Sputter deposition

Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by the phenomenon of sputtering. This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer.

Link to Acetron

Resputtering is re-emission of the deposited material during the deposition process by ion or atom bombardment.[1][2]

Sputtered atoms ejected from the target have a wide energy distribution, typically up to tens of eV (100,000 K). The sputtered ions (typically only a small fraction of the ejected particles are ionized &#; on the order of 1 percent) can ballistically fly from the target in straight lines and impact energetically on the substrates or vacuum chamber (causing resputtering). Alternatively, at higher gas pressures, the ions collide with the gas atoms that act as a moderator and move diffusively, reaching the substrates or vacuum chamber wall and condensing after undergoing a random walk. The entire range from high-energy ballistic impact to low-energy thermalized motion is accessible by changing the background gas pressure.

The sputtering gas is often an inert gas such as argon. For efficient momentum transfer, the atomic weight of the sputtering gas should be close to the atomic weight of the target, so for sputtering light elements neon is preferable, while for heavy elements krypton or xenon are used.[3] Reactive gases can also be used to sputter compounds.

The compound can be formed on the target surface, in-flight or on the substrate depending on the process parameters. The availability of many parameters that control sputter deposition make it a complex process, but also allow experts a large degree of control over the growth and microstructure of the film.

Uses

One of the earliest widespread commercial applications of sputter deposition, which is still one of its most important applications, is in the production of computer hard disks. Sputtering is used extensively in the semiconductor industry to deposit thin films of various materials in integrated circuit processing. Thin antireflection coatings on glass for optical applications are also deposited by sputtering. Because of the low substrate temperatures used, sputtering is an ideal method to deposit contact metals for thin-film transistors. Another familiar application of sputtering is low-emissivity coatings on glass, used in double-pane window assemblies. The coating is a multilayer containing silver and metal oxides such as zinc oxide, tin oxide, or titanium dioxide. A large industry has developed around tool bit coating using sputtered nitrides, such as titanium nitride, creating the familiar gold colored hard coat. Sputtering is also used as the process to deposit the metal (e.g. aluminium) layer during the fabrication of CDs and DVDs.

Hard disk surfaces use sputtered CrOx and other sputtered materials. Sputtering is one of the main processes of manufacturing optical waveguides and is another way for making efficient photovoltaic and thin film solar cells.[4][5]

In , researchers at IMEC built up lab superconducting qubits with coherence times exceeding 100 μs and an average single-qubit gate fidelity of 99.94%, using CMOS-compatible fabrication techniques such as sputtering deposition and subtractive etch.[6]

Sputter coating

Sputter coating in scanning electron microscopy is a sputter deposition process[clarification needed] to cover a specimen with a thin layer of conducting material, typically a metal, such as a gold/palladium (Au/Pd) alloy. A conductive coating is needed to prevent charging of a specimen with an electron beam in conventional SEM mode (high vacuum, high voltage). While metal coatings are also useful for increasing signal to noise ratio (heavy metals are good secondary electron emitters), they are of inferior quality when X-ray spectroscopy is employed. For this reason when using X-ray spectroscopy a carbon coating is preferred.[7]

Comparison with other deposition methods

An important advantage of sputter deposition is that even materials with very high melting points are easily sputtered while evaporation of these materials in a resistance evaporator or Knudsen cell is problematic or impossible. Sputter deposited films have a composition close to that of the source material. The difference is due to different elements spreading differently because of their different mass (light elements are deflected more easily by the gas) but this difference is constant. Sputtered films typically have a better adhesion on the substrate than evaporated films. A target contains a large amount of material and is maintenance free making the technique suited for ultrahigh vacuum applications. Sputtering sources contain no hot parts (to avoid heating they are typically water cooled) and are compatible with reactive gases such as oxygen. Sputtering can be performed top-down while evaporation must be performed bottom-up. Advanced processes such as epitaxial growth are possible.

