Abrading – Abrading process – Glass or stone abrading
Reexamination Certificate
2001-07-23
2003-07-22
Rose, Robert A. (Department: 3723)
Abrading
Abrading process
Glass or stone abrading
C451S042000, C451S037000
Reexamination Certificate
active
06595834
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to optical fluoride crystals, and particularly to optical fluoride crystals such as calcium fluoride, which have high transmission levels to below 200 nm light, such as produced by excimer lasers. In particular the invention relates to making optical fluoride crystals with improved transmission surfaces. The invention relates to the elimination of mid-spatial frequency roughness 1-1000 &mgr;m spatial wavelengths and high-spatial frequency<1 &mgr;m spatial wavelengths from optical fluoride crystal surfaces.
BACKGROUND
Applications of colloidal suspensions for polishing materials has become an exceedingly critical aspect of the final part formation of optical element components and blanks thereof. Silica and alumina colloids are formed through various techniques and typically require expensive precursor materials in order to ensure the highest purity products. Solutions are stabilized with buffer systems to pH and solids loading values that result in optimal surface finish attainment. Particle size distribution can be adjusted to control the final surface finish, as well as the ability to clean residue abrasive particles from workpiece surfaces after processing.
The level of final optical transmission surfaces currently available for optical fluoride crystals is not good enough for optical fluoride crystalline laser components and optical lithography elements.
SUMMARY OF INVENTION
The invention includes a method of making a wavelength &lgr;<200 nm optical fluoride crystal with the method including providing a fluoride crystal preform having a first and a second initial finished optical transmission surfaces having a ≦20 angstrom RMS surface roughness with a plurality of initial finished mid-spatial frequency roughness 1-1000 &mgr;m spatial wavelengths and initial finished high-spatial frequency roughness<1 &mgr;m spatial wavelengths and the initial finished fluoride preform having a low initial finish &lgr;<200 nm transmission LT (%/cm). The method includes providing a final polishing mid-spatial frequency and high-spatial frequency spatial wavelength removing colloidal particle solution having a pH≧9 and a plurality of colloidal particles and final polishing the initial finished surfaces with the mid-spatial frequency and high-spatial frequency spatial wavelength removing solution into first and second final polished calcium fluoride optical transmission surfaces with the mid-spatial frequency and high-spatial frequency spatial wavelength removing solution removing the initial finished spatial frequency spatial wavelengths to provide a final finished optical fluoride crystal having final finish high optical transmission surfaces free of the mid-spatial frequency roughness 1-1000 &mgr;m spatial wavelengths and the high-spatial frequency roughness<1 &mgr;m spatial wavelengths with a &lgr;<200 nm high transmission HT (%/cm), with HT>LT, and transmitting a final use wavelength &lgr;<200 nm light through the final finish high optical transmission surfaces.
The invention includes a method of making a <200 nm light transmitting optical fluoride crystal blank for transmitting less than 200 m light at a fluence less than 20 J/cm
2
. The method includes providing an optical fluoride crystal preform having a first and second initial finished parallel flat optical transmission surfaces having a ≦50 angstrom RMS surface roughness with an initial finished mid-spatial frequency (1-1000 &mgr;m spatial wavelength) roughness and an initial finished high-spatial frequency (<1 &mgr;m spatial wavelength) roughness, with the initial finished preform having a low initial finish &lgr;<200 nm transmission LT (%/cm). The method includes providing a final surface processing colloidal non-friable spherical abrasive particle solution having a pH≧9 and a plurality of colloidal non-friable spherical abrasive particles which have a mean particle size in the range from 20 to 300 nm. The method includes final polishing the initial finished surfaces with the final surface processing colloidal particle solution into a final polished optical transmission surface with the final polishing final surface processing colloidal particle solution removing the initial finished mid-spatial frequency roughness spatial wavelengths of 1-1000 &mgr;m and the initial finished high-spatial frequency roughness spatial wavelengths<1 &mgr;m to provide a final finished optical fluoride crystal blank having a final finish high optical transmission surface with a final finish &lgr;<200 nm high transmission HT (%/cm) with HT>LT.
The invention includes a method of making an ultraviolet &lgr;<200 nm qualified optical fluoride crystal qualified for use at a wavelength &lgr;<200 nm, and preferably at a fluence<20 J/cm
2
at the less than 200 nm wavelength &lgr;. The method comprises providing an optical fluoride crystal preform having a first initial finished flat optical transmission surface with a ≦50 angstrom RMS surface roughness with initial finished mid-spatial frequency roughness with 1-1000 &mgr;m spatial wavelengths and initial finished high-spatial frequency roughness with <1 &mgr;m spatial wavelengths, and the preform having a low initial finish &lgr;<200 nm transmission LT (%/cm). The method includes providing a final polishing mid-spatial frequency removing colloidal spherical abrasive particle solution, with the mid-spatial frequency removing solution having a pH≧9, a plurality of colloidal particles having a mean particle size in the range from 20 to 300 nm, preferably with the particles comprised of SiO
2
. The method includes final polishing the initial finished surface with the mid and high spatial frequency removing solution into a final polished optical transmission surface with the mid-spatial frequency and high-spatial frequency removing solution removing the initial finished mid-spatial frequency roughness 1-1000 &mgr;m spatial wavelengths and the initial finished high-spatial frequency roughness<1 &mgr;m spatial wavelengths to provide a final finished optical fluoride crystal having a final finish high optical transmission surface with a final finish &lgr;<200 nm high transmission HT (%/cm) with HT>LT and transmitting a final use wavelength &lgr;<200 nm light beam with a fluence<20 J/cm
2
through the final finish high optical transmission surfaces to provide a qualifying optical transmission measurement for said wavelength &lgr;<200 nm.
The invention includes a method of making a wavelength &lgr;<200 nm optical calcium fluoride crystal for use at fluences<20 J/cm
2
. The method includes providing a calcium fluoride crystal preform having a first and second initial finished flat optical transmission surfaces which have ≦50 angstrom RMS surface roughness with an initial finished mid-spatial frequency roughness 1-1000 &mgr;m spatial wavelengths and an initial finished high-spatial frequency roughness<1 &mgr;m spatial wavelengths, and the initial finished calcium fluoride preform having a low initial finish &lgr;<200 nm transmission LT (%/cm). The method includes providing a final polishing mid-spatial frequency and high-spatial frequency removing colloidal SiO
2
particle solution having a pH≧9 and a plurality of colloidal SiO
2
particles with a mean particle size in the range from 20 to 300 nm. The method includes final polishing the initial finished surfaces with the mid-spatial frequency and high-spatial frequency removing solution into first and second final polished calcium fluoride optical transmission surfaces with the spatial frequency removing solution removing the initial finished spatial frequency wavelengths to provide a final finished optical calcium fluoride crystal having final finish high optical transmission surfaces with a &lgr;<200 nm high transmission HT (%/cm) with HT>LT. The method includes transmitting a final use wavelength &lgr;<200 nm light beam with a fluence<20 J/cm
2
through the final finish high optical transmission surfac
Retherford Rebecca S.
Sabia Robert
Sokira Vincent P.
Corning Incorporated
Douglas Walter M.
Murphy Edward M.
Rose Robert A.
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