JPS5945173B2 - scanning electron microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a scanning electron microscope, and in particular to a scanning electron microscope of the type capable of obtaining scanning electron diffraction images in addition to scanning electron microscope images.

A scanning electron microscope scans a sample two-dimensionally with an electron beam focused by an objective lens, detects information signals that characterize the sample such as secondary electrons and reflected electrons obtained from the sample, and uses this signal to Based on this, an image of the scanned area of the sample is displayed on a cathode ray tube.

In such scanning electron microscopes, attempts have been made to obtain a scanning electron diffraction image by scanning an electron beam at an angle around a predetermined position on a sample.

In this scanning electron diffraction mode, the electron beam is generally focused on the focal plane of the objective lens in order to make the electron beam irradiating the sample parallel.

On the other hand, if the objective lens surface is scanned two-dimensionally with an electron beam, the objective lens also works as a deflector (so that the parallel electron beam that irradiates the sample is angularly scanned with one point on the sample as the center). will be done.

If the maximum value of the scanning angle at this time is selected to be thicker than the Bragg angle, and if secondary electrons, reflected electrons, or transmitted electrons obtained from the sample are detected by angle scanning,
Based on this detection signal, a scanning electron diffraction image can be displayed on a cathode ray tube.

However, in such a scanning electron diffraction mode, the scanning angle of the parallel electron beam exiting the objective lens extends to several degrees, so spherical aberration of the objective lens becomes a major problem.

This is because, if the scanning angle of the parallel electron beam is α and the spherical aberration coefficient is Cs, the spherical aberration ds is given by ds''Csα3.

If spherical aberration exists, the electron beam will irradiate a wide area of the sample, making it difficult to obtain a scanning electron diffraction image of a minute area of the sample with high resolution.

For this reason, some means for correcting spherical aberration is required.

Possible ways to do this include passing a correction current directly through the objective lens, or providing a correction lens made of a small coil separately from the objective lens, and passing the correction current through this.

However, since the angular scanning speed is on the order of tens of m5ec, the objective lens cannot fully respond to the high angular scanning speed by directly passing a correction current through the objective lens with large inductance, and therefore it is not possible to sufficiently correct spherical aberration. do not have.

In this respect, the latter approach is more preferred.

As belonging to the latter category, attempts have been made to provide a correction coil on the upper end surface of the objective lens.

However, according to the inventor's experimental research, it was found that a scanning electron diffraction image shows a spiral dist-'/"37 (Spiral
It was found that distortion occurs.

Furthermore, as a result of further investigation into the cause, we found that when the correction lens is installed at the upper end surface of the objective lens, the diameter of the correction lens must become smaller, so the effective magnetic field created by the correction lens is smaller than that of the electron optical axis. It was found that a portion away from this shows a thick (different) non-uniform intensity distribution, which causes a large difference in image rotation between the two portions, which causes the above-mentioned spiral distortion.

From this fact, the larger the diameter of the correction lens, the better.

The space between the objective lens and the sample can be considered as the space in which the large diameter correction lens is placed.

However, if a correction lens is placed in this space, the distance between the objective lens and the sample becomes long (therefore, the focal point of the objective lens becomes long and various aberrations increase, resulting in a decrease in performance in the scanning electron microscope mode).

An object of the present invention is to provide a scanning electron microscope suitable for correcting the spherical aberration of an objective lens in the scanning electron diffraction mode and eliminating spiral distortion in the scanning electron diffraction mode without deteriorating the performance in the scanning electron microscope mode. It is about providing.

A feature of the present invention is that it has an observation function in a scanning electron microscope mode and a scanning electron diffraction mode. In the scanning electron diffraction mode, the lens surface of the objective lens is two-dimensionally scanned with the electron beam, and the electron beam is directed to a predetermined position on the sample. In a scanning electron microscope configured to perform angular scanning around the center and further comprising a correction lens for correcting spherical aberration of the objective lens in the scanning electron diffraction mode, the correction lens is arranged above the objective lens; means for generating X-axis and Y-axis electron beam deflection signals for scanning the lens surface of the objective lens with the electron beam in the scanning electron diffraction mode; A scanning electron microscope characterized by comprising means for applying a signal proportional to the sum of the squares of the deflection signals to the correction lens.

