Coating Deposition Techniques

Coating applications: A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. • To improve surface properties of the substrate, such as: -Appearance -Adhesion -Wetability -Corrosion resistance -Wear resistance -Scratch resistance. • As manufacturing technique such as in printing processes and semiconductor device fabrication. • Industries/Fields in which coatings are of use: -Oil & Gas, Chemical Processing, Plastics, Textile, Automotive, Aviation & Aerospace, Food & Pharmaceutical, Microfabrication, Electronics, Photovoltaics. •

Various Coating Techniques (Catagories) : Chemical vapor deposition • Metalorganic vapour phase epitaxy • Electrostatic spray assisted vapour deposition (ESAVD) Physical vapor deposition • Cathodic arc deposition • Electron beam physical vapor deposition (EBPVD) • Ion plating • Ion beam assisted deposition (IBAD) • Magnetron sputtering • Pulsed laser deposition • Sputter deposition • Vacuum deposition • Vacuum evaporation, evaporation (deposition) Chemical and electrochemical techniques • Anodising • Electroless plating • Electroplating • Chromate conversion coating • Plasma electrolytic oxidation • Phosphate (coating) • Ion beam mixing • Pickled and oiled, a type of plate steel coating • Sol-gel Spraying • High velocity oxygen fuel (HVOF) • Plasma spraying • Thermal spraying • Plasma transferred wire arc thermal spraying

Others • Dip-coating • Epitaxy (Vapor-phase, Liquid-phase) • Vitreous enamel • Paint • Enamel paint • Silicate mineral paint • Polymer coatings, such as Teflon • Powder coating or Powder slurry coating • Fusion bonded epoxy coating (FBE coating) • Molecular beam epitaxy • Sheradizing • Spin coating • Hot Melt coating • Immersion (dip) coating • Roller coater • Overprint varnish • Water based coating • Acrylic based • Solvent based • Aqueous coating • UV Coating – curing

Chemical vapor deposition (CVD) • Chemical vapor deposition (CVD) is a chemical process used to produce high-purity, high-performance solid materials. • In a typical CVD process, the substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Frequently, volatile by-products are also produced, which are removed by gas flow through the reaction chamber. • The process is often used in the semiconductor industry to produce thin films.

Plasma Enhanced Chemical Vapor Deposition (PECVD) • • •

•

It is an excellent alternative for depositing a variety of thin films at lower temperatures than those utilized in CVD reactors without settling for a lesser film quality. For example, high quality silicon dioxide films can be deposited at 300 to 350 degrees centigrade while CVD requires temperatures in the range of 650 to 850 degrees centigrade to produce similar quality films. PECVD uses electrical energy to generate a glow discharge (plasma) in which the energy is transferred into a gas mixture. This transforms the gas mixture into reactive radicals, ions, neutral atoms and molecules, and other highly excited species. These atomic and molecular fragments interact with a substrate and, deposition takes place at the substrate. Since the formation of the reactive and energetic species in the gas phase occurs by collision in the gas phase, the substrate can be maintained at a low temperature. Hence, film formation can occur on substrates at a lower temperature than is possible in the conventional CVD process, which is a major advantage of PECVD. PECVD films have good adhesion and uniformity.

ELECTRONICS: PECVD systems have entrenched their place in electronics sector because of their flexibility in depositing many thin films such as: • Silicon Nitride (SiN) • Silicon Oxide (SiO) • Silicon Dioxide (SiO2) • Silicon OxyNitride (SiON) • Diamond Like Carbon (DLC) • Amorphous Silicon (A-Si) • Poly Silicon (poly-Si)

Physical vapor deposition (PVD) • Physical vapor deposition (PVD) techniques are methods to deposit thin films by the condensation of a vaporized form of the film material onto various work-piece surfaces. • These coating methods involve purely physical processes such as high temperature vacuum evaporation with subsequent condensation, or plasma sputter bombardment. • Do not involve chemical reactions at the surface to be coated as in chemical vapour deposition.

