Method for axial alignment of charged particle beam and charged particle beam system

A method for axial alignment of a charged particle beam relative to at least three stages of multipole elements and a charged particle beam system capable of making the axial alignment. Some parts of the orbit of the beam or the distributions of three astigmatic fields, or both, are simultaneously translated in a direction perpendicular to the optical axis such that astigmatisms of the same order and same type due to axial deviations between successive ones of the astigmatic fields cancel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of making axial alignment of a charged particle beam with at least three stages of multipole elements. The invention also relates to a charged particle beam system capable of making this axial alignment.

2. Description of Related Art

In a charged particle beam instrument (such as a transmission electron microscope (TEM) or scanning transmission electron microscope (STEM)), aberration correction is an essential technique for obtaining high spatial resolution. Especially, positive spherical aberration produced by an objective lens is a typical factor to limit spatial resolution (see H. Sawada et al., Journal of Electron Microscopy, vol. 58 (2009), pp. 341-347).

Today, it is widely known that this positive spherical aberration can be corrected by the use of negative spherical aberration produced by hexapole elements or the like. A hexapole element produces a three-fold astigmatism that is a second-order aberration. In spherical aberration correctors, hexapole elements of this construction are arranged in two stages such that their three-fold astigmatisms cancel out each other (see H. Rose,Optik, Vol. 85 (1990), pp. 19-24 and H. Haider et al., Nature, vol. 392 (1998), pp. 768-769).

The aforementioned aberration correction technique is fundamentally correction of third-order spherical aberration. Corrections up to the fourth-order aberration can be made by axial alignment, which can be achieved using an optical system. A fifth-order spherical aberration can be corrected if the distance between the objective lens and the aberration corrector is controlled optically. However, corrections up to still higher orders of aberrations cannot be achieved completely. On the other hand, correction of astigmatism of the same order (fifth order) (i.e., six-fold astigmatism) is not yet achieved. Therefore, in a spherical aberration corrector consisting of two stages of hexapole elements, six-fold astigmatism is a major aberration which remains after correction of third-order spherical aberration and which is a factor limiting spatial resolution. Where six-fold astigmatism cannot be corrected, further improvement of spatial resolution cannot be anticipated.

Therefore, in the aberration corrector of JP-A-2009-054565, three stages of three-fold fields are used to correct six-fold astigmatism. In this aberration corrector, three-fold symmetric fields produced by the middle and rear stages of multipole elements are distributed in certain rotational relationships to the three-fold symmetric field produced by the front stage of multipole element within the plane perpendicular to the optical axis.

When axial alignment of an electron beam with multipole elements is made, it is customary to deflect the beam using a deflector or the like. In the case of an imaging system aberration corrector, deflection for the axial alignment is done while grasping residual aberration in the aberration corrector having multipole elements by tilting the beam relative to an amorphous sample, taking an image of the sample, and subjecting the image to Fourier transformation. In the case of this illumination system aberration corrector, the processing is continued until a geometrical figure of a desired shape is obtained while monitoring variations in the distortion of the geometrical figure, known as a Ronchigram, which is observed on a diffraction plane when the electron beam is focused onto the sample. In the case of a two-stage multipole element system, the source of appearing aberration has been identified and so it is relatively easy to make axial alignment of the beam with both multipole elements.

In the case of axial alignment with three or more stages of multipole elements, if the axial alignment with the first stage of multipole element produces a new aberration, it is very difficult to identify which of the middle and rear stages of multipole elements has brought about the aberration. If a further stage of multipole element is added, the difficulty increases vastly. Accordingly, a quite long time would be consumed in making the axial alignment.

SUMMARY OF THE INVENTION

The present invention has been made to solve the foregoing problem. It is an object of the invention to provide a method of making axial alignment of a charged particle beam with at least three stages of multipole elements. It is another object to provide a charged particle beam system capable of making this axial alignment.

A first embodiment of the present invention provides a method of making axial alignment of a charged particle beam in a charged particle beam system. In this method, at least three astigmatic fields are produced. Parts of the orbit of the charged particle beam or distributions of the astigmatic fields, or both, are moved simultaneously along a direction perpendicular to the optical axis such that astigmatisms of the same order and same type due to axial deviations between successive ones of the astigmatic fields cancel out each other.

A second embodiment of the invention provides a method of making axial alignment of a charged particle beam in a charged particle beam system. In this method, at least 3 three-fold astigmatic fields are produced. A pair of transfer lenses consisting of two rotationally symmetric lens fields is formed between any adjacent two of the astigmatic fields such that planes on the three-fold astigmatic fields are transferred as equivalent planes to their downstream astigmatic fields. Thus, first axial alignments of the charged particle beam with the three-fold astigmatic fields are made simultaneously.

