JP6737539B2 - Charged particle beam device - Google Patents

Description

The present invention relates to a charged particle beam device for irradiating a sample with a charged particle beam, and more particularly to a charged particle beam device capable of obliquely irradiating a beam.

In the process of manufacturing a semiconductor device, a charged particle beam apparatus is used which measures the dimension of a pattern shape and inspects defects by irradiating an LSI with a charged particle beam and detecting secondary electrons generated from a sample, and particularly scanning. An electron microscope (Scanning Electron Microscope: SEM) is widely used. Conventionally, performance improvement and cost reduction of semiconductor devices have been achieved by improving the degree of integration by miniaturization, but in recent years, they are approaching their limits. In order to continue to improve the degree of integration, devices having a three-dimensional structure are being manufactured. In order to improve the yield of three-dimensional devices, inspection and measurement equipment that can acquire information in the height direction in addition to conventional plane information is required.

Therefore, it is possible to tilt the beam (deflect the beam from the ideal optical axis of the beam and irradiate the sample with the beam from a different direction from the ideal optical axis). Patent Document 1 discloses a charged particle beam device that tilts a beam and irradiates a sample. Further, in order to prevent resolution deterioration when the beam is tilted, in Patent Document 1, two-stage deflecting means for deflecting the charged particle beam in opposite directions in the converging magnetic field of the objective lens are provided, and the charged particle beam is used for the objective lens axis. A method for correcting off-axis chromatic aberration that occurs when tilted outside is disclosed.

JP-A-2000-348658 (corresponding US Pat. No. 6,452,175)

In a scanning electron microscope that observes a sample by inclining the beam, deflect secondary electrons off-axis and guide them to the detector side in order to perform energy discrimination and analysis of secondary electrons and highly efficient signal detection. Is desirable. On the other hand, the secondary electrons are emitted from the sample, converged by the objective lens, and then diverged. Secondary electrons that exhibit such behavior are detected on the detection surface of a detector with a finite width, or on secondary electron conversion electrodes that generate secondary electrons (tertiary electrons) by collision of electrons (secondary electrons). When trying to guide, highly efficient detection cannot be performed depending on the degree of divergence. Further, according to the study by the inventors, when the beam is tilted, a field curvature aberration occurs in the objective lens. Therefore, in order to correct this, an adjustment using the objective lens is necessary. It became clear that the behavior of the secondary electron changes. Patent Document 1 discloses a method for correcting aberration of primary electrons, but does not disclose a method for highly efficient detection of secondary electrons. Further, in the method disclosed in Patent Document 1, the primary electron is largely deflected by the two-stage deflector provided in the objective lens to cancel the aberration in the objective lens. Therefore, the secondary electrons are further deflected in the opposite direction to the primary electrons in the objective lens, and highly efficient detection cannot be expected. To detect secondary electrons with high efficiency, it is conceivable to appropriately control the trajectories of the secondary electrons. However, in the method of Patent Document 1, the influence of the deflection aberration of the deflector and the off-axis aberration of the objective lens is affected. Receive big. Therefore, it is difficult to control the secondary electron trajectories independently without affecting the primary electron trajectories during tilting.

Below, a charged particle beam device is proposed for the purpose of suppressing high-order off-axis chromatic aberration and the like that occur when the beam is tilted or when the charged particles emitted from the sample are deflected.

As one mode for achieving the above object, an objective lens that focuses a charged particle beam emitted from a charged particle source, a detector that detects charged particles, and a beam from a direction different from the ideal optical axis of the objective lens are used. Between the charged particle source and the objective lens, and one or more deflectors for deflecting the charged particle beam or for deflecting the charged particles emitted from the sample towards the detector to irradiate A charged particle beam apparatus including an aberration correction unit arranged and an optical element arranged between the aberration correction unit and the objective lens for focusing the charged particle beam that has passed through the aberration correction unit is proposed.

According to the above aspect, it is possible to suppress high-order off-axis chromatic aberration that occurs when the beam is tilted or when the charged particles emitted from the sample are deflected.

The figure which shows the optical system structure of a scanning electron microscope (schematic diagram of 1st Example). FIG. 3 is a schematic diagram of an optical system configuration of a second example. The schematic diagram of the optical system composition of a 3rd example. The schematic diagram of the optical system composition of the 4th example. The schematic diagram of the optical system composition of a 5th example. The block diagram of the secondary electron deflection EXB unit of a 6th Example. The block diagram of the Wien filter unit for secondary electron deflection of a 7th Example. The block diagram of the Wien filter unit for secondary electron deflection of an 8th Example. The flowchart of the beam inclination of a 9th Example. The figure showing the structure of an optical system control part. The figure showing adjustment of the focusing position of the secondary electron orbit of a 1st example. The figure showing the central orbit of a primary electron at the time of beam inclination of a 1st example.

The embodiments described below relate to a charged particle beam device capable of beam tilting, and particularly to a charged particle beam device that realizes suppression of aberrations and highly efficient detection of charged particles (secondary electrons, etc.). is there.

The pattern shapes and materials of semiconductor devices continue to diversify, and in the inspection and measurement of them, the yield of secondary electrons generated from the sample by irradiation with the primary electron beam is improved, and the material discrimination by energy discrimination, etc. The demand for analysis is increasing. A device having a function of controlling secondary electron trajectories is required for highly accurate discrimination and analysis. Therefore, it is necessary to inspect, measure, and analyze a three-dimensional device having a multilayer film structure in which different materials are stacked in the height direction, and it is considered that there will be an increasing demand for a device that can perform both tilt image acquisition and secondary electron analysis during tilt. To be

In order to obtain information in the height direction by SEM, it is sufficient to irradiate the sample with an inclined beam and acquire an image. There are a mechanical tilting method and an electric tilting method as a method of acquiring a tilted image with the SEM. For in-line measurement/inspection, there is a method to mechanically tilt the sample stage and column, but the working distance (working distance: WD) between the objective lens and the sample is long to prevent interference between the wafer and the column structure. Therefore, the focal length of the objective lens becomes long. Therefore, the aberration of the objective lens cannot be reduced and the resolution deteriorates. Further, the mechanical operation for tilting reduces the reproducibility of throughput, observation position, and tilting angle.

