Will PCM scale?
The activation energy for the three calculated seeded-bridge model straight lines is equal to or less than 2.7 eV; not unexpected, as Kalb reports values of 2.74, 2.35 and 1.89 eV for reducing germanium concentrations of GST respectively. This low value of activation energy and the unsatisfactory PCM performance of devices using these compositions is the reason why a number of doped and enriched versions of PCM active material were needed and developed.
It is interesting to note that the two Samsung papers, linked to the 17.5 nm × 7 nm structure and the data in figures 1 and 2, initially reported extrapolated PCM lifetime values from ETDR tests of 4.5 years and, later, 15 months at 85ºC. There are other reasons that might explain this, such as element separation or a less optimistic and more accurate extrapolation. Those changes may represent some evidence that the seeded-bridge model is accurate and crystal growth rates do not scale; i.e. PCM materials that demonstrated good ETDR results at higher lithographic nodes have not scaled. If the material was initially evaluated in a spherical PCM structure, that would exacerbate the problem because that structure is less sensitive to resistance changes caused by growth from the crystal electrode. The crystal growth scaling argument would also suggest that MLC-PCM devices are likely to show different ETDR test results for each data level.
For those who still see a commercial and competitive future for PCM and wish to pursue it, Parts 1 and 2 of this paper raise some important questions with respect to that future, especially in relation to lithographic scaling. Structure-dependant ETDR now adds a new problem to the list of other problems already facing PCM developers.
As suggested in Part 1, a move away from crystal electrodes back to the original all-metal electrodes might offer a route to improving ETDR performance for all materials and remove any need to trade off write bandwidth to achieve that performance. As always reduction to practice in a realistic PCM array environment will be the way to establish the validity of models of operation and scaling claims.
Note: I would like to acknowledge that in preparing Parts 1 and 2 of this paper I have been helped by a number of off-the-record comments and helpful critique from those working in the field.
References Part 2
(continuing sequentially from part 1)
 S. H. Lee, et al., “Highly Productive PCRAM Technology Platform and Full Chip Operation: Based on 4F2
(84-nm Pitch) Cell Scheme for 1 Gb and Beyond,” Proc. IEDM11-47.
 H Y Cheng et al., “A High Performance Phase Change Memory with Fast Switching Speed and High Temperature Retention by Engineering the Gex
Phase Change Material,” by Proc IEDM11-51.
 M J Kang et al., “PRAM Cell Technology and Characterization in 20nm node size,” Proc IEDM11-39; I.S Kim et al., “High Performance PRAM Cell Scalable To Sub-20nm,” by Proc VLSI 2010-208.
 J.A. Kalb, "Crystallization kinetics," in Phase change materials: Science and applications
(edited by S. Raoux and M. Wuttig, Springer (2009), Chapter 7.
About the author
Ron Neale is the former editor-in-chief of Electronic Engineering. Also, he is the co-author of "Nonvolatile and reprogrammable, the read-mostly memory is here," by R.G.Neale, D.L.Nelson and Gordon E. Moore, Electronics, pp56-60, Sept. 28, 1970.
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