Sputtering

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.

In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface after the material is bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space and can cause wear in precision components. However, it can be utilized in science and industry for precise etching, analytical techniques, and depositing thin film layers in the manufacture of optical coatings, semiconductor devices, and nanotechnology products. It is a physical vapor deposition technique.

Physics

When energetic ions collide with atoms of a target material, an exchange of momentum occurs between them. These ions, known as "incident ions," set off collision cascades in the target. If a collision cascade reaches the surface of the target with energy greater than the material'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 its backside; the atoms ejected in this fashion escape the surface binding energy "in transmission." The average number of atoms ejected per incident ion is called the "sputter yield." This yield depends on factors such as the collision angle, energy, ion mass, target atom mass, and target's surface binding energy.

The ions that cause sputtering come from various sources such as plasma, specially constructed ion sources, particle accelerators, outer space (solar wind), or radioactive materials (alpha radiation). 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 electron stripping at high energy, is implemented in the program TRIM.

Another mechanism of physical sputtering is called "heat spike sputtering." This occurs when the solid is dense enough, and the incoming ion heavy enough, that collisions occur very close to each other. The dense collisions induce a heat 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 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 (e.g., Ag, Au, Pb).

Preferential sputtering can occur when a multicomponent solid target is bombarded, and there is no solid-state diffusion. If the energy transfer is more efficient to one component, it sputters more efficiently. For example, if component A in an AB alloy is sputtered preferentially, the surface becomes enriched in component B during prolonged bombardment, eventually balancing the composition of the sputtered material back to AB.

Electronic Sputtering

Electronic sputtering can mean either sputtering induced by energetic electrons (e.g., in a transmission electron microscope) or sputtering due to high-energy ions that lose energy to the solid by electronic stopping power. Electronic sputtering yields high rates from insulators because the excitations aren't immediately quenched as they would be in conductors. For instance, on Jupiter's moon Europa, a MeV sulfur ion from Jupiter's magnetosphere can eject up to 10,000 H2O molecules.

Potential Sputtering

Potential sputtering occurs when multiply charged ions impact a solid surface, releasing stored potential energy. This sputtering process strongly depends on the charge state of the ion and can occur even at ion impact energies below the physical sputtering threshold. Potential sputtering has only been observed for certain targets and requires a minimum potential energy.

Etching and Chemical Sputtering

Ion milling or ion etching involves removing atoms by sputtering with an inert gas. Sputtering also plays a role in reactive-ion etching (RIE), where chemically active ions enhance the sputtering yield. Sputtering observed below the threshold energy of physical sputtering is often called chemical sputtering. At elevated temperatures, chemical sputtering of carbon occurs due to the weakening of bonds by incoming ions, which then desorb by thermal activation.

Applications and Phenomena

Sputter Cleaning

Sputter cleaning removes contaminants from solid surfaces using physical sputtering in a vacuum. It is often used in surface science, vacuum deposition, and ion plating. Sputter cleaning must be done carefully to avoid problems such as overheating, gas incorporation, surface damage, surface roughening, and recontamination. Plasma cleaning can be used for larger surfaces.

Film Deposition

Sputter deposition creates thin films by sputtering material from a target onto a substrate, like a silicon wafer or solar cell. Sputtering usually uses an argon plasma because argon won't react with the target material.

Sputter Damage

Sputter damage occurs during the deposition of transparent electrodes on optoelectronic devices, originating from the substrate's bombardment by energetic species. Various particles, such as sputtered atoms and negative ions, cause damage by imparting energy, dissociating bonds, or increasing substrate temperature. This can affect the functional properties of underlying layers and photoactive materials, impairing device performance.

Etching

In the semiconductor industry, sputtering is used to etch targets when a high degree of anisotropy is needed. However, sputter etching can cause wafer damage and requires high voltage.

For Analysis

Sputtering etches away target material in techniques like secondary ion mass spectrometry (SIMS), where the concentration and identity of sputtered atoms are measured to determine the composition of the target material.

In Space

Sputtering alters the physical and chemical properties of airless bodies like asteroids and the Moon. On icy moons like Europa, sputtering leads to the accumulation of oxygen-rich materials. On Mars, sputtering is one way the planet has lost most of its atmosphere.

