  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.
