Semester of Graduation
Master of Electrical Engineering (MEE)
Electrical Engineering and Computer Engineering
X-ray Phase contrast-based Talbot interferometer creates high contrast between weak and strong absorbing materials, which makes it effective in imaging soft tissues. However, its performance is bounded by the aspect-ratio, features and symmetry of its gratings. For 40 KeV energy X-rays, the analyzer grating thickness should be 100 µm or more to achieve > 90% absorption in order to obtain high contrast images. Moreover, the smaller period in grating is desired for higher resolution. Therefore, researchers are exploring various fabrication techniques to achieve greater aspect-ratio gratings. Utilizing modern LIGA techniques, the aspect-ratio of gratings can be improved with a simplified and precise fabrication process. This thesis focuses on the fabrication of gratings with aspect ratio of 25; 100 µm tall and 8 µm period with 50 % duty cycle. X-ray lithography, electroplating and micro-machining were used during the fabrication of these gratings. Also, a silicon nitride based membrane X-ray mask with grating patterns was fabricated to perform X-ray exposure. Multiple approaches were implemented to optimize the processing conditions and parameters for gratings fabrication. The thesis experimentally compared the adhesion of PMMA resist acting as a mold, in which metal gratings were electroplated, to Copper oxide and titanium oxide. For each of two oxides, wafers were prepared separately, starting with depositing copper (Cu) and titanium (Ti) as seed layers and later oxidizing them. Later, both the wafers were bonded with 2.5 mm thick PMMA resist wafer. They are further flycut down to 100 µm and later is exposed at and XRLM-1 beamline at CAMD/LSU. The resist development results are compared and adhesion was analyzed for both copper oxide and titanium oxide.
The adhesion in PMMA resist was better to copper oxide layer in comparison to the titanium oxide. However, titanium oxide is preferable because PMMA molds are damaged during copper oxide etching. Copper oxide, unlike titanium oxide, is not conductive which prevents electroplating of gratings; therefore, etching of copper oxide is required. Finally, the wafers were electroplated to nickel and later the resist is stripped. We have achieved gratings with an aspect ratio of ~21 with 4.64 µm period and 100 µm height.
Singh, Vikaram, "THE FABRICATION OF HIGH-ASPECT RATIO GRATINGS FOR TALBOT INTERFEROMETER WITH MEDICAL IMAGING APPLICATION." (2018). LSU Master's Theses. 4739.