Read operations on multi-bit memory cells in resistive cross...

Details Read operations on multi-bit memory cells in resistive cross... Read operations on multi-bit memory cells in resistive cross...

2002-09-03

2004-06-22

Elms, Richard (Department: 2824)

Static information storage and retrieval

Systems using particular element

Magnetoresistive

C365S173000

Reexamination Certificate

active

06754097

ABSTRACT:

BACKGROUND
Magnetic Random Access Memory (“MRAM”) is a non-volatile memory that is being considered for short-term and long-term data storage. MRAM has lower power consumption than short-term memory such as DRAM, SRAM and Flash memory. MRAM can perform read and write operations much faster (by orders of magnitude) than conventional long-term storage devices such as hard drives. In addition, MRAM is more compact and consumes less power than hard drives. MRAM is also being considered for embedded applications such as extremely fast processors and network appliances.
An MRAM device may include one or more arrays of memory cells, word lines crossing rows of memory cells, and bit lines crossing columns of memory cells. Each memory cell is at the cross point of a word line and a bit line.
Each memory cell may include a magnetoresistive device such as a magnetic tunnel junction. Each magnetoresistive device stores a logic value by setting its resistance to one of two states. The logic value stored in a selected magnetoresistive device may be read by determining the resistance state of the selected magnetoresistive device. The resistance state may be determined by causing a sense current to flow through the selected magnetoresistive device, and detecting the sense current.
The magnetoresistive devices of the array are coupled together through many parallel paths. The resistance seen at one cross point equals the resistance of the magnetoresistive device at that cross point in parallel with resistances of magnetoresistive devices in the other rows and columns. Thus each array of magnetoresistive devices may be characterized as a cross point resistor network.
Because the magnetoresistive devices are connected as a cross point resistor network, parasitic or sneak path currents can interfere with the read operations on selected magnetoresistive devices. Blocking devices such as diodes or transistors may be connected to the magnetoresistive device. These blocking devices can block the parasitic currents.
In the alternative, the parasitic currents may be dealt with by using a an “equipotential” method disclosed in assignee's U.S. Pat. No. 6,259,644. The equipotential method disclosed in U.S. Pat. No. 6,259,644 involves applying a potential to a selected line, and providing the same potential to a subset of unselected bit lines and unselected word lines. The parasitic currents are shunted so as not to interfere with read operations.


REFERENCES:
patent: 6191989 (2001-02-01), Scheuerlein
patent: 6259644 (2001-07-01), Tran et al.
patent: 6631085 (2003-10-01), Kleveland et al.
patent: 2002/0034096 (2002-03-01), Rosner et al.
patent: 1109170 (2001-06-01), None
patent: WO9918578 (1999-04-01), None

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