A third embodiment of the invention provides a method of making axial alignment of a charged particle beam in a charged particle beam system. In this method, 3 three-fold astigmatic fields are produced. The three-fold astigmatic fields are present on planes of multipole elements. A pair of transfer lenses for transferring the multipole element planes to their downstream multipole element planes is formed between any adjacent two of the astigmatic fields. The intensities of the three-fold astigmatic fields are varied simultaneously while maintaining the summation of three-fold astigmatisms produced by the three-fold astigmatic fields at zero at all times. Preferably, the difference between the phase angles of the three-fold astigmatic fields located on the most upstream side and on the most downstream side, respectively, of the 3 three-fold astigmatic fields is approximately (70.5/3)°.

A fourth embodiment of the invention provides a charged particle beam system having at least three stages of multipole elements for producing astigmatic fields and deflectors mounted between the multipole elements. The deflectors translate parts of the orbit of a charged particle beam simultaneously along a direction perpendicular to an optical axis such that astigmatisms of the same order and same type due to axial deviations between successive ones of the multipole elements cancel out each other.

A fifth embodiment of the invention provides a charged particle beam system having at least three stages of multipole elements for producing astigmatic fields and a moving mechanism for translating the at least three stages of multipole elements separately in a direction perpendicular to an optical axis. The moving mechanism translates the multipole elements simultaneously such that astigmatisms of the same order and same type due to axial deviations between successive ones of the multipole elements cancel out each other.

A sixth embodiment of the present invention provides a charged particle beam system having at least three stages of multipole elements for producing astigmatic fields, pairs of transfer lenses mounted between the multipole elements, each pair of transfer lenses producing a pair of rotationally symmetric fields to transfer planes equivalent to planes formed by the multipole elements to their downstream multipole elements, and a moving mechanism for translating the multipole elements or the transfer lenses, or both, separately in a direction perpendicular to an optical axis. The moving mechanism carries out the translations simultaneously such that astigmatisms of the same order and same type due to axial deviations between successive ones of the multipole elements cancel out each other.

A seventh embodiment of the present invention provides a charged particle beam system having at least three stages of multipole elements for producing three-fold astigmatic fields, pairs of transfer lenses mounted between successive ones of the multipole elements such that each pair of transfer lenses produces a pair of rotationally symmetric fields to transfer planes equivalent to planes formed by the multipole elements to their downstream multipole elements, and first deflectors mounted between the transfer lenses of each pair. The first deflectors simultaneously perform deflections for making parts of the orbit of the charged particle beam strike the optical axis obliquely.

An eighth embodiment of the invention provides a charged particle beam system having three stages of multipole elements for producing three-fold astigmatic fields, first deflectors mounted upstream of the multipole elements, and transfer lenses of a pair mounted upstream and downstream, respectively, of the first deflectors. The transfer lenses produce a pair of rotationally symmetric fields to transfer planes equivalent to planes formed by the multipole elements to their downstream multipole elements. The multipole elements vary the intensities of the three-fold astigmatic fields simultaneously while maintaining the summation of three-fold astigmatisms produced by the three-fold astigmatic fields at zero at all times. Preferably, the difference between the phase angles of the three-fold astigmatic fields produced respectively by the multipole elements located on the most upstream side and on the most downstream side, respectively, of the three stages of multipole elements is approximately (70.5/3)°.

According to the present invention, axial alignment can be made while paying attention to only certain aberration by simultaneously controlling parts of the orbit of the charged particle beam incident on the astigmatic fields or the astigmatic fields or the rotationally symmetric fields acting on the charged particle beam. In other words, aberrations other than the certain aberration that is produced when the parts of the orbit, astigmatic fields, or rotationally symmetric fields are controlled separately do not appear. Consequently, the axial alignment is facilitated. This reduces the burden on the operator and can contribute to a decrease in the alignment time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

A method of making axial alignment of a charged particle beam and its principle are described, the method being associated with a first embodiment of the present invention.

Referring toFIG. 1, as a comparative example for the present embodiment, an aberration corrector100having two stages of multipole elements111and112is assumed. In the corrector100, the multipole elements111and112are arrayed in a line along an optical axis OP. Each of the multipole elements111and112produces an N-fold astigmatic field (where N is an integer). For convenience of illustration, it is assumed that N=3. An N-fold astigmatic field means a field in which the intensity of the produced field varies with a period of 360/N degrees around the rotationally symmetric axis of the multipole element.

Preferably, each of the multipole elements111and112is made of a hexapole element or a dodecapole (12-pole) element. Note that no limitations are imposed on the number of poles. The produced three-fold astigmatic field is a static electric field, a static magnetic field, or a superimposition thereof.

Each of the multipole elements111and112has a thickness oft along the optical axis OP. The three-fold astigmatic field produced by each multipole element is termed the primary term of the field produced by the multipole element. Generally, a multipole element produces fields owing to higher-order terms other than the primary term though they are quite weak. In an ordinary multipole element having no thickness (so-called a thin multipole element), fields due to higher-order terms other than the primary term are neglected for the intended purpose of the multipole element or merely parasitic factors. However, if the thickness of the multipole element is increased, higher-order terms other than the primary term exhibit effects, which can be used for correction of spherical aberration. That is, a multipole element having thickness t along the optical axis OP creates a field owing to higher-order terms applicable to correction of spherical aberration other than the primary term. The thickness t may be different between the individual multipole elements.

