ECC and Signal Processing Technology for Solid State Drives and Multi-bit per cell NAND Flash Memories

NEW YORK, Jan. 4, 2012 /PRNewswire/ -- Reportlinker.com announces that a new market research report is available in its catalogue:

ECC and Signal Processing Technology for Solid State Drives and Multi-bit per cell NAND Flash Memories

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In December 2011, it was widely reported that Apple had acquired Anobit Technologies for half a billion dollars. What prompted Apple to spend that kind of money on a five year old startup? As the biggest consumer of NAND flash memories, this was clearly a strategic acquisition. The tie-up follows a less widely known acquisition of Storage Genetics by Micron Technology in 2010. Both Anobit and Storage Genetics were developing advanced ECC and signal processing technologies.

As bit errors increase as NAND flash memory scales below 2xnm process technology and transitions to 3-bit per cell architectures, traditional error correction codes such as BCH, RS and Hamming code will no longer be sufficient. These codes suffer from increased overhead in terms of coding redundancy and read latency as the number of errors corrected increases. In addition, the number of electrons stored in the memory cell is decreasing with each generation of flash memory resulting in reduced signal/noise requiring enhanced sensing techniques.

Digital signal processing technology has been employed in the magnetic recording industry since the early 1990's when partial-response maximum-likelihood technology (PRML) was commercialized. DSP technology is now being deployed in 3-bit per cell NAND flash memories and a concerted effort is being made by NAND flash manufacturers and a handful of startups to employ digital signal processing technology to improve the endurance and performance of next generation NAND flash memories and solid state drives. Signal processing technology will be essential for the continued scaling of NAND flash memories.

This research report examines the current state of ECC techniques and explores the technology, roadmap, market, cost as well as the key players and startups in the flash signal processing space.

Table of Contents

Contents

List of Figures

List of Tables

Executive Summary

Introduction

Hard Disk Drive Trends

The Read Channel

Peak-to-Peak Detection

Partial Response Maximum Likelihood (PRML)

Error Correction

Outlook

NAND Flash Memory Overview

NAND Flash Memory Introduction

Concept

NAND Flash Architecture

Scalability

NAND Flash Memory Reliability

Random Telegraph Noise

Program Noise

Dopant Fluctuation, Line Edge Roughness

Bit Errors

Program Disturb

Read Disturb

Charge Leakage

Shannon Limit

Error Correction Codes

Types of Codes

Block Codes

Convolutional Codes

Concatenated Codes

Hamming Code

BCH Code

Encoding

Decoding

Reed-Solomon Code

Encoding

Decoding

BCH and Reed-Solomon Considerations

Convolutional Codes

Encoding

Decoding

Performance

Trellis Coded Modulation

Concatenated TCM-BCH Coding

Turbo Codes

Encoding

Decoding

Performance

Low Density Parity Check (LDPC)

Representations for LDPC codes

Regular and Irregular LDPC codes

Constructing LDPC codes

Performance & Complexity

Decoding LDPC codes

Hard-decision Decoding

Soft-decision Decoding

Encoding

LDPC in NAND Flash Memories

Summary

Outlook

Signal Processing for Flash Memories

ECC for Flash Memories

Adaptive/Dynamic ECC Scheme

BCH and Shannon Limit

SLC Soft Detection

Test Mode Sequences

Soft Read

MLC Soft Detection

Sensing Schemes

Analog Sensing

Moving Reference

MLC Programming with ECC

Floating Gate Coupling Compensation

Fractional Reference Voltage Threshold

State Ordering

Statistics Collection

Inter-cell Interference Cancellation

Adaptive Program and Read

Data Scrambling

Asymmetric Coding

NAND Capacity Gains

Endurance Improvement

Controller Level Techniques

Summary

Competitive Landscape

Roadmap

Competencies of Flash Signal Processing Players

Players

Outlook

Market for Flash Signal Processing Technology

Cost

Consumer low end NAND applications (Removable storage)

