MULTISCALE, MULTIFUNCTION DIFFRACTIVE STRUCTURES WET ETCHED INTO FUSED-SILICA FOR HIGH-LASER DAMAGE THRESHOLD APPLICATIONS

We combined functionalities of two diffractive optics with almost 100x lateral and Vertical scale-length difference onto a single fused-sili ca surface. Fine-scale (2-mu m-period) gratings for beam sampling were printed in photoresist by interference Lithography and transferred to the substrate by a hydrofluoric acid etch. Subsequently, 115-mu m-lin ewidth stairstep gratings for color separation at focus were proximity printed and wet etched in a two-mask process. Line shapes of the lame llar sampling grating are remarkably preserved following etching of th e much deeper color separation grating structures with this nominally isotropic etch process. Model simulations of isotropic etching of topo graphical features show good agreement with the measured shape evoluti on of the sampling grating profiles, and the simulations reveal the se nsitivity of the final feature shape to its initial aspect ratio. As a rule of thumb, lamellar grating profiles can be etched approximately 0.08A(-2) times their modulation depth, where A is their initial aspec t ratio (height/width), before they evolve into a cusplike shape and b egin to lose height. (C) 1998 Optical Society of America.