Realistic limits to computation

As far as ultra-dense crossbars are related to correspondingly dense wire arrays, the crossbar route to tera-scale integration depends on the availability of preparation techniques for wire arrays with density of 106 cm−1 or more. This linear density implies, for a planar arrangement, a pitch of 10 nm or less, which not only is at the limits of the current technical possibilities, but also can modify appreciably the band structure of silicon. A dramatic increase of density could only be achieved if it were possible to organize the nanowires in a three-dimensional fashion still exploiting the planar technology. In this work processes are described for the fabrication of out-of-plane, vertically arranged, polycrystalline silicon nanowires via a rigorously top-down batch process. These techniques are consistent with the production of wire arrays with linear density (projected on the surface) larger than that achievable with any other proposed top-down process. Used for the fabrication of the bottom wire arrays of crossbars, these processes should eventually allow a cross-point amount per unit area in excess of 1012 cm−2, thus providing candidate technologies for ultra tera scale integration. The technique developed for such out-of-plane crossbars can be used to implement new functions like coils, solenoids and transformers.