Emission of surface atoms through energetic particle bombardment

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A commercial AJA Orion sputtering system at Cornell NanoScale Science and Technology Facility Ion thruster operating on iodine (yellow) using a xenon (blue) hollow cathode. High-energy ions emitted from plasma thrusters sputter material off the surrounding test chamber, causing problems for ground testing of high-power thrusters.[1]

In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas.[2] It occurs naturally in outer space, and can be an unwelcome source of wear 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, carry out analytical techniques, and deposit thin film layers in the manufacture of optical coatings, semiconductor devices and nanotechnology products. It is a physical vapor deposition technique.[3]

Physics

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When energetic ions collide with atoms of a target material, an exchange of momentum takes place between them.[2][4][5]

Sputtering from a linear collision cascade. The thick line illustrates the position of the surface, with everything below it being atoms inside of the material, and the thinner lines the ballistic movement paths of the atoms from beginning until they stop in the material. The purple circle is the incoming ion. Red, blue, green and yellow circles illustrate primary, secondary, tertiary and quaternary recoils, respectively. Two of the atoms happen to move out from the sample, i.e. they are sputtered.

These ions, known as "incident ions", set off collision cascades 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, 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 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 sources, particle accelerators, outer space (e.g. solar wind), or radioactive materials (e.g. alpha radiation).

A model for describing sputtering in the cascade regime for amorphous flat targets is Thompson's analytical model.[6] An algorithm that simulates sputtering based on a quantum mechanical treatment including electrons stripping at high energy is implemented in the program TRIM.[7]

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.[8] 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.[9] For animations of such a process see "Re: Displacement Cascade 1" in the external links 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

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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.[10] 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.[11]

Potential sputtering

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A commercial sputtering system

In the case of multiple charged projectile ions a particular form of electronic sputtering can take place that has been termed potential sputtering.[12][13] 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). 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[14] and requires a minimum potential energy.[15]

Etching and chemical sputtering

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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 (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 (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.[16]

Sputtering observed to occur below the threshold energy of physical sputtering is also often called chemical sputtering.[2][5] 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.[17] 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.[18]

Applications and phenomena

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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 (DC sputtering), voltages of 3-5 kV are used. When done with alternating current (RF sputtering), frequencies are around the 14 MHz range.

Sputter cleaning

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Surfaces of solids can be cleaned from contaminants by using physical sputtering in a vacuum. Sputter cleaning is often used in surface science, vacuum deposition and ion plating. In 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.[19][20][21] Sputter cleaning became an integral part of the ion plating process. When the surfaces to be cleaned are large, a similar technique, plasma cleaning, 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 is the one preferentially sputtered from the surface.

Film deposition

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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, solar cell, optical component, or many other possibilities.[22] Resputtering, in contrast, involves re-emission of the deposited material, e.g. SiO2 during the deposition also by ion bombardment.

Sputtered atoms are ejected into the gas phase but are not in their thermodynamic equilibrium 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 plasma because argon, a noble gas, will not react with the target material.

Sputter damage

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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 (values taken from[23]):

• 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+, Ar0, or In0) 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.[23]

Etching

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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 and high voltage use.

For analysis

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Another application of sputtering is to etch away the target material. One such example occurs in secondary ion mass spectrometry (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

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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

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Sputtering is a physical process in which atoms in a solid-state (target) are released and pass into the gas phase by bombardment with energetic ions (mainly noble gas ions). 

Introduction

Sputtering is usually understood as the sputter deposition, a high vacuum-based coating technique belonging to the group of PVD processes. Furthermore, sputtering in surface physics is used as a cleaning method for the preparation of high-purity surfaces and as a method for analyzing the chemical composition of surfaces.

Process of Sputtering

The principle of Sputtering is to use the energy of a plasma (partially ionized gas) on the surface of a target (cathode), to pull the atoms of the material one by one and deposit them on the substrate.

