Flash memory

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A disassembled USB flash drive. The chip on the left is flash memory. The controller is on the right. USB flash drive.JPG
A disassembled USB flash drive. The chip on the left is flash memory. The controller is on the right.

Flash memory is an electronic (solid-state) non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory are named after the NAND and NOR logic gates. The individual flash memory cells, consisting of floating-gate MOSFETs (floating-gate metal-oxide-semiconductor field-effect transistors), exhibit internal characteristics similar to those of the corresponding gates.

Integrated circuit electronic circuit manufactured by lithography; set of electronic circuits on one small flat piece (or "chip") of semiconductor material, normally silicon

An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material that is normally silicon. The integration of large numbers of tiny MOS transistors into a small chip results in circuits that are orders of magnitude smaller, faster, and less expensive than those constructed of discrete electronic components. The IC's mass production capability, reliability, and building-block approach to circuit design has ensured the rapid adoption of standardized IC's in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs.

Solid-state storage is a type of non-volatile computer storage that stores and retrieves digital information using only electronic circuits, without any involvement of moving mechanical parts. This differs fundamentally from the traditional electromechanical storage, which records data using rotating or linearly moving media coated with magnetic material.

Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retrieve stored information even after having been power cycled. In contrast, volatile memory needs constant power in order to retain data. Examples of non-volatile memory include flash memory, read-only memory (ROM), ferroelectric RAM, most types of magnetic computer storage devices, optical discs, and early computer storage methods such as paper tape and punched cards.

Contents

Flash memory is a type of floating-gate memory that was invented at Toshiba in 1980, based on EEPROM (electrically erasable programmable read-only memory) technology. Toshiba commercially introduced flash memory to the market in 1987. [1] While EPROMs had to be completely erased before being rewritten, NAND-type flash memory may be erased, written and read in blocks (or pages) which are generally much smaller than the entire device. NOR-type flash allows a single machine word (byte) to be written  to an erased location  or read independently. A flash memory device typically consists of one or more flash memory chips (each holding many flash memory cells) along with a separate flash memory controller chip.

Toshiba Japanese multinational electronics, electrical equipment and information technology corporation

Toshiba Corporation is a Japanese multinational conglomerate headquartered in Tokyo, Japan. Its diversified products and services include information technology and communications equipment and systems, electronic components and materials, power systems, industrial and social infrastructure systems, consumer electronics, home appliances, medical equipment, office equipment, as well as lighting and logistics.

EEPROM nonvolatile memory comprising arrays of floating-gate transistors used in computers, microcontrollers &c. to store relatively small amounts of data but allowing individual bytes to be erased/reprogrammed in-circuit through special programming signals

EEPROM (also E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory used in computers, integrated in microcontrollers for smart cards and remote keyless systems, and other electronic devices to store relatively small amounts of data but allowing individual bytes to be erased and reprogrammed.

Read-only memory non-volatile memory used in computers and other electronic devices; class of storage medium used in computers and other electronic devices

Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing software that is rarely changed during the life of the system, sometimes known as firmware. Software applications for programmable devices can be distributed as plug-in cartridges containing read-only memory.

The NAND type is found primarily in memory cards, USB flash drives, solid-state drives (those produced in 2009 or later), and similar products, for general storage and transfer of data. NAND or NOR flash memory is also often used to store configuration data in numerous digital products, a task previously made possible by EEPROM or battery-powered static RAM. One key disadvantage of flash memory is that it can only endure a relatively small number of write cycles in a specific block. [2]

Memory card electronic flash memory data storage device used for storing digital information

A memory card or memory cartridge is an electronic data storage device used for storing digital information, typically using flash memory. These are commonly used in portable electronic devices, such as digital cameras, mobile phones, laptop, computers, tablets, PDAs, portable media players, video game consoles, synthesizers, electronic keyboards and digital pianos.

USB flash drive Data storage device

A USB flash drive is a data storage device that includes flash memory with an integrated USB interface. It is typically removable, rewritable and much smaller than an optical disc. Most weigh less than 30 g (1 oz). Since first appearing on the market in late 2000, as with virtually all other computer memory devices, storage capacities have risen while prices have dropped. As of March 2016, flash drives with anywhere from 8 to 256 GB were frequently sold, while 512 GB and 1 TB units were less frequent. As of 2018, 2 TB flash drives were the largest available in terms of storage capacity. Some allow up to 100,000 write/erase cycles, depending on the exact type of memory chip used, and are thought to last between 10 and 100 years under normal circumstances.

Solid-state drive Data storage device

A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies as memory to store data persistently, typically using flash memory. It is also sometimes called a solid-state device or a solid-state disk, although SSDs lack the physical spinning disks and movable read-write heads used by the conventional electromechanical storage such as hard drives ("HDD") or floppy disks.

Example applications of flash memory include computers, PDAs, digital audio players, digital cameras, mobile phones, synthesizers, video games, scientific instrumentation, industrial robotics, and medical electronics. In addition to being non-volatile, flash memory offers fast read access times, although not as fast as static RAM or ROM. [3] Its mechanical shock resistance helps explain its popularity over hard disks in portable devices

Personal digital assistant The "Personal Digital Assistant" is used in many games.

A personal digital assistant (PDA), also known as a handheld PC, is a variety mobile device which functions as a personal information manager. PDAs have been mostly displaced by the widespread adoption of highly capable smartphones, in particular those based on iOS and Android.

Digital camera Camera that captures photographs or video in digital format

A digital camera or digicam is a camera that captures photographs in digital memory. Most cameras produced today are digital, and while there are still dedicated digital cameras, many more cameras are now being incorporated into mobile devices, portable touchscreen computers, which can, among many other purposes, use their cameras to initiate live video-telephony and directly edit and upload imagery to others. However, high-end, high-definition dedicated cameras are still commonly used by professionals.

A scientific instrument is, broadly speaking, a device or tool used for scientific purposes, including the study of both natural phenomena and theoretical research.

Although flash memory is technically a type of EEPROM, the term "EEPROM" is generally used to refer specifically to non-flash EEPROM which is erasable in small blocks, typically bytes.[ citation needed ] Because erase cycles are slow, the large block sizes used in flash memory erasing give it a significant speed advantage over non-flash EEPROM when writing large amounts of data. As of 2013, flash memory costs much less than byte-programmable EEPROM and had become the dominant memory type wherever a system required a significant amount of non-volatile solid-state storage.

History

Background

The origins of flash memory can be traced back to the development of the floating-gate MOSFET (FGMOS), also known as the floating-gate transistor. [4] [5] The original MOSFET (metal-oxide-semiconductor field-effect transistor), also known as the MOS transistor, was invented by Egyptian engineer Mohamed M. Atalla and Korean engineer Dawon Kahng at Bell Labs in 1959. [6] Kahng went on to develop a variation, the floating-gate MOSFET, with Simon Min Sze at Bell Labs in 1967. [7] They proposed that it could be used as floating-gate memory cells for storing a form of programmable read-only memory (PROM) that is both non-volatile and re-programmable. [7]

The floating-gate MOSFET (FGMOS), also known as a floating-gate transistor, is a type of MOSFET where the gate is electrically isolated, creating a floating node in DC, and a number of secondary gates or inputs are deposited above the floating gate (FG) and are electrically isolated from it. These inputs are only capacitively connected to the FG. Since the FG is completely surrounded by highly resistive material, the charge contained in it remains unchanged for long periods of time. Usually Fowler-Nordheim tunneling and hot-carrier injection mechanisms are used to modify the amount of charge stored in the FG.

MOSFET Transistor used for amplifying or switching electronic signals.

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), also known as the metal–oxide–silicon transistor (MOS transistor, or MOS), is a type of field-effect transistor that has an insulated gate and is fabricated by the controlled oxidation of a semiconductor, typically silicon. The voltage of the covered gate determines the electrical conductivity of the device; this ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. The MOSFET was invented by Egyptian engineer Mohamed M. Atalla and Korean engineer Dawon Kahng at Bell Labs in November 1959. It is the basic building block of modern electronics, and the most prolifically manufactured device in history, with an estimated total of 13 sextillion (1.3 × 1022) MOSFETs manufactured between 1960 and 2018.

