DUAL DIFFUSER LITHOGRAPHY TO FABRICATE DIMENSIONALLY CONTROLLED COMPLEX 3D MICROSTRUCTURES

Abstract

Fabrication of extensively used complex 3D icrostructures such as microlenses, microchannels, microtrapezoids, micro/nano tips etc with controlled dimensions in minimized process steps and minimum complexity has always been an objective of many researchers over the decades. Such complex microstructures have been in great demand due to the rapid developments in components of high-resolution projectors, photonic devices and microfluidic chips. To meet the fabrication needs of such patterns, intricate and expensive processes such as reactive ion etch, stereo lithography and ultraviolet micro stamping have been utilized. The processes are not only expensive but also have limitations in terms of controlling the pattern dimensions of these microstructures resulting in low throughput and efficiency. In this dissertation, a simple, convenient, cost effective and commercially applicable method, dual diffuser lithography (DDL) has been proposed for the fabrication of complex 3D microstructures by adding a pair of diffusers in the conventional photolithography process. The pair of diffusers rendering the Gaussian and Lambertian scattering phenomena was inserted in the photolithography tool above the photomask in the passage of incident Ultraviolet (UV) light. The pair of inserted diffusers diffuses the incident rays of light at wide angles before approaching the photoresist. The straight beam of light is initially diffused at certain angles using the first diffuser (Gaussian) and the diffused beam is re-diffused at wider angles with good uniformity using the second diffuser (Lambertian). The diffused rays of light possess different intensity at different points along the diffused angles resulting in exposure of photoresist in a curved manner. Photoresist (PR) patterns with µm to sub-µm dimensions were conveniently fabricated by just changing the exposure energy of the incident light. An increase in exposure energy of the UV light causes a decrease in the dimensions of the microstructures and vice versa. Microlens patterns with radius of curvature from 25 µm to as small as 0.50 µm and triangular patterns with tip of ~200 nm were successfully fabricated using the single step DDL process. The surface morphology of the fabricated patterns was analyzed using field emission scanning electron microscopy (FE-SEM) and 3D-profiler. The intensity profile of the UV light after passing through diffusers was analyzed using a deuterium light source with a spectrometer (λ = 360 nm) in the transmission efficiency analyzer. Surface morphology of the diffusers was studied using an optical microscope (OM) and 3Dprofiler. The variable curvature microlenses fabricated by the DDL process were then replicated in conductive and transparent PDMS for application in the field of ‘optofluidics’. The conductive and transparent PDMS was prepared by mixing silver (Ag) nanoparticles (<100 nm) with various percentages (5-20% wt.) in the PDMS base (sylgard A) and curing agent (sylgard B). The prepared Ag(n)-PDMS was spin coated on the Ni mold replicated from the PR mold to achieve a thickness of ~ 50 µm. The conductivity of the Ag(n)-PDMS mold was analyzed using a 4-point probe system. Transmission efficiency was measured using an ellipsometer by checking the differences in the intensities of light before and after passing through Ag(n)-PDMS layers. Electrowetting behavior was analyzed at different applied voltages using a contact angle analyzer. A relationship between amount of silver content, transmission efficiency and conductivity was studied. Diffusion process of UV light through the diffusers was explained in details with the help of diffuser analysis for better understanding of the diffusion phenomenon. Fabrication of various kinds of circular, line and triangular patterns was successfully conducted resulting in microlens, microchannels and micro/nano tip patterns respectively. Dimensional control over the fabricated patterns with the help of change in exposure energy and the fill factor was achieved. The fabricated variable focus microlenses were transferred to conductive and transparent Ag(n)-PDMS as an application of optofluidics. The proposed DDL method has high potential to be applied for the fabrication of microfluidic, photonic and electronic devices.