基于嵌入式3D堆叠闪存存储系统的可靠性关键技术研究

With the development of computer systems, the requirements on storage of systems become to design storages with high density and large space. NAND flash memory has been one of the most widely used storage devices. Recently, 3D NAND flash memory has been developed to match the requirements of advanced computer systems. However, 3D NAND flash memory has its disadvantage, which is the reliability issue. Currently, the reliability of 3D NAND flash memory has been degraded with its development, which will hold back the further development. Different from the reliability characteristics of planar NAND flash memory, the reason for the reliability issue of 3D NAND flash memory comes from at least two aspects: the stack design with more and more layers and the charge trap based storage cell. With these design and schemes, 3D NAND flash memory has serious read and write disturbance and retention loss. The reliability issue can be further degraded with the increases of storage layers and technology sizes. Improving the reliability issue of 3D NAND flash memory has been a key research topic for its further development. In the project, we will propose to optimize the reliability of 3D NAND flash memory by exploiting the operation and organization characteristics. First, we will design a reliability model based on several reliability factors, which will be very important for the optimization approaches. Second, we will propose to optimize the reliability issue from two aspects: reliability improvement for precise data on 3D NAND flash memory and reliability improvement for approximate data on 3D NAND flash memory. The objectives of the project are to optimize the performance and reliability. The outcome of the project will be very important for the further development of 3D NAND flash memory. In addition, it will provide technology support for different types of flash based storage devices.

随着计算机对存储设备容量和密度需求的不断提高，闪存作为最为广泛使用的存储器件之一，目前已经进入3D堆叠的发展阶段。然而，3D闪存却存在多种数据可靠性问题，阻碍了它的进一步发展。不同于2D闪存可靠性特征，3D闪存采用纵向堆叠和基于电荷陷阱的绝缘体构建，导致其在读写和数据保存过程中出现严重的电子干扰和电子泄露问题。随着尺寸和堆叠层次的不断发展，其可靠性也将进一步恶化。改善嵌入式3D闪存可靠性成为其发展过程中至关重要的研究课题。本项目将从3D闪存的操作和结构特征等方面展开可靠性优化研究。首先，根据3D闪存的可靠性影响因子特征分析，构建3D闪存可靠性模型，为其可靠性优化提供基础框架。然后在此基础上从两个方面展开可靠性优化：精确数据的可靠优化技术和近似数据的存储优化技术，最终达到存储的性能和可靠性优化的目标。本项目的成果将为3D闪存的进一步发展提供关键技术，为各类闪存设备的广泛应用提供技术支撑。

在过去的十余年间，3D闪存终于走上了舞台，成功替换了传统的2D闪存成为最为广泛使用的闪存存储设备。这是得益于其更高的存储密度和更低的价格。然而，随之产生的问题却不容忽视。首先，3D闪存的读性能严重受制于其访问延迟和可靠性，不同于传统的2D闪存，3D闪存的读延迟在不同页面甚至不同层次之间均存在较大差异，另外3D闪存由于其可靠性的差异，导致读过程需要更加长时间的纠错过程；第二，闪存的寿命和写性能也是一个关键问题。随着3D QLC的出现，单个闪存块只能磨损不到1000次，解决磨损导致的寿命问题则是另一个需要关注的问题。本项目中，我们在过去的4年间展开了深入和广泛的研究工作，力求改善和隐藏3D闪存的相关问题，实现其在实际系统的高效、高性能、高可靠性部署的目标。在国家自然科学基金的资助下，本项目从三个方面展开了大量深入的研究工作：1 通过研究3D闪存可靠性的特点，展开可靠性优化研究，2 通过3D闪存近似数据和制程差异，展开性能优化研究，3 针对3D闪存的特点，展开架构性优化设计研究。通过以上三个方面的深入研究工作，本项目取得了一系列成果，包括：实现了高密度闪存在实际部署过程中的高性能、高可靠性的优化目标；展开了一系列模拟仿真设计，加速了相关技术的验证；依托本项目已经发表论文38篇，其中国际期刊论文17篇，国际会议论文21篇，提交发明专利9项，其中国际专利1项，国内专利8项，已授权1项；培养了一批高水平的学术人才。相信通过这一系列工作的推进和实施，一定能够为3D闪存在系统部署过程中所存在的可靠性、性能和服务质量问题提供关键技术支撑。