A Study of Charging in the Presence of a Gaseous Environment

Charge accumulation in insulating or semiconducting samples due to electron beam irradiation is one of the key problems in electron microscopy. One of the most promising techniques for reducing the severity of such charging is to surround the sample with a low pressure atmosphere of a gas. The charging behavior of a number of materials, surrounded by a variety of gases, has been determined to identify the important factors which control charging under these conditions. The magnitude of the surface potential was deduced from an analysis of X-ray spectra from the surface. The relationship between surface charge, gas pressure, and gas type are measured, and the charging reduction efficiency (CRE) is compared.

In addition, the use of localized gas jets to alleviate charging without causing beam broadening has been investigated. The gas distribution emanating from the pipe is simulated by a molecular dynamics Monte Carlo model. The effect of the pipe shape on gas distribution is studied. A method to obtain a desired gas distribution by optimizing the gas jet arrangement is proposed.

As a part of this thesis, a simple technique which provides a rapid way of visualizing charging phenomena is described and its spatial characteristics is examined. The migration of small particles to form a Lichtenberg image is driven by the surface potentials, which are a direct function of the distribution of high-energy electrons at the surface. The Lichtenberg patterns qualitatively and quantitatively demonstrate the distribution of the surface electric field resulting from the surface charging. The combination of the PPM, nanoparticles, and the Lichtenberg technique might permit high-resolution direct metal imprinting.