19 FIG. 14: (a) TEM cross-section of a pillar cell with an FET ac- cess device. (b) Close-up TEM cross-section of GST/TiN pillar. The simulated RESET current dependence of this device is shown in Fig- ure 18(b). Reprinted with permission from Reference [138] ( c ○ 2006 IEEE). FIG. 13: TEM cross-sections of a mushroom cell PCM element in the (a) SET state and (b) RESET state. In the SET state, the phase change material is polycrystalline throughout. In the RESET state, a “mushroom cap” of amorphous phase change material restricts the current flow through the bottom electrode. Reprinted with permission from Reference [154] ( c ○ 2009 Springer). can be performed in various ways, including lithography followed by resist trimming[138]. In addition to the chal- lenges in controlling the size of the patterned islands, this cell structure also suffers from the drawback that the Reactive Ion Etch (RIE) of the phase change mate- rial can form a thin layer of altered alloy composition at the surface, strongly affecting the performance and yield of the cell[155]. This is discussed in further detail in Sec- tion III D. A modified version of the pillar cell structure is the pore cell[103], where a sub-lithographic hole formed in an insulating material atop the BEC is filled with the phase change material(Figure 15). Conformal filling of nanoscale holes with high aspect ratio is difficult using conventional PVD processes; hence development of CVD or Atomic Layer Deposition (ALD) technology for phase change materials will be necessary to enable continued scaling of pore cell devices[156]. As with nearly all PCM cell designs, the dimensions and aspect ratio of the phase change material region critically influences the RESET current. minimized and volume-minimized approaches. The µ - trench cell is an extension of the bridge concept, with a PCM element formed at the intersection of an underly- ing sidewall-deposited CVD TiN bottom electrode and a trench of phase change material formed at right an- gles across this electrode (Figure 16). The dash-confined cell[134] is an extension of the pore cell idea, except that the bottom electrode contact is formed by a spacer pro- cess. A CVD process is then employed to fill in the rect- angular sub-lithographic holes formed by the recess etch into the metal BEC (Figure 17). In all three of these cases, one critical dimension associated with the limiting cross-sectional aperture is controlled by a thin film depo- sition process. As a result, only one dimension inherits the variability of the lithography process. Conversely, of course, only one dimension will enjoy the associated scal- ing benefits as F shrinks from one technology node to the The µ -trench cell[139] and the dash-confined cell[134] are examples of PCM cells that combine the contact- The most advanced scaling demonstration of PCM technology to date was realized using the volume- confined bridge cell[42], which consists of a narrow line of ultrathin phase change material bridging two underlying electrodes (Figure 9). The cross-sectional area of this de- vice is determined by film thickness in one direction and by electron-beam lithography in the other, allowing the realization of functional cells with cross-sectional area of about 60 nm 2 and RESET current requirement of about 80 µ A. 3. Hybrid PCM cells FIG. 15: TEM cross-section of a 45 nm bottom CD low aspect ratio pore cell filled with a PVD GST process. The simulated RESET current dependence of this device is shown in Figure 18(c). Reprinted with permission from Reference [103], ( c ○ 2007 IEEE).