Ferroelectric Thin Film Depositions for Various Types of FeRAMs (Ferroelectric Random Access Memories)
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Ferroelectric Thin Film Depositions for Various Types of FeRAMs (Ferroelectric Random Access Memories) Yoshihisa Fujisaki1 and Hiroshi Ishiwara2 1 Central Research Laboratory, Hitachi Ltd. 1-280 Higashikoigakubo, Kokubunji, Tokyo 185-8601, JAPAN 2 Interdisciplinary Graduate School of Science and Engineering Tokyo Institute of Technology 4259 Nagatsuda, Midori-ku, Yokohama 226-8503, JAPAN
ABSTRACT Flash memories are now widely spread and conveniently used in mobile devices such as cellular phones, pagers, PDAs (Personal Digital Assistants), digital cameras and so on. And in the coming ubiquitous era, nonvolatile solid-state memories are expected to be more and more important for these mobile devices. However, Flash memories are not perfectly suitable for these mobile devices since their power consumptions are too high, writing speeds are two slow, programming endurances are limited up to 106 cycles. To resolve these problems, numbers of new nonvolatile solid-state memories are proposed and some of them are now under development. Among these newly emerging nonvolatile memories, FeRAMs (Ferroelectric Random Access Memories) are the only ones that are now in production. However, the process and materials to produce FeRAMs have not matured yet to support the ubiquitous technologies. In this study, we explore the process technologies and materials required for the future FeRAMs and obtained the result that the requirements can be satisfied by the known technologies at present.
INTRODUCTION Nonvolatile FeRAMs have received great attention in recent years because their writing speeds are much higher than those of Flash memories and their power consumption is much lower than those of other non-volatile memories [1,2]. LSIs as large as 64Mb have already been demonstrated [3] and memories with Mb scale integration are now in production. These devices have the cell structure composed of one ferroelectric capacitor and one transistor that is similar to DRAM cells. Hereafter, we call this structure as 1T1C type. For this structure, ferroelectric materials with large remnant polarizations such as PZT ((Pb, Zr)TiO3) and BLT ((Bi, La)4Ti3O12) are suitable. Another important structure for the nonvolatile FeRAM is a ferroelectric gate transistor. In
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this memory, one cell is composed of only one transistor with a ferroelectric gate insulator. This device is then called 1T type memory. Therefore, this is very suitable for the large-scale integration. It also has the features of low-power and high-speed operation. Memories with this structure are now under development, but recently great improvements have been reported [4,5]. Materials suitable for this structure should have relatively smaller remnant polarization and smaller coercive field compared to 1T1C type devices. SBT (SrBi2Ta2O9) is thought to be the primary candidate for this type of devices. We review the recent development of the film deposition techniques both for 1T1C and 1T types. We expect that the requirements for the low voltage operation are crucial
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