Ferroelectric memory devices

The ferroelectric hysteresis originates from the existence of irreversible polarization processes by polarization reversals of a single ferroelectric lattice cell (see Section 1.4.1). However, the exact interplay between this fundamental process, domain walls, defects and the overall appearance of the ferroelectric hysteresis is still not precisely known. The separation of the total polarization into reversible and irreversible contributions might facilitate the understanding of ferroelectric polarization mechanisms. Especially, the irreversible processes would be important for ferroelectric memory devices, since the reversible processes cannot be used to store information. 

Breakdown has long been a topic associated with ferroelectric memory devices, and is recognized as being a second major failure mechanism [226]. Breakdown can be... 

New natural polymers based on synthesis from renewable resources, improved recyclability based on retrosynthesis to reusable precursors, and molecular suicide switches to initiate biodegradation on demand are the exciting areas in polymer science. In the area of biomolecular materials, new materials for implants with improved durability and biocompatibility, light-harvesting materials based on biomimicry of photosynthetic systems, and biosensors for analysis and artificial enzymes for bioremediation will present the breakthrough opportunities. Finally, in the field of electronics and photonics, the new challenges are molecular switches, transistors, and other electronic components molecular photoad-dressable memory devices and ferroelectrics and ferromagnets based on nonmetals. 

There is considerable interest in developing new types of magnetic materials, with a particular hope that ferroelectric solids and polymers can be constructed— materials having spontaneous electric polarization that can be reversed by an electric field. Such materials could lead to new low-cost memory devices for computers. The fine control of dispersed magnetic nanostructures will take the storage and tunability of magnetic media to new levels, and novel tunneling microscopy approaches allow measurement of microscopic hysteresis effects in iron nanowires. 

The electrical characterization of polar media is crucial to investigate their suitability for ferroelectric memories, piezo- or pyroelectric devices and many other ferroelectric applications (see Chapter 3). Optical characterization of polar media is fundamental to investigate their ser-vicability for electro-optic devices or applications in the field of nonlinear optics (see Chapter 4). Additionally there are intentions to characterize polar media with a combination of both, electrical and optical methods, such as to understand ferroelectric phenomena that are influenced by the action of light. 

The progress in the development and integration of ferroelectric memories (FeRAM) leads to increasing demand for electrical characterization of sub-micron structures. This article will point out the measurement problems arising from the reduction of the ferroelectric capacitor size e.g. from memory cells or nanostorage devices. Procedures and solutions are presented to overcome these problems and to increase further the resolution and speed of ferroelectric characterization to be ahead of the technological demand. 

Several members of the MM O3 class of ternary metal oxides adopt the perovskite-type (CaTiOs) structure and are sought as worthy target materials possessing ferroelectric properties see Ferroelectricity) Among the more widely investigated members of this class are BaTiOs and SrTiOs. Clearly, use of these materials as potential memory device... 

Another general class of solids that has been prepared as thin films using PLD is ferroelectric materials. A potentially useful characteristic of ferroelectric materials is that they can be polarized by an electric field and retain this polarization when the field is removed. In ferroelectric thin films it may be possible to exploit this polarization phenomenon to make sensors, displays, and memory devices. A number of techniques have been used to prepare ferroelectric thin films. However, it has been difficult to control the stoichiometry (and correspondingly the properties) of these materials using thermal and sputtering techniques. In part, the difficulty in maintaining correct stoichiometry is due to the volatility of a component in the material (e.g. Pb in PbTiOs). 

Since the early suggestion that ferroelectric thin film materials could be the high dielectric layer in the capacitor of the ultra large scale integrated dynamic random access memory devices (ULSI DRAMS) made by Parker and Tasch, there has been a great deal of research effort to deposit multi-component ferroelectric oxide thin films as well as more recent industrial activity. The term ferroelectric indicates the property of certain materials that have remnant... 

