Patent Number: 
Section: description

This invention is described below in the context of multiple representative embodiments. However, it will be understood that the invention is not limited to these embodiments. Also, although these embodiments are described in the context of electron-beam systems, it will be understood that the general principles disclosed herein can be applied readily to charged-particle-beam (CPB) systems in general. FIG. 1 schematically depicts this embodiment of a secondary-electron mapping-projection apparatus. The components shown include an irradiation source 1, a specimen surface 2, a cathode lens 3, an aperture 4, a projection-optical system 5, and an imaging surface 6 (e.g., surface of a detector). An intermediate imaging plane is denoted by xe2x80x9cMxe2x80x9d, and the axis of the projection-optical system 5 is denoted by xe2x80x9cAX.xe2x80x9d The irradiation source 1 comprises an electron gun 1a and an electrostatic illumination-lens system 1b as its main elements. The irradiation source 1 irradiates the specimen surface 2 with an electron beam having a predetermined energy and beam current. Secondary electrons emitted from the specimen surface 2 are accelerated by an electric field formed between the specimen surface 2 and the cathode lens 3. An image of the specimen surface 2, as carried by the secondary electrons, is enlarged several-fold by the cathode lens 3 and formed at the intermediate imaging plane M inside the projection-optical system 5. The aperture 4 serves to minimize aberrations at the intermediate imaging plane M. The image of the specimen surface 2 formed at the intermediate imaging plane M is further enlarged by the projection-optical system 5, and a final image is formed on the imaging surface 6. The principal ray is denoted by the solid line. Further details of the projection-optical system 5 according to this embodiment are shown in FIGS. 2(a)-2(c). FIG. 2(a) depicts the disposition of electrodes along the axis AX; FIGS. 2(b) and 2(c) show equivalent lens systems to certain respective electrodes. The system shown in FIG. 2(a) includes a first front electrode 11, a second front electrode 12, a middle electrode 13, a first rear electrode 14, and a second rear electrode 15. In this embodiment, the first front electrode 11, the middle electrode, and the second rear electrode 15 are electrically grounded. In the FIG. 2(a) configuration, the middle electrode 13 is maintained at a constant position along the axis AX. Control of overall lens power is divided between a xe2x80x9cfrontxe2x80x9d power FP and a xe2x80x9crearxe2x80x9d power RP. The front power FP is controlled by the voltage impressed on the second front electrode 12, while the rear power RP is controlled by the voltage impressed on the first rear electrode 14. The front power RP causes the principal ray to intersect the axis AX near the center of the rear power (FIG. 2(b)); hence, the rear power is used to form the final image of the specimen surface 2 on the imaging surface 6. In addition, the cathode lens 3 is adjustable (with respect to power) so that the intermediate imaging plane M can be positioned at the center of the second front electrode 12, i.e., in the center of the front power. FIG. 2(b) shows exemplary ray traces when a front power is not used, wherein the principal ray is denoted by the solid line. The rays shown in the figure converge and form an image at the intermediate imaging plane M. However, at that location, the principal ray (from the specimen surface 2) is incident with a slight divergence. By employing a lens corresponding to the first rear electrode 14, the principal ray intersects the axis to the rear of the center (principal point or nodal point) of that lens (FIG. 2(b)). Consequently, pincushion distortion is produced as when a conventional Einzel lens alone is used. FIG. 2(c) shows exemplary ray traces when a controlled voltage also is applied to the second front electrode 12 so that the principal ray intersects the optical axis AX at the center of the lens corresponding to the first rear electrode 14. In this case, since the principal ray passes through at nearly the center of the rear power (i.e., paraxially), the onaxis field of the rear power is used mainly. Hence, distortion is nearly entirely corrected, and coma and transverse chromatic aberration also are reduced. This embodiment of the projection-optical system 5 is shown in FIGS. 3(a)-3(b). In this embodiment, a third front electrode 12xe2x80x2 is added (FIG. 3(a)), or a second rear electrode 14xe2x80x2 is added (FIG. 3(b)). Other components of this embodiment are similar to the corresponding components in the FIG. 2(a) embodiment and have the same respective reference numerals. Referring to FIG. 3(a), the respective voltages applied to the second front electrode 12 and the third front electrode 12xe2x80x2 desirably are controlled independently. By doing so, the magnitude of the front power FP and the position of the center of the front power FP can be controlled independently. The axial position of the intermediate imaging plane M can be varied by controlling the cathode lens 3 in FIG. 1 to vary the magnification. The principal ray can be made to intersect the optical axis essentially at the center of the rear power RP by aligning the position of the center of the front power FP with the position of the intermediate imaging plane M, and then adjusting the front power FP in the same way as described above in connection with FIGS. 2(a)-2(c) while maintaining this relationship. Hence, it is possible with this configuration to execute a zooming action that varies the magnification of