Pulsed Laser deposition • Pulsed laser deposition (PLD) is used for thin film deposition. • A physical vapor deposition, PVD technique where a high power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the target (in a plasma plume) which deposits it as a thin film on a substrate (such as a silicon wafer facing the target). This process can occur in ultra high vacuum or in the presence of a background gas. • When the laser pulse is absorbed by the target, energy is first converted to electronic excitation and then into thermal, chemical and mechanical energy resulting in evaporation, ablation, plasma formation and even exfoliation. The ejected species expand into the surrounding vacuum in the form of a plume containing many energetic species including atoms, molecules, electrons, ions, clusters, particulates and molten globules, before depositing on the typically hot substrate.

PLD The detailed mechanisms of PLD are very complex including the ablation process of the target material by the laser irradiation, the development of a plasma plume with high energetic ions, electrons as well as neutrals and the crystalline growth of the film itself on the heated substrate. The process of PLD can generally be divided into four stages: • Laser ablation of the target material and creation of a plasma – - The removal of atoms from the bulk material is done by vaporization of the bulk at the surface region. -The incident laser pulse penetrates into the surface of the material within the penetration depth. (This dimension is dependent on the laser wavelength and the index of refraction of the target material). -The strong electrical field generated by the laser light is sufficiently strong to remove the electrons from the bulk material of the penetrated volume. -The free electrons oscillate within the electromagnetic field of the laser light and can collide with the atoms of the bulk material thus transferring some of their energy to the lattice of the target material within the surface region. -The surface of the target is then heated up and the material is vaporized. • Dynamics of the plasma - The dependency of the plume shape on the pressure can be described in three stages: -The vacuum stage, where the plume is very narrow and forward directed; almost no scattering occurs with the background gases. -The intermediate region where a splitting of the high energetic ions from the less energetic species can be observed. -High pressure region where we find a more diffusion-like expansion of the ablated material. This scattering is also dependent on the mass of the background gas and can influence the stoichiometry of the deposited film. • The most important consequence of increasing the background pressure is the slowing down of the high energetic species in the expanding plasma plume. •

PLD • Deposition of the ablation material on the substrate: - The third stage is important to determine the quality of the deposited films. The high energetic species ablated from the target are bombarding the substrate surface and may cause damage to the surface by sputtering off atoms from the surface but also by causing defect formation in the deposited film. • Nucleation and growth of the film on the substrate surface - The nucleation process and growth kinetics of the film depend on several growth parameters including: -Laser parameters – several factors such as the laser energy, and ionization degree of the ablated material will affect the film quality, the stoichiometry, and the deposition flux. Generally, the nucleation density increases when the deposition flux is increased. -Surface temperature – The surface temperature has a large effect on the nucleation density. Generally, the nucleation density decreases as the temperature is increased. -Substrate surface – The nucleation and growth can be affected by the surface preparation as well as the roughness of the substrate. -Background pressure – Common in oxide deposition, an oxygen background is needed to ensure stoichiometric transfer from the target to the film. If, for example, the oxygen background is too low, the film will grow off stoichiometry which will affect the nucleation density and film quality

PLD • Technical aspects : • There are many different arrangements to build a deposition chamber for PLD. The target material which is evaporated by the laser is normally found as a rotating disc attached to a support. However, it can also be sintered into a cylindrical rod with rotational motion and a translational up and down movement along its axis. This special configuration allows not only the utilization of a synchronized reactive gas pulse but also of a multi-component target rod with which films of different multilayers can be created. • Some factors that influence deposition thickness: -Target material -Pulse energy of laser -Distance from target to substrate -Type of gas and pressure in chamber (oxygen, argon, etc.)

Sputter Deposition http://www.heraeustargets.com/en/technol ogy/_sputteringbasics/s puttering.aspx

Electroplating •

• • •

•

Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal. The process used in electroplating is called electrodeposition. The part to be plated is the cathode of the circuit In one technique, the anode is made of the metal to be plated on the part. Both components are immersed in a solution called an electrolyte containing one or more dissolved metal salts as well as other ions that permit the flow of electricity. A power supply supplies a direct current to the anode, oxidizing the metal atoms that comprise it and allowing them to dissolve in the solution. At the cathode, the dissolved metal ions in the electrolyte solution are reduced at the interface between the solution and the cathode, such that they "plate out" onto the cathode. The rate at which the anode is dissolved is equal to the rate at which the cathode is plated, vis-a-vis the current flowing through the circuit. In this manner, the ions in the electrolyte bath are continuously replenished by the anode. Other electroplating processes may use a non-consumable anode such as lead. In these techniques, ions of the metal to be plated must be periodically replenished in the bath as they are drawn out of the solution

