Local versus Global Planarization
Fine tuning the process is another difficulty. That is not to say that this process parameter is a roadblock, however, it becomes expensive to achieve a suitable balance between local or within-die and global or across-wafer uniformity. Adjusting the process either through changing the mechanics of the spin, or chemically by altering the slurry, is more art than science. It is necessary to adapt every run to maximize the output. Currently, the variation is quite good, within a few hundred angstroms across the wafer; if the within-die parameter is on, and vice versa. However, to achieve this, dummy structures must be added to each wafer in order to maintain uniform flatness. To create these dummy structures, valuable materials must be used including oxide, tungsten and metals. They simply are not part of the output.