Deposition

In a process known as film deposition, a thin film of some substance is deposited onto the wafer by means of either a chemical or a physical reaction.

Chemical methods

In one common method, known as chemical vapour deposition, the substrate is placed in a low-pressure chamber where certain gases are mixed and heated to 650–850 °C (1,200–1,550 °F) in order to form the desired solid film substance. The solid condenses from the mixed gases and “rains” evenly over the surface of a wafer. A special variant of this technique, known as epitaxy, slowly deposits silicon (or gallium arsenide) on the wafer to produce epitaxial growth of the crystal. Such films can be relatively thick (0.1 mm) and are commonly used for producing silicon-on-insulator substrates that lower the power requirements and speed the switching capabilities of CMOSs (described in the section Complementary metal-oxide semiconductors). Another variation, known as plasma-enhanced (or plasma-assisted) chemical vapour deposition, uses low pressure and high voltage to create a plasma environment. The plasma causes the gases to react and precipitate at much lower temperatures of 300 to 350 °C (600 to 650 °F) and at faster rates, but this method tends to sacrifice uniformity of deposition.

Two more chemical methods of deposition are electrodeposition (or electroplating) and thermal oxidation. In the former the substrate is given an electrically conducting coating and placed in a liquid solution (electrolyte) containing metal ions, such as gold, copper, or nickel. A wide range of film thicknesses can be built. In thermal oxidation the substrate is heated to 900–1,100 °C (1,650–2,000 °F), which causes the surface to oxidize. This process is often used to form a thin (0.0001-mm) insulating layer of silicon dioxide.

Physical methods

In general, physical methods of film deposition are less uniform than chemical methods; however, physical methods can be performed at lower temperatures and thus at less risk of damage to the substrate. A common physical method is sputtering. In sputtering, a wafer and a metal source are placed in a vacuum chamber, and an inert gas such as argon is introduced at low pressure. The gas is then ionized by a radio-frequency power source, and the ions are accelerated by an electric field toward the metal surface. When these high-energy ions impact, they knock some of the metal atoms loose from the surface to form a vapour. This vapour condenses on the surfaces within the chamber, including the substrate, where it forms the desired film.

In evaporation deposition, a metal source is heated in a vacuum chamber either by passing a current through a tungsten container or by focusing an electron beam on the metal’s surface. As metal atoms evaporate, they form a vapour that condenses on the cooler surface of the wafer to form a layer.

Finally, in casting, a substance is dissolved in a solvent and sprayed on the wafer. After the solvent evaporates, an extremely thin film (perhaps a single layer of molecules) of the substance is left behind. Casting is typically used to add a photosensitive polymer coating, called the photoresist layer.

Etching

A layer can be removed, in entirety or in part, either by etching away the material with strong chemicals or by reactive ion etching (RIE). RIE is like sputtering in the argon chamber, but the polarity is reversed and different gas mixtures are used. The atoms on the surface of the wafer fly away, leaving it bare.