What Are Thin Film Materials?
Thin film materials are high purity materials and chemicals used to form or modify thin film deposits and substrates. Examples include precursor gases, sputtering targets, and evaporation filaments.
Any technique that deposits a thin film material onto a substrate or previously deposited layers is considered thin-film deposition. "Thin" generally implies micro, nano, or atomic scales for characterizing deposited layers. Thin-film deposition can be specified as either chemical or physical deposition depending on the means by which the layer is deposited.
Thin film materials function in a variety of ways. Using a precursor gas method involves providing a metal-containing precursor to an activation zone, and activating the metal-containing precursor to form an activated precursor. The activated precursor gas is moved to a reaction chamber, and a film is deposited on a substrate using a cyclical deposition process, wherein the activated precursor gas and a reducing gas are alternately adsorbed onto the substrate.
In the sputtering target process, argon plasma is ignited in a vacuum chamber and argon ions are accelerated towards a negatively charged cathode by means of an electrical field. The argon ions infuse the target with high kinetic energy, resulting in the emission of atoms of the target material. The atoms diffuse through a vacuum chamber and condense as a thin layer on a substrate.
An evaporation filament has the advantages of efficient outgassing, easy reloading of metal charges, and automatic termination of evaporation before less volatile impurities can be evaporated. A falling evaporator film can be operated with very low temperature differences between the heating media and the boiling liquid. A falling evaporator film also has very short product contact times, typically just a few seconds per pass. These characteristics make a falling evaporator film particularly suitable for heat-sensitive products, and are the most frequently used type of film evaporator.
When selecting a thin film material, it is important to consider the type of product, material type, material properties, product dimensions, and any required or desired features. This guide is designed to help buyers with this process.
All of a thin-film material's specifications will depend on the type of product being considered.
- Chemical precursor is stock product in the form of a liquid, solid, or gaseous chemical precursor. This material undergoes a chemical change to be deposited on a solid surface.
- Electrochemical deposition materials are deposited onto a substrate in a wet electrochemical process akin to electroplating. ECD materials consist of metal salts or aqueous solutions.
- Evaporation materials consist of wire, sheet, or bulk solids which can be boiled or sublimed to produce vapors which are condensed onto a substrate.
- Sputtering targets are used in sputtering processes where atoms or molecules of sputtering target material are knocked off and sputtered onto a substrate in the form of a thin film deposition or sputter coating. These targets can specifically be planar or rotary shaped.
Thin films can include a large number of materials. Materials include metals, ceramics, polymers, and carbon-based compounds. Material selection will depend on the application that the product is being used for.
Depending on the application, different material properties may be important for a thin film material's application.
- Purity is the percentage of the main metal, alloy, or ceramic in a material. The percentage of impurities in a material is 100% minus the purityin percent. Purity can have a large impact on the properties of the thin-film in regards to the deposition process and its post-production application.
- Melting point or freezing point is the temperature or range of temperatures where a material changes between solid and liquid phases. Pure metals and eutectic alloys will melt at a distinct temperature.For alloys, the material may evaporate over a range of temperature if the phase diagram exhibits a liquid + solid phase region.
- Boiling, sublimation, or evaporation point is the temperature or range of temperatures where a material begins to change from a liquid to a gas.
- Electrical resistivity is thelongitudinal electricalresistance(ohm-cm) of a uniform rod of unit length and unit cross-sectional area. Electrical resistivity is the inverse of conductivity.High resistivity is a defining characteristic of a dielectric material.
- Index of refraction is the measure of the speed of light in a material. It is a ratio of speed of light in a vacuum to speed of light in the considered material. A material in which light travels very slowly will have a very high index of refraction.
- Transmission is the fraction of light of a specified wavelength that passes through a material. It can be specified as a percentage; a transmittance of 100% indicates no light has been reflected or absorbed.
- Theoretical material density is the mass per unit area for a material.The sintered or fired density is dependent on the theoretical density of 100% dense body and the actual porosity retained after processing.
Size and Dimensions
Sometimes it may be important to select a material based on specified dimensions, especially if a thin-film product must meet certain size requirements for a given process or application. Dimensions can include length, thickness, width or outer diameter, particle size, and mesh size. Particle size specifically refers to the diameter or other measured size of a granule, powder, or chopped fiber material. Mesh size designates a diameter range of grains or powder particles based on U.S. Standard mesh sizes.
For some applications, certain features may be important or necessary properties for thin-film materials.
- Anti-reflective, reflective, and transparent features for thin-film materials may be important for optical coating applications.
- Semiconductive, dielectric, and conductive features for thin-film materials may be important for electronics, microelectronics, fuel cell production, and photovoltaic or solar cell manufacturing.
- Low k or low loss tangent dielectric thin film coatings are important in semiconductor chip fabrication processes where dimensions are diminishing and cross talk problems are becoming a concern.
- Wear resistance featured thin-films may be particularly important for cutting tools or biotech applications.