Memory reading method for resistance drift mitigation

• US 8,144,508 B2
• Filed: 01/05/2011
• Issued: 03/27/2012
• Est. Priority Date: 02/24/2009
• Status: Active Grant

• Current Assignee: Globalfoundries US Incorporated (GlobalFoundries, Inc.)
• Original Assignee: International Business Machines Corporation
• Inventors: Franceschini, Michele M., Karidis, John P., Lastras, Luis A.
• Primary Examiner(s): Pham, Ly D
• Application Number:
  US12/984,682
• Publication Number:
  US 20110096594A1
• Time in Patent Office: 447 Days
• Field of Search: 365/148, 365/163
• US Class Current: 365/163
• CPC Class Codes: G11C 11/56 using storage elements with more than two stable states represented by steps, e.g. of voltage, current, phase, frequency
• CPC Class Codes: G11C 11/5678 using amorphous/crystalline phase transition storage elements
• CPC Class Codes: G11C 13/0004 comprising amorphous/crystalline phase transition cells
• CPC Class Codes: G11C 13/0033 Disturbance prevention or evaluation; Refreshing of disturbed memory data
• CPC Class Codes: G11C 13/004 Reading or sensing circuits or methods
• CPC Class Codes: G11C 2013/0052 Read process characterized by the shape, e.g. form, length, amplitude of the read pulse

Abstract

Techniques for reading phase change memory that mitigate resistance drift. One contemplated method includes apply a plurality of electrical input signals to the memory cell. The method includes measuring a plurality of electrical output signals from the memory cell resulting from the plurality of electrical input signals. The method includes calculating an invariant component of the plurality of electrical output signals dependent on the configuration of amorphous material in the memory cell. The method also includes determining a memory state of the memory cell based on the invariant component. In one embodiment of the invention, the method further includes mapping the plurality of electrical output signals to a measurements region of a plurality of measurements regions. The measurements regions correspond to memory states of the memory cell.

Claims

1. A method for operating a memory cell, a memory state of the memory cell being represented by a configuration of amorphous material in the memory cell, a resistance of the memory cell drifting over a period of time, the method comprising:

applying a plurality of electrical input signals to the memory cell;

measuring a plurality of electrical output signals from the memory cell resulting from the plurality of electrical input signals;

calculating an invariant component of the plurality of electrical output signals dependent on the configuration of amorphous material in the memory cell, the invariant component being substantially independent of a resistance drift characteristic of the memory cell over the period of time; and

determining a memory state of the memory cell based on the invariant component.

2. The method of claim 1, wherein the plurality of electrical input signals bias the memory cell at more than one point of the memory cell'"'"'s current-voltage characteristic relation.

3. The method of claim 1, wherein calculating the invariant component comprises:

mapping the plurality of electrical output signals to a measurements region containing the plurality of electrical output signals, the measurements region being one of a plurality of measurements regions, each measurements region of the plurality of measurements regions corresponding to a possible memory state of the memory cell; and

determining the invariant component to be equal to the measurements region to which the plurality of electrical output signals is mapped.

4. The method of claim 3, wherein the measurement regions are determined according to the steps of:

programming at least one memory cell at least once into each of a plurality of possible initial states;

applying the plurality of input electrical signals at least one time after each programming operation;

measuring the plurality of output electrical signals resulting from the plurality of electrical input signals;

associating the plurality of output electrical signals with the initial states into which the at least one cell was programmed; and

choosing one of the measurements regions for each of the plurality of possible initial states to contain a substantial majority of the plurality of output electrical signals associated with a desired initial state.

5. The method of claim 1, further comprising:

for each of a plurality of possible initial cell states, computing the probability of obtaining the plurality of measurements of electrical output signals conditional on said initial cell state based on a statistical model.

6. The method of claim 1, further comprising:

measuring a temperature of the memory cell; and

wherein the calculating an invariant component of the plurality of output electrical signals is dependent, at least in part, on the temperature of the memory cell and the period of time.

7. The method of claim 1, wherein calculating the invariant component comprises mapping the plurality of measurements of electrical output signals to a single valued quantity.

8. A memory controller for operating a memory cell, a memory cell state being represented by a configuration of amorphous material in the memory cell, a resistance of the memory cell drifting over a period of time, the memory controller comprising:

a biasing unit configured to bias a memory cell with a plurality of electrical input signals;

a measuring unit configured to measure a plurality of electrical output signals from the memory cell resulting from the plurality of electrical input signals;

a calculating unit configured to calculate an invariant component of the plurality of electrical output signals dependent on the configuration of amorphous material in the memory cell, the invariant component being substantially independent of a resistance drift characteristic of the memory cell over the period of time; and

a determining unit configured to determine a memory state of the memory cell based on the invariant component of the plurality of output electrical signals.

