More update on ion column design

In the last week or so, I carried out some more systematic study on electrostatic lens design. The overall goal of designing a lens is to meet the following criteria:

- achieve minimum spherical and chromatic aberration coefficients
- have the ability to focus with practical high voltage power supplies
- minimize the probability of micro-discharging between neighboring elements
- has to be manufacturable
- accommodate other physical restrictions, as such field of view, delfection, and etc.

The first step is optimizing the lens geometry for minimal aberrations. Basic geometrical factors include the thickness and bore size of each element, and the gaps between elements. To study the effect of one geometrical factor, one needs to systematically vary that factor, then compute the axial potential field distribution from it. Aberration coefficients can then be evaluated from the axial field. Python is a very convenient tool here for "glueing" multiple smaller modelling programs together. For each geometrical factor, I wrote a .py script to automate the parameter variation, and the extraction of the aberration coefficients.

Once a french ion optics researcher gave a talk at Zeiss, on the FIB that he designed. He stressed that it is simply impossible to design a FIB that can do everything, meaning that achieving the best resolution, and at the same time being a material hogging machine. I was not quite sure what he meant at the time. With more hands-on experience on column design, it becomes more obvious to me that some lenses have to be optimized for low current probes, while others have to be tweaked for larger current probes. Often times the two types of optimizations contradict each other, pushing one to an extreme will inadvertently compromise the other. As a result, at the beginning of a column design, one have to set out and ask, what is the end application of the column? Is it for high resolution imaging which usually uses a small probe size and probe current? Or for high-throughput material modification that would require a higher probe current?

I charged to design a condenser lens, which will magnify a source into a cross-over, and an objective lens that demagnify the the cross-over into a probe. The initial assessment of the column built out of the combination of the two lenses is promising, in terms of resolution performance.