Design of Multi-States Storage Medium for Phase Change Memory

A method has been introduced to realize multi-states storage in phase change memory (PCM) by using stacked phase change films with different amorphous resistivity. The amorphous resistivity of each phase change layer was selected to make the stacked films crystallize successively with the increase of current, and so obtain four resistance states. The thickness of each phase change layer was also optimized to ensure enough resistance difference between neighboring storage state. The thermal conduction simulation was performed, and the results indicate that the stacked Si–Sb–Te films with suitable composition, resistivity and thickness in each layer can crystallize successively with the increase of current and realize multi-states storage in PCM. A memory cell with stacked film structure (Si 16 Sb 33 Te 51 /Si 4 Sb 46 Te 50 /Si 11 Sb 39 Te 50 ) was prepared. The film thicknesses of Si 16 Sb 33 Te 51 , Si 4 Sb 46 Te 50 and Si 11 Sb 39 Te 50 were optimized to be 40, 80, and 20 nm, respectively. The current–voltage ( I – V ) characteristics and the resistance–current ( R – I ) characteristics were measured. The results confirmed the four-state storage capability of the memory cell with this stacked film structure.