DETECTING SWITCHING OF ACCESS ELEMENTS OF PHASE CHANGE MEMORY CELLS

• US 20060221734A1
• Filed: 03/30/2005
• Published: 10/05/2006
• Est. Priority Date: 03/30/2005
• Status: Active Grant

• Current Assignee: Ovonyx Memory Technology, LLC
• Original Assignee: Ovonyx Incorporated (Micron Technology, Inc.)
• Inventors: Bedeschi, Ferdinando, Resta, Claudio, Parkinson, Ward D., Gastaldi, Roberto
• Granted Patent: US 7,154,774 B2
• Time in Patent Office: Days
• US Class Current: 365/201
• CPC Class Codes: G11C 13/0004 comprising amorphous/crystalline phase transition cells
• CPC Class Codes: G11C 13/003 Cell access
• CPC Class Codes: G11C 13/004 Reading or sensing circuits or methods
• CPC Class Codes: G11C 13/0061 Timing circuits or methods
• CPC Class Codes: G11C 2013/0054 Read is performed on a reference element, e.g. cell, and the reference sensed value is used to compare the sensed value of the selected cell
• CPC Class Codes: G11C 2013/0071 Write using write potential applied to access device gate
• CPC Class Codes: G11C 2213/76 Array using an access device for each cell which being not a transistor and not a diode
• CPC Class Codes: G11C 7/062 Differential amplifiers of non-latching type, e.g. comparators, long-tailed pairs
• CPC Class Codes: G11C 7/08 Control thereof

Abstract

A memory includes a storage element (OUM) made of a phase-change material for storing a logic value and an access element (OTS) switching from a higher resistance condition to a lower resistance condition in response to a selection of the memory cell, the access element in the higher resistance condition decoupling the storage element from a read circuit and in the lower resistance condition coupling the storage element to the read circuit. The read circuit includes a sense amplifier to determine the logic value stored in the memory cell according to an electrical quantity associated with the memory cell. The read circuit further includes a detector that detects the switching of the access element by comparison to a delayed waveform or sensing a change in the column rate of change, and a circuit to enable the sense amplifier in response to the detection of the switching of the access element.

Claims

1. A memory comprising:

a phase change storage element and an access element to switch from a high impedance condition to a low impedance condition;

a sense amplifier couplable to said element;

a read circuit couplable to said element to detect the switching of the access element; and

a circuit to enable the sense amplifier in response to the detection of the switching of the access element.

2. The memory according to claim 1, wherein the access element includes a threshold switch element made of a single phase material.

3. The memory according to claim 1, wherein the read circuit includes a peak detector.

4. A method comprising:

switching an access element from a non-threshold condition to an threshold condition in response to selection of a memory cell;

detecting the switching of the access element, and

enabling the determination of a memory cell state in response to the detection of the switching of the access element.

5. The method according to claim 4, wherein detecting the switching of the access element includes identifying a peak rate of change of an electrical quantity.

6. The method according to claim 4, further comprising:

providing a read biasing current, a second read current lower than the read biasing current;

applying the biasing current to the memory cell in response to the selection of the memory cell;

detecting a first steady condition of the memory cell corresponding to the first read current;

sensing a first measuring voltage in response to the detection of the first steady condition;

applying the second read current to the memory cell in response to the sensing of the first measuring voltage;

detecting a second steady condition of the memory cell corresponding to the second read current;

sensing a second measuring voltage in response to the detection of the second steady condition; and

determining the logic value stored in the memory cell according to the first and the second measuring voltages.

7. The method according to claim 6, wherein detecting the switching of the access element and detecting the second steady condition include identifying a first change in rate of the electrical quantity, and the step of detecting the first steady condition includes identifying a second change of rate after the first change of rate.

8. The method according to claim 7, wherein the step of detecting each change of rate includes delaying the electrical quantity by a predetermined period and identifying the corresponding change of rate when the delayed electrical quantity reaches the electrical quantity within a predetermined offset, the offsets for the first and second changes of rate having opposite signs.

9. The method according to claim 4, wherein detecting the switching of the access element includes:

providing a derivative signal corresponding to a time-derivative of the electrical quantity, and

comparing the derivative signal with a threshold voltage.

10. The method according to claim 9, wherein determining the logic value stored in the memory cell includes comparing the derivative signal with a further threshold voltage.

11. A system comprising:

a controller; and

a phase change memory coupled to said controller including a storage element and an access element to switch from a high impedance condition to a low impedance condition, a sense amplifier couplable to said element, a read circuit couplable to said element to detect the switching of the access element and a circuit to enable the sense amplifier in response to the detection of the switching of the access element.

12. The system according to claim 11, wherein the access element includes a threshold switch element made of a single phase material.

13. The system according to claim 11, wherein the read circuit includes a rate of change detector.