Cryo focussed ion beam secondary ion mass spectrometry analysis of Galinstan

Abstract text (incl. figure legends and references)

Introduction

The combination of a focused ion beam and scanning electron microscope (FIB/SEM) with a time of flight secondary ion mass spectrometer (ToF-SIMS) [1] offers new possibilities for materials samples investigation. The combination of different information, such as highly sensitive 3D chemical mapping, in-situ sample preparation and high resolution imaging yield valuable insights into the samples. When adding cryo sample temperatures by using a cryo stage a number of experiments are possible for e.g. liquid metals, such as Galinstan, a Ga, In, Sn alloy.

Galinstan can be printed into 3D shapes at room temperature [2]. As it solidifies only at about -19°C [2], it is the general expectation that printed structures are stabilized by a thin galliumoxide layer, which forms around the alloy by passivation. As Galinstan is liquid at room temperature, it is generally difficult to investigate the oxide layer with conventional spectroscopic methods.

Objectives

In our study we want to detect the stabilizing oxide layer with ToF-SIMS facilitated by the Gallium ion beam in a FIB/SEM. To prevent material mixing within the liquid metal we solidified the printed Galinstan by cooling it down on a cryo stage. This allows also the study of the alloy demixing of Galinstan when cooled down, which can have an influence of Galinstan"s electrical properties at low temperatures.

Materials & Methods

Printed Galinstan lines are cooled down below the solidification temperature in-situ. Afterwards ToF-SIMS analyses of the sample surface can be conducted (Figure 1a). For the cross section analyses, the Galinstan is cut with the FIB and then tilted to an angle of 54° (Figure 1d). Either negative ions (GaO) or positive ions (Ga, In, Sn) can be detected by ToF-SIMS.

Results

We can visualize the enclosing galliumoxide layer in two different detection geometries: First, on the surface of the lines (Figure 1b,c) and second, along the line surface adjacent to a cross section (Figure 1e,f). As additional finding we see that the alloy separates into a Ga and In-Sn phase at temperatures lower than -25°C (Figure 2). However, we found a number of experimental sample and instrument parameters influencing the secondary ion signal, such as sample geometry, the presence of oxygen (matrix effect [3]) and the charging of the sample. Moreover, water freezing out on the sample surface during the cooling process can strongly influence the ion signal.

Conclusion

In our study we showed a first application of our cryo FIBSIMS instrumental combination yielding valuable results for the investigation of Galinstan. It was possible to visualize the galliumoxide layer and phase separation of the cooled down alloy. The secondary ion detection shows strong influence of the detection geometry and the investigated sample. In future, we want to expand this novel analysis tool to investigate fully printed, flexible electronics. This might yield valuable information for both the design of new devices and their quality control.

Acknowledgements: The author thanks Navid Hussain, Jasmin Aghassi-Hagmann, and Michael Hirtz (Karlsruher Institut für Technologie) for the samples. Research funded by DFG via the Excellence Cluster "3D Matter Made to Order" (EXC-2082/1-390761711)

[1] Pillatsch et al., Progr. Cryst. Growth Charact. Mater., 65(1):1–19, 2019

[2] Hussain et al., Adv. Mater. Technol., 29:2100650, 2021

[3] Priebe et al., J. Anal. Atom. Spectrom., 35(6):1156–1166, 2020