Electron beam oxidation of silicon nitride membranes in liquid phase transmission electron microscopy

Electron beam induced effects in liquid phase transmission electron microscopy (LPTEM) encompass changes introduced to both the liquid sample and the material encapsulating the liquid. Typically, LPTEM beam effect studies have focused on the interaction of the electron beam with the liquid itself, while the interaction with the encapsulating material has often not been considered. Here, the chemical changes of silicon nitride (SiN_x) thin films exposed to water, which are the conditions typically used in commercial LPTEM systems have been studied. Scanning transmission electron microscopy (STEM) was used in combination with electron energy-loss spectroscopy (EELS), from which the chemical and structural changes to the membrane over time were determined. The measurements reveal that the influence of the electron beam at high doses is significant, with SiN_x transforming into silicon oxide (SiO_x), when immersed in water and irradiated by the electron beam. The in situ EELS measurements indicate the release of both nitrogen (N₂) and oxygen (O₂) gas during irradiation, as well as the formation of an extended oxidized area of the membrane. Such changes can be reflected in measurements acquired from the liquid sample, and may in some cases instigate further chemical reactions in the liquid when the sample is subjected to high electron doses. For this reason, it can be necessary to identify the experimental conditions that limit the electron beam induced chemical transformation of SiN_x. Alternatively, for measurements where high electron doses are required, changes to the encapsulating SiN_x by the electron beam will also need to be taken into consideration in the analysis.