(Invited) ALD of Phase Change and Threshold Switching Materials for Next-Generation Nonvolatile Memory Devices

摘要

Memory technology continues to be at a crossroads. Established devices such as DRAM, Flash, and SRAM remain dominant, but each are facing increasingly formidable challenges in scaling, cost, and performance. Emerging memory technologies are attempting to displace these three or to find application spaces between those they traditionally support. However, no emerging memory has been able to demonstrate significant advantages over the incumbents, resulting in a proliferation of options such as phase change memory (PCM), filamentary resistive memory (ReRAM), ferroelectric memory (FERAM), magnetic memory (e.g. STT-MRAM and SOT-MRAM), and several others. Of these, the most successful has been PCM as evidenced by the commercial introduction of PCM based crosspoint memory in 2017. PCM based crosspoint memory has had limited market penetration in part due to the high manufacturing cost; the current layer-by-layer approach to PCM fabrication makes scaling difficult. In order to be cost competitive, crosspoint arrays will need to be manufactured in a style similar to that of 3D V-NAND where the active layers are deposited conformally inside deep features simultaneously forming hundreds of junctions. Chemical Vapor Deposition or Atomic Layer Deposition of these films is likely required to make this a reality. Some progress in materials for the PCM memory element has been reported; however, a remaining concern for crosspoint memory is the selector device, used in suppressing the sneak path current, consisting of a non-linear two-terminal device to properly select each memory element. In addition to high non-linearity, a selector device also requires high on-state current density J_(ON), fast switching time, high endurance, stability, high scalability and back-end-of-line (BEOL) integration. Several selector devices have been proposed to date, e.g., mixed-ionic-electronic-conduction (MIEC), field-assisted superlinear threshold (FAST), and amorphous silicon (a-Si) selectors, but Ovonic Threshold Switch (OTS) composed of chalcogenide multinary materials clearly appears as the best candidate to cover all the mentioned requirements for high-density crosspoint applications.