Deflection arrangement for separating two particle beams

Abstract: The invention relates to a deflection arrangement for separating two particle beams, with an electrostatic deflector and a magnetic deflector having a common optical axis and generating crossed electrostatic and magnetic deflection fields, wherein the two particle beams pass the deflection arrangements from opposite sides. The two deflectors are adapted to deflect one of the two beams achromatically by an angle a and the other beam by an angle β ≥ 3 α with respect to its angle of incidence, respectively.

Description: The invention is related to a deflection arrangement for separating two particle beams and a particle beam system with such a deflection arrangement. In particle beam systems, especially in high-resolution E-beam systems, there is the task of separating illuminating particles (primary particle beam) and signal particles (secondary particle beam) generated by the illuminating particle. This general task of separating two beams has to be performed both in scanning systems (like SEM's) and in parallel imaging systems (like LEEM's). The following components are frequently used as beam separators in known arrangements: magnetic sector fields, electrostatic and magnetic deflectors in one, two or three stages and Wien filter. In surface imaging tools having a wider beam, complete image fields are illuminated with an illumination particle beam, and signal particles, e.g. secondary particles, back-scattered particles, reflected particles or other particles released at the specimen are imaged by a detector on which a complete image of the surface area is generated. W. Telieps, "Surface Imaging with LEEM", Appl. Phys. A 44, 55-61, 1987, discloses such a surface imaging tool. The primary particle beam and the secondary particle beam are deflected by a magnetic sector field. The angle of incidence of the primary particle beam is α with respect to the optical axis and the beam will be deflected by the magnetic sector field onto the optical axis while the secondary particle beam reflected from the specimen will be deflected to the other side of the optical axis to separate the secondary particle beam from the primary particle beam. Magnetic sector fields, however, introduce chromatic aberration, which limits the resolution. In order to keep the resolution reasonable, only small separation angles can be achieved. EP-A-0 817 235 relates to a scanning electron microscope that obtains a scan image on the basis of secondary electrons emitted by irradiating an electron beam to a specimen. This microscope uses a Wien filter to separate the primary electron beam from the secondary electrons. A Wien filter generates a crossed electrostatic and magnetic deflection field, which, however, also introduces chromatic aberration. Only in the case that a crossover or an image of the surface is situated in the center of the Wien filter, chromatic aberration can be cancelled out. This, however, restricts the optical design. Electrostatic and magnetic deflectors can be arranged to an achromatic arrangement. This, however, requires at least a two stage or three stage arrangement as disclosed in EP-A-0 917 177. Optical systems with many deflection system, however, extend the length of the optical system which has a negative influence on the particle beam interaction and consequently on the image resolution. It is therefore an object of the invention to provide a deflection arrangement for separating two particle beams according to the preamble to claim 1 or a particle beam system with such a deflection arrangement according to the preamble to claim 8 which provides an achromatic operation and a good image resolution. The object is solved by the features of claims 1 and 8. Further embodiments of the invention are the subject matter of the subordinate claims. According to the invention, the deflection arrangement for separating two particle beams has an electrostatic deflector and a magnetic deflector having a common optical axis and generating crossed electrostatic and magnetic deflection fields, wherein the two particle beams pass the deflection arrangement from opposite sides. The two deflectors are adapted to deflect one of the two beams achromatically by an angle α and the other beam by an angle β ≥ 3α with respect to the angle of incidence, respectively. In one embodiment of the invention, the deflection arrangement is positioned in a crossover of one of the two beams, preferably in a crossover of the beam which is not deflected achromatically. With such an arrangement, the chromatic aberration can be cancelled out for both beams. In a prefered embodiment, the deflection fields of the electrostatic deflector and the magnetic deflector are superimposed, similar to an arrangement in a Wien filter. The major difference to a common Wien filter, however, is the excitation of the electrostatic and magnetic deflector: 1. The magnetic deflection is defined by: α m = C m * H * U 0 -0,5 dα m = - 0,5 * C m * H * U 0 -1,5 * dU 0 = - 0,5 * α m dU 0 /U 0 dα m /α m = - 0,5 dU 0 /U 0 α m = magnetic deflection angle C m = geometric data of the magnetic deflector H = magnetic field strength U 0 = acceleration voltage of the primary particle dα m = variation of the deflection angle by varying the primary energy by dU 0 2. The electrostatic deflection is defined by: α e = C e * U D * U 0 -1 dα e = -C e * U D * U 0 -2 * dU 