![Sputtering system](https://upload.wikimedia.org/wikipedia/commons/7/70/Sputtering_system.jpg)
In physics, sputtering is a phenomenon in which microscopic
particle
In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass.
They vary greatly in size or quantity, from ...
s of a solid material are ejected from its surface, after the material is itself bombarded by energetic
particle
In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass.
They vary greatly in size or quantity, from ...
s of a
plasma or
gas
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
.
It occurs naturally in
outer space
Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, pred ...
, and can be an unwelcome source of
wear
Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion). The study of wear and related processes is referred to as tribology.
Wear in ...
in precision components. However, the fact that it can be made to act on extremely fine layers of material is utilised in science and industry—there, it is used to perform precise
etching
Etching is traditionally the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio (incised) in the metal. In modern manufacturing, other chemicals may be used on other types ...
, carry out analytical techniques, and deposit
thin film
A thin film is a layer of material ranging from fractions of a nanometer ( monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ...
layers in the manufacture of
optical coatings,
semiconductor devices and
nanotechnology products. It is a
physical vapor deposition
Physical vapor deposition (PVD), sometimes called physical vapor transport (PVT), describes a variety of vacuum deposition methods which can be used to produce thin films and coatings on substrates including metals, ceramics, glass, and polym ...
technique.
Physics
When energetic ions collide with atoms of a target material, an exchange of
momentum takes place between them.
![linearcollisioncascadesput](https://upload.wikimedia.org/wikipedia/commons/9/91/Linearcollisioncascadesput.png)
These ions, known as "incident ions", set off
collision cascade
In condensed-matter physics, a collision cascade (also known as a displacement cascade or a displacement spike) is a set of nearby adjacent energetic (much higher than ordinary thermal energies) collisions of atoms induced by an energetic par ...
s in the target. Such cascades can take many paths; some recoil back toward the surface of the target. If a collision cascade reaches the surface of the target, and its remaining energy is greater than the target's surface
binding energy
In physics and chemistry, binding energy is the smallest amount of energy required to remove a particle from a system of particles or to disassemble a system of particles into individual parts. In the former meaning the term is predominantly use ...
, an atom will be ejected. This process is known as "sputtering". If the target is thin (on an atomic scale), the collision cascade can reach through to its back side; the atoms ejected in this fashion are said to escape the surface binding energy "in transmission".
The average number of atoms ejected from the target per incident ion is called the "sputter yield". The sputter yield depends on several things: the angle at which ions collide with the surface of the material, how much energy they strike it with, their masses, the masses of the target atoms, and the target's surface binding energy. If the target possesses a
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...
structure, the orientation of its axes with respect to the surface is an important factor.
The ions that cause sputtering come from a variety of sources—they can come from
plasma, specially constructed
ion source
An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.
Electron ionization
Electron ...
s,
particle accelerator
A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams.
Large accelerators are used for fundamental research in particle ...
s, outer space (e.g.
solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...
), or radioactive materials (e.g.
alpha radiation
Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an at ...
).
A model for describing sputtering in the cascade regime for amorphous flat targets is Thompson's analytical model.
An algorithm that simulates sputtering based on a quantum mechanical treatment including electrons stripping at high energy is implemented in the program
TRIM
Trim or TRIM may refer to:
Cutting
* Cutting or trimming small pieces off something to remove them
** Book trimming, a stage of the publishing process
** Pruning, trimming as a form of pruning often used on trees
Decoration
* Trim (sewing), or ...
.
Another mechanism of physical sputtering is called "heat spike sputtering". This can occur when the solid is dense enough, and the incoming ion heavy enough, that collisions occur very close to each other. In this case, the binary collision approximation is no longer valid, and the collisional process should be understood as a many-body process. The dense collisions induce a
heat spike (also called thermal spike), which essentially melts a small portion of the crystal. If that portion is close enough to its surface, large numbers of atoms may be ejected, due to liquid flowing to the surface and/or microexplosions.