Mohamed M. Atalla mechanical engineer

Mohamed Mohamed Atalla was an Egyptian-American engineer, physical chemist, cryptographer, inventor and entrepreneur. His pioneering work in semiconductor technology laid the foundations for modern electronics. Most importantly, his invention of the MOSFET in 1959, along with his earlier surface passivation and thermal oxidation processes, revolutionized the electronics industry. He is also known as the founder of the data security company Atalla Corporation, founded in 1972, which introduced the first hardware security module and was a pioneer in online security. He received the Stuart Ballantine Medal and was inducted into the National Inventors Hall of Fame for his important contributions to semiconductor technology as well as data security.

Early types of floating-gate memory included EPROM (erasable PROM) and EEPROM (electrically erasable PROM) in the 1970s. [7] However, early floating-gate memory required engineers to build a memory cell for each bit of data, which proved to be cumbersome, [8] slow, [9] and expensive, restricting floating-gate memory to niche applications in the 1970s, such as military equipment and the earliest experimental mobile phones. [4]

Invention and commercialization

Fujio Masuoka, while working for Toshiba, proposed a new type of floating-gate memory that allowed entire sections of memory to be erased quickly and easily, by applying a voltage to a single wire connected to a group of cells. [4] This led to Masuoka's invention of flash memory at Toshiba in 1980. [8] [10] [11] According to Toshiba, the name "flash" was suggested by Masuoka's colleague, Shōji Ariizumi, because the erasure process of the memory contents reminded him of the flash of a camera. [12] Masuoka and colleagues presented the invention of NOR flash in 1984, [13] [14] and then NAND flash at the IEEE 1987 International Electron Devices Meeting (IEDM) held in San Francisco. [15]

Toshiba commercially launched NAND flash memory in 1987. [1] [7] Intel Corporation introduced the first commercial NOR type flash chip in 1988. [16] NOR-based flash has long erase and write times, but provides full address and data buses, allowing random access to any memory location. This makes it a suitable replacement for older read-only memory (ROM) chips, which are used to store program code that rarely needs to be updated, such as a computer's BIOS or the firmware of set-top boxes. Its endurance may be from as little as 100 erase cycles for an on-chip flash memory, [17] to a more typical 10,000 or 100,000 erase cycles, up to 1,000,000 erase cycles. [18] NOR-based flash was the basis of early flash-based removable media; CompactFlash was originally based on it, though later cards moved to less expensive NAND flash.

NAND flash has reduced erase and write times, and requires less chip area per cell, thus allowing greater storage density and lower cost per bit than NOR flash; it also has up to 10 times the endurance of NOR flash. However, the I/O interface of NAND flash does not provide a random-access external address bus. Rather, data must be read on a block-wise basis, with typical block sizes of hundreds to thousands of bits. This makes NAND flash unsuitable as a drop-in replacement for program ROM, since most microprocessors and microcontrollers require byte-level random access. In this regard, NAND flash is similar to other secondary data storage devices, such as hard disks and optical media, and is thus highly suitable for use in mass-storage devices, such as memory cards and solid-state drives (SSD). Flash memory cards and SSDs store data using multiple NAND flash memory chips. The first NAND-based removable memory card format was SmartMedia in 1995, and many others have followed, including:

Later developments

A new generation of memory card formats, including RS-MMC, miniSD and microSD, feature extremely small form factors. For example, the microSD card has an area of just over 1.5 cm2, with a thickness of less than 1 mm.

NAND flash has achieved significant levels of memory density as a result of several major technologies that were commercialized during the late 2000s to early 2010s. [19]

Multi-level cell (MLC) technology stores more than one bit in each memory cell. NEC demonstrated quad-level cell (QLC) technology in 1996, with a 64  Mb flash memory chip storing 2-bit data per cell. STMicroelectronics also demonstrated quad-level cells in 2000, with a 64 Mb NOR flash memory chip. [20] In 2009, Toshiba and SanDisk introduced NAND flash chips with QLC technology storing 4-bit per cell and holding a capacity of 64 Gb. [21] [22] Samsung Electronics introduced triple-level cell (TLC) technology storing 3-bit per cell, and began mass-producing NAND chips with TLC technology in 2010. [23]

Charge trap flash (CTF) technology was developed during the 1990s to early 2000s. In 1991, NEC researchers including N. Kodama, K. Oyama and Hiroki Shirai described a type of flash memory with a charge trap method. [24] In 1998, Boaz Eitan of Saifun Semiconductors (later acquired by Spansion) patented a flash memory technology named NROM that took advantage of a charge trapping layer to replace the floating gate used in conventional flash memory designs. [25] In 2000, an Advanced Micro Devices (AMD) research team led by Richard M. Fastow, Egyptian engineer Khaled Z. Ahmed and Jordanian engineer Sameer Haddad (who later joined Spansion) demonstrated a charge-trapping mechanism for NOR flash memory cells. [26] CTF was later commercialized by AMD and Fujitsu in 2002. [27] 3D V-NAND (vertical NAND) technology stacks NAND flash memory cells vertically within a chip using 3D charge trap flash (CTP) technology. 3D V-NAND technology was first announced by Toshiba in 2007, [28] and was first commercially released by Samsung Electronics in 2013. [29] [30]

3D integrated circuit (3D IC) technology stacks integrated circuit (IC) chips vertically into a single 3D IC chip package. [19] Toshiba introduced 3D IC technology to NAND flash memory in April 2007, when they debuted a 16  GB THGAM embedded NAND flash memory chip, which was manufactured with eight stacked 2 GB NAND flash chips. [31] In September 2007, Hynix Semiconductor (now SK Hynix) introduced 24-layer 3D IC technology, with a 16 GB flash memory chip that was manufactured with 24 stacked NAND flash chips using a wafer bonding process. [32] Toshiba also used an eight-layer 3D IC for their 32 GB THGBM flash chip in 2008. [33] In 2010, Toshiba used a 16-layer 3D IC for their 128 GB THGBM2 flash chip, which was manufactured with 16 stacked 8 GB chips. [34] In the 2010s, 3D ICs came into widespread commercial use for NAND flash memory in mobile devices. [19]

As of August 2017, microSD cards with a capacity up to 400 GB (400 billion bytes) are available. [35] [36] The same year, Samsung combined 3D IC chip stacking with its 3D V-NAND and TLC technologies to manufacture its 512 GB KLUFG8R1EM flash memory chip with eight stacked 64-layer V-NAND chips. [37] In 2019, Samsung produced a 1  TB flash chip, with eight stacked 96-layer V-NAND chips and with QLC technology. [38] [39]

Principles of operation

A flash memory cell Flash cell structure.svg
A flash memory cell

Flash memory stores information in an array of memory cells made from floating-gate transistors. In single-level cell (SLC) devices, each cell stores only one bit of information. Multi-level cell (MLC) devices, including triple-level cell (TLC) devices, can store more than one bit per cell.

The floating gate may be conductive (typically polysilicon in most kinds of flash memory) or non-conductive (as in SONOS flash memory). [40]

Floating-gate MOSFET

In flash memory, each memory cell resembles a standard metal-oxide-semiconductor field-effect transistor (MOSFET) except that the transistor has two gates instead of one. The cells can be seen as an electrical switch in which current flows between two terminals (source and drain) and is controlled by a floating gate (FG) and a control gate (CG). The CG is similar to the gate in other MOS transistors, but below this, there is the FG insulated all around by an oxide layer. The FG is interposed between the CG and the MOSFET channel. Because the FG is electrically isolated by its insulating layer, electrons placed on it are trapped. When the FG is charged with electrons, this charge screens the electric field from the CG, thus, increasing the threshold voltage (VT1) of the cell. This means that now a higher voltage (VT2) must be applied to the CG to make the channel conductive. In order to read a value from the transistor, an intermediate voltage between the threshold voltages (VT1 & VT2) is applied to the CG. If the channel conducts at this intermediate voltage, the FG must be uncharged (if it was charged, we would not get conduction because the intermediate voltage is less than VT2), and hence, a logical "1" is stored in the gate. If the channel does not conduct at the intermediate voltage, it indicates that the FG is charged, and hence, a logical "0" is stored in the gate. The presence of a logical "0" or "1" is sensed by determining whether there is current flowing through the transistor when the intermediate voltage is asserted on the CG. In a multi-level cell device, which stores more than one bit per cell, the amount of current flow is sensed (rather than simply its presence or absence), in order to determine more precisely the level of charge on the FG.

