JPS58198841A - Focal point regulation device of stereoscopic scanning type electron microscope - Google Patents

Abstract

PURPOSE:To enable stereosopic observation of a sample with a big solid angle and high resolving-power by regulating a focal point synchronizing with two- dimensional scanning by means of two-dimensionally scanning refracted electron rays on the sample surface. CONSTITUTION:The image observation is performed by two-dimensionally scanning electron rays on a sample by means of a deflector 6 for scanning while being electrically controlled by a signal generator 9 for scanning and the signal generator 10 for refraction synchronized therewith. Synchronizing with said generators 9 and 10 for scanning and refraction, a focal point correction signal generator 11 is made to work for regulating a focal point of electron rays through an auxiliary lens 7. Said correction signal works as a function of the scanning positions (X and Y) while being + or - corresponding to + or - of a refraction angle t. Thereby, a gap of the focal point at the time of measurement can be removed thus to obtain high resolving-power.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for completely correcting defocus that occurs when an image is observed at a large solid angle in a stereoscopic scanning electron microscope.

In conventional stereoscopic scanning electron microscopy, defocusing that occurs when an electron beam is tilted in two specific directions is removed by completely refocusing.

However, as the solid angle increases, a focus shift occurs corresponding to the fixed point even during scanning, but this focus shift cannot be removed. Therefore, high-resolution observation was not possible.

SUMMARY OF THE INVENTION An object of the present invention is to provide a focus correction device that eliminates defocusing depending on the scanning position of an electron beam that occurs during three-dimensional observation of a sample image, and obtains an image with high resolution even at a large solid angle.

The correction device of the present invention is derived from deflection aberration theory. The deflector includes a deflector that tilts an electron beam in two directions for stereoscopic observation, and a scanning deflector for image observation. The former is tilted by an angle t (=t, +ita) with respect to the perpendicular to the sample surface, which is +1. At -1, the scanning deflector scans the electron beam over the sample surface in all two dimensions (any scanning point f c
= x + i y), and the image observation can be done. The amount of defocus (ΔZ) at this time is expressed by the following equation based on aberration theory.

ΔZ”KF IICC+LrCt+xyCt+kr t
t・・・・・・・・・・・・(1) Here, Kr and ky are the field curvature aberration coefficients of the scanning deflector and the tilting deflector for stereoscopic observation, respectively, and is the field curvature aberration coefficient that occurs in the composite motion of both deflectors.

Generally, the first term is sufficiently small compared to the other terms and can be ignored. The third term becomes a constant during electron beam scanning and is removed during normal focusing.

Also, since t only takes +t and -t, if we take all of these properties into consideration, we get ΔZ=±(ax+b)')...
...(2). Here, (x, y) are coordinate points during sample surface scanning, and a, b are coefficients. The sign of shi corresponds to the sign of shi t. The present invention corrects this amount of defocus ΔZk.

Hereinafter, one embodiment of the present invention will be explained with reference to FIG. The electron beam emitted from the object point 2 is narrowly focused onto the sample surface 4 by the King lens 8. This electron beam is guided to the trajectory 3 by the deflector 5.
The optical axis 1 can be tilted at - with respect to the optical axis 1. In this state, the electron beam is scanned two-dimensionally over the sample 4 by the scanning deflector 6, allowing image observation. After this, the same thing is done by tilting + in the direction symmetrical to the optical axis 1 at the same inclination angle as the orbit 3, and this operation is repeated.
This enables stereoscopic viewing. Such an operation is performed by a scanning signal generator 9 and a tilting signal generator 10 synchronized therewith.
Both are electrically controlled. In the present invention, the focus correction signal generator 11 is operated in synchronization with the generators 9 and 10, and the focus of the electron beam is adjusted through the auxiliary lens 7. This correction signal is determined by the scanning position (X) of the electron beam, as shown in equation (2).

y), and the sign of y corresponds to the sign of the slope t. That is, it is sufficient to change the sign of the correction signal amount when tilting in two directions. Although the coefficients a and b in equation (2) are measured in advance, they can be easily determined by scanning with a deflection larger than the actual scanning area.

In the present invention, the slope of shi t is simply shi a.

However, it goes without saying that it is also possible to use different coefficients in addition to changing the sign. A constant term may also be added.

In this case, it acts effectively on an electron optical system that deviates from the ideal system.

−10,000, the electron optical system is not limited to what is shown here,
Any optical system for stereoscopic observation may be used.

According to the present invention, it is possible to easily eliminate defocus that causes a decrease in resolution, and it is possible to perform stereoscopic observation of a sample image with a large solid angle and high resolution.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram showing an embodiment of the present invention. 5... Electron beam tilting deflector, 6... Scanning deflector, 7... Auxiliary lens, 8... King lens, 9... Scanning deflection signal generator, 10... Tilt signal generator, 1
1... Focus correction signal generator. Agent Patent Attorney Toshiyuki Usuda

Claims (1)

[Claims] 1. ``Means for constricting the electron beam generated from the electron beam generation source into a narrow image on the sample surface, and
In an apparatus equipped with a means for tilting the electron beam in the direction of the electron beam and a means for two-dimensionally scanning the tilted electron beam on the sample surface,
A focus adjustment device for a three-dimensional scanning electron microscope, which is characterized by adjusting the focus in synchronization with two-dimensional scanning.