FIB-notch: a novel method towards ultra-high throughput TEM sample preparation

Abstract text (incl. figure legends and references)

Transmission electron microscopy (TEM) has become a routinely used technique in a wide range of industries. With an ever-increasing number of analyses performed, lowest time-to-sample is now a central demand on sample preparation. Here, conventional focused ion beam (FIB) preparation routines are facing a fundamental physical limitation: milling is either performed rapidly at high currents or accurately at very low currents [1]. The final thinning and polishing steps thus become a bottleneck for the whole process.

FIB-Notch is a novel, FIB-based preparation approach designed to overcome the dichotomy between accurate and fast milling. It employs a special sample geometry, where a notch is cut into a thick lamella [2]. Upon glancing-angle milling of the sample, enhanced erosion occurs at the elevated notch faces, leading to the formation of a receded, electron-transparent terrace region (Fig. 1). The key advantage of this patented technique is that spatial localization is governed by the notch geometry rather than the ion beam profile, making the erosion process essentially self-aligned. This allows for rapid thinning to be performed at high beam currents in a quasi-broad ion beam setting, speeding up the preparation process significantly (Fig. 2).

In order to control the final thickness of the lamella in the terrace region, the notch geometry needs to be matched to the initial thickness of the lamella. Accurate determination of this parameter is thus required, preferably performed in-situ. A quantitative thickness measurement technique based on electron beam absorbed current (EBAC) [3] was developed to for this purpose.

First experiments performed on a VION plasma FIB and a customized Zeiss Auriga 40 instrument will be presented, where electron-transparent samples could directly be created with nA-range ion beam currents. The physical principles governing the erosion of a notched lamella will be discussed, which were studied analytically and numerically using a two-dimensional, kinetic model of sputter erosion. Finally, an outlook on further potential applications of the technique to tasks such as sample sectioning will be given. Future work will focus on validation of the thickness determination method and automation of the preparation process.

Figure 1. Erosion of a notched lamella (schematic depiction).

Figure 2. SEM micrograph of a TEM lamella prepared with the FIB-Notch technique, using a Xe plasma FIB at 180 nA.

Financial support by the European Union via the Europäischer Fonds für Regionale Entwicklung (EFRE) and the Investitionsbank Sachsen-Anhalt (Germany) through the project FIBNotch under the grant number 2004/00071 is gratefully acknowledged.

[1] Bassim N, Scott K. and Giannuzzi LA, MRS Bulletin 39, 317-325 (2014)

[2] Busch R, Krause M, Coyle S, and Höche T, Micron 107, 35-42 (2018)

[3] Moldovan, G. Microscopy and Microanalysis, 25(S2), 532-533. (2019)