In the leading-edge production measuring the geometrical dimensions with e-beam inspection (CD-SEM data) or scatterometry technology (OCD data) is one of the most time-consuming steps without adding value to the wafer. Hence the fabs want to limit the effort to minimize the costs per wafer. On the other hand, the output of the metrology steps is needed to feed the SPC and APC systems with sufficient information. We handle that trade-off with a new sampling scheme optimizer supporting CD-SEM and OCD data.
Generally, we can use the sampling scheme optimization for a set of different features and their measured parameters in parallel. Especially in logic, but also for memory, the focus and dose dependencies of several features may be different. Hence, we optimized the distribution of the measured sites to create a perfect representation of the systematic fingerprint for all important anchor features within one single sampling scheme.
For the verification of the approach we investigated two cases. The first case are dense CD measurements, which are usually needed to create and update intra-field dose corrections. We minimize the number of measured sites significantly and distribute the remaining sites over different fields to ensure a good coverage of the systematic effects. Finally, that allows us a much higher update frequency of the dose corrections and yields in smaller CDU values.
The second case optimized the throughput of an OCD metrology system. The applied high-density sampling scheme for the focus monitoring done on reference wafers takes a lot of time during measuring. That specific type of measurement is done for monitoring and updating the focus reference corrections. With our proposed solution, we can achieve the same quality with respect to the reference measurement with more 50% less measured sites.