Student Shanin S.V.

Kryvyi Rih Technical University

Application of FRAM

Main memory is one of computer basic components, which is vitally necessary for its efficient work. Memory, as an important attribute of computer systems architecture, was defined as extremely necessary thing, when computer was only on paper.

FRAM is a type of main memory.

Ferroelectric is an unique type of crystalline dielectric, possessing spontaneous polarization, which has place inside the crystal and lack of external electric field.

When electric field appears polarization can be observed between different stable positions. This property of ferroelectrics is used for creation of storage devices with random access (SDRA). In literature they were named FRAM – ferroelectric random access memory.

Basic material for elements, used in FeRAM, such as ferroelectric transistors and ferroelectric capacitor possessing besides variable ferromagnetic properties, are mixed poly-metallic oxides, cemented in ferroelectric ceramics. The most widely - spread is PZT family (Perovskite lead zirconate titanate), its general formula is Pb(Zr^x Ti^1-x ) O₃.

Now there is an enormous amount of various combinations of memory cell basic elements - ferromagnetic ferroelectric transistor and the same capacitor. But at further consideration of these combinations there are 4 basic type, the other cells types of FeRAM are only their combinations. There are mono-transistor cell of 1Т FeRAM, mono-capacitor cell of 1С FeRAM, named also SFRAM (statically read, non-volatile, ferroelectric random access memory is a complete analogue of SRAM), the most widespread transistor and capacitor cell of 1Т-1С FeRAM and the most stable from all above-stated double cell of 2T-2C FeRAM.

1Т FeRAM was used in one of the first work models of FeRAM, but it indexes were below the marks - the cell lost charge too quickly and passed to unpredictable state, that didn’t depend on energy, therefore works in the field of 1T were stopped.

Type of memory 1С FеRAM consists of 1С cells containing one ferromagnetic ferroelectric capacitor. The other name of 1С FеRAM is SFRAM - statically read, non-volatile, ferroelectric random access memory. This type of FeRAM is the closest according structure to ancient memory on ferromagnetic cores.

1Т-1С FeRAM is very close in structure to ordinary DRAM. As compared to 1Т structure, 1Т-1С structure is much more stable and can keep written information longer, but, the cell is larger.

2T-2C FeRAM this type of memory has an elementary cell which presents doubled cells of a 1T-1C type. These cells can be combined variously, therefore few variants can correspond the general formula of 2T-2C.

Energy-dependent chip of memory Ramtron FM18L08 (standards are already accessible) provides implementation of at least of ten billion of reading/record cycles, and access to every memory area can be carried out at a speed of 1 million times in a second during three hundred years. Working potential of chips FM18L08 is 2,7-3,6 in., and active current is 15 mA at access time 70 nsec.

Company Seiko Epson Corp. reported about development of organic FeRAM memory, formed on flexible base and requiring no feed for information storage, reading is performed without "destruction" of records data. The prototype of memory was marked as 1T and targets for future use in handheld highly-integrated electronic devices. These devices scarcely will be used in commercial purposes, and already their further modifications can be applied as tiny components for information storage future "palmtops" and devices similar to them.

For 1T device producing developers used a number of technologies, such as: forming and flow of source was performed through vacuum deposition, thin tapes getting - by means of centrifuging method, and getting of transistors gates was carried out through technology of "ink-jet printing". Area of every device 1T is 15 sqr. mm, and nine identical charts are formed on flexible one 4-inch base at once. It is needed to report that for description of transistors sizes, width of gate is 300 mkm, and length of gate – is from 25 to 45 mkm.

Originally, developers used pretty thick layers of dielectric for structure forming of field transistor - its thickness was about 0,78 mkm, but such decision required voltage about 75 for transistors functioning.

In this connection, thickness of dielectric tape was decreased to 0,13 mkm, at the same time level of required voltage decreased to 15. Further declining of dielectric thickness was limited by considerable degradation of descriptions of historicizes characteristics.