Static information storage and retrieval – Systems using particular element – Resistive
Reexamination Certificate
2001-04-04
2002-10-29
Tran, Michael (Department: 2818)
Static information storage and retrieval
Systems using particular element
Resistive
C365S185030
Reexamination Certificate
active
06473332
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrically operated, overwritable, multivalued, non-volatile resistive memory element.
More particularly, the present invention relates to an electrically operated, overwritable, multivalued, non-volatile resistive memory element where the memory element includes a two terminal non-volatile memory device in which a memory film material is included, and a circuit topology configuration is defined and to methods for using the device and to methods for constructing dense solid state memory systems.
2. Description of the Related Art
Because of simple two terminal structure and stepwise nonvolatile resistance levels, resistive memories, which usually take the form of a resistive layer located between two metallic electrodes, have attracted past attention for nonvolatile, multi-valued logic.
Several semiconductor-related resistive memory devices have been previously developed. One example is a filament-type semiconductor device, as disclosed in U.S. Pat. No. 3,886,577 to William D. Buckley. After an electrical pulse with sufficient high voltage and sufficient long duration (50V at 1-100 ms) is applied to an amorphous semiconductor thin film, a portion of the film may be set to a low resistance crystalline state, i e., after the electric pulse, a low resistivity filament of generally under 5-10 microns in size, may form in the film. Afterwards, if a high current and shorter duration electrical pulse (e.g., a 150 mA pulse of 10 &mgr;s) is applied, the formed filament portion may be reset to a high resistance state. Because of the required long duration pulse for setting, and high current pulse for resetting, such devices have low working speeds and high power consumption. In addition, these kinds of memory devices are basically a bistable device—only a small number of discrete nonvolatile resistance steps can be obtained.
Another example of a semiconductor resistive memory device, as disclosed in U.S. Pat. No. 5,541,869 to Mervyn J. Rose et al. and in a published paper “Quantized Electron Transport in Amorphous-silicon Memory Structure,” J. Hajto, A. E. Gage, and A. J. Snell, P. G. Lecomber and M. J. Rose, in Physical Review Letters, vol. 66, No. 14, PP.1918-1921, 1991, is the metal-amorphous silicon-metal (MSM) electrical memory switch. Such a device needs an initial “forming,” which comprises applying high voltage, short duration pulses (e.g., 200 ns-1000 ns) to disperse small metallic particles or atoms of V, Co, Ni, Pd, Fe and Mn in the amorphous silicon as the active species. The forming process reduces the resistance of the device from a very high value (in practice~10
9
to 10
11
&OHgr;) to a value within the range of 10
3
-10
6
&OHgr;. The resistance of the device can then be adjusted to a target value within this range. In addition to the troublesome initial forming step, this kind of device cannot store a large number of bits in one element. Furthermore, the adjustment of such a device's resistance to a target value depends on the application of pulses with specific voltage and duration so as to produce the desired resistance values, which for current applications requires additional complications of controlling pulse width and height.
Another kind of semiconductor resistive memory device, as disclosed in U.S. Pat. No. 5,406,509 to Stanford R. Ovshinsky; Qiuyi Ye; David A. Strand, and Wolodymyr Czubatj, is a resistive layer device, which uses chalcogenide memory material, instead of amorphous silicon. Chalcogenide-transition metal memory devices are also filament-type devices and exhibit switching characteristics (i.e., switching times, switching energies and resultant device resistance) similar to the electrical switching characteristics of Rose's MSM memory element described above, but can be directly overwritten.
In recent years, nonvolatile memory investigations have addressed ferroelectric and magnetoresistive materials, which are oxides and not semiconductors. In the oxide systems studied, the bit levels have generally been at 2 (high polarization-low polarization for ferroelectrics or high resistance-low resistance for magnetoresistive materials). Within these groups, the ferroelectric devices suffer from fatigue and retention problems. Moreover, reading a ferroelectric devices is information-destructive. See, e.g., “The Non-volatile Memory Challenge,” M. Dax,
Semiconductor International
, Sep., 1997, PP. 84-92.
In the case of the magnetoresistive oxide devices, magnetic switching fields are generally high and temperatures of operation are very low. See, e.g., “Giant Magnetoresistive Memory Effect in Nd
0.7
Sr
0.3
MnO
3
Films,” G. C. Xiong, Q. Li, H. L. Ju, S. M. Bhagat, S. E. lofland, R. L. Greene, and T. Venkatesan,
Appl. Phys. Lett
., Vol. 67, PP. 3031-3033, 1995.
Thus, there is a need in the art for new resistive memory systems that are capable of storing data in a non-volatile and multi-valued manner using traditional read voltages and relatively low write voltage pulses of relatively short to very short duration and good fatigue resistance properties.
SUMMARY OF THE INVENTION
The present invention provides a resistive memory devices including a thin film of a colossal magnetoresistance (CMR) material and two electrically conductive contacts in electrical communication with a portion of the film for reading from and writing to the portion of the film. The term “in electrical communication with” means that a pathway exists, that allows electromagnetic fields to flow between the elements. These pathways can be any known contacting means for allowing electromagnetic fields to flow between elements such as wires, conductive elements in an integrated circuit, or any other type of electric or magnetic component that allows electromagnetic fields to travel between to elements.
The present invention also provides a memory element including a portion of a thin film of a CMR material and two electrically conductive contacts in electrical communication with the portion of the film.
The present invention also provides a memory element array including a plurality of memory elements of this invention formed on a substrate in a column-row or array format.
The present invention also provides a memory apparatus including a plurality of memory elements of this invention.
The present invention also provides a method for storing and retrieving data including the steps of pulsing a memory element of this invention with an electric pulse sufficient to transform the element from an initial resistive state to a final resistive state and detecting the final resistive state with a read pulse, which is insufficient to change the final state of the element.
The present invention also provides a method for storing and retrieving data including the steps of pulsing a memory element array of this invention with electric pulses sufficient to transform at least one element in the array from an initial resistive state to a final resistive state and detecting the final resistive state of all elements with read pulses, which are insufficient to change the final state of any element in the array.
The present invention also relates to a computer including a digital processing unit and a memory apparatus or array of this invention.
The present invention also relates to a method for storing and retrieving multi-state data including the steps of pulsing a memory element array of this invention with electric pulses sufficient to transform at least one element in the array from an initial resistive state to a final resistive state and detecting the final resistive state of all elements with read pulses, which are insufficient to change the final state of any element in the array and where the initial and final state are selected from a group of resistive states separated from each other by a resistance sufficient to allow each state to be independently and consistently identified.
The present invention also provides an electrical pulse operated, directly overwritable, multi-valued non-volatile resis
Charlson E. Joseph
Ignatiev Alex
Liu Shangqing
Wu Naijuan
Strozier Robert W.
The University of Houston System
Tran Michael
LandOfFree
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