FIB Tailoring of SiNx Membranes for Enhanced-Clarity In Situ Closed-Cell TEM

In situ closed-cell transmission electron microscopy (TEM) enables real-time observation of dynamic processes, offering insight into the evolution of materials under realistic environments. However, the thickness of the silicon nitride membranes, enclosing the sample environment, critically limits image quality and analytical sensitivity. In this work, two focused ion beam (FIB) milling strategies are systematically investigated: (I) to enable precise thinning of membranes, and (II) to perforate, while preserving their functional integrity in both cases. For membrane thinning (I), an optimized set of FIB parameters, together with milling time calibration, is established to achieve reproducible thickness reduction. The milling behavior is found to depend primarily on the ion milling angle and membrane rotation. Milling angles ranging from normal to shallow incidence are examined, with the most uniform membrane morphologies obtained at intermediate angles. In addition, post-milling thermal treatment is shown to effectively mitigate ion implantation effects. For membrane perforation (II), localized membrane openings are fabricated via normal-incidence milling. The area of the openings is optimized to accommodate a range of sample types while maintaining compatibility with closed-cell operation. Based on these findings, FIB workflows are proposed to enhance the clarity and experimental flexibility of in situ closed-cell TEM studies.