18 spacer process[148], resist trimming[60], or by the key- hole transfer process[103], followed by Chemical Mechan- ical Polishing (CMP) for planarization. The processing of phase change materials is discussed in more detail in Section III D. Prototype devices can use e-beam pattern- ing to define the heater but this is too slow to be used commercially. The thin film of the phase change material (or a stack of different phase change materials[149] with varying alloy concentrations) can then be deposited over the planarized feature using standard techniques such as Physical Vapor Deposition (PVD) or Chemical Va- por Deposition (CVD). The top electrode contact (TEC) is also deposited, usually without breaking vacuum. The simplicity of the phase change materials portion of the process — and the ability to define the CD before any novel materials are introduced — represent two of the most attractive features of the mushroom cell. The deposited films are then patterned into islands us- ing conventional lithography to form individual cells, and isolated and encapsulated using thermally-insulating di- electric materials such as Si$_{3}$N$_{4}$[143]. A variety of mate- rials engineering techniques have been introduced to op- timize the cell performance, especially the minimization of RESET current. These include increasing the resis- tivity of either the electrode material[150] or the phase change material[133], and decreasing the thermal diffu- sive losses through both the top and bottom electrode region[130, 151]. Although cells with horizontal heater electrodes have been demonstrated[152], the vast majority of contact- minimized cells closely resemble the mushroom cell. One popular variant is the ring-electrode mushroom cell, where the heater electrode consists of a thin ring of metal surrounding a center dielectric core[153]. The incentive here is to reduce both RESET current and variability by decreasing the effective area, since compared to a normal heater the metal annulus has a smaller area which also scales only linearly with CD. FIG. 12: Phase-change device archetypes: (a) A typical contact- minimized cell, the mushroom cell, forces current to pass through a small aperture formed by the intersection of one electrode and the phase change material. (b) A typical volume-minimized cell, the “pore” cell, confines the volume of the phase change material in order to create a small cross-section within the PCM device. Reprinted with permission from Reference [32] ( c ○ 2008 IBM). 2. Volume-minimized cell 1. Contact-minimized cell The typical volume-minimized cell structures tend to be a bit more thermally efficient, offering the potential for lower RESET current requirements compared to the contact-minimized structures[41]. The most common contact-minimized cell structure is the mushroom cell, where a narrow cylindrical metal elec- trode contacts a thin film of phase change material. Fig- ure 13 shows TEM images of mushroom cells in the SET state (a) and in the RESET state (b). In the RESET state, an amorphous dome of the phase change material — resembling the cap of a mushroom, thus the name — plugs the critical current path of the memory cell, result- ing in an overall high resistance state for the cell. The bottom electrode contact (BEC), typically made of TiN, is the smallest and thus most critical dimension (CD) in this cell. It is common to see this described as the “heater,” although the cell works most efficiently when the heat is mostly generated in the phase change material at the top of this BEC. The sub-lithographic BEC can either be formed by a Significant research efforts have been spent explor- ing a variety of volume-minimized cell structures, ow- ing to their superior scaling characteristics. However, achieving such a structure can be a challenge, requir- ing the development of processing technologies that can successfully confine the phase change material within a sub-lithographic feature. The most obvious structure in this category is the pillar cell (Figure 14), where a nar- row cylinder of phase change material sits between two electrodes[138]. This cell is fabricated in a similar fashion to the mushroom cell, with a thin film stack of the phase change material and top electrode material deposited atop a bottom electrode, and then patterned. However, in the pillar cell, the BEC is large, and it is the phase change material that must be successfully and reliably patterned into sub-lithographic islands. This patterning