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Annular electrodes as PCM solution (Part 1)

Posted: 24 May 2013 ?? ?Print Version ?Bookmark and Share

Keywords:phase change memory? annular electrode? lithography?

A possible advantage is that for the annular device, there will be a flattening of the current density curve for Jb the as they approach the (d =2t) limiting conditions (figure 6, green and brown curves). Figure 6 is a section of the J = f(d) curve extracted from figure 2 for the sub-20-nm diameter devices.

It illustrates what might be possible with thermal coupling effects when the distance between the inside edges of an annulus electrode are less than 20 nm. I have made a best estimate for an annular electrode with a width of 3 nm. A word of caution: This is merely for illustrative purposes; the possibility of lesser or greater gains might be the reality.

There are ways to obtain a more realistic estimate of the value of Jc when the annulus is very thin and the advantages of annular coupling are in play. Consider the annulus as divided into a series of small close-coupled solid-electrode devices having an approximately square electrode with sides equal to the width of the electrode. A 20-nm-diameter device with a 3-nm annulus would provide approximately 18 such devices.

Figure 6: Segment of the J = f(d) curve extracted from figure 2 for sub-20-nm diameter devices shows that thermal coupling may reduce current density requirements.

Now consider such a device in isolation (figure 7). Determining the current density of this device from figures 2 or 6, or from the values provided in [5] would give a value of about 2 x 108 A/cm2. The device in isolation will suffer heat losses from all sides, but when assembled into an annulus, the main source of heat loss will only be from one side. This means that Jc for the annular device will be reduced by a factor of at least two to a value of 1 x 108 A/cm2. If we now calculate the value of Jb that will be approximately 5 x 107 A/cm2 slightly higher than the value obtained from simple geometric calculations from Jc at 20 nm diameter.

Figure 7: We can model an annular electrode (left) as a series of square electrodes (right). Adjacent device and thermal cross coupling from circular device means heat loss in only one direction.

It will be up to others to provide real numbers based on real devices. My simple first order analysis does not allow one to conclude that the annular electrode will allow PCM a safe route to sub-20-nm devices. This is just the start, though. In part 2 of this article, we will explore whether a newly proposed self-assembly technique might offer a solution to sub-20-nm PCM scaling problems.

1. P. Clarke, "IBM, SK Hynix sign phase-change memory deal," Memory Designline, (2012).
2. W. I. Park, B. K. You, et al., "Self-Assembled Incorporation of Modulated Block Copolymer Nanostructures in Phase-Change Memory for Switching Power Reduction," ACS Nano, 7 [3], (2013).
3. J.Y. Wu, M. Breitwisch, et al., "A low power phase change memory using thermally confined TaN/TiN bottom electrode," Proc IEDM 2011, pp43-46 (2011).
4. S H Lee et al., "Highly productive PCRAM technology platform and full chip operation: based on 4F2 (84 nm pitch P cell scheme for 1 Gb and beyond," Proc IEDM 2011, pp47-50 (2011).
5. R. Neale, "PCM Scalability: The Myth (Part 2)," Memory Designline, (2010); R. Neale, "PCM scalabilityMyth or realistic device projection?," Memory Designline, (2010).
6. IEDM 2008 Paper on ring electrodes.

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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