Some disadvantages of the sputtering process are that the process is more difficult to combine with a lift-off for structuring the film. This is because the diffuse transport, characteristic of sputtering, makes a full shadow impossible. Thus, one cannot fully restrict where the atoms go, which can lead to contamination problems. Also, active control for layer-by-layer growth is difficult compared to pulsed laser deposition and inert sputtering gases are built into the growing film as impurities. Pulsed laser deposition is a variant of the sputtering deposition technique in which a laser beam is used for sputtering. Role of the sputtered and resputtered ions and the background gas is fully investigated during the pulsed laser deposition process.[8][9]

Types of sputter deposition

Sputtering sources often employ magnetrons that utilize strong electric and magnetic fields to confine charged plasma particles close to the surface of the sputter target. In a magnetic field, electrons follow helical paths around magnetic field lines, undergoing more ionizing collisions with gaseous neutrals near the target surface than would otherwise occur. (As the target material is depleted, a "racetrack" erosion profile may appear on the surface of the target.) The sputter gas is typically an inert gas such as argon. The extra argon ions created as a result of these collisions lead to a higher deposition rate. The plasma can also be sustained at a lower pressure this way. The sputtered atoms are neutrally charged and so are unaffected by the magnetic trap. Charge build-up on insulating targets can be avoided with the use of RF sputtering where the sign of the anode-cathode bias is varied at a high rate (commonly 13.56 MHz).[10] RF sputtering works well to produce highly insulating oxide films but with the added expense of RF power supplies and impedance matching networks. Stray magnetic fields leaking from ferromagnetic targets also disturb the sputtering process. Specially designed sputter guns with unusually strong permanent magnets must often be used in compensation.

Ion-beam sputtering

Ion-beam sputtering (IBS) is a method in which the target is external to the ion source. A source can work without any magnetic field like in a hot filament ionization gauge. In a Kaufman source ions are generated by collisions with electrons that are confined by a magnetic field as in a magnetron. They are then accelerated by the electric field emanating from a grid toward a target. As the ions leave the source they are neutralized by electrons from a second external filament. IBS has an advantage in that the energy and flux of ions can be controlled independently. Since the flux that strikes the target is composed of neutral atoms, either insulating or conducting targets can be sputtered. IBS has found application in the manufacture of thin-film heads for disk drives. A pressure gradient between the ion source and the sample chamber is generated by placing the gas inlet at the source and shooting through a tube into the sample chamber. This saves gas and reduces contamination in UHV applications. The principal drawback of IBS is the large amount of maintenance required to keep the ion source operating.[11]

Reactive sputtering

In reactive sputtering, the sputtered particles from a target material undergo a chemical reaction aiming to deposit a film with different composition on a certain substrate. The chemical reaction that the particles undergo is with a reactive gas introduced into the sputtering chamber such as oxygen or nitrogen, enabling the production of oxide and nitride films, respectively.[12] The introduction of an additional element to the process, i.e. the reactive gas, has a significant influence in the desired depositions, making it more difficult to find ideal working points. Like so, the wide majority of reactive-based sputtering processes are characterized by an hysteresis-like behavior, thus needing proper control of the involved parameters, e.g. the partial pressure of working (or inert) and reactive gases, to undermine it.[13] Berg et al. proposed a significant model, i.e. Berg Model, to estimate the impact upon addition of the reactive gas in sputtering processes. Generally, the influence of the reactive gas' relative pressure and flow were estimated in accordance to the target's erosion and film's deposition rate on the desired substrate.[14] The composition of the film can be controlled by varying the relative pressures of the inert and reactive gases. Film stoichiometry is an important parameter for optimizing functional properties like the stress in SiNx and the index of refraction of SiOx.

Ion-assisted deposition

In ion-assisted deposition (IAD), the substrate is exposed to a secondary ion beam operating at a lower power than the sputter gun. Usually a Kaufman source, like that used in IBS, supplies the secondary beam. IAD can be used to deposit carbon in diamond-like form on a substrate. Any carbon atoms landing on the substrate which fail to bond properly in the diamond crystal lattice will be knocked off by the secondary beam. NASA used this technique to experiment with depositing diamond films on turbine blades in the s. IAD is used in other important industrial applications such as creating tetrahedral amorphous carbon surface coatings on hard disk platters and hard transition metal nitride coatings on medical implants.

High-target-utilization sputtering (HiTUS)

Sputtering may also be performed by remote generation of a high density plasma. The plasma is generated in a side chamber opening into the main process chamber, containing the target and the substrate to be coated. As the plasma is generated remotely, and not from the target itself (as in conventional magnetron sputtering), the ion current to the target is independent of the voltage applied to the target.

