JPH02816B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to an objective lens used in an electron microscope or the like, and particularly to an objective lens having three magnetic pole pieces.

[Prior art] If a high-resolution electron microscope is used in transmission image observation mode, images of atomic arrangement can be directly observed.
It can be used as a means of structural analysis of crystals with complex structures. On the other hand, if a high-resolution electron microscope is used in convergent electron diffraction mode, it is possible to determine a point group on a sample, so it has become common to use both observation modes in combination for structural analysis. In order to effectively use both observation modes in combination, it is necessary to obtain a transmission image and a convergent electron diffraction image of the same microscopic region of the sample. However, in order to obtain both images of the same microscopic region of the sample, the Z-direction position of the sample must be moved when switching the observation mode, and adjusting the amount of movement is not only complicated but also requires skill. It takes time. In order to solve such drawbacks, a yoke and a first, second, and third
a first excitation coil for forming a lens magnetic field between the first and second magnetic pole pieces;
a first power supply for supplying an excitation current to the excitation coil; a switching means for switching the direction of the excitation current supplied from the first power supply to the first excitation coil; A second excitation coil for forming a second lens magnetic field between the magnetic pole pieces, and a second power source for supplying an excitation current to the second excitation coil, the first excitation power source An objective lens was proposed in which the observation mode could be changed by changing the direction of the excitation current without moving the position of the sample (Japanese Patent Application No. 45064/1982). In such an objective lens, in order to bring the position of the sample and the position of the sample stage as close as possible and to avoid the influence of vibration as much as possible, the lower side of the tip of the second magnetic pole piece (the third magnetic pole The position of the top surface (on one side) must be placed above the position of the lower top surface on the root side, and furthermore, in order to obtain a relatively narrow focused electron beam when acquiring a diffraction image, the second The angle of inclination θ ₂ of the lower inclined surface of the tip of the magnetic pole piece with respect to the optical axis C must be between 60° and 80°. Therefore, in order to simultaneously satisfy these two additional conditions, the second
The base of the magnetic pole piece was made thin. Therefore, when the second excitation power supply is excited with the opposite polarity, the magnetic flux from the first excitation coil and the magnetic flux from the second excitation coil flow together in the second magnetic pole piece. The second magnetic pole piece causes magnetic saturation. Therefore, if things continue as they are, the second and third
Since the lens magnetic field strength between the second and third magnetic pole pieces becomes smaller than before switching, in order to keep the magnetic field strength between the second and third magnetic pole pieces almost the same as before switching,
Along with this switching, the excitation intensity of the second excitation power source is changed significantly. However, as this excitation intensity is changed significantly, the leakage magnetic field intensity also changes significantly, so in the past, it was necessary to adjust the correction current of the astigmatism correction lens, etc. each time this switching was made. It was troublesome.

[Object of the Invention] The present invention solves these conventional drawbacks and provides an objective lens for an electron microscope or the like that does not require adjusting the current supplied to the astigmatism correction lens even if the observation mode is switched. The purpose is to

[Structure of the Invention] The present invention includes a yoke, first, second, and third magnetic pole pieces attached to the yoke, and a first magnetic field for forming a lens magnetic field between the first and second magnetic pole pieces. an excitation coil, a first power supply for supplying an excitation current to the first excitation coil, and a first power supply from the first power supply.
a switching means for switching the direction of the excitation current supplied to the excitation coil; a second excitation coil for forming a second lens magnetic field between the second and third magnetic pole pieces; and a second power source for supplying an excitation current to the excitation coil, and the position of the top surface of one side of the third magnetic pole at the tip of the second magnetic pole piece is at the third position on the base side.
In an objective lens that is located on one side of the first magnetic pole above the position of the top surface of one side of the magnetic pole, and in which the inclined surface on the third magnetic pole side of the tip of the second magnetic pole piece forms an angle of 60° to 80° with respect to the optical axis, The thickness of the root portion of the second magnetic pole piece is greater than the thickness of the tip portion of the second magnetic pole piece, and an annular recess is formed on one side of the third magnetic pole on the tip side of the second magnetic pole piece. It is characterized by the formation of

[Example] Hereinafter, an example of the present invention will be described in detail based on the drawings.

