Physics 2 - Chemical vapour deposition, sol-gels, electro deposition and ball milling

Anna University

Engineering Physics 2

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Unit-5

Chemical vapour deposition, sol-gels, electro deposition and ball milling

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Introduction:

It has been established that properties of materials in nano scale differ from those at a larger scale. It seems to be important to prepare materials in the nano scale.

Here we are interested to know the following five methods to produce nanomaterials.

(i) Physical vapor deposition

(ii) Chemical vapour deposition

(iii) Sol-Gels

(iv) Electro deposition and

(v) Ball milling

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Learning Objective:

On completion of this topic you are able to understand the above mentioned five methods of producing nanomaterials.

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Physical Vapour Deposition (PVD)

Physical vapor deposition (PVD) is fundamentally a vaporization coating technique, involving transfer of material on an atomic level. It is an alternative process to electroplating. The process is similar to chemical vapor deposition (CVD) except that the raw materials/precursors, i.e. the material that is going to be deposited starts out in solid form, whereas in CVD, the precursors are introduced to the reaction chamber in the gaseous state. It incorporates processes such as sputter coating and pulsed laser deposition (PLD). PVD processes are carried out under vacuum conditions. The process involves four steps:

· Evaporation

· Transportation

· Reaction

· Deposition

Evaporation

During this stage, a target, consisting of the material to be deposited is bombarded by a high- energy source such as a beam of electrons or ions. This dislodges atoms from the surface of the target,

‘vaporizing’ them.

Transport

This process simply consists of the movement of ‘vaporized’ atoms from the target to the substrate to be coated and will generally is a straight-line affair.

Reaction

In some cases coatings will consist of metal oxides, nitrides, carbides and other such materials. In these cases, the target will consist of the metal. The atoms of metal will then react with the appropriate gas during the transport stage. For the above examples, the reactive gases may be oxygen, nitrogen and methane. In instances where the coating consists of the target material alone, this step would not be part of the process.

Deposition

This is the process of coating the required material on the substrate surface. Depending on the actual process, some reactions between target materials and the reactive gases may also take place at the substrate surface simultaneously with the deposition process.

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Uses of PVD Coatings

PVD coatings are preferred for numerous reasons. Some of the main points are:

· Improved hardness and wear resistance

· Reduced friction

· Improved oxidation resistance

The use of such coatings is aimed at improving efficiency through improved performance and longer component life. They may also allow coated components to operate in environments that the uncoated component would not otherwise have been able to perform.

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Advantages of the Physical Vapor Deposition Process

· Materials can be deposited with improved properties compared to the substrate material

· Almost any type of inorganic material can be used as well as some kinds of organic materials

· The process is more environmentally friendly than processes such as electroplating

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Disadvantages of the Physical Vapor Deposition Process

· It is a line of sight technique meaning that it is extremely difficult to coat undercuts and similar surface features

· High capital cost

· Some processes operate at high vacuums and temperatures requiring skilled operators

· Processes requiring large amounts of heat require appropriate cooling systems

· The rate of coating deposition is usually quite slow

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Applications

As mentioned previously, PVD coatings are generally used to improve hardness, wear resistance and oxidation resistance. Thus, such coatings are useful in a wide range of applications such as:

· Aerospace

· Automotive

· Surgical/Medical

· Dies and moulds for all manner of material processing

· Cutting tools

· Firearms

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Physical Vapor Deposition (PVD) by Sputtering

Sputtering is a mechanism by which atoms are dislodged from the surface of a material as a result of collision with high-energy particles. Thus, PVD by Sputtering is a term used to refer to a physical vapor deposition (PVD) technique wherein atoms or molecules are ejected from a target material by high-energy particle bombardment so that the ejected atoms or molecules can condense on a substrate as a thin film nano dimension. Sputtering has become one of the most widely used techniques for depositing various metallic films on wafers, including aluminum, aluminum alloys, platinum, gold, TiW, and tungsten.

Sputtering as a deposition technique may be described as a sequence of these steps: 1) ions are generated and directed at a target material; 2) the ions sputter atoms from the target; 3) the sputtered atoms get transported to the substrate through a region of reduced pressure; and 4) the sputtered atoms condense on the substrate, forming a thin film.

