Nano-RAM is a proprietary
computer memory
In computing, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and digital electronic devices. The term ''memory'' is often synonymous with the term '' primary storag ...
technology from the company Nantero. It is a type of
nonvolatile random-access memory based on the position of
carbon nanotubes
A scanning tunneling microscopy image of a single-walled carbon nanotube
Rotating single-walled zigzag carbon nanotube
A carbon nanotube (CNT) is a tube made of carbon with diameters typically measured in nanometers.
''Single-wall carbon na ...
deposited on a chip-like substrate. In theory, the small size of the nanotubes allows for very high density memories. Nantero also refers to it as NRAM.
Technology
The first generation Nantero NRAM technology was based on a three-terminal
semiconductor device where a third terminal is used to switch the memory cell between memory states. The second generation NRAM technology is based on a two-terminal memory cell. The two-terminal cell has advantages such as a smaller cell size, better scalability to sub-20 nm nodes (see
semiconductor device fabrication
Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically integrated circuit (IC) chips such as modern computer processors, microcontrollers, and memory chips such as NAND flash and DRAM that are p ...
), and the ability to
passivate the memory cell during fabrication.
In a non-woven fabric matrix of
carbon nanotubes
A scanning tunneling microscopy image of a single-walled carbon nanotube
Rotating single-walled zigzag carbon nanotube
A carbon nanotube (CNT) is a tube made of carbon with diameters typically measured in nanometers.
''Single-wall carbon na ...
(CNTs), crossed nanotubes can either be touching or slightly separated depending on their position. When touching, the carbon nanotubes are held together by
Van der Waals forces
In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and th ...
. Each NRAM "cell" consists of an interlinked network of CNTs located between two electrodes as illustrated in Figure 1. The CNT fabric is located between two metal electrodes, which is defined and etched by
photolithography, and forms the NRAM cell.
![NRAM fabric](https://upload.wikimedia.org/wikipedia/commons/8/8a/NRAM_fabric.png)
The NRAM acts as a resistive non-volatile
random-access memory
Random-access memory (RAM; ) 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 ...
(RAM) and can be placed in two or more resistive modes depending on the resistive state of the CNT fabric. When the CNTs are not in contact the
resistance state of the fabric is high and represents an "off" or "0" state. When the CNTs are brought into contact, the resistance state of the fabric is low and represents an "on" or "1" state. NRAM acts as a memory because the two resistive states are very stable. In the 0 state, the CNTs (or a portion of them) are not in contact and remain in a separated state due to the stiffness of the CNTs resulting in a high resistance or low current measurement state between the top and bottom electrodes. In the 1 state, the CNTs (or a portion of them) are in contact and remain contacted due to Van der Waals forces between the CNTs, resulting in a low resistance or high current measurement state between the top and bottom electrodes. Note that other sources of resistance such as contact resistance between electrode and CNT can be significant and also need to be considered.
To switch the NRAM between states, a small voltage greater than the read voltage is applied between top and bottom electrodes. If the NRAM is in the 0 state, the voltage applied will cause an electrostatic attraction between the CNTs close to each other causing a SET operation. After the applied voltage is removed, the CNTs remain in a 1 or low resistance state due to physical adhesion (Van der Waals force) with an
activation energy
In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...
(E
a) of approximately 5eV. If the NRAM cell is in the 1 state, applying a voltage greater than the read voltage will generate CNT phonon excitations with sufficient energy to separate the CNT junctions. This is the phonon driven RESET operation. The CNTs remain in the OFF or high resistance state due to the high mechanical stiffness (
Young's Modulus
Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied le ...
1 TPa) with an activation energy (E
a) much greater than 5 eV. Figure 2 illustrates both states of an individual pair of CNTs involved in the switch operation. Due to the high activation energy (> 5eV) required for switching between states, the NRAM switch resists outside interference like radiation and
operating temperature that can erase or flip conventional memories like
DRAM.
