Atomic layer deposition
Encyclopedia
Atomic layer deposition is a thin film deposition technique that is based on the sequential use of a gas phase chemical process
. The majority of ALD reactions use two chemicals, typically called precursors. These precursors react with a surface one-at-a-time in a sequential manner. By exposing the precursors to the growth surface repeatedly, a thin film is deposited.
of materials onto substrates of varying compositions. ALD is similar in chemistry to chemical vapor deposition
(CVD), except that the ALD reaction breaks the CVD reaction into two half-reaction
s, keeping the precursor materials separate during the reaction. Due to the characteristics of self-limiting and surface reactions, ALD film growth makes atomic scale deposition control possible. By keeping the precursors separate throughout the coating process, atomic layer control of film growth can be obtained as fine as ~0.1 Å
(10 pm) per cycle. Separation of the precursors is accomplished by pulsing a purge gas (typically nitrogen
or argon
) after each precursor pulse to remove excess precursor from the process chamber and prevent 'parasitic' CVD deposition on the substrate.
ALD was first published under name “Molecular Layering” in the early 1960s by Prof. S.I. Kol’tsov from Leningrad (Lensovet) Technological Institute (LTI). These ALD experiments were conducted under the scientific supervision of corresponding member of the Russian Academy of Sciences
Prof. V.B. Aleskovskii. The concept of the ALD process was first proposed by Prof. V.B. Aleskovskii in his Ph.D. thesis published in 1952. ALD had been developed and introduced worldwide with the name atomic layer epitaxy
(ALE) in the late 1970s. For thin film electroluminescent (TFEL) flat-panel displays, high quality dielectric and luminescent films were required on large-area substrates, thus the deposition method of ALD was developed. Interest in ALD has increased in steps in the mid-1990s and 2000s, with the interest focused on silicon-based microelectronics
. ALD is considered as one deposition method with the greatest potential for producing very thin, conformal films with control of the thickness and composition of the films possible at the atomic level. A major driving force for the recent interest is the prospective seen for ALD in scaling down microelectronic devices.
ALD can be used to deposit several types of thin films, including various oxide
s (e.g. Al2O3, TiO2, SnO2, ZnO, HfO2), metal nitride
s (e.g. TiN, TaN, WN, NbN), metals (e.g. Ru, Ir, Pt), and metal sulfide
s (e.g. ZnS).
Each reaction cycle adds a given amount of material to the surface, referred to as the growth per cycle. To grow a material layer, reaction cycles are repeated as many as required for the desired film thickness. One cycle may take time from 0.5 s to a few seconds and deposit between 0.1 and 3 Å
of film thickness. Due to the self-terminating reactions, ALD is a surface-controlled process, where process parameters other than the precursors, substrate, and temperature have little or no influence. And, because of the surface control, ALD-grown films are extremely conformal and uniform in thickness. These thin films can also be used in correlation with other common fabrication methods.
Other advantages of ALD are the wide range of film materials available, high density and low impurity level. Also, lower deposition temperature can be used in order not to affect sensitive substrates.
Although the selection of film materials grown by ALD is wide, many technologically important materials (Si
, Ge
, Si3N4
, several multi-component oxides, certain metals) cannot currently be deposited by ALD in a cost-effective way.
ALD is a chemical technique and thus there is always a risk of residues being left from the precursors. The impurity content of the films depends on the completeness of the reactions. In typical oxide processes where metal halides of alkyl compounds are used together with water as precursors, impurities found in the films are at the 0.1-1 atom % level.
) gate oxides, high-k memory capacitor dielectrics, ferroelectrics, and metals and nitrides for electrodes and interconnects. In high-k gate oxides, where the control of ultra thin films is essential, ALD is only likely to come in to wider use at the 45 nm technology. In metallizations, conformal films are required; currently it is expected that ALD will be used in mainstream production at the 65 nm node. In dynamic random access memories
(DRAMs), the conformality requirements are even higher and ALD is the only method that can be used when feature sizes become smaller than 100 nm.
, ZrO2
, and HfO2
has been one of the most widely examined areas of ALD. The motivation for high-k oxides comes from the problem of high tunneling currents through the currently used SiO2
metal-oxide-semiconductor field-effect transistor (MOSFET) gate dielectric when it is downscaled to a thickness of 1.0 nm and below. With the high-k oxide, a thicker gate dielectric can be made for the required capacitance density, thus the tunneling current can be reduced through the structure.
Intel Corporation
has reported using ALD to deposit high-k gate dielectric for its 45 nm CMOS technology
.