An objective lens114is mounted downstream of the multipole element112. A pair of transfer lenses115consisting of two axisymmetric lenses is mounted between the multipole elements111and112. Similarly, another pair of transfer lenses115is mounted between the multipole element112and the objective lens114. Each of the transfer lenses115transfers (projects) a plane (having a magnification of 1 times) equivalent to a plane formed by the multipole element111or112to the downstream multipole element112or to a coma-free plane of the objective lens114.

Plural stages (for example, two stages as shown inFIG. 1) of deflectors116are mounted between the transfer lenses115of each pair to deflect an electron beam120(i.e., a charged particle beam) by the use of a magnetic or electric field in two mutually perpendicular directions perpendicular to the optical axis OP such that some parts of the orbit of the beam120are translated.

Generally, axial deviation-induced aberrations in two N-fold astigmatic fields are (N−1)-fold astigmatisms. In particular, in the above-described configuration, if an axial deviation occurs in the two three-fold astigmatic fields, a new two-fold astigmatism occurs. This can be confirmed on a sample surface118. This two-fold astigmatism can be removed by translation of the parts of the orbit of the electron beam120performed by the deflectors116mounted between the multipole elements111and112. Consequently, the beam120passes on the rotationally symmetric axes of the multipole elements111and112, thus completing the axial alignment.

A method of making axial alignment of the three stages of multipole elements is next described by referring toFIG. 2. An aberration corrector10includes at least three stages of multipole elements11,12,13, a pair of transfer lenses15mounted between the multipole elements11and12, another pair of transfer lenses15mounted between the multipole elements12and13, a further pair of transfer lenses15mounted between the multipole element13and an objective lens14, and three sets of deflectors16. Each set of deflectors16is located between the transfer lenses15of respective one pair. Since the multipole elements11,12,13, the pairs of transfer lenses15, and the deflectors16are identical in structure and functions with the multipole elements111,112, the pairs of transfer lenses115, and the deflectors116, respectively, shown inFIG. 1, their description is omitted.

As described previously, the axial deviation-induced aberration in a three-fold astigmatic field is a two-fold astigmatism. If axial deviation-induced aberrations occur between the multipole elements11and12and between the multipole elements12and13, the axial deviation-induced aberrations are combined and appear on the sample surface18.

However, unlike the two stages of multipole elements111and112shown inFIG. 1, the three stages of multipole elements11,12, and13produce two axial deviation-induced aberrations of the same order and the same type (i.e., 2 two-fold astigmatisms). Furthermore, it is impossible to discern which one of the axial deviation-induced aberrations is due to axial deviation of the two three-fold astigmatic fields produced either by the multipole elements11and12or by the multipole elements12and13. In an attempt to remove these aberrations, it is conceivable to adjust the part of the orbit of the electron beam20incident on the multipole element12and then to adjust the part of the orbit incident on the multipole element13. The first adjustment frequently superimposes a new aberration onto the axial deviation-induced aberration on the sample surface18. Therefore, it is difficult to remove the axial deviation-induced aberrations while identifying the two-fold astigmatisms.

Accordingly, in the present embodiment, the electron beam is manipulated simultaneously between the multipole elements12,13, and14by making use of an optical action occurring by coupling of the 2 two-fold astigmatisms. As described in JP-A-2008-123999, in a case where 2 two-fold astigmatic fields are produced, a two-fold astigmatism and an axisymmetric dispersion occur. The dispersion is the action of a so-called concave lens. These actions are produced by an axial deviation between 2 three-fold astigmatic fields as exemplified in the present embodiment and by an axial deviation between other two N-fold astigmatic fields (N is an integer), as well as by an axial deviation between two-fold astigmatic fields.

In other words, the aforementioned concave lens action vanishes simultaneously with two-fold astigmatisms when the electron beam20passes on the rotationally symmetric axes of 2 two-fold astigmatic fields. In this case, the concave lens action that reduces the convex lens action of the objective lens vanishes and so the beam is most overfocused near the sample surface18. That is, in the case of the three stages of multipole elements11,12, and13shown inFIG. 2, parts of the orbit of the beam20are translated relative to the multipole elements12and13at the same time to cancel out the two-fold astigmatism produced by an axial deviation between the multipole elements11and12and the two-fold astigmatism produced by an axial deviation between the multipole elements12and13. The beam20exiting from the multipole element13is most overfocused around the sample surface18. If the manipulation is stopped at this point, the beam20will pass on the rotationally symmetric axes of the multipole elements11,12, and13, thus achieving an axial alignment.

Operations similar to the foregoing operations are performed in a case where there are four or more stages of multipole elements. That is, the parts of the orbit of the electron beam incident on the multipole elements, respectively, are translated simultaneously so as to cancel out appearing two-fold astigmatisms. The translational motions are stopped when the beam is most overfocused.

A charged particle beam system associated with a first embodiment of the present invention is next described by referring toFIGS. 3 and 4.