On the other hand, as an electric tilting method, there is a beam tilting method in which an electron beam is deflected by a deflector and tilted with respect to a sample. An electric beam tilting SEM is desired because it greatly surpasses the mechanical tilting method in terms of throughput, tilt angle reproducibility, tilt angle, and direction variability. However, in this method, when the beam is tilted, the off-axis aberration of the objective lens increases, the beam diameter increases, and the resolution deteriorates.

Therefore, in the embodiment described below, an SEM including an optical element capable of tilting a beam by a deflector (first deflector) and suppressing a resolution lowering factor generated due to the tilt will be described. Further, a trajectory of secondary electrons or the like deflected by a quadrature electromagnetic field generator (hereinafter, EXB) that generates a deflection electric field and a deflection magnetic field at right angles or a deflector such as a Wien filter (second deflector) An SEM equipped with an optical element that guides properly to the detector side regardless of the inclination will be described. More specifically, in order to discriminate the energy of secondary electrons and improve the analysis accuracy, it is necessary to align the incident trajectories of secondary electrons to the spectroscope. Also, in order to improve the signal yield, as many secondary electrons as possible must be guided to the detection surface of the detector. However, when the secondary electrons are spread on the secondary electron deflector, the trajectory changes greatly depending on the position on the EXB due to the influence of the secondary aberration, which deviates from the incident conditions necessary for highly accurate discrimination and analysis. Alternatively, secondary electrons may spread too much on the detection surface, which may reduce the yield. Therefore, it is desirable to minimize the geometrical aberration for the secondary electrons by forming the focal point of the secondary electrons on the secondary electron deflector.

Further, it is conceivable to adjust and correct the field curvature generated by the beam passing off the axis of the objective lens by weakening the current and voltage of the objective lens itself for each tilt angle, but the secondary electron The lens action received from the objective lens weakens with the tilt angle, and the focus position of the secondary electrons rises. Therefore, secondary electrons on the secondary electron deflector spread with the tilt angle, geometrical aberration increases, and discrimination accuracy decreases. Further, when the secondary electrons are deflected when the beam is tilted, chromatic dispersion also occurs with respect to the primary electrons, causing chromatic aberration other than the objective lens, which deteriorates the resolution during tilting.

In view of the above conditions, in the following embodiments, in order to mainly control the focusing position and the focusing angle of the charged particle beam source for supplying the charged particle beam and the charged particle beam emitted from the charged particle beam source. A plurality of condenser lenses, an objective lens for focusing the charged particle beam on the sample, a scanning means for scanning the charged particle beam on the sample, and detecting secondary electrons generated from the sample by irradiation of the charged particle beam. In a charged particle beam apparatus that includes a detector for tilting a beam to irradiate a sample to acquire a tilted image, a deflector that deflects primary electrons to tilt the beam (tilt deflector: first tilter). A deflector) and a secondary electron deflector (second deflector) which is disposed above the objective lens and separates secondary electrons generated from the sample from the primary charged particle beam and guides them to the detector. A lens (secondary electron focusing lens: charged particle focusing lens) disposed between the sample and the secondary electron deflector to focus secondary electrons at the position of the secondary electron deflector, and the secondary electron deflector. Aberration correction unit (aberration correction lens or multipole for aberration generation, deflector for aberration generation, and high-order chromatic aberration suppression optics for focusing primary electron beams having different energies on the main surface of the objective lens when the beam is tilted Element) and a lens (aberration characteristic compensating lens) for compensating a change in aberration characteristic due to the secondary electron focusing lens arranged above the secondary electron deflector will be described.

According to the above configuration, a three-dimensional observation technique capable of highly efficient signal detection and highly accurate material discrimination in the inspection and measurement of the semiconductor shape pattern becomes possible.

Hereinafter, a scanning electron microscope equipped with a beam tilting deflector will be described with reference to the drawings. In the following embodiments, a scanning electron microscope will be described as an example, but the following embodiments can also be applied to an ion beam irradiation device that irradiates an ion beam.

FIG. 1 is a schematic diagram of the optical configuration of the first embodiment. In the first embodiment, a case will be described in which a deflector and a lens are used to generate the aberration required for correcting the aberration of primary electrons when the beam is tilted.

First, the operation when the primary electrons are vertically incident on the sample and the focus position of the secondary electrons is not controlled will be described. A voltage is applied between the cathode 01 and the first anode 02 by the electron gun controller 100, and the primary electrons 41 are emitted at a predetermined current density. An accelerating voltage is applied between the cathode 01 and the second anode 03 by the electron gun control unit 100 to accelerate the primary electrons 41 to be ejected to the subsequent stage. The primary electrons 41 are focused on the point P1 on the optical axis 30 by the first condenser lens 04 controlled by the first condenser lens control unit 101, and then pass through the objective diaphragm 05 to remove unnecessary electrons. The probe current amount and the opening angle of the primary electron 41 are determined by the position of the point P1 and the diameter of the aperture.

After that, the second condenser lens 06 controlled by the second condenser lens controller 102 forms a crossover of the primary electrons 41 at the point P2 on the optical axis 30. The point P2 is set so as to coincide with the center position of the aberration generating deflector 08. Further, the primary electrons 41 are incident on the aberration correction lens 09 controlled by the aberration correction lens controller 105 to form a crossover at a point P3 on the object plane Zm of the objective lens. Since the high-order off-axis aberration suppressing lens 11 is arranged around the object plane Zm, the primary electrons 41 are not affected by the lens action of the high-order off-axis chromatic aberration suppressing lens 11.

Further, the electromagnetic field intensity of the secondary electron deflection EXB 22 controlled by the secondary electron deflection EXB control unit 110 is adjusted so that the central orbit of the primary electron 41 goes straight.

Further, the light enters the objective lens 14 controlled by the objective lens control unit 113. The objective lens 14 has an opening through which the electron beam passes, and the center of the opening is the ideal optical axis of the electron beam (passage trajectory of the electron beam when the beam is not deflected).

The booster electrode 33 controlled by the objective lens controller 113 is arranged, and the aberration of the objective lens 14 is reduced by applying the acceleration voltage. Further, the deceleration voltage controlled by the retarding voltage control power supply 34 is applied to the stage 15 controlled by the stage control unit 115, and a deceleration electric field is formed between the sample 16 and the objective lens 14 to generate the deceleration electric field. Aberration is further reduced. The primary electrons 41 incident on the objective lens 14 are focused on the sample 16 at a point Pi on the optical axis 30 to form a minute spot. The lens strength of the objective lens 14 is determined by the working distance measured by the sample height measuring device 120. The micro-spot of the primary electrons 41 with which the sample 16 is irradiated is planarly scanned on the sample 16 by the scanning deflector 13 controlled by the scanning deflector controller 111.

