Extraction uses the physical-design layers to identify the devices used and how they are interconnected. Therefore, IBM cannot recheck the schematic to physical verification. Extraction checking is very useful for catching incomplete or improperly connected devices, which can occur due to a missing physical-design layer. If any layers are missing, extraction will report that undefined devices exist in the design. Extraction can also find creatively designed devices that do not match any of those defined by the technology.
Many creative designs are acceptable, but some can cause manufacturing problems.
For these cases, the manufacturing engineers must review such designs to ensure they are acceptable in the specified technology. Again, discovering such errors as early in the design cycle as possible is important. Once all of the above-mentioned checks are completed, the results are reviewed. If no errors are found, the data is released for mask build. Ideally, all designs would fall into this category.