This dissertation is a study on the formation of conical tips with nanoscale sharpness on silicon films, as well as various microbumps or nano-sharp structures on metal films, as a result of localized single-pulse laser irradiation. Such conical tips with nanoscale structures are referred to as nanotips in this work. A Q-switched nanosecond-pulse Nd:YAG laser, emitting at its fourth harmonic of 266 nm, was employed. Irradiation of silicon-on-insulator, gold, platinum and other metal films, with thicknesses of several hundred nanometers, was studied in ambient air, low-vacuum or argon atmospheres. Individual circular laser spots, several micrometers in diameter, were used together with spot patterns generated through the use of a mask projection technique.
The laser based formation method studied and developed during this research allows for the fabrication of nano-tips, and microbumps, in a single step, at far less expense when compared to traditional lithography based techniques. It also offers precise control over the location of these structures unlike other laser based techniques. In addition, it eliminates the use of expensive and complex femtosecond or excimer lasers as well as their respective optical systems, which in principle, can be used to fabricate similar structures.
This laser based technique has proven capable of fabricating conical silicon tips in silicon-on-insulator films; where each of these tips is situated in the center of a circular depression with a depth, tens of nanometers below the original surface. The height of the tip apex is several hundred nanometers above the original surface with an estimated radius of curvature of 35-40 nm. Laser irradiation of gold films has produced microbumps with high-aspect-ratio protrusions with have heights of more than 3 µm above the original film surface, while the apex is estimated to have a radius of curvature of 5-10 nm. Irradiation of other metal films has produced various sized microbumps with and without vertical protrusions.
The formation dependencies of the nano-tips and microbumps on laser fluence, spot size and irradiation atmosphere are studied and discussed in detail. The resulting structures were examined through various techniques including atomic force (AFM), scanning electron (SEM) as well as scanning transmission electron microscopy (STEM). Cross-sections of selected tips/bumps were prepared, using a focused ion beam (FIB) technique, to understand the morphologies of these structures. The information obtained from structure characterization and the observed formation dependencies was used to clarify the formation mechanism of these structures.
In an effort to demonstrate the usefulness of these laser-fabricated structures for practical applications, arrays of gold tips were fabricated and tested to determine their field emission properties. The experimentally determined results were comparable to those obtained via numerical finite element modeling (FEM). The tests have shown that these laser fabricated gold tips can be used effectively as field emitting structures at low current levels. Higher emission currents should be achievable through the use of a more stable laser combined with a more uniform pinhole mask, resulting in more uniformly shaped protrusions.