Sputtering offers the following advantages over other PVD methods used in VLSI fabrication:

1) Sputtering can be achieved from large-size targets, simplifying the deposition of thin film with uniform thickness over large wafers;

2) Film thickness is easily controlled by fixing the operating parameters and simply adjusting the deposition time;

3) Control of the alloy composition, as well as other film properties such as step coverage and grain structure, is more easily accomplished than by deposition through evaporation;

4) Sputter-cleaning of the substrate in vacuum prior to film deposition can be done;

5) Device damage from X-rays generated by electron beam evaporation is avoided.

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Sputtering, however, has the following disadvantages too:

1) High capital expenses are required;

2) The rates of deposition of some materials (such as SiO2) are relatively low;

3) Some materials such as organic solids are easily degraded by ionic bombardment;

4) Sputtering has a greater tendency to introduce impurities in the substrate than deposition by evaporation because the former operates under a lesser vacuum range than the latter.

Sputtering yield, or the number of atoms ejected per incident ion, is an important factor in sputter deposition processes, since it affects the sputter deposition rate. Sputtering yield primarily depends on three major factors: 1) target material; 2) mass of the bombarding particles; and 3) energy of bombarding particles. In the energy range where sputtering occurs (10 to 5000 eV), the sputtering yield increases with particle mass and energy.

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Physical Vapor Deposition (PVD) by Evaporation

In PVD by sputtering, the material to be deposited as a film is converted into vapor by bombarding the source material with high-energy particles or ions. In PVD by evaporation, the conversion into vapor phase is achieved by applying heat to the source material, causing it to undergo evaporation. This is done in a high-vacuum environment, so that the vaporized atoms or molecules will be transported to the substrate with minimal collision interference from other gas atoms or molecules.

The rate of mass removal from the source material as a result of such evaporation increases with vapor pressure, which in turn increases with the applied heat. Vapor pressure greater than 1.5 Pa is needed in order to achieve deposition rates, which are high enough for manufacturing purposes.

In the semiconductor industry, PVD by evaporation has been used primarily in the deposition of aluminum (Al) and other metallic films on the wafer.

The advantages offered by evaporation for PVD are:

1) high film deposition rates;

2) less substrate surface damage from impinging atoms as the film is being formed, unlike sputtering that induces more damage because it involves high-energy particles; 3) excellent purity of the film because of the high vacuum condition used by evaporation; 4) less tendency for unintentional substrate heating.

The disadvantages of using evaporation for PVD are:

1) more difficult control of film composition than sputtering;

2) absence of capability to do in situ cleaning of substrate surfaces, which is possible in sputter deposition systems;

3) step coverage is more difficult to improve by evaporation than by sputtering; and

4) x-ray damage caused by electron beam evaporation can occur.

There are several ways by which heating is achieved in PVD by evaporation. The simplest (and one that has many disadvantages) is to employ resistive heating, wherein a wire of low vapor pressure metal such as tungsten is used to support strips of the material to be evaporated. The wire is then resistively heated, so that the metal to be deposited melts first and evaporates.

In electron beam evaporation, a high kinetic energy beam of electrons is directed at the material for evaporation. Upon impact, the high kinetic energy is converted into thermal energy, heating up and evaporating the target material, on the premise that the heat produced exceeds the heat lost during the process.

Evaporation can also be achieved by heating the source material with RF energy. This technique employs an RF induction-heating coil that surrounds a crucible containing the source. This method of evaporation is known as inductive heating evaporation

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Chemical Vapor Deposition (CVD)

Chemical Vapor Deposition (CVD) refers to the formation of a non-volatile solid film of nano dimension on a substrate from the reaction of vapor phase chemical reactants containing the right constituents. A reaction chamber is used for this process, into which the reactant gases are introduced to decompose and react with the substrate to form the film of nano dimension. Chemical vapor deposition is used in a multitude of semiconductor wafer fabrication processes, including the production of amorphous and polycrystalline thin films (such as polycrystalline silicon), deposition of SiO2 (CVD SiO2) and silicon nitride, and growing of single-crystal silicon epitaxial layers.

