Non-volatile memory

Computer memory types
Semiconductor memory
Volatile
RAM DRAM (e.g., DDR SDRAM) SRAM In development T-RAM Z-RAM TTRAM Historical Williams tube (1946-47) Delay line memory (1947) Selectron tube (1953)
Non-volatile
ROM Mask ROM PROM EPROM EEPROM NVRAM Flash memory Early stage NVRAM nvSRAM FeRAM MRAM PRAM In development CBRAM SONOS RRAM Racetrack memory NRAM Millipede memory Historical Drum memory (1932) Magnetic-core memory (1949) Plated wire memory (1957) Thin-film memory (1962) Twistor memory (~1968) Bubble memory (~1970)

Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, in the most basic sense, is computer memory that can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory, flash memory, ferroelectric RAM (F-RAM), EVMs (Electronic voting machines), most types of magnetic computer storage devices (e.g. hard disks, floppy disks, and magnetic tape), optical discs, and early computer storage methods such as paper tape and punched cards.

Non-volatile memory is typically used for the task of secondary storage, or long-term persistent storage. The most widely used form of primary storage today is a volatile form of random access memory (RAM), meaning that when the computer is shut down, anything contained in RAM is lost. Unfortunately, most forms of non-volatile memory have limitations that make them unsuitable for use as primary storage. Typically, non-volatile memory either costs more or performs worse than volatile random access memory.

Several companies are working on developing non-volatile memory systems comparable in speed and capacity to volatile RAM. IBM is currently developing MRAM (Magnetoresistive RAM). Not only would such technology save energy, but it would allow for computers that could be turned on and off almost instantly, bypassing the slow start-up and shutdown sequence. In addition, Ramtron International has developed, produced, and licensed ferroelectric RAM (F-RAM), a technology that offers distinct properties from other nonvolatile memory options, including extremely high endurance (exceeding 10¹⁶ for 3.3 V devices), ultra low power consumption (since F-RAM does not require a charge pump like other non-volatile memories), single-cycle write speeds, and gamma radiation tolerance. Other companies that have licensed and produced F-RAM technology include Texas Instruments, Rohm, and Fujitsu.

Non-volatile data storage can be categorized in electrically addressed systems (read-only memory) and mechanically addressed systems (hard disks, optical disc, magnetic tape, holographic memory, and such). Electrically addressed systems are expensive, but fast, whereas mechanically addressed systems have a low price per bit, but are slow. Non-volatile memory may one day eliminate the need for comparatively slow forms of secondary storage systems, which include hard disks.

Electrically addressed

Electrically addressed non-volatile memories based on charge storage can be categorized according to their write mechanism:

Mask-programmed ROM

Main article: Mask ROM

One of the earliest forms of non-volatile read-only memory, the mask-programmed ROM was prewired at the design stage to contain specific data; once the mask was used to manufacture the integrated circuits, the data was cast in stone (silicon, actually) and could not be changed.

The mask ROM was therefore useful only for large-volume production, such as for read-only memories containing the start up code in early microcomputers. This program was often referred to as the "bootstrap", as in pulling oneself up by one's own bootstraps.

Due to the very high initial cost and inability to make revisions, the mask ROM is rarely, if ever, used in new designs.

Programmable ROM

Main article: Programmable read-only memory

The next approach was to create a chip which was initially blank; the programmable ROM originally contained silicon or metal fuses, which would be selectively "blown" or destroyed by a device programmer or PROM programmer in order to change 0s to 1s. Once the bits were changed, there was no way to restore them to their original condition. Non-volatile but still somewhat inflexible.

Early PAL programmable array logic chips used a similar programming approach to that used in the fuse-based PROMs.

Newer Antifuse-based PROMs (which are also referred to as one-time-programmable (OTP) memory) are widely used in consumer and automotive electronics, radio-frequency identification devices (RFID), implantable medical devices, and high-definition multimedia interfaces(HDMI) due to their small footprint, reliability, fast read speed, and long data retention rates.

Erasable PROMs

Main article: EPROM

There are two classes of non-volatile memory chips based on EPROM technology.

UV-erase EPROM

The original erasable non-volatile memories were EPROM's; these could be readily identified by the distinctive quartz window in the center of the chip package. These operated by trapping an electrical charge on the gate of a field-effect transistor in order to change a 1 to a 0 in memory. To remove the charge, one would place the chip under an intense short-wavelength fluorescent ultraviolet lamp for 20–30 minutes, returning the entire chip to its original blank (all ones) state.

OTP (one-time programmable) EPROM

An OTP is electrically an EPROM, but with the quartz window physically missing. Like the fuse, PROM it can be written once, but cannot be erased.

Electrically erasable PROM

Main article: EEPROM

Electrically erasable PROMs have the advantage of being able to selectively erase any part of the chip without the need to erase the entire chip and without the need to remove the chip from the circuit. While an erase and rewrite of a location appears nearly instantaneous to the user, the write process is slightly slower than the read process; the chip can be read at full system speeds.

The limited number of times a single location can be rewritten is usually in the 10000-100000 range; the capacity of an EEPROM also tends to be smaller than that of other non-volatile memories. Nonetheless, EEPROMs are useful for storing settings or configuration for devices ranging from dial-up modems to satellite receivers.

