Active solid-state devices (e.g. – transistors – solid-state diode – Bulk effect device – Bulk effect switching in amorphous material
Reexamination Certificate
2002-09-04
2004-06-15
Fahmy, Wael (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Bulk effect device
Bulk effect switching in amorphous material
C315S101000, C315S105000, C315S163000
Reexamination Certificate
active
06750469
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-296102, filed on Sep. 27, 2001; the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to a phase-change nonvolatile storage device and its drive circuit, and more particularly to a phase-change nonvolatile storage device for storing and reproducing information by making use of resistance changes of a phase-change material contained in memory cells, and a drive circuit therefor.
Storage devices (memory) are used not only in computer systems but also in any devices and machines such as control systems of social infrastructures including electricity, gas, water, transport and communication services, and it is no exaggeration to say that the modern society does not work without storage devices. Ideal form of storage devices is to fully satisfy a high speed, low bit cost, nonvolatility, low power consumption and high reliability. With no such devices, however, it is the present status to set up a hierarchical memory structure optimum as a system.
Let a hierarchical memory structure of a personal computer be taken as an example. This hierarchical structure is set up with, in the order from the top-level memory: SRAM (static random access memory) for direct dialog with MPU (microprocessing unit), which has an ultra-high speed but has a very high bit unit price; DRAM (dynamic random access memory) that is not so speedy but has a relatively large capacity as solid memory and a lower bit unit price than SRAM; HDD (hard disk drive) that has an access speed lower by as much as several digits than DRAM but sufficiently high as mechanical access speed, large in capacity, low in bit unit price, but not being removable; and optical disk, floppy disk or magnetic tape that is lower in speed than HDD but very low in bit unit price and excellent in medium commutability and reliability.
Thus the current effort with storage devices is to optimize the capability as a system and its price by building such memory hierarchies. However, if an ideal storage device such as almighty memory (universal memory) combining the speed of DRAM and the capacity and nonvolatility of HDD appears, the system design will be greatly simplified, and it will be possible to realize a system having dramatically high capability and inexpensive.
Even when turning the eyes to individual storage devices from such almighty memory, there is the specific issue that DRAMs having led the electronic industry under the nickname “industrial rice” are close to the limit in the movement toward larger capacity. For example, the limit of DRAM and substitutional candidates of storage devices are explained in Nikkei Electronics No. 2001-2-12.
The limit of DRAM is a relative increase of the occupation area of the capacity caused by the continuous increase of the capacity, i.e. continuous miniaturization of memory cells, and it has become difficult to obtain a predetermined capacity (30 fF) with trench structures or stack structures. Candidates of storage devices substitutional for DRAM are three kinds of devices, namely, FeRAM (ferroelectric random access memory), MRAM (magnetoresistive random access memory) and PRAM (phase-change random access memory).
FeRAM stores and holds information by using residual polarization of a ferroelectric material, and its signal quantity is proportional to the quantity of the stored electric charge. Since the quantity of the stored electric charge is proportional to the area of the memory cell, structure of the ferroelectric storage portion of FeRAM is fated to become complicated in the three-dimensional configuration similarly to DRAM along with miniaturization of the memory cells.
MRAM makes use of a magnetoresistance effect. TMR (tunneling magnetoresistance effect) element and CPPGMR (current perpendicular to plane giant magnetoresistance effect) element, exhibiting a relatively large resistance change, are mainly subject to researches. An issue of MRAM is that miniaturization of the element results in an increase of the diamagnetism upon flux reversal and hence an increase of the recording current. Further, although the ratio of its resistance change is relatively large, it is only about 50%.
PRAM is the element that embodiments of the invention intend to handle. This is an element for recording information by using changes of the specific resistance of a phase-change material. Its principle has its origin in the disclosure of U.S. Pat. No. 3,271,591 of 1966 and the disclosure of U.S. Pat. No. 3,530,441, and it is often called Ovonic-memory named after the proponent, Dr. Ovshinsky. The entire contents of these references are incorporated herein by reference.
The principle of its operations is briefly explained below.
If a phase-change material contained in a recording cell is once melted by supplying a recording current of a level forming the amorphous phase to the memory cell, and thereafter quenched to carry over the amorphous state to the room temperature, then the amorphous state can be obtained. On the other hand, when such a phase-change material is annealed by supplying a recording current of a level forming the crystal state, then the phase-change material is crystallized, and the crystal state is obtained. In this manner, one of the amorphous state and the crystal state can be written in each cell.
Reproduction is carried out by supplying a cell with a current smaller than the amorphism-forming level and smaller than the crystallizing level and reading the difference in resistance between the amorphous state and the crystal state as a voltage change or current change. Since some kinds of phase-change materials have differences as large as two to three digits in specific resistance between the amorphous state and the crystal state. Therefore, the quality of reproduction signals therefrom is very high, and it is also possible to technically develop it for many-valued storage.
Structure of a PRAM cell is basically made up of an electrode and a phase-change material, and a diode or a transistor for selecting the cell is connected in series to each cell to form a matrix array. Here is no such problem that the storage portion becomes relatively bulky due to a progress of the cell miniaturization, which is inherent to DRAM and FeRAM. Also the problem of MRAM that miniaturization makes recording difficult does not exist. The phase-change storage portion of PRAM becomes smaller according to the scaling rule along with miniaturization of cells, and the recording current decreases in accordance with the miniaturization.
As such, PRAM has the excellent potential as a substitute for DRAM. Additionally, because of its availability for many-valued recording, it is positioned as a hopeful candidate of the “universal memory” mentioned above. Since the resistance change of PRAM reaches hundreds to thousands times, if 50% resistance changes obtained by MRAM are assigned to two signal levels, it is possible to store information of two hundreds to two thousands values in a single phase-change memory cell. Therefore, With a 1 Gb matrix for two-valued operation, storage of information substantially from 200 Gb to 2Tb will be possible. As such, PRAM must be just a hopeful candidate of universal memory having both the high speed of DRAM and the large capacity of HDD.
Regarding PRAM, improved techniques are disclosed by, in addition to the aforementioned literature, U.S. Pat. No. 5,341,328, U.S. Pat. No. 5,359,205, U.S. Pat. No. 5,534,711, U.S. Pat. No. 5,534,712, U.S. Pat. No. 5,596,522, U.S. Pat. No. 5,687,112 and U.S. Pat. No. 6,087,674, for example. The entire contents of these references are incorporated herein by reference.
SUMMARY OF THE INVENTION
The Inventor, however, made a review on operations of storage devices using phase-change materials, and found a basic problem. Then, through examination of operations of memory cells experimentally prepared on the basis of that knowledge, the Inventor co
Ichihara Katsutaro
Yoda Hiroaki
Fahmy Wael
Ha Nathan
Kabushiki Kaisha Toshiba
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