What Is an Atomic Layer Deposition?

Atomic layer deposition is a chemical process used in the manufacture of microprocessors, optical films, and other synthetic and organic thin films for sensors, medical devices, and advanced electronics where a layer of material only a few atoms in thickness is precisely deposited onto a substrate. There are several approaches and methods for depositing atomic layers, and it has become an essential feature of nanotechnology research and materials science research in electrical engineering, energy, and medical applications. The process often involves atomic layer epitaxy or molecular layer epitaxy, where a very thin layer of crystalline substance in the form of a metal or semiconducting silicon compound is attached to the surface of a thicker layer of similar material.

Thin-film deposition is an area of product research and production that requires the expertise of several scientific disciplines due to the fine layer of control that must be exercised to produce useful devices and materials. It often involves research and development in physics, chemistry, and various types of engineering from mechanical to chemical engineering. Research in chemistry determines how chemical processes take place at atomic and molecular levels and what the self-limiting factors are for the growth of crystals and metallic oxides, so that atomic layer deposition can consistently produce layers with uniform characteristics. Chemical reaction chambers for atomic layer deposition can produce deposition rates of 1.1 angstroms, or 0.11 nanometers of material per reaction cycle, by controlling the amount of various reactant chemicals and the temperature of the chamber. Common chemicals used in such processes include silicon dioxide, SiO₂; magnesium oxide, MgO; and tantalum nitride, TaN.

A similar form of thin film deposition technique is used to grow organic films, which usually starts with fragments of organic molecules such as various types of polymers. Hybrid materials can also be produced using organic and inorganic chemicals for use in products like stents that can be placed in human blood vessels and coated with time release medications to combat heart disease. Alberta researchers at the National Institute of Nanotechnology in Canada have created a similar thin film layer with a traditional stainless steel stent to prop open collapsed arteries as of 2011. The stainless steel stent is coated with a thin layer of glass silica that is used as a substrate to which to bind sugar carbohydrate material that is approximately 60 atomic layers in thickness. The carbohydrate then interacts with the immune system in a positive way to prevent the body from developing a rejection response to the presence of the steel stent in the artery.

There are hundreds of chemical compounds used in atomic layer deposition and they serve numerous purposes. One of the most widely researched as of 2011 is the development of high-k dielectric materials in the integrated circuit industry. As transistors get smaller and smaller, down below the 10 nanometer size, a process known as quantum tunneling where electrical charges leak across insulating barriers makes the traditional use of silicon dioxide for transistors impractical. High-k dielectric material films being tested in atomic layer deposition as replacements include zirconium dioxide, ZnO₂; hafnium dioxide, HfO₂; and aluminum oxide, Al₂O₃, as these materials demonstrate a much better resistance to tunneling.