the memory is implemented using a capacitor together with a signal transistor. A capacitor and transistor pair is called 1C-1T cell. The DRAM capacitor stores data by charging the capacitor. In short, exchange the dielectric from the DRAM capacitor to a ferroelectric material and you get F-RAM. One common ferroelectric dielectric is lead zicronate titanate, short PZT. Instead of storing data by the lack or presence of electrical change, the F-RAM stores data using positive and negative polarization.
Even though it has been around since the 1980’s, the F-RAM has not made a huge breakthrough. Comparing it to today’s Flash memories, the cost of the flash is about the half of the F-RAM. The Flash memory is well established, accepted and serves its purpose. However, Flash does not provide the same advantages such as the high write speed combined with the simple interfacing. There are many applications where an F-RAM would serve better then the flash (or backed-up RAM). In some cases the F-RAM can provide a non-volatile memory where the Flash would be too expensive or complicated (sometimes due to the need of Flash file system in order to extend the memory’s lifetime). Therefore, I think the F-RAM is an interesting alternative to other non-volatile technologies that may prove cost effective in some applications.
Mattias Almljung, Altran Technologies Sweden
Sources (2009-04-19):
http://www.eecg.toronto.edu/~ali/ferro/tutorial.html
http://en.wikipedia.org/wiki/Ferroelectric_RAM
MB85R1001, FUJITSU MICROELECTRONICS DATA SHEET
FM28V100, Ramton Data Sheet