Flash memory

Main article: Flash memory

The flash memory chip is a close relative to the EEPROM; it differs in that it can only be erased one block or "page" at a time. It is a solid-state chip that maintains stored data without any external power source. Capacity is substantially larger than that of an EEPROM, making these chips a popular choice for digital cameras and desktop PC BIOS chips.

Flash memory devices use two different logical technologies—NOR and NAND—to map data. NOR flash provides high-speed random access, reading and writing data in specific memory locations; it can retrieve as little as a single byte. NAND flash reads and writes sequentially at high speed, handling data in small blocks called pages, however it is slower on read when compared to NOR. NAND flash reads faster than it writes, quickly transferring whole pages of data. Less expensive than NOR flash at high densities, NAND technology offers higher capacity for the same-size silicon.

List of NOR Flash providers:

• Atmel
• Intel
• Greenliant Systems
• Macronix
• Micron Technology (formerly Numonyx)
• Silicon Storage Technology (SST) (now Microchip Technology)
• Spansion
• STMicroelectronics

List of NAND Flash providers:

• Hynix
• Intel
• Micron Technology
• Qimonda
• Renesas Electronics Corporation
• Samsung
• Spansion
• STMicroelectronics
• Toshiba

Ferroelectric RAM (F-RAM)

Main article: Ferroelectric RAM

Ferroelectric RAM (FeRAM, F-RAM or FRAM) is a random-access memory similar in construction to DRAM but (instead of a dielectric layer like in DRAM) contains a thin ferroelectric film of lead zirconate titanate [Pb(Zr,Ti)O₃], commonly referred to as PZT. The Zr/Ti atoms in the PZT change polarity in an electric field, thereby producing a binary switch. Unlike RAM devices, F-RAM retains its data memory when power is shut off or interrupted, due to the PZT crystal maintaining polarity. Due to this crystal structure and how it is influenced, F-RAM offers distinct properties from other nonvolatile memory options, including extremely high endurance (exceeding 10¹⁶ for 3.3 V devices), ultra low power consumption (since F-RAM does not require a charge pump like other non-volatile memories), single-cycle write speeds, and gamma radiation tolerance.

Magnetoresistive RAM (MRAM)

Main article: Magnetoresistive Random Access Memory

Magnetoresistive RAM is one of the newest approaches to non-volatile memory and stores data in magnetic storage elements called magnetic tunnel junctions (MTJ's). MRAM has an especially promising future as it seeks to encompass all the desirable features of the other popular types of memory (non-volatility, infinite endurance, high-speed reading/writing, low cost).

The 1st generation of MRAM, such as Everspin Technologies' 4 Mbit, utilized field induced writing. The 2nd generation is being developed mainly through two approaches: Thermal Assisted Switching (TAS) which is being developed by Crocus Technology, and Spin Torque Transfer (STT) which Crocus, Hynix, IBM, and several other companies are developing.

Mechanically addressed systems

Mechanically addressed systems utilize a contact structure ('head') to read and write on a designated storage medium. Since circuitry layout is not a key factor for data density, the amount of storage is typically much larger than for electrically addressed systems.

Tape

Main article: Magnetic tape

Hard disk

Main article: Hard disk

Optical disk

Main article: Optical disc

Nanodrive

Main article: IBM Millipede

Holographic storage

Main article: Holographic memory

Organic

Main articles: Organic electronics and ferroelectric memory

There are polymer printed ferroelectric memory.

Thin Film Electronics ("Thinfilm") produces rewriteable non-volatile organic memory based on ferroelectric polymers. Thinfilm successfully demonstrated roll-to-roll printed memories in 2009.

In Thinfilm's organic memory the ferroelectric polymer is sandwiched between two sets of electrodes in a passive matrix. Each crossing of metal lines is a ferroelectric capacitor and defines a memory cell. This gives a non-volatile memory comparable to ferroelectric RAM technologies and offer the same functionality as flash memory.

Specifications

Specification (March 2007)	2.5" HDD	1" microdrive	Flash memory	Optical disc	Tape	MRAM
Device model	Hitachi Travelstar 5k160	Hitachi Microdrive 3k8	Hynix HY27UH08AG5M	Blu-ray	HP Ultrium 960	Everspin (formerly of Freescale Semiconductor) MR2A16A
Density (GBit/cm2)	20.3	18.4	6.7	3.8	0.047	0.0021
Capacity (GByte)	160	8	2	50	400	0.004
Price per bit (Eur/GByte)	1.5	9.0	6.0	1.25	0.075	35000
Price per unit (Eur)	110	87	14	635	2340	17.4
Price per medium (Eur) (For removables)	nd	nd	nd	40	30	nd
Data rate (Mbit/s)	540	80	23	144	640	436
Access time (ms) (Average/typical)	11	12	0.025	180	72000	1.000035
Power consumption (W) (Average)	1.8	0.6	0.1	25	20	0.08
Form factor (h x w x d) (cm)	0.95x7x10	0.5x3x4	0.1x1.2x2	4x15x19	2x10x10	0.1x1x1.8