(DRAM) capacitor dielectrics has been similar to that of gate dielectrics: SiO2 has been widely used in the industry thus far, but it is likely to be phased out in the near future as the scale of devices are decreased. The requirements for the downscaled DRAM capacitors are good conformality and permittivity values above 200, thus the candidate materials are different from those explored for MOSFET gate dielectrics. (For example, Al2O3, ZrO2, and HfO2) The most extensively studied candidate has been (Ba,Sr)TiO3. ALD is a very promising method, which can satisfy the high conformal requirements of DRAM applications. A permittivity of 180 was measured for SrTiO3
and 165 for BaTiO3
when films thicker than 200 nm were post-deposition annealed, but when the film thickness was decreased to 50 nm, the permittivity decreased to only 100.
s, such as TiN
and TaN find potential use both as metal barriers
and as gate metals
. Metal barriers are used in modern Cu
-based chips to avoid diffusion of Cu into the surrounding materials, such as insulators and the silicon substrate, and also, to prevent Cu contamination by elements diffusing from the insulators by surrounding every Cu interconnection with a layer of metal barriers. The metal barriers have strict demands: they should be pure; dense; conductive; conformal; thin; have good adhesion towards metals and insulators. The requirements concerning process technique can be fulfilled by ALD. The most studied ALD nitride is TiN which is deposited from TiCl4 and NH3.
Chemical process
In a "scientific" sense, a chemical process is a method or means of somehow changing one or more chemicals or chemical compounds. Such a chemical process can occur by itself or be caused by somebody. Such a chemical process commonly involves a chemical reaction of some sort...
. The majority of ALD reactions use two chemicals, typically called precursors. These precursors react with a surface one-at-a-time in a sequential manner. By exposing the precursors to the growth surface repeatedly, a thin film is deposited.
Introduction
ALD is a self-limiting (the amount of film material deposited in each reaction cycle is constant), sequential surface chemistry that deposits conformal thin-filmsConformal film
A conformal film defines a morphologically uneven interface with another body and has a thickness that is the same everywhere along the interface. This is undoubtedly an idealization and may be used for abstract or theoretical purposes...
of materials onto substrates of varying compositions. ALD is similar in chemistry to chemical vapor deposition
Chemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...
(CVD), except that the ALD reaction breaks the CVD reaction into two half-reaction
Half-reaction
A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction.-Example:...
s, keeping the precursor materials separate during the reaction. Due to the characteristics of self-limiting and surface reactions, ALD film growth makes atomic scale deposition control possible. By keeping the precursors separate throughout the coating process, atomic layer control of film growth can be obtained as fine as ~0.1 Å
Å
Å represents various sounds in several languages. Å is part of the alphabets used for the Alemannic and the Bavarian-Austrian dialects of German...
(10 pm) per cycle. Separation of the precursors is accomplished by pulsing a purge gas (typically nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
or argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
) after each precursor pulse to remove excess precursor from the process chamber and prevent 'parasitic' CVD deposition on the substrate.
ALD was first published under name “Molecular Layering” in the early 1960s by Prof. S.I. Kol’tsov from Leningrad (Lensovet) Technological Institute (LTI). These ALD experiments were conducted under the scientific supervision of corresponding member of the Russian Academy of Sciences
Russian Academy of Sciences
The Russian Academy of Sciences consists of the national academy of Russia and a network of scientific research institutes from across the Russian Federation as well as auxiliary scientific and social units like libraries, publishers and hospitals....
Prof. V.B. Aleskovskii. The concept of the ALD process was first proposed by Prof. V.B. Aleskovskii in his Ph.D. thesis published in 1952. ALD had been developed and introduced worldwide with the name atomic layer epitaxy
Atomic layer epitaxy
Atomic layer epitaxy or Atomic Layer Chemical Vapor Deposition , now more generally called Atomic Layer Deposition , is a specialized form of epitaxy that typically deposit alternating monolayers of two elements onto a substrate. The crystal lattice structure achieved is thin, uniform, and...
(ALE) in the late 1970s. For thin film electroluminescent (TFEL) flat-panel displays, high quality dielectric and luminescent films were required on large-area substrates, thus the deposition method of ALD was developed. Interest in ALD has increased in steps in the mid-1990s and 2000s, with the interest focused on silicon-based microelectronics
Microelectronics
Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture of very small electronic components. Usually, but not always, this means micrometre-scale or smaller,. These devices are made from semiconductors...
. ALD is considered as one deposition method with the greatest potential for producing very thin, conformal films with control of the thickness and composition of the films possible at the atomic level. A major driving force for the recent interest is the prospective seen for ALD in scaling down microelectronic devices.