FIG. 3schematically shows one example of charged particle beam system associated with embodiments of the invention. The charged particle beam system, indicated by numeral30, is a transmission electron microscope, for example, and has an electron gun31, a first condenser lens32, the aforementioned aberration corrector10, a second condenser lens33, an objective lens assembly14including a sample stage (sample chamber), an intermediate/projector lens system34, an observational chamber35, and a power supply36for controlling the beam within the electron-optical column by applying voltages or currents to the optics. An aberration corrector and deflectors for axial alignment may be appropriately installed around the condenser lenses32and33to correct astigmatisms produced by the lenses32and33.

Furthermore, the charged particle beam system30includes a controller37that controls the power supply36to set the voltages or currents applied to the optics including the first and second condenser lenses32,33and the aberration corrector10. Data stored in the storage portion (not shown) within the controller37or values entered into the input portion of the controller37by an operator are used as those set values. Furthermore, the controller37displays an observation image obtained by the observational chamber35.

The electron gun31produces an electron beam (not shown) in response to applied voltage and current values. A high voltage of several kV to hundreds of kV is impressed on the gun31. The emitted electron beam (not shown) is accelerated toward the downstream first condenser lens32up to a desired energy. The beam is focused by the first condenser lens32. Astigmatism is removed by the aberration corrector10. Then, the beam is again focused by the second condenser lens33. Astigmatic correcting elements, deflectors, apertures, and other components are appropriately installed upstream and downstream of the first condenser lens32and second condenser lens33.

After passing through the aberration corrector10, the beam is further focused in the objective lens assembly14and made to impinge on a sample placed on the sample stage. The beam transmitted through the sample is magnified by the intermediate/projector lens system34and hits a fluorescent screen (not shown) in the observational chamber35. The sample image (real-space image, diffraction image, or the like) projected onto the fluorescent screen is picked up by an imaging device such as a CCD camera (not shown) and outputted as an image data to the controller37.

FIG. 4schematically shows another example of the charged particle beam system associated with embodiments of the invention. The charged particle beam system is indicated by numeral30′ and is a transmission electron microscope that is similar in configuration with the charged particle beam system30ofFIG. 3except that the aberration corrector10is installed between the objective lens assembly14and the intermediate/projector lens system34. That is, the beam transmitted through the sample is subjected to an aberration correction made by the aberration corrector10.

An operation for making axial alignment of the electron beam in a manner associated with the present embodiment using the aberration corrector10is next described by referring toFIGS. 2-4. First, the controller37controls the multipole elements11,12, and13to produce three-fold astigmatisms. The controller37then controls the deflectors16to deflect parts of the orbit of the electron beam20such that the parts of the orbit hit the multipole elements12and13at given angles of incidence.

The controller37then controls the deflectors16to translate the parts of the orbit of the beam20incident on the multipole elements12and13simultaneously in a direction perpendicular to the optical axis OP such that two-fold astigmatism produced by an axial deviation between the multipole elements11and12and two-fold astigmatism produced by an axial deviation between the multipole elements12and13cancel out each other. Because of the translational motions, the angles of incidence of parts of the orbit of the beam20to the multipole elements12and13do not vary. The controller37confirms the focal condition on the sample surface18during the translation motions. When the most overfocused state is achieved, the translational motions are stopped. The overfocused condition can be checked, for example, by performing known image processing on the observed image.

When the electron beam20is most overfocused on the sample, the two astigmatisms are completely removed. The beam20is allowed to pass on the rotationally symmetric axes of the multipole elements11,12, and13.

Second Embodiment

A method of making axial alignment of a charged particle beam and a charged particle beam system associated with a second embodiment of the invention are next described.

The charged particle beam system associated with the present embodiment is similar to the system associated with the first embodiment except that the electron beam20is controlled differently than by the controller37of the first embodiment. Therefore, only the control provided by the controller37is described below.

In the method of making axial alignment in accordance with the first embodiment, parts of the orbit of the beam are translated so as to cancel out plural two-fold astigmatisms. A check is made as to whether the axial alignment has been achieved from variations of the focal condition during the translational motions. On the other hand, in the present embodiment, hexapole astigmatism that is a higher-order aberration produced by two adjacent three-fold astigmatic fields is noticed. As described below, when axial deviations occur among the multipole elements11,12, and13producing three-fold astigmatic fields, five-fold astigmatism is produced from six-fold astigmatisms which are produced, respectively, between the multipole elements11and12and between the multipole elements12and13.

The generation of the five-fold astigmatism is now described by referring back toFIG. 1. It is assumed that the multipole elements111and112have thickness t along the optical axis OP and that the objective lens114has a focal distance off Let M be a magnification provided by the multipole elements111,112and objective lens114. Six-fold astigmatism (see non-patent document 3) is given by

A6=A~32⁢A~3214⁢⁢M6⁢f6⁢t7(1)
where Ã3is a three-fold astigmatism per unit length produced in each of the multipole elements111and112.

