Understanding Process Corner (Corner Lots)

Process Lots (or corner lots) are special-modified-wafers that help verifying chip design robustness to accommodate process variations that statistically occur in wafer production over the years.

One of the products that semiconductor foundries offer is process lots (also called: corner lots, split lots or skewed lots). Corner lots wafers are a group of wafers which have been skewed by the fab to different corners.

The purpose of process lots is to help you find out whether your design will be immune to process variations in the future. A successful corner lot exercise includes production of process lots in all different corners. Thereafter these wafers need to pass the electrical test (by the ATE) at all corners.

The industry is using two-letter designation to describe the different corners, where the first letter refers to the NMOS device, and the second refers to the PMOS device.  There are 5 classic corners:

• FF (fast fast)
• SF (slow fast)
• SS (slow slow)
• FS (fast slow)
• TT (typical typical)

The FF corner, for instance, is obtained by skewing both P and N devices to the fast corner. The TT corner is the center corner where wafers are normally produced (e.g. typical process parameters).

In typically chip project schedule, corner lots should be produced just after tapeout or before releasing to production.

Silicon foundries are also offering skewing of different parameters to see the effect of specific parameters on memory blocks for example. This will require deeper discussions with the process engineers to help identifying parameters in the process that can provide benefits.