Sputtering is a process whereby atoms are ejected from a solid target material due to
bombardment of the target by energetic particles. It only happens when the kinetic energy of
the incoming particles is much higher than conventional thermal energies. This process
can lead, during prolonged ion or plasma bombardment of a material, to significant erosion of materials,
and can thus be harmful. On the other hand, it is commonly utilized for thin-film deposition,
etching and analytical techniques.
Physics of sputtering Physical sputtering is driven by momentum
exchange between the ions and atoms in the materials, due to collisions.
The incident ions set off collision cascades in the target. When such cascades recoil and
reach the target surface with an energy greater than the surface binding energy, an atom would
be ejected, known as sputtering. If the target is thin on an atomic scale the collision cascade
can reach the back side of the target and atoms can escape the surface binding energy
"in transmission". The average number of atoms ejected from the target per incident ion is
called the sputter yield and depends on the ion incident angle, the energy of the ion,
the masses of the ion and target atoms, and the surface binding energy of atoms in the
target. For a crystalline target the orientation of the crystal axes with respect to the target
surface is relevant. The primary particles for the sputtering process
can be supplied in a number of ways, for example by a plasma, an ion source, an accelerator
or by a radioactive material emitting alpha particles.
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.
A different mechanism of physical sputtering is heat spike sputtering. This may occur when
the solid is dense enough, and then the incoming ion heavy enough, that the collisions occur
very close to each other. Then the binary collision approximation is no longer valid,
but rather the collisional process should be understood as a many-body process. The
dense collisions induce a heat spike, which essentially melts the crystal locally. If
the molten zone is close enough to a surface, large numbers of atoms may sputter due to
flow of liquid to the surface and/or microexplosions. Heat spike sputtering is most important for
heavy ions with energies in the keV–MeV range bombarding dense but soft metals with
a low melting point. 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 External
links. Physical sputtering has a well-defined minimum
energy threshold equal to or larger than the ion energy at which the maximum energy transfer
of the ion to a sample atom equals the binding energy of a surface atom. This threshold typically
is somewhere in the range 10–100 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, and/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 be AB.
Electronic sputtering The term electronic sputtering can mean either
sputtering induced by energetic electrons, 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, 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 H2O molecules. Potential sputtering
In the case of multiply 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 is liberated when the ions recombine during impact on a
solid surface. 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" or "ion etching". Sputtering can also play a role in reactive
ion etching, 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 equipment to enhance
the sputter rates. The mechanisms causing the sputtering enhancement are not always
well understood, but for instance 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. 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
Film deposition
Sputter deposition is a method of depositing thin films by sputtering that involves eroding
material from a "target" source onto a "substrate" e.g. a silicon wafer. Resputtering, in contrast,
involves re-emission of the deposited material, e.g. SiO2 during the deposition also by ion
bombardment. Sputtered atoms ejected into the gas phase
are not in their thermodynamic equilibrium state, and tend to deposit on all surfaces
in the vacuum chamber. A substrate placed in the chamber will be coated with a thin
film. Sputtering usually uses an argon plasma. 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 is needed and selectivity is not
a concern. One major drawback of this technique is wafer damage.
For analysis Another application of sputtering is to etch
away the target material. One such example occurs in secondary ion mass spectrometry,
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 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. 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 has lost most of its atmosphere
and that Mercury continually replenishes its tenuous surface-bounded exosphere.
References
External links Thin Film Evaporation Guide
What is Sputtering? - an introduction with animations
Sputtering Basics - animated film of a sputtering process
Free molecular dynamics simulation program capable of modeling sputtering
American Vacuum Society short courses on thin film deposition
Reactive Sputtering Using A Dual-Anode Magnetron System
H. R. Kaufman, J. J. Cuomo and J. M. E. Harper. "Technology and applications of broad-beam
ion sources used in sputtering. Part I. Ion source technology". Journal of Vacuum Science
and Technology 21: 725–736. Bibcode:1982JVST...21..725K. doi:10.1116/1.571819. (The original paper
on Kaufman sputter sources.) Re: Displacement Cascade 1. YouTube. 2008.