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The term high-κ dielectric refers to a material with a high
dielectric constant The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insula ...
(κ,
kappa Kappa (uppercase Κ, lowercase κ or cursive ; el, κάππα, ''káppa'') is the 10th letter of the Greek alphabet, representing the voiceless velar plosive sound in Ancient and Modern Greek. In the system of Greek numerals, has a value ...
), as compared to
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 ...
. High-κ dielectrics are used in semiconductor manufacturing processes where they are usually used to replace a silicon dioxide
gate dielectric A gate dielectric is a dielectric used between the gate and substrate of a field-effect transistor (such as a MOSFET). In state-of-the-art processes, the gate dielectric is subject to many constraints, including: * Electrically clean interface to ...
or another dielectric layer of a device. The implementation of high-κ gate dielectrics is one of several strategies developed to allow further miniaturization of microelectronic components, colloquially referred to as extending
Moore's Law Moore's law is the observation that the number of transistors in a dense integrated circuit (IC) doubles about every two years. Moore's law is an observation and projection of a historical trend. Rather than a law of physics, it is an empi ...
. Sometimes these materials are called "high-k" (pronounced "high kay"), instead of "high-κ" (high kappa).


Need for high-κ materials

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 ...
() has been used as a gate oxide material for decades. As metal-oxide-semiconductor field-effect transistors (MOSFETs) have decreased in size, the thickness of the silicon dioxide gate dielectric has steadily decreased to increase the gate capacitance (per unit area) and thereby drive current (per device width), raising device performance. As the thickness scales below 2  nm, leakage currents due to tunneling increase drastically, leading to high power consumption and reduced device reliability. Replacing the silicon dioxide gate dielectric with a high-κ material allows increased gate capacitance without the associated leakage effects.


First principles

The gate oxide in a
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 ...
can be modeled as a parallel plate capacitor. Ignoring quantum mechanical and depletion effects from the Si substrate and gate, the
capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...
of this parallel plate
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...
is given by : C=\frac where * is the capacitor area * is the relative dielectric constant of the material (3.9 for
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 ...
) * is the
permittivity of free space Vacuum permittivity, commonly denoted (pronounced "epsilon nought" or "epsilon zero"), is the value of the absolute dielectric permittivity of classical vacuum. It may also be referred to as the permittivity of free space, the electric const ...
* is the thickness of the capacitor oxide insulator Since leakage limitation constrains further reduction of , an alternative method to increase gate capacitance is alter κ by replacing silicon dioxide with a high-κ material. In such a scenario, a thicker gate oxide layer might be used which can reduce the leakage current flowing through the structure as well as improving the gate dielectric reliability.


Gate capacitance impact on drive current

The drain current for a
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 ...
can be written (using the gradual channel approximation) as :I_ = \frac \mu\, C_\text\frac where * is the width of the transistor channel * is the channel length * is the channel carrier mobility (assumed constant here) * is the capacitance density associated with the gate dielectric when the underlying channel is in the inverted state * is the voltage applied to the transistor gate * is the
threshold voltage The threshold voltage, commonly abbreviated as Vth or VGS(th), of a field-effect transistor (FET) is the minimum gate-to-source voltage (VGS) that is needed to create a conducting path between the source and drain terminals. It is an important s ...
The term is limited in range due to reliability and room temperature operation constraints, since a too large would create an undesirable, high electric field across the oxide. Furthermore, cannot easily be reduced below about 200 mV, because leakage currents due to increased oxide leakage (that is, assuming high-κ dielectrics are not available) and subthreshold conduction raise stand-by power consumption to unacceptable levels. (See the industry roadmap, which limits threshold to 200 mV, and Roy ''et al.'' ). Thus, according to this simplified list of factors, an increased requires a reduction in the channel length or an increase in the gate dielectric capacitance.


Materials and considerations

Replacing the silicon dioxide gate dielectric with another material adds complexity to the manufacturing process. Silicon dioxide can be formed by
oxidizing Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
the underlying silicon, ensuring a uniform, conformal oxide and high interface quality. As a consequence, development efforts have focused on finding a material with a requisitely high dielectric constant that can be easily integrated into a manufacturing process. Other key considerations include band alignment to
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 ...
(which may alter leakage current), film morphology, thermal stability, maintenance of a high
mobility Mobility may refer to: Social sciences and humanities * Economic mobility, ability of individuals or families to improve their economic status * Geographic mobility, the measure of how populations and goods move over time * Mobilities, a conte ...
of charge carriers in the channel and minimization of electrical defects in the film/interface. Materials which have received considerable attention are hafnium silicate, zirconium silicate, hafnium dioxide and zirconium dioxide, typically deposited using
atomic layer deposition Atomic layer deposition (ALD) is a thin-film deposition technique based on the sequential use of a gas-phase chemical process; it is a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals called precursors (a ...
. It is expected that defect states in the high-κ dielectric can influence its electrical properties. Defect states can be measured for example by using zero-bias thermally stimulated current, zero-temperature-gradient zero-bias thermally stimulated current spectroscopy, or inelastic electron tunneling spectroscopy (IETS).