High-power impulse magnetron sputtering (HiPIMS)

HiPIMS is a method for physical vapor deposition of thin films which is based on magnetron sputter deposition. HiPIMS utilizes extremely high power densities of the order of kW/cm2 in short pulses (impulses) of tens of microseconds at low duty cycle of < 10%.

Gas flow sputtering

Gas flow sputtering makes use of the hollow cathode effect, the same effect by which hollow cathode lamps operate. In gas flow sputtering a working gas like argon is led through an opening in a metal subjected to a negative electrical potential.[15][16] Enhanced plasma densities occur in the hollow cathode, if the pressure in the chamber p and a characteristic dimension L of the hollow cathode obey the Paschen's law 0.5 Pa·m < p·L < 5 Pa·m. This causes a high flux of ions on the surrounding surfaces and a large sputter effect. The hollow-cathode based gas flow sputtering may thus be associated with large deposition rates up to values of a few μm/min.[17]

Structure and morphology

In J. A. Thornton applied the structure zone model for the description of thin film morphologies to sputter deposition. In a study on metallic layers prepared by DC sputtering,[18] he extended the structure zone concept initially introduced by Movchan and Demchishin for evaporated films.[19] Thornton introduced a further structure zone T, which was observed at low argon pressures and characterized by densely packed fibrous grains. The most important point of this extension was to emphasize the pressure p as a decisive process parameter. In particular, if hyperthermal techniques like sputtering etc. are used for the sublimation of source atoms, the pressure governs via the mean free path the energy distribution with which they impinge on the surface of the growing film. Next to the deposition temperature Td the chamber pressure or mean free path should thus always be specified when considering a deposition process.

Since sputter deposition belongs to the group of plasma-assisted processes, next to neutral atoms also charged species (like argon ions) hit the surface of the growing film, and this component may exert a large effect. Denoting the fluxes of the arriving ions and atoms by Ji and Ja, it turned out that the magnitude of the Ji/Ja ratio plays a decisive role on the microstructure and morphology obtained in the film.[20] The effect of ion bombardment may quantitatively be derived from structural parameters like preferred orientation of crystallites or texture and from the state of residual stress. It has been shown recently [21] that textures and residual stresses may arise in gas-flow sputtered Ti layers that compare to those obtained in macroscopic Ti work pieces subjected to a severe plastic deformation by shot peening.

See also

• Coating

References

Further reading

• The Foundations of Vacuum Coating Technology by D. Mattox
• William D. Westwood (). Sputter Deposition, AVS Education Committee Book Series. Vol. 2. ISBN 978-0---1.
• Kiyotaka Wasa & Shigeru Hayakawa (). Handbook of sputter deposition technology principles, technology and applications. Noyes Publications. ISBN .

Written By Matt Hughes &#; President &#; Semicore Equipment, Inc.
Published: 24 November

Sputtering is the thin film deposition manufacturing process at the core of today&#;s semiconductors, disk drives, CDs, and optical devices industries. On an atomic level, sputtering is the process whereby atoms are ejected from a target or source material that is to be deposited on a substrate &#; such as a silicon wafer, solar panel or optical device &#; as a result of the bombardment of the target by high energy particles.

The verb &#;To Sputter&#; comes from the Latin word Sputare meaning to &#;To emit saliva with noise.&#; While the word sputtering sounds funny to those who associate it with stammering and speech impediments, in Peter J. Clarke changed the course of history when he developed the first &#;Sputter gun&#; that catapulted the semiconductor industry by enabling the accurate and reliable deposition of materials on an atomic level using a charged plasma stream of electrons and ions in a vacuum environment.

The sputtering process begins when a substrate to be coated is placed in a vacuum chamber containing an inert gas &#; usually Argon &#; and a negative charge is applied to a target source material that will be deposited onto the substrate causing the plasma to glow.

Free electrons flow from the negatively charged target source material in the plasma environment, colliding with the outer electronic shell of the Argon gas atoms driving these electrons off due to their like charge. The inert gas atoms become positively charged ions attracted to the negatively charged target material at a very high velocity that &#;Sputters off&#; atomic size particles from the target source material due to the momentum of the collisions. These particles cross the vacuum deposition chamber of the sputter coater and are deposited as a thin film of material on the surface of the substrate to be coated.

