PhD Thesis Defenses

Nanofabrication processes for high-aspect ratio X-ray zone plates


The field of nanofabrication is vast, and there are many methods available to facilitate the fabrication of different types of structures. Nanostructured materials make up critical infrastructure in fields like medical technology, energy storage and production, computer science and more. These can come in many different shapes, including particles, crystallites, patterned structures and more.

A common material for nanostructures of more deterministic natures is silicon. Processing silicon can be done in many ways, and within nanofabrication, methods are sorted as either top-down or bottom-up. Each term describes the process of reaching the desired size of the structure by either breaking down a bulk (top-down) or building up from smaller particles (bottom-up). Through combining different methods of nanofabrication, advanced nanostructures for complicated applications can be made.

One area where such nanostructures are needed is X-ray imaging. Nanostructures make up the optical components responsible for the focusing of the light. For this purpose, there are many options available. One is zone plates, which are circular gratings with radially decreasing features that through diffraction focus the X-rays to a spot. The resolution of a zone plate is determined by the size of the outermost zone. As X-ray imaging is a technique commonly used for imaging of very small samples, such as viruses, cells, particles, or crystals, a zone plate needs to offer the possibility to resolve small features. This leads to the outermost zone of the zone plate needing to be very small, preferably on the nanoscale. For efficiency, a high aspect ratio is desirable. To achieve this, precise nanofabrication is key, and the processes for fabricating these devices are many, complex, and depend on good control and optimization to get the best results.

The purpose of this thesis is to investigate and improve these methods for the fabrication of zone plate nanostructures for X-ray imaging purposes. Among these are electron beam lithography, electroplating, lift-off methods and metal-assisted chemical etching – MACE.

An optimized and controlled electroplating process was evaluated for filling of direct-written zone plate structures in CSAR62, an e-beam lithography resist, with gold in a miniaturized sulphite-based bath. This bottom-up approach yielded free-standing zone plates on silicon nitrate membranes with a height between 400 and 450 nm and an outermost feature size of 40 nm, which constitutes an aspect ratio of 1:10.

MACE is a promising process for the fabrication of zone plate nanostructures and has the capability to achieve high aspect ratios. It has, however proven to be complex in nature and difficult to control fully. In this thesis, the MACE process is explored further in regard to how to facilitate the fabrication of these kinds of structures. This includes the single-layer lift-off process for MACE, which was optimized and evaluated to raise the yield of usable samples for etching. It was found that despite the tiny process window available, cyclic processing was crucial to the success of the method. Furthermore, starting materials were evaluated in regards to N and P-type silicon, and the resulting etched structures were studied. Lastly, an investigation into a lift-off-free process was conducted, where the more conventional evaporation to lift-off method was replaced by an electroplated bi-layer catalyst. This was then etched in vapour phase and subsequently evaluated. The aspect ratio from initial image tests was approximated to 1:50.


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