Laser Sputter Deposition

Laser sputter deposition is a thin film deposition technique for silicon wafers, which uses high-energy pulsed laser radiation to ablate or knock-off target molecules and deposit them as a thin film layer on a wafer substrate. This technique was first tried by Smith and Turner in 1965. Three years later, Breech and Cross wrote a paper on laser vaporization and atom excitation.

At that time, quality of laser sputter deposition was far behind than that of the layers deposited by other techniques like molecular beam epitaxy and chemical vapor deposition. In 1987 laser sputter deposition became an established technique when Dijkkamp and his group used laser deposition on high-temperature superconductors (YBa2Cu3O7). Their work marked the starting point for the industry to utilize laser sputter deposition for layers of high quality crystals.

Since then, all types of oxides, nitrides, or even the metallic systems, and polymers have been deposited through laser.

How does laser sputter deposition work?

In laser sputter deposition, the target material undergoes a number of energy conversions before it is deposited on the substrate. First, when the radiated laser pulse hits the target, the energy is transferred as an electronic excitation, and then converted to thermal, chemical and mechanical energy.

All these energy conversions cause the target to evaporate, ablate, create plasma, and exfoliate.

The whole process can be divided in the following stages:

a.      Laser Sputtering of the Target Material

Laser sputtering uses a high-energy excimer laser pulse with very short wavelength like the KrF laser at 248nm. As the high-energy laser pulse is directed towards the target material causing energy transfer between the free electrons and the atoms. This will result in an extreme increase in temperature and in the evaporation of a part of the target.

 b.      Expansion of Material through Plasma

The vaporized material will expand towards the substrate in the plasma located in parallel to the target material. This expansion is dependent on the background pressure. An increase in background pressure slows down the excited particles, providing a higher chance for the deposited material to resputter. Thus, the deposition rate will decrease and material’s stoichiometry will be altered.

 c.       Deposition of the Sputtered Particles

Once the sputtered particles start bombarding the substrate surface, they will cause some of the substrate’s atoms to be knocked-off of its surface. Deposition begins when the condensation rate, caused by the collision of the sputtered particles of the target material and the substrate, is high enough to obtain thermal equilibrium.  It is important to have of continuous flow of sputtered particles to maintain thermal equilibrium.

d.      Growth of the Deposited Material

There are three growth techniques that can be used in laser sputter deposition: step flow, layer by layer, and 3D growth. In step flow growth, once the atoms of the target material reach the substrate surface, they will diffuse. Step flow growth is usually used on substrates with high crystal miscuts since the atomic steps on the substrate surface are formed from miscuts on the crystal.

Layer-by-layer technique, on the other hand, continuously grows islands of material until the islands overlap each other, a phenomenon known as coalescence. During coalescence, there is a huge density of material on the crater of the substrate surface. This whole process is performed for every additional layer.

In 3D growth mode, islands of material are formed on top of each other. This technique is somehow similar to layer-by-layer except for the fact that the deposition does not appear in a layered fashion because materials roughen every time it is added on top of the island.

Growth Modes for Laser Sputter Deposition

Figure 1. Growth Modes for Laser Sputter Deposition: (a) Step Flow, (b) Layer-by-layer, (c) 3D Growth.

The Characteristics of Laser Sputter Deposition

In comparison with other deposition techniques, laser sputter deposition is much more flexible as shown by the following parameters:

Target Size

Since the target size of the laser beam on laser sputter deposition is small, it is possible for this technique to deposit complex compounds. Such capability is usually useful for research purposes, especially when the preparation for the target material using other techniques would be expensive, like in the case of superconductor films.

Target Material Type

A deposition process with different types of target material must be operated in an ultra high vacuum chamber with the laser source located outside the chamber.  Some of the materials that can be prepared using laser sputter deposition are listed in Table 1.

Material Type Compound Reference
High-temperature superconductors YBa2Cu3O7

BiSrCaCuO

TlBaCaCuO

MgB2

Dijkkamp (1987)

Guarnieri (1988)

Foster (1990)

Shinde (2001)

Oxides SiO2 Fogarassy (1990)
Carbides SiC Balooch (1990)
Nitrides TiN Biunno (1989)
Ferroelectric materials Pb(Zr,Ti)O3 Kidoh (1991)
Diamond-like carbon C Martin (1990)
Buckminster fullerene C60 Curl and Smalley (1991)
Polymers Polyethylene, PMMA Hansen and Robitaille (1988)
Metallic systems 30 alloys/multilayer

FeNdB

Krebs and Bremert (1993)

Geurtsen (1996)

Table 1. Some of the materials prepared for the first time by laser sputter deposition.

