Editor's Note: In this follow-up paper on phase change memory (PCM), Memory Designline asked Ron Neale to expand on some of the points he made in his earlier feature and to address some of the comments from EETimes’ expert readers that were generated as a result of the first article.
PCM structures: the similarities and differences
What was apparent from the comments received after publication of Part 1
is a clear need to ensure that any claims for PCM scaling performance must be accompanied by a description of the device structure. Without that, there is room for misunderstanding. Even worse there is scope when fundamental problems are reported for one structure for PCM advocates to claim it is not valid for a different structure.
In  the analysis and prediction of current density trends with scaling, the focus was on the "pore" and "link" PCM structures. One comment from a reader suggested that the concern regarding the very high current densities predicted in  for PCM at photolithographic nodes of 30nm and lower was unwarranted because they were already reliably in use in the third one of the generic PCM structures.
The third PCM structure
To complete the work of , the third generic PCM structure the "μTrench," "dome," "mushroom" or "match head" will be explored here in relation to current density effects of scaling. The names dome and mushroom were coined because of the attractive end on view of the active region most often shown in publicity pictures and micrographs. A generalized three-dimension cutaway diagram is shown in Figure 1.
Figure 1: Cutaway section of PCM trench (dome) structure. Red outline shows hard and soft reset.