© 2020 Elsevier B.V. Amorphous silicon carbonitride (a-SiCN) films were prepared with surface-modified layers of silicone resin and silicone grease, fabricated by low-energy nitrogen ion beam irradiation, for use as diaphragms in an environmental-cell transmission electron microscope (E-TEM). Conditions used during nitrogen ion irradiation for surface modification of silicone polymers were accelerating voltages of 0.8–1.5 kV, ion current densities of 0.70–2.10 μA/cm2, and ion doses of 4.8–8.0 × 1016 ions/cm2. The modified layers, obtained by etching the unmodified layers with hexane, were evaluated by X-ray photoelectron spectroscopy (XPS). The atomic ratio of nitrogen was higher in the modified layer using silicone grease than in the modified layer using silicone resin. The best thinnest and nitriding condition on the silicone grease was obtained through nitrogen ion irradiation using is the case of an accelerating voltage of 0.8 kV, an ion current density of 0.7 μA/cm2, and an ion dose of 4.8 ions/cm2. However, the silicone-grease-based modified layer exhibited much more deflection than the silicone-resin-based modified layer. This is thought to be because the non-modified layer of silicone resin is a tougher material than silicone grease.