Reconfigurable circuits have been widely used in the semiconductor industry for field programmable gate arrays (FPGAs) and for repair of a defective memory element. The FPGA consists of a set of simple, configurable logic blocks in an array with interspersed switches that can rearrange the interconnection between the logic blocks.
Reconfigurable circuits are also expected to play a significant role in three-dimensional (3D) integration technology that is presently being developed. Three-dimensional integration fabricates multilayer structures which are vertically stacked one upon another that can form a single chip combination with different functionalities. In these multilayered and multifunctional structures, reconfigurable circuit connection is typically needed to provide controllable logic functionality, memory repair, data encryption as well as other functions.
The programmable via is an enabling technology for high-performance reconfigurable logic applications without the trade offs in low logic gate density and power. Phase change materials are an attractive option for this application, but to date, have drawn the most attention from semiconductor memory developers as a possible replacement for flash memory.
Phase change materials are typically ternary alloys of germanium (Ge), antimony (Sb) and tellurium (Te), with a typical composition being Ge2Sb2Te5. Other compositions such as GeSb and GeSb4 (including substitution/addition of other elements, i.e., dopants) are under active investigation.
At room temperature, and up to moderately elevated temperatures, phase change materials are stable in two phases, a crystalline phase, which is a moderately good conductor of electricity, and an amorphous phase, which is insulating. The phases are interconverted by thermal cycling. The thermal cycling consists of (i) the “RESET” (or OFF) pulse, which is the conversion of the phase change material from a crystalline phase to an amorphous phase. In this thermal cycle, the temperature is raised above the melting point of the phase change material, followed by a rapid quench in a time t1 as a result of which the disordered arrangement of atoms in the melt is retained. (ii) The “SET” (or ON) pulse, in which an anneal at a lower temperature is performed for a somewhat longer time t2 which enables the conversion from the amorphous phase back into the crystalline phase.
The programmable via is comprised of a phase change material (P CM), which can be switched between resistive (OFF-amorphous) and conductive (ON-crystalline) states with an integrated heating element. The switching process is typically accomplished by a current pulse passed through the heating element which bisects the via containing the phase change material. The OFF switching operation is accomplished by an abrupt high-current pulse to melt and quench/amorphize a thin region of the phase change via adjacent to the heating element. In the ON switching operation, a relatively low current, but longer, pulse is applied through the heating element to anneal the amorphous PCM to the crystalline state.
In the prior art, only the top half of the via (plus the heating element) is implemented and introduced, where the via resistance has been measured between the top contact of the via and one of the heating element contacts.
Although the concept of programmable via structures and their fabrication have been purposed, there has been no disclosure of a circuit design or system design which describes the use and incorporation of such devices to achieve better performance of PCM switching.