15 FIG. 9: (a) Scanning electron microscope image of a phase change bridge and its TiN electrodes. (b) Cross-sectional transmission elec- tron microscope image of a 3nm-thick GeSb bridge. Reprinted with permission from Reference [42] ( c ○ 2006 IEEE). a complex manner, with the transistor supplying power to the PCM device as if it were a load resistor. The added complexity is that the dynamic resistance of the PCM device itself will in turn be a strong function of this supplied power[122]. Thus too tight a focus on RESET current can end up optimizing into a shallow minimum in RESET current which in fact is quite disadvantageous in terms of RESET power[128]. D. Scalability of prototype PCM devices protection. Since so many interlocking parameters influence the performance of PCM devices, especially as they become ultra-small, an important part of scalability studies is the fabrication and testing of prototype PCM devices. Be- cause only a modest number of such prototype devices are typically fabricated and tested in research environ- ments, these types of experiments cannot hope to predict the actual device reliability statistics (yield, endurance, and resistance distributions) that can be expected from full arrays. However, prototype devices are an extremely important test for the scalability of PCM — if you can- not get any ultra-small devices to operate correctly, then this is a bad sign for the future of the technology. Here we will report on the properties of one such prototype device: the bridge cell. The phase change bridge cell is a relative simple testing vehicle for studying novel phase change materials[42], ex- tending the line-device concept that had been introduced earlier[77] to ultra-small dimensions. In these studies, two 80nm-thick TiN electrodes separated by a planarized dielectric layer were typically used as the contacts, with a thin phase change bridge fabricated to connect the two electrodes. Bridge devices have been fabricated from various materials including undoped and doped Ge-Sb with 15 at. % Ge, Ge$_{2}$Sb$_{2}$Te$_{5}$, Ag- and In-doped Sb$_{2}$Te, Ge-Te with 15 at. % Ge, and thin Sb phase change materials[34, 42, 78]. After fabrication of these TiN bottom electrodes us- ing KrF lithography and chemical-mechanical polishing, a thin layer of phase change material (down to 3nm thick) was deposited by sputter deposition and capped with a thin SiO$_{2}$ layer to prevent oxidation. Electron- beam lithography was used to define the phase change bridge itself. Bridge widths (set by the e-beam lithog- raphy) varied between 20nm and 200nm, and the length (determined by the spacing between the underlying elec- trodes) ranged from 20nm to 500nm. Figure 9(a) shows a scanning electron microscope image of the phase change bridge and the TiN electrodes, while Figure 9(b) shows a cross-sectional transmission electron microscope image of a Ge-Sb bridge that is only 3nm thick. Negative pho- toresist was used to define the bridge so that the resist did not need to be removed after the fabrication process. Ar ion-milling was applied to transfer the exposed pho- toresist pattern into the phase change material, and a 50 nm thick layer of SiO$_{2}$ was subsequently deposited for Several device parameters were measured as a func- tion of device geometry. Current-voltage (I-V) curves re- vealed typical PCM behavior with threshold switching. By modifying the fabrication procedure so that the phase change material never experienced any temperatures over 120 $^{◦}$C, bridges that remained in the amorphous-as- deposited phase could be produced[34, 35]. This allowed the precise measurement of threshold switching as a func- tion of device length in a well-known geometry. Each material was found to have a unique threshold field, mea- sured by determining the threshold voltage as a function of device length. These fields were 8, 19, 39, 56 and 94 V/ µ m for Ge-Sb with 15 at. % Ge, Ag- and In-doped Sb$_{2}$Te, Ge-Te with 15 at. % Ge, Ge$_{2}$Sb$_{2}$Te$_{5}$, and thin Sb phase change materials, respectively[34, 35]. No leveling- off of the threshold voltage with length was observed, although the shortest bridge devices were 20nm, as op- posed to the nanowire devices where amorphous plugs shorter than 10nm were studied (Section II A). Unfor- tunately, line-edge-roughness in the TiN electrodes (see Figure 9(a)) led to shorts between the long TiN elec- trodes for separations smaller than 20nm. Figure 10 demonstrates the scaling behavior of bridge devices in terms of the RESET current. Both measured RESET These devices could be cycled through more than 30,000 SET-RESET cycles and the stored data was shown to survive temperature excursions up to 175 $^{◦}$C[42]. These ultra-small devices — down to devices with cross-sectional apertures as small as 60nm 2 — corre- spond to effective switching areas that will not be encoun- tered by mainstream device technology until the 22nm node, which Flash is expected to reach in 2015[5]. Thus these bridge-device demonstrations show that PCM will remain not only functional but robust through at least the 22nm technology node.