There is a different application for ferroelectric materials in solid state memory devices the ferroelectric RAM (FeRAM). However, in this chapter, the FERAM and related issues will not be considered because the subject would be too diverse. 

The ferroelectric effect is an electrical phenomenon. Parhcular materials, including the ternary oxides (Ba,Sr)Ti03, Pb(Zr,Ti)03 and (Bi,La)Ti03, exhibit a spontaneous dipole moment which can be switched between equivalent states by an external electric held. Ferroelectric thin hlms are of importance for the production of nonvolahle ferroelectric random access memory devices (FeRAM) °. Two possibilities to synthesize such mixed metal oxides are given by the CVD and ALD methods. Table 10 shows the preparation methods of such materials synthesized from metal enolates recently. 

Recently, efforts have been devoted to the fabrication and characterization of PbZri- Ti c03 family thin films for their potential applications in nonvolatile memory devices (See Ref. 17, for example). Partly because of the convenient stoichiometry control during processing, it was found that chemical methods, such as sol-gel and metal organic decomposition (MOD), are superior to physical means in many aspects. To appreciate better the science and technology of ferroelectric thin-film fabrication, it is important to give a brief account of the past efforts and the present status and, it is hoped, shed some light on the future. 

Ferroelectric materials have numerous microelectronics applications, including capacitors (7,2), nonvolatile memory devices (2-4 electrooptic devices 1,4) and many others (5). The ferroelectrics described in this paper are Pb(ZrxTii x)03 (PZT), BaTi03, and YMn03. Here we investigate the use of a photochemical method for the direct deposition of these complex materials. 

Ferroelectric crystals (especially oxides in the form of ceramics) are important basic materials for technological applications in capacitors and in piezoelectric, pyroelectric, and optical devices. In many cases their nonlinear characteristics turn out to be very useful, for example in optical second-harmonic generators and other nonlinear optical devices. In recent decades, ceramic thin-film ferroelectrics have been utilized intensively as parts of memory devices. Liquid crystal and polymer ferroelectrics are utilized in the broad field of fast displays in electronic equipment. 

Ferroelectric materials, especially polyciystalhne ceramics, are utihzed in various devices such as high-permittivity dielectrics, ferroelectric memories, pyroelectric sensors, piezoelectric transducers, electrooptic devices, and PTC (positive temperature coefficient of resistivity) components. 

Ferroelectric liquid crystals (FLC) have attracted attention because of their high speed response and memory effect (7-5). The characteristics of fast response and memory effect make them suitable in electro-optical device applications, such as display, light valve and memory devices. Ferroelectric side chain liquid crystalline polymers (FLCPs) exhibit desirable mechanical properties of polymers and electro-optical properties of low molecular weight FLC, which have been investigated extensively Corresponding author. 

Synthesized ferroelectric niobate thin films with preferred orientation along the polar and optic axis can satisfy several requirements for various appUcations in piezoelectric or elecroacoustic transducers, high-frequency surface-acoustic-wave (SAW) devices, pyroelectric infrared detectors, ferroelectric memory cells, ferroelectric photoconductive displays, two dimensional special light modulators or optical waveguide devices, etc. 

C-V Characteristics of a Ferroelectric Thin Film Sample. In the semiconductor industry, C-V curves are frequently utilized for studying the characteristics of devices. In general, a C- V curve can show ferroelectric memory effect of a ferroelectric thin film sample. A voltage applied on the sample is swept from 0 to a positive value (for example, -1-5 V), then from -h5 to -5 V and returned from to -i-5 V. It is demonstrated that the C-V curve has a counterclockwise hysteresis loop, probably due to the ferroeletric nature of the sample. This C- V hysteresis loop can indirectly reflect that a sample may have ferroelectric characteristics (Tokumitsu, 2002). However, the hysteresis characteristics of a C-V curve is not only caused by spontaneous polarization, but also included other effects, like space charge and interface charge. 

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