SONOS, short for "silicon–oxide–nitride–oxide–silicon", more precisely, "
polycrystalline silicon"—"
silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
"—"
silicon nitride"—"silicon dioxide"—"
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...
",
[
]
is a
cross sectional structure of
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
(metal-oxide-semiconductor field-effect transistor), realized by P.C.Y. Chen of
Fairchild Camera and Instrument
Fairchild Camera and Instrument Corporation was a company founded by Sherman Fairchild. It was based on the East Coast of the United States, and provided research and development for flash photography equipment. The technology was primarily used ...
in 1977.
This structure is often used for
non-volatile memories, such as
EEPROM
EEPROM (also called E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory used in computers, usually integrated in microcontrollers such as smart cards and remote keyless systems, or ...
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 (Si
3N
4 or Si
9N
10) instead of "
polysilicon
Polycrystalline silicon, or multicrystalline silicon, also called polysilicon, poly-Si, or mc-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry.
Polysilicon is produce ...
-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
Macronix International Co., Ltd. (MCIX; often shortened to Macronix) is an integrated device manufacturer in the non-volatile memory (NVM) market. The company manufactures NOR Flash, NAND Flash, and ROM products for the consumer, communication ...
,
Toshiba
, commonly known as Toshiba and stylized as TOSHIBA, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. Its diversified products and services include power, industrial and social infrastructure systems, ...
,
United Microelectronics Corporation an
Floadia
Description
A SONOS
memory cell is formed from a standard
polysilicon
Polycrystalline silicon, or multicrystalline silicon, also called polysilicon, poly-Si, or mc-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry.
Polysilicon is produce ...
N-channel
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
with the addition of a small sliver of silicon nitride inserted inside the transistor's gate oxide. The sliver of nitride is non-conductive but contains a large number of charge trapping sites able to hold an
electrostatic
Electrostatics is a branch of physics that studies electric charges at rest ( static electricity).
Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for ...
charge. The nitride layer is electrically isolated from the surrounding transistor, although charges stored on the nitride directly affect the conductivity of the underlying transistor channel. The oxide/nitride sandwich typically consists of a 2 nm thick oxide lower layer, a 5 nm thick silicon nitride middle layer, and a 5–10 nm oxide upper layer.
When the polysilicon control gate is biased positively,
electrons
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have n ...
from the transistor source and drain regions
tunnel through the oxide layer and get trapped in the silicon nitride. This results in an energy barrier between the drain and the source, raising the
threshold voltage V
t (the gate-source voltage necessary for current to flow through the transistor). The electrons can be removed again by applying a negative bias on the control gate.
A SONOS memory array is constructed by fabricating a grid of SONOS transistors which are connected by horizontal and vertical control lines (
wordlines and
bitlines) to peripheral circuitry such as
address decoder
In digital electronics, an address decoder is a binary decoder that has two or more inputs for address bits and one or more outputs for device selection signals. When the address for a particular device appears on the address inputs, the decoder as ...
s and
sense amplifier In modern computer memory, a sense amplifier is one of the elements which make up the circuitry on a semiconductor memory chip ( integrated circuit); the term itself dates back to the era of magnetic core memory. A sense amplifier is part of the re ...
s. After storing or erasing the cell, the controller can measure the state of the cell by passing a small voltage across the source-drain nodes; if current flows the cell must be in the "no trapped electrons" state, which is considered a
logical "1". If no current is seen the cell must be in the "trapped electrons" state, which is considered as "0" state. The needed voltages are normally about 2 V for the erased state, and around 4.5 V for the programmed state.
Comparison with Floating-Gate structure
Generally SONOS is very similar to traditional
FG (floating gate) type memory cell,
but hypothetically offers higher quality storage. This is due to the smooth homogeneity of the Si
3N
4 film compared with polycrystalline film which has tiny irregularities. Flash requires the construction of a very high-performance insulating barrier on the gate leads of its transistors, often requiring as many as nine different steps, whereas the oxide layering in SONOS can be more easily produced on existing lines and more easily combined with CMOS logic.
Additionally, traditional flash is less tolerant of oxide defects because a single shorting defect will discharge the entire polysilicon
floating gate. The nitride in the SONOS structure is non-conductive, so a short only disturbs a localized patch of charge. Even with the introduction of new insulator technologies this has a definite "lower limit" around 7 to 12 nm, which means it is difficult for flash devices to scale smaller than about 45 nm linewidths.
But,
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 ser ...
-
Micron
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Un ...
group have realized 16 nm planar flash memory with traditional FG technology.
SONOS, on the other hand, requires a very thin layer of insulator in order to work, making the gate area smaller than flash. This allows SONOS to scale to smaller linewidth, with recent examples being produced on 40 nm fabs and claims that it will scale to 20 nm. The linewidth is directly related to the overall storage of the resulting device, and indirectly related to the cost; in theory, SONOS' better scalability will result in higher capacity devices at lower costs.
