Static information storage and retrieval – Read only systems – Resistive
Reexamination Certificate
2001-10-10
2003-08-19
Ho, Hoai (Department: 2818)
Static information storage and retrieval
Read only systems
Resistive
C365S189090
Reexamination Certificate
active
06608773
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to electrically programmable memory arrays. More specifically, the present invention relates to circuitry for reading data from an array of programmable resistance elements.
BACKGROUND OF THE INVENTION
Programmable resistance memory elements formed from materials that can be programmed to exhibit at least a high or low stable ohmic state are known in the art. Such programmable resistance elements may be programmed to a high resistance state to store, for example, a logic ONE data bit or programmed to a low resistance state to store a logic ZERO data bit.
One type of material that can be used as the memory material for programmable resistance elements is phase-change material. Phase-change materials may be programmed between a first structural state where the material is generally more amorphous (less ordered) and a second structural state where the material is generally more crystalline (more ordered). The term “amorphous”, as used herein, refers to a condition which is relatively structurally less ordered or more disordered than a single crystal and has a detectable characteristic, such as high electrical resistivity. The term “crystalline”, as used herein, refers to a condition which is relatively structurally more ordered than amorphous and has lower electrical resistivity than the amorphous state.
The phase-change materials may be programmed between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. That is, the programming of such materials is not required to take place between completely amorphous and completely crystalline states but rather the material can be programmed in incremental steps reflecting (1) changes of local order, or (2) changes in volume of two or more materials having different local order so as to provide a “gray scale” represented by a multiplicity of conditions of local order spanning the spectrum between the completely amorphous and the completely crystalline states. For example, phase-change materials may be programmed between different resistive states while in crystalline form.
A volume of phase-change material may be programmed between a more ordered, low resistance state and a less ordered, high resistance state. A volume of phase-change is capable of being transformed from a high resistance state to a low resistance state in response to the input of a single pulse of energy referred to as a “set pulse”. The set pulse is sufficient to transform the volume of memory material from the high resistance state to the low resistance state. It is believed that application of a set pulse to the volume of memory material changes the local order of at least a portion of the volume of memory material. Specifically, it is believed that the set pulse is sufficient to change at least a portion of the volume of memory material from a less-ordered amorphous state to a more-ordered crystalline state.
The volume of memory material is also capable of being transformed from the low resistance state to the high resistance state in response to the input of a single pulse of energy which is referred to as a “reset pulse”. The reset pulse is sufficient to transform the volume of memory material from the low resistance state to the high resistance state. While not wishing to be bound by theory, it is believed that application of a reset pulse to the volume of memory material changes the local order of at least a portion of the volume of memory material. Specifically, it is believed that the reset pulse is sufficient to change at least a portion of the volume of memory material from a more-ordered crystalline state to a less-ordered amorphous state.
The use of phase-change materials for electronic memory applications is known in the art. Phase-change materials and electrically programmable memory elements formed from such materials are disclosed, for example, in U.S. Pat. Nos. 5,166,758, 5,296,716, 5,414,271, 5,359,205, 5,341,328, 5,536,947, 5,534,712, 5,687,112, and 5,825,046 the disclosures of which are all incorporated by reference herein. Still another example of a phase-change memory element is provided in U.S. patent application Ser. No. 09/276,273, the disclosure of which is also incorporated herein by reference.
It is important to be able to accurately read the resistance states of programmable resistance elements which are arranged in a memory array. The present invention descibes an apparatus and method for accurately determining the resistance states of programmable resistance elements arranged as memory cells in a memory array. Background art circuitry is provided in U.S. Pat. No. 4,272,833 which describes a reading apparatus based upon the variation in the threshold levels of memory elements, and U.S. Pat. No. 5,883,827 which describes an apparatus using a fixed resistance element to generate reference signals. Both U.S. Pat. Nos. 4,272,833 and 5,883,827 are incorporated by reference herein.
SUMMARY OF THE INVENTION
One aspect of the present invention is a memory system, comprising: A memory system, comprising: a memory cell comprising a programmable resistance element programmable to at least a first resistance state and a second resistance state, the memory cell interconnecting a row line and a column line; and a power line, distinct from the row line and the column line, coupling the memory cell to a power source.
Another aspect of the present invention is a method of operating a memory cell, the memory cell interconnecting a column line and a row line, the memory cell including a programmable resistance element programmable to at least a first resistance state and a second resistance state, the programmable element having at least a first and second terminal, the first terminal coupled or selectively coupled to the column or the row line, the second terminal coupled or selectively coupled to a third line distinct from the column and row lines, the method comprising the steps of: reading data from the programmable element, the reading step comprising the step of developing a first potential difference across the programmable element; and writing data to the programmable element, the writing step comprising the step of developing a second potential difference across the programmable element.
REFERENCES:
patent: 5883827 (1999-03-01), Morgan
patent: 6034882 (2000-03-01), Johnson et al.
Lowrey Tyler
Wicker Guy C.
Ho Hoai
Ovonyx Inc.
Schlazer Philip H.
Siskind Marvin S.
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