Then, it is assumed that the multipole elements11,12, and13are arranged along the optical axis in three stages as shown inFIG. 2. In the same way as the foregoing, each of the multipole elements11,12, and13creates a three-fold astigmatic field. As described previously, the objective lens assembly14ofFIG. 2is the same as the objective lens114ofFIG. 1.

When an axial deviation occurs between the multipole elements of the optical system shown inFIG. 2, the two adjacent multipole elements11and12produce a six-fold astigmatism A6. Similarly, the adjacent multipole elements12and13produce another six-fold astigmatism A6. If these 6-fold astigmatisms A6produce an axial deviation T, its wave aberration is given by

Re⁢{16⁢A6⁢(ω+T)_6}=Re⁢{16⁢A6⁡(ω_6+6⁢⁢T⁢⁢ω_5+15⁢⁢T_2⁢ω_4+20⁢⁢T_3⁢ω_3+15⁢⁢T_4⁢ω_2+6⁢⁢T_5⁢ω+T_6)}(2)
where ω is a complex angle. The second term of the right side of Eq. (2) contains the fifth power of the complex angle ω, and it can be seen that it represents a five-fold astigmatism. This is identically equal to

Therefore, the second term of the right side of Eq. (2) means that a five-fold astigmatism whose amount is 5A6T is produced. That is, if the axial deviation T occurs between the 2 six-fold astigmatisms, each produced by two adjacent ones of the three stages of multipole elements11,12, and13, then the above-described five-fold astigmatism is produced. Accordingly, in a case where the five-fold astigmatism (or, axial deviation-induced aberration) on the sample surface18has vanished, the electron beam20passes on the central axes (rotationally symmetric axes) of the multipole elements11-13.

Fundamentally, an axial deviation-induced aberration is produced by the presence of two adjacent three-fold astigmatic fields accompanied by an axial deviation. Therefore, the number of the multipole elements (3-fold astigmatic fields) is not limited to three. For example, where four stages of multipole elements are arranged, a five-fold astigmatism is produced by axial deviations between three six-fold astigmatisms. Therefore, in this case, when the five-fold astigmatism vanishes on the sample, the charged particle beam passes on the central axes of the multipole elements, in the same way as the foregoing.

Axial alignment of an electron beam by a method associated with the present embodiment is next described. First, the controller37controls the multipole elements11,12, and13to produce three-fold astigmatic fields. Then, the controller37controls the deflectors16located between the multipole elements to deflect parts of the orbit of the electron beam20such that they hit the multipole elements at given angles of incidence.

The controller37controls the deflectors16positioned between the multipole elements to translate simultaneously those parts of the orbit of the beam20which hit the multipole elements12and13, respectively, to translate these parts of the orbit in a direction perpendicular to the optical axis OP such that the two-fold astigmatism produced due to axial deviation of the multipole elements11and12and the two-fold astigmatism produced due to axial deviation of the multipole elements12and13cancel out each other. Because of the translational motions, the angles of incidence of the beam20to the multipole elements12and13do not vary. The controller37checks the five-fold astigmatism appearing on the sample surface18during the translational motions. When the five-fold astigmatism vanishes, the translational motions are stopped. This five-fold astigmatism can be checked, for example, by performing known image processing on the observed image.

Where the five-fold astigmatism vanishes on the sample surface18, the electron beam20is passing on the rotationally symmetric axes of the multipole elements12-13.

Third Embodiment

A method of making axial alignment of a charged particle beam in a manner associated with a third embodiment of the invention and a charged particle beam system for implementing the method are next described.

Referring still toFIGS. 1-3, a charged particle beam system associated with the present embodiment has a moving mechanism19for moving the multipole elements11,12,13or at least transfer lens pairs15between the multipole elements, in addition to the configuration of the charged particle beam system30or30′ of the first or second embodiment. Concomitantly, the controller37controls movement made by the moving mechanism19. The third embodiment is similar to the first and second embodiments in other respects and so only the control provided by the moving mechanism19and controller37is described below.

The moving mechanism19is mounted, for example, in the electron optical column (not shown) of the charged particle beam system30or30′, and is a manipulator for independently moving the multipole elements11,12,13or at least the two transfer lens pairs15between the multipole elements in two directions perpendicular to the optical axis OP.

In the first and second embodiments, the aforementioned parts of the orbit of the electron beam20are translated to cancel out the two-fold astigmatisms produced between the multipole elements11,12, and13. In the present embodiment, instead of translating parts of the orbit of the beam20, the multipole elements11,12, and13are moved independently and simultaneously by the use of the moving mechanism19such that the two-fold astigmatisms cancel out each other. The multipole elements11-13are moved in directions perpendicular to the optical axis OP. Alternatively, the transfer lens pairs15between the multipole elements may be moved by the moving mechanism independently and simultaneously. Also, in this case, the transfer lenses15are moved in directions perpendicular to the optical axis OP.

The controller37controls the multipole elements11,12, and13to produce three-fold astigmatic fields. The controller37then controls the deflectors16to deflect the electron beam20such that the beam20enters the multipole elements at given angles of incidence.

