A theoretical and experimental study of liquid metal ion sources and their application to focused ion beam technology

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Puretz, Joseph
Oregon Health & Science University
Gallium; Ion beam lithography
thesis / dissertation description
A key parameter of liquid metal ion source (LMIS) operation is the current-voltage (I(V)) relationship. Some issues surrounding this relationship, which is complicated by severe space-charge near the emitter, are examined for a LMIS. A simple technique is introduced which relies on the principle of momentum conservation within a simple diode structure and enables the solution of space charge problems without resorting to the Poisson equation. As an illustration of this approach, the result of Stern-Gossling-Fowler is easily derived. This method is then applied to the problem of modeling the I(V) characteristic of a LMIS. A simple and physically intuitive criterion is derived which demonstrates that the ratio of the viscous drag force on the emitter to the force of the ion beam impinging on the collector determines whether the ion current is dominated by the flow of liquid metal or by the space charge in the beam. This explains, for the first time, how these two factors are related. Another result is that, contrary to currently accepted thinking, it is not the flow impedance per se which influences the ion current but the product of the film thickness and the flow impedance. Since the advent of LMIS, the understanding of their fundamental properties and their application to focused ion beam (FIB) technology has followed parallel developments. One of the goals of the research presented in this dissertation has been to expand the applications of the liquid metal ion source. The gallium LMIS in particular has been found to be the most suitable ion source for many applications. Yet despite its apparent simplicity, the opportunity exists for producing ion emitters which operate poorly. The major difficulties encountered during their fabrication, testing and operation are due primarily to contamination by oxygen, carbon bearing gases and backsputtering of material to the source, particularly from the extraction electrode. The application of this source to FIB lithography and micromachining has been examined. Particularly exciting is the FIB micro machining of semiconductor lasers which has resulted in the fabrication of novel devices, including the first diode lasers fabricated by focused ion beam micromachining.