To do this, a plasma is created by ionization of a pure gas (usually Argon) by means of a potential difference (pulsed DC), or electromagnetic excitation (MF, RF); this plasma is composed of Ar+ ions which are accelerated and confined around the target due to the presence of a magnetic field. Each ionized atom, by striking the target, transfers its energy and rips an atom, having enough energy to be projected to the substrate.

The plasma is created at relatively high pressures (10-1 - 10-3 mbar), but it is necessary to start from a lower pressure before the introduction of Argon, to avoid contamination due to the residual gases.

The diversity of sputtering target shapes (circular, rectangular, Delta, tubular...) and the materials used allows creating all types of thin layers, including alloys during a single run.

Basics of the Sputtering Process

When bombarding a surface with ions, various effects may occur, depending on the ions used and their kinetic energy:

1. Material is removed from the bombarded target (cathode). This is the sputtering described here.

2. The ions are incorporated into the target material and enter there, possibly a chemical compound. This effect is then called (reactive) ion implantation.

3. The ions condense on the bombarded substrate, where they form a layer: ion beam deposition.

If a material removal is intended, the ions must have a certain minimum energy. The impinging ion transmits its impulse to atoms of the bombarded material (target), which then - similar to the billiards - trigger further collisions. After several collisions, some of the target atoms have a momentum away from the target interior. If such an atom is sufficiently close to the surface and has sufficiently high energy, it leaves the target.

The sputter yield depends essentially on the kinetic energy and mass of the ions and on the binding energy of the surface atoms and their mass. In order to eject an atom from the target, the ions must have material-dependent minimum energy (typically 30-50 eV). Above this threshold, the yield increases. However, the initially strong increase flattens rapidly, since at high ion energies, this energy is deposited even deeper into the target and thus barely reaches the surface. The ratio of the masses of ion and target atom determines the possible momentum transfer. For light target atoms, maximum yield is achieved when the mass of target and ion approximately match. However, as the mass of the target atoms increases, the maximum of the yield shifts to ever higher mass ratios between the ion and the target atom.

The ion bombardment generates not only neutral atoms, but also secondary electrons and, to a lesser extent, secondary ions and clusters of different masses. The energy distribution of the dissolved atoms has a maximum at half the surface binding energy, but falls to high energies only slowly, so that the average energy is often an order of magnitude above. This effect is exploited in analysis methods of surface physics and thin-film technology as well as for the production of thin layers (sputter deposition).

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Types of Sputtering

The main types of Sputtering are discussed below:

1. DC diode sputtering

With a DC voltage = 500 - V, an argon low-pressure plasma is ignited between a target and a substrate. Positive argon ions precipitate atoms out of the target, which then migrates to the substrate and condenses there.

Limitations

Only electrical conductors can be sputtered, as otherwise an opposing field builds up and the sputtering process stops. The other limitation is that only low sputtering rates are achieved since only a few argon ions are formed.

2. RF sputtering

In radio frequency sputtering, a high-frequency alternating field is applied instead of the DC electric field. The necessary high-frequency voltage source is connected in series with a capacitor and the plasma. The capacitor serves to separate the DC component and to keep the plasma electrically neutral.

The alternating field accelerates the ions and the electrons alternately in both directions. From a frequency of approximately 50 kHz, the ions can no longer follow the alternating field due to their much smaller charge-to-mass ratio.

The electrons oscillate in the area of the plasma and there are more and more collisions with argon atoms. This results in a high plasma rate, a consequence of which is the possible pressure reduction to about 10-1 - 10-2 Pa) with the same sputtering rate. This allows the production of thin layers with a different microstructure than would be possible at higher pressures.

The positive ions move through a superimposed negative offset voltage on the target in the direction of the target and solve as in DC sputtering by collision atoms from the target material. The subsequent sputter deposition corresponds to that of other sputtering methods.

Benefits

1. Insulators (e.g. aluminum oxide or boron nitride) and semiconductors can also be sputtered

2. the substrate heats up less

3. Due to the oscillating electrons, the sputtering rate at the same chamber pressure is about 10 times higher than with DC sputtering.