Fowler–Nordheim tunneling

The process of moving electrons from the control gate and into the floating gate is called Fowler–Nordheim tunneling, and it fundamentally changes the characteristics of the cell by increasing the MOSFET’s threshold voltage. This, in turn, changes the drain-source current that flows through the transistor for a given gate voltage, which is ultimately used to encode a binary value. The Fowler-Nordheim tunneling effect is reversible, so electrons can be added to or removed from the floating gate, processes traditionally known as writing and erasing. [41]

Internal charge pumps

Despite the need for relatively high programming and erasing voltages, virtually all flash chips today require only a single supply voltage and produce the high voltages using on-chip charge pumps.

Over half the energy used by a 1.8 V NAND flash chip is lost in the charge pump itself. Since boost converters are inherently more efficient than charge pumps, researchers developing low-power SSDs have proposed returning to the dual Vcc/Vpp supply voltages used on all the early flash chips, driving the high Vpp voltage for all flash chips in a SSD with a single shared external boost converter. [42] [43] [44] [45] [46] [47] [48] [49]

In spacecraft and other high-radiation environments, the on-chip charge pump is the first part of the flash chip to fail, although flash memories will continue to work  in read-only mode  at much higher radiation levels. [50]

NOR flash

NOR flash memory wiring and structure on silicon NOR flash layout.svg
NOR flash memory wiring and structure on silicon

In NOR flash, each cell has one end connected directly to ground, and the other end connected directly to a bit line. This arrangement is called "NOR flash" because it acts like a NOR gate: when one of the word lines (connected to the cell's CG) is brought high, the corresponding storage transistor acts to pull the output bit line low. NOR flash continues to be the technology of choice for embedded applications requiring a discrete non-volatile memory device. The low read latencies characteristic of NOR devices allow for both direct code execution and data storage in a single memory product. [51]

Programming

Programming a NOR memory cell (setting it to logical 0), via hot-electron injection Flash-Programming.svg
Programming a NOR memory cell (setting it to logical 0), via hot-electron injection
Erasing a NOR memory cell (setting it to logical 1), via quantum tunneling Flash erase.svg
Erasing a NOR memory cell (setting it to logical 1), via quantum tunneling

A single-level NOR flash cell in its default state is logically equivalent to a binary "1" value, because current will flow through the channel under application of an appropriate voltage to the control gate, so that the bitline voltage is pulled down. A NOR flash cell can be programmed, or set to a binary "0" value, by the following procedure:

  • an elevated on-voltage (typically >5 V) is applied to the CG
  • the channel is now turned on, so electrons can flow from the source to the drain (assuming an NMOS transistor)
  • the source-drain current is sufficiently high to cause some high energy electrons to jump through the insulating layer onto the FG, via a process called hot-electron injection.

Erasing

To erase a NOR flash cell (resetting it to the "1" state), a large voltage of the opposite polarity is applied between the CG and source terminal, pulling the electrons off the FG through quantum tunneling. Modern NOR flash memory chips are divided into erase segments (often called blocks or sectors). The erase operation can be performed only on a block-wise basis; all the cells in an erase segment must be erased together. Programming of NOR cells, however, generally can be performed one byte or word at a time.

NAND flash memory wiring and structure on silicon Nand flash structure.svg
NAND flash memory wiring and structure on silicon

NAND flash

NAND flash also uses floating-gate transistors, but they are connected in a way that resembles a NAND gate: several transistors are connected in series, and the bit line is pulled low only if all the word lines are pulled high (above the transistors' VT). These groups are then connected via some additional transistors to a NOR-style bit line array in the same way that single transistors are linked in NOR flash.

Compared to NOR flash, replacing single transistors with serial-linked groups adds an extra level of addressing. Whereas NOR flash might address memory by page then word, NAND flash might address it by page, word and bit. Bit-level addressing suits bit-serial applications (such as hard disk emulation), which access only one bit at a time. Execute-in-place applications, on the other hand, require every bit in a word to be accessed simultaneously. This requires word-level addressing. In any case, both bit and word addressing modes are possible with either NOR or NAND flash.

To read data, first the desired group is selected (in the same way that a single transistor is selected from a NOR array). Next, most of the word lines are pulled up above the VT of a programmed bit, while one of them is pulled up to just over the VT of an erased bit. The series group will conduct (and pull the bit line low) if the selected bit has not been programmed.

Despite the additional transistors, the reduction in ground wires and bit lines allows a denser layout and greater storage capacity per chip. (The ground wires and bit lines are actually much wider than the lines in the diagrams.) In addition, NAND flash is typically permitted to contain a certain number of faults (NOR flash, as is used for a BIOS  ROM, is expected to be fault-free). Manufacturers try to maximize the amount of usable storage by shrinking the size of the transistors.

Writing and erasing

NAND flash uses tunnel injection for writing and tunnel release for erasing. NAND flash memory forms the core of the removable USB storage devices known as USB flash drives, as well as most memory card formats and solid-state drives available today.

The architecture of NAND Flash means that data can be read and programmed in pages, typically between 4 KiB and 16 KiB in size, but can only be erased at the level of entire blocks consisting of multiple pages and MB in size. When a block is erased all the cells are logically set to 1. Data can only be programmed in one pass to a page in a block that was erased. Any cells that have been set to 0 by programming can only be reset to 1 by erasing the entire block. This means that before new data can be programmed into a page that already contains data, the current contents of the page plus the new data must be copied to a new, erased page. If a suitable page is available, the data can be written to it immediately. If no erased page is available, a block must be erased before copying the data to a page in that block. The old page is then marked as invalid and is available for erasing and reuse. [52]

Vertical NAND

Vertical NAND (V-NAND) or 3D NAND memory stacks memory cells vertically and uses a charge trap flash architecture. The vertical layers allow larger areal bit densities without requiring smaller individual cells. [53] It is also known as 3D NAND or BiCS Flash. 3D NAND was first announced by Toshiba in 2007. [28] V-NAND was first commercially manufactured by Samsung Electronics in 2013. [29] [30] [54] [55]

Structure

V-NAND uses a charge trap flash geometry (which was commercially introduced in 2002 by AMD and Fujitsu) [27] that stores charge on an embedded silicon nitride film. Such a film is more robust against point defects and can be made thicker to hold larger numbers of electrons. V-NAND wraps a planar charge trap cell into a cylindrical form. [53]

The hierarchical structure of NAND Flash starts at a cell level which establishes strings, then pages, blocks, planes and ultimately a die. A string is a series of connected NAND cells in which the source of one cell is connected to the drain of the next one. Depending on the NAND technology, a string typically consists of 32 to 128 NAND cells. Strings are organised into pages which are then organised into blocks in which each string is connected to a separate line called a bitline (BL) All cells with the same position in the string are connected through the control gates by a wordline (WL) A plane contains a certain number of blocks that are connected through the same BL. A Flash die consists of one or more planes, and the peripheral circuitry that is needed to perform all the read/ write/ erase operations.

An individual memory cell is made up of one planar polysilicon layer containing a hole filled by multiple concentric vertical cylinders. The hole's polysilicon surface acts as the gate electrode. The outermost silicon dioxide cylinder acts as the gate dielectric, enclosing a silicon nitride cylinder that stores charge, in turn enclosing a silicon dioxide cylinder as the tunnel dielectric that surrounds a central rod of conducting polysilicon which acts as the conducting channel. [53]

Memory cells in different vertical layers do not interfere with each other, as the charges cannot move vertically through the silicon nitride storage medium, and the electric fields associated with the gates are closely confined within each layer. The vertical collection is electrically identical to the serial-linked groups in which conventional NAND flash memory is configured. [53]

Construction

Growth of a group of V-NAND cells begins with an alternating stack of conducting (doped) polysilicon layers and insulating silicon dioxide layers. [53]

The next step is to form a cylindrical hole through these layers. In practice, a 128  Gibit V-NAND chip with 24 layers of memory cells requires about 2.9 billion such holes. Next, the hole's inner surface receives multiple coatings, first silicon dioxide, then silicon nitride, then a second layer of silicon dioxide. Finally, the hole is filled with conducting (doped) polysilicon. [53]

Performance

As of 2013, V-NAND flash architecture allows read and write operations twice as fast as conventional NAND and can last up to 10 times as long, while consuming 50 percent less power. They offer comparable physical bit density using 10-nm lithography but may be able to increase bit density by up to two orders of magnitude. [53]

Limitations

Block erasure

One limitation of flash memory is that, although it can be read or programmed a byte or a word at a time in a random access fashion, it can be erased only a block at a time. This generally sets all bits in the block to 1. Starting with a freshly erased block, any location within that block can be programmed. However, once a bit has been set to 0, only by erasing the entire block can it be changed back to 1. In other words, flash memory (specifically NOR flash) offers random-access read and programming operations but does not offer arbitrary random-access rewrite or erase operations. A location can, however, be rewritten as long as the new value's 0 bits are a superset of the over-written values. For example, a nibble value may be erased to 1111, then written as 1110. Successive writes to that nibble can change it to 1010, then 0010, and finally 0000. Essentially, erasure sets all bits to 1, and programming can only clear bits to 0. [56] Some file systems designed for flash devices make use of this rewrite capability, for example Yaffs1, to represent sector metadata. Other flash file systems, such as YAFFS2, never make use of this "rewrite" capability -- they do a lot of extra work to meet a "write once rule".

