| Résumé | Contamination is a long standing problem in electron microscopy [1,2,3]. It can arise from poor vacuum, from sample handling and storage, or from the nature of the sample nature. We are systematically investigating candidate sources of contamination using a common TEM substrate: 10-nm thick Si₃N₄ membrane, with typical storage methods and practices. We use a dry-pumped Hitachi H 9500 ETEM whose base pressure is 1.4x10⁻⁵ Pa without liquid nitrogen (LN₂). The measured residual gas is composed mainly of water vapor (8x10⁻⁷ Pa) and N2 (about 3x10⁻⁷ Pa), both decreasing to about 2x10⁻⁷ Pa with LN₂. We use electron energy-loss spectroscopy (EELS), energy-filtering thickness maps and bright-field TEM imaging in the H9500 to characterize the contamination build up, and SEM imaging in Hitachi S 5500 to image the surface morphology of the contamination dots. The Si₃N₄ membranes were examined in the TEM as received, after cleaning using a ZONE cleaner [4] with LN₂ trap of the microscope, either cooled (ZONE-LN₂) or at room temperature (ZONE-RT), or without LN₂ in the cold trap and after heating the Si₃N₄ membrane on a hot plate at 50 ºC (Hot-50C) prior to inserting in the microscope. A new Si₃N₄ membrane was used for each experiment and the variation of the contamination rate was evaluated for 5-min increments of the cleaning process. The membrane was irradiated by a probe of 37 nm FWHM with 0.83 nA total current for 300 seconds, resulting in a 250 C/cm² irradiation dose. Five areas were measured for each condition and a thickness map and a bright-field image was then collected from the irradiated area using a broad illumination. For each condition, the experiment was performed on five locations of the Si₃N₄ membrane and the volume of resulting contamination measured in nm³ by summing over a 134x134 nm² thickness map, subtracting the contribution of the SiN membrane. |
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