0 = -α e * dU 0 /U 0 dα e /α e = -dU 0 /U 0 α e = electrostatic deflection angle C e = geometric data or the electrostatic deflector U D = electrostatic deflection voltage U 0 = acceleration voltage of the primary particle dα e = variation of the deflection angle by varying the primary energy by dU 0 3. Superposition of electrostatic and magnetic deflector: dα = ( -α e - 0,5 * α m ) * dU 0 /U 0 dα = variation of the total deflection angle of the superimposed electrostatic and magnetic deflector by varying the primary energy by dU 0 An achromatic deflector can be realized if dα = 0. This is true, if the excitation of the magnetic deflector is double as high in amplitude as that one of the electrostatic deflector. The deflection direction of both systems is opposite. The effect of such an arrangement is that the magnetic deflector tries to the deflect the beam by an angle of 2 α, while the electrostatic deflector bends the beam back by α. The resulting deflection power of this arrangement is α. Since the chromatic aberration of the electrostatic deflector is twice as high as that one of the magnetic deflector, the resulting deflection is achromatic, independent of the beam trace. For the beam coming from the opposite direction, electrostatic and magnetic deflection forces are working in the same direction, which means that this beam is deflected by at least 3 α. The total separation angle of the two beams is consequently at least 4 α. The achromatic deflection is only true for one beam. For the other beam, the energy dispersion of the two superimposed deflectors will even add. This, however, is in most cases no drawback, because resolution requirements for one of the two beams are of less importance. In case that chromatic compensation is requried for both beams, the arrangement and the corresponding excitation of the two deflectors offers also a solution. In this case, the beam which is not achromatically deflected has to provide a crossover in the center of the deflection arrangement. In this case, the chromatic aberration of the second beam is also compensated. Further advantages and embodiments of the invention will now be described in more detail with reference to the drawings. In the drawings Fig.1 shows a schematic representation of the deflection arrangement according to a first embodiment, Fig.2 shows a schematic representation of the deflection arrangement according to a second embodiment and Fig.3 shows a schematic representation of a particle beam system with a deflection arrangement according to the invention. Fig.1 discloses a deflection arrangement for separating two particle beams with an electrostatic deflector 1 and a magnetic deflector 2 having a common optical axis and generating crossed electrostatic and magnetic deflection fields E, B, wherein two particle beams 4, 5 pass the deflection arrangement from opposite sides. The two deflectors 1, 2 are adapted to deflect one of the two beams 4 achromatically by an angle α and the other beam 5 by an angle β ≥ 3α with respect to its angle of incidence, respectively. The deflection fields E, B of the electrostatic deflector 1 and the magnetic deflector 2 are superimposed. The excitation of the magnetic deflector 2 is twice as high in amplitude as the excitation of the electrostatic deflector 1. The deflection direction of the two deflectors 1, 2 is opposite. As a consequence, the first beam 4, whose angle of incidence with respect to the optical axis 3 is α, will be deflected by the magnetic deflector 2 by an angle of - 2 α, while the electrostatic deflector 1 bends the beam back by α. Accordingly, the resulting deflection of the first beam 4 is α. If the angle of incidence of the first beam is α, the exit angle is 0° with respect to the optical axis 3. Since the chromatic aberration of the electrostatic deflector 1 is double as high as that one of the magnetic deflector 2, the resulting deflection is achromatic. The second particle beam 5 enters the deflection arrangement from below and its angle of incidence is 0° with respect to the optical axis 3. In this case, both deflectors will deflect the particle beam in the same direction, which means that the second beam 5 is deflected by β ≥ 3 α. Consequently, the total separation angle of the first and second beam 4, 5 is at least 4 α. It should be noted that the second particle beam 5 in the embodiment according to Fig.1 ist not chromatically compensated. In contrary, energy dispersion of the two superimposed deflectors 1, 2 will even add. This, however, is in most cases no drawback, because resolution requirements for one of the two beams are of less importance. In the case that chromatic compensation is required for both beams, the deflection arrangement is to be positioned in a crossover of the beam, which is not deflected achromatically. Fig.2 discloses such an arrangement, where the deflection arrangement is positioned within the crossover of the second beam 5. Furthermore, the angle of incidence of the first particle beam 4 which is to be deflected achromatically, is 0°. Consequently, its exit angle is α with respect to the optical axis 3. Because of the arrangement of the deflection arrangement in a crossover of the second beam 5, the chromatic aberration of the second beam 5 can also be compensated. If the two beams 4, 5 have the same energy, the angle β of the second beam 5 is 3 α. In the case that the energy of the first beam 4 is