MICROFABRICATION OF X-RAY OPTICS FOR MICROSCOPY AND PHASE CONTRAST IMAGING

Abstract

The short wavelength of x-ray radiation (λ = 10Å to λ = 0.1Å) gives it the ability to penetrate deeper into matter and makes it highly useful for applications in microanalysis and imaging, where it is desirable to probe beyond the surface of the material under analysis. Hard x-rays generally refer to photon energies above 10keV, while soft x-rays are between 0.1 and 10 keV. This dissertation investigates the development of two types of optical elements suitable for hard x-ray imaging: compound refractive lenses for hard x-ray microscopy and diffraction gratings for hard x-ray phase contrast imaging. Hard x-ray lenses are useful elements in x-ray microscopy and in creating focused illumination for analytical applications such as x-ray fluorescence imaging. Recently, polymer compound refractive lenses for focused illumination in the soft x-ray regime (< 10 keV) have been created with nano-printing. However, there are no such lenses yet for hard x-rays, particularly of short focal lengths for benchtop microscopy. In the first section of this dissertation, I report the first instance of a nano-printed lens for hard x-ray microscopy, and evaluate its imaging performance. The lens consists of a spherically focusing compound refractive lens designed for 22 keV photon energy, with a tightly packed structure to provide a short total length of 1.8 mm and a focal length of 21.5 mm. The resulting lens technology was found to enable benchtop microscopy at 74x magnification and 1.1 µm pixel resolution. It was used to image and evaluate the focal spots of tungsten-anode micro-focus x-ray sources. In the second section of the dissertation, I describe the fabrication of nanometric x-ray gratings that will advance x-ray phase contrast imaging technologies. X-ray phase contrast imaging, unlike conventional x-ray attenuation imaging, measures the phase shift and scattering of the wavefront by the imaged material. High quality x-ray gratings are essential components in benchtop phase contrast imaging systems. Fabrication of such gratings is challenging, especially when the pitch of the gratings shrinks to less than half a micrometer. The fabrication of x-ray gratings typically involves creating a free-standing high-aspect-ratio mold, which is then filled with high atomic number elements. I report the fabrication of silicon grating molds of 400 nm pitch and up to 10 µm height, achieving a structural aspect ratio of 50. The technology of deep silicon etching was based on the Bosch process with a novel and optimized two-layer masking process.