Methods for Threshold Voltage Setting of String Select Transistors in Channel Stacked NAND Flash Memory

학위논문 (박사)-- 서울대학교 대학원 공과대학 전기·컴퓨터공학부, 2017. 8. 박병국.Since recent mobile electronic devices such as tablets, laptops, smartphones, or solid-state drives (SSDs) have started to adopt the NAND flash memory as their main data storage device, the demand for low-cost and high-density NAND flash memories has experienced a rapid increase. However, some problems such as the limitations of photolithography technology, cell-to-cell interference, and reduction of the number of electrons stored in floating gates have hindered the downscaling of floating-gate NAND flash memories. To overcome the NAND scaling issues, several types of three-dimensional (3D) stacked charge-trap NAND flash memories, which have been developed based on the bit-cost scalable (BiCS) technology introduced by Toshiba, have been widely investigated, owing to their scalability, ease of fabrication, and coupling-free characteristics. 3D-stacked NAND flash memory architectures can be divided into two categories. The first is the gate-stacked NAND flash memory, in which current flows through a vertical channel while the gates are shared horizontally by all the strings. The second category consists of channel-stacked NAND flash memories, in which the current flows through the horizontally stacked channel and the gates are shared vertically by all the strings. In 3D-stacked NAND flash memory architectures. The channel-stacked type presents several outstanding advantages in terms of minimal unit cell size, bit line (BL) pitch scaling, use of a single-crystalline Si channel by Si/SiGe epitaxial growth process, and degradation characteristics of read currents caused by the increase in the number of stacked layers. However, compared with the gate-stacked type, the channel-stacked type presents critical issues that hinder its use in commercial applications, such as complex array architectures and decoding of the stacked layers. To overcome these problems, our group has recently reported channel-stacked arrays with layer selection by multilevel (LSM) operation. However, the array architecture and operation scheme setting the string select transistors (SSTs) with multilevel states should be simplified further to enable commercialization. In this dissertation, a simplified channel-stacked array with LSM operation is proposed. In addition, new SST threshold voltage (Vth) setting methods to set all the SSTs on each layer to the targeted Vths values are introduced and verified by using technology computer-aided design (TCAD) simulations and measurements in fabricated pseudo-SLSM. Furthermore, various disturbance phenomena that could occur during basic memory operations such as erase, program, and read are analyzed, and schemes for mitigating these disturbances are proposed and verified.Chapter1 Three-Dimensional Stacked NAND Flash Memory 1 1.1 Introduction to Three-Dimensional Stacked NAND Flash Memory 1 1.2 Gate Stack Type NAND Flash Memory 8 1.3 Channel Stack Type NAND Flash Memory 17 1.4 Comparison between Gate Stack Type NAND Flash and Channel Stack Type NAND Flash 24 Chapter2 Channel Stacked NAND Flash Memory with Layer Selection by Multilevel Operation 28 2.1 LSM and Channel Stacked NAND Flash Architecture Design 28 2.2 Operation Scheme of Channel Stacked NAND Flash Memory with LSM 36 2.2.1 Stacked SST Initialization to Enable LSM 36 2.2.2 Read Operation with LSM 38 2.2.3 Program/Erase Operation with LSM 42 2.3 Comparison with Conventional Channel Stacked NAND Flash Memory Architecture 47 Chapter3 Methods for Setting String Select Transistors for Layer Selection in Channel Stacked NAND Flash Memory 50 3.1 Method for Setting SST Vth Using One Erase Operation 50 3.2 Method for Setting SST Vth Using Dummy SSTs 60 Chapter4 Reliability Issues During LSM in Channel Stacked NAND Flash Memory 69 4.1 Program Disturbance in SLSM 69 4.2 Read Disturbance in SLSM 84 Chapter5 Application to General NAND Flash Memory 95 Chapter6 Conclusions 103 Bibliography 106 Abstract in Korean 119Docto