A 200-kV STEM/SEM Produces 1 Å SEM Resolution

Introduction “Second best no more” was the title of an article written by David C. Joy for Nature Materials [1]. The article was for highlighting a breakthrough made by a team formed between Brookhaven National Laboratory (BNL, USA) and Hitachi High Technologies Corporation (HHT, Japan). Sub-angstrom secondary electron (SE) images were obtained on a Hitachi HD-2700, which is a combined scanning electron microscope (SEM) and scanning transmission electron microscope (STEM) operating at a maximum accelerating voltage of 200 kV. For the first time, single atoms and atomic lattices on crystal surfaces are unambiguously presented in SE images. Like any standard 30 kV SEM, the 200 kV HD-2700 STEM/ SEM probes the specimen with an electron beam focused into a tiny spot on a specimen. The electron probe scans across a specimen area, and SE signals emitted from the illuminated specimen area are collected by a high-sensitivity SE detector installed above the specimen position, resulting in images that carry structural information of material surfaces. In terms of the image formation process, the 200 kV SE imaging is similar to the conventional 30 kV SE imaging—the most popular imaging mode for SEM. The difference is the resolution power. The 200 kV HD-2700 delivers sub-angstrom (0.8 Å) SE imaging resolution; whereas, the best achievable resolution on a conventional 30 kV SEM is about an order of magnitude worse. The BNL/ HHT team received a 2010 Microscopy Today Innovation Award because this team, according to the award statement, was the first to observe isolated single atoms with secondary electron imaging, and these observations point to a new mechanism for secondary electron emission. This article describes the high-voltage, high-resolution STEM/SEM and shows its benefits for secondary electron microscopy applications.