The Pulsed Nature of Laser

Some complex compounds such as polymer-metals require different multilayered conditions for the deposition of each component. One example is the deposition of polycarbonate silver. This kind of deposition is not possible with other techniques because of the big gap between the energy requirements of the two components. But with laser sputter deposition, the pulsed nature of the laser allows the polycarbonates to be deposited at low laser fluence of 60mJ/cm2 and the silver crystals at much higher laser fluence, about 80 times higher at 5J/cm2.

Conclusion

With the continuous advancements in laser technology, laser sputter deposition has emerged in the industry as a competitive technique for depositing thin films of complex stoichiometry but because of the small target size usable, it is not practical for large-scale layer formation.

Honeywell starts to sell new CuMn sputtering targets for sputtering machines

sputtering

US maker Honeywell has formally announced today that it will start to sell a new kind of copper manganese sputtering targets for sputtering equipment. The main application of the targets will likely be semiconductor manufacturing.

Director of Honeywell department for sputtering target development Chris LaPietra has stated that the new targets were developed using the company proprietary Equal Channel Angular Extrusion technology, originally developed for other materials such as aluminium and now used for the new CuMn alloy.

The benefits of the new technology are very few impurities, extremely small grain size (in the order of less than 100μm), improved strength among others.

The small grain size is particularly important as it contributes to solve the issue of plasma drop out that causes the deposition of defective thin-film on wafers and therefore decreases yield and increases the process costs. The new sputtering target manufacturing technology allows to increase the target life expectancy by 100%, effectively doubling it, and therefore reducing sensibly the cost of ownership of the whole system

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How to sputter indium gallium nitride at low temperatures

Physical vapor deposition through sputter technique

According to a new article published on the Applied Physics Letter, an authoritative publication covering sputter and other deposition technologies, a group of researchers from both the University of Tokyo and the CREST technology agency led by Dr. Eiji Nakamura has proven a strong reduction in the temperature used to sputter indium gallium nitride over a glass substrate using a process called pulsed sputter deposition.

The team aims at creating a new and revolutionary method to sputter InGaN for LED applications combining the newly developed InGaN thin film deposition by PSD with the use of a graphene buffer layer on an amorphous substrate.

The Mg-doped GaN layers have been deposited over a semi-insulating GaN at 480°C, the team measured a p-type conductivity with hole concentration 3.0×10^17.

The application aimed for is InGaN light emitting diodes (LEDs), where low-temperature InGaN deposition is part of the required process.

Until now, you could sputter InGaN over silicon or other substrates by chemical vapor deposition (CVD), with the issue that CVD usually requires deposition temperatures of over 1000°C and most substrates can not withstand such heat.

For example, glass can withstand temperatures up to 500°C, as it softens at 550°C or over depending on glass type.

The new technique will allow to sputter on glass substrates, possibly at industrial scale, on large areas up to few squared meters.

The method created by the Japanese group of researchers has been able to achieve such low temperature deposition thanks to the pulsed sputter process that allows the deposited metals to have an enhanced surface migration of the growth precursors.

Moreover, at temperatures under 500°C it is usually very challenging to achieve p-type conductivity by magnesium doping; the team however used a particular stoichiometric ratio.

The film showed a surface roughness of 0.67nm and a good surface morphology with less defects than usual CVD-deposited InGaN thin films.

Thicknesses from 30nm to 300nm have been tested, measured internal quantum efficiency was at about 24%.

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Thin film deposition techniques for MEMS processing

Thin film deposition techniques for MEMS processing

One of the fundamental steps in microelectromechanical systems (MEMS) processing is the deposition of thin films of material with thicknesses ranging from a few nanometers to about 100 micrometers.

Deposition techniques in MEMS processing fall into two categories, the first being techniques that deposit a thin film based on chemical reactions. These include chemical vapor deposition (CVD), epitaxy, thermal oxidation, and electrodeposition.

All of these processes utilize the production of solid materials from chemical reactions in a gas or liquid matrix or through reactions with the substrate material.

The second category is the direct physical deposition of materials on the substrate, such as casting and physical vapor deposition (PVD).

Chemical deposition technologies

Among chemical deposition techniques, CVD is one of the most important.
The basic principle of CVD is that the substrate is placed inside a reactor where source gases are supplied, and when the chemical reaction between the gases takes place, the product of that reaction is a solid material that condenses on the surface of the substrate inside the reactor. CVD is ideally used when the thin film to be produced should have good step coverage. One drawback of CVD is that it can produce hazardous products during the process.

