JPS6338825B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electron lens for an electron microscope, and in particular to an electron lens improved so that the amount of heat generated per unit time remains approximately constant while the magnetomotive force is varied stepwise or continuously. It is something.

An example of the structure of an electron lens used in a conventional electron microscope is shown in FIG. This consists of a yoke 1 which is made of a cylindrical body as shown in FIG. 1 and has a lens gap 2 at the center of its inner circumference, and an excitation coil 5 wound around a coil winding frame 3 disposed inside the yoke 1. and a cooling means 4 disposed adjacent to the outside of the coil winding frame 3 in order to cool down the Joule heat generated when the excitation coil 5 is excited by conducting current. conducts current through an electrical circuit as shown in FIG. This electrical circuit consists of power supply 7
, a control element 8 such as a transistor connected to this power supply 7, an excitation coil 5 and an amplifier 9 that are each separately connected to this control element 8, and a variable resistor 10 that controls the operation of this amplifier 9. .
Then, by operating the variable resistor 10,
The magnetomotive force can be adjusted by changing the strength of the current flowing through the exciting coil 5. The joule heat generated from the excitation coil 5 is cooled down by the cooling means 4.

However, in the electron lens having the structure shown above, the cooling operation by the cooling means is still insufficient, and it is difficult to completely thermally isolate the excitation coil 5 and the yoke 1. In particular, for objective lenses such as scanning electron microscopes and X-ray microanalyzers, the excitation coil 5 is made to have a relatively small cross-sectional area in order to reduce the working distance.
A mini lens in which a large current flows through the excitation coil 5 is sometimes used. In this case, the Joule heat generated from the excitation coil 5 is extremely large, and it is difficult to completely insulate the space between the excitation coil 5 and the yoke 1, so if the magnetomotive force of the lens is changed, it will be transmitted to the yoke 1. The amount of heat also changes significantly. For this reason, the lens gap, lens aperture, etc. change due to thermal expansion, and focus drift is likely to occur.
Furthermore, in high-resolution electron microscopes, analytical electron microscopes, and the like, there are cases where it is desired to significantly change the excitation of a mini-lens, objective lens, etc. provided near the sample. When such changes occur, the sample holder or sample stage expands or contracts under the influence of heat, causing sample drift, which becomes a major hindrance to image observation.

In order to reduce such focus drift and sample drift caused by changes in the amount of heat generated in the excitation coil 5, the excitation coil 5 has conventionally been constructed from two coils, so that the magnetomotive force of the lens is
When , each coil generates a magnetomotive force of Jo/2 pointing in the same direction, while the magnetomotive force of the lens is set to 0.
When you want to increase the magnetomotive force of the lens, generate a magnetomotive force of Jo/2 in one coil, and conduct a current in the opposite direction to the other coil to generate a magnetomotive force of -Jo/2. There is an example of a configuration in which the amount of heat generated from each coil is the same when Jo is 0.

However, in such a conventional example, the sum of the amount of heat generated from the coil is the same only when the magnetomotive force of the lens is set to Jo and 0, but in any other excitation, the amount of heat generated from the coil is the same. The sum of quantities cannot be made constant. This is because current of the same strength always flows through the two coils, and the only effect on one coil is to reverse the polarity of the other coil. In a lens that uses such a method of reversing the coil polarity, by increasing the number of coils, it is possible to increase the types of magnetomotive force that can be obtained under the condition that the total amount of heat generation is constant.
It is not practical because the magnetomotive force of the lens is subject to certain limitations, the number of coils is limited due to structural issues, and the number of circuits that reverse the magnetism of the coil increases in the coil excitation circuit, making it complicated. There was a problem.

The present invention has been made with attention to such conventional problems, and its purpose is to configure an excitation coil with two coils, change the current flowing through these coils in multiple stages or continuously, By providing an electronic lens in which the magnetomotive force of the lens is changed in accordance with this change, and the sum of the amount of heat generated from each coil is approximately constant for various lens magnetomotive forces during this period, the said It is an object of the present invention to provide an electron lens that minimizes focus drift or specimen stage drift caused by changes in excitation of the electron lens, thereby solving the above-mentioned conventional problems.

Embodiments of the present invention will be described in detail based on the accompanying drawings.