Electroless plating • • •

• •

Electroless plating, also known as chemical or auto-catalytic plating, is a nongalvanic type of plating method that involves several simultaneous reactions in an aqueous solution, which occur without the use of external electrical power. The most common electroless plating method is electroless nickel plating, although silver, gold and copper layers can also be applied in this manner. Usually an electrolytic cell (consisting of two electrodes, electrolyte, and external source of current) is used for electrodeposition. In contrast, an electroless deposition process uses only one electrode and no external source of electric current. However, the solution for the electroless process needs to contain a reducing agent so that the electrode reaction has the form:

A benefit of this approach over electroplating is that power sources and plating baths are not needed, reducing the cost of production. The technique can also plate diverse shapes and types of surface. The downside is that the plating process is usually slower and cannot create such thick plates of metal. As a consequence of these characteristics, electroless deposition is quite common in the decorative arts.

Conversion coatings • Conversion coatings are coatings for metals where the part surface is converted into the coating with a chemical or electro-chemical process. Examples include chromate conversion coatings, phosphate conversion coatings, bluing, black oxide coatings on steel, and anodizing. • They are used for corrosion protection, increased surface hardness, to add decorative colour and as paint primers. Conversion coatings may be very thin, on the order of 0.00001". Thick coatings, up to 0.002", are usually built up on aluminium alloys, either by anodizing or chromate conversion

Anodizing • • •

• • • •

Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called "anodizing" because the part to be treated forms the anode electrode of an electrical circuit. Anodizing increases corrosion resistance and wear resistance, and provides better adhesion for paint primers and glues than does bare metal. Anodic films can also be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add interference effects to reflected light. Thick coatings are normally porous, so a sealing process is often needed to achieve corrosion resistance. Long immersion in boiling hot deionized water or steam is the simplest sealing process. The oxide is converted into its hydrated form, and the resulting swelling reduces the porosity of the surface. Cold sealing, where the pores are closed by impregnation of a sealant in a roomtemperature bath, is more popular due to energy savings. Anodized aluminium surfaces, for example, are harder than aluminium but have low to moderate wear resistance that can be improved with increasing thickness or by applying suitable sealing substances. Anodic films are generally much stronger and more adherent than most types of paint and metal plating, but also more brittle. This makes them less likely to crack and peel from aging and wear, but more susceptible to cracking from thermal stress.

Anodizing • Aluminium -Process • Preceding the anodization process, the alloys are cleaned. • The anodized aluminium layer is grown by passing a direct current through an electrolytic solution, with the aluminium object serving as the anode (the positive electrode). • The current releases hydrogen at the cathode (the negative electrode) and oxygen at the surface of the aluminium anode, creating a build-up of aluminium oxide. Alternating current and pulsed current is also possible but rarely used. • Aluminium anodizing is usually performed in an acid solution which slowly dissolves the aluminium oxide. The acid action is balanced with the oxidation rate to form a coating with nanopores, 10-150 nm in diameter. These pores are what allows the electrolyte solution and current to reach the aluminium substrate and continue growing the coating to greater thickness beyond what is produced by autopassivation. • However, these same pores will later permit air or water to reach the substrate and initiate corrosion if not sealed. They are often filled with colored dyes and/or corrosion inhibitors before sealing. Because the dye is only superficial, the underlying oxide may continue to provide corrosion protection even if minor wear and scratches may break through the dyed layer. • Conditions such as electrolyte concentration, acidity, solution temperature, and current must be controlled to allow the formation of a consistent oxide layer. Harder, thicker films tend to be produced by more dilute solutions at lower temperatures with higher voltages and currents. The film thickness can range from under 0.5 micrometers for bright decorative work up to 150 micrometers for architectural applications.