Embedded Consumer NAND applications

Consumer SSD NAND applications

Enterprise SSD NAND applications

Applications

Summary

References

Terminology

About the Authors

About Forward Insights

Services

Contact

Report Offerings

List of Figures

Figure 1. HDD Areal Density Trend

Figure 2. HDD Performance Trend

Figure 3. Storage Pricing Trends

Figure 4. HDD Communications Model

Figure 5. Readback Signal

Figure 6. Peak Detection

Figure 7. Effect of Increased Recording Density

Figure 8. PRML Read Channel Architecture

Figure 9. Superposition of Pulses

Figure 10. Maximum Likelihood Detection

Figure 11. Partial Response Schemes

Figure 12. Eye Diagrams for PR4 and EPR4 Channels

Figure 13. Eye Diagram for EPR4 (1,7) Channel

Figure 14. Waveform and Interleaved Sequences for RLL (0,4/4)

Figure 15. Diminishing Returns on 512B Sector ECC

Figure 16. ECC Gains with Increased Sector Size

Figure 17. Signal Processing and Coding Trends

Figure 18. LDPC vs. PR4, EPR4 and NPML

Figure 19. Iterative Decoding

Figure 20. NAND Flash Cell Programming

Figure 21. Multi-level Storage in NAND Flash

Figure 22. NAND Flash Cell Reading

Figure 23. NAND Flash Cell Erase

Figure 24. NAND Cell Architecture

Figure 25. NAND Cell String

Figure 26. NAND Flash Memory Architecture

Figure 27. 8Gb NAND Flash Memory Organization

Figure 28. Cross-talk and Coupling Ratio

Figure 29. Inter-cell Interference

Figure 30. Multi-bit per Cell NAND Flash Endurance

Figure 31. Electrons Stored on the Floating Gate

Figure 32. NAND Flash Bit Size Trend

Figure 33. Vt Fluctuation due to RTN

Figure 34. Number and Amplitude of Trap Sites

Figure 35. Dependence of Traps on P/E Cycles

Figure 36. S/N in Multi-level NAND Flash Memories

Figure 37. ISPP and DVt Spread

Figure 38. Effect of Program Injection

Figure 39. sVt vs. Channel Length due to Random Discrete Dopants in the Source and Drain of Double Gate MOSFETs

Figure 40. Effect of Gate Oxide Thickness on Vt

Figure 41. Line Edge Roughness

Figure 42. Uncorrectable BER vs. Raw BER

Figure 43. Flash Error Rate Surface

Figure 44. Program Disturb

Figure 45. RBER vs. P/E Cycling

Figure 46. Read Disturb

Figure 47. RBER vs. Number of Reads

Figure 48. Effect of Number of Electrons per Bit on Retention Time

Figure 49. RBER vs. Retention

Figure 50. MLC NAND Flash Endurance & Data Retention

Figure 51. Soft Decoded Multi-bit/cell Limits and Hard Decoded 1bit/cell Limit vs, Shannon Limit

Figure 52 Code Gain and Distance Limit

Figure 53. Representation of the Hamming Coding and Decoding

Figure 54. Encoding and Decoding for Systematic Hamming

Figure 55. Representation of the matrix M systematically built on the basis of the data flow (mi) going through the memory. The sorting indexes x (1-512) and y (1-8) univocally identify each bit of the matrix M and are therefore suitable for building the parity matrix H.

Figure 56. Representation of the matrix H. The sorting indexes x (1-512) and y (1-8), univocally identify each row of the matrix H (a). For the calculation of the parity and of the syndrome, the matrix H is seen as the composition ((Y,X)T,I). The matrix H is completed by adding the necessary row and column for the code extension.

Figure 57. Representation of the matrix M systematically built on the basis of the data flow going through the memory. The sorting indexes x (0-511) and y (0-7) univocally identify each bit of the matrix M and are therefore suitable for building the parity matrix

Figure 58. Representation of the matrix H. The sorting indexes x (0-511) and y (0-7), univocally identify each row of the matrix H (a). For the parity and the syndrome calculation, the matrix H is seen as the composition ((Y,Y',X,X')T,I) (b).

