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used in Figure 24 can be programmed into relatively tight resistance distributions using iterative write attempts, where the length of the trailing edge of the pulse is care- fully controlled based on the just-accumulated prior ex- perience with that cell. Such iterative write schemes and other considerations of Multi-Level Cell programming are discussed in more detail in Section V A. 

As PCM devices scale into future technology nodes, variability can be expected to become an even more im- 

## 6. PCM scaling and variability 

portant consideration. Difficulties in scaling lithography and processing techniques may lead to even looser spec- ifications on relative CD uniformity in future technology nodes, since the acceptable range of control will be driven by considerations for conventional CMOS devices and not by PCM technology. At future technology nodes, PCM cell designs which depend on control over film thickness may no longer be as attractive, if CD is shrinking while both the minimum thickness t and the achievable con- trol of this variable, Δ t , remain relatively static. As the number of atoms participating in the active portion of the PCM cell decreases, any variations in local dop- ing or stoichiometry can be expected to affect individual cells, reducing yield and further broadening distributions. Eventually, in the same way that Poissonian effects have arrived for CMOS devices (e.g., as the number of dopants in the channel becomes countable)[172], such few-atom effects can be expected to be observable for PCM. That said, all logic and memory devices seeking to operate in this regime will have some variant of these effects — the winners will be those with not-yet-known physics which keeps those effects sufficiently unlikely, or those amenable to not-yet-invented engineering techniques which can fi- nesse these issues to an acceptable degree. 

Most studies related to the integration and process- ing of PCM technology focus on the popular variant, Ge$_{2}$Sb$_{2}$Te$_{5}$, here abbreviated simply as GST, along with various choices of dopant. This reflects the fact that GST possesses many favorable characteristics and has thus re- ceived the bulk of attention of technologists attempting to take PCM technology from research to development. Although these studies reveal numerous characteristics which might be considered less than perfect, the mere ex- istence of such a large base of knowledge also represents a substantial hurdle for any alternative phase-change me- terial hoping to challenge GST. 

The vast majority of phase change materials reported in the literature have been deposited by sputtering, i.e. Physical Vapor Deposition (PVD). Sputter deposition from multiple targets makes it very simple to try dif- ferent compositions. However, sputtered films typically do not have good step coverage and cannot completely fill high aspect-ratio vias without keyhole formation[103]. A number of interesting PCM cell structures have been discussed (Section III A) that call for confining the phase change volume inside a contact hole (or “pore”) formed in a dielectric, primarily to reduce the RESET current. It is therefore essential to examine alternatives to sput- ter deposition for GST and other phase change materials that can successfully fill higher aspect-ratio vias. 

## D. Processing 

## 1. Deposition 

