The third PCM structure (continued)
The PCM cell is defined by a cross over of the active material and a sub-lithographic electrode, usually of a sidewall deposited thin film that can be of a width many times less than the photolithographic node of the process used. The active material and its overlay electrode are deposited (sputtered or CVD) in an orthogonal direction into the smallest possible width, slot, or trench, cut into a dielectric and in an array spaced at the minimum feature pitch.
As Figure 1 illustrates, a more appropriate name for this structure might be "half barrel." With lithographic scaling, the two side walls of the structure will move inwards, as indicated by black arrows in Figure 1, with very little change needed to the sub-lithographic lower electrode. The lower electrode is usually described as the heater/electrode. It is used as a heater to localize the dome half (barrel) on the lower heater/electrode, and its contact surface may need to reach the 600oC required to melt the chalcogenide.
Figure 1 illustrates two versions of the lower surface of the molten region for reset in relation to the heater/electrode (the red colored regions). One is with all of the molten active material in contact with the dielectric surface. The second, representing the extreme of a low current reset, creates a match head structure.
Clearly, if there is any diffusion or reaction between the dielectric surface and the molten chalcogenide, conditions are created for tracking, which results in data retention failure or reset failure. This is especially true if the remaining gap between the top corner of the heater/electrode and crystallized GST upper electrode is very small. As a derivative structure, a version of the match-head shape can also be formed in the dielectric surface.
Maximum current density for these devices must be calculated using the length x width of the heater electrode surface. At that contact surface, the current density is at its highest value, reducing with the radial flow outward to the crystallized material serving as the second electrode. This creates a special difficulty with the structure; most of the measured resistance of a PCM device is as a result of the region close to the heater/electrode. The measured resistance is dominated by the logarithmic term (Integral 1/t), where t is the thickness of the lower heater/electrode.
Therefore, any changes in the resistance of the active material close to the heater/electrode, caused by electro-migration or reaction with the heater/electrode, alloying or diffusion, will have a more significant effect on two terminal electrical resistance values. In which case, there is likely to be an even more significant reliability consequence. The optimum reset current may become ineffective in resetting the device, a reset failure. Or it may result in the creation of a match head soft reset, leading to the possibility of tracking failure described earlier.