Thin Film Materials and Technology

Thin film technology—the growth or deposition of mechanical strengthening, optical, electronic, magnetic, or semiconductor materials in an ultrathin layer—resulted from rapid development of materials science and technology in the late twentieth century. Thin film technology aided the development of devices such as transistors and microelectronic components such as diodes, capacitors, sensors, and resistors, microelectromechanical systems (MEMs), and solar cells.

Thin films enable transparent conductive coatings for touch screens and thin conductive films on magnetic read-write heads that allow increased magnetic data storage capacity. In aerospace engineering thin films contribute to strengthening against wear friction and corrosion. They are also used to coat microcircuits and optical lenses to withstand stress and extreme temperatures, protect them from damage and wear, give antireflection or polarizing coatings, and improve durability and performance.

Chemists create thin films, usually only a few micrometers or less thick but potentially nanometers for monolayers, through some method of depositing atoms from a source material target onto a foundation called a substrate (often a silicon wafer, but could be metal or glass). Deposition techniques and material sources can be adjusted to design thin films with tailored properties or thickness to meet specific industrial needs and conditions.

Thin films technology is based on research and processes related to vacuum, gas diffusion, and thin material layers that had gradually developed in the nineteenth century. In 1852, Sir William Robert Grove observed in experiments involving electric discharges between electrodes in a low-pressure atmosphere that metal from one of the electrodes was deposited on the glass walls containing the electrodes and the gas. This ‘‘vapor deposition’’ of metal films, also reported by Michael Faraday in 1854 and Julius Plücker in 1858, is now known to be caused by sputtering (see below). By 1887, Robert Nahrwold heated platinum wires inside a vacuum to deposit material for thin films.

The first commercial use of thin films was in 1901, when mass production of Edison’s ‘‘gold molded’’ cylinder phonograph records was enabled by a vacuum coating process that deposited gold vapor from gold electrodes. With awareness of possible industrial uses for vacuums, research into vacuum equipment accelerated, and then shifted to applications by the mid-twentieth century. Innovation of existing thin film processes increased in the mid-twentieth century, quickly escalating from the 1960s through the turn of the century, with chemists adapting processes and materials to fabricate new thin films to create desired structures.

Developments paralleled microelectronics demands, particularly the need for transistors and integrated circuits to have pure layers with no microstructural defects that could affect electronic properties. Thin film technology also progressed as processes and underlying knowledge of materials such as electroceramics advanced. For example, discovery of high-temperature superconducting oxides (ceramics) in the late 1980s stimulated development of new thin film deposition techniques, owing to potential applications in superconducting electronics.

Technologists consider vacuum evaporation to be the most efficient and productive thin film deposition method. Physical vapor deposition (PVD) processes usually involve depositing material from a vaporized solid or liquid target source by moving atoms through a vacuum or low- pressure gas or plasma to condense onto a substrate.

Vacuum process technology relies on vacuums that are as empty as possible of any particles and gases that might interfere with materials being deposited within the vacuum. Technologists heat a selected source material in a vacuum so that sublimation or evaporation, by thermal or electron beam heating, results in atoms or molecules forming a film on a substrate.

Substrate materials are often metals or glass composed of aluminum, silicon, or beryllium that are smooth, mechanically strong, chemically stable, and have desired thermal qualities. The quality of thin films is diminished with exposure to any moisture or contaminants that enter the vacuum if seals leak, pressure is not maintained, or the chamber is not cleaned.

Molecular beam epitaxy (MBE) methods in which the film crystal structure is ‘‘ordered’’ as it is deposited (growth of the deposited crystal is oriented by the lattice structure of the substrate) were developed by several chemists in the U.S. (including Alfred Cho and John Arthur at Bell Labs), Europe, and Asia during the 1960s and 1970s.

Vapors from heated sources form beams that condense onto a heated substrate to create thin crystalline films just as in vacuum evaporation. However the timing and content of these beams can be carefully controlled by shutters in front of the heated sources that can close and block the atomic beams. In the 1960s, this assisted the subsequent development of integrated circuitry.

The molecules are placed one layer at a time on the substrate, producing monolayer films that are more suitable for tiny electronics applications than films in which the substrate and polycrystalline film are not so evenly matched and the film’s grain size is larger than the circuits for which the films will be used. This process occurs in an ultrahigh vacuum compared to other thin film deposition techniques, and as a result, films tend to be cleaner than those created by other methods.

Because MBE relies on vacuum pressures and hygienic measures often too unstable to sustain, technologists devised technology in which various aspects of MBE deposition occur separately in a series of connecting chambers which process crystalline wafer substrates. After being decontaminated and prepared, a wafer moves on a platform along a track and is heated from 500 to 900° C before entering the growth chamber where films are formed.

Sputtering deposition differs from high-vacuum techniques because a rare gas such as argon is always moving in the vacuum chamber. Gas flow and a throttle valve determine gas pressure in the chamber. The rare gas is ionized in the electrical field formed by the substrate and diode or magnetron target in the chamber, creating an ionized plasma with an overall neutral charge. The accelerated ions strike the target causing its atoms to be ablated, and the target atoms are directed towards the substrate.

Technologists often choose sputtering to create thin films from refractory sources including tungsten because deposition can occur at less than the materials’ normal melting point. Sputtering also appeals because technologists can design films similar to alloy materials or compound sources. This method efficiently shifts atoms from targets to substrates to produce a film almost identical to its source. Evaporation deposition techniques are not as consistent because fluctuating vapor pressure affects how source material is deposited.

Unlike PVD methods where material is removed from a solid target, chemical vapor deposition (CVD) uses reactive carrier gases to form new material on a heated substrate. These gases either break down into reactive precursors or interact, and the resulting materials coat substrates. CVD offers chemists the capability to create a large variety of thin films suitable for numerous uses. CVD technologists use a reactor that provides the energy necessary to cause chemical reactions to deposit material to form films. They have innovated processes for specific material sources such as metal-organic CVD for organometallic materials used in semiconductors.

Plasma is the energy source for reactions in plasma-enhanced CVD, in which vaporized compounds in the plasma trigger the coating of substrates. A vacuumless process, atmospheric pressure CVD moves substrates on a belt through the chamber to deposit materials for films used to coat silicate glasses. In this technique, coating colors can be achieved by the selection of specific compound sources. Diamond thin films that increase the surface hardness of cutting tools have been a popular product of CVD methods. Based on work patented by American William G.

Eversole in the 1950s and developed in England by John C. Angus, H.A. Will, and W.S. Stanko in the next decade, CVD methods have continued to improve throughout the late twentieth century for optical and electronic usages.

Ion beam deposition (IBD) refers to several interconnected processes that create predictable quality and characteristics in thin films. Kasturi L. Chopra and M. R. Randlett first used ion beam sputtering to make thin films in the mid-1960s. Chemists appropriated techniques for IBD, using ion beams that are usually broad and high energy to prepare substrates prior to deposition by removing contaminants.

They then utilize ion beams to deposit materials by sputtering techniques as described above or as an aid for other methods to achieve optimal film production. Typically, the ion beam is aimed at a metal. As a result, target material sputters onto substrates, forming thin films. Often, an additional ion source known as the ion assist source (IAD) provides energy in ions that thicken and stabilize the films’ surfaces and enhance their strength and capabilities. Engineers have appropriated IBD to reinforce magnetic heads with carbon films.