In some cases the material being deposited reacts with the gaseous environment or a codeposited species to form a film of a compound material such as an oxide, a nitride, carbide, or a carbonitride. In CVD processing, the use of a plasma to fragment the chemical vapor precursor in the vapor phase allows the decomposition or reduction processes to proceed at lower temperatures than with thermal activation alone. PECVD can be performed at pressures as low as those used in PVD processing (low-pressure PECVD, LP-PECVD), where the precursor vapor is decomposed mainly in the plasma. In some cases a hybrid deposition process of PVD and LP-PECVD is used to deposit alloys, composites, or compounds. An example is metal carbonitrides where the carbon comes from a chemical vapor precursor such as acetylene; the nitrogen comes from a gas; and the metal from evaporation, sputtering, or arc vaporization of a solid or liquid surface.
Metal nitride, carbide, and silicide films generally are electrically conductive (Si3N4 and AlN are important exceptions). In some applications, films of these refractory materials are used to provide diffusion barriers between materials. For example, in semiconductor metallization, aluminum or gold electrode material will diffuse into the silicon during high-temperature processing. An electrically conductive titanium nitride film deposited on the silicon surface before the metal electrode is deposited will prevent the diffusion. Generating stable, electrically conductive, nonrectifying, metal semiconductor contacts of metals or metal-silicide compounds is an important aspect of semiconductor device fabrication. Metal nitrides such as tantalum nitride (TaN) are used as thin film resistor materials. Nontransparent electrically conductive oxides such as chromium trioxide (Cr2O3), lead oxide (PbO), and ruthenium oxygen (RuO) are used as electrodes in high-temperature oxidizing atmospheres.
Superconductors are materials that have close to zero electrical resistivity below some critical temperature (Tc). Low- Tc (less than [<] 10 Kelvin [K]) superconductors are often metals. A typical high- Tc (greater than [>] 50 K) superconductor material is a mixture of oxides (yttrium-bismuth-copper [Y-Bi-Cu] oxides, YBCO). High- Tcsuperconductor thin films are often deposited by laser ablation in vacuum.
Transparent Electrical Conductors
Transparent conductive oxide (TCO) films, such as indium trioxide (In2O3), tin dioxide (SnO2), zinc oxide (ZnO) and an alloy of indium oxide and tin oxide (ITO), have numerous applications such as heaters on windows for defrosting, antistatic coatings on display screens, electrodes on flat panel displays and electrochromic devices, and electrodes on both flexible (resistive screen) and rigid (capacitive screen) touch screens. Electrical resistivity for the TCO films can vary from greater than 1,000 ohms per "square" to less than 10 ohms per square with good optical transmission.
Electrically insulating films are used to electrically isolate conducting components in semiconductor devices, and as a dielectric within capacitors. Common insulator film materials are silicon dioxide (SiO2), aluminum trioxide (Al2O3), tantalum pentoxide (Ta2O5), silicon nitride (Si3N4), and aluminum nitride (AlN). Interposing a thin oxide film between a metal film and a semiconductor allows the formation of the technologically important metal-oxide-semiconductor (MOS) device. Thick coatings of SiO2, with its low coefficient of thermal expansion, can be rf sputter deposited. Insulating layers of SiO2, silicon nitride (Si2N3), and glass are deposited by PECVD for encapsulation and insulation layers in semiconductor processing.
Optical films, usually multilayer films ("stacks"), are films that affect the optical transmission or reflection of a surface. They are generally alternating layers of materials having high (germanium [Ge], Si, TiO2, zirconium dioxide [ZrO2], SiO, cerium dioxide [CeO2]) and low (magnesium fluoride [MgF2], SiO2) indices of refraction. A major application is the antireflection (AR) coatings on lenses. Optical film stacks can be used as optical filters. Neutral density or gray filters reduce the light intensity equally for all wavelengths; broadband filters affect the transmission of radiation over a wide wavelength range, while narrow or monochromatic filters affect transmission over a very narrow wavelength region. An example of a broadband filter is an "edge filter" that "cuts off" the ultraviolet (UV) emitted by a mercury vapor lamp. Examples of narrow-band filters are the color filters used in photography and in projectors.
Some film stacks are a special type of optical film that has a color that is related to the angle-of-observation (OVIDs). These films allow holographic-like imaging. These OVID films are used as security devices to prevent counterfeiting. These films are an outgrowth of the interference-colored films used for decorative films and, when pulverized, as pigments.
Thermal Control Coatings
The composition of the thermal control coatings on windows differs with the end result desired. If the object is to keep solar radiation from entering through the window, a multilayer film of glass-TiO2-Cr-TiO2 may be used (solar control coating). If the object is to keep heat in the room, a thin film of silver can be used to reflect 85% to 95% of the low-temperature infrared radiation back into the room (low-E coating). One such "double-E coating" is glass-ZnO-Ag-(Ti)-ZnO-Ag-(Ti)-ZnO-TiO2. The ZnO provides an antireflective coating.
Other types of thermal control coatings are used to absorb solar radiation (solar absorbers), selectively adsorb solar radiation and not emit infrared radiation (selective solar absorbers), or to have a high emissivity to enhance cooling by radiation. Thermal barrier coatings are used to reduce the thermal transport from a hot environment to the substrate. Zirconium oxide (ZrO2) stabilized with calcium oxide (CaO), MgO, or Y2O3 is used as a thermal barrier coating on aircraft engine turbine blades.