Semester of Graduation

Summer 2024

Degree

Master of Science (MS)

Department

Physics and Astronomy

Document Type

Thesis

Abstract

Purpose: High prevalence of lung diseases leads to many people worldwide needing spirometry and imaging testing. The typical imaging done uses chest radiography and/or low-dose CT. Lung tissue is difficult to visualize, making them substandard for monitoring lung disease progression. An improvement on these systems is x-ray interferometry which produces additional images that provide better visualization of changes in the lung tissue. The set-up of x-ray interferometry builds upon other imaging systems in that it includes diffraction gratings.

Materials: This thesis focused on implementing the fabrication processes to create diffraction gratings for a lung imager prototype that is currently under development. Diffraction gratings can be made through microfabrication using lithography and etch tools. Silicon wafers went through photolithographic processes using photoresist and a developer to form the grating pattern. Dry or plasma etch was used to evaluate different recipes to produce deep, vertical trenches. The processes were initially developed at the Nanofabrication Facility at the LSU Center for Microstructures and Devices. Once the processes were understood, production of the gratings was carried out at the Center for Nanoscale Science and Technology at NIST, where a direct-write lithography and deep silicon etch tool were used with similar recipes established at CAMD.

Results: The developed recipes allowed the fabrication of gratings with the desired properties of feature size width, sharp cornered features, spacing between features (pitch), and high aspect ratio depths. Characterization using imaging and other tools showed that a fast etch rate, vertical walls, and consistent fabrication could be achieved. Etch rates of 2.7 um/min and 3.4 um/min for phase and absorption gratings respectively were achieved.

Discussion: We have created phase and absorption diffraction gratings suited for our interferometry design. These processes should be adaptable to other models of lithography and etch tools. Lessons learned along the way include using a high resolution light source to pattern sharp features and a dedicated deep silicon etch tool as this avoids gas contamination leading to significantly improved fabrication quality. The processes developed and demonstrated in this thesis will facilitate the fabrication of similar grating designs for future interferometry development projects.

Date

5-16-2024

Committee Chair

Matthews, Kenneth L. II

Download

Share

COinS