Observation of zeolite surface by electron energy selection in SEM deceleration method

抄録

Nanoporous materials such as zeolites and nanostructured composite materials are becoming more important in the field of catalysis. The understanding of the surface structure characteristics and the precise analysis of the local arrangement of elements in these material systems are indispensable for the deep understanding of the structure performance relationships, which depends on the availability of sophisticated analysis techniques. However, the zeolites are usually insulating and electron beam sensitive material. Therefore, it is one of the difficult samples to observe fine details of the surface.

In order to solve these problems and obtain surface information that governs the properties, the development of a high-resolution low landing voltage SEM is desirable. On the other hand, in the low landing voltage SEM, the effects of objective lens aberration and chromatic aberration become significant, and the electron probe diameter increases, resulting in a decrease in spatial resolution. For this reason, we have been working to improve the spatial resolution of the SEM under low landing voltage conditions. We have developed an objective lens called the super hybrid lens (SHL): a compound lens consisting of both magnetic and electrostatic lenses. The SHL is capable of producing a small probe size even at low incident voltages (for example 0.7 nm at 1 keV) and moreover, beam deceleration mode allows imaging down to 10 eV. However, it was difficult to reduce electron charge-up effect even under low voltage conditions at high magnified imaging. Therefore, we have developed an electron energy selectable detector which is installed in the electrostatic lens of SHL, the so-called upper hybrid detector (UHD). The UHD enables the reduction of the electron charge-up effect during high resolution observation, owing to detecting higher energy electrons that are expected to reduce the electron charge-up. Figure 1 shows high magnification images of Zeolite ZSM-5. The image of UHD clearly shows fine steps on crystal surfaces, while a conventional detector so-called upper electron detector (UED) image shows an uncleared surface owing to detecting secondary electrons below 70 eV even at landing energy of 500 eV.

In this report, we will introduce a recent low landing voltage SEM for observing the fine structures of materials such as zeolites.