The controller37then translates the multipole elements11,12, and13independently and simultaneously by controlling the moving mechanism19such that the two-fold astigmatism produced by an axial deviation between the multipole elements11and12and the two-fold astigmatism produced by an axial deviation between the multipole elements12and13cancel out each other. Because of the translational motions, the angles of incidence of the parts of the orbit of the beam20hitting the multipole elements12and13do not vary. The controller37checks the focal condition or five-fold astigmatism appearing on the sample surface18during the translational motions. When the beam is most overfocused or the five-fold astigmatism vanishes, the controller stops the translational motions.

As described already in the first and second embodiments, when the beam is most overfocused or the five-fold astigmatism vanishes on the sample surface18, the two-fold astigmatisms are completely removed. The electron beam20is passing on the rotationally symmetric axes of the multipole elements11,12, and13.

In the axial alignment making method and charged particle beam system associated with the first through third embodiments, a variation in the focal state or five-fold astigmatism is utilized by translating parts of the orbit of the electron beam between multipole elements, the multipole elements, or transfer lens pairs in this way. Translational motions are done such that two-fold astigmatisms, i.e., axial deviation-induced aberrations, produced between the multipole elements cancel out each other. Consequently, axial alignment can be made while paying attention to only the focal state or five-fold astigmatism irrespective of the number of stages of multipole elements. Accordingly, the operation for making axial alignment is facilitated. This can contribute to a decrease in the observation time. Furthermore, the time taken to manipulate the charged particle beam system is shortened. Hence, the burden on the operator can be reduced.

The aberration corrector10may be installed in an imaging optical system configured including an intermediate lens and a projector lens. Aberration correctors may be installed in both the imaging optical system and in a condenser optical system configured including the first condenser lens. In any case, the above-described advantageous effects can be obtained.

Fourth Embodiment

A method of making axial alignment of a charged particle beam in a manner associated with a fourth embodiment of the invention and a charged particle beam system for implementing the method are described.

In the charged particle beam system associated with the present embodiment, an aberration corrector40shown inFIG. 5is installed instead of the aberration corrector10of the first embodiment. Accordingly, the system of the fourth embodiment is similar to the charged particle beam system30or30′ of the first embodiment except that the aberration corrector40is installed and that the controller37controls the electron beam20in a different manner. The aberration corrector40and the control provided by the controller37are described below. Description of other configurations are omitted because they are identical with their counterparts of the first embodiment.

As shown inFIG. 5, the aberration corrector40associated with the fourth embodiment has at least three stages of multipole elements41,42,43, a condenser lens51that is an axisymmetric lens mounted upstream of the multipole element41placed in the most front stage, pairs of transfer lenses45installed downstream of the multipole elements41,42, and43, respectively, deflectors46mounted upstream of the condenser lens51, and deflectors47,48, and49mounted between the transfer lenses45of their respective pairs. Each pair of transfer lenses45consists of two stages of axisymmetric lenses. Each of the multipole elements41,42, and43produces a three-fold astigmatic field, and is identical in structure with the multipole elements11,12, and13described in the first embodiment. The pairs of transfer lenses45are identical in structure with the pairs of transfer lenses15described in the first embodiment. The deflectors46-49are identical in structure with the deflectors16described in the first embodiment.

The condenser lens51focuses the electron beam20, which has been deflected by the deflectors46located upstream of the condenser lens51, toward the multipole element41.

In the present embodiment, a first deflecting operation consisting of deflecting steps (1)-(3) and a second deflecting operation consisting of a deflection step (4) as described below are simultaneously carried out to achieve axial alignment together with correction of four-fold astigmatism. Angles θ1-θ4given below are made between the optical axis OP and the electron beam20. InFIG. 5, a counterclockwise direction is taken as a positive direction.

(1) As shown inFIG. 5, the controller37controls the deflectors46to deflect the electron beam20propagating on the optical axis OP once away from the axis OP by angle θ1. Then, the beam20is deflected toward the optical axis OP by angle −θ2such that the beam strikes the multipole element41at an angle.

(2) The controller37controls the deflectors47to deflect the beam20exiting from the multipole element41at an angle of −θ2such that the beam approaches the optical axis OP at an angle of θ3and hits the multipole element42at an angle.

(3) The controller37controls the deflectors48to deflect the beam20leaving from the multipole element42at an angle of θ3such that the beam approaches the optical axis OP at an angle of −θ4and strikes the multipole element43at an angle.

(4) The controller37controls the deflectors49to deflect the beam20going out of the multipole element43at an angle of −θ4such that the beam travels on the optical axis OP. The angles θ1to θ4are varied simultaneously in such a way that other first-order, second-order, and third-order aberrations are not produced.

While the deflecting steps (1)-(4) are being carried out simultaneously, only the four-fold astigmatism on the sample surface18varies. Where the deflecting steps are stopped when the four-fold astigmatism has disappeared, the electron beam20passes through the centers P of the multipole elements41,42, and43, the centers being along the optical axis OP. That is, correction of the four-fold astigmatism and axial alignment of the beam20with the multipole elements41,42, and43are achieved.