Although data structures in flash memory cannot be updated in completely general ways, this allows members to be "removed" by marking them as invalid. This technique may need to be modified for multi-level cell devices, where one memory cell holds more than one bit.

Common flash devices such as USB flash drives and memory cards provide only a block-level interface, or flash translation layer (FTL), which writes to a different cell each time to wear-level the device. This prevents incremental writing within a block; however, it does help the device from being prematurely worn out by intensive write patterns.

Memory wear

Another limitation is that flash memory has a finite number of program  erase cycles (typically written as P/E cycles). Most commercially available flash products are guaranteed to withstand around 100,000 P/E cycles before the wear begins to deteriorate the integrity of the storage. [57] Micron Technology and Sun Microsystems announced an SLC NAND flash memory chip rated for 1,000,000 P/E cycles on 17 December 2008. [58]

The guaranteed cycle count may apply only to block zero (as is the case with TSOP  NAND devices), or to all blocks (as in NOR). This effect is mitigated in some chip firmware or file system drivers by counting the writes and dynamically remapping blocks in order to spread write operations between sectors; this technique is called wear leveling. Another approach is to perform write verification and remapping to spare sectors in case of write failure, a technique called bad block management (BBM). For portable consumer devices, these wear out management techniques typically extend the life of the flash memory beyond the life of the device itself, and some data loss may be acceptable in these applications. For high-reliability data storage, however, it is not advisable to use flash memory that would have to go through a large number of programming cycles. This limitation is meaningless for 'read-only' applications such as thin clients and routers, which are programmed only once or at most a few times during their lifetimes.

In December 2012, Taiwanese engineers from Macronix revealed their intention to announce at the 2012 IEEE International Electron Devices Meeting that they had figured out how to improve NAND flash storage read/write cycles from 10,000 to 100 million cycles using a "self-healing" process that used a flash chip with "onboard heaters that could anneal small groups of memory cells." [59] The built-in thermal annealing was to replace the usual erase cycle with a local high temperature process that not only erased the stored charge, but also repaired the electron-induced stress in the chip, giving write cycles of at least 100 million. [60] The result was to be a chip that could be erased and rewritten over and over, even when it should theoretically break down. As promising as Macronix’s breakthrough might have been for the mobile industry, however, there were no plans for a commercial product to be released any time in the near future. [61]

Read disturb

The method used to read NAND flash memory can cause nearby cells in the same memory block to change over time (become programmed). This is known as read disturb. The threshold number of reads is generally in the hundreds of thousands of reads between intervening erase operations. If reading continually from one cell, that cell will not fail but rather one of the surrounding cells on a subsequent read. To avoid the read disturb problem the flash controller will typically count the total number of reads to a block since the last erase. When the count exceeds a target limit, the affected block is copied over to a new block, erased, then released to the block pool. The original block is as good as new after the erase. If the flash controller does not intervene in time, however, a read disturb error will occur with possible data loss if the errors are too numerous to correct with an error-correcting code. [62] [63] [64]

X-ray effects

Most flash ICs come in ball grid array (BGA) packages, and even the ones that do not are often mounted on a PCB next to other BGA packages. After PCB Assembly, boards with BGA packages are often X-rayed to see if the balls are making proper connections to the proper pad, or if the BGA needs rework. These X-rays can erase programmed bits in a flash chip (convert programmed "0" bits into erased "1" bits). Erased bits ("1" bits) are not affected by X-rays. [65] [66]

Some manufacturers are now making X-ray proof SD [67] and USB [68] memory devices.

Low-level access

The low-level interface to flash memory chips differs from those of other memory types such as DRAM, ROM, and EEPROM, which support bit-alterability (both zero to one and one to zero) and random access via externally accessible address buses.

NOR memory has an external address bus for reading and programming. For NOR memory, reading and programming are random-access, and unlocking and erasing are block-wise. For NAND memory, reading and programming are page-wise, and unlocking and erasing are block-wise.

NOR memories

NOR flash by Intel IPhone 3G teardown - Intel 3050M0Y0CE -3303.jpg
NOR flash by Intel

Reading from NOR flash is similar to reading from random-access memory, provided the address and data bus are mapped correctly. Because of this, most microprocessors can use NOR flash memory as execute in place (XIP) memory, meaning that programs stored in NOR flash can be executed directly from the NOR flash without needing to be copied into RAM first. NOR flash may be programmed in a random-access manner similar to reading. Programming changes bits from a logical one to a zero. Bits that are already zero are left unchanged. Erasure must happen a block at a time, and resets all the bits in the erased block back to one. Typical block sizes are 64, 128, or 256  KiB.

Bad block management is a relatively new feature in NOR chips. In older NOR devices not supporting bad block management, the software or device driver controlling the memory chip must correct for blocks that wear out, or the device will cease to work reliably.

The specific commands used to lock, unlock, program, or erase NOR memories differ for each manufacturer. To avoid needing unique driver software for every device made, special Common Flash Memory Interface (CFI) commands allow the device to identify itself and its critical operating parameters.

Besides its use as random-access ROM, NOR flash can also be used as a storage device, by taking advantage of random-access programming. Some devices offer read-while-write functionality so that code continues to execute even while a program or erase operation is occurring in the background. For sequential data writes, NOR flash chips typically have slow write speeds, compared with NAND flash.

Typical NOR flash does not need an error correcting code. [69]

NAND memories

NAND flash architecture was introduced by Toshiba in 1989. [70] These memories are accessed much like block devices, such as hard disks. Each block consists of a number of pages. The pages are typically 512, [71] 2,048 or 4,096 bytes in size. Associated with each page are a few bytes (typically 1/32 of the data size) that can be used for storage of an error correcting code (ECC) checksum.

Typical block sizes include:

While reading and programming is performed on a page basis, erasure can only be performed on a block basis. [74]

NAND devices also require bad block management by the device driver software or by a separate controller chip. SD cards, for example, include controller circuitry to perform bad block management and wear leveling. When a logical block is accessed by high-level software, it is mapped to a physical block by the device driver or controller. A number of blocks on the flash chip may be set aside for storing mapping tables to deal with bad blocks, or the system may simply check each block at power-up to create a bad block map in RAM. The overall memory capacity gradually shrinks as more blocks are marked as bad.

NAND relies on ECC to compensate for bits that may spontaneously fail during normal device operation. A typical ECC will correct a one-bit error in each 2048 bits (256 bytes) using 22 bits of ECC, or a one-bit error in each 4096 bits (512 bytes) using 24 bits of ECC. [75] If the ECC cannot correct the error during read, it may still detect the error. When doing erase or program operations, the device can detect blocks that fail to program or erase and mark them bad. The data is then written to a different, good block, and the bad block map is updated.

Hamming codes are the most commonly used ECC for SLC NAND flash. Reed-Solomon codes and Bose-Chaudhuri-Hocquenghem codes are commonly used ECC for MLC NAND flash. Some MLC NAND flash chips internally generate the appropriate BCH error correction codes. [69]

Most NAND devices are shipped from the factory with some bad blocks. These are typically marked according to a specified bad block marking strategy. By allowing some bad blocks, manufacturers achieve far higher yields than would be possible if all blocks had to be verified to be good. This significantly reduces NAND flash costs and only slightly decreases the storage capacity of the parts.

