In
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way ...
manufacturing, a low-κ is a material with a small
relative dielectric constant (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 ...
) relative 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 ...
. Low-κ
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 ...
material implementation is one of several strategies used to allow continued scaling of microelectronic devices, colloquially referred to as extending
Moore's law. In digital
circuits, insulating dielectrics separate the conducting parts (wire
interconnects and
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 ...
s) from one another. As components have scaled and transistors have gotten closer together, the insulating dielectrics have thinned to the point where charge build up and
crosstalk adversely affect the performance of the device. Replacing the silicon dioxide with a low-κ dielectric of the same thickness reduces
parasitic capacitance
Parasitic capacitance is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other. When two electrical conductors at different voltages ...
, enabling faster switching speeds (in case of
synchronous circuits) and lower heat dissipation. In conversation such materials may be referred to as "low-k" (spoken "low-kay") rather than "low-κ" (low-kappa).
Low-κ materials
In
integrated circuits
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tin ...
, and
CMOS
Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSF ...
devices, silicon dioxide can readily be formed on surfaces of Si through
thermal oxidation, and can further be deposited on the surfaces of conductors using
chemical vapor deposition
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (subst ...
or various other thin film fabrication methods. Due to the wide range of methods that can be used to cheaply form silicon dioxide layers, this material is used conventionally as the baseline to which other low permittivity dielectrics are compared. The relative dielectric constant of SiO
2, the insulating material still used in
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 ...
chips, is 3.9. This number is the ratio of the
permittivity of SiO
2 divided by permittivity of vacuum, ε
SiO2/ε
0, where ε
0 = 8.854×10
−6 pF/μm. There are many materials with lower relative dielectric constants but few of them can be suitably integrated into a manufacturing process. Development efforts have focused primarily on the following classes of materials:
Fluorine-doped silicon dioxide
By doping SiO
2 with fluorine to produce fluorinated silica glass, the relative dielectric constant is lowered from 3.9 to 3.5. Fluorine-doped oxide materials were used for the
180 nm
The 180 nm process refers to the level of MOSFET (CMOS) semiconductor process technology that was commercialized around the 1998–2000 timeframe by leading semiconductor companies, starting with TSMC and Fujitsu, then followed by Sony, Tosh ...
and
130 nm technology nodes.
Organosilicate glass or OSG (Carbon-doped oxide or CDO)
By doping SiO
2 with carbon, one can lower the relative dielectric constant to 3.0, the density to 1.4 g/cm
3 and the thermal conductivity to 0.39 W/(m*K). The
semiconductor industry
The semiconductor industry is the aggregate of companies engaged in the design and fabrication of semiconductors and semiconductor devices, such as transistors and integrated circuits. It formed around 1960, once the fabrication of semiconduc ...
has been using the organosilicate glass dielectrics since the
90 nm technology node.
Porous silicon dioxide
Various methods may be employed to create voids or pores in a silicon dioxide dielectric.
Voids can have a relative dielectric constant of nearly 1, thus the dielectric constant of the porous material may be reduced by increasing the porosity of the film. Relative dielectric constants lower than 2.0 have been reported. Integration difficulties related to porous silicon dioxide implementation include low mechanical strength and difficult integration with etch and polish processes.
Porous organosilicate glass (carbon-doped oxide)
Porous organosilicate materials are usually obtained by a two-step procedure
where the first step consists of the co-deposition of a labile organic phase (known as porogen) together with an organosilicate phase resulting in an organic-inorganic
hybrid material. In the second step, the organic phase is decomposed by
UV curing or
annealing at a temperature of up to 400°C, leaving behind pores in the organosilicate low-κ materials. Porous organosilicate glasses have been employed since the
45 nm technology node.
Spin-on organic polymeric dielectrics
Polymeric dielectrics are generally deposited by a spin-on approach, which is traditionally used for the deposition of
photoresist materials, rather than
chemical vapor deposition
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (subst ...
. Integration difficulties include low mechanical strength,
coefficient of thermal expansion (CTE) mismatch and thermal stability. Some examples of spin-on organic low-κ polymers are
polyimide,
polynorbornenes,
benzocyclobutene
Benzocyclobutene (BCB) is a benzene ring fused to a cyclobutane ring. It has chemical formula .
BCB is frequently used to create photosensitive polymers. BCB-based polymer dielectrics may be spun on or applied to various substrates for us ...
, and
PTFE
Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chem ...
.
Spin-on silicon based polymeric dielectric
There are two kinds of silicon based polymeric dielectric materials,
hydrogen silsesquioxane and methylsilsesquioxane.
Air gaps
The ultimate low-κ material is air with a relative permittivity value of ~1.0. However, the placement of air gaps between the conducting wires compromises the mechanical stability of the integrated circuit making it impractical to build an IC consisting entirely of air as the insulating material. Nevertheless, the strategic placement of air gaps can improve the chip's electrical performance without compromising critically its durability. For example, Intel uses air gaps for two interconnect levels in its
14 nm
The 14 nm process refers to the MOSFET technology node that is the successor to the 22nm (or 20nm) node. The 14nm was so named by the International Technology Roadmap for Semiconductors (ITRS). Until about 2011, the node following 22nm was expec ...
FinFET technology.
See also
*
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 ...
*
High-κ dielectric
*
Relative static permittivity
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 insul ...
References
External links
Nasa on Low-kThe evolution of interconnect technology for silicon integrated circuitry
{{DEFAULTSORT:Low-K Dielectric
Semiconductor fabrication materials
Materials science