the image in a continuous manner while minimizing aberrations. In the configuration shown in FIG. 3(b), by independently controlling the voltages impressed on the first rear electrode 14 and the second rear electrode 14xe2x80x2, the magnitude of the rear power RP and the position of the center of the rear power RP can be controlled independently. Hence, it is possible with this configuration to execute a zooming action that varies the magnification of the image in a continuous manner while keeping the position of the intermediate imaging plane M constant. In this case, the front power FP also is changed as the center of the rear power RP is shifted so that the principal ray intersects the axis at essentially the center of the rear power. Thus, zooming action can be realized while keeping aberrations low. A projection-optical system according to this embodiment is shown in FIGS. 4(a)-4(b), wherein FIG. 4(a) depicts the respective dispositions of the constituent electrodes, and FIG. 4(b) depicts the equivalent lens system. Components shown in FIG. 4(a) that are the same as respective components in Representative Embodiment 1 have the same respective reference numerals and are not described further. The FIG. 4(a) configuration includes a quadrupole electrode 16 (instead of a second front electrode 12) situated between the first front electrode 11 and the middle electrode 13. Between the irradiation source 1 (not shown in FIG. 4(a) but see FIG. 1) and the specimen surface 2 is a beam separator (e.g., an ExB, not shown) serving as a xe2x80x9cdown-lightingxe2x80x9d irradiation system for irradiating a specimen 2 with an electron beam. The angle of the principal ray at the intermediate image plane M differs in orthogonal directions of the transverse section of the light flux (termed astigmatism in the pupil space). In other words, in an x-y-z rectangular coordinate system where the optical axis is the z-axis AX, the intermediate imaging plane M is an x-y plane, and the magnetic field of the beam separator is oriented in the direction of the x-axis. Aberration is corrected in the intermediate imaging plane M. As shown in FIG. 4(b), the angle of principal rays (solid lines) is different in the x-direction compared to the y-direction. To cause the principal rays to intersect the optical axis at the center of the rear power RP, the principal rays are deflected by the angle xcex81 in the x-direction and by the angle xcex82(xcex81 greater than xcex82) in the y-direction. In order to accomplish this, respective voltages are impressed on the poles of the quadrupole electrode 16 to create a lens power that is stronger in the x-direction than in the y-direction. A typical voltage-impression scheme for the poles is depicted in FIG. 5. The main voltage V1 and the astigmatism-adjustment voltage V2 (V1 greater than V2 greater than 0) that comprise the overall power are overlapped so that the impressed voltage is greater in the x-direction. As a result, principal rays of different angles collectively can form an image at the center of the rear power with correction of the various aberrations. A secondary-electron mapping-projection apparatus according to this embodiment is depicted in FIG. 6, and comprises an irradiation column 21, a beam separator (e.g., an ExB) 22, a first front lens 23, a second front lens 24, an aperture 26, a projection lens 27, a detector surface (imaging surface) 28, and an intermediate imaging plane M1. An electron irradiation beam having a predetermined transverse area and profile is irradiated by the irradiation column 21 at an angle to the optical axis AX. The irradiation beam is directed to the optical center of the beam separator 22 and is deflected by the beam separator 22 so as to propagate along the optical axis AX from the beam separator 22 to a specimen surface 25. The irradiation beam passes through the first front lens 23, the aperture 26, and the second front lens 24 to the specimen surface 25 at which the irradiation beam is perpendicularly incident. Secondary electrons emitted from the specimen surface 25 are formed by the front lenses 23, 24 into an image at the intermediate imaging plane M1 (situated at the optical center of the beam separator 22). The aperture 26 serves to reduce aberrations in the intermediate image. Unlike the conventional system shown in FIG. 9, the FIG. 6 embodiment has a front lens system, located between the beam separator 22 and the specimen surface 25, that includes a two-stage lens system (first and second front lenses 23, 24, respectively) configured as a bi-directionally telecentric system. In other words, the specimen surface 25 is situated at the forward focal position of the second front lens 24 (note convergence of dotted lines), and the aperture 26 is positioned at the rearward focal position of the second front lens 24 and at the forward focal position of the first front lens 23. The intermediate imaging plane M1 is formed at the rearward focal position of the first front lens 23. Between the first front lens 23 and the projection lens 27, the principal ray (solid line) is parallel to the optical axis AX and thus does not diverge. As a result, the ray is incident to the projection lens 27 more paraxially (at a point closer to the optical axis AX) than in conventional systems (compare the height of the principal ray passing through the lens 47 (FIG. 9) with the height of the principal ray passing through the lens 27 (FIG. 6)). Consequently, pincushion distortions are suppressed. Also, because the rays that are used for forming an image at the detector surface 28 are closer to being paraxial rays, other aberrations are suppressed