When executing software from NAND memories, virtual memory strategies are often used: memory contents must first be paged or copied into memory-mapped RAM and executed there (leading to the common combination of NAND + RAM). A memory management unit (MMU) in the system is helpful, but this can also be accomplished with overlays. For this reason, some systems will use a combination of NOR and NAND memories, where a smaller NOR memory is used as software ROM and a larger NAND memory is partitioned with a file system for use as a non-volatile data storage area.

NAND sacrifices the random-access and execute-in-place advantages of NOR. NAND is best suited to systems requiring high capacity data storage. It offers higher densities, larger capacities, and lower cost. It has faster erases, sequential writes, and sequential reads.

Standardization

A group called the Open NAND Flash Interface Working Group (ONFI) has developed a standardized low-level interface for NAND flash chips. This allows interoperability between conforming NAND devices from different vendors. The ONFI specification version 1.0 [76] was released on 28 December 2006. It specifies:

The ONFI group is supported by major NAND flash manufacturers, including Hynix, Intel, Micron Technology, and Numonyx, as well as by major manufacturers of devices incorporating NAND flash chips. [77]

Two major flash device manufacturers, Toshiba and Samsung, have chosen to use an interface of their own design known as Toggle Mode (and now Toggle V2.0). This interface isn't pin-to-pin compatible with the ONFI specification. The result is a product designed for one vendor's devices may not be able to use another vendor's devices. [78]

A group of vendors, including Intel, Dell, and Microsoft, formed a Non-Volatile Memory Host Controller Interface (NVMHCI) Working Group. [79] The goal of the group is to provide standard software and hardware programming interfaces for nonvolatile memory subsystems, including the "flash cache" device connected to the PCI Express bus.

Distinction between NOR and NAND flash

NOR and NAND flash differ in two important ways:

These two are linked by the design choices made in the development of NAND flash. A goal of NAND flash development was to reduce the chip area required to implement a given capacity of flash memory, and thereby to reduce cost per bit and increase maximum chip capacity so that flash memory could compete with magnetic storage devices like hard disks.[ citation needed ]

NOR and NAND flash get their names from the structure of the interconnections between memory cells. [80] In NOR flash, cells are connected in parallel to the bit lines, allowing cells to be read and programmed individually. The parallel connection of cells resembles the parallel connection of transistors in a CMOS NOR gate. In NAND flash, cells are connected in series, resembling a CMOS NAND gate. The series connections consume less space than parallel ones, reducing the cost of NAND flash. It does not, by itself, prevent NAND cells from being read and programmed individually.

Each NOR flash cell is larger than a NAND flash cell  10 F2 vs 4 F2  even when using exactly the same semiconductor device fabrication and so each transistor, contact, etc. is exactly the same size  because NOR flash cells require a separate metal contact for each cell. [81]

When NOR flash was developed, it was envisioned as a more economical and conveniently rewritable ROM than contemporary EPROM and EEPROM memories. Thus random-access reading circuitry was necessary. However, it was expected that NOR flash ROM would be read much more often than written, so the write circuitry included was fairly slow and could erase only in a block-wise fashion. On the other hand, applications that use flash as a replacement for disk drives do not require word-level write address, which would only add to the complexity and cost unnecessarily.[ citation needed ]

Because of the series connection and removal of wordline contacts, a large grid of NAND flash memory cells will occupy perhaps only 60% of the area of equivalent NOR cells [82] (assuming the same CMOS process resolution, for example, 130  nm, 90 nm, or 65 nm). NAND flash's designers realized that the area of a NAND chip, and thus the cost, could be further reduced by removing the external address and data bus circuitry. Instead, external devices could communicate with NAND flash via sequential-accessed command and data registers, which would internally retrieve and output the necessary data. This design choice made random-access of NAND flash memory impossible, but the goal of NAND flash was to replace mechanical hard disks, not to replace ROMs.

AttributeNANDNOR
Main applicationFile storageCode execution
Storage capacityHighLow
Cost per bitBetter
Active powerBetter
Standby powerBetter
Write speedGood
Read speedGood

Write endurance

The write endurance of SLC floating-gate NOR flash is typically equal to or greater than that of NAND flash, while MLC NOR and NAND flash have similar endurance capabilities. Examples of endurance cycle ratings listed in datasheets for NAND and NOR flash, as well as in storage devices using flash memory, are provided. [83]

Type of flash memoryEndurance rating (erases per block)Example(s) of flash memory or storage device
SLC NAND100,000Samsung OneNAND KFW4G16Q2M, Toshiba SLC NAND Flash chips [84] [85] [86] [87] [84] [88] , Transcend SD500, Fujitsu S26361-F3298
MLC NAND5,000 to 10,000 for medium-capacity applications;
1,000 to 3,000 for high-capacity applications [89]
Samsung K9G8G08U0M (Example for medium-capacity applications), Memblaze PBlaze4 [90] , ADATA SU900, Mushkin Reactor
TLC NAND1,000Samsung SSD 840
3D SLC NAND100,000Samsung Z-NAND [91] [91]
3D MLC NAND6,000 to 40,000Samsung SSD 850 PRO, Samsung SSD 845DC PRO [92] [93] , Samsung 860 PRO
3D TLC NAND1,000 to 3,000Samsung SSD 850 EVO, Samsung SSD 845DC EVO, Crucial MX300 [94] [95] [96] ,Memblaze PBlaze5 900, Memblaze PBlaze5 700, Memblaze PBlaze5 910/916,Memblaze PBlaze5 510/516 [97] [98] [99] [100] , ADATA SX 8200 PRO (also being sold under "XPG Gammix" branding, model S11 PRO)
3D QLC NAND100 to 1,000Samsung SSD 860 QVO SATA, Intel SSD 660p, Samsung SSD 980 QVO NVMe, Micron 5210 ION, Samsung SSD BM991 NVMe [101] [102] [103] [104] [105] [106] [107] [108]
3D PLC NANDUnknownIn development by Intel and Toshiba. [89]
SLC (floating-gate) NOR100,000 to 1,000,000Numonyx M58BW (Endurance rating of 100,000 erases per block);
Spansion S29CD016J (Endurance rating of 1,000,000 erases per block)
MLC (floating-gate) NOR100,000Numonyx J3 flash

However, by applying certain algorithms and design paradigms such as wear leveling and memory over-provisioning, the endurance of a storage system can be tuned to serve specific requirements. [3] [109]

In order to compute the longevity of the NAND flash, one must account for the size of the memory chip, the type of memory (e.g. SLC/MLC/TLC), and use pattern.

3D NAND performance may degrade as layers are added. [91]

Flash file systems

Because of the particular characteristics of flash memory, it is best used with either a controller to perform wear leveling and error correction or specifically designed flash file systems, which spread writes over the media and deal with the long erase times of NOR flash blocks. [110] The basic concept behind flash file systems is the following: when the flash store is to be updated, the file system will write a new copy of the changed data to a fresh block, remap the file pointers, then erase the old block later when it has time.

In practice, flash file systems are used only for memory technology devices (MTDs), which are embedded flash memories that do not have a controller. Removable flash memory cards and USB flash drives have built-in controllers to perform wear leveling and error correction so use of a specific flash file system does not add any benefit.

Capacity

Multiple chips are often arrayed to achieve higher capacities [111] for use in consumer electronic devices such as multimedia players or GPSs. The capacity of flash chips generally follows Moore's Law because they are manufactured with many of the same integrated circuits techniques and equipment.

Consumer flash storage devices typically are advertised with usable sizes expressed as a small integer power of two (2, 4, 8, etc.) and a designation of megabytes (MB) or gigabytes (GB); e.g., 512 MB, 8 GB. This includes SSDs marketed as hard drive replacements, in accordance with traditional hard drives, which use decimal prefixes. [112] Thus, an SSD marked as "64  GB" is at least 64 × 10003 bytes (64 GB). Most users will have slightly less capacity than this available for their files, due to the space taken by file system metadata.

The flash memory chips inside them are sized in strict binary multiples, but the actual total capacity of the chips is not usable at the drive interface. It is considerably larger than the advertised capacity in order to allow for distribution of writes (wear leveling), for sparing, for error correction codes, and for other metadata needed by the device's internal firmware.

