The present invention relates to a phase change memory.
Phase change memories use a class of materials that have the property of switching between two phases having distinct electrical characteristics, associated to two different crystallographic structures of the material, and precisely an amorphous, disordered phase and a crystalline or polycrystalline, ordered phase. The two phases are hence associated to resistivities of considerably different values.
Currently, the alloys of elements of group VI of the periodic table, such as Te or Se, referred to as chalcogenides or chalcogenic materials, can be used advantageously in phase change memory cells. The currently most promising chalcogenide is formed from an alloy of Ge, Sb and Te (Ge2Sb2Te5).
In the chalcogenides, the resistivity may vary by two or more orders of magnitude when the material passes from the amorphous (more resistive) phase to the crystalline (more conductive) phase, and vice versa.
Phase change can be obtained by locally increasing the temperature. Below 150° C., both the phases are relatively stable, with an amorphous bit tending to gradually crystallize over a period of years if held at 150° C. Starting from an amorphous state, and raising the temperature above 200° C., where there is a rapid nucleation of the crystallites and, if the material is kept at the crystallization temperature for a sufficiently long time, it undergoes a phase change and becomes crystalline. Preferably, the material is cooled off slowly to better assure the crystalline state. To bring the chalcogenide back to the amorphous state it is necessary to raise the temperature above the melting temperature (approximately 600° C.) and then rapidly cool off the chalcogenide.