as well, even during wide-field, low-magnification imaging. The image formed at the intermediate imaging plane Ml is enlarged and re-formed on the detector surface 28 by the projection lens 27. The magnification of the image formed on the detector surface 28 is changed by changing the axial position of the projection lens 27. A wide-field, low-magnification image is obtained by reducing the power of the projection lens 27. Typically, the projection lens 27 actually comprises multiple lenses to facilitate making changes in image magnification. This embodiment is shown in FIG. 7, in which components that are similar to corresponding components shown in FIG. 6 have the same respective reference numerals. (It is noted that, in this embodiment, item 27 is termed a xe2x80x9cfirst rear lensxe2x80x9d whereas item 27 is termed simply a xe2x80x9cprojection lensxe2x80x9d in FIG. 6.) The FIG. 7 embodiment further comprises a second rear lens 29 and a third rear lens 30. xe2x80x9cM2xe2x80x9d denotes a second intermediate imaging plane, and xe2x80x9cM1xe2x80x9d is a first intermediate imaging plane. The respective structures and positions of the irradiation column 21, beam separator 22, aperture 26, and irradiation column 21 are similar to respective structures and positions of corresponding components of the system shown in FIG. 9. The second front lens 24 forms an image, of secondary electrons emitted from the specimen surface 25, at the first intermediate imaging plane M1 located at the optical center of the beam separator 22. The first rear lens 27 and second rear lens 29 collectively form a relay optical system. The relay optical system forms an image, of the image at the first intermediate imaging plane M1, at the second intermediate imaging plane M2. The image at the second intermediate imaging plane M2 is reformed by the third rear lens 30 on the detector surface 28. The lenses 27, 29, and 30 collectively comprise a projection-optical system. The relay optical system suppresses divergence of the principal ray (solid line) and thus corrects distortion aberrations. Namely, the principal ray incident to the third rear lens 30 is essentially parallel with the optical axis AX and passes through the third rear lens paraxially (near the optical axis AX). The magnification of the image created by the projection-optical system can be varied in a continuous manner by changing the power balance in the relay optical system. (When the first and second intermediate image planes are fixed, the xe2x80x9cpower balancexe2x80x9d is the respective focal lengths of the lenses 27, 29, relative to each other.) Changing the power balance changes the axial position of the second intermediate imaging plane M2. Image magnification also can be varied by changing the power balance between the relay optical system and the third rear lens 30, or by changing the power of the second rear lens 30. This makes it possible to obtain images at magnifications ranging from wide-field at low magnification to narrow-field at high magnification. The relay optical system desirably is configured to be bi-directionally telecentric whenever the magnification of the image is small and the optical field is large in the relay optical system (i.e., at the wide-angle end of the zoom range). In such a configuration, the focal point of the first rear lens 27 is set at the position of the first intermediate imaging plane M1, and the focal point of the second rear lens 29 is set at the position of the second intermediate imaging plane M2. With such a configuration, the principal ray is made incident to the third rear lens 30 nearly parallel with the optical axis AX under conditions that otherwise would be susceptible to the generation of excessive distortion aberrations. Thus, this configuration suppresses such distortion aberrations. This embodiment is depicted in FIG. 8, in which components that are similar to corresponding components in the FIG. 7 embodiment have the same respective reference numerals. In the FIG. 8 embodiment, the optical system (xe2x80x9cfront optical systemxe2x80x9d) between the beam separator 22 and the specimen surface 25 is similar to the corresponding optical system shown in FIG. 6, and the optical system (xe2x80x9crear optical systemxe2x80x9d or xe2x80x9cprojection-optical systemxe2x80x9d) between the beam separator 22 and the detector surface 28 is similar to the corresponding optical system shown in FIG. 7. Hence, the FIG. 8 embodiment has the advantages of both the FIG. 6 embodiment and the FIG. 7 embodiment. Namely, the front optical system in this embodiment is configured to be bi-directionally telecentric. A principal ray (solid line) passing through the first intermediate imaging plane M1 is parallel to the optical axis AX. The principal ray enters the rear optical system non-divergently and not excessively off-axis. As a result, conditions that otherwise would produce distortion aberrations are not present. In addition, the axial position of the second intermediate imaging plane M2 can be changed by the relay optical system in a continuous manner without causing any divergence of the principal ray. As a result, essentially no distortion aberrations are generated in the rear optical system. If a higher magnification in a narrow field is required than can be achieved by the rear optical system, then the bi-directional telecentricity of the front optical system can be exploited. Whereas the invention has been described in connection with multiple representative embodiments, it will be apparent that the invention is not limited to those embodiments. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.