Figure 59. BER/EDR for Hamming ECC

Figure 60. BCH Encoding and Decoding

Figure 61. BERout vs. Eb/N0 for a 2KB Page SLC NAND Flash Device for Hamming and BCH Code

Figure 62. Channel Capacity - Hamming Code and BCH Code for 1, 2, 3, 4 bits/cell.

Figure 63. Structure of a Reed-Solomon Code

Figure 64. R (Ratio) vs. t (Number of Error Corrected) for BCH

Figure 65. Channel Capacity - BCH vs. Reed-Solomon

Figure 66. BERout vs. Eb/N0 for a 2KB Page SLC Device for Reed-Solomon Code

Figure 67. Convolutional Encoder

Figure 68. Constraint Length 3 Convolutional Encoder State Diagram

Figure 69. Constraint Length 3 Convolutional Encoder Trellis Diagram

Figure 70. Constraint Length 3 Convolutional Encoder Trellis Diagram for Input Sequence (1 0 0 1 1 0 1 0)

Figure 71. Convolutional Code Performance (Upper Limit)

Figure 72. Convolutional code R = 1/2, k = 3, 8, 14 on Channel Capacity Plane.

Figure 73. TCM Error Correction System

Figure 74. BER Performance TCM, BCH & Hamming Code for 16-bit, 32-bit and 64-bit User Data

Figure 75. Program (a) and Read (b) in TCM Error Correction System

Figure 76. t for BCH Code

Figure 77. BER handled by BCH Code as a Function of Deviation of Vt Width Distribution s

Figure 78. Cell Storage Efficiency

Figure 79. Basic Turbo Encoder

Figure 80. Basic Turbo Code Decoder

Figure 81. Performance of Turbo Code. BER given by Iterative Decoding (p = 1,..18) at a rate R = 1/2 Encoder, Memory v = 4, generators G1 = 37, G2 = 21, with interleaving 256 x 256.

Figure 82. LDPC Performance as a Function of Block Length

Figure 83. Tanner Graph of Parity Check Matrix

Figure 84. Overview over messages received and sent by the c-nodes in step 2 of the message passing algorithm

Figure 85. Step 3 of the described decoding algorithm. The v-nodes use the answer messages from the c-nodes to perform a majority vote on the bit value.

Figure 86. a) Illustrates the calculation of rji(b) and b) qij(b)

Figure 87. Approaching the Shannon Limit

Figure 88. Page Size + Spare Area Trend

Figure 89. Codewords of Four Operation Modes for Adaptive BCH ECC

Figure 90. Dynamic ECC Codeword Transition Scheme

Figure 91. Dynamic ECC codeword transition by (a) monitoring the number of errors (b) monitoring the number of W/E cycles

Figure 92. Shannon Limit and BCH ECC for Multi-level NAND Flash Memories

Figure 93. SLC Soft Detection

Figure 94. SLC Soft Decoding (5 – 3 Level) with LDPC vs. Hard Decision + BCH 32-bit correction

Figure 95. SLC vs. MLC Soft Decoding Limits

Figure 96. DSM Sensing

Figure 97. Linear Dependency Between N and Icell in the DSM Page Buffer Architecture

Figure 98. 4-Level Distribution with References R1, R2 and R3

Figure 99. Widened 4-Level Distribution with References

Figure 100. Shifted 4-Level Distribution with References

Figure 101. Effect of Read Disturb on 4-Level Distribution

Figure 102. Effect of Temperature on Vt Distribution

Figure 103. Moving Reference Algorithm

Figure 104. Moving Reference ECC

Figure 105. Program Page Sequence to Minimize FG Coupling

Figure 106. WL0 after Programming of Page 0

Figure 107. WL0 after Programming of Page 1

Figure 108. Marginal cell can be moved to the wrong distribution (red arrow) if an error occurs in the initial read inside the program flow

Figure 109. 8-level and 16-level Programming

Figure 110. Progamming Sequence for 4bit/cell NAND flash and ECC Activity

Figure 111. Observed Cell and Neighbouring Cells

Figure 112. Observed Cell Programmed at 1V and Neighboring Cells Programmed at -2V.

