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Addressing PCM elevated temp data retention

Posted: 01 Sep 2015 ?? ?Print Version ?Bookmark and Share

Keywords:phase change memory? PCM? Ge? segregation model? Elevated temperature data retention?

For the joint STMicroelectronics and CEA-LETI phase change memory (PCM) paper at VLSI2015, the word forming appears to have been replaced by gentler and less prejudicial terms like "initial seasoning procedure," "initialisation" and "electrical activation." These terms mean the final part of the PCM memory cell fabrication is carried out electrically in situ.

Such word games should not detract from what is by any standards a very impressive piece of detailed analytical work which looked at the problem of composition changes or segregation in "dome" and "edge" type PCM cells and demonstrated how they can be used to advantage to solve the problems of PCM elevated temperature data retention up to 240C.

Elevated temperature data retention is a fundamental problem for PCM. In principle, the solution to the problem is simple: start with a Ge+ rich or doped active GST material composition that in its amorphous state has a high crystallisation temperature. The problem is during operation the Ge will move out of the active volume, especially in the case of the dome or half-cylinder PCM; cell structures and elevated temperature data retention properties become compromised. In this case, the element separation is normal to the direction of current flow. In the past. most of the focus on element separation problems in PCM structures were associated with electro-migration occurring in the direction of current flow.

The authors of Operation Fundamentals in 12Mb Phase Change Memory Based on Innovative Ge-Rich GST Materials Featuring High Reliability Performance [ref 1] have discovered and explained an interesting effect. If the starting material is made very geranium rich (GST +45%Ge) Ge++, the germanium will segregate as expected, leaving the composition of the active core of the device with a Ge+ composition required (GST+25%Ge) for its superior elevated temperature data retention properties, with a crystallisation temperature of 250C. Instead of continuing, the segregation process stops because the high concentration gradient of Ge surrounding the active core acts to oppose any further segregation as illustrated in figure 1. My illustration in figure 1 summarising the results is supported in [ref 1] by extensive STEM/EELS results.

Figure 1: Cross section illustrating how a very enriched segregated layer of Ge (blue) builds up along the cylindrical active region, the resulting concentration gradient acts to oppose any further segregation.

Although the authors indicate that after only one RESET pulse Ge segregation has occurred, it is not clear how many more pulses are needed to complete the process. Perhaps the element of time is implied in their words "initial seasoning procedure."

Outside of the segregated GE rich (Ge+++) layer, there are a number of features of the cell structures reported that are not part of the normal bill of fare for those familiar with the operation of dome and half-cylinder PCM cell structures. Normally in this type of structure only the region within the dome or H-C is involved in the operation of the device, here during the initial forming a region that reaches through the cylindrical crystal surface to the upper electrode is modified from its as-deposited poly-crystalline state.

This may have something to do with the fact segregated Ge has a different resistance from the deposited GST and changes the normal radial current distribution from the cylindrical surface.

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