Anodizing • Anodized titanium is used in a recent generation of dental implants. An anodized oxide layer has a thickness in the range of 30 nanometers (1.2×10−6 in) to several micrometers. • Anodizing titanium generates an array of different colours without dyes, for which it is sometimes used in art, and jewelleries. The colour formed is dependent on the thickness of the oxide (which is determined by the anodizing voltage); it is caused by the interference of light reflecting off the oxide surface with light travelling through it and reflecting off the underlying metal surface. • Titanium nitride coatings can also be formed, which have a brown or golden color and have the same wear and corrosion benefits as anodization

Colors achievable through anodization of Titanium

Thermal spraying • •

• • • • • • • • •

Thermal spraying techniques are coating processes in which melted (or heated) materials are sprayed onto a surface. The "feedstock" (coating precursor) is heated by electrical (plasma or arc) or chemical means (combustion flame). Thermal spraying can provide thick coatings (approx. thickness range is 20 micrometers to several mm, depending on the process and feedstock), over a large area at high deposition rate as compared to other coating processes such as electroplating, physical and chemical vapour deposition. Coating materials available for thermal spraying include metals, alloys, ceramics, plastics and composites. They are fed in powder or wire form, heated to a molten or semimolten state and accelerated towards substrates in the form of micrometer-size particles. Combustion or electrical arc discharge is usually used as the source of energy for thermal spraying. Resulting coatings are made by the accumulation of numerous sprayed particles. The surface may not heat up significantly, allowing the coating of flammable substances. Coating quality is usually assessed by measuring its porosity, oxide content, macro and micro-hardness, bond strength and surface roughness. Generally, the coating quality increases with increasing particle velocities. Several variations of thermal spraying are distinguished: Plasma spraying Detonation spraying Wire arc spraying Flame spraying High velocity oxy-fuel coating spraying (HVOF) Warm spraying Cold spraying

Thermal spraying • •

• •

• •

Detonation Thermal Spraying Process The Detonation gun basically consists of a long water cooled barrel with inlet valves for gases and powder. Oxygen and fuel (acetylene most common) is fed into the barrel along with a charge of powder. A spark is used to ignite the gas mixture and the resulting detonation heats and accelerates the powder to supersonic velocity down the barrel. A pulse of nitrogen is used to purge the barrel after each detonation. This process is repeated many times a second. The high kinetic energy of the hot powder particles on impact with the substrate result in a build up of a very dense and strong coating. Plasma spraying In plasma spraying process, the material to be deposited (feedstock) — typically as a powder, sometimes as a liquid suspension or wire — is introduced into the plasma jet, emanating from a plasma torch. - In the jet, where the temperature is of the order of 10,000 K, the material is melted and propelled towards a substrate. There, the molten droplets flatten, rapidly solidify and form a deposit. The deposits remain adherent to the substrate as coatings; free-standing parts can also be produced by removing the substrate. There are a large number of technological parameters that influence the interaction of the particles with the plasma jet and the substrate and therefore the deposit properties. These parameters include feedstock type, plasma gas composition and flow rate, energy input, torch offset distance, substrate cooling, etc.

Spin coating • •

•

•

• •

• • •

Spin coating is a procedure used to apply uniform thin films to flat substrates. An excess amount of a solution is placed on the substrate, which is then rotated at high speed in order to spread the fluid by centrifugal force. A machine used for spin coating is called a spin coater, or simply spinner. Rotation is continued while the fluid spins off the edges of the substrate, until the desired thickness of the film is achieved. The applied solvent is usually volatile, and simultaneously evaporates. So, the higher the angular speed of spinning, the thinner the film. The thickness of the film also depends on the concentration of the solution and the solvent. Spin coating is widely used in microfabrication, where it can be used to create thin films with thicknesses below 10 nm. Although different engineers count things differently, there are four distinct stages to the spin coating process. Deposition of the coating fluid onto the wafer or substrate This can be done by using a nozzle and pouring the coating solution or by spraying it onto the surface. A substantial excess of coating solution is usually applied compared to the amount that is required. Acceleration of the substrate up to its final, desired, rotation speed Spinning of the substrate at a constant rate; fluid viscous forces dominate the fluid thinning behavior Spinning of the substrate at a constant rate; solvent evaporation dominates the coating thinning behavior.

Your Questions!