"Three-Dimensional Memory Chip Technology" - Learning Record (1)

Chapter 1 Ecology of NAND Memory

1.1 Changes in the memory industry

It can be said that the past 10 years are the 10 years with the greatest changes in the history of the entire storage industry.

1.1.1 Integration of NAND and memory suppliers

As shown in Figure 1.1, in the past 6 years, 95% of the global memory supply has been concentrated in 5 manufacturers.

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1.1.2 NAND technology development

In the past 10 years, the transformation of memory technology has two directions:

• One is to use an advanced lithography machine to continue shrinking in the X and Y directions of the 2D plane
• The other is to use advanced etching tools in 3D space to increase the number of layers in the Z direction

1.1.3 Changes in NAND Application Mode

The final factor causing major changes in the NAND ecosystem comes from the NAND market segments. Compared with 2011, the NAND market application has changed greatly in 2015, as shown in Figure 1.3.

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• Consumer Grade SSD Growth
• Enterprise SSD Growth
• Removable storage such as memory card and USB is gradually decreasing

1.2 SSD

1.2.1 Enterprise SSD

As shown in Figure 1.4, the enterprise SSD market is the most interesting segment of the NAND application market.

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Figure 1.5 shows a typical storage pyramid, with DRAM in the first echelon as the fastest external storage.

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Due to the following reasons, PCIe SSD is developing very rapidly

• Provides the highest performance and lowest latency
• Demand for server-based storage grows, cloud providers tend to connect servers directly via PCIe
• Use 2.5in drive or M.2 to improve manufacturing efficiency and reduce costs
• The growth of enterprise applications has led to the increase of PCIe enterprise ecosystem technology

The benefits of PCIe-based SSDs are fully realized, including:

• PCIe has better performance and lowest latency compared to SATA and SAS. High-end PCIe SSD can provide more than one million I/O throughput per second, and the read latency can be on the order of 10us
• Leverage customer base economics as PCIe is hot in consumer SSDs
• In the storage industry ecosystem, growing support for PCIe and increased investment in NVMf and related technologies require larger deployments

The performance advantages of PCIe SSD combined with traditional DAS, SAN and NAS storage indicate that standardized PCIe NVMe storage will become the trend of enterprise SSD interface in the next few years. Table 1.1 provides the actual performance evaluation results of different SSD interfaces used in enterprises.

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1.2.2 Build-Your-Own and Customized SSD

Build-Your-Own (BYO) SSD is a model that is customized by a large number of consumers instead of standard SSD. As shown in Figure 1.7, the motivations for customers to customize SSDs mainly include the following points.

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• NAND Cost Advantage: Leveraging Raw NAND and SSD Price Arbitrage
• Operational advantages: By simplifying SSD development and shortening the development cycle, it can be more seamlessly connected with other platform conversion and development, and optimize the SSD functions that operators are most concerned about.
• Time-to-market (TTM) advantage: joint certification can start quickly, custom BYO SSD can reduce certification cycle and allow faster mass production
• Technical advantages: One of the biggest advantages of the BYO model. As shown in Figure 1.8, the parameter size can be controlled by yourself.

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The three most common SSD indicators are modified: endurance; performance/power consumption; NAND Flash cost

1 Scratch resistance

The number of erasing (Program/Erase, P/E) is the main indicator of the resistance to erasing and writing. For different types of NAND, this indicator is different. The industry standard is:

• The SLC specification is 1bit/cell, and the number of P/E is 10000 or higher
• The MLC specification is 2bit/cell, and the number of P/E is 3000
• The TLC specification is 3bit/cell, and the number of P/E is usually less than 1000

OP is the part other than Flash storage in SSD, which can perform various management functions through the Flash controller.

2 Performance/Power Consumption

NAND performance varies greatly between SLC, MLC, and TLC. For example, BYO SSD will be configured as follows in order to meet the application performance level endurance requirements:

• cheaper NAND
• Higher number of channels for performance optimization
• Configure powerful ECC for increased endurance
• Minimum OP configuration

3 NAND Flash cost

The total cost of enterprise-level BYO custom SSD is still dominated by NAND cost. As shown in Figure 1.9, for a 2TB enterprise SSD, its controller is equipped with more Flash channels, stronger ECC, and relatively low cost.

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1.2.3 Economic Effect of SSD Controller

The key flexible controller (high-level) design points include: the number of Flash channels; DRAM interface; supported enterprise functions; functions and components that do not require special processing can be turned off independently; powerful ECC supports multiple storage types; flexible support NAND protocol and special instructions.

1.2.4 Consumer SSD

Consumer grade SSDs have a different market, mostly driven by cost, which has to do with using the lowest cost NAND type.

