1. Technical Field
The present invention relates generally to phase change memories that use chalcogenide materials.
2. Description of the Related Art
Phase change memory devices use phase change materials, i.e., materials that may be electrically switched between a generally amorphous and a generally crystalline state, for electronic memory application. One type of memory element utilizes a phase change material that is electrically switched between a structural state of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. The state of the phase change materials is also non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until changed by another programming event, as that value represents a phase or physical state of the material (e.g., crystalline or amorphous). The state is unaffected by removing electrical power.
Thus, the use of phase-change storage elements has already been proposed in memory arrays formed by a plurality of memory cells arranged in rows and columns. In order to prevent the memory cells from being affected by noise caused by adjacent memory cells, generally each memory cell comprises a phase-change storage element and a selection element (such as an MOS transistor or a diode), coupled to the phase-change storage element.
A phase-change storage element comprises a resistive element (also called a heater) and a programmable element made of a chalcogenide, also called memory element. Generally, the resistive element and the programmable element are formed by physical stacking of layers including an ovonic unified memory (OUM).
The memory cells are addressed through metal lines or connections of copper, forming row and column lines. Therefore, when the cells are made using the OUM technology, their manufacturing should be compatible with copper backend step. That is, migration of copper into the chalcogenide layers forming the programmable elements should be prevented in order not to compromise the electrical characteristics of the memory cell.