Apparatus and methods for predicting and/or calibrating memory yields

1. A method comprising:

determining a model of a memory cell;

identifying a subset of parameters of the memory cell that influence a stability performance of the memory cell;

determining a refined model of the memory cell based on the identified subset of parameters;

simulating operation of variations of the refined model using different combinations of values of the parameters;

determining a predicted probability that the simulated memory cell will operate correctly based on the simulated operation;

calculating a predicted yield percentage of a memory array comprised of a plurality of the simulated memory cells based on the predicted probability the simulated memory cell will operate correctly; and

determining a minimum number of repair elements that would change the predicted yield percentage of the memory array with redundancy to an acceptable level.

2. The method of claim 1 wherein determining a model includes determining a first characterization of a memory cell device, wherein the first characterization is based on a plurality of parameters each having a distribution about a nominal value.

3. The method of claim 2 wherein identifying a subset of parameters includes identifying a subset of the parameters that influence a stability performance of a memory cell that is modeled using a simulated circuit that includes a plurality of the first characterized memory cell devices, wherein the subset of the parameters are identified based on the parameters influencing the stability performance more than remaining parameters influence the stability performance in a simulation.

4. The method of claim 3 wherein determining a refined model includes determining a second characterization of the memory cell device, wherein the second characterization is based on the identified subset of parameters.

5. The method of claim 4 wherein simulating operation includes simulating operation of variations of the memory cell modeled using a simulated circuit that includes a plurality of the second characterized memory cell devices, wherein simulating is performed for each different combination of values of the parameters, varied over a predefined range of the parameters' distributions, of each of the identified subset of parameters of each of the second characterized memory cell devices.

6. The method of claim 1 further comprising determining an observed probability that a memory cell operates correctly based on testing a manufactured memory array of the memory cells for voltage dependent failures.

7. The method of claim 6 further comprising refining the refined model of the memory cell based on any difference between the predicted probability and the observed probability.

8. The method of claim 1 further comprising determining an observed probability that a memory cell operates correctly based on testing a manufactured memory array of the memory cells for failures due to device defects.

9. The method of claim 8 further comprising refining the refined model of the memory cell based on any difference between the predicted probability and the observed probability.

10. A method comprising:

determining a first model of a memory cell device, wherein the first model is based on a plurality of parameters each having a distribution about a nominal value;

identifying a subset of the parameters that influence a stability performance of a memory cell that is modeled using a simulated circuit that includes a plurality of the first modeled memory cell devices, wherein the subset of the parameters are identified based on the parameters influencing the stability performance more than remaining parameters influence the stability performance in a simulation;

determining a second model of the memory cell device, wherein the second model is based on the identified subset of parameters;

simulating operation of variations of the memory cell modeled using a simulated circuit that includes a plurality of the second modeled memory cell devices, wherein simulating is performed for each different combination of values of the parameters, varied over a predefined range of the parameters' distributions, of each of the identified subset of parameters of each of the second modeled memory cell devices;

determining a predicted probability the simulated memory cell will operate correctly based on the simulated operation of the memory cell variations;

calculating a predicted yield percentage of a memory array comprised of a plurality of the simulated memory cells based on the predicted probability the simulated memory cell will operate correctly; and

determining a number of repair elements that would change the predicted yield percentage of the memory array with redundancy to an acceptable level.

11. The method of claim 10 further comprising:

determining an observed probability that a memory cell operates correctly based on testing a manufactured memory array of the memory cells for voltage dependent failures; and

refining the second model of the memory cell device based on any difference between the predicted probability and the observed probability.

12. An apparatus comprising:

a storage device; and

a processor coupled to the storage device adapted to:

determine a model of a memory cell,

identify a subset of parameters of the memory cell that influence a stability performance of the memory cell,

determine a refined model of the memory cell based on the identified subset of parameters,

simulate operation of variations of the refined model using different combinations of values of the parameters,

determine a predicted probability that the simulated memory cell will operate correctly based on the simulated operation,

calculate a predicted yield percentage of a memory array comprised of a plurality of the simulated memory cells based on the predicted probability the simulated memory cell will operate correctly, and

determine a minimum number of repair elements that would change the predicted yield percentage of the memory array with redundancy to an acceptable level.

13. The apparatus of claim 12 wherein the processor adapted to determine a model includes the processor being adapted to determine a first characterization of a memory cell device, wherein the first characterization is based on a plurality of parameters each having a distribution about a nominal value.

14. The apparatus of claim 13 wherein the processor adapted to identify a subset of parameters includes the processor being adapted to identify a subset of the parameters that influence a stability performance of a memory cell that is modeled using a simulated circuit that includes a plurality of the first characterized memory cell devices, wherein the subset of the parameters are identified based on the parameters influencing the stability performance more than remaining parameters influence the stability performance in a simulation.

15. The apparatus of claim 14 wherein the processor adapted to determine a refined model includes the processor being adapted to determine a second characterization of the memory cell device, wherein the second characterization is based on the identified subset of parameters.

16. The apparatus of claim 15 wherein the processor adapted to simulate operation includes the processor being adapted to simulate operation of variations of the memory cell modeled using a simulated circuit that includes a plurality of the second characterized memory cell devices, wherein simulating is performed for each different combination of values of the parameters, varied over a predefined range of the parameters' distributions, of each of the identified subset of parameters of each of the second characterized memory cell devices.

17. The apparatus of claim 12 wherein the processor is further adapted to determine an observed probability that a memory cell operates correctly based on data received from testing a manufactured memory array of the memory cells for voltage dependent failures.

18. The apparatus of claim 17 wherein the processor is further adapted to refine the refined model of the memory cell based on any difference between the predicted probability and the observed probability.

19. The apparatus of claim 12 wherein the processor is further adapted to determine an observed probability that a memory cell operates correctly based on data received from testing a manufactured memory array of the memory cells for failures due to device defects.

20. The apparatus of claim 19 wherein the processor is further adapted to refine the refined model of the memory cell based on any difference between the predicted probability and the observed probability.