Sputtering only takes place when the kinetic energy of the bombarding particles is extremely high, much higher than normal thermal energies in the &#;Fourth state of nature&#; plasma environment. This can allow a much more pure and precise thin film deposition on the atomic level than can be achieved by melting a source material with conventional thermal energies.

The number of atoms ejected or &#;Sputtered off&#; from the target or source material is called the sputter yield. The sputter yield varies and can be controlled by the energy and incident of angle of the bombarding ions, the relative masses of the ions and target atoms, and the surface binding energy of the target atoms. Several different methods of physical vapor deposition are widely used in sputter coaters, including ion beam and ion-assisted sputtering, reactive sputtering in an Oxygen gas environment, gas flow and magnetron sputtering.

What is Magnetron Sputtering?

Because ions are charged particles, magnetic fields can be used to control their velocity and behavior. John S. Chapin is credited with inventing the first planar magnetron sputtering source with a patent filed in . While conventional diode sputtering can deposit extremely thin films down to the atomic scale, it tends to be slow and most effective with small substrates. The bombardment of the substrate can also create overheating or damage to the object to be coated.

Magnetron sputtering deposition uses magnets behind the negative cathode to trap electrons over the negatively charged target material so they are not free to bombard the substrate, allowing for faster deposition rates.

The most common magnetron sputter cathode/target shapes are circular and rectangular. Rectangular magnetrons are most commonly used in larger scale &#;In-line&#; systems where substrates scan linearly past the targets on some type of conveyor belt or carrier. Circular sputtering magnetrons are more commonly found in smaller scale &#;Confocal&#; batch systems or single wafer stations. Read More&#;

The company is the world’s best semiconductor sputtering supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

What is Reactive Sputtering?

Reactive Sputtering is the process of adding a gas to the vacuum chamber that undergoes a chemical reaction before coming into contact with the materials to be coated. Gases like Nitrogen or Oxygen which are normally stable and inert under normal circumstances become ionized and reactive in the plasma environment as a result of the high energy collisions.

When this happens, the gas can react chemically with the target material cloud and create a molecular compound which then becomes the thin film coating. For example, a silicon target reactively sputtered with oxygen gas can produce a silicon oxide film, or with nitrogen can produce a silicon nitride film which are at the heart of the semiconductor industry. Read more&#;

What is Co-Sputtering?

Co-Sputtering is where two or more target materials are sputtered at once in the vacuum chamber and is often used with Reactive Magnetron Sputtering to produce thin films that are compounds such as alloys or composites.

It is widely used in the optical and architectural glass industries. By utilizing Reactive Co-Sputtering of two target materials such as Silicon and Titanium with dual Magnetron Sputtering, the refractive index or shading effect of the glass can be carefully and precisely controlled on applications ranging from large scale surfaces such as skyscraper architectural glass to sunglasses. It is also widely used producing solar panels. Read more&#;

Types of Sputtering Power Sources

There are several different types of power sources used to bombard the target material to sputter the atoms &#; including DC and RF Sputtering, Pulsed DC, MF, AC and the newly evolving HIPIMS sputtering techniques.

DC or Direct Current Sputtering is the simplest and most frequently used with electrically conductive target materials like metals because it is easy to control and relatively low cost in power consumption. When possible, DC Sputtering can be a relatively inexpensive, cost effective solution for coating a wide range of decorative metal coatings. Read more&#;

However, DC Sputtering has limitations when it comes to dielectric target materials &#; coatings which are non-conducting insulating materials that can take on a polarized charge. Examples of common dielectric coating materials include Aluminum Oxide, Silicon Oxide and Tantalum Oxide.

During DC Sputtering, the gas in the vacuum chamber becomes ionized. As a result, positive ions are produced which accumulate on the surface of the target face giving it a positive charge. This dielectric buildup of a positive charge over time can terminate the discharge of sputtering atoms.

Several methods have been developed to alternate or pulse the sputtering power source to &#;clean&#; or neutralize the target surface and prevent it from developing a positive charge.