In 2005, Toshiba and SanDisk developed a NAND flash chip capable of storing 1 GB of data using multi-level cell (MLC) technology, capable of storing two bits of data per cell. In September 2005, Samsung Electronics announced that it had developed the world’s first 2 GB chip. [113]

In March 2006, Samsung announced flash hard drives with a capacity of 4 GB, essentially the same order of magnitude as smaller laptop hard drives, and in September 2006, Samsung announced an 8 GB chip produced using a 40 nm manufacturing process. [114] In January 2008, SanDisk announced availability of their 16 GB MicroSDHC and 32 GB SDHC Plus cards. [115] [116]

More recent flash drives (as of 2012) have much greater capacities, holding 64, 128, and 256 GB. [117]

A joint development at Intel and Micron will allow the production of 32-layer 3.5 terabyte (TB) NAND flash sticks and 10 TB standard-sized SSDs. The device includes 5 packages of 16 × 48 GB TLC dies, using a floating gate cell design. [118]

Flash chips continue to be manufactured with capacities under or around 1 MB, e.g., for BIOS-ROMs and embedded applications.

In July 2016, Samsung announced the 4TB Samsung 850 EVO which utilizes their 256 Gb 48-layer TLC 3D V-NAND. [119] In August 2016, Samsung announced a 32 TB 2.5-inch SAS SSD based on their 512 Gb 64-layer TLC 3D V-NAND. Further, Samsung expects to unveil SSDs with up to 100 TB of storage by 2020. [120]

Transfer rates

Flash memory devices are typically much faster at reading than writing. [121] Performance also depends on the quality of storage controllers which become more critical when devices are partially full. [121] Even when the only change to manufacturing is die-shrink, the absence of an appropriate controller can result in degraded speeds. [122]

Applications

Serial flash

Serial Flash: Silicon Storage Tech SST25VF080B IPhone 3G teardown - Silicon Storage Tech SST25VF080B-3309.jpg
Serial Flash: Silicon Storage Tech SST25VF080B

Serial flash is a small, low-power flash memory that provides only serial access to the data - rather than addressing individual bytes, the user reads or writes large contiguous groups of bytes in the address space serially. Serial Peripheral Interface Bus (SPI) is a typical protocol for accessing the device. When incorporated into an embedded system, serial flash requires fewer wires on the PCB than parallel flash memories, since it transmits and receives data one bit at a time. This may permit a reduction in board space, power consumption, and total system cost.

There are several reasons why a serial device, with fewer external pins than a parallel device, can significantly reduce overall cost:

There are two major SPI flash types. The first type is characterized by small pages and one or more internal SRAM page buffers allowing a complete page to be read to the buffer, partially modified, and then written back (for example, the Atmel AT45 DataFlash or the Micron Technology Page Erase NOR Flash). The second type has larger sectors. The smallest sectors typically found in an SPI flash are 4 kB, but they can be as large as 64 kB. Since the SPI flash lacks an internal SRAM buffer, the complete page must be read out and modified before being written back, making it slow to manage. SPI flash is cheaper than DataFlash and is therefore a good choice when the application is code shadowing.

The two types are not easily exchangeable, since they do not have the same pinout, and the command sets are incompatible.

Most FPGAs are based on SRAM configuration cells and require an external configuration device, often a serial flash chip, to reload the configuration bitstream every power cycle. [123]

Firmware storage

With the increasing speed of modern CPUs, parallel flash devices are often much slower than the memory bus of the computer they are connected to. Conversely, modern SRAM offers access times below 10  ns, while DDR2 SDRAM offers access times below 20 ns. Because of this, it is often desirable to shadow code stored in flash into RAM; that is, the code is copied from flash into RAM before execution, so that the CPU may access it at full speed. Device firmware may be stored in a serial flash device, and then copied into SDRAM or SRAM when the device is powered-up. [124] Using an external serial flash device rather than on-chip flash removes the need for significant process compromise (a manufacturing process that is good for high-speed logic is generally not good for flash and vice versa). Once it is decided to read the firmware in as one big block it is common to add compression to allow a smaller flash chip to be used. Typical applications for serial flash include storing firmware for hard drives, Ethernet controllers, DSL modems, wireless network devices, etc.

Flash memory as a replacement for hard drives

One more recent application for flash memory is as a replacement for hard disks. Flash memory does not have the mechanical limitations and latencies of hard drives, so a solid-state drive (SSD) is attractive when considering speed, noise, power consumption, and reliability. Flash drives are gaining traction as mobile device secondary storage devices; they are also used as substitutes for hard drives in high-performance desktop computers and some servers with RAID and SAN architectures.

There remain some aspects of flash-based SSDs that make them unattractive. The cost per gigabyte of flash memory remains significantly higher than that of hard disks. [125] Also flash memory has a finite number of P/E cycles, but this seems to be currently under control since warranties on flash-based SSDs are approaching those of current hard drives. [126] In addition, deleted files on SSDs can remain for an indefinite period of time before being overwritten by fresh data; erasure or shred techniques or software that work well on magnetic hard disk drives have no effect on SSDs, compromising security and forensic examination.

For relational databases or other systems that require ACID transactions, even a modest amount of flash storage can offer vast speedups over arrays of disk drives. [127] [128]

In May 2006, Samsung Electronics announced two flash-memory based PCs, the Q1-SSD and Q30-SSD were expected to become available in June 2006, both of which used 32 GB SSDs, and were at least initially available only in South Korea. [129] The Q1-SSD and Q30-SSD launch was delayed and finally shipped in late August 2006. [130]

The first flash-memory based PC to become available was the Sony Vaio UX90, announced for pre-order on 27 June 2006 and began shipping in Japan on 3 July 2006 with a 16Gb flash memory hard drive. [131] In late September 2006 Sony upgraded the flash-memory in the Vaio UX90 to 32Gb. [132]

A solid-state drive was offered as an option with the first MacBook Air introduced in 2008, and from 2010 onwards, all models shipped with an SSD. Starting in late 2011, as part of Intel's Ultrabook initiative, an increasing number of ultra-thin laptops are being shipped with SSDs standard.

There are also hybrid techniques such as hybrid drive and ReadyBoost that attempt to combine the advantages of both technologies, using flash as a high-speed non-volatile cache for files on the disk that are often referenced, but rarely modified, such as application and operating system executable files.

Flash memory as RAM

As of 2012, there are attempts to use flash memory as the main computer memory, DRAM. [133]

Archival or long-term storage

It is unclear how long flash memory will persist under archival conditions  i.e., benign temperature and humidity with infrequent access with or without prophylactic rewrite. Datasheets of Atmel's flash-based "ATmega" microcontrollers typically promise retention times of 20 years at 85 °C (185 °F) and 100 years at 25 °C (77 °F). [134]

An article from CMU in 2015 writes that "Today's flash devices, which do not require flash refresh, have a typical retention age of 1 year at room temperature." And that temperature can lower the retention time exponentially. The phenomenon can be modeled by the Arrhenius equation. [135] [136]

FPGA configuration

Some FPGAs are based on flash configuration cells that are used directly as (programmable) switches to connect internal elements together, using the same kind of floating-gate transistor as the flash data storage cells in data storage devices. [123]

Industry

One source states that, in 2008, the flash memory industry includes about US$9.1 billion in production and sales. Other sources put the flash memory market at a size of more than US$20 billion in 2006, accounting for more than eight percent of the overall semiconductor market and more than 34 percent of the total semiconductor memory market. [137] In 2012, the market was estimated at $26.8 billion. [138]

Manufacturers

The following are the largest NAND flash memory manufacturers, as of the first quarter of 2019. [139]

