Memory reading method for resistance drift mitigation

• US 7,929,338 B2
• Filed: 02/24/2009
• Issued: 04/19/2011
• Est. Priority Date: 02/24/2009
• Status: Active Grant

• Current Assignee: Globalfoundries US Incorporated (GlobalFoundries, Inc.)
• Original Assignee: International Business Machines Corporation
• Inventors: Karidis, John P., Franceschini, Michele M., Lastras, Luis A.
• Primary Examiner(s): Pham; Ly D
• Application Number:
  US12/392,032
• Publication Number:
  US 20100214830A1
• Time in Patent Office: 784 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 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 method comprising:

applying at least one periodic input signal to the memory cell;

measuring an amplitude of at least one harmonic frequency resulting from the at least one periodic input signal;

calculating an invariant component based on the amplitude of the at least one harmonic frequency resulting from the at least one periodic input signal, the invariant component being 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 the memory state of the memory cell based on the invariant component.

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

mapping the amplitude of at least one harmonic frequency to a measurements region containing the amplitude of the at least one harmonic frequency, the measurements region being one of a plurality of measurements regions, the plurality of measurements regions corresponding to possible outcomes for the amplitude of the at least one harmonic frequency at different memory states of the memory cell; and

determining the invariant component to be equal to the measurements region to which the amplitude of the at least one harmonic frequency is mapped.

3. The method of claim 2, where the measurements regions are determined through the steps of:

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

applying at least one periodic input signal at least one time after each programming operation;

measuring the amplitude of at least the one harmonic frequency resulting from the at least one periodic input signal;

associating the amplitude of the at least one harmonic frequency 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 amplitudes of the at least one harmonic frequencies associated with a desired initial state.

4. 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 at least one periodic input signal to the memory cell;

measure an amplitude of at least one harmonic frequency resulting from the at least one periodic input signal;

calculate an invariant component based on the amplitude of the at least one harmonic frequency resulting from the at least one periodic input signal, the invariant component being 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 the memory state of the memory cell based on the invariant component.