FIG. 1 shows an embodiment of the main part of a scanning electron microscope based on the present invention.

The electron beam emitted from the electron gun 1 is focused on the sample 5 by the first and second converging lenses 2 and 3 and the objective lens 4 in the scanning electron microscope mode.

The electron beam irradiating the sample 5 is two-dimensionally deflected by the two-stage deflection devices 6 and 7 provided inside the objective lens A, and the deflection fulcrum of this deflection is always at the center of the lens surface of the objective lens 4. It is done in such a way that it exists in a certain position.

Therefore, the sample 5 is two-dimensionally scanned by the focused electron beam.

By this scanning, information signals characterizing the sample 5, such as secondary electrons and reflected electrons, are generated from the sample 5, and this information signal is detected by the detector and introduced into the cathode ray tube as a brightness modulation signal.

Furthermore, the two-dimensional scanning of the sample 5 by the electron beam is performed in synchronization with the two-dimensional scanning of the cathode ray tube screen by the electron beam, and therefore, the scanning electron microscope image of the scanned area of the sample 5 is displayed on the screen of the cathode ray tube. is displayed.

A removable objective diaphragm 8 is arranged on the lens surface of the objective lens 4.

This has the function of limiting the aperture angle of the electron beam that irradiates the sample in the scanning electron microscope mode and the function of removing scattered electrons.

Here, limiting the aperture angle of the electron beam that irradiates the sample reduces the influence of various aberrations of the objective lens (and increases the depth of focus) so that the entire scanning area of the sample 5 is focused. means to do so.

In the scanning electron diffraction mode, the objective aperture 8 is removed from the electron optical axis, and the electron beam emitted from the electron gun 1 is
and is converged onto the focal plane of the objective lens 4 by the second converging lenses 2 and 3.

Therefore, the electron beam focused on the focal plane is made parallel by the objective lens 4 and irradiates the sample.

On the other hand, if the operation of the lower deflector 7 is stopped and the upper deflector 6 is used to two-dimensionally scan the lens surface of the objective lens 4 with an electron beam, the objective lens 4 will also function as a deflector. Thus, the electron beam irradiating the sample 5 is angularly scanned with one point on the sample 5 as the center.

Secondary electrons, reflected electrons, or transmitted electrons obtained from the sample 5 are detected by performing angular scanning with the maximum scanning angle wider than the Bragg angle, and this detection signal is used as a cathode ray in the same way as in the scanning electron microscope mode. If the two-dimensional scanning of the lens surface of the objective lens 4 is synchronized with the scanning of the screen of the cathode ray tube while introducing it into the tube as a brightness modulation signal,
A scanning electron diffraction image is displayed on the screen.

In scanning electron diffraction mode, since the scanning angle is large,
The spherical aberration of the objective lens 4 poses a particularly big problem.

In order to correct this spherical aberration, a correction lens 9 is shown in FIG.
-F of the objective lens 4, the lower pole 4a.

4b.

Therefore, spherical aberration is removed by flowing a correction current through this correction lens.

The space between the upper and lower poles 4as4b of the objective lens 4 is generally very large compared to other parts of the space within the objective lens.

The space is particularly large in the diameter direction of the objective lens.

Therefore, when a lens for correcting spherical aberration is placed here, the diameter of the lens can be increased.

When this diameter is large, the magnetic field strength distribution in a plane perpendicular to the electron optical axis, which is generated by this lens, becomes almost uniform.

Therefore, the spiral distortion described above hardly occurs.

Also, since correcting the spherical aberration of the objective lens 4 with the correction lens 9 means correcting the focus of the objective lens 4, it is necessary to arrange the correction lens 9 on the lens surface of the objective lens 4 for this purpose. is the most efficient (meaning that spherical aberration can be corrected).

From this point as well, it can be said that the embodiment of FIG. 1 has significant advantages.

Even if the correction lens 9 is provided between the lower poles 4a and 4b of the objective lens 4 as in the embodiment shown in FIG. 1, there is no need to increase the distance (working distance) between the objective lens 4 and the sample 5. .

Therefore, the objective lens can be operated to obtain a short focus in the scanning electron microscope mode, and thus also without resulting in a performance loss in the scanning electron microscope mode.

