Practical Electron Microscopy and Database

In a Focused Ion Beam (FIB) system, the convergence semi-angle refers to the half-angle at which the ion beam narrows as it approaches the focal point on the sample surface. [1-5] This angle plays a critical role in determining the beam’s resolution, depth of focus, and milling characteristics. For conventional gallium-based FIB systems, the convergence semi-angle typically ranges from approximately 0.1 to 1 degree, depending on various factors such as beam current, lens configuration, aperture size, and working distance. A smaller convergence angle is generally associated with higher spatial resolution and improved depth of focus, making it suitable for high-precision imaging, nanofabrication and TEM sample preparation tasks. In contrast, larger convergence angles are employed at higher beam currents to achieve faster material removal, though this comes at the cost of reduced resolution. The specific convergence semi-angle can vary with the system design and operational settings, and it is often detailed in the manufacturer’s technical documentation for each configuration.

For Thermo Fisher Scientific (formerly FEI) FIB systems such as the Helios and Scios DualBeam platforms, the convergence semi-angle also plays a critical role in determining beam resolution, depth of focus, and milling efficiency. The convergence semi-angle in these systems is not fixed but varies depending on the selected ion beam current, acceleration voltage, and aperture size. Under low beam current conditions, which are typically used for high-resolution imaging and fine milling, the convergence semi-angle is approximately 0.1° to 0.3°. Conversely, at higher beam currents—often employed for rapid material removal—the semi-angle can increase to around 0.5° to 1.0° or more. This tunability is achieved by adjusting the ion column optics and changing the beam-limiting apertures, allowing users to trade off between resolution and throughput depending on the application. While specific values are not always disclosed in standard user manuals, these ranges are consistent with the known optical behavior of Thermo Fisher's FIB columns and are supported by empirical performance data and modeling tools. These characteristics make the Helios and similar systems highly versatile for both analytical and sample preparation tasks in materials science and semiconductor failure analysis.

[1] Giannuzzi, L. A., & Stevie, F. A. (2005). Introduction to focused ion beams: Instrumentation, theory, techniques and practice. Springer.
[2] Orloff, J., Utlaut, M., & Swanson, L. (2003). High resolution focused ion beams: FIB and its applications. Springer.
[3] Langford, R. M., & Petford-Long, A. K. (2001). Preparation of transmission electron microscopy cross-section specimens using focused ion beam milling. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 19(4), 2186–2193. https://doi.org/10.1116/1.1387082.
[4] Nieminen, J., & Konno, T. J. (2004). Effect of beam convergence angle on FIB-induced damage. Microscopy and Microanalysis, 10(S02), 150–151. https://doi.org/10.1017/S1431927604881252.
[5] Thermo Fisher Scientific. (n.d.). Helios G4 DualBeam User Guide [Application note]. Thermo Fisher Scientific. (Access via internal documentation or company website.)