Method of fabricating high efficiency binary planar optical elements

• US 4,846,552 A
• Filed: 02/09/1988
• Issued: 07/11/1989
• Est. Priority Date: 04/16/1986
• Status: Expired due to Fees

• Current Assignee: United States Of America As Represented By The Secretary Of The Air Force
• Original Assignee: United States Of America As Represented By The Secretary Of The Air Force
• Inventors: Swanson, Gary J., Veldkamp, Wilfrid B.
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): Lerner, Martin
• Application Number:
  US07/157,087
• Time in Patent Office: 518 Days
• Field of Search: 350/3.61, 350/162.2, 350/162.22, 350/162.23, 350/162.17, 250/492.2, 250/492.22
• US Class Current: 359/572
• CPC Class Codes: G02B 5/1809 with pitch less than or comparable to the wavelength
• CPC Class Codes: G02B 5/1857 using exposure or etching means, e.g. holography, photolithography, exposure to electron or ion beams
• CPC Class Codes: G02B 5/1876 Diffractive Fresnel lenses; Zone plates; Kinoforms G02B5/1842, G02B5/1847 take precedence; optical systems having diffractive correction means G02B27/0037; Fresnel lenses operating by refraction G02B3/08
• CPC Class Codes: Y10S 359/90 Methods

Abstract

The high diffraction efficiency regime of binary gratings occurs at periodicities on the order of a wavelength in order to produce grating which diffract radiation with efficiencies of greater than 90%, a fabrication procedure is disclosed which uses halographic and very large scale integration techniques which allow fine control over the periodicity and depth parameters to produce binary planar optical elements having a ratio of λ/T greater than one (where λ equals the wavelength of an illuminating wavefront, and T equals the grating periodicity). Additionally, the disclosed process produces high optical quality diffractive elements with phase precision of as high as λ/100. These diffractive elements include laser beam multiplexers, beam profile shapers, and binary lenses which are lossless optical transfer functions.

Claims

1. A process of fabricating a high efficiency binary planar optical element which diffracts an illuminating wavefront having a wavelength with a distance of λ

, said binary planar optical element having a reflective grating with a periodicity of distance T formed by a plurality of rectangular grooves and plurality of rectangular walls, said fabricating process comprising the steps of;

preparing a mask by etching a lithographic mask pattern upon a positive resist layer which coats a first substrate, said lithographic mask pattern being prepared with feature sizes as small as 0.25 microns, and a positional accuracy of 0.05 microns using an electron beam pattern generator;

producing a finished mask by chromium etching said lithographic mask pattern on said positive resist on said first substrate to produce a finished mask pattern which has transparent areas corresponding to all troughs of said plurality of rectangular grooves and darkened areas corresponding to all walls of said rectangular grooves, said transparent areas and said darkened areas corresponding to said reflective grating with said periodicity of distance T wherein said distance T approaches said wavelength distance λ

said darkened areas being coated with a layer of chromium which covers only said darkened areas, said layer of chromium blocking illumination which comes from above said mask while said transparent areas permitting illumination to pass through said mask;

preparing a second substrate for receipt of said lithographic mask pattern by coating said second substrate with a thick layer of resist, said thick layer of resist being coated by an aluminum layer, said aluminum layer being coated by a layer of photoresist;

placing said finished mask over said second substrate, said second substrate having been already coated on its top surface with said layer of photoresist, wherein said second substrate is a material selected from a group consisting of;

quartz, silicon, plastic, and glass;

exposing the finished mask to incoherent ultraviolet light to print the lithographic mask pattern on said layer of photoresist;

wet etching the lithographic mask pattern onto said aluminum layer on said second substrate by exposing said second substrate to a wet etchant that transmits the lithographic mask pattern onto said aluminum layers;

forming by reactive-ion etching troughs and walls in said thick layer of resist of said second substrate corresponding to said troughs and walls of said finished mask, said troughs and walls on said second substrate defining a plurality of rectangular grooves on said second substrate; and

depositing a reflective coating on said rectangular grooves in said thick layer of resist, said aluminum layer and said second substrate, said reflective coating forming said reflective grating of said binary planar optical element, and wherein said reflective coating is a material selected from a metal group consisting of;

chromium, gold, aluminum, copper, nickel and silver.