At this time, among the secondary electrons having a wide range of energy generated by the primary electrons 41 scanning the sample, the secondary electrons 42 having the energy of interest are subjected to a strong lens action by the objective lens 14. As a result, it is once focused by the action of the lens field on the sample side of the objective lens 14. After that, it is affected by the rest of the lens field. At this time, the secondary electrons 42 are focused at a certain position according to the working distance, the lens strength of the objective lens 14, the acceleration electric field of the booster electrode 33, and the deceleration electric field of the sample 16. After that, the secondary electrons 42 are deflected by the secondary electron deflection EXB 22 controlled by the secondary electron deflection EXB control unit 110, enter the detector 17 controlled by the detector control unit 107, and are detected as a signal. .. The detection signal is calculated by the optical system control unit 116 and displayed on the image display unit 117 as an SEM image. When moving the field of view of the SEM image, the sample stage controlled by the stage control unit 114 is moved, or the image shift deflector 18 controlled by the scanning deflector control unit 111 is operated to arrive at the primary electron 41 on the sample. Change the position. The control device 116 includes a storage medium (not shown), and executes the control described in the embodiments described later based on the optical condition stored in the storage medium.

In the present embodiment, a secondary electron focusing lens 21 is arranged below the secondary electron deflecting EXB 22. The secondary electron focusing lens 21 is on the opposite side of the object surface Zm of the objective lens 14 and is inclined. The aberration characteristic compensating lens 20 is disposed below the optical deflector 08.

In order to simultaneously correct the primary off-axis chromatic aberration of the primary electrons at the time of beam tilt and the deflection coma, it is necessary that the ratios of the deflection comatic aberration and the axial chromatic aberration of the objective lens system and the aberration generating lens 09 are equal. The condition is the following expression (1).

However C _s ^OBJ and C _c ^OBJ is spherical aberration coefficient and the chromatic aberration coefficient of the objective lens 14 of the magnetic field, accelerating electric field by booster electrodes 33, retarding field of the synthetic lens, as defined by the objective lens paraboloid Zm. Further, C _s ^COR and C _c ^COR are a spherical aberration coefficient and a chromatic aberration coefficient of the aberration generating lens 09 defined by the objective lens object surface Zm. In this embodiment, the aberration generating lens 09 that satisfies the formula (1) when the secondary electron focusing lens 21 and the aberration characteristic compensating lens 20 are OFF is arranged.

Next, a method of focusing the secondary electrons 42 of the energy of interest when the tilt angle is 0° on the center of the secondary electron deflection EXB 22 will be described. FIG. 11A is a diagram showing the central orbit of the primary electrons when the beam is tilted when the secondary electron converging lens 21 and the aberration characteristic compensating lens 20 are OFF. The secondary electrons 42 of the energy of interest are focused at a position outside the object surface Zm of the objective lens.

Next, current and voltage are applied to the secondary electron focusing lens 21 to operate it. At this time, the voltage and current of the secondary electron focusing lens 21 operate in cooperation with the current change of the objective lens 14. Since the primary electron and the secondary electron have greatly different energies, the refractive power of the same lens is different. The intensity of the objective lens 14 and the secondary electron focusing lens 21 is simultaneously adjusted so that the primary electrons are focused on the point Pi on the sample and the secondary electrons 42 are focused on the point P3 on the object plane Zm of the objective lens 14. (FIG. 11B).

The operation of the secondary electron focusing lens 21 causes the objective lens system when the beam is tilted to be a combined lens of the secondary electron focusing lens 21 in addition to the objective lens 14, the booster electrode 33, and the retarding electric field. Therefore, the spherical aberration coefficient and chromatic aberration coefficient of the objective lens system change due to the electromagnetic field of the secondary electron focusing lens 21. Since the electromagnetic field strength is different for each tilt angle, these aberration coefficients change for each tilt angle.

Next, the aberration characteristic compensation lens 20 is operated. The aberration characteristic compensating lens 20 and the aberration generating lens 09 are adjusted at the same time in accordance with changes in the secondary electron focusing lens 21 and the objective lens 14 so that the following equation (2) holds while maintaining the focusing point P3 of the primary electrons. Then, the following equation (2) is established.

Secondary electron focusing lens 21 to the objective lens system of spherical and chromatic aberration coefficients ^_C's OBJ defined by the objective lens material surface Zm, including _the synthesis of ^{C'c OBJ,} for aberration lens 09 and the aberration characteristic compensator lens 20 The spherical aberration coefficient and the chromatic aberration coefficient of the lens are C′ _s ^COR and C′ _c ^COR , respectively.

By this operation, the focusing position of the secondary electron when there is no inclination with respect to the arbitrary working distance, the lens strength of the objective lens 14, the acceleration electric field of the booster electrode 33, and the deceleration electric field strength of the sample 16 is set to the EXB 22 for secondary electron deflection. It can be set to the center P3.

Next, the operation when the beam is tilted will be described. FIG. 12 shows the central orbit of primary electrons when the beam is tilted. The primary electrons 43 are deflected by the aberration generating deflector 08 centered on the object point P2 of the aberration correcting lens 09 and pass off the axis of the aberration correcting lens 09. Then, it intersects with the optical axis at the center point P3 of the symmetry plane Zm of the objective lens 14, is swung back by the tilting deflector 12 whose center coincides with P3, passes through the axis of the objective lens 14 and tilts to the sample 16. While being irradiated.

At this time, the primary electrons that have passed the off-axis of the aberration generating lens 09 are subjected to primary off-axis chromatic aberration and are dispersed into primary electrons 44 with high energy and primary electrons 45 with low energy. The high-order off-axis chromatic aberration suppressing lens 11 arranged on the object plane of the objective lens 14 has a lens strength set so as to focus the primary electrons dispersed by the aberration generating lens 09 on the main surface of the objective lens 14. Since the expression (2) is established, the primary off-axis chromatic aberration and the deflection coma aberration are corrected and the high-order off-axis chromatic aberration is suppressed. When changing the inclination angle and the inclination direction, the deflection angle and the deflection direction of the primary electron 43 by the aberration generating deflector 08 and the inclination deflector 12 are changed.