In the combinations of the angles θ1-θ4, the polarity (positive or negative) of each angle may be reversed. For instance, in the deflecting step (1), if the beam20is deflected by an angle of −θ1, the deflection angle used thereafter is θ2.

As described also in the first embodiment, if parts of the orbit of the electron beam respectively hitting three or more stages of multipole elements are manipulated separately, various aberrations will appear on the sample surface18. Therefore, it is difficult to identify aberrations used for axial alignment, thereby lengthening the adjustment. In the present embodiment, an axial alignment is made which uses a variation of four-fold astigmatism by employing simultaneous deflection of the parts of the orbit of the beam hitting the multipole elements. Consequently, it is easy to judge whether or not the axial alignment has been achieved. Accordingly, the operation for the axial alignment is facilitated. This can contribute to a decrease in the observation time. Furthermore, the charged particle beam system can be manipulated in a reduced time. As a result, the burden on the operator can be reduced.

For the sake of illustration, in the description of the present embodiment, an arrangement of three stages of multipole elements is taken as an example. The number of the multipole elements is not restricted to three. An axial alignment of four or more stages of multipole elements can be made while paying attention to only four-fold astigmatism.

Fifth Embodiment

A method of making axial alignment of a charged particle beam in accordance with a fifth embodiment of the invention and a charged particle beam system for implementing the method are described. The present embodiment is similar to the fourth embodiment except that the deflecting steps (2) and (3) of the first deflecting operation for the electron beam20are replaced by deflecting steps 2′ and 3′ for translating parts of the orbit of the beam relative to two multipole elements.

(1′) As shown inFIG. 6, the controller37controls the deflectors46to deflect the electron beam20propagating on the optical axis OP such that the beam once goes away from the optical axis OP at an angle of θ1and then to deflect the beam toward the optical axis OP at an angle of −θ2and make the beam impinge on the multipole element41at an angle.

(2′) The controller37controls the deflectors47to deflect the beam20exiting from the multipole element41at an angle of −θ2in such a way that the beam moves toward the optical axis OP at an angle of θ3and then to translate a part of the orbit of the beam20a distance of r3in a direction perpendicular to the optical axis OP such that the beam strikes the multipole element42at an angle.

(3′) The controller37controls the deflectors48to deflect the beam20going out of the multipole element42at an angle of θ3such that the beam approaches the optical axis OP at an angle of −θ4and to translate a part of the orbit of the beam20a distance of r4in a direction perpendicular to the optical axis OP, thereby making the beam hit the multipole element43at an angle.

(4′) The controller37controls the deflectors49to deflect the beam20exiting from the multipole element43at an angle of −θ4such that the beam propagates on the optical axis OP.

The angles θ1-θ4and distances r3, r4are so set that other first-order, second-order, third-order, and fourth-order aberrations are not produced, and the angles and distances are varied at the same time.

While the deflections and translational motions are being made at the same time, only five-fold astigmatism on the sample surface18varies. Where the deflections are stopped at the instant when the five-fold astigmatism vanishes, the electron beam20passes through the centers P of the multipole elements41,42, and43taken along the optical axis OP. That is, correction of the five-fold astigmatism and axial alignment of the beam20with the multipole elements have been achieved.

In this way, in the axial alignment method and charged particle beam system associated with the fifth embodiment, variations of the five-fold astigmatism caused by simultaneous deflections and translational motions of the parts of the orbit of the electron beam hitting the multipole elements are utilized. Because of the deflections and the simultaneous translational motions, only the five-fold astigmatisms vary. Therefore, it is easy to judge whether or not an axial alignment has been achieved. Consequently, in the same way as in the fourth embodiment, it is easy to perform the operation for the axial alignment. This can contribute to a decrease in the observation time. Furthermore, the charged particle beam system can be manipulated in a reduced time. Hence, operator's burden can be reduced.

For convenience of illustration, in the description of the present embodiment, an arrangement of three stages of multipole elements is taken as an example. The number of stages of multipole elements is not restricted to three. An axial alignment of four or more stages of multipole elements can be made while paying attention to only four-fold astigmatism.

Furthermore, in the present embodiment, an axial alignment can also be attained by reversing the direction of propagation of the electron beam20and simultaneously effecting the aforementioned deflections and translational motions.

Sixth Embodiment

A method of making axial alignment of a charged particle beam in a manner associated with a sixth embodiment of the invention and a charged particle beam system for implementing the method are described.

The charged particle beam system associated with the present embodiment is similar in configuration to the charged particle beam system of the fourth embodiment except that three-fold astigmatic fields produced by the multipole elements41,42, and43are varied in intensity at the same time. Description of the configurations of the charged particle beam system associated with the present embodiment is omitted because the configurations are the same as their counterparts of the fourth embodiment, except for the manner in which the multipole elements41,42, and43are controlled by the controller37.

