Mechanism of Laser Induced Compaction
Fused Silica is the material of choice for UV optical systems such as the projection optics in microlithography systems. Unfortunately fused silica is not stable under UV irradition and undergoes compaction and color center formation. The UV damage properties of fused silica have been the subject of a number of studies. The detailed microscopic mechanism for color center formation is complex and remains obscure but the fact that compaction must be a generic property of a glassy system seems to have been overlooked by most researchers although Krajnovich, et. al., do briefly discuss compaction with respect to a specific glass model. The purpose of this note is to briefly discuss how the basic mechanism of compaction follows directly from the modern theoretical understanding of glassy systems. This understanding of the basic mechanism of compaction fits the experimental data quite well. It also allows for the prediction of some of qualitative properties of compaction which have not so far been experimentally determined, such as, the absence of a minimum damage threshold for the onset of compaction and the eventual long term cessation of compaction after sufficient shrinkage has occurred.