US Patent 7787291 Programming a multilevel phase change memory cell

Multilevel phase change memory cells may be programmed forming amorphous regions of amorphous phase change material in a storage region of the phase change memory cell. Crystalline paths of crystalline phase change material are formed through the amorphous regions of amorphous phase change material. Lengths of the crystalline paths are controlled so that at least a first crystalline path has a first length in a first programming state and a second crystalline path has a second length, different from the first length, in a second programming state.