Ferroelectric memories

Memory) (128 KB) on the MIMOSII electronics board. Firmware parameters and the instrument logbook are stored in the nonvolatile memory ferroelectric RAM (FRAM) on the electronics board. There are three individual FRAMs on the MIMOS II electronics board with three identical copies of these parameters to ensure parameter integrity. The copies are compared with each other from time to time to verify that they are identical. If one copy deviates from the other two, it is replaced by a copy of the other two identical parameter sets. All parameters can be adjusted during mission operations. 

Also of significant interest are ferroelectric oxides. Ferroelectrics such as PbZrTiO (PZT) are useful in permanent memories. Ferroelectrics also are under development in capacitor and display applications. Ferroelectrics surpass SMO thin film applications for the immediate future. In fact, ferroelectrics are being produced in the 150 mm diameter range. 

The number of appHcatioas utilizing the ferroelectric respoase of ceramic materials has beeaHmited. Memory products are uader developmeat and are anticipated to be commercially available duting the early to mid-1990s. 

Numerous uses for PZT/PLZT thin films are under investigation. The device that, as of this writing, is closest to commercialization is a nonvolatile memory. This device, which utilizes a ferroelectric thin-film capacitor integrated onto siUcon circuitry, provides memory retention when the power is off because of the polarization retention of the ferroelectric capacitor. One and zero memory states arise from the two polarization states, — and +F, of the ferroelectric. Because PZT is radiation-hard, the devices are also of interest for military and space appHcations. 

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. 

In Table 1 we compare performance data for reported MRAM and FRAM prototypes. The small Fujitsu FRAM is not a prototype it is in large-scale production and found in the memory board of every Sony Playstation 2, as part of the Toshiba memory system. The main advantages of FRAMs over EEP-ROMs or Elash memory are in the WRITE times (100 ns for FRAM, versus 1 xs for Flash and 10 xs for EEPROM), and energy per 32-bit WRITE (1 nj for ERAM versus 1 or 2 mj for EEPROM or Plash). Note that parameters such as READ time or WRITE time for PRAMs are dependent upon actual cell architecture they are not limited by the intrinsic switching time of the ferroelectric thin film, which is typically 220 ps [3]. 

In addition to requiring high dielectric films for DRAM capacitors (dynamic random access memories) and for the active memory elements in FRAMs, the microelectronics industry has a stated demand for a replacement for Si02 gate oxides very soon. The leading candidate is hafnia (Hf02), and there are significant opportunities for the ferroelectrics community to contribute to the solution of this problem. 

Kim K (2004) High density integration. In Ishiwara H, Okuyama M, Arimoto Y (eds) Ferroelectric random access memories. Top Appl Phys 93 165-176... 

A Comparison of Magnetic Random Access Memories (MRAMs) and Ferroelectric Random Access Memories (FRAMs)... 

The fully ionic solids (region I) afforded band insulators, 1 1 Mott insulators with ground states of antiferromagnets (E b(21) and F b(22) in Fig. 1) or spin-Peierls systems, ferroelectrics, ferromagnets, spin-ladders, and nonlinear transport materials (switching and memory). 

Funakubo H (2004) Recent development in the preparation of ferroelectric thin films by MOCVD. Ferroelectric Random Access Memories Fundamentals and Applications 93,95-103... 

Barium titanate is one example of a ferroelectric material. Other oxides with the perovskite structure are also ferroelectric (e.g., lead titanate and lithium niobate). One important set of such compounds, used in many transducer applications, is the mixed oxides PZT (PbZri-Ji/Ds). These, like barium titanate, have small ions in Oe cages which are easily displaced. Other ferroelectric solids include hydrogen-bonded solids, such as KH2PO4 and Rochelle salt (NaKC4H406.4H20), salts with anions which possess dipole moments, such as NaNOz, and copolymers of poly vinylidene fluoride. It has even been proposed that ferroelectric mechanisms are involved in some biological processes such as brain memory and voltagedependent ion channels concerned with impulse conduction in nerve and muscle cells. 

Useful applications have been found lor the varied effects of these crystal changes. One of the first came from the properly of selectively reflecting visible light because this is lempcraiurv-dependent. the property can be used as a temperature detector, and in gel lurm liquid crystals have been used lor the early detection of those cancers which cause hot spots in the body. Applications of the smectic modifications arise from their ferroelectric properties this phase can function as a fast-switching light-valve device with memory. This kind of application requires some... 

An alternative structure that has also been widely investigated both for high temperature piezoelectric, as well as for ferroelectric memory applications is the bismuth layer structure family as shown in Figure 1.14 for SrBi2Ta209 (sbt), e.g. [8], The structure consists of perovskite layers of different thicknesses, separated by Bi20 + layers. It has been shown that when the perovskite block is an even number of octahedra thick, the symmetry imposes a restriction on the polarization direction, confining it to the a-b plane. In contrast, when the perovskite block is an odd number of octahedra thick, it is possible to develop a component of the polarization along the c axis (nearly perpendicular to the layers). This could be used in... 

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. 

Furthermore, the history of a hysteresis loop plays an important role in the determination of lifetime and reliability of ferroelectric capacitors, especially for applications in ferroelectric memories. Three main effects are characterized in particular as changes in the hysteresis loop under various conditions, which are described later in this chapter as fatigue, retention, and imprint with the corresponding ways to measure these effects. 

The failure mechanism within a memory cell is either due to the inability of the programming voltage to switch the ferroelectric material because of an increase of the coercive voltage (write failure) or due to a decrease in the difference of Ps and Pns. This means the two different states of remanent polarization cannot be distinguished by the memory sense amplifier (read failure). This case is shown in Figure 3.19. 

M. Grossmann, Imprint An Important Failure Mechanism of Ferroelectric Thin Films in View of Memory Applications. Dissertation. RWTH-Aachen, 2001. published by vdi Verlag, Fortschritt-Berichte vdi Reihe 9 Elektronik/Mikro- und Nanotechnik. 

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. 

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