A basic CVD process consists of the following steps:

1) a predefined mix of reactant gases and diluents inert gases are introduced at a specified flow rate into the reaction chamber;

2) the gas species move to the substrate;

3) the reactants get adsorbed on the surface of the substrate;

4) the reactants undergo chemical reactions with the substrate to form the film; and

5) the gaseous by- products of the reactions are desorbed and evacuated from the reaction chamber.

During the process of chemical vapor deposition, the reactant gases not only react with the substrate material at the wafer surface (or very close to it), but also in gas phase in the reactor's atmosphere. Reactions that take place at the substrate surface are known as heterogeneous reactions, and are selectively occurring on the heated surface of the wafer where they create good- quality films of nano dimension.

Reactions that take place in the gas phase are known as homogeneous reactions. Homogeneous reactions form gas phase aggregates of the depositing material, which adhere to the surface poorly and at the same time form low-density films with lots of defects. In short, heterogeneous reactions are much more desirable than homogeneous reactions during chemical vapor deposition.

A typical CVD system consists of the following parts:

1) sources of and feed lines for gases;

2) mass flow controllers for metering the gases into the system;

3) a reaction chamber or reactor;

4) a system for heating up the wafer on which the film is to be deposited; and

5) temperature sensors.

In this technique, the precursor vapor is passed through a hot walled reactor. The precursor decomposes and nano particles nucleate in the gas phase. The nano particles are carried by the gas stream and collected on a cold finger. The size of the nano particles is determined by the particle residence time, temperature of the chamber, precursor composition and pressure.

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Laser Ablation

Ablation, in the broadest sense, is removal of material because of the incident light. This method involves melting of material to be deposited by a suitable laser, vaporization and deposition of nano particles on the substrates.

If the removal is by vaporization, special attention must be given to the plume. The plume will be a plasma-like substance consisting of molecular fragments, neutral particles, free electrons and ions, and chemical reaction products. The plume will be responsible for optical absorption and scattering of the incident beam and can condense on the surrounding work material and/or the beam delivery optics. Normally, the ablation site is cleared by a pressurized inert gas, such as nitrogen or argon.

If the material to be ablated has a poor absorption, such as diamond, but a thermally converted form of the material has relatively good absorption, such as graphite, then it is normal to cover the diamond surface with a thin coating of graphite. The laser will ablate the graphite and in doing so the surface of the underlying diamond will be converted to graphite allowing efficient absorption. Sequentially, the graphite is ablated and a new layer of diamond is converted.

The ability of the material to absorb laser energy limits the depth to which that energy can perform useful ablation. The absorption depth of the material and the heat of vaporization of the work material determine ablation depth. The depth is also a function of beam energy density, the laser pulse duration, and the laser wavelength. Laser energy per unit area on the work material is measured in terms of the energy fluency.

There are several key parameters to consider for laser ablation. The first is selection of a wavelength with a minimum absorption depth. This will help ensure a high-energy deposition in a small volume for rapid and complete ablation. The second parameter is short pulse duration to maximize peak power and to minimize thermal conduction to the surrounding work material. The third parameter is the pulse repetition rate. If the rate is too low, all of the energy which was not used for ablation will leave the ablation zone allowing cooling. If the residual heat can be retained, thus limiting the time for conduction, by a rapid pulse repetition rate, the ablation will be more efficient. More of the incident energy will go toward ablation and less will be lost to the surrounding work material and the environment. The fourth parameter is the beam quality. Beam quality is measured by the brightness (energy), the focus ability, and the homogeneity. The beam energy is of no use if it cannot be properly and efficiently delivered to the ablation region. Further, if the beam is not of a controlled size, the ablation region may be larger than desired with excessive slope in the sidewalls.

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SOL-GEL PROCESS

The sol-gel process involves the formation of colloidal suspension of particles called “sol"and transition of the colloidal “sol" into a solid called the "gel" phase. The sol-gel process allows the fabrication of thinfilms with a large variety of properties. Solgel chemistry is a remarkably versatile approach for fabricating materials.