RF or Radio Frequency Sputtering alternates the electrical potential of the current at radio frequencies to avoid a charge build up. By alternating the current in this manner, each phase of the cycle has the effect of reversing the buildup when the current is only flowing continuously in one direction.  As with DC Magnetron Sputtering, RF Magnetron sputtering coaters increases the growth of the thin film by increasing the percentage of target atoms which become ionized. Read more&#;

Pulsed DC Sputtering is where the target is bombarded with powerful voltage spikes to clean the target face and prevent the buildup of a dielectric charge. These voltage spikes which clean the target surface are usually set at frequencies ranging from 40 to 200 KHz. Read more&#;

HIPIMS or High Power Impulse Magnetron Sputtering is a newly evolving sputtering technique which also uses a high current voltage spike to greatly increase the ionization of the sputtering target. Compared to traditional sputtering processes, ionized atoms in HIPIMS systems have significantly higher energies capable of producing very dense thin film coatings. Read more&#;

MF or Mid Frequency AC Sputtering is usually used for depositing non-conductive thin film coatings. Two cathodes are used with an AC current switched back and forth between them which cleans the target surface with each reversal of the current. Read more&#;

Matt Hughes is President of Semicore Equipment Inc, one of the world&#;s leading suppliers of high performance PVD coating equipment including RF, DC and Pulsed DC, HIPIMS and AC Sputtering Systems.

What is Sputtering? Video Script

Exactly how does the sputtering process work?

First your coating materials are placed on a magnetron in a solid form called a target. For highly pure coatings you need a clean environment with only materials of your choosing.

This is why the chamber is evacuated, to remove almost every molecule from the chamber. Then the chamber is backfilled with a process gas.

Which gas is selected is based on the type of material to be deposited; Common process gasses include argon, oxygen, and nitrogen.

Now the conditions are ready to begin the process. A negative electrical potential is applied to the target material to be sputtered which is the magnetron cathode &#; and the positive anode or ground is the chamber body.

This electrical potential will cause free electrons to accelerate away from the magnetron. When these electrons collide with a process gas atom they strip the gas atom of an electron creating a positively charged process gas ion.The positively charged ion is accelerated toward the magnetron.

This ion carries enough energy with it to &#;knock off&#; or &#;sputter&#; some of the magnetrons target material. Target material will then collect on surfaces in the path that the magnetron is directed. This is how &#;sputtered&#; material collects on your substrate.

The light from the plasma is created when the ions recombine with free electrons into a lower energy state. Positively charged ions recombine with free electrons to create a neutral atom again.

The plasma glow is created when the ions recombine with free electrons into a lower energy state. When a free electron recombines with an ion it has a voltage; the ion needs less voltage, so this &#;excess voltage&#; is let off as light. The light is the plasma glow that is seen during processing.

This thin film deposition process continues at a constant rate until the desired thickness is achieved and the power is removed from the cathode.

This amazing atomic reaction known as &#;sputtering&#; is what makes Semicore a leader in custom vacuum equipment.

Semicore Equipment, Inc. is a leading worldwide supplier of sputtering equipment for the electronics, optical, solar energy, medical, automotive, military and related high technology industries. Please allow our support staff to answer any questions you have regarding &#;What is sputtering?&#; and how to implement the best equipment and techniques for your specific needs &#; whether it be DC, RF, Pulsed DC or HIPIMS equipment &#; by contacting us at or calling 925-373-.

Related Articles

DC electrical current typically in the -2 to -5 kV range is applied to the target coating material that is the cathode or point at which electrons enter the system known as the negative bias. A positive charge is also applied to the substrate to be coated which become the anode. The electrically neutral argon gas atoms are first ionized colliding with the target which eject atoms off into the plasma &#; a hot gas?like state consisting of roughly half gas ions and half electrons that emits the visible plasma glow&#; Read More

RF Sputtering can be used for the coating of dielectric or insulative materials that can take on a charge that results in arcing in the vacuum chamber with convention DC Sputtering. However, RF Sputtering deposition rates are slower than DC Sputtering rates and have higher power costs and so is usually used on smaller substrates to be coated. &#; Read More

Compared to conventional DC Sputtering, arcing can be greatly decreased or even eliminated by pulsing the DC voltage in the 10&#;350?kHz range with duty cycles in the 50&#;90% range.  A Pulsed DC electrical current typically in the few hundreds of volts range is applied to the target coating material. the voltage is either turned off or reversed with a low voltage short duration cycle to &#;cleanse&#; the target of any charge buildup&#; Read More

For more information, please visit sio2 sputtering.