Determining metrology sampling decisions based on fabrication simulation

1. A method for determining metrology sampling rates for workpieces in a process flow, comprising:

determining a current status of the process flow;

simulating future processing of the workpieces in the process flow based on the current status of the process flow over a predetermined time horizon to predict metrology sampling rates for the workpieces;

implementing sampling rules during the simulating that consider capacity constraints of a metrology resource in the process flow; and

sampling actual workpieces in the process flow based on the predicted sampling rates to collect metrology data associated with the actual workpieces.

2. The method of claim 1 , wherein implementing the sampling rules further comprises implementing the sampling rules to maintain a substantially constant queue depth for the metrology resource.

3. The method of claim 1 , wherein implementing the sampling rules during the simulating further comprises implementing a backward sampling rule each time a workpiece exists the metrology resource.

4. The method of claim 3 , wherein implementing the backward sampling rule further comprises selecting a workpiece for sampling in the metrology resource responsive to a current sampling rate of workpieces having a process characteristic matching the selected workpiece being less than an average sampling rate.

5. The method of claim 3 , further comprising implementing the backward sampling rule in the process flow.

6. The method of claim 1 , wherein implementing the sampling rules during the simulating further comprises implementing a forward sampling rule each time a workpiece exists a processing resource and enters the metrology resource.

7. The method of claim 6 , wherein implementing the forward sampling rule comprises sampling the workpiece entering the metrology resource responsive to a queue length of the metrology resource being less than a predetermined threshold.

8. The method of claim 6 , wherein implementing the forward sampling rule comprises sampling the workpiece entering the metrology resource responsive to a current sampling rate of workpieces having a process characteristic matching the workpiece being less than a minimum sampling rate.

9. The method of claim 6 , wherein implementing the forward sampling rule comprises sampling the workpiece entering the metrology resource responsive to a current sampling rate of workpieces having a process characteristic matching the workpiece being less than an average sampling rate.

10. The method of claim 6 , wherein implementing the forward sampling rule comprises not sampling the workpiece entering the metrology resource responsive to a queue length of the metrology resource being greater than a predetermined threshold.

11. The method of claim 6 , wherein implementing the forward sampling rule comprises not sampling the workpiece entering the metrology resource responsive to a current sampling rate of workpieces having a process characteristic matching the workpiece being greater than a maximum sampling rate.

12. The method of claim 6 , further comprising implementing the forward sampling rule in the process flow.

13. The method of claim 1 , further comprising:

grouping the workpieces based on a processing characteristic;

assigning sampling weighting factors to the groupings of workpieces, the sampling weighting factors determining a relative importance of collecting metrology data for the associated grouping; and

selecting workpieces for sampling during the simulating based at least in part on the sampling weighting factors.

14. The method of claim 13 , wherein the workpieces comprise semiconductor wafers, and the process characteristic comprises a layer to be processed on each workpiece.

15. The method of claim 13 , wherein the workpieces comprise semiconductor wafers, and the process characteristic comprises a layer to be processed on each workpiece and a tool in the process flow used to process the layer.

16. The method of claim 13 , further comprising, during the simulating:

determining an average unit sampling rate for all of the groupings;

determining a unit sampling rate for each grouping based on the sampling weighting factor associated with the grouping;

comparing the unit sampling rate to the average unit sampling rate; and

implementing the sampling rules based on the comparison.

17. The method of claim 16 , wherein implementing the sampling rules further comprises selecting a workpiece for sampling responsive to the unit sampling rate for its associated grouping being less than the average unit sampling rate.

18. The method of claim 16 , wherein implementing the sampling rules further comprises not selecting a workpiece for sampling responsive to the unit sampling rate for its associated grouping being greater than the average unit sampling rate.

19. The method of claim 1 , further comprising processing workpieces in the process flow after determining the predicted sampling rates.

20. A system, comprising:

a plurality of tools for processing workpieces in a process flow; and

a sampling unit operable to determine a current status of the process flow, the sampling unit being further operable to employ a fabrication simulation model to simulate future processing of the workpieces in the process flow based on the current status of the process flow over a predetermined time horizon to predict metrology sampling rates for the workpieces and implement sampling rules during the simulating that consider capacity constraints of a metrology resource in the process flow; and

a workflow controller operable to sample actual workpieces in the process flow based on the predicted metrology sampling rates.

21. A sampling controller comprising a processing device operable to execute instructions that when implemented perform a method, comprising:

determining a current status of a process flow;

simulating future processing of workpieces in the process flow based on the current status over a predetermined time horizon to predict sampling rates of the workpieces;

implementing sampling rules during the simulating that consider capacity constraints of a metrology resource in the process flow; and

communicating the predicted sampling rates to an entity operable to control the process flow.