Research On Key Technologies Of 3D Quad-level-cell NAND Flash

Due to the fast and innovative development,NAND Flash technology is pushing the development of whole IC industry.It is a high monopolistic market.China will face a serious security problem if there is no independent research & development abilities of Flash memory.In this paper,3D QLC NAND Flash whose data levels are up to 16 voltage ranges and with 15 read levels and 15 program levels is studied.The work mainly includes:Firstly,the basic NAND Flash technologies are presented in detailed and 3D NAND designs are studied.(1)NAND Flash are mainly based on 2 types of technologies-charge trap(CT)and floating gate(FG).The architectures,physical characters and the principles of program/read/erase operations of CT-based and FG-based NAND Flash are analyzed.Meanwhile,the designs of SLC/MLC/TLC NAND are analyzed and studied,including the mapping between different voltage settings and data coding,the implementation of different program algorithms,etc.(2)NAND Flash technology has been innovated from two dimension(2D)which is based on planar design with photolithography technology to three dimension(3D)which is based on vertical layers with etching technology.Also,the stereoscopic structure,the multi-layer design in Z axis and the detailed manufacture of 3D NAND Flash have been studied in this paper.Secondly,the work principles and chip designs of 3D QLC NAND Flash are studied.(1)The technology studies of 3D QLC NAND Flash include: the behaviors of 4 bits/cell with 16 voltage level,15 program levels and 15 read levels,the basic program algorithms(QLC-1),the read algorithm and flow,and the data mapping based on the “half-change” Gray coding(Gray-1).For the data storage of 4 pages,the enhanced program algorithms(QLC-2 ~ QLC-5)have been presented.(2)The chip-level design of 3D QLC NAND is studied as well which includes the research of pillar,QLC program/read/erase operations,3D manufacture based on etching technology,the cost impact by multi-layer design.Moreover,whole design flow of 3D QLC NAND Flash are studied further.Thirdly,because NAND Flash stores the data by charging the memory cell to the defined voltage levels,once the number of stored electrons is changed,the data value may be wrong.The different types of data errors are discussed in this paper.And new enhanced Gray coding(Gray-2 ~ Gray-5)which are based on different program algorithms(QLC-2 ~ QLC-5)are studied and designed.A 4k bits/page QLC NAND model with 16 voltage ranges,15 program voltages and 15 read voltages is built by MATLAB.After introducing the voltage bias in QLC memory cells to imitate the actual behavior of QLC NAND Flash to generate data errors,the error rates which are based on different program algorithms and new enhanced Gray codings are computed and compared.For the different program algorithms,the study shows that the related enhanced Gray coding can improve the efficiency and keep more balance in system-level application for the data error correction of QLC 4 pages.Hence,the maximum error data rate of QLC 4 pages will drop 25% at least.Lastly,when NAND Flash is applied in system,because of the data errors,the powerful ECC(Error Correction Code)algorithms are needed either in software solution or in peripheral controller.For 3D QLC NAND Flash application,Low-Density-Parity-Check(LDPC)is necessary since LDPC coding has the good performance and efficiency of error correction.This paper studies the solution of generating the soft-bit message for LDPC Belief Propagation(BP)decoding in QLC NAND design for the system level application to improve the efficiency of error correction.After analyzing QLC NAND's 16 voltage levels and the critical areas which are near the 15 read voltages,the situation of error probability in the critical areas are researched.Then based on log-likelihood ratio(LLR),with the new reading operation by shifting read voltage,QLC NAND Flash chip reads out the new data 0 or 1,and meanwhile provides different LLR values for LDPC BP decoding to improve the error correction efficiency.In this paper,the main design fields of 3D QLC NAND Flash are studied.Especially,the enhanced program algorithms,the related enhanced Gray codings,and the data mapping will be important for QLC NAND's development.The researches of QLC NAND will not only realize the next generation NAND memory chip,but also can be helpful to the research and development of Chinese 3D NAND industry.