Melissa Zaverton

Sensitivity analysis and optimization method for the fabrication of one-dimensional beam-splitting phase gratings.

A method to design one-dimensional beam-spitting phase gratings with low sensitivity to fabrication errors is described. The method optimizes the phase function of a grating by minimizing the integrated variance of the energy of each output beam over a range of fabrication errors. Numerical results for three 1x9 beam splitting phase gratings are given. Two optimized gratings with low sensitivity to fabrication errors were compared with a grating designed for optimal efficiency. These three gratings were fabricated using gray-scale photolithography. The standard deviation of the 9 outgoing beam energies in the optimized gratings were 2.3 and 3.4 times lower than the optimal efficiency grating.

Freeform Diffractive Structure Writing through Maskless Lithography

Printing of diffractive structures on 3D surfaces has been demonstrated using focal adjustment during i-line maskless lithography. A diffractive pupil for asymmetric astrometric distortion correction was fabricated on spherical mirrors with diameters up to 50mm.

Micro-optics fabrication by mask-based and mask-less mixed lithography process towards 3D optical circuits

We utilized a hybrid lithography technique in the fabrication of a 45 degree micro mirror coupler to be used for a 3D optical circuit. The hybrid process combines traditional mask-based lithography techniques with mask-less methods. The result is a CMOS compatible process that can be used for fabrication of integrated micro-optics.

Freeform surface relief diffractive optic for photovoltaic spectrum splitting

A surface relief diffractive optical element (DOE) for photovoltaic (PV) spectrum splitting is fabricated and tested. The optic is designed using a modified Gerchberg-Saxton algorithm. The module consists of a DOE followed by a 3.3 cm focal length lens. Alternating side-by-side PV cells - Indium Gallium Phosphide and Silicon - are placed at the collection plane. The DOE is fabricated in photopolymer using grayscale lithography. Optical efficiency and spectral distribution are measured with a scanning spectrometer. Two-bandgap conversion efficiency of 25.4% is achieved using the fabricated DOE. Simulations show that 28.4% conversion efficiency is possible with this type of optical element, which approaches the maximum possible conversion efficiency of the two-cell combination used (32.4%).

High contrast and metal-less alignment process for all-polymer optical interconnect devices

A polymer-based flat, flexible and parallel optical interconnect has become an attractive approach for short-range data transfer. For such a device, a low cost fabrication technique is required for light couplers to redirect light from source to waveguides. Recently, we demonstrated a mask-less gray scale lithography process, which used a CMOS compatible polymer for a 45-degree mirror coupler. Polymer materials such as epoclad and AP2210B can be used to fabricate flexible substrates and waveguides, respectively. We propose an all-photopolymer lithography process to fabricate the flexible and parallel optical interconnect in conjunction with the mirror couplers. In the process, a buried polymer structure is used to precisely align the mirror coupler to waveguides, which make it possible to avoid an additional metallization process. However, the contrast of such buried fiducial mark is low since such the structure is a phase structure. As a result, it is not feasible to use the buried polymer structure as an alignment mark with conventional amplitude based imaging modalities. To increase the contrast of these buried alignment marks, we propose a feature specific alignment system for which the shape and depth of the buried alignment marks are optimized for phase-based imaging such as phase contrast and Schlieren imaging. Our results show that an optimized alignment mark provides a significant contrast enhancement while using a phase contrast imaging system compared to that of a conventional imaging system. In addition, we have fabricated an optimized alignment mark specifically for use with a Schlieren imaging system.

Flexible micro-optics fabrication by direct laser writing toward CMOS compatible 3D optical circuit

We demonstrated a 45 degree micro mirror by a direct laser writing method. A flat, smooth and clearly defined mirror surface has been fabricated despite of the finite size and long tail of the point spread function of the exposure tool.

Process optimization for a 3D optical coupler and waveguide fabrication on a single substrate using buffer coat material

We have developed a hybrid lithography process necessary to fabricate a vertical optical coupler and an array of waveguide structures using the same buffer coat material on a single substrate. A virtual vernier scale built into the process enables precise alignment of both structures.

Optical characterization of CMOS compatible micro optics fabricated by mask-based and mask-less hybrid lithography

We report a CMOS compatible fabrication and optical characterization of the micrometer scale optical coupler, a 45° mirror-based optical coupler for inter-layer optical coupling. A newly proposed mask-based and mask-less hybrid lithography process enables accurate surface profile of the micrometer sized 45° mirror by using a CMOS compatible buffer coat material. Surface profile inspected by an optical interferometry agrees well with SEM based inspection results. Experimental and theoretical results for routing and coupling of laser beam in 90° will be discussed.

Central aperture detection for auto direct read-write photoresist fabrication and inspection

Abstract. We show the design for a laser scanning microscopy defect detection system based upon the idea that the light can reflect off a photoresist-laden fused-silica sample containing defects, allowing height and depth information to be obtained through changes in light intensity. Image registration using predefined points is employed. Image processing techniques involving median and deconvolution filtering are used. Results show that the 2.1-μm resolution of these defects is obtainable, and receiver operating characteristic curves are used for quantifying results. Discriminabilities of 0.73 are achieved. Preliminary results for larger-array patterns through stitching processes are also shown.

Characterization of photoresist and simulation of a developed resist profile for the fabrication of gray-scale diffractive optic elements

We have characterized a photoresist used for the fabrication of gray-scale diffractive optic elements in terms of Dills and Macks model parameters. The resist model parameters were employed for the simulations of developed resist profiles for sawtooth patterns executed by solving the Eikonal equation with the fast-marching method. The simulated results were shown to be in good agreement with empirical data.