LDW-GLASS PHOTOMASK BLANKS

DOE MADE OF LDW-GLASSES: LASER WRITTEN DOE PATTERN IN LDW-GLASS BECOMES MULTIPHASE LEVEL DOE UPON 10 MIN. ETCHING IN HF SOLUTION.

LDW-Glass is ideally suited for generating DOE's in glass using a laser beam direct write process with a following etch step. Phase levels of DOE are immediately visible as gray levels in LDW-glass upon laser beam exposure. The multi-gray levels in LDW-glass were transformed into multi-phase levels, i.e., depth variation of surface relief in LDW-glass upon a wet chemical etching or a dry etching process. Before the etching process, the image in LDW-glass causes an absorption or amplitude modulation of am incoming light beam. The residual optical density in DOEs made of colored LDW-glasses ca be heat erased by baking in as oven at temperatures up to 300°C

At a given laser write velocity, there exists an etch-threshold intensity I_ItTH below which the laser beam exposed area is not ethced preferentially in HF solution. The value of the etch threshold energy density is less that that of the erasure-theshold energy density and is invariably less than the abrasion-threshold energy density. Using a write beam energy density just above the etch-threshold energy density, LDW-glass shows little or no evidence of change until it is etched in a HF solution. Prior to HF etch, the laser exposure above the etch-threshold energy density may be exhibited as a bump (i.e. a surface deformation) of a few nm height at the are of laser beam exposure.

Colorless LDW glasses which are transparent in the visible spectral range and absorp photons of wavelengths shorter than about 300nm are ideally suited for generating DOEs in glass using a laser beam direct write process with a following etch step. The baking step to eliminate the residual optical density is no longer necessary when a colorless LDW-glass and an eximer laser are employed.

The transformation of a laser beam written image in LDW-glass into a surface relief pattern is accomplished in a simple step:

• Chemical ethcing in HF soultion (or dry etching); 5 to 10 min. in 1.25% soultion

When the DOE is generated in a colored LDW glass, DOE is made colorless and transparent with the folowwing heat step:

• Heat the etched LDW-glass plate in a box oven for a duration up to about 30 minutes at temperatures up to 300°C to render the entire LDW-glass plate colorless and transparent, i.e. to heat erase the remaining gray levels within the newly made DOE.