JPS5913141B2 - Objective lenses for scanning electron microscopes, etc. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an objective lens magnetic pole piece for a scanning electron microscope, an X-ray microanalyzer, etc., which can have an extremely small spherical aberration coefficient.

In order to improve the resolution of a scanning electron microscope or the like, it is necessary to reduce various aberration coefficients of the objective lens, especially the spherical aberration coefficient.

FIG. 1 is a schematic diagram showing a cross section of an objective lens of a conventionally widely used scanning electron microscope.

In the figure, 1 indicates a lens yoke in which an excitation coil 2 is wound, and 3 and 4 indicate upper and lower magnetic pole pieces, respectively.

An electron beam 5 passes through holes drilled in the upper and lower magnetic pole pieces 3 and 4, and a magnetic field symmetrical with respect to the optical axis 6 is formed between the upper and lower magnetic pole pieces.

7a and 7b are deflection coils for scanning the electron beam on the sample 8, and 9 is a deflection coil for scanning the electron beam on the sample 8;
Detection means for detecting secondary electrons and reflected electrons is shown, the output of which is supplied to a cathode ray tube (not shown).

A broken line 10 indicates the X-rays generated from the sample 8, and the X-rays enter an X-ray spectrometer (not shown) at an extraction angle β and are separated into spectra.

Zo indicates the distance between the main surface of the objective lens and the sample surface, and this distance is approximately equal to the focal length of the objective lens.

FIG. 2 shows the measured value of the spherical aberration coefficient Cs in such a lens.

In the figure, the horizontal axes are the upper and lower magnetic pole piece diameters D2. The top obtained when changing the D10 dimension.

It shows the half width d of the lens magnetic field formed between the lower magnetic pole pieces 3 and 4, and is the result of actually measuring the spherical aberration coefficient C8 using Zo as a parameter.

From this figure, the spherical aberration coefficient Cs is the half width d of the lens magnetic field and the distance Z between the lens principal surface and the sample.

It can be seen that it depends on both.

Z from Figure 2.

The method of reducing the spherical aberration coefficient by reducing the spherical aberration coefficient generally requires shortening the work distance (the distance between the sample and the magnetic pole piece 4), which limits the range in which the sample can be tilted and the size of the sample.

The sample position indicated by 8' in FIG. 1 is very close to the lower pole piece 4.

Furthermore, when the work distance is shortened, the X-ray extraction angle becomes smaller from β to β', which has the disadvantage that correction calculations in quantitative measurement of sample elements by X-ray spectroscopy become inaccurate.

On the other hand, one way to increase the half-width of the lens magnetic field is to increase the hole diameter of the lower magnetic pole piece, but the lens magnetic field extends downward through this hole, which has a negative effect on the sample and the electron beam detector (for example, when observing magnetic materials. Secondary electrons from the sample are trapped in the objective lens.

), it is necessary to make the hole diameter of the lower pole piece as small as possible.

Therefore, conventionally, as shown in Fig. 3, the hole diameter D1 of the lower magnetic pole piece 3 is made small and the hole diameter D2 of the upper magnetic pole piece 3 is made large, and the distance S between both magnetic pole pieces is increased, and the relational expression D2>2・Dl is I try to fulfill it.

Furthermore, FIG. 4 shows the magnetic field distribution along the optical axis 6 in the apparatus of FIG.

In Fig. 4, the axial magnetic field distribution when the pole piece spacing S' is relatively small is represented by a, and the magnetic field distribution when the magnetic pole piece is moved to the broken line position and the magnetic pole spacing S〃 is increased is represented by b. and the half-width of these magnetic field distributions is d 1 * d 2
Indicated by

As shown in FIG. 5, the value of the half width d can be increased by increasing the distance between the upper and lower magnetic pole pieces.

However, as the gap S is increased, the magnetic field distribution extends to above the upper magnetic pole piece 3, as shown by the curved line in FIG. 4, and mutual interference occurs between the objective lens magnetic field and the deflection magnetic field due to the deflection collar a.

This results in distortions, especially in scanned images at low magnifications.

Furthermore, since the distribution curve is basically bell-shaped, the main surface of the objective lens moves toward the upper magnetic pole piece 3 regardless of the magnetic field strength.

At the same time, Z is approximately equal to the focal length f of the objective lens.

becomes longer, and the chromatic aberration coefficient of the objective lens, which is approximately proportional to the focal length, also increases.

Therefore, the influence of not only spherical aberration due to the objective lens but also chromatic aberration cannot be ignored, and the beam diameter on the sample also becomes larger.

Therefore, for these reasons, there is a limit to increasing the distance S between the upper and lower magnetic pole pieces.

