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Annular electrodes as PCM solution (Part 2)

Posted: 28 May 2013 ?? ?Print Version ?Bookmark and Share

Keywords:self-assembly? sub-lithographic? fabrication process?

These results raise the first possible limiting question: What is the minimum size surface disc is it possible to form using the S-A technique? Reference 2 provides examples of disks of the order 20 nm with the smallest disks estimated by this writer from the published micrographs as approximately 15 nm.

How the structures grow
For a circular aperture of 20-nm diameter with a fill factor of 50%, the disc diameter is 10 nm, 7 nm, and 5 nm for two, five, and eight disks respectively; for sub-20-nm apertures these numbers will scale.

Compared with a cross-section view, the plan view in figure 3 of a conceptual 20-nm-diameter device with 5 nm oxide disks gives a better idea of the shape of the electrode surface. Cross section views of S-A devices can lead to the misleading conclusion that S-A devices are a series ofand can be treated assmall, isolated devices, which the plan view quickly refutes. Only if the packing arrangement results in all the disks touching can the surface be considered as a set of discrete devices.

Figure 3: Plan view of core device (green), electrodes (grey), and oxide disks (yellow) during the growth of the molten region during reset for self-assembled device shows how the initiating molten hotspot forms (a) and starts to expand (b) to encompass the central core annual region (c). It then further expands along all exposed electrode surfaces (d), expanding into oxide surface (e) to completion.

The side elevation as shown in the original paper gives a false impression. In reality, the electrodes shown in figure 3 consist of the spaces between the oxide disks. The resulting structure can be considered as a large number of overlapping annular electrode devices or a device with a single electrode formed as an interconnected hexagonal network. Although figure 3 is a plan view of a seven-oxide-disc PCM device, much of what follows will be applicable to larger devices that extend in two dimensions. In this small device, the white coloured areas will also be electrode surface and in an extended device will be part of the adjacent overlapping annulus pattern.

The electrode surface coloured green is the core device. Based on the initiating molten hotspot (IMH) model, the sequence of events would be as follows. First, the IMH forms at some point on the green electrode surface at the mid-point between three oxide disks, it then expands to cover the surface of the core annulus electrode. Next, quite rapidly, the molten material extends to cover all the exposed electrode surface, with fill-in over the oxide in the central region. That expansion will also be 3-D, occurring in a direction normal to the plane of the electrode. The growth of the molten region then proceeds to completion with extension over the oxide and exposed electrode surface, as shown in figure 3. As described in the IMH model, a twofold role for the reset pulse is required. In the early part of the reset pulse, the current density must be sufficient to raise the temperature of a substantial volume of the crystallized active material to a temperature of (Tm-T) and the hotspot to Tm, the melting temperature.

It is interesting to compare these results with a PCM device using an annular electrode and to understand why the reset current and current density will be different. For a 20-nm aperture, a 3-nm-wide annular electrode can be considered as a fill factor of approximately 50%. That is, in effect, the equivalent to a single oxide disc with a diameter of approximately 14 nm.

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