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really need to be. 

There are several ways a sub-lithographically sized aperture can be defined. Not surprisingly, the degree to which the aperture size can be controlled varies for each of these methods[154]. A direct way to reduce the size of a lithographically-defined hole is to implement a collar process, as shown in Figure 19. Similarly, a subtractive method can be used (as in the Pillar cell scheme[138]), where an island of lithographically-patterned photoresist is trimmed in size using RIE and then used as a mask to transfer the sub-lithographic pattern down into the underlying layers. Unfortunately, in both of these tech- niques, any variability in the diameter of the original hole 

## 2. Sources of structural variability 

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(or photoresist pillar) introduced by either lithography or etch (or resist development) transfers directly to the fi- nal CD. Thus the fractional variability (ΔCD / CD) can become uncomfortably large. 

The µ -trench cell[139], the ring bottom-electrode Mushroom cell[153], the dash-confined cell[134] and the Bridge cell[42] define one of the dimensions of the cross- sectional area of the aperture through film deposition. The thickness of such deposited film can be tightly con- trolled, especially for CVD and even more so for ALD techniques. This thickness can easily be much thinner than the lithographic dimension F (at least for current and near-future technology nodes). This method of defin- ing one dimension of the aperture by film thickness is inherently decoupled from any lithographic variability. However, for these schemes, lithography is still needed to define the “other” dimension of the aperture. 

A keyhole-transfer process has been developed for PCM devices which decouples both dimensions of the final aperture from lithography. Here, a keyhole is de- fined within a lithographically-defined hole by film depo- sition. Typically, a keyhole is undesired and indicates a failure to fill the hole. However, the advantage is that the dimension of the keyhole itself can be tightly con- trolled by accurate control over the hole depth (through film deposition) and over the etched undercut into the sidewalls of the hole, despite poor control over the ac- tual lithographically-defined diameter of the hole itself. Figure 20 describes this process and shows how the key- hole process can produce identical sub-lithographic holes (30nm) despite significant variation in the much larger lithographically-defined holes (243nm and 302nm). 

The keyhole-transfer process has been experimentally demonstrated to decouple the final aperture size from the initial lithographically-defined hole size[103]. Over a span of initial lithographically-defined hole sizes, the dis- tribution of RESET current was found to be consistently 