Additionally, the voltage needed to bias the gate during writing is much smaller than in traditional flash. In order to write flash, a high voltage is first built up in a separate circuit known as a
charge pump, which increases the input voltage to between 9 V to 20 V. This process takes some time, meaning that writing to a flash cell is much slower than reading, often between 100 and 1000 times slower. The pulse of high power also degrades the cells slightly, meaning that flash devices can only be written to between 10,000 and 100,000 times, depending on the type. SONOS devices require much lower write voltages, typically 5–8 V, and do not degrade in the same way. SONOS does suffer from the converse problem however, where electrons become strongly trapped in the ONO layer and cannot be removed again. Over long usage this can eventually lead to enough trapped electrons to permanently set the cell to the "0" state, similar to the problems in flash. However, in SONOS this requires on the order of a 100 thousands write/erase cycles,
10 to 100 times worse compared with legacy FG memory cell.
History
Background
The original
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
(metal-oxide-semiconductor field-effect transistor, or MOS transistor) was invented by Egyptian engineer
Mohamed M. Atalla and Korean engineer
Dawon Kahng at
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mul ...
in 1959, and demonstrated in 1960.
Kahng went on to invent the
floating-gate MOSFET with
Simon Min Sze at Bell Labs, and they proposed its use as a
floating-gate (FG)
memory cell, in 1967. This was the first form of
non-volatile memory based on the injection and storage of charges in a floating-gate MOSFET,
which later became the basis for
EPROM (erasable
PROM),
EEPROM
EEPROM (also called E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory used in computers, usually integrated in microcontrollers such as smart cards and remote keyless systems, or ...
(electrically erasable PROM) and
flash memory
Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both use ...
technologies.
Charge trapping at the time was an issue in MNOS transistors, but
John Szedon and Ting L. Chu revealed in June 1967 that this difficulty could be harnessed to produce a nonvolatile memory cell. Subsequently, in late 1967, a
Sperry Sperry may refer to:
Places
In the United States:
* Sperry, Iowa, community in Des Moines County
*Sperry, Missouri
* Sperry, Oklahoma, town in Tulsa County
*Sperry Chalet, historic backcountry chalet, Glacier National Park, Montana
*Sperry Glacier ...
research team led by H.A. Richard Wegener invented the
metal–nitride–oxide–semiconductor transistor The metal–nitride–oxide–semiconductor or metal–nitride–oxide–silicon (MNOS) transistor is a type of MOSFET (metal-oxide-semiconductor field-effect transistor) in which the oxide layer is replaced by a double layer of nitride and oxide. I ...
(MNOS transistor), a type of MOSFET in which the
oxide
An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
layer is replaced by a double layer of
nitride and oxide.
Nitride was used as a trapping layer instead of a floating gate, but its use was limited as it was considered inferior to a floating gate.
Charge trap (CT) memory was introduced with MNOS devices in the late 1960s. It had a device structure and operating principles similar to floating-gate (FG)
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remember ...
, but the main difference is that the charges are stored in a conducting material (typically a doped
polysilicon
Polycrystalline silicon, or multicrystalline silicon, also called polysilicon, poly-Si, or mc-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry.
Polysilicon is produce ...
layer) in FG memory, whereas CT memory stored charges in localized traps within a
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the ma ...
layer (typically made of
silicon nitride).
Development
SONOS was first conceptualized in the 1960s. MONOS is realized in 1968 by
Westinghouse Electric Corporation
The Westinghouse Electric Corporation was an American manufacturing company founded in 1886 by George Westinghouse. It was originally named "Westinghouse Electric & Manufacturing Company" and was renamed "Westinghouse Electric Corporation" in ...
.
[
][
]
In the early 1970s initial commercial devices were realized using
PMOS transistors and a metal-nitride-oxide (
MNOS) stack with a 45 nm nitride storage layer. These devices required up to 30V to operate.
In 1977, P.C.Y. Chen of
Fairchild Camera and Instrument
Fairchild Camera and Instrument Corporation was a company founded by Sherman Fairchild. It was based on the East Coast of the United States, and provided research and development for flash photography equipment. The technology was primarily used ...
introduced a SONOS
cross sectional structured
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
with
tunnel silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
of 30
Ångström thickness for
EEPROM
EEPROM (also called E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory used in computers, usually integrated in microcontrollers such as smart cards and remote keyless systems, or ...
.
[
] According to
NCR Corporation's patent application in 1980, SONOS structure required +25 volts and −25 volts for writing and erasing, respectively.
[
]
It was improved to +12 V by PMOS-based MNOS (metal-nitride-oxide-semiconductor) structure.
By the early 1980s, polysilicon
NMOS-based structures were in use with operating voltages under 20 V. By the late 1980s and early 1990s PMOS SONOS structures
were demonstrating program/erase voltages in the range of 5–12 volts.
On the other hand, in 1980,
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 ser ...
realized highly reliable
EEPROM
EEPROM (also called E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory used in computers, usually integrated in microcontrollers such as smart cards and remote keyless systems, or ...
with double layered
polysilicon
Polycrystalline silicon, or multicrystalline silicon, also called polysilicon, poly-Si, or mc-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry.
