Recent Progress in Ferroelectric Random Access Memory Technology
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0997-I08-05
Recent Progress in Ferroelectric Random Access Memory Technology Hiroshi Ishiwara1, and Hiroshi Ishiwara2 1 Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259-J2-67 Nagatsuda, Midoriku, Yokohama, 226-8503, Japan 2 Tokyo Institute of Technology, Yokohama, 226-8502, Japan
ABSTRACT In this paper, ferroelectric materials suitable for realizing high-density 1T1C-type (capacitor-type) FeRAM are first reviewed. It is found in BiFeO3 (BFO) films formed by chemical solution deposition that leakage current density at a low electric field increases by substitution of Mn and Cr atoms for Fe atoms. But, it is also found that the breakdown characteristic is much improved by substitution of these atoms. Because of the better breakdown characteristic, the leakage current densities in the 3 and 5% Mn-substituted films are lower than that in an undoped BFO film at an applied electric field of 1MV/cm at room temperature, and thus well saturated hysteresis loops in P-E (polarization vs. electric field) characteristics are observed in these films. Next, recent technological progress in transistor-type FeRAM, in which data are stored in a single ferroelectric-gate FET(field effect transistor), is discussed. It is demonstrated that the data retention time of ferroelectric-gate FETs is much improved by use of HfO2-based buffer layers which are inserted between the ferroelectric film and Si substrate for preventing interdiffusion of constituent elements. Particular attention is paid to FETs with a Pt/SrBi2Ta2O9/HfO2/Si gate structure, in which the data retention time longer than 30 days has been attained. Finally, the cell structure and operation principle of 1T (one transistor)-type FeRAM are discussed. INTRODUCTION Ferroelectric random access memory (FeRAM) is one of the most promising non-volatile memories, because the power consumption of FeRAM is lowest among various semiconductor memories and because the operation speed is as fast as that of dynamic RAM (DRAM). At present, the maximum capacity of commercially available FeRAM is 1 Mb. However, there are still several problems in high-density integration of FeRAM cells. FeRAM has two different cell structures. One is 1T1C-type (capacitor-type) cell, which is composed of one transistor and one ferroelectric capacitor, as shown in Fig.1(a) and the other is 1T-type (FET-type) cell, which is composed of a single ferroelectric-gate FET (field effect transistor). These FETs are connected in either NOR type (Fig.1(b)) or NAND type (Fig.1(c)) in a memory cell array. In the former 1T1C cell the polarization reversal current is detected to read the stored data, while in the latter 1T cell the polarization direction of the ferroelectric film is read out nondestructively by the drain current of the FET.
In this paper, recent progress in FeRAM technology is discussed from viewpoints of materials and devices. First, novel ferroelectric materials suitable for realizing high-density 1T1C-type FeRAM are reviewed. Then, recent technological p
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