FIG. 3 and FIG. 4 are diagrams showing an embodiment of the present invention. The electronic lens according to this embodiment includes a yoke 1, an excitation coil 5 which is composed of an inner or first coil 5a and an outer or second coil 5b, which are wound around a coil winding frame 3 disposed inside the yoke 1. and,
In order to cool down the Joule heat generated from the excitation coil 5, the coil winding frame 3 is provided with a cooling means 4 adjacent to the outside thereof. As shown in FIG. 3, the first coil 5a constituting the excitation coil 5 has a winding cross-sectional area of a and b, and the distance from the lens center axis 6 to the winding center is set to _r1. It has been done. On the other hand,
The cross-sectional area of the winding of the second coil 5b is set to a and b like the first coil 5a, and the distance from the lens center axis 6 to the winding center is set to _r2 .
Although the first coil 5a and the second coil 5b have the same cross-sectional area as described above, they may have different cross-sectional areas.

In the electronic lens in which the excitation coil 5 is constituted by the first and second coils 5a and 5b, the magnetomotive force of each coil 5a and 5b is adjusted appropriately to obtain a lens magnetomotive force of an arbitrary magnitude. By setting , it is possible to obtain the lens magnetomotive force while keeping the total amount of heat generated from the excitation coil 5 constant at all times per unit time. For example, the Joule heat generated in each coil 5a, 5b per unit time (the amount of heat generated is W) can be estimated as follows.

Generally the coil is Jc (AT: Ampere Turn)
When generating a magnetomotive force of is inversely proportional to the cross-sectional area of A coil is usually made by winding a thin conductive metal wire many times, but here, the cross-sectional area of the coil bundle (denoted as A and B) is the cross-sectional area of the coil, and the distance from the central axis of the lens to the center of the cross-section of the coil bundle is Consider the case where the coil is wound at a distance (r). When the resistivity of the coil is ρ, the following equation holds true.

W=ρ2πrJc ² /A・B...(1) The resistivity ρ is a constant that depends on the type of conductor, the proportion of the conductor in the coil cross section A and B, the coil temperature, etc., and can be determined by experiment. Can be done.
Based on this equation (1), the amount of heat generated in each coil 5a, 5b of the present invention is determined.

If the magnetomotive force of the first coil 5a is J ₁ and the magnetomotive force of the second coil 5b is J ₂ , and if the total amount of heat generated in each coil is W _T , the following equation holds true.

W _T =k ₁ J ₁ ² +k ₂ J ₂ ² (2) Here, k ₁ and k ₂ are constants determined by the coil shape of each coil 5a, 5b and other factors.

Further, the magnetomotive force J generated in the lens gap 2 is given by J=J ₁ +J ₂ (3). Place the lens coil on the lens center axis 6.
J ₁ and J ₂
can take positive or negative values.

Now, if the maximum value of the lens magnetomotive force is J _M , the minimum value (W _T min) of the total heat generation amount W _T is given by Equation (2) and
It can be obtained from (3), and is given by W _T min=k ₁・k ₂ /k ₁ +k ₂・J _M ² ……(4).

Also, the excitation force J ₁ , J ₂ of each coil at J=J _M
is given by J ₁ =K ₂ /K ₁ +K ₂ J _M , J ₂ =K ₁ /K ₁ +K ₂ J _M ...(5).

At this time, from equations (2), (3), and (4), for the lens magnetomotive force J in the range of _0JJM , each coil 5a,
5b, the total amount of heat generated per unit time is always expressed as formula (4)
It is possible to find the values of the magnetomotive force J ₁ and J ₂ of each coil, which is W _T min given by . As an example, for each coil 5a, 5b, a=15mm,
Let b=20mm, and r ₁ for the first coil 5a.
= 20 mm, and for the second coil 5b, r ₂ = 40 mm, and the total heat generation amount W _T is always constant for the lens magnetomotive force in the range of 0 J J _M when J _M = 2000 (AT). The values of J ₁ and J ₂ of the coils 5a and 5b are determined as shown in FIG. In this graph, the vertical axis indicates the magnetomotive forces J ₁ and J ₂ given to each of the first coil 5a and the second coil 5b, respectively. Further, the horizontal axis indicates the magnetomotive force J of the entire exciting coil 5, which is obtained by adding the above J ₁ and J ₂ . That is, in this graph, J ₁ at the left end
is approximately -900 (AT), J ₂ is approximately -900 (AT), and J =
J ₁ + J ₂ is 0, and J ₁ is about -100 in the center
(AT) J ₂ is approximately 1100 (AT), and J = J ₁ + J ₂ is abbreviated.
1000 (AT). Furthermore, at the right end, J ₁ is approximately
1200 (AT) J ₂ becomes approximately 800 (AT), and J = J ₁ + J ₂ is
2000 (AT). When J ₁ and J ₂ are determined so as to obtain a desired value of J, the calorific value determined by K ₁ J ₁ ² +K ₁ J ₂ ² is set to be always constant.
In this way, when obtaining the magnetomotive force of the exciting coil 5 with a certain value of 0J2000 (AT), if the magnetomotive force of each coil 5a and 5b is set to J ₁ and J ₂ shown in this graph, the magnetomotive force of the exciting coil 5 can be obtained. The amount of heat generated can be kept constant, and focus drift and sample stage drift due to changes in heat generation due to changes in excitation can be prevented.