Next, the exciting current of the objective lens 14 is adjusted in order to correct the curvature of field due to passing off the axis of the objective lens 14. Since the lens strength of the objective lens 14 becomes weaker, the focus point of the secondary electron 42 of the energy of interest shifts upward from the point P3 of the object plane Zm of the objective lens 14.

Next, the secondary electron focusing lens 21 and the objective lens 14 are adjusted at the same time as in the case of no tilt, and the secondary electrons 42 of the objective lens 14 are focused while focusing the primary electrons to the point Pi on the sample. It is focused on a point P3 on the object surface Zm. Since this adjustment differs depending on the tilt angle, the aberration coefficient changes for each tilt angle, and the ratio thereof also changes. Let C′ _s ^OBJ (θ) and C′ _c ^OBJ (θ) be the functions of the spherical surface and the chromatic aberration of the objective lens system defined by the objective lens object surface Zm including the secondary electron focusing lens 21 as a function of the tilt angle θ. ..

Next, the aberration characteristic compensation lens 20 is operated. While the focusing point P3 of the primary electrons is always maintained, the aberration characteristic compensating lens 20 and the aberration are generated for each tilt angle in accordance with the changes of the secondary electron focusing lens 21 and the objective lens 14 so that the following expression (3) is established. The lens 09 is adjusted at the same time.

However, the spherical aberration coefficient and the chromatic aberration coefficient of the combined lens of the aberration generating lens 09 and the aberration characteristic compensating lens 20 defined by the objective lens object surface Zm when the tilt angle is θ are C′ _s ^COR (θ) and C, respectively. ^Let ' _c ^COR (θ).

According to this embodiment, the primary off-axis chromatic aberration and the deflection coma aberration of the primary electrons are simultaneously corrected with respect to an arbitrary beam tilt angle and tilt direction, the occurrence of higher-order off-axis chromatic aberration is suppressed, and the secondary electron focusing point is secondary. The position of the electron deflection EXB22 can be fixed, and the geometrical aberration received by the secondary electrons can be minimized. Since the size of the detection surface of the detector 17 is finite, the strengths of the secondary electron converging lens 21 and the objective lens 14 are set so that the secondary electron trajectories that expand with distance from the optical axis do not deviate from the detection surface. By adjusting, it becomes possible to realize highly efficient detection of secondary charged particles regardless of the degree of beam tilt.

Further, when the secondary electrons are deflected by the secondary electrons EXB22, chromatic dispersion occurs in the primary electrons. In this embodiment, since the secondary electron deflection EXB22 is arranged so that its center coincides with the object surface Zm of the objective lens, when the secondary electron 42 is deflected by the secondary electron deflection EXB22, it becomes a primary electron. The generated chromatic dispersion is focused on the sample by the objective lens 14 and becomes zero. In addition, when the tilt angle of the primary electrons and the deflection angle of the secondary electrons are both low angle tilt and deflection, the high-order off-axis chromatic aberration due to the chromatic dispersion of the primary electrons by the secondary electrons EXB22 does not become apparent. Further, since the primary electrons are focused at the center of the secondary electron deflection EXB22, the focusing action, the astigmatism imaging action and the secondary aberration of the secondary electron deflection EXB22 on the primary electrons are minimized.

In this embodiment, the object surface Zm of the objective lens 14 always coincides with the center of the secondary electron deflection EXB22 even if the inclination angle is changed. The resolution deterioration at the time of beam tilt due to the operation of EXB does not become apparent.

According to the present embodiment, while correcting the aberration of the primary electron at the time of beam tilt, the geometric aberration that the secondary electron receives from the secondary electron deflector 22 is minimized, and the beam tilt is capable of highly accurate energy discrimination of the secondary electron. An optical system can be provided.

In this embodiment, the tilting deflector 12 and the secondary electron deflecting EXB 22 that have different tilting fields and different tilting fields are arranged on the object lens surface Zm. Therefore, the secondary electron deflection EXB generates a deflection field so as to cancel the deflection action given to the secondary electrons by the tilting deflector 12 while maintaining the condition that the primary electrons go straight.

Further, in this embodiment, the tilting deflector 12 and the secondary deflecting EXB 22 may be one deflection unit.

In this embodiment, the secondary electron focusing lens 21 is arranged above the objective lens 14 and the aberration characteristic compensating lens 20 is arranged below the aberration generating lens 09, but it may be arranged on the opposite side. ..

In the present embodiment, when the energy of the secondary electron of interest is changed by changing the energy region of the secondary electron for discrimination, or when the acceleration voltage, the retarding voltage, the working distance, etc. are changed, there is no inclination. The adjustment of the focusing position of the secondary electron 42 at that time may be performed again.

Further, in the present embodiment, a plurality of detectors 17 may be arranged, and a detector having different detection characteristics, a detector having an energy filter function, a spectroscope, etc. may be arranged at the same time. At this time, when the detectors are switched, the deflection direction and angle of the secondary electrons 42 by the secondary deflection EXB may be set individually for each detector.

Further, in this embodiment, the focusing point of the secondary electron 42 is fixed to the object surface Zm of the objective lens 14 by adjusting the strength of the objective lens 14 and the secondary electron focusing lens 21, but instead of the objective lens 14, the booster electrode 33 is used. The acceleration voltage or the retarding voltage of the sample 16 may be adjusted in cooperation with the secondary electron focusing lens 21.

The aberration characteristic compensating lens 20, the secondary electron focusing lens 21, the aberration generating lens 09, and the high-order off-axis chromatic aberration suppressing lens 11 in the present embodiment may be any of an electrostatic lens, a magnetic field lens, and an electromagnetic superposition lens. Further, the aberration generating deflector 08 and the tilt deflector 12 may be either an electrostatic deflector or a magnetic field deflector.

This embodiment provides a preferable configuration in the case where the secondary electron deflecting EXB 22 cannot be arranged on the object plane Zm of the objective lens 14 due to design restrictions in the first embodiment. FIG. 2 is a schematic diagram of the configuration of the optical system of this embodiment. In this embodiment, the secondary electron deflecting EXB 22 is arranged between the high-order off-axis chromatic aberration suppressing lens 11 and the secondary electron focusing lens 21 arranged at the object plane position Zm of the objective lens 14. Further, a dispersion adjustment EXB23 is newly arranged between the secondary electron deflection EXB22 and the higher-order off-axis chromatic aberration suppressing lens 11. The detector 17 is arranged between the dispersion adjusting EXB 23 and the secondary electron deflecting EXB 22.