![NRAM contact point](https://upload.wikimedia.org/wikipedia/commons/f/f0/NRAM_contact_point.png)
NRAMs are fabricated by depositing a uniform layer of CNTs onto a prefabricated array of drivers such as transistors as shown in Figure 1. The bottom electrode of the NRAM cell is in contact with the underlying
via (electronics)
A via (Latin for ''path'' or ''way'') is an electrical connection between copper layers in a printed circuit board. Essentially a via is a small drilled hole that goes through two or more adjacent layers; the hole is plated with copper that fo ...
connecting the cell to the driver. The bottom electrode may be fabricated as part of the underlying via or it may be fabricated simultaneously with the NRAM cell, when the cell is photolithographically defined and etched. Before the cell is photolithographically defined and etched, the top electrode is deposited as a metal film onto the CNT layer so that the top metal electrode is patterned and etched during the definition of the NRAM cell. Following the dielectric passivation and fill of the array, the top metal electrode is exposed by etching back the overlying dielectric using a smoothing process such as
chemical-mechanical planarization. With the top electrode exposed, the next level of metal wiring interconnect is fabricated to complete the NRAM array. Figure 3 illustrates one circuit method to select a single cell for writing and reading. Using a cross-grid interconnect arrangement, the NRAM and driver, (the cell), forms a memory array similar to other memory arrays. A single cell can be selected by applying the proper voltages to the word line (WL), bit line (BL), and select lines (SL) without disturbing the other cells in the array.
Characteristics
NRAM has a density, at least in theory, similar to that of DRAM. DRAM includes capacitors, which are essentially two small metal plates with a thin insulator between them. NRAM has terminals and electrodes roughly the same size as the plates in a DRAM, the nanotubes between them being so much smaller they add nothing to the overall size. However it seems there is a minimum size at which a DRAM can be built, below which there is simply not enough charge being stored on the plates. NRAM appears to be limited only by
lithography
Lithography () is a planographic method of printing originally based on the immiscibility of oil and water. The printing is from a stone (lithographic limestone) or a metal plate with a smooth surface. It was invented in 1796 by the German a ...
. This means that NRAM may be able to become much denser than DRAM, perhaps also less expensive. Unlike DRAM, NRAM does not require power to "refresh" it, and will retain its memory even after power is removed. Thus the power needed to write and retain the memory state of the device is much lower than DRAM, which has to build up charge on the cell plates. This means that NRAM might compete with DRAM in terms of cost, but also require less power, and as a result also be much faster because write performance is largely determined by the total charge needed. NRAM can theoretically reach performance similar to SRAM, which is faster than DRAM but much less dense, and thus much more expensive.
Comparison with other non-volatile memory
Compared with other
non-volatile random-access memory (NVRAM) technologies, NRAM has several advantages. In
flash memory, the common form of NVRAM, each cell resembles a
MOSFET transistor with a control gate (CG) modulated by a floating gate (FG) interposed between the CG and the FG. The FG is surrounded by an insulating dielectric, typically an oxide. Since the FG is electrically isolated by the surrounding dielectric, any electrons placed on the FG will be trapped on the FG which screens the CG from the channel of the transistor and modifies the threshold voltage (VT) of the transistor. By writing and controlling the amount of charge placed on the FG, the FG controls the conduction state of the MOSFET flash device depending on the VT of the cell selected. The current flowing through the MOSFET channel is sensed to determine the state of the cell forming a
binary code where a 1 state (current flow) when an appropriate CG voltage is applied and a 0 state (no current flow) when the CG voltage is applied.
After being written to, the insulator traps electrons on the FG, locking it into the 0 state. However, in order to change that bit, the insulator has to be "overcharged" to erase any charge already stored in it. This requires higher voltage, about 10 volts, much more than a battery can provide. Flash systems include a "
charge pump
A charge pump is a kind of DC-to-DC converter that uses capacitors for energetic charge storage to raise or lower voltage. Charge-pump circuits are capable of high efficiencies, sometimes as high as 90–95%, while being electrically simple c ...
" that slowly builds up power and releases it at higher voltage. This process is not only slow, but degrades the insulators. For this reason flash has a limited number of writes before the device will no longer operate effectively.
