Description:
This invention relates to an improvement in an objective lens pole piece for electron microscope and similar devices, and, particularly, provides pole pieces that diminish both spherical and chromatic aberration while allowing sufficient space for specimen manipulation and for the incorporation of various attachments near a specimen. The performance of an electron microscope, particularly with respect to its resolving power, resides in the degree of spherical and chromatic aberration which can be eliminated. To minimize these aberrations in conventional type objective lenses, the gap between the pole pieces and the aperture diameter of these pole pieces are made extremely small. However, by making the gap and the aperture diameter extremely small, the space between the pole pieces is insufficient to meet the requirements for the various attachments which must be inserted therein.
In our copending application, Ser. No. 663,678, referred to above, we disclosed and claimed objective lenses that diminish spherical and chromatic aberrations while providing a large aperture in the upper pole piece. In this specification, we disclose and claim objective lenses that diminish the same aberrations while providing a large gap between the pole pieces. These results were achieved using relatively small amounts of excitation current in the pole pieces.
We have found that both the spherical and chromatic aberrations may be reduced by forming the lower pole piece with a frustoconical shape and by minimizing the diameter of the lower pole face. By so constructing the lower pole piece, the magnetic field of the lens is concentrated immediately at the top of the lower pole piece. The magnetic flux at the face of the lower pole piece increases more rapidly than the flux entering the frustoconical sidewalls of said pole piece, thereby making the field distribution near the top of the lower pole piece similar to the usual symmetrical objective lens pole piece having extremely small gap and aperture diameter. This construction has resulted in minimizing the aberration and focal length utilizing a relatively small excitation current.
According to this invention, a magnetic electron objective lens has upper and lower pole pieces spaced apart by a distance S. The pole pieces have apertures aligned along a common axis. The lower pole piece has a pole face normal to the axis having a diameter D2 which is equal to or greater than the diameter b1 of the aperture of the upper pole piece. The lower pole pieces have sidewalls that have at least one conical taper between 40° and 60° with respect to the axis. The diameter b1 of upper pole piece and the diameter D2 of the lower pole face are equal to or less than the distance S between the pole pieces. It is preferable according to this invention to maintain the diameter b2 of the aperture of the lower pole piece as small as possible and at least smaller than one-half the diameter D2 of the lower pole face. According to a preferred embodiment of the invention, the gap S is between 4 and 20 mm. wide. The lower pole face D2 is preferably within 6 mm. in diameter.
In the accompanying drawings, we have shown preferred embodiments of the invention in which:
FIG. 1 is a vertical section through the pole pieces of an objective lens of our construction;
FIG. 2 is a partial section of the objective lens shown in FIG. 1 including an axial field distribution curve between the upper and lower pole pieces;
FIG. 3 is a partial vertical section through another embodiment of our invention;
FIG. 4 is a partial vertical section through still another embodiment of our invention; and
FIGS. 5 and 6 are graphs showing the variations in the chromatic aberration coefficient of the asymmetrical objective lens according to this invention in which the frustoconical surface is 60°.
Referring to FIGS. 1, 2 and 3, 1 represents an asymmetrical objective lens pole piece. Upper pole piece 2 is spaced from lower pole piece 3 by means of a nonmagnetic material 4. Pole pieces 2 and 3 have beam passages 7 and 8, respectively, that are coincident with each other. These pole pieces are connected to the end of the objective lens yokes 5 and 6. Lens coil 11, excited by an energizing current, is adapted to excite the pole pieces greater than that required for saturation. The upper pole piece 2 has an aperture diameter b1 equal to or smaller than the diameter D2 of the lower pole face. Conical surface 9 of the lower pole piece 3 has an angle θ with respect to the lens axis 10.
The magnetic field distribution located at the gap between upper pole piece 2 and lower pole piece 3 of the asymmetrical objective lens is shown in FIG. 2. As is apparent from the FIG., the point where the field strength becomes maximum (Bmax) is considerably lower than one-half the distance between pole pieces 2 and 3. In the region of saturation of the lens pole pieces, the maximum field and half width d of the image side field distribution are not only varied by increasing the excitation current of the pole pieces, but are also altered by changing the geometry of the pole pieces, such as lower pole face diameter D2, frustoconical surface angle θ, gap S, and aperture diameters b1 and b2.
As explained in our copending application, Ser. No. 663,678, referred to above, the spherical and chromatic aberration coefficient to magnetic lenses can be considerably reduced by increasing the excitation current J(KA). It is, of course, desirable to minimize excitation current. Further objects and advantages of this invention would be apparent from a study of the following examples.
