Current Status of Ferroelectric Random-Access Memory

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Current Status of

Ferroelectric Random-Access Memory

Yoshihiro Arimoto and Hiroshi Ishiwara Abstract The current status of ferroelectric random-access memory (FeRAM) technology is reviewed in this article. Presented first is the status of conventional FeRAM, in which the memory cells are composed of ferroelectric capacitors to store the data and cell-selection transistors to access the selected capacitors. Discussed next are recent developments in the field. Pb(Zrx,Ti1–x)O3 (PZT) and SrBi2Ta2O9 (SBT) films are being used to produce 0.13 m and 0.18 m FeRAM cells, respectively, with a stacked capacitor configuration; these cells are easily embedded into logic circuits. A new class of FeRAM called 6T4C—containing static RAM (SRAM) cells composed of six transistors (6T) and four ferroelectric capacitors (4C)—has been commercially produced. This type of FeRAM features a nondestructive readout operation, unlimited read/write cycling, and a fast access time of less than 10 ns. Lastly, the status of field-effect-transistor (FET)-type FeRAM is reviewed, emphasizing that the data retention time of a ferroelectric-gate FET has been improved to more than a month in recent studies. Keywords: ferroelectric random-access memory, FeRAM, nonvolatile static random-access memory, NVSRAM.

Introduction Ferroelectric random-access memory (FeRAM) is a type of nonvolatile randomaccess memory that uses a ferroelectric film as a capacitor for storing data. FeRAM can achieve high-speed read/write operations comparable to that of dynamic RAM (DRAM), without losing data when the power is turned off (unlike DRAM). In addition to nonvolatility and high-speed operation, FeRAM cells offer the advantages of easy embedding into LSI logic circuits (LSI  large-scale integration, the technology that allows thousands of transistors per chip) and low power consumption, perhaps their greatest advantage for many applications. The market for FeRAM has expanded rapidly; FeRAM-embedded LSI circuits have been used in smart cards, radiofrequency identification (RFID) tags, and as a replacement for BBSRAM (batterybacked-up static RAM), which is used in

MRS BULLETIN/NOVEMBER 2004

various devices to protect data from an unexpected power failure, as well as in many other SoC (system on a chip) applications. FeRAM cells have been extensively developed to improve their density, speed, read/write endurance, and reliability. A memory cell, where one bit of data is stored, is composed of a cell-selection transistor and a capacitor for 1T1C (one transistor, one capacitor)-type FeRAM. A major problem encountered when reducing the size of the memory cell is preventing reliability degradation. The reliability of FeRAM cells is dependent on the materials used (for the ferroelectric film, electrode, interlayer dielectric, etc.), the fabrication process, the device structure, the memory cell circuit, and the operation sequence. Various types of FeRAM are compared in Table I. By optimizing these parameters, new types of FeRAM, designated by the number of transistors (T)

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