  1. Toshiba Memory 35.1%
  2. Samsung Electronics 29.9%
  3. Micron Technology 16.5%
  4. SK Hynix 9.5%
  5. Intel 8.5%

Shipments

Flash memory shipments (est. manufactured units)
Year(s)Discrete flash memory chips Flash memory data capacity (gigabytes) Floating-gate MOSFET memory cells (billions)
199226,000,000 [140] 3 [140] 24 [lower-alpha 1]
199373,000,000 [140] 17 [140] 139 [lower-alpha 1]
1994112,000,000 [140] 25 [140] 203 [lower-alpha 1]
1995235,000,000 [140] 38 [140] 300 [lower-alpha 1]
1996359,000,000 [140] 140 [140] 1,121 [lower-alpha 1]
1997477,200,000+ [141] 317+ [141] 2,533+ [lower-alpha 1]
1998762,195,122 [142] 455+ [141] 3,642+ [lower-alpha 1]
199912,800,000,000 [143] 635+ [141] 5,082+ [lower-alpha 1]
20002004134,217,728,000 (NAND) [144] 1,073,741,824,000 (NAND) [144]
20052007?
20081,226,215,645 (mobile NAND) [145]
20091,226,215,645+ (mobile NAND)
20107,280,000,000+ [lower-alpha 2]
20118,700,000,000 [147]
20125,151,515,152 (serial) [148]
2013?
2014?59,000,000,000 [149] 118,000,000,000+ [lower-alpha 1]
20157,692,307,692 (NAND) [150] 85,000,000,000 [151] 170,000,000,000+ [lower-alpha 1]
2016?100,000,000,000 [152] 200,000,000,000+ [lower-alpha 1]
2017?148,200,000,000 [lower-alpha 3] 296,400,000,000+ [lower-alpha 1]
2018?231,640,000,000 [lower-alpha 4] 463,280,000,000+ [lower-alpha 1]
1992201845,358,454,134+ memory chips758,057,729,630+ gigabytes2,321,421,837,044 billion+ cells

In addition to individual flash memory chips, flash memory is also embedded in microcontroller (MCU) chips and system-on-chip (SoC) devices. [156] Flash memory is embedded in ARM chips, [156] which have sold 150 billion units worldwide as of 2019, [157] and in programmable system-on-chip (PSoC) devices, which have sold 1.1 billion units as of 2012. [158] This adds up to at least 151.1 billion MCU and SoC chips with embedded flash memory, in addition to the 45.4 billion known individual flash chip sales as of 2015, totalling at least 196.5 billion chips containing flash memory.

Flash scalability

Due to its relatively simple structure and high demand for higher capacity, NAND flash memory is the most aggressively scaled technology among electronic devices. The heavy competition among the top few manufacturers only adds to the aggressiveness in shrinking the floating-gate MOSFET design rule or process technology node. [63] While the expected shrink timeline is a factor of two every three years per original version of Moore's law, this has recently been accelerated in the case of NAND flash to a factor of two every two years.

ITRS or company201020112012201320142015201620172018
ITRS Flash Roadmap 2011 [159] 32 nm 22 nm 20 nm18 nm 16 nm
Updated ITRS Flash Roadmap [160] 17 nm15 nm 14 nm
Samsung [159] [160] [161]
(Samsung 3D NAND) [160]
35–20 nm [23] 27 nm21 nm
(MLC, TLC)
19–16 nm
19–10 nm (MLC, TLC) [162]
19–10 nm
V-NAND (24L)
16–10 nm
V-NAND (32L)
16–10 nm12–10 nm12–10 nm
Micron, Intel [159] [160] [161] 34–25 nm25 nm20 nm
(MLC + HKMG)
20 nm
(TLC)
16 nm16 nm
3D NAND
16 nm
3D NAND
12 nm
3D NAND
12 nm
3D NAND
Toshiba, WD (SanDisk) [159] [160] [161] 43–32 nm
24 nm (Toshiba) [163]
24 nm19 nm
(MLC, TLC)
15 nm15 nm
3D NAND
15 nm
3D NAND
12 nm
3D NAND
12 nm
3D NAND
SK Hynix [159] [160] [161] 46–35 nm26 nm20 nm (MLC)16 nm16 nm16 nm12 nm12 nm

As the MOSFET feature size of flash memory cells reaches the 15-16 nm minimum limit, further flash density increases will be driven by TLC (3 bits/cell) combined with vertical stacking of NAND memory planes. The decrease in endurance and increase in uncorrectable bit error rates that accompany feature size shrinking can be compensated by improved error correction mechanisms. [164] Even with these advances, it may be impossible to economically scale flash to smaller and smaller dimensions as the number of electron holding capacity reduces. Many promising new technologies (such as FeRAM, MRAM, PMC, PCM, ReRAM, and others) are under investigation and development as possible more scalable replacements for flash. [165]

Timeline

Date of introductionChip nameCapacity (bits)Flash type Cell type Manufacturer(s) Process AreaRef
1984?? NOR SLC Toshiba ?? [13]
1985?256 kb NORSLCToshiba2,000 nm ? [20]
1987?? NAND SLCToshiba?? [1]
1989?1 Mb NORSLC Seeq, Intel ?? [20]
4 MbNANDSLCToshiba 1,000 nm
1991?16 MbNORSLC Mitsubishi 600 nm ? [20]
1993DD28F032SA32 MbNORSLC Intel ?280 mm² [166] [167]
1994?64 MbNORSLC NEC 400 nm? [20]
1995?16 MbDINORSLCMitsubishi, Hitachi ?? [20] [168]
NANDSLCToshiba?? [169]
32 MbNANDSLCHitachi, Samsung, Toshiba?? [20]
34 Mb Serial SLC SanDisk
1996?64 MbNANDSLC Hitachi, Mitsubishi400 nm? [20]
QLC NEC
128 MbNANDSLCSamsung, Hitachi?
1997?32 MbNORSLCIntel, Sharp 400 nm? [170]
NANDSLCAMD, Fujitsu 350 nm
1999?256 MbNANDSLCToshiba 250 nm ? [20]
MLC Hitachi
2000?32 MbNORSLCToshiba250 nm? [20]
64 MbNORQLC STMicroelectronics 180 nm
512 MbNANDSLCToshiba?? [171]
2001?512 MbNANDMLCHitachi?? [20]
1 Gb NANDMLC Samsung
Toshiba, SanDisk160 nm? [172]
2002?512 Mb NROM MLC Saifun 170 nm? [20]
2 GbNANDSLCSamsung, Toshiba?? [173] [174]
2003?128 MbNORMLCIntel 130 nm ? [20]
1 GbNANDMLCHitachi
2004?8 GbNANDSLCSamsung60 nm? [173]
2005?16 GbNANDSLCSamsung50 nm? [23]
2006?32 GbNANDSLCSamsung 40 nm
April 2007THGAM128 Gb Stacked NANDSLCToshiba56 nm252 mm² [31]
September 2007?128 GbStacked NANDSLC Hynix ?? [32]
2008THGBM256 GbStacked NANDSLCToshiba43 nm353 mm² [33]
2009?32 GbNAND TLC Toshiba 32 nm 113 mm² [21]
64 GbNANDQLCToshiba, SanDisk43 nm? [21] [22]
2010?64 GbNANDSLCHynix 20 nm ? [175]
TLCSamsung20 nm? [23]
THGBM21 Tb Stacked NANDQLCToshiba32 nm374 mm² [34]
2011KLMCG8GE4A512 GbStacked NANDMLCSamsung?192 mm² [176]
2013??NANDSLC SK Hynix 16 nm ? [175]
128 Gb V-NAND TLCSamsung 10 nm ? [162]
2015?256 GbV-NANDTLCSamsung?? [23]
2017?512 GbV-NANDTLCSamsung?? [37]
768 GbV-NANDQLCToshiba?? [177]
KLUFG8R1EM4 TbStacked V-NANDTLCSamsung?150 mm² [37]
2018?1 TbV-NANDQLCSamsung?? [178]
1.33 TbV-NANDQLCToshiba?158 mm² [179] [180]
2019?512 GbV-NANDQLCSamsung?? [38] [39]
1 TbV-NANDTLCSK Hynix?? [181]
eUFS (1 TB)8 TbStacked V-NANDQLCSamsung?150 mm² [38] [39] [182]

See also

Notes

  1. 1 2 3 4 5 6 7 8 9 10 11 12 13 Single-level cell (1-bit per cell) up until 2009. Multi-level cell (up to 4-bit or half-byte per cell) commercialised in 2009. [21] [22]
  2. Flash memory chip shipments in 2010:
    • NOR – 3.64 billion [146]
    • NAND – 3.64 billion+ (est.)
  3. Flash memory data capacity shipments in 2017:
  4. Flash memory data capacity shipments in 2018 (est.)
    • NAND NVM – 140 exabytes [153]
    • SSD – 91.64 exabytes [155]

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Computer memory physical device used to store information for immediate use in a digital electronic device

In computing, memory refers to a device that is used to store information for immediate use in a computer or related computer hardware device. It typically refers to semiconductor memory, specifically metal-oxide-semiconductor (MOS) memory, where data is stored within MOSFET memory cells on a silicon integrated circuit chip. The term "memory" is often synonymous with the term "primary storage". Computer memory operates at a high speed, for example random-access memory (RAM), as a distinction from storage that provides slow-to-access information but offers higher capacities. If needed, contents of the computer memory can be transferred to secondary storage; a very common way of doing this is through a memory management technique called "virtual memory". An archaic synonym for memory is store.