FIG. 1 is for showing one embodiment of the present invention. In the figure, 1 is a yoke that serves as a path for magnetic flux to form a lens magnetic field. This yoke 1 includes an upper magnetic pole piece 2 and a middle magnetic pole piece 2. A piece 3 and a lower pole piece 4 are attached. 5 is a first excitation coil for forming a first lens magnetic field between the upper magnetic pole piece 2 and the middle magnetic pole piece 3 (first gap G ₁ ); Excitation current is supplied from an excitation power source 6 via a switch 7 . This switch 7 is for switching the direction of the excitation current supplied to the first excitation coil 5. 8 is a second excitation coil for forming a second lens magnetic field between the middle magnetic pole piece 3 and the lower magnetic pole piece 4 (second gap G ₂ );
An excitation current is supplied to this excitation coil 8 by a second excitation power source 9 . 10 and 11 are spacers made of nonmagnetic material. Reference numeral 12 denotes a top-entry type sample tilting device, and by this sample tilting device, the sample 13 is arranged at an angle between the middle pole piece 3 and the bottom pole piece 4. The sample tilting device 12 has a substantially conical shape, and in order to insert the sample tilting device 12, the hole diameter b ₁ of each of the upper, middle, and lower magnetic pole pieces 2, 3, 4,
The relationship b ₁ > b ₂ > b ₃ holds between b ₂ and b ₃ . Also, the gap distance between the middle magnetic pole piece 3 and the lower magnetic pole piece 4 is S ₂ , the diameter of the top surface of the lower magnetic pole piece 4 is D ₃ , and the lower inclined surface of the middle magnetic pole piece 3 and the upper inclined surface of the lower magnetic pole piece 4 Let the inclination angles with respect to the optical axis C be θ ₂ and θ ₃ respectively, in order to reduce the spherical aberration coefficient Cs and the chromatic aberration coefficient Cc,
Applying the conditions known to ordinary electron lenses having two magnetic pole pieces, the following relationships hold: b ₂ >S ₂ , b ₂ >D ₃ and 50°≦θ ₃ ≦70°. Furthermore, 60°≦θ ₂ ≦80° (condition 1), which is a condition for obtaining a relatively narrow convergent electron beam during diffraction image acquisition, is satisfied. Further, in order to suppress the influence of vibration as much as possible, the lower top surface T ₁ of the tip of the middle magnetic pole piece 3 is arranged above the lower top surface T ₂ of the base thereof (condition 2). Furthermore, unlike the conventional
In the vicinity of the top surface _T1 , an annular recess U is formed on the lower pole piece 4 side. is constructed so that it is thicker than the thickness of its tip.

In the objective lens configured as described above, the accelerating voltage is set to 200KV, and an excitation current is supplied from the first power supply 6 to the first excitation coil 5 to open the first gap.
The excitation intensity of G ₁ is set to the same polarity as the second gear G ₂ .
In addition to exciting only 500AT, the second excitation power supply 9
supplying an excitation current to the second excitation coil 8,
When the excitation intensity of the second gap _G2 is changed from 10KAT, the sample position z _p is required for the electron beam transmitted through the sample to satisfy the imaging condition, and the incident electron beam is imaged on the sample surface. I used a computer to calculate how the position z _i that satisfies the conditions (z _p and z _i are based on the top surface of the lower magnetic pole piece 4) changes.
It was found that z _p and z _i change as shown by solid lines o and i, respectively, in FIG. However, in FIG. 2, the horizontal axis indicates the excitation intensity (KAT) of the second excitation coil 8, and the vertical axis indicates the position (mm) of z _p or z _i . Therefore, it is clear from this figure that in the state shown at point A where the second excitation coil 8 is excited to about 12100 AT, both z _p and z _i are at the position Z of 2.2 mm. Therefore, under this excitation condition, when a parallel electron beam EB is incident on the first lens _L1 formed in the first gap _G1 as shown in FIG. 3a, the electron beam EB becomes The electron beam EB, which is narrowly focused and incident on the surface of the sample 13 by the second lens L ₂ formed between the first lens L ₁ and the second gap G ₂ , is transmitted through the sample 13 and passes through the second gap G 2 . G ₂
The third lens L ₃ formed at
A convergent electron diffraction image can be observed. On the other hand, if the switch 7 is switched to reverse the excitation polarity of the first excitation coil 5 to the excitation polarity of the second excitation coil 8, the sample position satisfies the above-mentioned conditions for forming an image of the electron beam transmitted through the sample. z _p and the position z _i that satisfies the conditions for the electron beam incident on the sample to form an image on the sample surface,
In FIG. 2, it was found that the values changed as shown by dotted lines o and i, respectively. From the characteristics shown by this dotted line, when the sample 13 is placed at the position Z, the excitation strength of the second excitation coil 8 is slightly increased by 100 AT from the excitation intensity shown at point A, and the second
The excitation intensity corresponding to point B in the figure (i.e.,
12200AT), it can be seen that only the above-mentioned imaging conditions are satisfied. In this case, as shown in FIG. 3b, the electron beam is once converged by a converging lens (not shown) in the previous stage, and then passed through the first lens.
It is made into a parallel beam by L ₁ and enters the sample 13,
The electron beam EB transmitted through the sample 13 is focused by the third lens _L3 onto the object plane of a distant intermediate lens (not shown). Therefore, this point B
It can be seen that a transmission image of the sample 13 can be obtained under excitation corresponding to . Next, the first excitation coil 5
The combined spherical aberration coefficient of lenses L ₁ , L ₂ , and L ₃ when the excitation intensity of the second excitation coil 8 is changed with the excitation intensity of the second excitation coil 8 being the same as the excitation polarity of the second excitation coil 8 Regarding Cs and chromatic aberration coefficient Cc,
Similarly, when calculations were performed using an electronic computer, the relationship between b ₂ > S ₂ , b ₂ > D ₃ and 50°≦θ ₃ ≦70°, as described above, resulted in the combination of lenses L ₁ , L ₂ , L ₃ The magnitudes of the spherical aberration coefficient Cs and the chromatic aberration coefficient Cc are respectively the fourth
As shown by solid lines Cs and Cc in the figure, it was found that the values were small in the entire region. When the excitation intensity of the second excitation coil 8 is changed while the excitation intensity of the first excitation coil 5 is excited with the opposite polarity to that of the second excitation coil 8, the characteristics are almost the same, and it is practically used. It turned out that there was no problem at all.