In this embodiment, the intensity of the objective lens 14 and the secondary electron focusing lens 21 is adjusted so that the secondary electrons 42 are focused on the center point Ps of the secondary electron deflection EXB 22 and the object surface Zm of the objective lens 14 is fixed. Is adjusted. The aberration generating lens 09 and the aberration characteristic compensating lens 20 fix the object surface Zm of the objective lens 14 and adjust the lens strength for each inclination angle so that the expression (3) is always satisfied. This corrects the primary off-axis chromatic aberration and deflection coma aberration when the beam of the primary electrons is tilted, suppresses the high-order off-axis chromatic aberration, and fixes the secondary electron focusing position at the center Ps of the secondary electron deflecting EXB 22. The secondary electrons 42 are deflected toward the detector 17 by the secondary electron deflection EXB 22.

However, since the secondary electron deflecting EXB 22 is away from the object plane Zm of the objective lens 14, the chromatic dispersion given to the primary electrons by the secondary electron deflecting EXB becomes apparent. The EXB23 for dispersion adjustment causes the primary electrons of average energy to go straight, and the origin of dispersion of the primary electrons having different energies emitted from the EXB22 for secondary electron deflection is virtually the center point P3 of the object surface Zm of the objective lens 14. Adjusted to be. As a result, the chromatic dispersion of the primary electrons generated in the dispersion adjusting EXB 23 and the secondary electron deflecting EXB 22 is focused on the sample 16 by the objective lens 14 and becomes zero and does not become apparent.

In this embodiment, since the primary electrons spread on the secondary electron deflection EXB 22 and the dispersion adjustment EXB 23, they are subjected to the lens function and astigmatism function of the EXB. However, in this embodiment, the strengths of the secondary electron deflection EXB 22 and the dispersion adjustment EXB 23 are fixed regardless of the inclination angle and the inclination direction. Therefore, the astigmatism imaging action can be corrected by adjusting the astigmatism corrector 07 when there is no inclination. Further, the objective lens 14, the secondary electron converging lens 21, the aberration generating lens 09, and the aberration characteristic compensating lens 20 in the non-tilted state may be adjusted in consideration of the lens functions of the secondary electron deflecting EXB 22 and the dispersion adjusting EXB 23. Good.

In the present embodiment, in the first embodiment, when the tilt angle is 10° and the secondary electron deflection angle is 10° or more, the chromatic dispersion given to the primary electrons by the secondary electron deflection EXB 22 is high when the beam is inclined. Provided is a suitable configuration that does not cause high-order off-axis chromatic aberration when the secondary off-axis chromatic aberration is to be manifested.

FIG. 3 is a schematic diagram of the configuration of this embodiment. In this embodiment, the secondary electron deflecting EXB 22 is arranged between the high-order off-axis chromatic aberration suppressing lens 11 and the secondary electron focusing lens 21 arranged at the object plane position Zm of the objective lens 14. The detector 17 is arranged between the high-order off-axis chromatic aberration suppressing lens 11 and the secondary electron deflection EXB 22. Further, the second dispersion compensation EXB 32 is arranged at the same position as the high-order off-axis chromatic aberration suppressing lens 11. A dispersion compensating first EXB 31, which is exactly the same EXB as the secondary electron deflecting EXB 22, is arranged at a symmetrical position with the object plane Zm of the objective lens 14 in between.

In this embodiment, the objective lens 14, the secondary electron focusing lens 21, the aberration generating lens 09, and the aberration characteristic compensation lens 20 are adjusted by the secondary electron deflecting EXB 22 and the dispersion compensating first EXB 31 as in the second embodiment. The focusing point of the secondary electrons 42 is fixed at the center point Ps of the secondary electron deflecting EXB 22 after the lens action is taken into consideration.

The secondary electron deflection EXB 22 and the dispersion compensation first EXB 31 are exactly the same EXB, and the strength of the generated dipole electromagnetic field is also the same. However, when the high-order off-axis chromatic aberration suppressing lens 11 is a magnetic lens, the direction of generation of the dipole electromagnetic field is rotated by that rotation angle.

The action of these EXBs on the primary electrons will be described. The first EXB 31 for dispersion compensation causes chromatic dispersion in primary electrons. This chromatic dispersion occurs from the center P _A of the first dispersion compensation EXB 31 as a starting point. Second EXB32 for dispersion compensation, including the lens action of the first EXB31 chromatic dispersion higher shaft outer chromatic aberration suppression lens 11 generated by the dispersion compensation dipole to focus to the center P _S of the secondary electron deflecting EXB22 Generate a place. Further, the chromatic dispersion of the secondary electron deflecting EXB22 is added, and the chromatic dispersion is completely corrected after passing through the secondary electron deflecting EXB22. Therefore, the chromatic dispersion of the secondary electron deflection EXB on the main surface of the objective lens 14 is zero, and high-order off-axis chromatic aberration does not occur even when the large-angle tilt and the large-angle secondary electron are deflected.

Further, in this embodiment, since the electromagnetic field strengths, directions, and distributions of the secondary electron deflecting EXB 22 in which the primary electrons are spread and the dispersion compensating first EXB 31 are symmetrical with respect to the focusing plane Zm of the primary electrons, the secondary electron deflecting EXB 22 is The secondary aberration of the dispersion compensating first EXB 31 with respect to the primary electrons is simultaneously corrected.

The present embodiment is not limited to the symmetrical arrangement of the secondary electron deflection EXB 22 and the dispersion compensation first EXB 31. The three EXBs of the secondary electron deflection EXB22, the dispersion compensation second EXB32, and the dispersion compensation first EXB31 are arranged in order from the bottom. When the dipole field strength and direction of each EXB are appropriately set, the electromagnetic field distribution, Regardless of the arrangement, chromatic dispersion due to EXB can be completely corrected, and large-angle tilt and large-angle secondary electron deflection can be realized within a range where the secondary aberration of EXB does not become apparent.

In the first, second, and third embodiments, the aberration for correcting the aberration of the objective lens at the time of beam tilt is generated by the aberration generating lens 09 and the aberration generating deflector 08 that passes primary electrons off the axis thereof. The lens is used to suppress the high-order off-axis chromatic aberration when the beam is tilted.