Epitaxy is a chemical process that is highly similar to CVD, the only difference being that it uses ordered semiconductor crystals as the substrate and that the thin film grows with the same crystallographic orientation as the substrate. Epitaxy has the advantage of high growth rate of the material, enabling the formation of films that are much thicker than usual. It’s a technology that is usually used for the deposition of silicon used in producing silicon on insulator substrates.

One of the most basic chemical deposition techniques is thermal oxidation, where the substrate surface is simply oxidized in an oxygen-rich atmosphere at high temperatures.

Thermal oxidation is also the only deposition method where the substrate is consumed, this means that the formation of the film is downwards into the substrate and that the oxidation time is increases as the film becomes thicker.

Thermal oxidation is a simple process that is typically used to produce films for electrical insulation, but it has the drawback of being of somewhat limited use in MEMS components.

Electrodeposition is a method that is limited only to electrically conductive materials. There are two kinds of electrodeposition: electroplating and electroless plating: electroplating involves the application of an electric potential between two electrodes in an electrolyte solution, while electroless plating utilizes a more complex chemical solution.

Electrodeposition is suitable for producing films of metals such as copper, nickel, and gold.

Physical Deposition Technologies

One of the physical deposition technologies is casting.
Casting involves the dissolution of the material to be deposited in a solvent, then applying it to the substrate by spraying it.

A thin film of the target material remains on the substrate after the solvent has evaporated.

Casting is particularly useful for polymeric materials that can be dissolved in organic solvents, it is typically used to apply photoresistive materials to substrates in photolithography.

PVD is a broad term for a number of deposition techniques where a material is released or removed from a source and then transferred to the substrate. It is the more common technology used for the deposition of metals as it can be performed at lower process risk and is cheaper, as compared to CVD.

PVD does have drawbacks, such as the inferior quality of its films as compared to films produced by CVD as well as lower step coverage.

There are several deposition methods that fall under the category of PVD, but the two most important techniques are evaporation and sputtering.

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PVD (physical vapor deposition) through sputtering

PVD (physical vapor deposition) through sputtering

Sputtering is one of the methods by which physical vapor deposition (PVD) is achieved. PVD is a process where a thin film of material is deposited on the surface of a substrate. In sputter deposition, high-energy particles are utilized to remove or eject atoms or molecules from the surface of a target material. The ejected atoms are removed from the target material and deposit on a substrate as a thin film. Sputtering is one of the most common methods used to deposit various thin metallic films on wafer substrates, with gold, platinum, aluminum, and tungsten among the common target materials.

PVD by sputtering can be described as a series of four steps. High-energy particles, which are ions of a gas, are generated and directed at a target material. When the high-energy particles collide with the target material, atoms of the target material are sputtered from the target. These sputtered atoms travel through a region of reduced pressure toward the substrate and then condense on the surface of the substrate to form a thin film of the sputtered target material. The high energy particles used in the sputtering process are usually ions of an inert sputtering gas, such as neon. Neon is used to sputter light elements, while krypton or xenon is preferred when sputtering heavier elements. However, reactive gases can also be used. The high-energy ions utilized in PVD sputter deposition are produced via glow discharges, which is a type of self-sustaining plasma generated by applying an RF field to a pressurized sputtering gas. There are many types of sputter deposition, such as ion-beam sputtering, gas-flow sputtering, high-power impulse magnetron PVD sputtering, reactive sputtering, ion-assisted deposition, and high-target-utilization sputtering.

PVD sputtering deposition offers several advantages as compared to other deposition techniques. One of this is that materials with very high melting points can be easily sputtered, while the evaporation of such materials in a Knudsen cell or a resistance evaporator is very difficult. Sputtered films also have better adhesion on the wafer as compared to evaporated films and their composition is also close to that of the target material. PVD sputtering also simplifies the deposition of thin films over large wafers as it can be used on large-size target materials. PVD sputter deposition is a complex process due to the availability of many parameters, but this also enables a large degree of control over factors such as film thickness, film growth, alloy composition, and film microstructure. The substrate can also be sputter cleaned in vacuum before film deposition is performed. Finally, sputter deposition avoids device damage that can happen in electron beam evaporation due to the X-rays generated.

However, sputtering also has its disadvantages, one of which is the high capital expenses required to start and run a sputtering facility. PVD sputtering also has a higher tendency to introduce impurities in the substrate as compared to deposition via evaporation. Other materials are also incompatible with sputtering deposition particularly when they are easily degraded by ionic bombardment, such as organic solids. Finally, some important materials also have relatively low rates of deposition via sputtering, such as SiO2.

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