A multilevel microfabrication process has been developed to produce silicon Fresnel lenses for terahertz waves. A repeated binary fabrication process was used to create lenses with up to eight levels in complexity and these lenses have been compared to both less complex structures and refractive optic lenses. The microfabrication required deep reactive ion etching and multilevel resist processing using SU8 photoresist. At the design frequency of 1 THz an eight-level lens showed significant improvement in intensity at the focus and had much reduced energy lost into sidelobes compared with refractive lenses.

Multilevel silicon diffractive optics for terahertz waves

Tubes with Controllable Internal Nanotopography

Three-dimensional microstructures find applications in diffractive optical elements, photonic elements, etc., and can be efficiently fabricated by e-beam lithography. Good process control and efficient proximity effect correction are important for achieving the desired structures. With polymethylmethacrylate as the resist, a process optimization of different develop conditions is carried out to identify a process that is most conductive to gray scale features. A novel proximity effect correction scheme called effective dose-depth (EDD) method is proposed. Using the EDD method for grating design and the optimized process, blazed gratings have been fabricated with excellent uniformity and low surface roughness.

Process optimization and proximity effect correction for gray scale e-beam lithography

Thermal reflow of polymers for innovative and smart 3D structures: a review

Nanoscale embossing of polymers using a thermoplastic die

Diffractive optics are of increasing interest as they can be used to make compact optical components of great complexity. However, they require precision of fabrication on a scale significantly below the operating wavelength in order to achieve high efficiency. We describe a new technique for making multilevel diffractive optics in a single step by the patterning of solid poly methyl methacrylate (PMMA) as opposed to conventional thin films. We have characterized the process to optimize it for development, dose, and beam current. Using the technique we have made blazed diffraction gratings for use in transmission or reflection with efficiencies as high as 83% at a wavelength of 514 nm. The method is excellent for rapid prototyping of designs and we anticipate that it may be used to make embossing masters for lens fabrication into other materials.

Iman I. Khandaker

Papers

Efficient diffractive optics made by single-step electron beam lithography in solid PMMA