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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?

The next step is to link the area related current density of figure 3 to device characteristics. The green and brown curves in figure 2 show the resulting device characteristics, i.e. Jb for devices with annular contact thickness of 3 nm and 5 nm. Any damaging effects at the active material-electrode interface remain because Jc has not changed. For a 50% reduction, the gains are not significant and for a 10-nm diameter device, they hardly appear to be worth the effort.

In the past, most of the annular devices reported have had diameters of the order 40 nm or greater. As will be discussed in a later section, thermal coupling may provide some cause for optimism to those wishing to pursue sub-20-nm PCM developments.

The initiating molten hotspot (IMH) model
The physical model I propose to account for the "flat" part of the current density characteristics is based on the concept that during the early part of the reset pulse, an initial minimum-size molten hot spot forms at some defect on the surface or within the matrix of the crystal (figure 4). The model suggests that this hot spot is on the order 3 nm to 5 nm in diameter.

Figure 4: During the early part of the reset pulse, a molten hot spot (red dot) on the order 3 to 5 nm forms within a region of the crystalline material that has risen to near-melting temperature (orange); this effect may account for the quasi-constant current density shown in figure 2. The dotted lines illustrate the effects of an aperture that defines the reset volume of the device.

My model suggests that the magnitude of the reset current is determined by the thermal conditions required to form the initiating molten hot spot in the crystallized active material. These thermal conditions are necessary to preheat a substantial volume of the crystallized active material to a near-melting temperature Tpre defined as

Tpre = TmT [2]

where Tm is the material's melting temperature; then to raise the hot spot to Tm.

Figure 4(a) illustrates the equi-potentials and current-flow contours from a PCM electrode structure, showing the region at Tpre and the hot spot at Tm. This model assumes a PCM device with a hemisphere-like reset volume. It is important to note that the hemisphere structure is losing popularity in favour of designs in which an aperture or pore (dotted lines in figure 4) limits the volume. The latter structure serves the need for more closely packed PCM arrays.

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