MEMORY PROGRAMMING

• US 20100214829A1
• Filed: 02/24/2009
• Published: 08/26/2010
• Est. Priority Date: 02/24/2009
• Status: Active Grant

• Current Assignee: GlobalFoundries, Inc.
• Original Assignee: International Business Machines Corporation
• Inventors: Lastras-Montano, Luis A., Franceschini, Michele M., Breitwisch, Matthew J., Elfadel, Ibrahim M., Sharma, Mayank, Mittelholzer, Thomas, Cheek, Roger W., Karidis, John P., Chiras, Stefanie R.
• Granted Patent: US 8,023,345 B2
• Time in Patent Office: Days
• US Class Current: 365/163
• CPC Class Codes: G11C 13/0004 comprising amorphous/crystalline phase transition cells
• CPC Class Codes: G11C 13/0064 Verifying circuits or methods
• CPC Class Codes: G11C 13/0069 Writing or programming circuits or methods
• CPC Class Codes: G11C 2013/0092 Write characterized by the shape, e.g. form, length, amplitude of the write pulse
• CPC Class Codes: G11C 29/02 Detection or location of defective auxiliary circuits, e.g. defective refresh counters
• CPC Class Codes: G11C 29/021 in voltage or current generators
• CPC Class Codes: G11C 29/028 with adaption or trimming of parameters

Abstract

Systems, methods, and devices for iteratively writing contents to memory locations are provided. A statistical model is used to determine a sequence of pulses to write desired contents to a memory location. The contents can be expressed as a resistance value in a range to store one or more bits in a memory cell. For phase change memory, an adaptive reset pulse and one or more annealing pulses are selected based on a desired resistance range. Reading the resistance value of the memory cell can provide feedback to determine adjustments in an overall pulse application strategy. The statistical model and a look up table can be used to select and modify pulses. Adaptively updating the statistical model and look up table may reduce the number of looping iterations to shift the resistance value of the memory cell into the desired resistance range.

Claims

1. A method for iteratively writing contents to memory locations, the method comprising:

receiving desired contents for a memory location to write through a plurality of write signals, desired accuracy for the memory location, and conditions on a number of iterations to the memory location;

initializing a statistical model associated with the memory location and an iterations counter;

selecting one of the write signals to be applied to the memory location, the selecting responsive to the desired contents, the desired accuracy, the iterations counter, the conditions on the number of iterations, and the statistical model;

applying the write signal to the memory location;

reading current contents of the memory location;

determining whether to continue with an other iteration or to halt, the determining responsive to the current contents of the memory location, the desired contents, the desired accuracy, the iterations counter and the conditions on the number of iterations; and

in response to determining to continue with an other iteration;

updating the statistical model based on the desired contents, the desired accuracy, the iterations counter, the current contents of the memory location, and the statistical model; and

performing the selecting, the applying, the reading, and the determining.

2. The method of claim 1, wherein the statistical model associated with the memory location describes the probability of obtaining the desired contents given an applied write signal and the write signal is selected to maximize the probability of obtaining the desired contents within a given accuracy.

3. The method of claim 1, wherein the write signal for a first set of iterations is selected to enhance statistical model quality by updating a modeled distribution of an ensemble of memory locations included in the statistical model, and the write signal for a second set of iterations is selected to maximize the probability of obtaining the desired contents within a given accuracy.

4. The method of claim 1, wherein the statistical model has known and unknown parameters, and the statistical model shares parameters with a statistical model associated with an other memory location.

5. The method of claim 1, wherein the selecting is implemented by estimating unknown parameters of the memory location and then using the estimated parameters to obtain a write signal that maximizes the probability of obtaining the desired contents within a given accuracy.

6. The method of claim 1, wherein the selecting is further responsive to the current contents of the memory location.

7. The method of claim 1, wherein the statistical model describes random variables that are jointly Gaussian.

8. A system for iteratively writing contents to memory locations, the system comprising:

a write apparatus to interpret one or more write control signals, generate a write signal, and apply the write signal to store desired contents at a selected memory location;

a read apparatus to read current contents of the selected memory location; and

write control circuitry in communication with the write apparatus and the read apparatus, the write control circuitry comprising;

an iterations counter to count a number of write attempts;

halt logic to determine when to stop writing;

a statistical model block to store a statistical model of the memory location; and

a write signal selector to select the one or more write control signals responsive to the desired contents, a desired accuracy, the iterations counter, conditions on a number of iterations, the statistical model, and the current contents of the memory location.

9. A method for programming a phase change memory (PCM) cell, comprising:

receiving a desired resistance range for the PCM cell, the PCM cell having a state with a resistance value; and

looping to determine adjustments to the resistance value of the PCM cell, the looping comprising;

reading the resistance value of the PCM cell;

applying an annealing pulse in response to determining that the resistance value of the PCM cell is greater than the desired resistance range, the annealing pulse shaped responsive to the resistance value of the PCM cell and the desired resistance range;

applying an adjustable reset pulse to reset the state of the PCM cell in response to determining that the resistance value of the PCM cell is less than the desired resistance range, the adjustable reset pulse shaped responsive to the desired resistance range; and

repeating the looping until one or more termination criteria are met.

10. The method of claim 9 further comprising:

applying an initial instance of the adjustable reset pulse prior to the looping.

11. The method of claim 9 wherein the termination criteria comprise:

the resistance value of the PCM cell being within the desired resistance range; and

performing a maximum number of looping iterations.

12. The method of claim 9 wherein the adjustable reset pulse is an electrical pulse having a trailing tail with a slope, a leading edge with a slope, a peak height and peak width, and further wherein the adjustable reset pulse is shaped by changing one or more of the slope of the trailing tail, the slope of the leading edge, the peak height and the peak width responsive to the desired resistance range.

13. The method of claim 9 wherein the annealing pulse is shaped further responsive to an elapsed time between a previously applied pulse and the reading of the resistance value of the PCM cell.

14. The method of claim 9 wherein the annealing pulse is selected among a finite collection of available pulses.

15. The method of claim 9 wherein the annealing pulse is shaped responsive to a difference between the resistance value of the PCM cell and the desired resistance range.

16. The method of claim 9 further comprising:

assigning a cost to each pulse;

determining a statistical model for a probable outcome of applying each pulse as a function of an initial state of the PCM cell; and

selecting a pulse application sequence for the PCM cell that minimizes a total expected cost until one or more of the termination criteria are met.

17. The method of claim 16 wherein the cost of each pulse is assigned responsive to at least one of:

a duration, a current, and total energy of each pulse.

18. The method of claim 16 wherein selecting the annealing pulse in the pulse application sequence further comprises:

computing an index for accessing a table as a function of the resistance value of the PCM cell;

retrieving at least one stored value in the table at the computed index; and

selecting the annealing pulse responsive to the desired resistance range and the at least one retrieved stored value.

19. The method of claim 16 wherein the statistical model includes one or more unknown parameters, and the method further comprises updating the statistical model using the resistance value of the PCM cell, and a next pulse to apply is chosen to minimize the total expected cost according to the updated statistical model until one or more of the termination criteria are met.

20. The method of claim 9 wherein shaping the adjustable reset pulse responsive to the desired resistance range further comprises:

determining an index associated with the desired resistance range; and

using the index to lookup one or more adjustable reset pulse shape characteristics in a table.