The excitation force of a predetermined excitation coil 5 is selected on such a graph, and each coil 5a, 5 is adjusted accordingly.
FIG. 4 shows an electric circuit that generates predetermined magnetomotive forces J ₁ and J ₂ at magnetomotive force J 1 and J 2 . The electrical circuit shown here is
Each coil 5
The excitation of a and 5b is switched. This electric circuit includes a power variable means 30 consisting of a control element 8 connected to a power source 7, an amplifier 9 connected to the control element 8 and amplifying the current flowing to the control element 8, and contacts each having a different output. 11a to 11f, and an excitation correlation value setting circuit 12 which changes the excitation strength of the first coil 5a by converting the input current to the amplifier 9, and a contact point for switching the excitation correlation value setting circuit 12. 1
1a to 11f, a first coil excitation device 31 consisting of a switch mechanism 11, and similar to the first coil excitation device, a power variable means 40 consisting of a control element 18 and an amplifier 19, and a contact 21.
It incorporates a second coil excitation device 41 comprising an excitation correlation value setting circuit 22 having coils a to 21f and changing the excitation strength of the second coil 5b, and a switch mechanism 21. The first coil excitation device is connected to the first coil 5a via a relay switch 13 which operates as a polarity switching mechanism operated by power from a relay power supply 14 provided separately from the power supply 7. The relay switch 13 constitutes a polarity switching mechanism, and includes switch pieces 13a and 13b connected to each terminal of the first coil, and switch pieces 13a and 13b connected to each terminal of the first coil.
It has contacts 13c, 13d, 13e, 13f that connect the terminals 3a, 13b to the positive or negative terminal of the power source 7, and when the switch is off, one switch piece is connected to the positive terminal and the other switch piece is connected to the negative terminal. coil 5a
is excited to a polarity opposite to that of the second coil 5b, and the connection state to the positive and negative poles is changed by relay operation to switch the polarity of the first coil 5a. This relay switch 13 also has contacts 15a to 1.
A relay operating switch circuit 15 is connected to the relay power supply 14 via a relay operating switch circuit 15 having 5f, and is connected to the contacts 15a to 15f.
The on/off operation is performed by the operation of the . The switch mechanisms or circuits 11, 15, and 21 are all operatively connected to each other and assume corresponding operating positions during excitation switching of the excitation coil 5 in six stages.

Therefore, for example, as shown in FIG. 4, when the switch mechanism 11 contacts the contact 11b, the switch mechanism 21 contacts the contact 21b, and the relay operating switch circuit 15 contacts the contact 15b to turn off the relay switch 13. Therefore, the first and second coils 5a and 5b generate magnetomotive forces J ₁ and J ₂ of predetermined magnitude with mutually different polarities. In the next step, when the switch mechanism 11 contacts the contact 11e, the switch mechanism 21 contacts the contact 21e, while the relay operating switch circuit 15 contacts the contact 11e.
5e to turn on the relay switch 13, the first and second coils 5a and 5b generate magnetomotive forces J ₁ and J ₂ of the same polarity and a predetermined magnitude. In the former and latter operations, the first and second
Since the magnetomotive forces J ₁ and J ₂ of the coils both satisfy the graph shown in FIG. 5, the total amount of heat generated per unit time in the excitation coil 5 is kept constant. Even when the switch mechanism or circuits 11, 15, and 21 come into contact with other contacts, the total amount of heat generated per unit time in the excitation coil 5 is kept constant as described above.

In addition, in the range where the change of each coil magnetomotive force J ₁ and J ₂ with respect to the lens magnetomotive force J is considered linear, the electric circuit is slightly changed, and the total heat generation amount is kept almost constant, and the magnetomotive force J is continuously changed. can be changed. Furthermore, if the graph of J ₁ and J ₂ shown in Fig. 5 is approximated by a broken line, an electric circuit that can continuously change the magnetomotive force J over the entire range from 0 to the maximum value can be constructed. It is also possible to do so.

