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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?

Hynix1 did provide I = f (V) characteristics of the complete PCM cell, including the isolating device. From that, I have estimated an approximate value of the forward voltage drop of the poly-silicon diode, the basis of which is explained in figure 3. It is assumed that the PCM cell characteristics consist of three components: the constant voltage across the molten chalcogenide (approx 0.5 V), the forward voltage across the poly-silicon diode (approx 1 V), and the ohmic series resistance of the metalcontacts and metal interconnect. It would therefore appear that the poly-silicon diode is operating at a constant forward voltage of 1V, over the reset current range, with a current density of greater than 1 107 A/cm2. However, it is difficult to determine the temperature of the diode when the device is being reset.

The volume of material is about twice that of the PCM and is dissipating twice the power, suggesting the temperature of the diode is not too far removed from that of the PCM and must give some cause for concern.

Lifetime of more than one device
The IBM/Macronix write/erase lifetime guarantee of 108 cycles for their "golden composition" provides an opportunity to revisit the subject of write/erase lifetime. For PCM, the guarantee of 108 cycles is only of real value if it also guarantees all the other characteristics of the device will remain in tolerance at any point in the write/erase lifetime.

For me, the rules by which all claims for number of PCM write/erase cycles should be tested are:

a) A read-verify step should be included after each set/reset

b) More than one or two devices, preferably part of more than one array, are evaluated and the statistical distribution of set/reset resistance published.

c) The read voltage or current used to obtain the set/reset state resistance window should be clearly specified.

d) Write/erase lifetime testing should be carried out at both room and elevated temperature.

e) Elevated temperature data retention must be evaluated at the start, middle, and close to the end of the write/erase lifetime. Where possible, other PCM device parameters should also be presented.

f) If multiple iterative reset or set steps are used to produce a target set/reset resistance value, it should be made clear.

g) The write/erase lifetime testing should be done with the PCM device monolithically integrated into a memory array; the results for test patterns within the array are acceptable providing that is made clear.

h) For write/erase lifetime tests, in relation to the volume of material returned to the amorphous state by a given reset pulse(s), an indication of the volume fraction of that material that is crystallized during the set pulse should be provided.

I have earlier expressed concerns over the lack of understanding of the reasons for changes in the set/reset resistance values in the form of statistical jumps, cyclic behaviors, divergence, or the parallel changes in resistance and mixtures of those characteristics, in the results reported by different workers. Figure 4 summarizes the form or types of structure for the write/erase characteristics collected from the literature; the artificial flat curves obtained by multiple-try iterative write or erase have been omitted.

Figure 4: The unexplained structure of PCM resistance curves with write/erase lifetime.

If real progress is to be made, the question that must be answered is how the different curves are related to the structure, operating conditions, and composition of the active material. Are the causes a signature of something of a fundamental nature, such as the element separation; or from the same effect the permanent crystallization of some volume of the PCM material? Are they the result of a repositioning of the central core of the active volume in the plane of the contact electrode surface or normal to it, or a result of changes at the active material-contact interface?

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