The sol is made of solid particles of a diameter of few hundred nm, usually inorganic metalsalts, suspended in a liquid phase. In a typical sol-gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, then the particles condense in a new phase, the gel, in which a solid macromolecule is immersed in a solvent.

Three reactions are generally used to describe the sol-gel process: hydrolysis, alcohol condensation, and water condensation. However, the characteristics and properties of a particular sol- gel inorganic network are related to a number of factors that affect the rate of hydrolysis and condensation reactions, such as, pH, temperature and time of reaction, reagent concentrations, catalyst nature and concentration, aging temperature and time, and drying. Of the factors listed above, pH, nature and concentration of catalyst and temperature have been identified as most important. Thus, by controlling these factors, it is possible to vary the structure and properties of the sol-gel-derived inorganic network over wide ranges. The solutions exhibit a strong concentration dependence on the intrinsic viscosity and a power law dependence of the reduced viscosity .The sol solution is deposited on the substrates by spraying, dipping or spinning

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Spin Coating

Spin Coating involves the acceleration of a liquid puddle on a rotating substrate. The coating material is deposited at the center of the substrate. The physics behind spin coating involves a balance between centrifugal forces controlled by spin speed and viscous forces, which are determined by solvent viscosity

There are four distinct stages in the spin coating process. Stage 3 (flow controlled) and Stage 4 (evaporation controlled) are the two stages that have the most impact on final coating thickness.

• In the first stage the coating fluid is deposited onto the wafer or substrate.

• In the second stage the substrate is accelerated up to its final, desired, rotation speed.

• In the third stage the substrate is spinning at a constant rate and fluid viscous forces dominate fluid thinning behavior.

• In the fourth stage the substrate is spinning at a constant rate and solvent evaporation dominates the coating thinning behavior.

• After completion of each coating the coating is baked at appropriate temperature. The process of is repeated 5 to 10 times depending upon the required thickness.

• Finally after completion of the above process, the film is annealed at appropriate temperature.

• Then the film can be characterized with XRD, SEM, TEM, etc.,

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Applications of Spin Coating

Some technologies that depend heavily on high quality spin coated layers are: Photo resist for defining patterns in microcircuit fabrication.

• Dielectric/insulating layers for microcircuit fabrication

• Magnetic disk coatings - magnetic particle suspensions, head lubricants, etc.

• Flat screen display coatings. - Antireflection coatings, conductive oxide, etc.

• Compact Disks – DVD, CD ROM, etc.

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Ball mill

A ball mill, a type of grinder, is a cylindrical device used in grinding (or mixing) materials like ores, chemicals, ceramic raw materials and paints. Ball mills rotate around a horizontal axis, partially filled with the material to be ground plus the grinding medium. Different materials are used as media, including ceramic balls, flint pebbles and stainless steel balls. An internal cascading effect reduces the material to a fine powder. Industrial ball mills can operate continuously, fed at one end and discharged at the other end. Large to medium-sized ball mills are mechanically rotated on their axis, but small ones normally consist of a cylindrical capped container that sits on two drive shafts (pulleys and belts are used to transmit rotary motion). High-quality ball mills are potentially expensive and can grind mixture particles to as small as 0.01 µm, enormously increasing surface area and reaction rates. There are many types of grinding media suitable for use in a ball mill, each material having its own specific properties and advantages. Common in some applications are stainless steel balls. While usually very effective due to their high density and low contamination of the material being processed, stainless steel balls are unsuitable for some applications.

Ball mill

Lead antimony grinding media with aluminum powder.

High-energy ball milling, the only top-down approach for nanoparticle synthesis, has been used for the generation of magnetic, catalytic and structural nanoparticles. The technique, which is already a commercial technology, has been considered dirty because of contamination problems from ball-milling processes. However, the availability of tungsten carbide components and the use of inert atmosphere and/or high vacuum processes have reduced impurities to acceptable levels for many industrial applications. Common drawbacks include the low surface area, the highly polydisperse size distributions, and the partially amorphous state of the as-prepared powders.