1.3 Evolution of NAND Technology: 3D NAND

The discussion on the evolution of NAND technology can be understood through two important criteria: the need to reduce the cost per bit to continue to increase the proportion of NAND in the total storage; To explore new 3D NAND.

1.3.1 3D NAND technology

The evolution process of 2D NAND nodes has been described in detail above, and 2D NAND can no longer meet the demand for cost reduction brought about by the rapidly expanding market. The 3D evolution is from 32-48-64 layers, which can continue to shrink for several generations like the shrinking speed of previous 2D NAND technology.

1 3D NAND cost and price positioning

When suppliers choose 3D NAND technology, they mainly consider the following aspects:

• Comparison with the cost structure of the latest 2D NAND products
• Yield rate of 3D NAND products
• 3D TLC NAND is capable of meeting the technical specifications of the latest 2D MLC nodes
• Wafer cost between 32-layer and 48-layer products
• Differences in Manufacturing Yield Between Floating Gate Technology and Charge Trapping Technology Between Different 3D NAND Products

2 Technical Specifications

The key indicators corresponding to each generation are:

• Reliability (P/E times) decreased
• Increased Bit Error Rate (BER)
• ECC requirements increase

3 quadruple cell (QLC, 4bits/cell)

If 3D TLC can replace 2D MLC, the question is what to replace 2D TLC with. NAND suppliers have repeatedly announced in the industry that they will develop QLC 3D NAND products.

1.3.2 3D NAND products are dominated by TLC

The net effect of the ramp-up in 3D NAND production is TLC products, which currently represent a relatively small portion of the overall industry, but are expected to become supplier-dominated products in the next few years.

1.3.3 Floating Gate Technology VS Charge Trapping Technology

As shown in Figure 1.14, an interesting technology debate in NAND constituent cells is whether the memory cell uses floating gate (FG) technology or charge trapping (CT) technology. Most NAND suppliers choose CT technology, but Intel and Micron cooperated to develop 3D NAND and chose the technology based on floating gate.

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1.3.4 Packaging Innovation: TSV NAND

1 interface chip

It can distinguish the needs of each generation of NAND and provide a simpler unified interface for the system layer, such as eMMC products. The final packaging of the chip adopts standard BGA and MCP packaging. In addition to the original NAND chip, a logic chip needs to be plugged in to: reduce I/O power consumption and capacitance; perform ECC functions; handle more advanced functions, such as garbage collection and other NAND Management matters, providing a simpler, higher-level interface to the host processor.

2 TSV packages

TSV packages are used for higher density applications. TSV technology is like a steel frame in a tall building, as shown in Figure 1.15, which reduces the connection length by about 10 times. The advantages of short wiring lengths are as follows:

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• Performance, the parasitic capacitance becomes smaller, which can support higher transmission speed and improve signal integrity (SI).
• Low power consumption, the IO voltage can be reduced from 1.8V to 1.2V, and the NAND core voltage can be reduced from 3.3V to 1.8V.

1.4 New Memory Technology

From the perspective of new storage technologies, it is observed in Figure 1.17 that the current storage industry has invested a lot in two directions:

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• An alternative to DRAM, meeting non-volatility and lower cost per bit than DRAM
• Storage Class Memory (SCM) with higher performance than NAND

At present, the mainstream new memory includes the following three types

• Magnetoresistive Random Access Memory (MRAM)
• Phase Change Memory (PCRAM)
• Resistive RAM (ReRAM)

1.5 What do we expect in the next 5 years

As the NAND industry will transform into 3D NAND in the next few years, it will face the following problems:

• The landscape of storage vendors. Question: Will there be further consolidation in the storage industry over the next few years due to continued acquisitions of leaders in the storage or HDD space?
• 3D NAND leader. Question: Which suppliers will be the first to ramp up production in 3D NAND products, and how will other NAND players respond?
• 3D NAND will account for a large proportion in 2017. Question: By 2017 or 2018, has NAND transitioned to 3D NAND (50% and above for 3D NAND)? When will 3D wafers reach 50%?
• storage cell technology. Question: Will the second or third generation 3D NAND products adopt CT technology or FG technology?
• Economics of NAND cells. Question: When will QLC (4bit/ell) be introduced to the market and what will be the technical specifications compared to 2D TLC?
• Enterprise-grade SSDs. Question: Who will be the leader in PCle SSD (consumer and enterprise)? Will the BYO SSD model be popularized by large cloud operators?
• 3D cross memory. Question: Will Intel and Micron's 3DXPoint technology be widely accepted in a few years? Will Intel use this technology extensively in its server architecture?
• New type of storage. Question: Which new memory will be successful for longer in the next generation? Will it be successful in the mainstream market or in niche applications? Will MRAM be able to achieve the cost-effectiveness of gigabyte-scale products?

致谢《三维存储芯片技术》

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