The unit for generating the correction aberration and suppressing the high-order off-axis chromatic aberration at the time of beam inclination in the present embodiment is not limited to the above configuration, and a multipole element may be used. In the present embodiment, a case will be described in which an aberration correcting multipole element is used in an oblique aberration generating unit and a multipole element capable of generating a quadrupole field is used as an optical element for suppressing high-order off-axis chromatic aberration.

FIG. 4 is a schematic diagram of the configuration of this embodiment. In the present embodiment, in the optical system configuration of the first embodiment, instead of the aberration generating lens 09, the aberration generating multipole element 50 is arranged between the aberration characteristic compensating lens 20 and the second condenser lens 06. .. On the object plane Zm of the objective lens 14, the tilting deflector 12, the secondary electron deflecting EXB 22 and the high-order off-axis chromatic aberration suppressing multipole element 51 are arranged.

The primary electrons are appropriately spread on the aberration generating multipole element 50, and are focused at the center point P3 of the high-order off-axis chromatic aberration suppressing multipole element 37 so as not to generate extra aberrations for the primary electrons.

The aberration generating multipole element 50 is adjusted so as to generate chromatic aberration and coma aberration equivalent to the objective lens 14 on the object surface Zm of the objective lens 14.

Similar to the first embodiment, the objective lens 14 and the secondary electron focusing lens 21 focus the primary electrons on the sample according to the beam tilt angle and the secondary electrons 42 are the central points of the secondary electron deflection EXB 22. Work together to focus on P3.

Further, the primary electrons are always focused on the center point P3 of the high-order off-axis chromatic aberration suppressing multipole element 51 for each tilt angle and direction, and chromatic dispersion and secondary coma aberration of the same amount and opposite sign to those of the objective lens system are generated. In addition, the strengths of the aberration generating multipole element 50 and the aberration characteristic compensating lens 20 are adjusted in cooperation for each tilt angle.

The high-order off-axis chromatic aberration suppressing multipole element 51 generates a quadrupole field. By the astigmatic imaging action of the quadrupole field, the chromatic aberration generated by the aberration generating multipole element 50 for correcting the chromatic aberration when the beam is tilted is focused on the main surface of the objective lens 14. As a result, the occurrence of high-order off-axis chromatic aberration is suppressed. When changing the tilt angle and tilt direction of the beam, the multipole intensity and direction of the aberration generating multipole element 50 may be changed, and the quadrupole field direction of the high-order off-axis chromatic aberration suppressing multipole element 51 may be changed.

In this embodiment, since the secondary electron deflecting EXB22 is arranged on the object plane Zm of the objective lens 14, the chromatic dispersion given to the primary electrons by the secondary electron deflecting EXB22 is zero on the sample 16 as in the first embodiment. Since it does not become apparent, there is no problem when tilting at a low angle and deflecting secondary electrons at a low angle.

When large-angle tilting and secondary-electron large-angle deflection are carried out, two stages of dispersion-compensating EXBs may be arranged above the secondary-electron deflecting EXB 22 as in the third embodiment.

A unit in which a deflector is added between the second condenser lens 06 and the aberration generating multipole element 50 is used as an aberration generating unit, and primary electrons are passed off the axis of the aberration generating multipole element 51 to generate chromatic aberration and coma. The ratio may be adjusted.

The aberration-generating multipole element 50 and the high-order aberration suppressing multipole element 51 in this embodiment may be any of electrostatic type, magnetic field type, and electromagnetic superposition type.

In this embodiment, a case where a Wien filter is used as an aberration generating unit at the time of beam tilt and a high-order off-axis chromatic aberration suppressing optical element will be described.

FIG. 5 is a schematic diagram of the configuration of this embodiment. In this embodiment, instead of the aberration generating lens 09, the aberration generating Wien filter 36 is arranged between the aberration characteristic compensating lens 20 and the second condenser lens 06. The tilting deflector 12 and the higher-order off-axis chromatic aberration suppressing Wien filter 37 are arranged on the object plane Zm of the objective lens 14. The deflection field of the tilting deflector 12 may be superimposed on the high-order off-axis chromatic aberration suppressing Wien filter 37. The secondary electron deflecting EXB 22 is arranged between the secondary electron focusing lens 22 and the high-order off-axis chromatic aberration suppressing Wien filter 37, and the detector 17 includes the secondary electron deflecting EXB 22 and the high-order off-axis chromatic aberration suppressing. It is arranged between the Wien filters 37.

The primary electrons are appropriately spread on the aberration generating Wien filter 36, and are focused at the center point P3 of the higher-order off-axis chromatic aberration suppressing Wien filter 37 so as not to generate extra aberrations for the primary electrons.

The aberration generating Wien filter 36 generates chromatic dispersion of primary electrons and secondary aberration according to the intensity and direction of the dipole field. In this configuration, of the secondary aberrations that occur, the secondary coma aberration is used to correct the aberration when the beam of the objective lens 14 is tilted. The aberration generating Wien filter 36 is adjusted so as to generate chromatic dispersion equivalent to the objective lens 14 and second-order coma aberration in the object plane Zm of the objective lens 14.

Similar to the first embodiment, the objective lens 14 and the secondary electron focusing lens 21 focus the primary electrons on the sample according to the beam tilt angle and the secondary electrons 42 are the central points of the secondary electron deflection EXB 22. Work together to focus on P _S.

In addition, primary electrons are always focused at the center point P3 of the Wien filter 37 for suppressing off-axis chromatic aberration for each tilt angle and direction, and chromatic dispersion and secondary coma aberration of the same amount and opposite sign to those of the objective lens system are generated. In addition, the strengths of the aberration generating Wien filter 36 and the aberration characteristic compensating lens 20 are adjusted in cooperation for each inclination angle.

The high-order off-axis chromatic aberration suppressing Wien filter 37 operates so as to generate a dipole field so that the chromatic dispersion generated by the aberration generating Wien filter 36 is focused on the main surface of the objective lens 14. When changing the inclination angle and the inclination direction, the generation intensity and direction of the dipole field and the quadrupole field of the aberration generating Wien filter 36 and the high-order off-axis chromatic aberration suppressing Wien filter 37 may be changed.

Next, correction of chromatic dispersion of secondary electron orbits will be described. In this embodiment, the aberration compensating first EXB 31 in the third embodiment plays the role of the aberration generating Wien filter 36, and the dispersion compensating second EXB 32 plays the role of the higher-order off-axis chromatic aberration suppressing Wien filter 37. The aberration generating Wien filter 36 and the higher-order off-axis chromatic aberration suppressing Wien filter 37 generate chromatic dispersion in the deflection direction of the secondary electrons so as to correct the chromatic dispersion generated in the primary electrons by the operation of the secondary electron deflector 22. It suffices to superimpose the dipole fields so that The intensity and direction of the superposed field are set according to the energy of the secondary electron of interest, the deflection direction, and the deflection angle. This suppresses the occurrence of high-order off-axis chromatic aberration of primary electrons when the beam is tilted.

