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A diffusion barrier is usually a thin coating of material used to prevent diffusion. Diffusion occurs when molecules move from an area of high concentration to an area of low concentration so that an equal number occurs in both areas. Diffusion happens whether the molecules are in a gas, liquid or solid state, and can lead to the contamination of one product by another.
A diffusion barrier is usually only micrometers thin, and is used to enhance the shelf life of metal-containing products by slowing their corruption from other products nearby. These types of barriers are used in a variety of commercial applications, so effective and inexpensive barriers are highly sought-after, especially by the electronics industry. Although oxygen and hydrogen gas diffusion barriers exist, the majority of diffusion barriers are metals.
A good diffusion barrier has physical and chemical properties that vary depending on the metal components used to make the barrier. The thinner the diffusion barrier, and the more uniform the coating, the more effective the barrier. The metals in the barrier must be non-reactive to the materials around it, so they do not diffuse into and corrupt the metals the barrier is supposed to be protecting. Furthermore, the diffusion barrier must be able to strongly adhere to what it is protecting to provide a secure barrier that will completely prevent diffusion by any molecules.
The different materials used to make diffusion barriers offer different advantages, and care must be taken to optimize the thickness, reactivity and adherence of the barrier. Metals differ in their reactivity and adherence, with some metals providing a high degree of non-reactivity but low adherence, or vice versa. Some barriers may have multiple layers to accommodate the need for both non-reactive and adhesive metals. Alternatively, a combination of metals, called alloys, may be used to form the barrier. A number of metals have been used in the creation of diffusion barriers, including aluminum, chromium, nickel, tungsten and manganese.
Diffusion barriers have been commonly used in the manufacturing of electronics for decades. They are used to preserve the integrity of internal copper wiring from the silica insulation that surrounds it. This serves to prolong the lifespan of the electronic device by preventing circuit failure that would occur if the copper and silica came in contact. So far, the technology for creating and depositing diffusion barriers has allowed for increased speed of consumer electronics; however, new alloys and barrier deposition techniques are being investigated for use in new generations of electronics.