Jonathan Page at MSC Gleichmann explores how Ferroelectric RAM (FRAM) combines the performance of fast static RAM with the advantages of non-volatile memory for fast speed data intensive applications
The choice of memory technology for a given application will depend on a wide range of features and parameters. Key among these are read/write access speed and the issue of volatility and these two will often dictate a particular memory technology at an early point in the decision-making process.
Conventional Random Access Memory (RAM), both static and dynamic, offers fast random read/ write access but is volatile so needs power to maintain data. Non-volatile memory such as E2PROM or Flash is much slower because data needs to be erased on a byte or sector block access basis before new data can be written.
Ferroelectric Random Access Memory (FRAM) combines the best of both worlds. It uses a non-volatile technology that is capable of providing fast random read/write access, closer to SRAM in performance. Unlike other non-volatile memories, FRAM doesn’t suffer from limited life in terms of endurance (the number of times it can be erased and re-written).
The construction of FRAM and DRAM memory cells is similar. A DRAM cell comprises one transistor and one capacitor. The capacitor stores an electrical charge; a charged cell generally represents binary value ‘1’ and no charge ‘0’. The transistor is used to charge and discharge the capacitor and read out the stored data value.
An FRAM cell replaces the DRAM capacitor’s normal dielectric with a thin film of ferroelectric material such as PZT (lead zirconate titanate). The PZT compound exhibits a non-linear relationship between applied electric field and the apparent stored charge.
This is because semi-permanent dipoles, created in the crystal structure, retain their polarisation state after the electric field is removed, giving rise to the hysteresis loop characteristic. It is this behaviour, where two stable states can exist following a change of polarisation, which makes ferroelectric memories non-volatile.
FRAM offers several major advantages over E2PROM and Flash including: speed, endurance, low power consumption, security and radiation tolerance.
The write cycle time of FRAM is considerably faster than E2PROM, this is largely because FRAM overwrites data in a single cycle. The FRAM core also operates at CMOS voltage levels so doesn’t need the booster required in conventional non-volatile memory.
This greatly increases its writing speed but also reduces stress on the ferroelectric material and consumes less power. Of course real-world applications, with large amounts of data and transfer rates limited by standard interfaces like SPI, don’t always realise these raw performance gains. Even so, representative side-by-side tests have shown FRAM to be considerably faster than either E2PROM or Flash memory.
Non-volatile memory
Conventional non-volatile memory has limited endurance i.e. the number of read/write cycles before material fatigue kicks in and the memory becomes non-functional. This is typically capped at 100,000 to 1,000,000 cycles but can be as low as 5,000 cycles for small geometry, high capacity Flash memories.
Hence Flash and E2PROM are not best suited to applications that require frequent data logging. But FRAM has one million times the endurance of E2PROM so is ideal for applications like smart meters that are constantly recording and backing up data and where long life is important and replacing memory is neither easy nor practical.
To put this in perspective, if a system requires access to non-volatile memory once every second then conventional Flash memory will wear out in less than two days. In the same situation FRAM would last for well over 300 years, which is pretty infinite.
Because it operates at CMOS voltage levels, FRAM avoids the requirement for a power-hungry voltage booster circuit and hence provides a low-power memory solution, consuming approximately 200 times less power than a comparable E2PROM device.
Another energy-related benefit from using FRAM in many applications is the elimination of a back-up battery; this reduces cost, enables a smaller circuit board footprint and is more environmentally friendly, lowering CO2 emissions during manufacturing by as much as 65 percent.
Low power and no need for a battery, coupled with high endurance and long life, make FRAM the perfect solution for metering applications. FRAM is also ideal for passive RFID systems where the tags are powered by energy derived from the external reader or writer. The power efficiency of FRAM can also help extend the effective range of passive RFID systems.
Conventional non-volatile memories store binary data using a charge accumulation principle that alters the physical structure of the device, which means that the 1s and 0s can be detected by an electron microscope. FRAM doesn’t use charge accumulation so is inherently a much more secure memory technology as it is immune to scanning in this way.
FRAM’s radiation tolerance is particularly important in the medical field where the typical dosage for pharmaceutical sterilisation processes is 20 kilogray (kGy). The fatal dose for E2PROM is 2kGy so FRAM’s ability to withstand 45kGy, more than twice the usual dosage, make it the obvious choice and even means it can be irradiated twice without data corruption!
Radiation-hard FRAM-based RFID is proving to be indispensable as the medical industry ramps its use of e-pedigree tracking devices in the battle against counterfeiting and the re-use of disposable medical equipment.
FRAM is a non-volatile memory technology whose key features are speed, endurance, low power consumption, security and radiation tolerance. These qualities make FRAM perfect for applications like smart cards, RFID, security, metering and other applications that require high-performance, non-volatile memory.
Jonathan Page is Field Applications Engineer at MSC Gleichmann
MSC Gleichman