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Electrodeposition

Electrodeposition technique is used to electroplate a material. In many liquids called electrolytes (aqueous solutions of salts, acids etc.,), when current is passed through two electrodes immersed inside the electrolyte, certain mass of the substance liberated at one electrode gets deposited on the surface of the other. By controlling the current and other parameters, it is possible to deposit even a single layer of atoms. Nanostructured films of copper, platinum, nickel, gold etc., can be produced by electrodeposition. The film thus obtained is mechanically robust, highly flat and uniform. Since these films have larger surface areas, they exhibit quite different and favorable electrical properties. They have wide range of applications. These include batteries, fuel cells, solar cells, magnetic read heads, etc.,

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Check your understanding

1. What is the basic principle of Chemical vapor Deposition method of producing nanomaterial?

Ans :

Chemical Vapor Deposition (CVD) refers to the formation of a non-volatile solid film of nano dimension on a substrate from the reaction of vapor phase chemical reactants containing the right constituents.

2. Explain briefly sol-gel process.

Ans :

The sol-gel process involves the formation of colloidal suspension of particles called “sol"and transition of the colloidal “sol" into a solid called the "gel" phase. The sol-gel process allows the fabrication of thinfilms with a large variety of properties. Solgel chemistry is a remarkably versatile approach for fabricating materials.

The sol is made of solid particles of a diameter of few hundred nm, usually inorganic metalsalts, suspended in a liquid phase. In a typical sol-gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, then the particles condense in a new phase, the gel, in which a solid macromolecule is immersed in a solvent.

3. Explain electrodeposition.

Electrodeposition technique is used to electroplate a material. In many liquids called electrolytes (aqueous solutions of salts, acids etc.,), when current is passed through two electrodes immersed inside the electrolyte, certain mass of the substance liberated at one electrode gets deposited on the surface of the other.

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Summary:

• Chemical Vapor Deposition (CVD) refers to the formation of a non-volatile solid film of nano dimension on a substrate from the reaction of vapor phase chemical reactants containing the right constituents. A reaction chamber is used for this process, into which the reactant gases are introduced to decompose and react with the substrate to form the film of nano dimension. Chemical vapor deposition is used in a multitude of semiconductor wafer fabrication processes, including the production of amorphous and polycrystalline thin films (such as polycrystalline silicon), deposition of SiO2 (CVD SiO2) and silicon nitride, and growing of single-crystal silicon epitaxial layers.

• A basic CVD process consists of the following steps: 1) a predefined mix of reactant gases and diluents inert gases are introduced at a specified flow rate into the reaction chamber; 2) the gas species move to the substrate; 3) the reactants get adsorbed on the surface of the substrate; 4) the reactants undergo chemical reactions with the substrate to form the film; and 5) the gaseous by- products of the reactions are desorbed and evacuated from the reaction chamber.

• The sol-gel process involves the formation of colloidal suspension of particles called “sol"and transition of the colloidal “sol" into a solid called the "gel" phase. The sol-gel process allows the fabrication of thinfilms with a large variety of properties. Solgel chemistry is a remarkably versatile approach for fabricating materials.

The sol is made of solid particles of a diameter of few hundred nm, usually inorganic metal salts, suspended in a liquid phase. In a typical sol-gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, then the particles condense in a new phase, the gel, in which a solid macromolecule is immersed in a solvent.

• High-energy ball milling, the only top-down approach for nanoparticle synthesis, has been used for the generation of magnetic, catalytic and structural nanoparticles.

• Electrodeposition technique is used to electroplate a material. In many liquids called electrolytes (aqueous solutions of salts, acids etc.,), when current is passed through two

electrodes immersed inside the electrolyte, certain mass of the substance liberated at one electrode gets deposited on the surface of the other. By controlling the current and other parameters, it is possible to deposit even a single layer of atoms. Nanostructured films of copper, platinum, nickel, gold etc., can be produced by electrodeposition.

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Reference:

1. Engineering Physics-II by M.N.Avadhanulu and P.G.Kshirsagar

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