A magnetic lens 55 according to this invention was provided with an upper pole piece aperture having a diameter b1 of 2 mm. The lower pole piece had a 2mm. pole face diameter D2, a 0.5mm. aperture and a conical sidewall having an angle θ with the axis of 60°. An almost identical lens 56 was provided with a 4mm. lower pole face diameter D2. FIG. 5 illustrates graphically the chromatic aberration coefficient Cf as a function of excitation current J(KA) for the lens described above. The curves for each lens is marked 55 and 56, respectively. FIG. 5 establishes that decreasing the diameter D2 decreases the chromatic aberration. Furthermore, the smallest possible aberration coefficient obtainable is the same or less than the coefficient for a lens having the same lower pole geometry (D2, θ and i b2) as referred to in FIG. 5 in our copending application, Ser. No. 663,678. It will be noted from the curves in FIG. 5 in this application that with a small excitation current the chromatic aberration coefficient can be reduced to less than 1mm.
FIG. 6 illustrates graphically the chromatic aberration coefficient Cf as a function of excitation current J(KA) for three additional lenses 65, 66 and 67. All three lenses were constructed such that the gap between the pole pieces was 10mm., the diameter of the upper aperture b1 was 2mm. and the slope of the sidewalls of the lower pole pieces was 60°. However, the size of the faces and apertures of the lower pole pieces were varied. In lens 65, D2 was 2mm. and b2 was 0.5mm.; in lens 66, D2 was 4mm. and b2 was 1mm.; and in lens 67, D2 was 6mm. and b2 was 1mm. The curves for these lenses are marked 65, 66 and 67. FIGS. 5 and 6 establish that the smallest possible aberration coefficient does not change even when S is changed from 4mm. to 10mm. However, an increase in excitation current is necessary. In other words, the very small aberration coefficient is dependent upon D2 and θ; that is to say, the coefficient becomes small when D2 is small regardless of the size of S. It is also apparent that, in order to decrease the coefficient when S is constant, the diameter D2 should be decreased.
According to this invention, very small aberration coefficients which are obtainable with conventional lenses utilizing very small apertures and gaps, are obtainable by increasing only the excitation current, even if the gaps are large. These same results are also obtainable for other types of aberration coefficients, such as off-axial astigmatism and comma aberration. Furthermore, the eccentricity of the aperture diameter and the inclination of the aperture are not considered, compared with the conventional objective lens, since the shape of the field distribution that affects the electron beam passing through the aperture of the poles is chiefly dependent upon the geometry of the lower pole piece top.
We have, therefore, confirmed from our investigation with our novel objective lens pole pieces that the smallest possible aberration coefficients depend upon parameters D2, b2 and θ. Accordingly, these parameters must be taken into account to reduce or eliminate spherical and chromatic aberrations. That is, in general, the chromatic aberration coefficient diminishes by enlarging the conical surface angle θ and by reducing the ratio of the lower pole face diameter D2 to gap S. With respect to the spherical aberration coefficient, a large surface angle θ decreases the coefficient for larger excitation current. A small angle will reduce the coefficient at small excitation currents. It is undesirable to make the frustoconical surface angle too small, since the curve of the chromatic aberration coefficient rises. Also, if the conical surface angle is made too large, the concentration of the magnetic field near the top of the lower pole piece weakens, resulting in an environment of large currents. Consequently, best results are obtained at frustoconical angles θ between 40° and 60°. When the gap between the poles is made very large, the concentration of the magnetic field of the lower pole becomes weak and the excitation current must be increased. Moreover, we have confirmed that it is preferable to make the aperture of the lower pole piece very small, since this effects the maximum field. This is usually made less than one-half the lower pole face diameter D2.
An alternative embodiment of our invention is shown in FIG. 4. The frustoconical surface of pole piece 61 has two angles, θ1 and θ2, with respect to the lens axis. This cone-shaped lens has a magnetic field distribution concentrated at the top of the lower pole piece which is greater than the concentration for the pole pieces shown in FIGS. 1, 2 and 3. Because of this concentration, a much smaller aberration coefficient can be obtained than in the lens of FIG. 1 from the same amount of excitation current.
By employing the pole pieces of the present invention, both spherical and chromatic aberrations can be reduced without the requirement of large excitation current. Furthermore, diminution of other aberrations such as astigmatism based on the eccentricity of the poles is obtained since the shape of the field distribution depends almost entirely upon the geometry of the lower pole pieces. Moreover, there is very little field disturbance due to the accuracy of the lower pole piece surface, since the field distribution depends largely upon the conical surface magnetic charge.
As described herein above, we have experimented at 100 kv. accelerating voltage, but it may be possible to obtain similar results in the case of different accelerating voltages.
While we have shown and described preferred embodiments of our invention, it is to be understood that it may be otherwise embodied within the scope of the appended claims.