Applying a rubidium-focused ion beam

A focused ion beam (FIB) is a powerful multipurpose tool for analyzing and processing microscale samples. In a typical FIB setup, ions are accelerated to more than five hundred times the speed of sound before they impact a sample.

Meanwhile, the landing spot of the ions on the sample surface ranges from a few to less than two hundred nanometers. The interaction between the high-speed incoming ions and the sample produces secondary particles, often collected for imaging purposes, and removes some sample material.

With specialized chemicals, a FIB can also be used to grow metal nanowires. Combining the small landing spot, the ion-sample interaction properties, and the nanostructure-making capability allows a FIB to perform tasks such as imaging semiconductor devices or editing circuits in a smartphone processor.

The current state of FIB systems

Gallium (Ga) is currently the most common element used for ion generation in FIB systems. Unfortunately, the Ga-FIB system has reached a limit that the setup cannot focus a meaningful ion current onto a spot smaller than five nanometers.

This limit restricts FIB applications for the ever-shrinking semiconductor devices. In addition, Ga has some undesired properties, such as leaving a high contamination level in the analyzed sample or uneven surfaces in the fabricated structure.

Several types of FIB systems using noble gas elements can circumvent some of the issues associated with the gallium-FIB combination. However, these systems sacrifice the final spot size for the beam current, or vice versa. This dilemma forces FIB users to choose between imaging resolution and process throughput time.

The rubidium-FIB

The Coherence and Quantum Technology group in the Department of Applied Physics and Science Education at TU/e has already developed a prototype FIB system based on laser-cooled rubidium (Rb) atoms.

For his PhD project, Yang Li emphasized the application of the Rb-FIB and identified some differences with the Ga-FIB system.

The most prominent advantage of Rb ions is that they can induce a higher number of secondary electrons than Ga, which should lead to better image qualities.

Another strength of Rb ions is that although they arrive at a sample with a slower speed, they can remove sample material at rates like those of Ga ions. These properties give confidence that the Rb-FIB system, combined with future tool optimization, can be an adequate tool and fill a niche left open by the Ga-FIB.

Title of PhD thesis: Characterization and Application of an Ultracold Rubidium Focused Ion Beam. Supervisors: Edgar Vredenbregt and Peter Mutsaers.