Susumu Maruse

Theoretical considerations on electron optical brightness for thermionic, field and T-F emissions

Abstract General theoretical considerations are made on electron optical brightness. The relation between the electron optical brightness and the energy distribution function of electrons emitted from the cathode surface is studied. A general expression, which is valid in various electron emission regions ranging from thermionic to field emission, is derived for the electron optical brightness for a general energy distribution at the emitting cathode. The result is then specialized to the case of the axial brightness, that is, the brightness on the axis in the axial direction. The axial brightness is found to be related to the tangential energy distribution function at zero tangential energy. The axial brightness is numerically evaluated over a wide range of the cathode temperature and field strength as well as work function, including the thermionic and field emission cases. The simplified analytical expressions of the axial brightness both for Schottky and field emission are also given together with the limits of their applicabilities. The useful numerical data for emission parameters necessary to evaluate the axial brightness are given. The theoretical values of the axial brightness are compared with the experimental ones in the case of the field emission gun.

High Resolution and Spectroscopic Cathodoluminescent Images in Scanning Electron Microscope

It was shown that the radiation collection system collecting the transmitted radiation through the specimen from the back side (i.e., the side opposite the surface struck by the electron beam) was effective for the formation of cathodoluminescent (CL) images in SEM. This system provided an image resolution of the order of the electron beam spot size (~500 A) for a CaWO4 crystal at an accelerating voltage of 20 kV. This collection system was also especially useful for the formation of spectroscopic CL images with a monochromator. The spectroscopic images of the mixed powder specimen of CaWO4 and ZnS were obtained separately in consequence of differences in their characteristic spectral components.

Correction of the spherical aberration of the CTEM objective lens using a foil lens

Abstract A foil lens has been applied to the objective lens of a conventional transmission electron microscope and its characteristics in correcting the spherical aberration have been investigated experimentally. Special polepieces of the objective lens were designed so that the foil lens can easily be inserted into them. The spherical aberration of a compound lens consisting of the objective lens and the foil lens was measured from the displacement of the {200} dark field image of a magnesium oxide crystal from its bright field image for various voltages of the foil lens. The experiment has shown that the spherical aberration of the objective lens, being excited at NI √V = 19.6 A V −1 2 at 100 kV, is reduced and becomes negative as the foil lens voltage is increased. The experimental result is in good agreement with calculations involving the third- and fifth-order spherical aberrations.

Relativistic Considerations on Electron Optical Brightness

On the basis of Liouvilles theorem, the relativistically corrected expressions are derived for the differential brightness in the orthogonal curvilinear coordinate system, both in terms of the total and normal momentum representations and of the total and normal velocity representations. The result is specialized to the case of the axial brightness for thermionic and field emission. It is shown that the relativistically corrected axial brightness is given by R*=j0e\varPhi*/πkT for thermionic emission, and is given by R*=j0e\varPhi*/πd for field emission, where \varPhi*=\varPhi(1+e\varPhi/2m0c2) is the relativistically corrected accelerating voltage. It is also demonstrated that the differential brightness is a fundamental quantity characterizing the quality of the electron gun.

The Ion-Induced Emission Electron Microscope and an Image Contrast Due to Specimen Contamination

The ion-induced emission electron microscope is constructed, for which an ion source making use of high frequency gas discharge is developed. This ion source – compared with the conventional one of d.c. discharge–affords a stable, and finely focused ion beam with a high density (3 mA/cm2), besides facilitates regulation of the extracted ion current. In this emission electron microscope an improved image contrast due to selective contamination is observed on a carbon steel surface. The contrast is attributed to the rate of contamination growth on the specimen surface at its early stage. The rate is found to depend on the kind of bombarding ions and the crystallographical properties of the surface.

Measurement of axial geometrical aberrations of the probe-forming lens by means of the shadow image of fine particles

Abstract A method using a shadow image of fine particles, proposed for the stigmation and the measurement of spherical aberration of the probe-forming lens, was further developed for the accurate measurement of spherical aberration and of non-rotationally symmetrical axial aberrations. From the radii of the rings of infinite magnification in a pair of shadow images, the coefficient of two- or three-fold astigmatism or axial coma can be measured together with the spherical aberration coefficient. A theoretical evaluation and an experiment carried out with a probe-forming lens of 1.5 nm resolution showed that the error of measurement is within several percent for the spherical aberration coefficient and is less than 30 nm for the coefficient of two-fold astigmatism. The three-fold astigmatism remaining after the correction of spherical aberration was also measured and was found to be of considerable importance in limiting the performance of the spherically corrected lens.

The Mean Brightness of the Electron Microscope Gun

As the brightness obtained in the measurement is always the mean brightness averaged over a certain aperture angle and a certain area, it is not equal to the Langmuirs theoretical value except for a special operating condition of the electron gun and in general becomes lower than the theoretical value. The amount of decrease of the measured brightness depends on the sizes of the apertures used in the measurement and the electron optical characteristics of the electron gun. When the bias voltage is much reduced, the electron gun may be considered approximately as the plane, spherical or cylindrical diode system. From this point of view, the expressions for the mean brightness in these diode systems are derived both for thermionic and field emissions. It is also shown that these results will be able to apply to the calculation of the mean brightness of the triode electron gun.

Rotatable Magnetic Anisotropy in Electron-Microscope Specimens of Permalloy

The rotatable anisotropy observed in electron microscope specimens of Permalloy is investigated by Lorentz microscopy and torque method. The rotatable anisotropy of this kind is attributed to the thinness of the substrate supporting the Permalloy film, and it disappears when the thick (≥1,100A) SiO films are used as substrates. Permalloy films evaporated on such thick SiO films can be observed by a high voltage electron microscope and thus it is possible to infer the properties of the films evaporated on glass from those of the electron microscope films.