1. Field of the Invention
The present invention relates to a charged particle radiation apparatus or the like, i.e., an apparatus for observing or measuring, for example, the shape or the material of a specimen to be observed by applying an electron beam to the specimen and by utilizing a physical phenomenon such as generation of secondary electrons obtained from the specimen.
2. Background Art
In scanning electron microscopes, which are one of kinds of charged particle radiation apparatus, bending of an electron beam during scanning and a deformation or disturbance in a resulting image occur frequently under the influence of a magnetic field externally generated and flowing in.
According to JP Patent Publication (Kokai) No. 2004-014909A or JP Patent Publication (Kokai) No. 2-278642 A (1995) for example, a technique to solve such a problem by covering the periphery of a scanning electron microscope with a magnetic shield has been proposed. A method of bending a flat sheet of ferromagnetic magnetic material into a simple cylinder and covering the periphery of a scanning electron microscope with the cylinder to limit the influence of a magnetic field has also been practiced.
Also, a method of multiply, for example, doubly or triply wrapping cylindrical shields to improve the magnetic field suppression effect in the case of using the above-described shield or the like has been practiced.
A shield formed as a multiple type for the purpose of improving the magnetic field suppression effect necessitates a correspondingly increased space. In particular, an attempt to further improve the magnetic field suppression effect entails an increase in thickness of the shield surrounding the apparatus, and the practicability of such an attempt is thought to be considerably low from consideration of mounting and cost performance.
A magnetic shield with which a high magnetic field suppression effect is realized in a restricted space and an apparatus using the magnetic shield, particularly a charged particle radiation apparatus are described below.
To achieve the above-described object, an apparatus wherein a shield for shielding against an external magnetic field is formed of a plurality of plate portions made of a magnetic material, the plate portions being disposed on the circumference of a circle whose center corresponds to a center of the space so that each plate portion has a surface direction set different from a line tangent to the circle, is proposed.
In one form of the apparatus, the outer periphery of the tubular shield is covered with a ferromagnetic material plate bent into a triangular wave form. The new shield in triangular form is characterized by having a slanting surface having a mount angle not equal to 0° from the direction of line tangent to the tubular shielding surface, as shown in
The above-described shield is capable of reducing the influence of an externally generated magnetic field on an apparatus such as a scanning electron microscope.
One form of a magnetic shield will be described with reference to the drawings.
In the example shown in
In the example shown in
With the arrangement in which the plurality of plate portions having surfaces parallel to the optical axis of an electron beam (the ideal optical axis when the electron beam is not deflected) are disposed along the cylindrical body (the column of the electron microscope and the shield surrounding the column) and one of opposite sides of each plate portion is set away from the optical axis relative to the other side, the magnetic field suppression effect can be improved on a straight line connecting the locus of passage of the electron beam and the external magnetic field generation source. One of the effects of the arrangement is that the shield W has a thickness of W=(W1/cos θ) (W1 is the thickness in the direction perpendicular to the surface direction of the shield W). That is, the shield can be made substantially thicker in comparison with the case where a shielding member in plate form is simply disposed around the column. Another of the effects is that magnetic lines to be suppressed are prevented from entering perpendicularly to the surface of the shield W.
Also, since the shielding member itself is provided in a monocoque construction by bending a magnetic member into a triangular shape, the shield can be easily installed with an appropriate space between the shield W and the shield C2 being provided, only by wrapping the member around a cylindrical shielding member without requiring any special supporting member or the like.
A structure such as that of the shield W is particularly effective in application to an electron beam barrel which is made cylindrical in principle. For example, if a triangular shielding member is applied to shielding around a body in box form such as a cubic or rectangular body, the value of W1 varies largely depending on the position of an external magnetic field generation source. In some region, the thickness of the shield W is substantially the same as W1 as seen from the center of the body in box form. In such a case, therefore, the desired magnetic field suppression effect cannot be expected, in contrast with the case where a plate member is omnidirectionally applied to a cylindrical body. In the case of application to a cylindrical body, the effect of suppressing magnetic fields in all directions with stability can be expected. Further, since the shield W is of such a construction that it projects from the shield C2, the effect of suppressing reaching of magnetic fields to the column can be improved.
In the present embodiment, a plate member having the shape of a right-angled triangle as seen along the direction irradiation of the electron beam is adopted. However, such a plate member is not exclusively used. For example, a plate member having the shape of an equilateral triangle may alternatively be used.
The shielding member may be constructed so as to cover the entire column or may be selectively applied to a portion where the influence of an external magnetic field is particularly considerable.
When a magnetic field generated on the outside changes with time on the periphery of the objective lens 6 and the specimen 7, the position at which the primary electron beam 2 reaches the specimen 7 surface is thereby influenced to change along the Y-direction according to the wave of the change of the magnetic field. When this change is in synchronization with the scanning signal for example, a first scanning line 17a, a subsequent scanning line 17b and another subsequent scanning line 17c undulate in conformity with substantially the same waveform. In a resulting SEM image, a line pattern to be seen in straight form undulates in wave form, as shown in
In actuality, in many scanning electron microscopes, the scanning signal is synchronized with the period of an alternating-current power supply and is synchronized with power supply noise generated in a different unit operated by the same power supply on the periphery of the microscope. In such a case, the microscope is easily influenced by the power supply noise.
While the present embodiment is described as a scanning electron microscope in one form of a charged particle radiation apparatus by way of example, the present invention is not limited to the described microscope. For example, the present invention can be applied to other charged particle radiation apparatuses including focused ion beam (FIB) apparatuses. However, the electron beam is more susceptible to a magnetic field than the ion beam used in FIB apparatuses. For this characteristic reason, the present invention can be applied to electron microscope with high technical advantages.
Number | Date | Country | Kind |
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2008-020041 | Jan 2008 | JP | national |