Subsurface structure in polished fused silica and diamond turned single crystal211 silicon

摘要

The surface and near surface structure of glass and other optical materials is211u001egreatly influenced by the nature of the processes used to generate that surface. 211u001eIn high quality optics, the effects of process changes are often subtle and 211u001ecannot be seen with conventional metrology. The presence of process induced 211u001edamage in the near surface region is felt in a number of ways. Damage thresholds 211u001efor optics subjected to high fluences are a particular problem in UV or high-211u001epowered laser systems. In high quality glass, the chemical and material 211u001ecomposition of the outermost layer is influenced principally by the grinding, 211u001elapping and polishing processes used in fabrication. Performance in high fluence 211u001eapplications is often dominated by these process-induced inhomogeneities in the 211u001efirst few hundred nanometers of material. Each succeeding step in a process is 211u001edesigned to remove the damage from the previous operation. However, any force 211u001eagainst the surface, no matter how slight will leave evidence of damage. 211u001eFabrication processes invariably create dislocations, cracks and plastic 211u001edeformation between 100 nm and 500 nm below the surface. In glass polishing, the 211u001efirst 100 nm is comprised of material redeposited from the polishing solution. 211u001eThis redeposition layer is responsible for the extremely smooth surfaces that can 211u001ebe generated on glass. Unfortunately, this layer also conceals many flaws present 211u001ein the deeper surface regions.