Researchers have developed a number of alternatives to the Ga LMIS in recent years. For example, the He gas field ion source (GFIS) and the Ne GFIS are much brighter than the LMIS and can produce smaller spot sizes. Plasma sources are also now available, and while these are not as bright as the LMIS or GFIS, they do have a higher current and employ high sputter-yield, low-contamination heavy ion species such as Xe.

Sources based on ionized laser-cooled atoms are also promising for producing brighter and better beams. Here, neutral atoms are cooled to microkelvin temperatures thanks to momentum transfer from laser light and then ionized via a focused laser beam to create a very high brightness ion beam. This technique can be used to produce beams with an inherently narrow energy spread for laser cooling over 27 ionic species, many of which cannot easily be cooled with alloy LMIS, GFIS or even plasma sources.

LoTIS produces high-performance Cs cold atomic beam

At the moment, there are two types of cold atom ion source. The first is a magneto-optical trap ion source (MOTIS) in which atoms are cooled and collected in a 3D trap made up of a quadrupole magnetic field and three pairs of counter-propagating laser beams incident from three orthogonal directions. The cooled atoms are then ionized by one or more extra lasers and these types of sources have been successfully produced using Cr, Rb and Li. The second consists of an atomic beam of neutral atoms that is cooled to microkelvin temperatures in two transverse directions before being ionized. This approach, which has proven successful with Cs ions, produces a beam with a higher current.

Now, Steele and colleagues have gone a step further and have produced a high-performance Cs cold atomic beam ion source in a so-called LoTIS configuration. The beam can not only be used to mill nanoscale materials at high speed and with high resolution thanks to its higher flux of ions and smaller spot size with a larger sputtering rate per ion, it could also prove very useful for nanoscale secondary ion mass spectrometry (SIMS) applications, say the researchers.

Much smaller focal spot

“The LoTIS is a configuration of laser beams, magnetic fields and electrodes that convert a room-temperature gas (in this case of Cs atoms) into a very cold and dense beam of ions,” explains Steele. “By taking nearly all of the heat away from the atoms first and then photoionizing them, it creates an ion beam that can be focused into a much smaller focal spot than other ion sources.

“Performance-wise, the Cs LoTIS is much brighter (by an order of magnitude) than the gallium liquid metal ion source that is most often used for nanomachining, and many orders of magnitude better than existing Cs+ ion sources,” he adds. “It is a very different approach to achieving high brightness in that it creates ions over a few cubic microns, rather than trying to create a large number of ions from a few cubic nanometre volume, but it has similar phase-space densities (brightness) because the ions are so cold.”

Improving circuit-editing, nanofabrication and nanomachining

The Cs+ LoTIS could replace many existing commercial ion-source technologies, he tells “The fact that it can deliver a 1 nm spot size means that it could improve circuit-editing, nanofabrication and nanomachining FIB applications. It also induces much lower levels of damage in the materials being edited, fabricated or machined, which might make it the tool of choice for transmission electron microscopy (TEM) sample preparation as well.”

And that is not all: since it can provide beam currents ranging from less than 1 pA to more than 1 nA, it could be used in the same sorts of areas that Ga LMIS is today.

The researchers say that they will continue to optimize their FIB source to achieve even smaller spot focus spot sizes and higher beam energies and will test the performance of LoTIS in many of the applications mentioned above – that is, in circuit editing, nanomachining, sample preparation and SIMS. “We are also looking to collaborate with other researchers as well as industry professionals to overcome the latest problems being encountered in the field of FIBs today.”

The work is detailed in Nano Futures 1 015005.