Heat spike sputtering is most important for heavy ions (e.g. Xe or Au or cluster ions) with energies in the keV–MeV range bombarding dense but soft metals with a low melting point (Ag, Au, Pb, etc.). The heat spike sputtering often increases nonlinearly with energy, and can for small cluster ions lead to dramatic sputtering yields per cluster of the order of 10,000.
For animations of such a process see "Re: Displacement Cascade 1" in the
external links
An internal link is a type of hyperlink on a web page to another page or resource, such as an image or document, on the same website or domain.
Hyperlinks are considered either "external" or "internal" depending on their target or destinatio ...
section.
Physical sputtering has a well-defined minimum energy threshold, equal to or larger than the ion energy at which the maximum energy transfer from the ion to a target atom equals the binding energy of a surface atom. That is to say, it can only happen when an ion is capable of transferring more energy into the target than is required for an atom to break free from its surface.
This threshold is typically somewhere in the range of ten to a hundred
eV.
''Preferential sputtering'' can occur at the start when a multicomponent solid target is bombarded and there is no solid state diffusion. If the energy transfer is more efficient to one of the target components, or it is less strongly bound to the solid, it will sputter more efficiently than the other. If in an AB alloy the component A is sputtered preferentially, the surface of the solid will, during prolonged bombardment, become enriched in the B component, thereby increasing the probability that B is sputtered such that the composition of the sputtered material will ultimately return to AB.
Electronic sputtering
The term ''electronic sputtering'' can mean either sputtering induced by energetic electrons (for example in a transmission electron microscope), or sputtering due to very high-energy or highly charged heavy ions that lose energy to the solid, mostly by electronic
stopping power, where the electronic excitations cause sputtering.
Electronic sputtering produces high sputtering yields from
insulators
Insulator may refer to:
* Insulator (electricity), a substance that resists electricity
** Pin insulator, a device that isolates a wire from a physical support such as a pin on a utility pole
** Strain insulator, a device that is designed to work ...
, as the electronic excitations that cause sputtering are not immediately quenched, as they would be in a conductor. One example of this is Jupiter's ice-covered moon
Europa, where a MeV sulfur ion from Jupiter's magnetosphere can eject up to 10,000 H
2O molecules.
Potential sputtering
![Sputterer](https://upload.wikimedia.org/wikipedia/commons/f/f6/Sputterer.JPG)
In the case of multiple charged projectile ions a particular form of electronic sputtering can take place that has been termed ''potential sputtering''.
In these cases the potential energy stored in multiply charged ions (i.e., the energy necessary to produce an ion of this charge state from its neutral atom) is liberated when the ions recombine during impact on a solid surface (formation of
hollow atoms
Hollow Atoms (discovered in 1990 by a French team of researchers around Jean-Pierre Briand) are short-lived multiply excited neutral atoms which carry a large part of their Z electrons (Z ... projectile nuclear charge) in high-n levels while inn ...
). This sputtering process is characterized by a strong dependence of the observed sputtering yields on the charge state of the impinging ion and can already take place at ion impact energies well below the physical sputtering threshold. Potential sputtering has only been observed for certain target species
and requires a minimum potential energy.
Etching and chemical sputtering
Removing atoms by sputtering with an inert gas is called ''
ion milling
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
'' or ''ion etching''.
Sputtering can also play a role in
reactive-ion etching
Reactive-ion etching (RIE) is an etching technology used in microfabrication. RIE is a type of dry etching which has different characteristics than wet etching. RIE uses chemically reactive plasma to remove material deposited on wafers. The pl ...
(RIE), a plasma process carried out with chemically active ions and radicals, for which the sputtering yield may be enhanced significantly compared to pure physical sputtering. Reactive ions are frequently used in
secondary ion mass spectrometry
Secondary-ion mass spectrometry (SIMS) is a technique used to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions. ...