Each of the three stages of multipole elements41,42, and43(seeFIG. 5) associated with the present embodiment is, for example, a dodecapole (12-pole) element that produces a three-fold astigmatic field. In this optical system, the first multipole element41on the most upstream side, the second multipole element42located at an intermediate position, and the third multipole element43on the most downstream side produce three-fold astigmatisms which are herein denoted by A31, A32, and A33, respectively.

H. Rose,Optik, Vol. 85 (1990), pp. 19-24 and H. Haider et al., Nature, vol. 392 (1998), pp. 768-769) state that, when the summation of these three-fold astigmatisms A31, A32, and A33becomes equal to 0 under the condition where the summation of any two of them does not cancel the summation of other two of them, the whole three-fold astigmatism is null.

FIGS. 7A-7Band8A-8D show vectorial representations of three-fold fields. In the case of vectorial representations of a three-fold field, it is customary to use values that are three times greater than actual rotational relations of angles in the three-fold field within an optical or physical multipole element. According to this custom, when three stages of three-fold fields optically cancel out each other and the three-fold astigmatism becomes null, if vectorial representations are used, the combination of three vectors is geometrically null, which is easy to understand.

The main aberration remaining under the condition where the whole three-fold astigmatism is 0 is spherical aberration. Accordingly, if the intensity distribution of the three-fold astigmatic fields produced by the multipole elements41,42, and43is so varied that the summation of the 3 three-fold astigmatisms A31, A32, and A33is kept at 0 at all times, variations of the spherical aberration are prevalent. InFIGS. 7A and 7B, variations of the three-fold astigmatisms A31, A32, and A33are represented vectorially.

In particular, the distributions (amplitudes) of the three-fold astigmatic fields produced by the multipole elements41-43are varied while the summation of the 3 three-fold astigmatisms A31, A32, and A33is maintained at 0 by controlling the three-fold astigmatic fields produced by the multipole elements41-43by the use of the controller37. If the variations are brought to a stop when the spherical aberration in the objective lens is reduced maximally, an axial alignment in which the whole spherical aberration has been most corrected can be accomplished.

If the three-fold astigmatic fields produced by the three stages of multipole elements are manipulated separately as in the related art, varied aberrations appear on the sample surface18. This makes it difficult to identify aberrations used for axial alignment and lengthens the adjustment. In the present embodiment, an axial alignment is made using variations of spherical aberration caused by simultaneous adjustment of the three-fold astigmatic fields produced by the multipole elements. Therefore, it is easy to judge whether or not the axial alignment has been achieved. Accordingly, the operation for the axial alignment is facilitated. This can contribute to a decrease in the observation time. Furthermore, the charged particle beam system is manipulated in reduced time. Consequently, operator's burden can be alleviated.

Where the intensities or rotational angles of three-fold fields are not set appropriately, three-lobe aberration that is a fourth-order aberration remains as shown inFIGS. 8A-8D.FIGS. 8A-8Bare schematic diagrams showing states in which the phase angle difference between the three-fold astigmatism A31produced by the first multipole element41and the three-fold astigmatism A33produced by the third multipole element43is not (70.5/3)°.FIGS. 8C-8Dare schematic diagrams showing states in which the phase angle difference between the three-fold astigmatism A31produced by the first multipole element41and the three-fold astigmatism A33produced by the third multipole element43is (70.5/3)° but the aberrations A31and A33are not equal in intensity.

When the summation of the three-fold astigmatisms A31, A32, and A33produced by the multipole elements41,42, and43is 0 and the phase angle difference between the three-fold astigmatism A31produced by the first multipole element41and the three-fold astigmatism A33produced by the third multipole element43deviates from (70.5/3)° as shown inFIGS. 8A and 8B, three-lobe aberration remains. On the other hand, when the three-fold fields are so set that the summation of the three-fold astigmatisms A31, A32, and A33produced by the multipole elements41-43is 0 and, at the same time, the phase angle difference between the three-fold astigmatism A31produced by the first multipole element41and the three-fold astigmatism A33produced by the third multipole element43is (70.5/3)° but the astigmatisms A31and A33are not equal in intensity as shown inFIGS. 8C and 8D, three-lobe aberration remains. Accordingly, the distribution of the three-fold astigmatic fields produced by the multipole elements41,42, and43is so varied that the astigmatisms A31and A33are equal in intensity and that the phase angle difference between the three-fold astigmatism A31produced by the first multipole element41and the three-fold astigmatism A33produced by the third multipole element43is approximately (70.5/3)°. If this variation is brought to a stop when the spherical aberration in the objective lens or three-lobe aberration has been reduced to a minimum, an axial alignment in which the two aberrations have been most corrected as a whole can be achieved.

While embodiments of the present invention have been described so far, charged particle beam systems of the embodiments are not restricted to transmission electron microscopes. The charged particle beam systems of the embodiments can also be applied to scanning electron microscopes, scanning transmission electron microscopes, ion microscopes, focused ion microscopes, and similar other instruments.

Having thus described our invention with the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.