Silica with low compaction under high energy irradiation

Abstract

Fused silica stepper lens for photolithographic application is disclosed which is resistant to laser-induced damage, specifically, compaction or densification which can lead to an increase in the optical path length of the lens.

1 . Stepper lens made from fused silica glass which is resistant to compaction when exposed to a dose of high intensity excimer radiation.  
     
     
         2 . Fused silica stepper lens which is resistant to compaction when exposed to high intensity excimer radiation, said lens being made by a process comprising the steps of: 
 (a) preparing a solution which contains at least one silicon-containing organic compound having the formula SI(OR) 4  or SIR(OR) 3 , where R is an alkyl group;    (b) polymerizing the silicon in the solution to form a SiO 2  gel;    (c) drying the gel at a rate which causes the gel to fragment into granules having a mean particle size less than about one millimeter;    (d) sintering the granules at a temperature less than about 1150° C., the density of the granules after sintering being approximately equal to their maximum theoretical density;    (e) forming a green body from the sintered granules;    (f) drying and partially sintering the green body in a chamber by (i) raising the temperature of the chamber to above about 1000° C., and (ii) introducing chlorine gas into the chamber and/or subjecting the chamber to a vacuum and/or purging the chamber with an inert gas; and    (g) fully sintering the green body in a chamber by raising the temperature of the chamber to a temperature above about 1720° C., while purging the chamber with helium or applying a vacuum to the chamber.    
     
     
         3 . Stepper lens according to  claim 2  wherein the process includes the additional step after step (g) of hot isostatic pressing the fully sintered green body to a chamber by raising the temperature of the chamber to above about 1150° C. and introducing an inert gas into the chamber at a pressure above about 100 psig.  
     
     
         4 . Stepper lens according to  claim 2  wherein the solution of step (a) contains tetraethylorthosilicate having the formula Si(O(C 2 H 5 ) 4 .  
     
     
         5 . Fused silica stepper lens which is resistant to compaction when exposed to high intensity excimer radiation, said lens being made by a process comprising the steps of: 
 (a) depositing on a starting member a coating of flame hydrolysis-produced glass soot to form a soot preform;    (b) consolidating the soot preform to form a dense glass layer free from particle boundaries; and    (c) forming said dense glass layer into a stepper lens;    said consolidation step being characterized in that it comprises heating said soot preform to a temperature within the consolidation temperature range for a time sufficient to cause said soot particles to fuse and form a dense glass layer, and simultaneously subjecting the soot preform to a stream of a substantially dry, hydrogen-free, chlorine containing atmosphere.