Thin film deposition: what is it?

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Thin-film deposition is a technique used in industry to apply a thin coating to a particular design part made from a target material and to infuse its surface with certain properties. Thin film coatings are applied to change the optical properties of glass, the corrosive properties of metals and the electrical properties of semiconductors. A variety of deposition techniques are employed, usually to add atoms or molecules, one layer at a time, to a wide variety of materials that lack the essential surface properties provided by thin coatings. Any design that requires a coating of minimal volume and weight can benefit from thin-film deposition that exposes a target material to an energized liquid, gas, or plasma environment.

The first raw metal coatings were used in the first millennium to improve the reflective properties of mirror glass. The 1600s saw the development of more refined coating techniques by Venetian glassmakers. Only in the 1800s did precision methods exist for applying thin coatings, such as electroplating and vacuum deposition.

Electroplating is a form of chemical deposition in which the part to be coated is attached to an electrode and immersed in a conductive solution of metal ions. When a current passes through the solution, ions are drawn to the surface of a part to slowly create a thin layer of metal. Semi-solid solutions called sol-gels are another means of chemical deposition of thin films. As long as the coating particles are small enough, they will remain in suspension in the gel long enough to organize into layers and provide a uniform coating when the liquid fraction is removed in a drying step.

Vapor deposition is a technique for creating thin film deposition in which a part is coated in an energized gas or plasma, usually in a partial vacuum. In the vacuum chamber, atoms and molecules diffuse evenly and create a coating of constant purity and thickness. With chemical vapor deposition, on the other hand, the piece is placed in a reaction chamber occupied by the coating in gaseous form. The gas reacts with the target material to create the desired coating thickness. In plasma deposition, the coating gas is superheated into an ionic form which then reacts with the atomic surface of the part, typically at elevated pressures.

In sputter deposition, a source of pure coating material in solid form is energized by heat or electron bombardment. Some of the atoms in the solid source break off and are suspended uniformly around the surface of the part in an inert gas, such as argon. This type of thin-film deposition is useful for visualizing fine features on small parts that are sputtered with gold and viewed through an electron microscope. In coating the part for later study, gold atoms are removed from a solid source on top of the part and fall onto its surface via an argon gas filled chamber.

The applications of thin film deposition are diverse and expanding. Optical coatings on glass lenses and plates can improve transmission, refraction and reflection properties, producing ultraviolet (UV) filters in eyeglasses and anti-reflection glass for framed photos. The semiconductor industry uses thin coatings to provide better conductance or insulation for materials such as silicon wafers. Ceramic thin films are anticorrosive, hard and insulating; although brittle at low temperatures, they have been used successfully in sensors, integrated circuits, and more complex designs. Thin films can be deposited to form ultra-small ‘smart’ structures such as batteries, solar cells, drug delivery systems and even quantum computers.

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