ALD can be used to deposit several types of thin films, including various oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s (e.g. Al2O3, TiO2, SnO2, ZnO, HfO2), metal nitride
Nitride
In chemistry, a nitride is a compound of nitrogen where nitrogen has a formal oxidation state of −3. Nitrides are a large class of compounds with a wide range of properties and applications....
s (e.g. TiN, TaN, WN, NbN), metals (e.g. Ru, Ir, Pt), and metal sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
s (e.g. ZnS).
ALD process
The growth of material layers by ALD consists of repeating the following characteristic four steps:- Exposure of the first precursor, typically an organometallic compound.
- Purge or evacuation of the reaction chamber to remove the non-reacted precursors and the gaseous reaction by-products.
- Exposure of the second precursor – or another treatment to activate the surface again for the reaction of the first precursor, such as a plasmaPlasma (physics)In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
. - Purge or evacuation of the reaction chamber.
Each reaction cycle adds a given amount of material to the surface, referred to as the growth per cycle. To grow a material layer, reaction cycles are repeated as many as required for the desired film thickness. One cycle may take time from 0.5 s to a few seconds and deposit between 0.1 and 3 Å
Å
Å represents various sounds in several languages. Å is part of the alphabets used for the Alemannic and the Bavarian-Austrian dialects of German...
of film thickness. Due to the self-terminating reactions, ALD is a surface-controlled process, where process parameters other than the precursors, substrate, and temperature have little or no influence. And, because of the surface control, ALD-grown films are extremely conformal and uniform in thickness. These thin films can also be used in correlation with other common fabrication methods.
Advantages
Using ALD, film thickness depends only on the number of reaction cycles, which makes the thickness control accurate and simple. Unlike CVD, there is less need of reactant flux homogeneity, which gives large area (large batch and easy scale-up) capability, excellent conformality and reproducibility, and simplifies the use of solid precursors. Also, the growth of different multilayer structures is straight forward. These advantages make the ALD method attractive for microelectronics for manufacturing of future generation integrated circuits.Other advantages of ALD are the wide range of film materials available, high density and low impurity level. Also, lower deposition temperature can be used in order not to affect sensitive substrates.
Limitations
The major limitation of ALD is its slowness; usually only a fraction of a monolayer is deposited in one cycle. Fortunately, the films needed for future-generation ICs are very thin and thus the slowness of ALD is not such an important issue.Although the selection of film materials grown by ALD is wide, many technologically important materials (Si
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
, Ge
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
, Si3N4
Silicon nitride
Silicon nitride is a chemical compound of silicon and nitrogen. If powdered silicon is heated between 1300° and 1400°C in an atmosphere of nitrogen, trisilicon tetranitride, Si3N4, is formed. The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen...
, several multi-component oxides, certain metals) cannot currently be deposited by ALD in a cost-effective way.
ALD is a chemical technique and thus there is always a risk of residues being left from the precursors. The impurity content of the films depends on the completeness of the reactions. In typical oxide processes where metal halides of alkyl compounds are used together with water as precursors, impurities found in the films are at the 0.1-1 atom % level.
ALD in microelectronics
In microelectronics, ALD is studied as a potential technique to deposit high-k (high permittivityPermittivity
In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...
) gate oxides, high-k memory capacitor dielectrics, ferroelectrics, and metals and nitrides for electrodes and interconnects. In high-k gate oxides, where the control of ultra thin films is essential, ALD is only likely to come in to wider use at the 45 nm technology. In metallizations, conformal films are required; currently it is expected that ALD will be used in mainstream production at the 65 nm node. In dynamic random access memories
Dynamic random access memory
Dynamic random-access memory is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1...
(DRAMs), the conformality requirements are even higher and ALD is the only method that can be used when feature sizes become smaller than 100 nm.
Gate oxides
Deposition of the high-k oxides Al2O3Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...
, ZrO2
Zirconium dioxide
Zirconium dioxide , sometimes known as zirconia , is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the rare mineral baddeleyite. The high temperature cubic crystalline form is rarely found in nature as mineral tazheranite O2...
, and HfO2
Hafnium(IV) oxide
Hafnium oxide is the inorganic compound with the formula HfO2. Also known as hafnia, this colourless solid is one of the most common and stable compounds of hafnium. It is an electrical insulator with a band gap of approximately 6 eV. Hafnium dioxide is an intermediate in some processes that...
has been one of the most widely examined areas of ALD. The motivation for high-k oxides comes from the problem of high tunneling currents through the currently used SiO2
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
metal-oxide-semiconductor field-effect transistor (MOSFET) gate dielectric when it is downscaled to a thickness of 1.0 nm and below. With the high-k oxide, a thicker gate dielectric can be made for the required capacitance density, thus the tunneling current can be reduced through the structure.