higher than the second beam 5, the deflection angle β of the second beam 5 is higher than 3 α. Of course, it is not absolutely necessary to have superimposed deflection fields of the electrostatic and magnetic deflectors. It would also be possible to arrange the two deflectors close together along the optical axis 3. The embodiments of the Figs.1 and 2 differ from each other in the direction of the two particle beams and in that in one case, the deflection arrangement is arranged within the crossover of the particle beam which is not deflected achromatically. However, it is obvious that it would also be possible to arrange the embodiment according to Fig.1 in a crossover of the second particle beam 5 and not to arrange the embodiment according to Fig.2 in the crossover of the second particle beam 5. The deflection arrangement described above can be used advantageously in a particle beam system, i.e. an electron beam measuring system. Fig.3 discloses schematically a particle beam system with a source 6 for generating a primary particle beam 4, an optical unit, comprising an objective lens 7 for directing the primary particle beam 4 to a specimen 8, a detector 9 for detecting a secondary particle beam 5, consisting of secondary, backscattered and/or reflected particles released at the specimen 8 and a deflection arrangement between the specimen 8 and the detector 9 for separating the primary particle beam 4 and the secondary particle beam 5. The deflection arrangement, for example, is formed by one of the two deflection arrangements according to Figs.1 or 2. Accordingly, the deflection arrangement has an electrostatic deflector 1 and a magnetic deflector 2 having a common optical axis 3 and generating crossed electrostatic and magnetic deflection fields E, B, wherein the primary particle beam 4 and the secondary particle beam 5 pass the deflection arrangement from opposite sides. The two deflectors are adapted to deflect one of the two beams achromatically by an angle α and the other beam by an angle β ≥ 3 α with respect to its angle of incidence, respectively. In Fig.3, it is the primary particle beam which is deflected achromatically, However, it would also be possible that the particle beam which is deflected achromatically is the secondary particle beam. The deflection arrangement according to the invention is short and compact in order not to extend the length of the particle beam system. Furthermore, it is simple in design and guarantees an unsophisticated operation. Furthermore, at least one beam, if necessary both beams can be deflected achromatically, which guarantees a good resolution of the particle beam system.

Claims: Deflection arrangement for separating two particle beams (4, 5) with an electrostatic deflector (1) and a magnetic deflector (2) having a common optical axis and generating crossed electrostatic and magnetic deflection fields (E, B), wherein the two particle beams pass the deflection arrangement from opposite sides, characterised in that the two deflectors are adapted to deflect one of the two beams (4) achromatically by an angle α and the other beam (5) by an angle β ≥ 3 α, with respect to its angle of incidence, respectively. Deflection arrangement according to claim 1, characterised in that the deflection arrangement is positioned in a crossover of one of the two beams. Deflection arrangement according to claim 1, characterised in that the deflection arrangement is positioned in a crossover of the beam which is not deflected achromatically. Deflection arrangement according to one or more of the preceding claims, characterised in that the deflection fields (E, B) of the electrostatic deflector (1) and the magnetic deflector (2) are superimposed. Deflection arrangement according to one or more of the preceding claims, characterised in that the angle of incidence of the beam (4), which is to be deflected achromatically, is α and its exit angle is 0° with respect to the optical axis. Deflection arrangement according to one or more of the preceding claims, characterised in that the angle of incidence of the beam (4), which is to be deflected achromatically, is 0° and its exit angle is α with respect to the optical axis. Deflection arrangement according to one or more of the preceding claims, characterised in that the angle β of the other beam is 3 α if the two beams (4, 5) have the same energy. Particle beam system with a source (6) for generating a primary particle beam (4), an optical unit (7) for directing the primary particle beam (4) to a specimen (8), a detector for detecting a secondary particle beam (5), consisting of secondary, backscattered and/or reflected particles and a deflection arrangement between the specimen and the detector for separating the primary particle beam (4) and the secondary particle beam (59, characterised by a deflection arrangement according to one or more of the claims 1 to 6. Corpuscular beam apparatus according to claim 8, characterised in that the particle beam which is deflected achromatically is the secondary particle beam (5). Corpuscular beam apparatus according to claim 8, characterised in that the particle beam which is deflected achromatically is the primary particle beam (4).

Inventors: FROSIEN JUERGEN DR, FROSIEN, JUERGEN, DR., Frosien, Jürgen, Dr.
Applicants: ADVANTEST CORP, ADVANTEST CORPORATION
Application year: 1999
Main classification: H01J37
IPC codes: H01J37, G01N23, G01Q60