12-state multi-level cell storage implemented in a 128 Mb phase change memory chip

Abstract

128 Mb Phase Change Memory (PCM) chips show potential for many applications in artificial intelligence. A PCM cell often has a sandwich structure that consists of a TiN bottom electrode, a phase change material, and a top metal. TiN films prepared by atomic layer deposition have high thermal stability, and a WN coating layer on the TiN electrode can prevent oxidation in the electric and thermal field, achieving high endurance of the TiN electrode over 10¹¹ cycles. In the phase change material of carbon-doped Ge₂Te₂Te₅ (CGST), C–C chains and C clusters precipitate at the Ge₂Te₂Te₅ (GST) grain boundaries, which effectively refines the grain size of GST. The C confinement enhances the Ge/Sb atomic migration barrier and suppresses the composition segregation in the Reset/Set operation process and the atomic relaxation of the CGST material. As a result, the endurance and conductivity-drift of the PCM chip were enhanced. Finally, stability over 5 × 10⁸ cycles and 12 multi-level stable states were achieved in the 128 Mb PCM chip. This work presents a step towards the realization of large-scale and energy-efficient neuromorphic computing systems.