In order to solve the above-mentioned problem, the present inventor prototyped an objective lens in which the spherical aberration coefficient was reduced while keeping the chromatic aberration coefficient low by generating the objective lens magnetic field not by a single magnetic pole gap but by two magnetic pole gaps. did.

FIG. 6 shows a cross-sectional view of the main part of an example of such an objective lens, and compared to FIG. A major feature is that the magnetic pole pieces 11 are installed.

These pole pieces are attached to the yoke 1 by spacers 12.13 and coupling members 14.15 made of non-magnetic material.

The astigmatism correction coil 16 is incorporated inside the auxiliary pole piece 11 .

Since the coil 16 is supplied with a direct current, there is no mutual interference between the objective lens magnetic field and the astigmatism correction magnetic field.

The inner diameter D3 of the auxiliary magnetic pole piece 11 is approximately equal to the inner diameter D2 of the upper magnetic pole piece 3 and approximately twice or more than the inner diameter D1 of the lower magnetic pole piece 4.

Therefore, the following equation (1) holds true.

By reducing the value of Dl in this way, it is possible to prevent the objective lens magnetic field from reaching the vicinity of the sample.

FIG. 7 is a diagram showing the magnetic field distribution on the optical axis in the apparatus of FIG. 6.

In the figure, the curve C represents the magnetic field distribution generated in the gap S1, and the curve e represents the magnetic field distribution generated in the gap S2.Since the half-value width d4 of the curve g is larger than the half-value width d3 of the curve C, the gap S1 and 82
size and top.

A large half-width can be obtained by appropriately selecting the distance t between the bottom pole pieces.

FIG. 8 shows the results of an experiment in which the relationship between the distance t and the distance d in the half l chain was determined using the value of lL as a parameter under the conditions of DI=10 mm and D2=20 mm.

In order to compare this experimental result with the experimental result shown in FIG. 5, the curve shown in FIG. 5 is shown as a dashed curve in FIG.

Hence from the comparison of these curves! , ) When the following equation is satisfied in the area of 15 mm, it can be seen that the half-width of the magnetic field due to multiple gaps is larger than the half-width of the magnetic field due to a single gap.

s2 s2 し机乍6・-;6E 1.5 or moreExample 9Bare 7: O curve l In the line /! , = 20mm, the value of the half width is m = 1.
It can be seen that the value is smaller than the value of the curve 51.

Furthermore, as S2 increases, the axial magnetic field reaches the state of ``-=>1.5'' and extends through the upper pole piece to the deflection coil, causing the aforementioned undesirable mutual interference, and furthermore, the axial magnetic field As shown in FIG. 9, two clearly separated peaks P1 and P2 appear in the distribution.

Regarding such a lens magnetic field distribution, the relationship shown in Figure 2 no longer holds, and as the half-width d increases, the spherical aberration coefficient CS also increases, and the principal surface of the objective lens also moves upward. Therefore, the focal length of the objective lens also increases, and the chromatic aberration coefficient of the objective lens also increases.

Therefore, for the above reasons, the value of Ei must meet the following conditions:
−jゎIt's so big, so big.

8. Under the various constraints mentioned above required for the objective lens of a scanning electron microscope, the present inventor proposed a method for further reducing the spherical aberration coefficient by using a single composite magnetic field obtained by three or more magnetic pole gaps. We tried using this as an objective lens and found that when the gap between each magnetic pole satisfies certain conditions, an objective lens with better performance than conventional ones can be obtained.

FIG. 10 shows the main part of an objective lens having three magnetic pole gaps that satisfy the conditions described later in the present invention, in which two auxiliary magnetic pole pieces 19.20 are spacers 13.2 made of non-magnetic material.
1 and a coupling member 15.22 to the upper pole piece 3, and is configured to obtain a single combined magnetic field through the three pole gaps.

FIG. 11 is a schematic diagram showing the axial magnetic field distribution in the objective lens of FIG. 10, and the curve j in the figure shows the combined objective lens magnetic field intensity distribution.

It is necessary to avoid a large upward movement of the position of the principal surface of the objective lens magnetic field for the reasons mentioned above, and the following equation (4), which corresponds to the above-mentioned equation (3), is derived as a condition for this purpose.

Here, G1 represents the sum of the distances between the magnetic pole gaps existing in the lower half region between the upper and lower magnetic pole pieces, and G2 represents the total sum of the distances between the magnetic pole gaps existing in the upper half region between the upper and lower magnetic pole pieces.

In the device shown in Fig. 11, G is S1+S2, G22S
3, but the above (4) also applies when there are four or more magnetic pole gaps.
It is necessary to satisfy the condition of the formula, and it was confirmed that the main surface of the objective lens is located below the center between the upper and lower magnetic pole pieces.

