The Substrate of EB Coating

The substrate on which the film deposition takes place is ultrasonically cleaned and fastened to the substrate holder. The substrate holder is attached to the manipulator shaft. The manipulator shaft moves translationally to adjust the distance between the ingot source and the substrate. The shaft also rotates the substrate at a particular speed so that the film is uniformly deposited on the substrate. A negative bias D.C. voltage of 200 V – 400 V can be applied to the substrate. Often, focused high energy electrons from one of the electron guns or infrared light from heater lamps is used to preheat the substrate. Heating of the substrate allows for increased adatom - substrate and adatom - film diffusion by giving the adatoms sufficient energy to overcome kinetic barriers. If a rough film, such as metallic nanorods, is desired substrate cooling with water or liquid nitrogen may be employed to reduce diffusion lifetime, positively bolstering surface kinetic barriers.To further enhance film roughness, the substrate may be mounted at a steep angle with respect to the flux to achieve geometric shadowing where incoming line of sight flux lands onto only higher parts of the developing film. This method is known as glancing angle deposition (GLAD) or oblique angle deposition (OAD).

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Material Evaporation Methods of EB Coating

Refractory carbides like titanium carbide and borides like titanium boride and zirconium boride can evaporate without undergoing decomposition in the vapor phase. These compounds are deposited by direct evaporation. In this process these compounds, compacted in the form of an ingot, are evaporated in vacuum by the focused high energy electron beam and the vapors are directly condensed over the substrate.

Certain refractory oxides and carbides undergo fragmentation during their evaporation by the electron beam, resulting in a stoichiometry that is different from the initial material. For example, alumina, when evaporated by electron beam, dissociates into aluminum, AlO3 and Al2O. Some refractory carbides like silicon carbide and tungsten carbide decompose upon heating and the dissociated elements have different volatilities. These compounds can be deposited on the substrate either by reactive evaporation or by co-evaporation. In the reactive evaporation process, the metal is evaporated from the ingot by the electron beam. The vapors are carried by the reactive gas, which is oxygen in case of metal oxides or acetylene in case of metal carbides. When the thermodynamic conditions are met, the vapors react with the gas in the vicinity of the substrate to form films. Metal carbide films can also be deposited by co-evaporation. In this process, two ingots are used, one for metal and the other for carbon. Each ingot is heated with a different beam energy so that their evaporation rate can be controlled. As the vapors arrive at the surface, they chemically combine under proper thermodynamic conditions to form a metal carbide film.

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Process of EB Coating

In an EBPVD system, the deposition chamber must be evacuated to a pressure of at least 7.5 x 10−5 Torr (10−4 hPa) to allow passage of electrons from the electron gun to the evaporation material which can be in the form of an ingot or rod.[1] Alternatively, some Modern EBPVD systems utilize an arc suppression system and can be operated at vacuum levels as low as 5.0 x 10−3 Torr, for situations such as parallel use with magnetron sputtering.[2] Multiple types of evaporation materials and electron guns can be used simultaneously in a single EBPVD system, each having a power from tens to hundreds of kW. Electron beams can be generated by thermionic emission, field electron emission or the anodic arc method. The generated electron beam is accelerated to a high kinetic energy and directed towards the evaporation material. Upon striking the evaporation material, the electrons will lose their energy very rapidly.[3] The kinetic energy of the electrons is converted into other forms of energy through interactions with the evaporation material. The thermal energy that is produced heats up the evaporation material causing it to melt or sublimate. Once temperature and vacuum level are sufficiently high, vapor will result from the melt or solid. The resulting vapor can then be used to coat surfaces. Accelerating voltages can be between 3 kV – 40 kV. When the accelerating voltage is between 20 kV – 25 kV and the beam current is a few amperes, 85% of the electron's kinetic energy can be converted into thermal energy. Some of the incident electron energy is lost through the production of X-rays and secondary electron emission.

There are three main EBPVD configurations, electromagnetic alignment, electromagnetic focusing and the pendant drop configuration. Electromagnetic alignment and electromagnetic focusing use evaporation material that is in the form of an ingot while the pendant drop configuration uses a rod. Ingots will be enclosed in a copper crucible or hearth[4] while a rod will be mounted at one end in a socket. Both the crucible and socket must be cooled. This is typically done by water circulation. In the case of ingots, molten liquid can form on its surface which can be kept constant by vertical displacement of the ingot. The evaporation rate may be on the order of 10−2 g/cm2 sec.

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Introduction of EB Coating

Electron Beam Physical Vapor Deposition or EBPVD is a form of physical vapor deposition in which a target anode is bombarded with an electron beam given off by a charged tungsten filament under high vacuum. The electron beam causes atoms from the target to transform into the gaseous phase. These atoms then precipitate into solid form, coating everything in the vacuum chamber (within line of sight) with a thin layer of the anode material.

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Advantages of Vacuum Coating

Advantages of Vacuum Coating
Very high transfer efficiencies
High speed coating
Low application costs

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View of Vacuum Coating

Exit template view of a remote vacuum coating head

The amount of vacuum applied to the system regulates the resulting coating thickness applied to the board. In a perfect system too much vacuum would remove all the coating from the board leaving none behind at all. Conversely, a non-existent vacuum would allow the coating head to fill sufficiently with coating where it would run out of the space created between template and product.
Vacuum coating differs from vacuum deposition in that the vacuum employed here is intended to vary the amount of coating by sucking off the excess and creating varying coating thicknesses between 0.001 and 0.008 in (25 and 200 µm) in thickness. Vacuum deposition is applying a coating while the substrate is under a vacuum and applies the coating in very precise layers.

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Process of Vacuum Coating

The product passes through the portion of the coater known as the application chamber at a constant speed of up to 500' per minute (15m/min). As it enters the chamber, it passes through a template which has the same shape hole, or profile, in itself to the shape of the product passing through. As it exits the chamber, it passes through another template called the exit template that also has a similar matching profile.

Paint is drawn from a reservoir by a diaphragm pump, filtered for large particles, and delivered through a hole into the bottom of the coater head. The fluid delivery diaphragm pump is by definition low shear and under low pressure. The level of the coating then rises within the head until the part passing through is completely surrounded or immersed.

A vacuum is applied to the reservoir and the application chamber. The reservoir is a sealed environment attached to the coating head. The coating head contains the only area for air to inrush and that is the space between the product and the template profile. It is this inrush of air that is used to strip excess coating from the product and this removal of excess coating is what determines the wet film thickness applied. The amount removed is subject to the vacuum relief valve, the size of the templates, the linear speed of the product, and the viscosity of the coating. This application and removal method stratifies the coating on removal and then it is drawn up and over a baffle and drains to the reservoir.

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