In the scanning electron microscope mode, the use of the objective diaphragm 8 is essential for the reasons mentioned above, and it is therefore necessary to ensure that the correction lens 9 does not interfere with the insertion of this objective diaphragm 8.

For this reason, in the embodiment shown in FIG. 1, the correction lens 9 is divided into two lens parts 9a and 9b along the lower direction L, and the objective diaphragm 8 can be inserted between these lens parts.

In order to correct the spherical aberration of the objective lens 40, it is necessary to flow a correction current through the correction lens 9, and this correction current is determined as follows.

The occurrence of spherical aberration ds means that there is a difference in the focal length of the objective lens 4 for the paraxial electron beam and the focal length for the off-axis electron beam, and therefore, these differences,
In other words, if the focus change is Δf, the difference between these and the scanning angle α of the electron beam is Δf ``” d3 / (1-Cs
The relationship (r/f)2 holds true.

Here, r is the off-axis distance of the electron beam on the lens surface of the objective lens 4, and f is the focal length of the objective lens 4.

On the other hand, the focal length f of the objective lens 4 is generally inversely proportional to the square of its excitation current H, but since the excitation current that must be changed to correct spherical aberration is insignificant, approximately Δf = can be considered as 1Δ■.

Considering this, ΔI=CB K2 (r/f
) 2 = 3 r 2, therefore, if a correction current is applied to the correction coil 9 so as to satisfy this equation, the objective lens 40
Spherical aberration can be substantially corrected.

Further, this r2 is given by r22aX2+bY2, where X and Y are the X-axis and Y-axis deflection currents (sawtooth currents) given to the deflection device 6 in the scanning electron diffraction mode.

However, a and b are constants.

FIG. 2 is a principle diagram of an embodiment for generating such a correction signal.

The deflection device 6 has an X beam consisting of a sawtooth wave necessary for two-dimensionally scanning the lens surface of the objective lens 4 with an electron beam.
A deflection current generator 10 is connected that generates axis and Y-axis deflection currents.

The X-axis and Y-axis deflection currents X and Y generated from this
are squared by squared routes 11 and 12, respectively,
Furthermore, they are added in the adder circuit 13 (aX2
+bY2).

The output of this adder circuit is passed through an amplifier 14 to a correction lens 9.
The spherical aberration of the objective lens 4 that occurs during angular scanning is thereby corrected moment by moment.

According to the present invention, the spherical aberration of the objective lens is corrected in the scanning electron diffraction mode, and the spiral distortion in the scanning electron diffraction mode can be removed without deterioration of the performance in the scanning electron microscope mode. An electron microscope is provided.

[Brief explanation of the drawing]

FIG. 1 is a schematic sectional view of the main parts of a scanning electron microscope based on the present invention (one embodiment), and FIG. 2 is the principle of an embodiment of the correction signal generation system to be applied to the correction lens of FIG. 1. It is a figure. 4... Objective lens, 5... Sample, 6 and 1...
Deflection device, 8...Objective aperture, 9...Correction lens, 1
0... Deflection current generator, 11 and 12... Square circuit, 13... Addition circuit.

Claims (1)

[Claims] 1. It has an observation function in a scanning electron microscope mode and a scanning electron diffraction mode, and in the above-mentioned scanning electron microscope mode, an electron beam is focused by an objective lens, and the sample is examined with the focused electron beam. configured to scan in two dimensions,
In the scanning electron diffraction mode, the lens surface of the objective lens is two-dimensionally scanned with the electron beam, and the electron beam is angularly scanned around a predetermined position of the sample; Furthermore, in a scanning electron microscope equipped with a correction lens for correcting spherical aberration of the objective lens in the scanning electron diffraction mode, the correction lens is disposed between the upper and lower poles of the objective lens, and further means for generating X-axis and Y-axis electron beam deflection signals for scanning the lens surface of the objective lens with the electron beam in the scanning electron diffraction mode; means for applying a signal to the correction lens. 2. According to claim 1, the correction lens is divided in a direction along the electron beam so as to allow insertion of an objective lens diaphragm for use in the scanning electron microscope mode. Scanning electron microscope.