Fabrication of extremely smooth blazed diffraction gratings

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004.Includes bibliographical references (p. 103-106).High efficiency diffraction gratings are important in a variety of applications, such as optical telecommunications, lithography, and spectroscopy. Special interest has been placed on blazed diffraction gratings for their ability to enhance diffraction intensity at the specular reflection angle off the blazed facets. In this thesis I will report a novel process for fabricating extremely smooth blazed diffraction gratings with 200 nm-period. The blazed grating is fabricated using interference lithography and anisotropic etching, then replicated using nanoimprint lithography. This process was developed for fabricating the off-plane blazed diffraction gratings for the NASA Constellation-X x-ray space telescope. In order for x-rays to reflect effectively through grazing incidence reflection, the gratings will be coated with high atomic number materials, such as gold. Deposition of thin metal film often develops residual stress that adds out-of-plane distortion. In this thesis the out-of-plane distortions due to thin metal films are analyzed using wavefront aberration functions known as the Zernike polynomials. The thin film stress is proved to be linearly related to the change of the Z₂₁ Zernike coefficient. The anisotropic material properties of silicon are taken into account in the derivation, and a prediction of lattice dependent distortion is proposed.by Chih-Hao Chang.S.M

Fabricating sub-wavelength periodic nanostructures

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.Includes bibliographical references (p. 227-234).Periodic nanostructures have many exciting applications, including high-energy spectroscopy, patterned magnetic media, photonic crystals, and templates for self-assembly. Interference lithography (IL) is an attractive method for fabricating such structures, as it offers several advantages including large exposure area and high spatial-phase coherence. However, the spatial resolution of IL is limited, and the smallest attainable period is roughly half the wavelength of the light used. To overcome this wavelength-limited resolution, we have developed a multilevel interference lithography process that is capable of fabricating sub-wavelength periodic nanostructures over large areas. In this process, multiple grating levels with different phase-offsets are overlaid and spatial-phase aligned to a common reference grating. Each grating level is pattern-transferred into a single hard mask layer, resulting in spatial-frequency multiplication. To ensure high grating overlay accuracy, each grating level is aligned to the reference grating with various interferometric techniques. In addition, an image-reversal process with plasma etch trimming was developed to control the linewidth of each grating level to nanometer-repeatability. Extensive optical simulations using rigorous coupled-wave analysis were used to examine the intensity distribution of exposures over multilayer periodic structures. The immediate goal of this work is to extend the wavelength-limited resolution of interference lithography with high precision metrology and well-controlled fabrication processes. Using this multilevel process, we have successfully fabricated 50 nm-period gratings using light with 351.1 nm wavelength. This process presents a general scheme for overlaying periodic nanostructures, and can be used to fabricate more complex 2D and 3D geometries.by Chih-Hao Chang.Ph.D