Polysilicon is produce ...
structure, which is named ''FLOTOX'',
both for erase and write cycling endurance and for data retention term.
[
]
SONOS has been in the past produced by
Philips Semiconductors
NXP Semiconductors N.V. (NXP) is a Dutch semiconductor designer and manufacturer with headquarters in Eindhoven, Netherlands. The company employs approximately 31,000 people in more than 30 countries. NXP reported revenue of $11.06 billion in 2 ...
,
Spansion
Spansion Inc. was an American-based company that designed, developed, and manufactured flash memory, microcontrollers, mixed-signal and analog products, and system-on-chip (SoC) solutions.Reuters.Spansion Inc." July 26, 2010.By Mark LaPedus, EE ...
,
Qimonda
Qimonda AG ( ) was a German memory company split out of Infineon Technologies (itself a spun off business unit of Siemens AG) on 1 May 2006 to form at the time the second largest DRAM company worldwide, according to the industry research firm ...
and
Saifun Semiconductors
Spansion Inc. was an American-based company that designed, developed, and manufactured flash memory, microcontrollers, mixed-signal and analog products, and system-on-chip (SoC) solutions.Reuters.Spansion Inc." July 26, 2010.By Mark LaPedus, EE ...
.
Recent efforts
In 2002,
AMD and
Fujitsu
is a Japanese multinational information and communications technology equipment and services corporation, established in 1935 and headquartered in Tokyo. Fujitsu is the world's sixth-largest IT services provider by annual revenue, and the la ...
, formed as
Spansion
Spansion Inc. was an American-based company that designed, developed, and manufactured flash memory, microcontrollers, mixed-signal and analog products, and system-on-chip (SoC) solutions.Reuters.Spansion Inc." July 26, 2010.By Mark LaPedus, EE ...
in 2003 and later merged with
Cypress Semiconductor in 2014, developed a SONOS-like ''MirrorBit'' technology based on the license from
Saifun Semiconductors, Ltd.'s ''NROM technology''.
[
][
]
As of 2011
Cypress Semiconductor developed SONOS memories for multiple processes,
and started to sell them as
IP to embed in other devices.
UMC has already used SONOS since 2006 and has licensed Cypress for 40 nm and other nodes. Shanghai Huali Microelectronics Corporation (HLMC) has also announced
to be producing Cypress SONOS at 40 nm and 55 nm.
In 2006, Toshiba developed a new double
tunneling layer technology with SONOS structure, which utilize Si
9N
10 silicon nitride.
[
][
]
Toshiba
, commonly known as Toshiba and stylized as TOSHIBA, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. Its diversified products and services include power, industrial and social infrastructure systems, ...
also researches MONOS ("Metal-Oxide-Nitride-Oxide-Silicon") structure for their 20 nm node
NAND gate type
flash memories.
[
]
Renesas Electronics
is a Japanese semiconductor manufacturer headquartered in Tokyo, Japan, initially incorporated in 2002 as Renesas Technology, the consolidated entity of the semiconductor units of Hitachi and Mitsubishi excluding their dynamic random-access ...
uses MONOS structure in 40 nm node era.
[
][
]
which is the result of collaboration with
TSMC
Taiwan Semiconductor Manufacturing Company Limited (TSMC; also called Taiwan Semiconductor) is a Taiwanese multinational semiconductor contract manufacturing and design company. It is the world's most valuable semiconductor company, the world' ...
.
[
]
While other companies still use
FG (floating gate) structure.
[
]
For example,
GlobalFoundries use floating-gate-based split-gate ''SuperFlash ESF3'' cell for their 40 nm products.
[
]
Some new structure for FG (floating gate) type flash memories are still intensively studied.
[
]
In 2016,
GlobalFoundries developed FG-based 2.5V Embedded flash macro.
In 2017,
Fujitsu
is a Japanese multinational information and communications technology equipment and services corporation, established in 1935 and headquartered in Tokyo. Fujitsu is the world's sixth-largest IT services provider by annual revenue, and the la ...
announced to license FG-based ''ESF3/FLOTOX'' structure,
which is originally developed by
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 ser ...
in 1980, from
Silicon Storage Technology for their
embedded non-volatile memory solutions.
[
][
][
]
As of 2016,
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 ser ...
-
Micron
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Un ...
group have disclosed that they stayed traditional FG technology in their 3-dimensional NAND flash memory.
[
]
They also use FG technology for 16 nm planar NAND flash.
[
]
See also
*
Polycrystalline silicon
*
Silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
*
Silicon nitride
*
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...
*
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
*
Charge trap flash
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 use ...
*
Floating-gate MOSFET
*
EEPROM
EEPROM (also called E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory used in computers, usually integrated in microcontrollers such as smart cards and remote keyless systems, or ...
*
Flash memory
Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both use ...
References
External links
*
*
Gutmann (2001) papaer: "Data Remanence in Semiconductor Devices" USENIX
{{emerging technologies, topics=yes, infocom=yes
Computer memory
Non-volatile memory
Emerging technologies
MOSFETs