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PCM progress report no. 6: Recent advances in phase change memory (Part 2)

Posted: 15 Mar 2012 ?? ?Print Version ?Bookmark and Share

Keywords:phase change memory? array? physical vapor deposition?

Part 1 of this series reviews structural and materials advances, focusing on both benefits and challenges.

In the 2011 IEEE International Electron Devices Meeting1, Hynix presented what was clearly top billed as a phase change memory (PCM) fabrication platform and then used it to demonstrate how, at the 42-nm node, they could fabricate 1-Gb PCM array with a 4F2 cell size.

Although the paper provided a great deal of detail by way of characteristics at the cell level, little was provided of the overall array performance, especially in terms of power dissipation, read/write data bandwidths, performance and write/erase lifetime. The 6.07-mm by 5.47-mm array was configured as 16 partitions of 64 Mb, with further subdivision of each of the 16 partitions into 32 Mats of 2 Mb. The active PCM material was stated to be a germanium-antimony-tellurium (GeSbTe) based alloy.

Shown in figure 1 is a cutaway illustration of two bits of the array. Of particular note are: a self-aligning cell structure; the poly-silicon matrix isolation device and the square, edge of film contact, to the phase change material.

Figure 1: Simplified cutaway section of two bits of Hynix 1-Gb PCM.

Hynix addressed the challenging problem for PCM arrays of the matrix isolation device (figure 1). It was claimed their thin film poly-silicon diode will eventually allow for the construction of 3D stacked PCM matrices; a claim that ignores any considerations of thermal cross talk discussed in part 1 of this story.

The ability to use a deposited film matrix-isolating element does offer fabrication advantages; Intel2 demonstrated the use of chalcogenide threshold switches in that role for a stacked PCM array, while IBM3 proposed the use of an ionic conduction switch as a possible matrix-isolating device. The former appears to have been abandoned and as far as I am aware the latter has not been pursued in the direction of any commercial PCM offering.

Figure 2: Confining the active region, the growing complexity of the PCM electrode structure.

Hynix informs us that they consider the use of physical vapor deposition (PVD) and atomic layer deposition (ALD) deposition techniques will facilitate the scaling of their structure. Also, the cell structure offers a degree of flexibility in terms of the chalcogenide-electrode interface. For example, as well as the original flat square edge electrode surface, by removing some of the dielectric core (colored yellow) inside the edge electrode it is possible to produce a more confining electrode structure (figure 2d), or even with scaling use a solid electrode surface.

Figure 3: I-V Characteristics of Hynix cell with (right) component parts that are summed in series.

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