Fabrication Of Large-size Diffraction Gratings

Large-size diffraction gratings play important roles in many areas of science andtechnology. The pulse compressors in contemporary high-power chirped-pulseamplification laser systems for inertial confinement fusion urgently demand formeter-sized gratings. To fabricate large-area gratings of high-quality diffractionwavefronts, the traditional method of holographic exposure requires large-aperturecollimating lenses. Recently researchers have developed the optical mosaic techniqueto fabricate gratings much larger than the aperture of exposure system. The essentialproblem of this technique is to study the high-accuracy control of the relative positionand attitude between the grating substrate and the exposure beams, during consecutiveholographic exposures on different areas of a large-size monolithic substrate. Tocompete with the traditional single-exposure technique, this new technique should alsopossess stability and practicality for manufacture, and applicability to meter-sizefabrication. This dissertation develops a latent-image-based optical mosaic techniquethat meets the stringent requirements for fabricating large-size diffraction gratings.We analyzed the influences of the mosaic errors and established the mosaicconditions. Employing the interference fringes formed by the diffractions of theexposure beams from the latent image gratings that were recorded in photoresist, werealized the high-accuracy alignments of the relative position and attitude between thesubstrate and the exposure beams, in the orders of10nm and0.1μrad, respectively.The alignments include the adjustment between different exposures and the automatedfringe-locking during each exposure. The proposed fully self-referencing techniquecombines the exposure, adjustment, and fringe-locking systems together, and eliminatesvarious random and accumulative mosaic errors that would otherwise be caused by therelative drifts between these systems. We conducted many kinds of mosaicexperiments and fabricated many mosaic gratings, with different mosaic times,directions, sizes, exposure intensities, single-exposure time, etc. Their diffractionwavefronts are close to those limited by the substrate flatness, exposure aberration, anddiffraction wavefront from single-exposed gratings, while the mosaic errors are clearlybetter than0.05λ at the mosaic seams. For example, the measured peak-valley value of the wavefront error of a1×4mosaic grating in50×(30+30+30+30) mm~2area is0.05λ, while the errors near its three mosaic seams are all lower than0.03λ; the error of a1×2mosaic grating in90×(80+80) mm~2area is0.14λ, while the error near its mosaicseam is lower than0.05λ. These high-quality experimental results proved theaccuracy, stability, practicality, and success rates of the mosaic technique.Ion-beam etching is the final procedure in the fabrication of mosaic gratings. Theetching route we used is as the following. First, Ar is employed as the working gas toetch through the Cr layer. Then, CHF3is employed as the working gas to reactivelyetch the top SiO2layer of the multilayer dielectric stack, with the end-point detection bymonitoring the TM polarized diffractions. We fabricated a1×2mosaic multilayerdielectric grating with the area of70×(40+40) mm~2, the diffraction efficiency of95.4±0.6%, and the peak-valley value of the wavefront error of0.10λ. This resultconfirmed the effectiveness of the etching process and the whole mosaic technical route.Concerning the application of the developed technique to meter-size gratingmosaic, we analyzed the advanced technologies for direct transplantation and the mainremaining problems as well as their possible solutions. The results demonstrate thatthe current optical mosaic technique can be applied to larger-size grating fabrication.