Figure 113. Vt Shift due to FG Coupling

Figure 114. Fractional Reference Vt for 8-level Cell Memory

Figure 115. Serially Ordered 16-level Cell Memory

Figure 116. 16-level Cell Memory Ordering for Optimizing Bit Errors

Figure 117. Pattern Dependency of Data Retention and Program Disturb Errors

Figure 118. Proportion of the data retention error to the program disturb error

Figure 119. Asymmetric Coding

Figure 120. Improvement in Data Retention Error due to Asymmetric Coding

Figure 121. Probability Processing vs. Digital Processing

Figure 122. LEC Footprint

Figure 123. Performance of LDPC on 3 Bits / Cell Flash Memory

Figure 124. DensBits ECC under Hard Decoding

Figure 125. DensBits ECC under Soft Decoding

Figure 126. DensBits Hard and Soft Decoding vs. Hamming

Figure 127. DensBits Soft Decoding vs. BCH 24-bit/1kB

Figure 128. Reverse Concatentation Read Channel Coding

Figure 129. Marvell Flash Read Channel based on Conventional HDD Read Channel

Figure 130. Marvell Flash Read Channel

Figure 131. Block Diagram of Controller and Flash Memory

Figure 132. Effect of Storage Genetics Processing Engine

Figure 133. Storage Genetics Correction Engine - 4 bit/cell NAND

Figure 134. ClearNAND

Figure 135. HB and SB Decoding Process

Figure 136. OLEA-BCH vs. Normal BCH Architecture, 8kB page size; 8KB, BSC channel, random error model; 8 BCH blocks per page; Code Rate: 0.9609, 24bits/1024B

Figure 137. PAC LDPC vs. Normal LDPC Architecture, AWGN channel; Code Rate: 0.9362; 3-bit soft information; min-sum algorithm; 8 iterations

Figure 138. Weight Control Code, 4kB Page Size; 15/16bit / 512Byte BCH; Raw ER : 1.8 X 10-3

Figure 139. Maximum Frequency and Power Consumption vs. Supply Voltage

Figure 140. a) Normalized Voltage Supply vs. Parallelism, b) BER vs. Maximum Number of Iterations under 4-bit quantized minsum decoding - Reed-Solomon based (6, 32)-regular 2048-bit LDPC code

Figure 141. LDPC Decoder Die Photo

Figure 142. A Parallel RS-LDPC (2048,1723) for 10GBASE-T Ethernet

Figure 143. Applications Utilizing NAND Flash

Figure 144. Flash Signal Processing Cost Adder

Figure 145. NAND Flash Solution by Implementation

Figure 146. TAM for Advanced ECC/FSP Chips

List of Tables

Table 1. Capacities of (0,G/I)

Table 2. Comparison of Detection Methods

Table 3. TCM vs. Hamming & BCH

Table 4. Silicon Area (mm2)

Table 5. ECC Requirements for Multi-level NAND Flash Memories

Table 6. Adaptive ECC Scheme Performance

Table 7. Capacity Increase – LDPC vs. BCH

Table 8. Endurance Increase – LDPC vs. BCH

Table 9. Controller-level Techniques

Table 10. Planned Deployment of DSP Techniques by NAND Flash Vendor

Table 11. Planned Product Deployment of NAND Flash Vendors Employing DSP Techniques

Table 12. Competencies of Flash Signal Processing Players

Table 13. Coding Gain

Table 14. DensBits ECC Hardware Implementation

Table 15. Comparison of DBECC, LDPC and BCH

Table 16. Endurance Specifications with ECC Engine

Table 17. EZ-NAND Architecture vs. Traditional Architecture

Table 18. LDPC Error Correction Capability

Table 19. Siglead SSD Controllers

Table 20. Comparison of LDPC Decoders

Table 21. Core Area for 2.5 million Gates

To order this report:Portable Music Industry: ECC and Signal Processing Technology for Solid State Drives and Multi-bit per cell NAND Flash Memories

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