(SIMS) equipment to enhance the sputter rates. The mechanisms causing the sputtering enhancement are not always well understood, although the case of fluorine etching of Si has been modeled well theoretically.
Sputtering observed to occur below the threshold energy of physical sputtering is also often called chemical sputtering.
The mechanisms behind such sputtering are not always well understood, and may be hard to distinguish from chemical
etching
Etching is traditionally the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio (incised) in the metal. In modern manufacturing, other chemicals may be used on other types ...
. At elevated temperatures, chemical sputtering of carbon can be understood to be due to the incoming ions weakening bonds in the sample, which then desorb by thermal activation.
The hydrogen-induced sputtering of carbon-based materials observed at low temperatures has been explained by H ions entering between C-C bonds and thus breaking them, a mechanism dubbed ''swift chemical sputtering''.
Applications and phenomena
Sputtering only happens when the kinetic energy of the incoming particles is much higher than conventional thermal energies (
≫ 1
eV). When done with
direct current
Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...
(DC sputtering), voltages of 3-5 kV are used. When done with
alternating current
Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
(
RF sputtering), frequencies are around the 14 MHz range.
Sputter cleaning
Surfaces of solids can be cleaned from contaminants by using physical sputtering in a
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...
. Sputter cleaning is often used in
surface science,
vacuum deposition
Vacuum deposition is a group of processes used to deposit layers of material atom-by-atom or molecule-by-molecule on a solid surface. These processes operate at pressures well below atmospheric pressure (i.e., vacuum). The deposited layers can r ...
and
ion plating
Ion plating (IP) is a physical vapor deposition (PVD) process that is sometimes called ''ion assisted deposition'' (IAD) or ''ion vapor deposition'' (IVD) and is a modified version of ''vacuum deposition''. Ion plating uses concurrent or periodi ...
. In 1955 Farnsworth, Schlier, George, and Burger reported using sputter cleaning in an ultra-high-vacuum system to prepare ultra-clean surfaces for low-energy electron-diffraction (LEED) studies. Sputter cleaning became an integral part of the
ion plating
Ion plating (IP) is a physical vapor deposition (PVD) process that is sometimes called ''ion assisted deposition'' (IAD) or ''ion vapor deposition'' (IVD) and is a modified version of ''vacuum deposition''. Ion plating uses concurrent or periodi ...
process. When the surfaces to be cleaned are large, a similar technique,
plasma cleaning
Plasma cleaning is the removal of impurities and contaminants from surfaces through the use of an energetic plasma or dielectric barrier discharge (DBD) plasma created from gaseous species. Gases such as argon and oxygen, as well as mixtures such ...
, can be used. Sputter cleaning has some potential problems such as overheating, gas incorporation in the surface region, bombardment (radiation) damage in the surface region, and the roughening of the surface, particularly if ''over done.'' It is important to have a ''clean''
plasma in order to not continually recontaminate the surface during sputter cleaning. Redeposition of sputtered material on the substrate can also give problems, especially at high sputtering pressures. Sputtering of the surface of a compound or alloy material can result in the surface composition being changed. Often the species with the least mass or the highest
vapor pressure
Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phas ...
is the one preferentially sputtered from the surface.
Film deposition
Sputter deposition is a method of
depositing thin film
A thin film is a layer of material ranging from fractions of a nanometer ( monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ...
s by sputtering that involves eroding material from a "target" source onto a "substrate" e.g. a silicon
wafer
A wafer is a crisp, often sweet, very thin, flat, light and dry biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. Wafers can also be made into cookies with cream flavoring sandwiched between them. They ...
, solar cell, optical component, or many other possibilities.
Resputtering, in contrast, involves re-emission of the deposited material, e.g. SiO
2 during the deposition also by ion bombardment.
Sputtered atoms are ejected into the gas phase but are not in their
thermodynamic equilibrium
Thermodynamic equilibrium is an axiomatic concept of thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls. In the ...
state, and tend to deposit on all surfaces in the vacuum chamber. A substrate (such as a wafer) placed in the chamber will be coated with a thin film. Sputtering deposition usually uses an
argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as ...
plasma because argon, a noble gas, will not react with the target material.