Use in industry

Industry has employed oxynitride gate dielectrics since the 1990s, wherein a conventionally formed silicon oxide dielectric is infused with a small amount of nitrogen. The nitride content subtly raises the dielectric constant and is thought to offer other advantages, such as resistance against dopant diffusion through the gate dielectric. In 2000, Gurtej Singh Sandhu and Trung T. Doan of
Micron Technology Micron Technology, Inc. is an American producer of computer memory and computer data storage including dynamic random-access memory, flash memory, and USB flash drives. It is headquartered in Boise, Idaho. Its consumer products, includin ...
initiated the development of
atomic layer deposition Atomic layer deposition (ALD) is a thin-film deposition technique based on the sequential use of a gas-phase chemical process; it is a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals called precursors (a ...
high-κ
films A film also called a movie, motion picture, moving picture, picture, photoplay or (slang) flick is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, feelings, beauty, or atmospher ...
for
DRAM Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory that stores each bit of data in a memory cell, usually consisting of a tiny capacitor and a transistor, both typically based on metal-oxid ...
memory devices. This helped drive cost-effective implementation of
semiconductor memory Semiconductor memory is a digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to devices in which data is stored within metal–oxide–semiconductor (MOS) memory cells on a si ...
, starting with 90-nm
node In general, a node is a localized swelling (a " knot") or a point of intersection (a vertex). Node may refer to: In mathematics * Vertex (graph theory), a vertex in a mathematical graph * Vertex (geometry), a point where two or more curves, line ...
DRAM. In early 2007,
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 ...
announced the deployment of
hafnium Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri M ...
-based high-κ dielectrics in conjunction with a metallic gate for components built on
45 nanometer Per the International Technology Roadmap for Semiconductors, the 45 nm process is a MOSFET technology node referring to the average half-pitch of a memory cell manufactured at around the 2007–2008 time frame. Matsushita and Intel started mas ...
technologies, and has shipped it in the 2007 processor series codenamed Penryn. At the same time, IBM announced plans to transition to high-κ materials, also hafnium-based, for some products in 2008. While not identified, the most likely dielectric used in such applications are some form of nitrided hafnium silicates (). and are susceptible to crystallization during dopant activation annealing.
NEC is a Japanese multinational information technology and electronics corporation, headquartered in Minato, Tokyo. The company was known as the Nippon Electric Company, Limited, before rebranding in 1983 as NEC. It provides IT and network soluti ...
Electronics has also announced the use of a dielectric in their 55 nm ''UltimateLowPower'' technology. However, even is susceptible to trap-related leakage currents, which tend to increase with stress over device lifetime. This leakage effect becomes more severe as hafnium concentration increases. There is no guarantee, however, that hafnium will serve as a de facto basis for future high-κ dielectrics. The 2006 ITRS roadmap predicted the implementation of high-κ materials to be commonplace in the industry by 2010.


See also

*
Low-κ dielectric In semiconductor manufacturing, a low-κ is a material with a small relative dielectric constant (κ, kappa) relative to silicon dioxide. Low-κ dielectric material implementation is one of several strategies used to allow continued scaling of micr ...
*
Silicon–germanium SiGe ( or ), or silicon–germanium, is an alloy with any molar ratio of silicon and germanium, i.e. with a molecular formula of the form Si1−''x''Ge''x''. It is commonly used as a semiconductor material in integrated circuits (ICs) for heter ...
*
Silicon on insulator In semiconductor manufacturing, silicon on insulator (SOI) technology is fabrication of silicon semiconductor devices in a layered silicon–insulator–silicon substrate, to reduce parasitic capacitance within the device, thereby improving per ...


References


Further reading


Review article
by Wilk ''et al.'' in the
Journal of Applied Physics The ''Journal of Applied Physics'' is a peer-reviewed scientific journal with a focus on the physics of modern technology. The journal was originally established in 1931 under the name of ''Physics'', and was published by the American Physical So ...
*Houssa, M. (Ed.) (2003) ''High-k Dielectrics'' Institute of Physics
CRC Press Online
*Huff, H.R., Gilmer, D.C. (Ed.) (2005) ''High Dielectric Constant Materials : VLSI MOSFET applications'' Springer *Demkov, A.A, Navrotsky, A., (Ed.) (2005) ''Materials Fundamentals of Gate Dielectrics'' Springer *"High dielectric constant gate oxides for metal oxide Si transistors" Robertson, J. (''Rep. Prog. Phys.'' 69 327-396 2006) ''Institute Physics Publishing'' High dielectric constant gate oxides] *Media coverage of March, 2007 Intel/IBM announcement
BBC NEWS, Technology, Chips push through nano-barrier
*Gusev, E. P. (Ed.) (2006) "Defects in High-k Gate Dielectric Stacks: Nano-Electronic Semiconductor Devices", Springer {{DEFAULTSORT:High-k dielectric High-κ dielectrics, Electronic engineering Transistors Semiconductor fabrication materials MOSFETs