According to the present embodiment, even when the Wien filter is used as the aberration generating unit and the optical element for suppressing the higher-order off-axis chromatic aberration, the aberration correction of the primary electrons at the time of beam tilting and the control of the focusing position for the highly accurate discrimination of the secondary electrons can be performed. compatible.

A unit in which a deflector is added between the second condenser lens 06 and the aberration generating Wien filter 36 is used as an aberration generating unit, and primary electrons are passed off the axis of the aberration generating Wien filter 36 to generate a secondary coma of chromatic dispersion. The generation ratio of aberration may be adjusted.

Further, in the first, second, third, fourth, and present examples, the aberration generating optical element may be any of a lens, a multipole element, and a Wien filter. Further, the optical element for suppressing high-order off-axis chromatic aberration may be a lens, a multipole element capable of generating a quadrupole field, or a Wien filter.

In this example, an example of a configuration for aligning the centers of the high-order chromatic aberration suppressing lens 11 and the secondary electron deflection EXB 22 in the same position in the configuration of the first example will be described.

FIG. 6 is a sectional view showing the configuration of the lens and EXB in this embodiment. 6A is an XZ sectional view, and FIG. 6B is an XY sectional view at the position of the object plane Zm of the objective lens 14. Paramagnetic metal electrodes 150 to 153 are arranged with the object plane Zm as the center of symmetry in the Z direction, and deflection coils 154 to 157 not using a ferromagnetic core for winding are arranged outside the electrodes to form EXB. A coil 158 for a magnetic field circular lens is arranged outside the EXB, and these components are casing by a circular lens magnetic path 159 made of a ferromagnetic metal. Although not shown, each electrode and coil are connected to a power source for supplying a deflection dipole electric field, a deflection dipole magnetic field, a voltage for generating a circular lens magnetic field, and a current. These power supplies are controlled by the secondary electron deflection EXB control unit 110.

In the configuration of the present embodiment, the electrodes for generating the deflection bipolar electric field of EXB are arranged at the innermost side, so that the generated electric field can be prevented from being electrostatically shielded. In addition, by using paramagnetic metal as the electrode material, the generation of the dipole magnetic field and the circular lens magnetic field is not hindered. Further, since the ferromagnetic core is not used in the winding form of the coils 154 to 157 for generating the dipole magnetic field, the deflection dipole magnetic field is generated without hindering the generation of the circular lens magnetic field by the circular lens coil 158 arranged at the outermost side. it can.

In the present embodiment, the number of electrodes of the secondary electron deflection EXB is set to eight poles or twelve poles, or the azimuth distribution of the number of turns of the deflection coil is adjusted to generate a multipole field of hexapole field or more. You may suppress it.

In the present embodiment, a second configuration for aligning the centers of the high-order chromatic aberration suppressing lens 11 and the secondary electron deflecting EXB 22 at the same position in the configurations of the first to third embodiments will be described. ..

In this embodiment, the electrode of the secondary electron deflection EXB and the magnetic pole are made to be a common electromagnetic pole, and the distribution of the electric field and the magnetic field are made to coincide with each other to form a Wien filter unit. FIG. 7 is a sectional view showing the configuration of the Wien filter unit. In this embodiment, four electromagnetic poles 201, 202, 203 and 204 are arranged by rotating 90° in the azimuth direction. Each electromagnetic pole is made of a ferromagnetic metal, and a coil for exciting a magnetic field is wound around it. Although not shown, a DC power source is independently connected to each electromagnetic pole and coil.

When the same voltage is applied to the electromagnetic poles 201 to 204, the four electromagnetic poles have the same potential and an electrostatic circular lens field is generated. However, an electrostatic octupole field is generated at the same time. When the voltage is positive, an accelerating lens field is generated. When the voltage is negative, a decelerating lens field is generated.

A bipolar electric field can be generated in an arbitrary direction by independently superposing the voltage on each of the electromagnetic poles 201 to 204 while applying the voltage for generating the electrostatic circular lens field to all the electromagnetic poles. In addition, a bipolar magnetic field can be generated in any direction by appropriately passing a current through the coil for exciting the electromagnetic pole.

The present embodiment is also suitable as the configuration of the EXB and Wien filter used in the first, second, third and fourth embodiments.

In the Wien filter unit shown in the seventh embodiment, an electrostatic octopole field is generated together with the electrostatic circular lens field. The octupole field produces third-order geometrical aberrations and third-order chromatic aberrations. When the beam tilt angle and the secondary electron deflection angle increase, these aberrations may become apparent. Therefore, here, a Wien filter unit using eight electromagnetic poles will be described as a more preferable example. FIG. 8 is a sectional view showing the structure of this embodiment.

The Wien filter unit is composed of eight electromagnetic poles 211 to 218 and a coil for exciting the electromagnetic poles. A power source for supplying voltage and current is independently connected to each electromagnetic pole and coil.

When the same voltage is applied to the eight electromagnetic poles, an electrostatic hexagonal field is generated together with the electrostatic circular lens field. The electrostatic hexapole field aberrations are the 7th-order geometrical aberration and the 7th-order chromatic aberration. These aberrations are negligible in practical use and can be ignored. In this configuration, by independently applying the voltage and the current to the eight electromagnetic poles, it is possible to generate the dipole electric field, the dipole magnetic field and the quadrupole electric field, and the quadrupole magnetic field in arbitrary directions in addition to the electrostatic circular lens field.

Further, the number of electromagnetic poles may be 12, and a circular lens field, a dipole field, a quadrupole field, and a hexapole field may be arbitrarily generated.

Further, a circular lens magnetic field, an octupole magnetic field, and a 16-pole magnetic field can be generated in the same manner as a current is applied to the electromagnetic pole exciting coil of the Wien filter unit so that all electromagnetic poles are excited to the N pole or the S pole. ..