Moores law Heuristic law stating that the number of transistors on a integrated circuit doubles every two years

Moore's law is the observation that the number of transistors in a dense integrated circuit doubles about every two years. The observation is named after Gordon Moore, the co-founder of Fairchild Semiconductor and CEO of Intel, whose 1965 paper described a doubling every year in the number of components per integrated circuit, and projected this rate of growth would continue for at least another decade. In 1975, looking forward to the next decade, he revised the forecast to doubling every two years, a compound annual growth rate (CAGR) of 41.4%.

An EPROM, or erasable programmable read-only memory, is a type of programmable read-only memory (PROM) chip that retains its data when its power supply is switched off. Computer memory that can retrieve stored data after a power supply has been turned off and back on is called non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in digital circuits. Once programmed, an EPROM can be erased by exposing it to strong ultraviolet light source. EPROMs are easily recognizable by the transparent fused quartz window in the top of the package, through which the silicon chip is visible, and which permits exposure to ultraviolet light during erasing.

Phase-change memory (also known as PCM, PCME, PRAM, PCRAM, OUM and C-RAM or CRAM is a type of non-volatile random-access memory. PRAMs exploit the unique behaviour of chalcogenide glass. In the older generation of PCM, heat produced by the passage of an electric current through a heating element generally made of TiN was used to either quickly heat and quench the glass, making it amorphous, or to hold it in its crystallization temperature range for some time, thereby switching it to a crystalline state. PCM also has the ability to achieve a number of distinct intermediary states, thereby having the ability to hold multiple bits in a single cell, but the difficulties in programming cells in this way has prevented these capabilities from being implemented in other technologies with the same capability.

Semiconductor memory is a digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to MOS memory, where data is stored within metal-oxide-semiconductor (MOS) memory cells on a silicon memory chip. MOS memory accounts for about 25% of the integrated circuit semiconductor industry. There are numerous different types of implementations using various MOS technologies. The two main types of random-access memory (RAM) are static RAM (SRAM), which uses several MOSFETs per memory cell, and dynamic RAM (DRAM), which uses a single MOSFET and MOS capacitor per cell. Non-volatile memory uses floating-gate memory cells, which consist of a single floating-gate MOSFET per cell.

In computing, a hybrid drive is a logical or physical storage device that combines a faster storage medium such as solid-state drive (SSD) with a higher-capacity hard disk drive (HDD). The intent is adding some of the speed of SSDs to the cost-effective storage capacity of traditional HDDs. The purpose of the SSD in a hybrid drive is to act as a cache for the data stored on the HDD, improving the overall performance by keeping copies of the most frequently used data on the faster SSD.

The 32 nanometer (32 nm) node is the step following the 45 nanometer process in CMOS (MOSFET) semiconductor device fabrication. "32 nanometer" refers to the average half-pitch of a memory cell at this technology level. Toshiba produced commercial 32 Gb NAND flash memory chips with the 32 nm process in 2009. Intel and AMD produced commercial microchips using the 32 nanometer process in the early 2010s. IBM and the Common Platform also developed a 32 nm high-κ metal gate process. Intel began selling its first 32 nm processors using the Westmere architecture on 7 January 2010.

Transistor count the number of transistors in a device

The transistor count is the number of transistors on an integrated circuit (IC). It typically refers to the number of MOSFETs on an IC chip, as all modern ICs use MOSFETs. It is the most common measure of IC complexity. The rate at which MOS transistor counts have increased generally follows Moore's law, which observed that the transistor count doubles approximately every two years.

Charge trap flash (CTF) is a semiconductor memory technology used in creating non-volatile NOR and NAND flash memory. It is a type of floating-gate MOSFET memory technology, but differs from the conventional floating-gate technology in that it uses a silicon nitride film to store electrons rather than the doped polycrystalline silicon typical of a floating-gate structure. This approach allows memory manufacturers to reduce manufacturing costs five ways:

  1. Fewer process steps are required to form a charge storage node
  2. Smaller process geometries can be used
  3. Multiple bits can be stored on a single flash memory cell.
  4. Improved reliability
  5. Higher yield since the charge trap is less susceptible to point defects in the tunnel oxide layer

SONOS, short for "silicon–oxide–nitride–oxide–silicon", more precisely, "polycrystalline silicon"—"silicon dioxide"—"silicon nitride"—"siicon dioxide"—"silicon", is a cross sectional structure of MOSFET (metal-oxide-semiconductor field-effect transistor), realized by P.C.Y. Chen of Fairchild Camera and Instrument in 1977. This structure is often used for non-volatile memories, such as EEPROM and flash memories. It is sometimes used for TFT LCD displays. It is one of CTF (charge trap flash) variants. It is distinguished from traditional non-volatile memory structures by the use of silicon nitride (Si3N4 or Si9N10) instead of "polysilicon-based FG (floating-gate)" for the charge storage material. A further variant is "SHINOS" ("silicon"—"hi-k"—"nitride"—"oxide"—"silicon"), which is substituted top oxide layer with high-κ material. Another advanced variant is "MONOS" ("metal–oxide–nitride–oxide–silicon"). Companies offering SONOS-based products include Cypress Semiconductor, Macronix, Toshiba, United Microelectronics Corporation and Floadia.

The 22 nanometer (22 nm) node is the process step following the 32 nm in MOSFET (CMOS) semiconductor device fabrication. The typical half-pitch for a memory cell using the process is around 22 nm. It was first demonstrated by semiconductor companies for use in RAM memory in 2008. In 2010, Toshiba began shipping 24 nm flash memory chips, and Samsung Electronics began mass-producing 20 nm flash memory chips. The first consumer-level CPU deliveries using a 22 nm process started in April 2012.

In electronics, a multi-level cell (MLC) is a memory cell/element capable of storing more than a single bit of information, compared to a single-level cell (SLC) which can store only one bit per memory cell/element. A memory cell typically consists of a single MOSFET, thus multi-level cells reduce the number of MOSFETs required to store the same amount of data as single-level cells.

A three-dimensional integrated circuit is a MOS integrated circuit (IC) manufactured by stacking silicon wafers or dies and interconnecting them vertically using, for instance, through-silicon vias (TSVs) or Cu-Cu connections, so that they behave as a single device to achieve performance improvements at reduced power and smaller footprint than conventional two dimensional processes. The 3D IC is one of several 3D integration schemes that exploit the z-direction to achieve electrical performance benefits, in microelectronics and nanoelectronics.

Random-access memory Form of computer data storage

Random-access memory is a form of computer memory that can be read and changed in any order, typically used to store working data and machine code. A random-access memory device allows data items to be read or written in almost the same amount of time irrespective of the physical location of data inside the memory. In contrast, with other direct-access data storage media such as hard disks, CD-RWs, DVD-RWs and the older magnetic tapes and drum memory, the time required to read and write data items varies significantly depending on their physical locations on the recording medium, due to mechanical limitations such as media rotation speeds and arm movement.

Memory cell (computing) part of computer memory

The memory cell is the fundamental building block of computer memory. The memory cell is an electronic circuit that stores one bit of binary information and it must be set to store a logic 1 and reset to store a logic 0. Its value is maintained/stored until it is changed by the set/reset process. The value in the memory cell can be accessed by reading it.

An open-channel solid state drive is a solid-state drive which does not have a firmware Flash Translation Layer implemented on the device, but instead leaves the management of the physical solid-state storage to the computer's operating system. The Linux 4.4 kernel is an example of an operating system kernel that supports open-channel SSDs which follow the NVM Express specification. The interface used by the operating system to acces open-channel solid state drives is called LightNVM.

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