NRAM reads and writes are both "low energy" in comparison to flash (or DRAM for that matter due to "refresh"), meaning NRAM could have longer battery life. It may also be much faster to write than either, meaning it may be used to replace both. Modern phones include flash memory for storing phone numbers, DRAM for higher performance working memory because flash is too slow, and some SRAM for even higher performance. Some NRAM could be placed on the CPU to act as the
CPU cache
A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. A cache is a smaller, faster memory, located closer to a processor core, whic ...
, and more in other chips replacing both the DRAM and flash.
NRAM is one of a variety of new memory systems, many of which claim to be "
universal
Universal is the adjective for universe.
Universal may also refer to:
Companies
* NBCUniversal, a media and entertainment company
** Universal Animation Studios, an American Animation studio, and a subsidiary of NBCUniversal
** Universal TV, a ...
" in the same fashion as NRAM – replacing everything from flash to DRAM to SRAM.
An alternative memory ready for use is
ferroelectric RAM
Ferroelectric RAM (FeRAM, F-RAM or FRAM) is a random-access memory similar in construction to DRAM but using a ferroelectric layer instead of a dielectric layer to achieve non-volatility. FeRAM is one of a growing number of alternative non-vo ...
(FRAM or FeRAM). FeRAM adds a small amount of a ferro-electric material to a DRAM cell. The state of the field in the material encodes the bit in a non-destructive format. FeRAM has advantages of NRAM, although the smallest possible cell size is much larger than for NRAM. FeRAM is used in applications where the limited number of writes of flash is an issue. FeRAM read operations are destructive, requiring a restoring write operation afterwards.
Other more speculative memory systems include
magnetoresistive random-access memory
Magnetoresistive random-access memory (MRAM) is a type of non-volatile random-access memory which stores data in magnetic domains. Developed in the mid-1980s, proponents have argued that magnetoresistive RAM will eventually surpass competing tec ...
(MRAM) and
phase-change memory
Phase-change memory (also known as PCM, PCME, PRAM, PCRAM, OUM (ovonic unified memory) and C-RAM or CRAM (chalcogenide RAM)) is a type of non-volatile random-access memory. PRAMs exploit the unique behaviour of chalcogenide glass. In PCM, heat pr ...
(PRAM). MRAM is based on a grid of
magnetic tunnel junctions
Tunnel magnetoresistance (TMR) is a magnetoresistive effect that occurs in a magnetic tunnel junction (MTJ), which is a component consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough (typically a fe ...
. MRAM's reads the memory using the
tunnel magnetoresistance
Tunnel magnetoresistance (TMR) is a magnetoresistive effect that occurs in a magnetic tunnel junction (MTJ), which is a component consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough (typically a fe ...
effect, allowing it to read the memory both non-destructively and with very little power. Early MRAM used field induced writing, reached a limit in terms of size, which kept it much larger than flash devices. However, new MRAM techniques might overcome the size limitation to make MRAM competitive even with flash memory. The techniques are
Thermal Assisted Switching (TAS), developed by
Crocus Technology
Crocus Technology, founded in 2006, is a venture-capital-backed semiconductor startup company developing magnetoresistive random-access memory (MRAM) technology. The company's products originated in a Grenoble-based Spintec laboratory and its te ...
, and
Spin-transfer torque
Spin-transfer torque (STT) is an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve can be modified using a spin-polarized current.
Charge carriers (such as electrons) have a property known as spin ...
on which Crocus,
Hynix
SK hynix Inc. is a South Korean supplier of dynamic random-access memory (DRAM) chips and flash memory chips. Hynix is the world's second-largest memory chipmaker (after Samsung Electronics) and the world's third-largest semiconductor company. ...
,
IBM, and other companies were working in 2009.
PRAM is based on a technology similar to that in a writable CD or DVD, using a phase-change material that changes its magnetic or electrical properties instead of its optical ones. The PRAM material itself is scalable but requires a larger current source.
History
Nantero was founded in 2001, and headquartered in
Woburn, Massachusetts
Woburn ( ) is a city in Middlesex County, Massachusetts, United States. The population was 40,876 at the 2020 census. Woburn is located north of Boston. Woburn uses Massachusetts' mayor-council form of government, in which an elected mayor is ...
. Due to the massive investment in flash
semiconductor fabrication plant
In the microelectronics industry, a semiconductor fabrication plant (commonly called a fab; sometimes foundry) is a factory where devices such as integrated circuits are manufactured.