9. The memory controller of claim 8, wherein the plurality of electrical input signals bias the memory cell at more than one point of the memory cell'"'"'s current-voltage characteristic relation.

10. The memory controller of claim 8, wherein the plurality of electrical input signals is part of a continuous input signal applied to the memory cell.

11. The memory controller of claim 8, wherein the calculating unit is further configured to:

map the plurality of electrical output signals to a measurements region containing the plurality of electrical output signals, the measurements region being one of a plurality of measurements regions, each measurements region of the plurality of measurements regions corresponding to a possible memory state of the memory cell; and

determine the invariant component to be equal to the measurements region to which the plurality of electrical output signals is mapped.

12. The memory controller of claim 8, wherein the calculating unit is further configured to, for each of a plurality of possible initial cell states, compute the probability of obtaining the plurality of measurements of electrical output signals conditional on said initial cell state based on a statistical model.

13. The memory controller of claim 8, wherein the calculating unit is further configured to:

calculate a single valued quantity based on the plurality of electrical output signals, the single valued quantity being a value dependent on the configuration of amorphous material in the memory cell, each single valued quantity corresponding to one of the possible memory cell states; and

determine the invariant component to be equal to the single valued quantity calculated from the plurality of electrical output signals.

14. A computer program product embodied in a computer usable memory comprising:

computer readable program codes coupled to the computer usable medium for operating a memory cell, a memory cell state being represented by a configuration of amorphous material in the memory cell, a resistance of the memory cell drifting over a period of time, the computer readable program codes configured to cause the program to;

apply a plurality of electrical input signals to the memory cell;

measure a plurality of electrical output signals from the memory cell resulting from the plurality of electrical input signals;

calculate an invariant component of the plurality of electrical output signals dependent on the configuration of amorphous material in the memory cell, the invariant component being substantially independent of a resistance drift characteristic of the memory cell over the period of time; and

determine a memory state of the memory cell based on the invariant component.

15. The computer program product of claim 14, wherein the plurality of electrical input signals bias the memory cell at more than one point of the memory cell'"'"'s current-voltage characteristic relation.

16. The computer program product of claim 14, further comprising computer readable program codes configured to cause the program to:

map the plurality of electrical output signals to a measurements region containing the plurality of electrical output signals, the measurements region being one of a plurality of measurements regions, each measurements region of the plurality of measurements regions corresponding to a possible memory state of the memory cell; and

determine the invariant component to be equal to the measurements region to which the plurality of electrical output signals is mapped.

17. The computer program product of claim 16, further comprising computer readable program codes configured to cause the program to:

program at least one memory cell at one time into each of a plurality of possible memory states;

for each possible memory state, apply a plurality of electrical input signals;

for each possible memory state, measure a plurality of electrical output signals resulting from the plurality of electrical input signals;

for each possible memory state, associate the plurality of electrical output signals with the memory state; and

for each possible memory state, assign a measurements region for each of the plurality of memory states, the measurements region containing a substantial majority of the plurality of electrical output signals associated with the memory state.

18. The computer program product of claim 14, further comprises computer readable program codes configured to cause the program to for each of a plurality of possible memory states, compute a probability that the plurality of electrical output signals corresponds to the memory state.

19. The computer program product of claim 14, further comprising computer readable program codes configured to cause the program to:

calculate a single valued quantity based on the plurality of electrical output signals, the single valued quantity being a value dependent on the configuration of amorphous material in the memory cell, each single valued quantity corresponding to one of the possible memory cell states; and

determine the invariant component to be equal to the single valued quantity calculated from the plurality of electrical output signals.

20. The computer program product of claim 14, wherein the plurality of electrical input signals is part of a continuous input signal applied to the memory cell.

21. A memory device comprising:

a plurality of memory cells, each memory cell including a memory state represented by a configuration of amorphous material in the memory cell, a resistance of the memory cell drifting over a period of time; and

a controller configured to;

(a) receive an address specifying a memory location to be read;

(b) select the memory cell specified;

(c) apply a plurality of electrical input signals to the memory cell;

(d) measure a plurality of electrical output signals from the memory cell resulting from the plurality of electrical input signals;

(e) calculate an invariant component of the plurality of electrical output signals dependent on the configuration of amorphous material in the memory cell, the invariant component being substantially independent of a resistance drift characteristic of the memory cell over the period of time; and

(f) determine a memory state of the memory cell based on the invariant component.