In addition, this embodiment is most suitable as the configuration of the EXB and Wien filter used in the first, second, third, fourth, and fifth embodiments. Since the quadrupole field can be freely generated in addition to the dipole field, it is possible to cancel the astigmatic imaging action of the Wien filter while achromatizing the quadrupole. It is also most suitable as the configuration of the high-order off-axis chromatic aberration suppressing multipole element in the fourth embodiment.

In this embodiment, a control flow of primary electrons and secondary electrons when the beam is tilted will be described. FIG. 10 is a diagram showing the configuration of the optical system controller 116. FIG. 9 is a flowchart of the beam tilt of the primary electrons according to this embodiment.

The optical condition setting unit 301 sets the optical conditions (accelerating voltage, booster voltage, retarding voltage, energy of secondary electrons to be focused) observed in step 001.

In step 002, the sample stage is moved to the observation position.

In step 003, the working distance is measured by the sample height measuring device 120.

In step 004, a detector for image acquisition is selected.

In step 005, the tilt angle and tilt direction are input.

In step 006, according to the optical condition set in step 001, the detector selected in step 004, the tilt angle and tilt direction input in step 005, the deflector operation condition recording unit 303, the lens operation condition recording unit 304, and the EXB operation condition are recorded. The operating condition table recorded in the recording unit 305 and the astigmatism corrector operating condition recording unit 306 is read out, and based on the working distance measured in step 003, the operating condition calculating unit 302 uses each lens, each EXB or Wien filter, each. The voltage and current of the multipole element, each deflector, and astigmatism corrector are determined and set through each control unit.

In step 007, fine adjustment of focus and stigma is performed. At this time, in step 005, the focus fine adjustment is performed in step 005, in which the lens strengths of the objective lens 14 and the secondary electron focusing lens 20 are changed in conjunction with each other based on the calculated operating conditions, and the focus position of the secondary electrons is not changed. , Is executed so that only the focus of the primary electron is changed.

In step 008, a tilted SEM image is acquired.

In step 009, it is determined whether or not the tilt angle and the direction need to be changed.

In step 010, it is judged whether or not the detector for acquiring an image needs to be changed.

In step 110, it is determined whether or not the sample observation position needs to be changed, and if it is necessary to change, the process returns to step 002.

01: cathode, 02: first anode, 03: second anode, 04: first condenser lens, 05: objective diaphragm, 06: second condenser lens, 07: astigmatism corrector, 08: aberration-deflecting deflector, 09: aberration generating lens, 11: high-order chromatic aberration suppressing lens, 12: tilting deflector, 13: scanning deflector, 14: objective lens, 15: sample stage, 16: sample, 17: detector, 18: image Shift deflector, 20: Aberration characteristic compensating lens, 21: Secondary electron focusing lens, 22: Secondary electron deflecting EXB, 23: Dispersion adjusting EXB, 30: Optical axis, 31: Dispersion compensation first EXB, 32: second dispersion compensation EXB, 33: booster electrode, 34: retarding voltage application power source, 35: orbit adjustment deflector, 36: Wien filter for aberration generation, 37: Wien filter for suppressing high-order off-axis chromatic aberration, 41: Primary electron, 42: secondary electron, 43: central orbit of primary electron of average energy, 44: central orbit of primary electron of low energy, 45: central orbit of primary electron of high energy, 50: multipole element for aberration correction, 51: multipole for suppressing higher-order off-axis chromatic aberration, 100: electron gun control unit, 101: first condenser lens control unit, 102: second condenser lens control unit, 103: astigmatism corrector control unit, 104: aberration generation Deflector control unit, 105: aberration generation lens control unit, 106: aberration characteristic compensation lens control unit, 107: detector control unit, 108: high-order chromatic aberration suppressing lens control unit, 109: tilt deflector control unit , 110: secondary electron deflection EXB control unit, 111: scanning deflector control unit, 112: secondary electron focusing lens control unit, 113: objective lens control unit, 114: sample stage control unit, 116: optical system control Part, 120: sample height measuring instrument, 150: electrode, 151: electrode, 152: electrode, 153: electrode, 154: coil, 155: coil, 156: coil, 157: coil, 158: circular lens magnetic field coil, 159: magnetic path, 201: electromagnetic pole, 202: electromagnetic pole, 203: electromagnetic pole, 204: electromagnetic pole, 211: electromagnetic pole, 212: electromagnetic pole, 213: electromagnetic pole, 214: electromagnetic pole, 215: electromagnetic pole, 216: Electromagnetic pole, 217: Electromagnetic pole, 218: Electromagnetic pole, 301: Optical condition setting unit, 302: Operating condition calculation unit, 303: Deflector operating condition recording unit, 304: Lens operating condition recording unit, 305: EXB operation Condition recording unit, 306: Astigmatism corrector operating condition recording unit

Claims (4)

An objective lens that focuses a charged particle beam emitted from a charged particle source, a detector that detects charged particles, and a charged particle beam that irradiates the beam from a direction different from the ideal optical axis of the objective lens. One or more deflectors for deflecting or deflecting the charged particles emitted from the sample toward the detector; an aberration correction unit arranged between the charged particle source and the objective lens; and the aberration correction. An optical element is provided between the unit and the objective lens to focus the charged particle beam that has passed through the aberration correction unit, and the optical element focuses the charged particle beam having different energies on the main surface of the objective lens. A charged particle beam device, which is a focusing lens, a Wien filter, or a multipole that generates a quadrupole field . An objective lens that focuses a charged particle beam emitted from a charged particle source, a detector that detects charged particles, and a charged particle beam that irradiates the beam from a direction different from the ideal optical axis of the objective lens. One or more deflectors for deflecting or deflecting the charged particles emitted from the sample toward the detector; an aberration correction unit arranged between the charged particle source and the objective lens; and the aberration correction. A charging element that is disposed between the unit and the objective lens, includes an optical element that focuses the charged particle beam that has passed through the aberration correction unit, and that deflects the charged particles emitted from the sample among the one or more deflectors. The particle deflector is arranged between the objective lens and the object surface of the objective lens, and a dispersion deflector for dispersing the charged particle beam is arranged between the charged particle deflector and the optical element. A charged particle beam device characterized by the above. In claim 2 ,
A control device for controlling the dispersion deflector is provided, wherein the control device controls the dispersion origin of the charged particle beam that has passed through the dispersion deflector so as to be substantially the object plane of the objective lens. A charged particle beam device characterized by controlling a dispersion deflector . In claim 2 ,
The charged particle beam apparatus , wherein the dispersion deflector is an orthogonal electromagnetic field generator .

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