Intel Corporation
Intel Corporation
Intel Corporation is an American multinational semiconductor chip maker corporation headquartered in Santa Clara, California, United States and the world's largest semiconductor chip maker, based on revenue. It is the inventor of the x86 series of microprocessors, the processors found in most...
has reported using ALD to deposit high-k gate dielectric for its 45 nm CMOS technology
45 nanometer
Per the International Technology Roadmap for Semiconductors, the 45 nm technology node should refer to the average half-pitch of a memory cell manufactured at around the 2007–2008 time frame....
.
DRAM capacitors
The development of dynamic random access memoryDynamic random access memory
Dynamic random-access memory is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1...
(DRAM) capacitor dielectrics has been similar to that of gate dielectrics: SiO2 has been widely used in the industry thus far, but it is likely to be phased out in the near future as the scale of devices are decreased. The requirements for the downscaled DRAM capacitors are good conformality and permittivity values above 200, thus the candidate materials are different from those explored for MOSFET gate dielectrics. (For example, Al2O3, ZrO2, and HfO2) The most extensively studied candidate has been (Ba,Sr)TiO3. ALD is a very promising method, which can satisfy the high conformal requirements of DRAM applications. A permittivity of 180 was measured for SrTiO3
Strontium titanate
Strontium titanate is an oxide of strontium and titanium with the chemical formula SrTiO3. At room temperature, it is a centrosymmetric paraelectric material with a perovskite structure...
and 165 for BaTiO3
Barium titanate
Barium titanate is the inorganic compound with the chemical formula BaTiO3. Barium titanate is a white powder and transparent as larger crystals...
when films thicker than 200 nm were post-deposition annealed, but when the film thickness was decreased to 50 nm, the permittivity decreased to only 100.
Transition-metal nitrides
Transition-metal nitrideNitride
In chemistry, a nitride is a compound of nitrogen where nitrogen has a formal oxidation state of −3. Nitrides are a large class of compounds with a wide range of properties and applications....
s, such as TiN
Titanium nitride
Titanium nitride is an extremely hard ceramic material, often used as a coating on titanium alloys, steel, carbide, and aluminium components to improve the substrate's surface properties....
and TaN find potential use both as metal barriers
Barrier metal
A barrier metal is a material used in integrated circuits to chemically isolate semiconductors from soft metal interconnects, while maintaining an electrical connection between them...
and as gate metals
Metal gate
A metal gate, in the context of a lateral Metal-Oxide-Semiconductor MOS stack, is just that—the gate material is made from a metal.For decades, the industry had moved away from metal as the gate material in the MOS stack due to fabrication complications...
. Metal barriers are used in modern Cu
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
-based chips to avoid diffusion of Cu into the surrounding materials, such as insulators and the silicon substrate, and also, to prevent Cu contamination by elements diffusing from the insulators by surrounding every Cu interconnection with a layer of metal barriers. The metal barriers have strict demands: they should be pure; dense; conductive; conformal; thin; have good adhesion towards metals and insulators. The requirements concerning process technique can be fulfilled by ALD. The most studied ALD nitride is TiN which is deposited from TiCl4 and NH3.
Metal films
Motivations of an interest in metal ALD are:- Cu interconnects and W plugs, or at least Cu seed layers for Cu electrodeposition and W seeds for W CVD,
- transition-metal nitrides (e.g. TiN, TaN, WN) for Cu interconnect barriers
- noble metals for ferroelectric random access memoryFerroelectric RAMFerroelectric RAM is a random-access memory similar in construction to DRAM but uses a ferroelectric layer instead of a dielectric layer to achieve non-volatility. FeRAM is one of a growing number of alternative non-volatile memory technologies that offer the same functionality as Flash memory...
(FRAM) and DRAM capacitor electrodes - high- and low-work function metals for dual-gate MOSFETs.
See also
- Atomic layer epitaxyAtomic layer epitaxyAtomic layer epitaxy or Atomic Layer Chemical Vapor Deposition , now more generally called Atomic Layer Deposition , is a specialized form of epitaxy that typically deposit alternating monolayers of two elements onto a substrate. The crystal lattice structure achieved is thin, uniform, and...
- Chemical vapor depositionChemical vapor depositionChemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...
- Thin-film deposition
- High-k dielectricHigh-k DielectricThe term high-κ dielectric refers to a material with a high dielectric constant κ used in semiconductor manufacturing processes which replaces the silicon dioxide gate dielectric...