However, if the gap S2 in FIG. 11 is filled with a magnetic material and a single auxiliary magnetic pole piece is used between the upper and lower magnetic pole pieces as shown by the broken line 20', the magnetic poles will be separated as shown by the broken line curve. We experimentally determined the conditions to prevent multiple peaks from occurring in the magnetic field distribution when using a magnetic strip.

As a result, in addition to the condition of equation (4) above, the auxiliary magnetic pole gap spacing S 2 # S 3 compared to the main magnetic pole gap spacing S1 at the lowest end satisfies the following (5) and (6). It has been found that it is necessary to satisfy the condition that the size is within a certain range as shown in the formula.

Furthermore, the inventor found that better results could be obtained by making the upper inner diameter of the auxiliary pole piece adjacent to the upper magnetic pole piece 3 larger than the lower inner diameter. An example of the distribution is shown in FIG.

That is, although the device shown in FIG. 12 uses a single auxiliary magnetic pole piece 11, the inner diameter d of the portion close to the magnetic pole side
3□ is larger than the inner diameter D3 of other parts.

Curve 1 shows the on-axis magnetic field distribution with half width d6 obtained in this example device.

The curve indicated by the broken line in the figure shows the inner diameter of the auxiliary pole piece 17 as D.
It shows the axial magnetic field distribution obtained when the value is 3, and it can be seen that the half-width d7 is smaller than the half-width d6.

Incidentally, instead of the auxiliary magnetic pole piece 170, the magnetic pole piece 18 shown in FIG.
Substantially the same effect can be obtained by using a shape in which the inner diameter is partially tapered.

Also, when using multiple auxiliary magnetic pole pieces instead of a single auxiliary magnetic pole piece, by partially changing the inner diameter of the upper magnetic pole piece and the adjacent correction magnetic pole piece, it is possible to obtain a structure similar to that shown in FIG. 12. A similar effect can be obtained.

As explained in detail above, the present invention makes it possible to obtain a high-resolution image by reducing the spherical aberration coefficient of the objective lens used in a scanning electron microscope. It was confirmed through experiments that the amount decreases over time.

This means that complex astigmatism correction operations using an astigmatism correction device become somewhat easier.

[Brief explanation of drawings]

Figure 1 is a cross-sectional diagram showing a conventional objective lens, Figure 2 is a schematic diagram showing the spherical aberration coefficient with respect to the half-width of an axial magnetic field, Figure 3 is a schematic diagram showing the main parts of a conventional lens, and Figure 4 is its axis. A schematic diagram showing the upper magnetic field distribution, FIG. 5 is a diagram showing the relationship between magnetic pole spacing and half width, FIG. 6 is a cross-sectional view showing an objective lens using multiple magnetic pole piece spacing, and FIGS. FIG. 8 is a schematic diagram showing the change in the spherical aberration coefficient of an objective lens having two magnetic pole gaps. FIG. 10 is a cross-sectional diagram showing an embodiment of the present invention. Figure 11 is a schematic diagram showing the axial magnetic field distribution of the lens in Figure 10, Figures 12 and 13.
The figures are schematic diagrams showing other embodiments of the invention, respectively. 1: Objective lens yoke, 2: Excitation coil, 3: Upper magnetic pole piece, 4: Lower magnetic pole piece, 5: Electron beam, 6: Optical axis, 7a, 7b:
Deflection coil, 8.8': Deflection coil, 9: Electron beam detector, 10: X-ray, 11, 17.18° 19.20:
Auxiliary pole piece, 12.13.21 Varnish spacer-115,
22: Connecting member.

Claims (1)

[Claims] 1. An electron lens system for narrowly focusing an electron beam generated from an electron gun onto a sample, and a hole diameter D2 of a magnetic pole piece above an objective lens (on the electron gun side) in the electron lens system. In an apparatus in which the hole diameter D1 of the lower (sample side) magnetic pole piece is larger than the hole diameter D1, a plurality of auxiliary magnetic pole pieces are provided between the upper and lower magnetic pole pieces, and the upper and
The sum G2 of the gaps between adjacent magnetic pole pieces existing in the upper half area between the lower magnetic pole pieces is the upper sum G1, and the sum G1 of the gaps between adjacent magnetic pole pieces existing in the lower half area between the lower magnetic pole pieces. 1
, an objective lens for a scanning electron microscope, etc., characterized by being smaller than 5 times. 2 G1 and G2 are G1-S1+S2 and G2-S
3 such that the distance S1. S2. Claim 1, characterized in that two of the auxiliary magnetic pole pieces are provided at a distance of S3 from each other from below, and are configured to satisfy the following two relational expressions: Objective lenses for scanning electron microscopes, etc. described in . 3. An objective lens for a scanning electron microscope or the like as set forth in claim 1, wherein the hole diameter of the auxiliary magnetic pole piece adjacent to the upper magnetic pole piece is larger than the hole diameter of the upper magnetic pole piece.