PVD: Sputtering and Thermal Evaporation

Any wafer fabrication involves deposition of layers of materials, which will serve as a conductor, an insulator, or a buffer. Thin film metal layers like aluminum interconnects are deposited in the wafer substrate through Physical Vapor Deposition (PVD) – a process which uses vacuum deposition technique.

One of the earliest PVD techniques is the thermal evaporation. In thermal evaporation, the metal is heated to its vaporization point. It will then evaporate to the wafer to form the thin film.  However, since evaporated materials tend to be directional, thermal evaporation provides poor coverage.

The poor coverage problem in thermal evaporation was solved when sputtering emerged. Sputtering is one of the most widely used PVD techniques by the semiconductor industry in deployment of thin film metal layers over the wafer substrate. It injects the metal layer in the substrate by ‘knocking-off’ the metal atoms out of the metal material and bombards the ‘knocked-off’ ions on to the wafer. This grants sputtering a better coverage of deposition than thermal evaporation.

Physical Vapor Deposition Techniques

Figure: Physical vapor deposition techniques: (a) thermal evaporation and (b) sputtering

Following are the main differences of thermal evaporation and sputtering:

1. Coverage Area

As mentioned, sputtering has a larger coverage area than thermal evaporation because of the nature of their distribution method. The vapors used in thermal evaporation are directional; while the ion bombardment of the sputtering is ‘rain-like’.

2. Deposition Rate Control Factors

In thermal evaporation, deposition rate can be controlled by the amount of heat supplied on the material (vaporization point). On the other hand, sputtering controls its rate through gas pressure, temperature, the potential difference between the material, which acts as the cathode of system and the wafers placed on the system’s anode.

3. Deposition Rate

When talking about deposition rate, the more controlled the rate is (lower number of layers/second), the better. Sputtering can trim down its deployment of metal layers up to one atomic layer per second. Whereas thermal injection can only control it to hundreds or thousands of atomic layers per second.

4. Choice of Material

Sputtering has a wider range of choices for material than thermal evaporation.

5. Decomposition of Material

The uniformity of decomposition and erosion of the material in sputtering makes it more efficient than thermal evaporation.

6. Equipment Cost

Operating using sputtering will cost more than thermal evaporation because the latter only needs a vacuum chamber with precise thermometers; while the former requires twice or thrice of the energy used in thermal deposition to excite the ion of the material.

7. Surface Damage

Surface damage has higher possibility in sputtering. Its ion particle bombardment can induce damage in the substrate.

THERMAL EVAPORATION

SPUTTERING

Coverage Area poor coverage large coverage
Deposition Rate Control Factors temperature only gas pressure,temperature, andpotential difference between the material and the wafer
Deposition Rate thousand atomic layers per sec one atomic layer per sec
Choice of Material Limited wide range of variety
Decomposition of Material High Low (uniform)
Equipment Cost Cheaper more expensive
Surface Damage very low ion particle bombardment can induce damage

Considering the pros and cons between these two techniques, sputtering will clearly prevail over thermal evaporation for industrial applications. Thermal evaporation, on the other hand, is most of the time preferred in laboratory experiments.

Aside from being the main choice for thin film deposition in semiconductor industry, sputtering is also used in fabrication of compact disks, large area displays, and other deposition process on blades and gears.

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The evolution of physical vapor deposition technology from thermal evaporation to sputtering

PVD (physical vapor deposition) through sputtering

Sputtering deposition is one of the main physical vapor deposition techniques to deposit thin films of metals or other materials over a substrate.

The technique is useful for microelectronics applications and has been widely popular in the industry for several reasons as it can be seen as the natural evolution of thermal evaporation, one of the earliest techniques for thin film physical vapor deposition.

While thermal evaporation requires only a relatively simple vacuum chamber where the material is heated in a controlled way and evaporated, sputtering needs much larger and complicated machinery that allows the substrate to be literally “bombarded” by ions in order to eject particles that get deposited over a substrate placed in front of the target itself
Sputtering is now the deposition method of choice for industrial applications while thermal evaporation is little more than a lab experiment as sputtering allows deposit a large variety of materials in a much tightly controlled way.

While with thermal evaporation the deposition rate can only roughly be controlled by setting the material source at a predetermined temperature, sputtering allows to control more factors such as: the gas pressure inside the chamber, the potential difference between anode and cathode, the target and substrate temperatures, etc.

The main differences between thermal evaporation and sputtering are:

  1. With thermal evaporation only a limited number of materials can be deposited, with sputtering the variety is much larger;
  2. With sputtering it is possible to control the deposition speed down to almost one atomic layer per second while with thermal deposition hundreds if not thousands of atomic layers are deposited in a second. In industrial applications, where the deposition rate must be strictly controlled, thermal evaporation is then not an option
  3. It is possible to deposit larger areas with sputtering than with thermal evaporation
  4. The decomposition of the target material and its erosion during sputtering are uniform and therefore the process makes better use of the target material, which in many cases can be expensive (gold, ruthenium, etc.)

However, sputtering has also some disadvantages if compared with thermal evaporation as:

  1. It needs by far more expensive machinery and devices than thermal evaporation, which basically needs only a simple vacuum chamber coupled with a precise thermometer
  2. The substrate can be damaged due to particle hit during sputtering

From the above considerations, it is clear that the advantages of sputtering over thermal evaporation far outweigh the disadvantages for industrial applications.

Nowadays, sputtering is widely considered the physical vapor deposition method of choice for compact disks, large area displays, and even for applications not related with microelectronics, such as deposition of TiN, CrN, TiC or CrAlN over saw blades and bearing gears.

The sputtering process mechanism works as follows: during the deposition, the disc of materials which needs to be deposited (called as the “target”) has a negative potential and it is bombarded by positive ions of inert gases such as argon or xenon. By purely kinetic energy transfer between the ions colliding on the target and the atoms on the target surface, the latter get ejected from the target surface and collide with the substrate, creating a thin film of material over it.

Keeping all other variables fixed, sputtering deposition rate is proportional to the plasma energy within a certain range and therefore it can be easily controlled.

This is the main reason why sputtering deposition is superior to other thin film physical vapor deposition methods such as thermal evaporation

However, as hinted above, the whole process of sputtering requires a much larger system as it takes at least two or three order of magnitude more energy to liberate one atom from the target during sputtering than during thermal evaporation.

Moreover, the target and the substrate require considerable cooling and therefore a proper refrigerating system needs to be put in place.

The whole sputtering station needs to be into vacuum, and to be kept free from contamination from atmospheric element.

The formation of sustained plasma is also crucial to the reliability of the whole process.
The reason why the conditions of the plasma need to be carefully controlled are several, probably one of the most important parameters is plasma pressure, a factor that is pivotal to the reliability of the whole process.

If the pressure of the plasma is too high, a large number of the sputtered atoms cannot pass through the gas and get reflected back to the cathode, while if the pressure is too low, the number ionizing collisions between secondary electrons released from the cathode and the inert gas is not enough to keep the plasma sustained at the needed levels.

Another point that needs to be carefully considered is the power supply for the whole system as the type of power supply is heavily dependent on the type of material that needs to be sputtered.

For conductors, a typical DC power supply can be used, for insulating materials an RF power supply is needed. Insulating materials are usually harder to be sputtered and therefore sometimes sputtering may not be the best physical vapor deposition method, plasma lased deposition is a another possible option

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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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