Therefore, by exciting the first excitation coil 5 with the same polarity as the second excitation coil 8 and adjusting the output current of the second excitation power supply 9, the current can be approximately 12000 AT corresponding to the second point A. For example, a focused electron beam diffraction image of the sample 13 can be obtained, the switch 7 is switched to excite the first excitation coil 5 with the opposite polarity to the second excitation coil 8, and in this state, the second excitation coil 5 is excited with the opposite polarity to the second excitation coil 8. Coil 8 is approximately 12200AT corresponding to point B in Figure 2.
If the excitation is performed with All you need to do is make a small change.

The embodiment described above is only one embodiment of the present invention, and other embodiments may be adopted when implementing the present invention. For example, in the above-described embodiment, the sample is inserted from above, but it may also be inserted from a direction perpendicular to the optical axis C.

[Effect] As described above, in the objective lens of the present invention, when switching the direction of the excitation current supplied from the first excitation power source to the first excitation coil and switching the observation mode, Since the base part of the magnetic pole piece is thick, almost no magnetic saturation occurs in this part, so it is only necessary to change the excitation current supplied to the second excitation coil by a small amount of about 100 AT. Therefore, not only is the magnification kept substantially constant, but there is no need to adjust the current supplied to the astigmatism correction coil every time this switching is performed, and the operation can be simplified.

[Brief explanation of drawings]

Fig. 1 is a diagram showing an embodiment of the present invention, and Fig. 2 shows the z-direction position zo of the sample that satisfies the condition that the electron beam transmitted through the sample forms an image, and the position zo of the sample where the electron beam incident on the sample forms an image. FIG. 3 is a diagram illustrating the change in position zi that satisfies the condition of image formation on a plane in relation to the excitation intensity of the second excitation coil with respect to the two excitation polarities of the first coil. is a diagram showing an optical diagram when obtaining a convergent electron beam diffraction image and an optical diagram when obtaining a transmission image. Figure 4 shows the spherical aberration coefficient Cs due to changes in the excitation intensity of the second excitation coil. , and is a diagram showing changes in the chromatic aberration coefficient Cc. 1: Yoke, 2: Upper magnetic pole piece, 3: Middle magnetic pole piece,
4: Lower magnetic pole piece, 5, 8: Excitation coil, 6, 9: Excitation power supply, 7: Switch, 10, 11: Spacer,
12: sample tilting device, 13: sample, T ₁ , T ₂ : top surface, U: recess.

Claims (1)

[Claims] 1 a yoke; a first yoke attached to the yoke;
second and third magnetic pole pieces, a first excitation coil for forming a lens magnetic field between the first and second magnetic pole pieces, and a first excitation coil for supplying an excitation current to the first excitation coil. A second lens magnetic field is formed between a first power source, a switching means for switching the direction of an excitation current supplied from the first power source to the first excitation coil, and the second and third magnetic pole pieces. a second excitation coil for supplying an excitation current to the second excitation coil, and a second power supply for supplying an excitation current to the second excitation coil, the position of the top surface of one side of the third magnetic pole at the tip of the second magnetic pole piece being Located on one side of the first magnetic pole above the position of the top surface of one side of the third magnetic pole on the base side, and so that the inclined surface on the third magnetic pole side at the tip of the second magnetic pole piece forms an angle of 60° to 80° with respect to the optical axis. In the objective lens, the thickness of the root portion of the second magnetic pole piece is thicker than the thickness of the tip portion of the second magnetic pole piece, and one side of the third magnetic pole on the tip side of the second magnetic pole piece. An objective lens for an electron microscope, etc., characterized in that an annular concave portion is formed in the objective lens.