Phase change memory engineering and integration with CMOS technology

Abstract

Phase change random access memory (PCRAM) is one of the strongest contenders to replace the current Flash memory technology, which is reaching its fundamental scaling limits. PCRAM is an electrically-induced thermally-activated device in which joule heating plays an important role in phase transformation and data storage. Not only does PCRAM exhibit fast switching speed and high endurance, it also has excellent scaling capability and compatibility with complementary metal-oxide-semiconductor (CMOS) technology. Materials engineering may be performed to tailor the properties of the phase-change material for improvement of memory device characteristics. This thesis summarizes work on advanced materials and device engineering for PCRAM technology. The energy band alignment between Ge-Sb-Te based phase change materials and common microelectronic materials such as dielectrics and metals was first investigated. Significant Fermi level pinning at the interface between phase change materials and metals was discovered. The results are useful for calculation of leakage currents between closely spaced cells as well as the contact resistance in PCRAM. Nitrogen-doped Ge2Sb2Te5 PCRAM devices were fabricated and the dependence of the electrical characteristics on nitrogen content in Ge2Sb2Te5 was investigated next. Two regimes of the crystallization process were observed, depending on the nitrogen content in Ge2Sb2Te5. Introduction of a small amount of nitrogen in Ge2Sb2Te5 was found to be useful for optimization of device performance. The criterion for fast switching and good device performances were evaluated based on material and device characterization. Energy band alignment studies show that metal silicides could possibly offer better contacts than conventional heater materials. Various metal silicides were investigated to assess their suitability as a contact material in PCRAM devices. Memory cells with metal silicide contacts and optimized nitrogen-doped Ge2Sb2Te5 were fabricated. Good device performance was achieved. Further improvement to this structure was made by inserting a thin dielectric layer at the interface between the silicide and phase change layer. The low thermal conductivity dielectric layer reduces thermal diffusion, thus enabling reduction of the reset current. Exploration of advanced materials and device designs opens up new avenues for compact PCRAM device design and integration with CMOS technology.