Sputter Damage
Sputter damage is usually defined during transparent electrode deposition on optoelectronic devices, which is usually originated from the substrate's bombardment by highly energetic species. The main species involved in the process and the representative energies can be listed as (the values are taken from:
* Sputtered atoms (ions) from the target surface (∼10 eV), the formation of which mainly depends on the binding energy of the target material;
* Negative ions (originating from the carrier gas) formed in the plasma (∼5–15 eV), the formation of which mainly depends on the plasma potential;
* Negative ions formed at the target surface (up to 400 eV), the formation of which mainly depends on the target voltage;•
* Positive ions formed in the plasma (∼15 eV), the formation of which mainly depends on the potential fall in front of a substrate at floating potential;
* Reflected atoms and neutralized ions from the target surface (20–50 eV), the formation of which mainly depends on the background gas and the mass of the sputtered element.
As seen in the list above, negative ions (e.g., O
− and In
− for ITO sputtering) formed at the target surface and accelerated toward the substrate acquire the largest energy, which is determined by the potential between target and plasma potentials. Although the flux of the energetic particles is an important parameter, high-energy negative O
− ions are additionally the most abundant species in plasma in case of reactive deposition of oxides. However, energies of other ions/atoms (e.g., Ar
+,Ar
0, or In
0) in the discharge may already be sufficient to dissociate surface bonds or etch soft layers in certain device technologies. In addition, the momentum transfer of high-energy particles from the plasma (Ar, oxygen ions) or sputtered from the target might impinge or even increase the substrate temperature sufficiently to trigger physical (e.g., etching) or thermal degradation of sensitive substrate layers (e.g. thin film metal halide perovskites).
This can affect the functional properties of underlying charge transport and passivation layers and photoactive absorbers or emitters, eroding device performance. For instance, due to sputter damage, there may be inevitable interfacial consequences such as pinning of the Fermi level, caused by damage-related interface gap states, resulting in the formation of Schottky-barrier impeding carrier transport. Sputter damage can also impair the doping efficiency of materials and the lifetime of excess charge carriers in photoactive materials; in some cases, depending on its extent, such damage can even lead to a reduced shunt resistance.
Etching
In the semiconductor industry sputtering is used to etch the target. Sputter etching is chosen in cases where a high degree of etching
anisotropy
Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...
is needed and selectivity is not a concern. One major drawback of this technique is wafer damage and high voltage use.
For analysis
Another application of sputtering is to etch away the target material. One such example occurs in
secondary ion mass spectrometry
Secondary-ion mass spectrometry (SIMS) is a technique used to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions. ...
(SIMS), where the target sample is sputtered at a constant rate. As the target is sputtered, the concentration and identity of sputtered atoms are measured using
mass spectrometry. In this way the composition of the target material can be determined and even extremely low concentrations (20 µg/kg) of impurities detected. Furthermore, because the sputtering continually etches deeper into the sample, concentration profiles as a function of depth can be measured.
In space
Sputtering is one of the forms of space weathering, a process that changes the physical and chemical properties of airless bodies, such as asteroids and the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
. On icy moons, especially
Europa, sputtering of photolyzed water from the surface leads to net loss of hydrogen and accumulation of oxygen-rich materials that may be important for life. Sputtering is also one of the possible ways that
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
has lost most of its
atmosphere and that
Mercury continually replenishes its tenuous surface-bounded
exosphere.
References
External links
Thin Film Evaporation Guide- an introduction with animations
Sputtering Basics - animated film of a sputtering processFree molecular dynamics simulation program (Kalypso) capable of modeling sputtering
American Vacuum Society short courses on thin film deposition
* (The original paper on Kaufman sputter sources.)
*
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Coatings
Materials science
Thin film deposition
Surface science