Mitsugu Sato

Lattice imaging at an accelerating voltage of 30kV using an in-lens type cold field-emission scanning electron microscope.

We reported investigation of lattice resolution imaging using a Hitachi SU9000 conventional in-lens type cold field emission scanning electron microscope without an aberration corrector at an accelerating voltage of 30kV and discuss the electron optics and optimization of observation conditions for obtaining lattice resolution. It is possible to visualize lattice spacings that are much smaller than the diameter of the incident electron beam through the influence of the superior coherent performance of the cold field emission electron source. The defocus difference between STEM imaging and lattice imaging is found to increase with spherical aberration but it is possible to reduce the spherical aberration by reducing the focal length (f) of the objective lens combined with an experimental sample stage enabling a shorter distance between the objective lens pre-field and the sample. We demonstrate that it is possible to observe the STEM image and crystalline lattice simultaneously. STEM and Fourier transform images are detected for Si{222} lattice fringes and reflection spots, corresponding to 0.157nm. These results reveal the potential and possibility for a measuring technique with excellent precision as a theoretically exact dimension and established the ability to perform high precision measurements of crystal lattices for the structural characterization of semiconductor materials with minimal radiation beam damage.

Snorkel-type conical objective lens with E cross B field for detecting secondary electrons

A new optical system has been developed which employs a snorkel type conical objective lens that allows high resolution imaging at high tilt angles, up to 45 degrees. An E cross B field for detecting secondary electrons is utilized in this optical system in order to avoid influence upon the primary beam from the extraction field generated by the usual scintillator secondary electron detector. Spatial resolution of better than 4 nm at an accelerating voltage of 1 kV has been obtained from a secondary electron image, with a working distance of 3 mm.

Depth of field at high magnifications of scanning electron microscopes

Depth of field (DoF) in scanning electron microscopes is evaluated at high magnifications where the image resolution is limited by the probe size. The calculation of DoF is made in terms of the information passing capacity of an optical system or entropy of the image estimated along the optical axis near focus. Electron diffraction, source brightness (or source size), and signal-to-noise ratio are taken into account in the calculations of the DoF. The DoF at high magnifications is proportional to Rmin2 where Rmin is resolution of the system, and degrades due to the lack of brightness and aberrations. The DoF in semiconductor applications is also estimated.

Beam characteristics for various sizes of annular aperture on scanning electron microscope

Using an analogy between light optics and electron optics, we have calculated beam characteristics such as the beam profile and the optical transfer function for several sizes of annular and circular apertures on a scanning electron microscope (SEM). It has been found that an annular aperture improves the image quality with regard to several kinds of image resolution and the depth of focus at the price of good low-frequency (nu) contrast. In contrast with conventional circular-aperture SEM images, a combination of a low-nu-pass filtered, circular-aperture SEM image with a high-nu-pass filtered, annular-aperture SEM image has the potential to enhance the image quality in terms of both the image resolution and the depth of focus.

A Study of Beam Sensitive Materials Using High Resolution, ULV Scanning Electron Microscopy

Low voltage scanning electron microscopy has become common both for topmost surface imaging and reducing beam damage [1]. Lately, high resolution, ultra-low-voltage (ULV) imaging (less than 500 V) has been realized by beam retarding [2] and/or boosting [3] techniques. In this study, some beam sensitive materials are observed by the Hitachi S-4800, which employs a cold field emission source, snorkel type objective lens and a retarding function [4].

Approximation method of resolution based on information-passing capacity (IPC) of an electron optical system

Spatial resolution of an optical system can be evaluated accurately taking into account of signal-to-noise ratio in terms of the information-passing capacity (IPC) of an optical system. In order to determine resolution of an optical system including effects of aberrations and source size, the IPC is approximated in terms of various analytical functions determined by numerically computed results of the IPC. This approximation method (the IPC method) allows to estimate resolution of an optical system under geometrical condition (wavelength equals 0) and diffraction limited condition (wavelength does not equal 0). The calculated resolution well represents the behavior of the actual SEM images obtained at various beam convergence half-angles using a field emission type in-lens SEM.

New evaluation method for the depth of field in terms of the information-passing capacity

A new evaluation method for the depth of field in a scanning electron microscope (SEM) images in terms of the quality of an optical image is introduced. The depth of field, in our method, is evaluated by calculating the image resolution along the optical axis defined in terms of the information passing capacity (IPC) of an optical system. The IPC corresponds to the mean information content included in an optical image, i.e., the quality of the image, evaluated based on the theory of Linfoot. The depth of field in a high resolution observation evaluated by our method depends on the accelerating voltage of the primary beam and signal- to-noise ratio of the image. The calculated results has agreed well with experiment.