Fabs require many expensive devices to function. Estimate ...
s, no alternative memory has replaced flash in the marketplace, despite predictions as early as 2003 of the impending speed and density of NRAM.
In 2005, NRAM was promoted as
universal memory
Universal memory refers to a computer data storage device combining the cost benefits of DRAM, the speed of SRAM, the non-volatility of flash memory along with infinite durability, and longevity. Such a device, if it ever becomes possible to deve ...
, and Nantero predicted it would be in production by the end of 2006.
In August 2008,
Lockheed Martin acquired an exclusive license for government applications of Nantero's intellectual property.
By early 2009, Nantero had 30 US patents and 47 employees, but was still in the engineering phase. In May 2009, a radiation-resistant version of NRAM was tested on the
STS-125
STS-125, or HST-SM4 (Hubble Space Telescope Servicing Mission 4), was the fifth and final Space Shuttle mission to the Hubble Space Telescope (HST) and the last solo flight of the Space Shuttle ''Atlantis''.
The launch of the Space Shuttle ''Atl ...
mission of the US .
The company was quiet until another round of funding and collaboration with the Belgian research center
imec
Interuniversity Microelectronics Centre (IMEC) is an international research & development organization, active in the fields of nanoelectronics and digital technologies, with headquarters in Belgium. Luc Van den hove has served as President an ...
was announced in November 2012.
Nantero raised a total of over $42 million through the November 2012 series D round.
Investors included
Charles River Ventures
Charles River Ventures (CRV) is a venture capital firm focused on early-stage investments in technology. The firm was founded in 1970 to commercialize research that came out of MIT. Its name comes from the Charles River.
History
The firm has ra ...
,
Draper Fisher Jurvetson
Draper Fisher Jurvetson (DFJ) is an American venture capital firm focused on investments in enterprise, consumer and disruptive technologies. In January 2019, DFJ Venture, the early-stage team, spun out and formed Threshold Ventures. DFJ Growth ...
,
Globespan Capital Partners,
Stata Venture Partners and
Harris & Harris Group.
In May 2013, Nantero completed series D with an investment by
Schlumberger.
''
EE Times'' listed Nantero as one of "10 top startups to watch in 2013".
31 Aug 2016: Two Fujitsu semiconductor businesses are licensing Nantero NRAM technology with joint Nantero–Fujitsu development to produce chips in 2018. They will have several thousand times faster rewrites and many thousands of times more rewrite cycles than embedded flash memory.
See also
*
RAM
Ram, ram, or RAM may refer to:
Animals
* A male sheep
* Ram cichlid, a freshwater tropical fish
People
* Ram (given name)
* Ram (surname)
* Ram (director) (Ramsubramaniam), an Indian Tamil film director
* RAM (musician) (born 1974), Dutch
* ...
*
Magnetoresistive random-access memory
Magnetoresistive random-access memory (MRAM) is a type of non-volatile random-access memory which stores data in magnetic domains. Developed in the mid-1980s, proponents have argued that magnetoresistive RAM will eventually surpass competing tec ...
*
Phase-change memory
Phase-change memory (also known as PCM, PCME, PRAM, PCRAM, OUM (ovonic unified memory) and C-RAM or CRAM (chalcogenide RAM)) is a type of non-volatile random-access memory. PRAMs exploit the unique behaviour of chalcogenide glass. In PCM, heat pr ...
*
Ferroelectric RAM
Ferroelectric RAM (FeRAM, F-RAM or FRAM) is a random-access memory similar in construction to DRAM but using a ferroelectric layer instead of a dielectric layer to achieve non-volatility. FeRAM is one of a growing number of alternative non-vo ...
*
Nanocrystal
A ''nanocrystal'' is a material particle having at least one dimension smaller than 100 nanometres, based on quantum dots
(a nanoparticle) and composed of atoms in either a single- or poly-crystalline arrangement.
The size of nanocrystals dist ...
References
External links
Nantero's NRAM page
{{DEFAULTSORT:Nano-Ram
Non-volatile random-access memory
Nanomaterials
Emerging technologies