Downstream Plasma Technology for Cleaning TEM Samples on Carbon Films

Abstract

Introduction With the advent of modern scanning/transmission electron microscopy (S/TEM) capable of higher resolution, better contrast, and faster throughput, it is imperative to ensure the cleanliness of the TEM sample under the ultrahigh vacuum conditions of the microscopes [1–5]. It is well known that sample contamination can severely deteriorate the quality of electron microscopy analysis of materials, especially as the sample regions of interest decrease in size. The adverse effects of sample contamination include obscuring the area of the sample being analyzed by buildup of a carbonaceous layer, interfering with focusing and astigmatism correction, and generating unexpected microanalysis signals [2, 3]. A variety of cleaning methods, including electron beam flooding, heating and/or cooling, ultraviolet light exposure, and plasma cleaning, have been developed to minimize sample contamination [4, 5]. Among them, plasma cleaning is considered the most effective way to prepare samples for electron microscopy. As shown in Figure 1a, a plasma can be described as an ionized gaseous state created by direct current (DC), radio frequency (RF), or microwave glow discharge, in which electrons, ions, and radicals coexist. The interaction of these plasma species with a solid surface causes three basic phenomena that lead to surface cleaning: heating from the electron-specimen interaction, sputtering from the ion-specimen interaction, and etching from the radical-specimen interaction [2]. Although the combination of all three plasma species is efficient in terms of cleaning rates, it can cause irreversible surface modification and undesirable heating. More problematically, as the plasma cleaning process removes the hydrocarbon contamination layer, it removes other carbon structures at the same time. This can be an issue for TEM carbon film users because the carbon film is extensively used as a support on TEM grids for both materials science and biological applications. For example, our laboratory routinely loads from 10 to 50 ex-situ lifted-out TEM samples on a carbon film Cu-grid in order to support the high volume wafer-based manufacturing process [6]. If the carbon support film is damaged before the hydrocarbon contamination layer on the TEM samples is removed, valuable information in TEM samples will be lost. Therefore, it is essential to develop a method that removes only the hydrocarbon contamination layer while preserving the carbon support film. Downstream oxygen plasma is one technology that has been extensively used in modern semiconductor and