In the above embodiment, an electronic lens in which the excitation coil 5 is composed of two first and second coils 5a and 5b is given as a specific example, but the excitation coil 5 may be composed of three or more coils. Of course, it may also be configured by For example, in a symmetric objective lens, the sample holder is inserted in a direction perpendicular to the optical axis, that is, the lens center axis 6, so
The lens coil is usually divided into upper and lower parts, and each coil is connected in series. In this case, even if you change the coils to conduct different currents and change the excitation of the upper and lower coils in the same way as above so that the total heat generation amount is constant, the two coils will simply flow upward. Since the upper part and the lower part are simply arranged separately, the flow of heat changes greatly.

Therefore, as a modification of the present invention, an electron lens having a structure as shown in FIG. 6 can be made to solve the above-mentioned drawbacks. This electronic lens has coils 5 and 25 arranged vertically separately.
Further divided into four coils 5a, 5b, 25
a and 25b serve as an exciting coil. If the coils 5a and 25a generate a magnetomotive force J of _J1 , and the coils 5b and 25b generate a magnetomotive force _J2 , when the lens magnetomotive force is changed over a relatively wide range, However, the drift of the sample stage can be significantly reduced.

Therefore, the operability of an apparatus such as an analytical electron microscope that requires changing objective lens excitation over a relatively wide range is improved.

As described above, according to the present invention, an electronic lens is provided in which the excitation coil is divided and the current flowing through the divided coils is regulated to meet predetermined conditions, so that the total heat generated per unit time in the excitation coil is The amount can now be kept constant at all times,
It has become possible to significantly reduce the focus drift of objective lenses such as SEM and XMA. Furthermore, in high-resolution electron microscopes, analytical electron microscopes, and the like, it has become possible to significantly reduce focus drift and sample stage drift caused by changes in the magnetomotive force of objective lenses, etc.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an example of the structure of a conventional electronic lens. FIG. 2 is an electric circuit diagram used in the conventional example of the electronic lens. FIG. 3 is a sectional view showing the structure of an electron lens according to an embodiment of the present invention. FIG. 4 is a diagram showing a control circuit for generating coil magnetomotive forces J ₁ and J ₂ correlated to an arbitrary lens magnetomotive force J in the embodiment of the present invention. Figure 5 shows how to make the sum of the heat generation amount per unit time in two coils constant for any lens magnetomotive force J from 0 to 2000 (AT) in one embodiment of the present invention. FIG. 3 is a diagram showing an example of the correlation between the magnetomotive forces J ₁ and J ₂ of each coil. FIG. 6 is a diagram showing a modification of the present invention applied to an objective lens. DESCRIPTION OF SYMBOLS 1... Yoke, 2... Lens gap, 3... Coil winding frame, 4... Cooling means, 5... Excitation coil, 6
... Lens center axis, 7 ... Power supply, 8, 18 ... Control element, 9, 19 ... Amplifier, 12, 22 ... Excitation correlation value setting circuit, 13 ... Relay switch, 1
1, 21...Switch mechanism, 15...Relay operating switch circuit, 30, 40...Power variable means, 3
1,41...Coil excitation device.

Claims (1)

[Claims] 1. Inner and outer coils 5 having a common center
The coil 5 of the electronic lens 5 having a and 5b
a, 5b, power variable means 30, 40 that change the power applied to the coils 5a, 5b in order to change the excitation strength of each coil 5a, 5b, and an excitation correlation value that controls the power variable means 30, 40. A setting circuit 12, 22 and a coil excitation device 31, 41 consisting of a switch mechanism 11, 21 are connected, and at least one coil and a coil excitation device 31, 4 are connected.
1, a polarity switching mechanism 13 is provided between the polarity switching mechanism 13 and the two coil excitation devices 31,
41, even if the magnetomotive force of the lens is changed many times or continuously, each coil 5a, 5b per unit time
An electronic lens characterized in that the electronic lenses are linked together so that the sum of the amount of heat generated by the two is almost constant. 2. The fixed value of the total amount of heat generated by each coil was made to be approximately equal to the minimum value determined according to the dimensions and resistivity of each lens current variable coil with respect to the required maximum magnetomotive force of the lens. An electronic lens according to claim 1, characterized in that: