Surfaces of solids can be cleaned from contaminants by using physical sputtering in a vacuum. Sputter cleaning is often used in surface science, vacuum deposition and ion plating. In 1955 Farnsworth, Schlier, George, and Burger reported using sputter cleaning in an ultra-high-vacuum system to prepare ultra-clean surfaces for low-energy electron-diffraction (LEED) studies.  Sputter cleaning became an integral part of the ion plating process. When the surfaces to be cleaned are large, a similar technique, plasma cleaning, can be used. Sputter cleaning has some potential problems such as overheating, gas incorporation in the surface region, bombardment (radiation) damage in the surface region, and the roughening of the surface, particularly if over done. It is important to have a clean plasma in order to not continually recontaminate the surface during sputter cleaning. Redeposition of sputtered material on the substrate can also give problems, especially at high sputtering pressures. Sputtering of the surface of a compound or alloy material can result in the surface composition being changed. Often the species with the least mass or the highest vapor pressure is the one preferentially sputtered from the surface.