Limitations

1. Relatively low coating rates

2. The RF generation is more expensive than a DC voltage source

3. For large rectangular cathodes (> 1m), unevenness in the plasma density (layer thickness distribution) may occur.

3. DC Triode Sputtering

The target is placed as a third electrode outside the plasma chamber. Plasma generation and sputtering process are decoupled.

4. Magnetron

While in the simple cathode sputtering, only an electric field is applied, an additional magnetic field is arranged at the magnetron sputter behind the cathode plate. Due to the superimposition of the electric field and the magnetic field, the charge carriers no longer move parallel to the electric field lines but are deflected onto a spiral path (exact cycloid orbits) - they now circle over the target surface. This prolongs their path and increases the number of impacts per electron. The electron density is highest at the point where the magnetic field is parallel to the target surface. This causes a higher ionization in this area. Since the ions are hardly deflected by the magnetic field due to their mass, the largest sputtering on the target takes place directly in the area below. The erosion trenches are typical of magnetron sputtering form on the target.

The effectively higher ionization capacity of the electrons leads to an increase of the noble gas ion number and thus also of the sputtering rate. As more target material is atomized, this leads to significantly higher coating rates at the same process pressure. Since the layer growth and thus the layer properties in addition to the temperature depends primarily on the process pressure, you can start with the same growth rates, the process pressure by up to one hundred times lower than in conventional sputtering. This results in less scattering of the material on the way to the substrate and a denser layer.

Magnetron sputtering is the most widely used method in microelectronics for producing metal layers.

5. Reactive sputtering

Reaction gas (e.g., oxygen or nitrogen) is added to the Argon gas. As with the argon gas, ions of the reaction gas are formed, which react with the sputtered layer atoms in the vacuum chamber. The resulting reaction products are then deposited on the substrate surface. Reactive sputtering is available as a DC and HF variant.

Applications of Sputtering Targets

The phenomenon of Sputtering is employed in the semiconductor industry for depositing thin films of different constituents on silicon wafers. The process is also employed in optical applications by depositing a thin layer on glass. The process of sputtering occurs at extremely low temperatures, due to which, it is the perfect method for depositing thin films of different constituents. This application of the Sputtering process is the most important one and commonly used.

The process of Sputtering has various advantages, from which, the one that stands out is that the concentration of deposited film is similar to the raw material. This is unusual, as indicated earlier, the yield of spray is dependent on the atomic weight of the species. This is the reason that the alloy components are expected to be deposited one by one, which must affect the concentration of the deposited film. Although it is factual that the constituents are sprayed at diverse speeds, being a surface phenomenon, the vaporization of a species preferentially enriches the surface with atoms of the remaining ones, which effectively compensates for the difference in abrasion speeds. Thus, deposited films have a similar concentration as of the raw material.

Another use of the sputtering process is the erosion of white material. An example is found in secondary ion mass spectroscopy, in which the target is sprayed at a constant speed. As this occurs, the identity and concentration of the evaporated atoms is determined by mass spectrometry. In this way, the concentration of the examined material can be estimated and enormously low concentrations of impurities can be recognized. In addition, as the spray is attacking deeper and deeper layers, it is possible to obtain a concentration profile depending on the depth.

Conclusion

Sputtering is a physical process in which the vaporization occurs of a solid material by bombarding it by ion energy. This is a process widely used in the formation of thin films on materials, engraving techniques, erosion of white material and analytical techniques. The sputtering is mainly caused by the momentum exchange between atoms and ions of the material, due to collisions. The Sputtering process can be thought of as a pool game at the atomic level, with the ions (white ball) hitting a cluster of densely packed atoms (billiard balls). Although the primary collision pushes atoms further in the cluster, subsequent collisions among atoms can cause a few atoms near the surface being expelled. The number of atoms expelled from the surface per incident ion is the sputter yield and is a significant measure of the efficiency of the Sputtering process. Some factors that influence this parameter are the energy of the incident ions, their masses and those of the target atoms and the bond energy of the solid.

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