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Introduction

Sputtering is a technique involving the ejection of atoms from a solid target through bombardment with energetic ions. It is a high-vacuum coating technique in the group of PVD (Physical Vapor Deposition) processes. Additionally, in surface physics, sputtering is used for surface purification and chemical composition analysis.

Process of Sputtering

Sputtering uses the energy of plasma to remove atoms from a target (cathode) and deposit them onto a substrate. A plasma, typically created by ionizing argon gas, contains Ar+ ions that accelerate toward the target, causing the ejection and deposition of target atoms onto the substrate. The process requires maintaining low pressures to avoid contamination from residual gases.

Sputtering targets come in various shapes and materials, allowing for different types of thin layers, including alloys, in a single run.

Basics of the Sputtering Process

When a surface is bombarded with ions, several effects can occur:

1. Material is removed from the bombarded target (cathode).
2. The ions incorporate into the target material, possibly forming a chemical compound (reactive ion implantation).
3. The ions condense on the bombarded substrate, forming a layer (ion beam deposition).

If material removal is intended, ions must have sufficient energy. The impinging ion transmits its impulse to atoms of the target material, causing further collisions. This collision cascade results in some atoms near the surface being ejected if they have enough energy.

The sputter yield is influenced by the ion's kinetic energy, ion mass, surface atom binding energy, and the mass of the target. Above the minimum energy threshold (30-50 eV), yields increase, but high ion energies deposit energy deeper into the target, reducing the surface ejection.

Ion bombardment also generates secondary electrons, ions, and clusters. The energy distribution of dissolved atoms peaks at half the surface binding energy, falling off slowly to higher energies. This effect is exploited in surface physics analysis methods, thin-film technology, and sputter deposition.

Types of Sputtering

Main types of sputtering include:

1. DC Diode Sputtering

A DC voltage (500 - V) ignites argon low-pressure plasma between a target and a substrate, causing positive argon ions to precipitate atoms from the target onto the substrate.

Limitations:

• Only electrical conductors can be sputtered.
• Low sputtering rates due to few argon ions.

2. RF Sputtering

Uses a high-frequency alternating field instead of a DC electric field. The alternating field accelerates ions and electrons, increasing the plasma rate and allowing for pressure reduction while maintaining high sputtering rates.

Benefits:

• Can sputter insulators and semiconductors.
• Less substrate heating.
• Higher sputtering rates at the same chamber pressure.

Limitations:

• Relatively low coating rates.
• More expensive RF generation compared to DC voltage source.
• Uneven plasma density in large rectangular cathodes.

3. DC Triode Sputtering

The target is placed as a third electrode outside the plasma chamber, decoupling plasma generation and the sputtering process.

4. Magnetron Sputtering

Combines an electric field with a magnetic field to deflect charge carriers onto spiral paths over the target surface, increasing ionization and sputtering rates. Commonly used in microelectronics for producing metal layers.

5. Reactive Sputtering

Adds reaction gases (e.g., oxygen or nitrogen) to argon gas, ionizing and reacting with sputtered layer atoms to form reaction products that deposit on the substrate surface. Available as both DC and RF variants.

Applications of Sputtering Targets

Sputtering is used in the semiconductor industry for depositing thin films of different materials on silicon wafers and in optical applications by coating glass. The process operates at low temperatures, making it suitable for various thin-film applications.

Sputtering offers the advantage of depositing films with concentrations similar to the raw material. Despite different deposition rates for alloy components, the surface phenomenon compensates for differences in abrasion speeds. Sputtered films have similar concentrations as the raw material.

Sputtering is also used for material erosion. For example, secondary ion mass spectrometry (SIMS) involves sputtering a target at a constant rate and measuring the identity and concentration of sputtered atoms to determine the target's composition. This technique can detect extremely low concentrations of impurities and provide concentration profiles as a function of depth.

Conclusion

Sputtering is a physical process of vaporizing solid material by bombarding it with ion energy. It is widely used for thin-film deposition, etching, material erosion, and analytical techniques. Sputtering results from momentum exchange between atoms and ions of the material due to collisions. Factors like incident ion energy, ion mass, and target bonding energy influence sputtering efficiency.

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