JP3123861B2 - Electron beam equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron beam apparatus such as an electron microscope or an electron probe microanalyzer (hereinafter referred to as "EPMA"), and particularly to a diameter of an electron probe (hereinafter simply referred to as "probe") on a sample surface. And an electron beam apparatus whose probe current is adjustable.

[0002]

2. Description of the Related Art Conventionally, in an electron beam apparatus having an analysis function such as elemental analysis such as EPMA, an electron gun, a focusing lens system, an objective aperture, an objective lens and a sample position are arranged in this order. The so-called upper objective diaphragm method is adopted, and the probe current and the probe diameter at the sample position can be adjusted.

FIG. 1 is a diagram showing a schematic configuration example of an electron beam apparatus employing an upper objective diaphragm system. In the figure, reference numeral 1 denotes an electron gun,
CL1 is the first focusing lens, CL2 is the second focusing lens, A
P is the objective aperture, OL is the objective lens, 2 is the sample, 3 is the first
The coil of the focusing lens CL1 and 4 are the second focusing lens CL2.
5 is a coil of the objective lens OL, 6 is a control unit,
Reference numeral 7 denotes an operation unit, and O denotes an optical axis. In FIG. 1, each lens shows only its main surface, and the coil of each lens shows only one side.

The electron beam from the electron gun 1 is divided into CL1 and CL2.
Only the electron beam that has been focused and passed through the AP is irradiated on the surface of the sample 2 by the OL.

Now, considering the case where a desired minute area of the sample 2 is to be analyzed by the EPMA, in this case, it is necessary to narrow the probe narrowly. Therefore, the control unit 6 performs analysis for analyzing the minute area. Mode (hereinafter, this analysis mode is referred to as a first analysis mode). Therefore, when the first analysis mode is instructed by the operation unit 7, the control unit 6 determines that the opening angle is the optimum opening angle α based on the set probe current, acceleration voltage, and the like, that is, the given condition. Then, the excitation currents of CL1, CL2 and OL are determined so that the opening angle becomes such that the probe diameter is minimized, and the determined excitation currents are supplied to the coils 3, 4, and 5.

As a result, the probe irradiates the sample 2 with an optimum opening angle as indicated by reference numeral 8 in FIG. 1, so that the probe diameter on the sample 2 is minimized, and the analysis of a minute area can be performed well. It is possible to do.

However, when it is desired to analyze the average composition within a relatively large area of the sample 2, when it is expected that the surface of the sample 2 has relatively large irregularities, or when a probe having a high intensity is irradiated, When it is predicted that the sample 2 will be damaged, the irradiation is performed by expanding the probe diameter on the surface of the sample 2 to several μm to several hundred μm. In this specification, the probe diameter is a value of the radius of the probe.

Accordingly, the control unit 6 is also provided with an analysis mode (hereinafter, this analysis mode is referred to as a second analysis mode) for analyzing the probe while expanding the probe diameter. When instructed, the control unit 6 sets the probe diameter on the sample 2 to a set value based on the set probe current, acceleration voltage, and the like, for example, as indicated by reference numeral 9 in FIG. The focal position of the probe is changed by controlling the focal length of the OL.

[0009]

By the way, in the electron beam apparatus, conventionally, it is usual that the probe current can be arbitrarily changed completely independently of the selection of the analysis mode. When the change of the probe current is instructed from the unit 7, the control unit 6 controls the electron gun 1 so as to give the set probe current.

Therefore, although the probe current can be changed even in the second analysis mode, there is a problem that changing the probe current in the second analysis mode changes the probe diameter on the sample 2.

This will be described as follows. FIG. 2 is an enlarged view of the ray diagram in the vicinity of the OL and the sample 2 in FIG. 1. _Assuming that the probe current is now set to I _P0 ,
At this time, the beam diameter on the OL main surface is R ₀ .
Note that the beam diameter R on the OL main surface is a function of the probe current I _P , which is represented as R (I _P ). Since the characteristics of the electron gun 1 and the focusing lenses CL ₁ and CL ₂ , the diameter of the AP, and the characteristics of the OL are known, the function R (I _P ) is eventually obtained.
Is known.

When the analysis is performed in the first analysis mode, the control unit 6 controls the CL1 so that the probe irradiates the sample 2 at the optimum opening angle α as shown by the dashed line 11 in FIG.
, CL2 and OL are determined.

When the second analysis mode is set, the controller 6 extends the focal length of the OL so that the probe diameter on the sample 2 becomes the designated value r. As a result, the electron beam is focused on the point U by the OL as shown by the solid line 12, so that the probe diameter becomes r on the sample 2.

If the probe current is increased in this state, the optimum opening angle is also increased, so that the beam diameter on the OL main surface increases with the increase in the probe current. Assuming that the beam diameter changes from R ₀ to R ′, in this case, since the focal length of the OL is not changed, the electron beam is focused on the point U as shown by the broken line 13, and thus the probe on the sample 2 The diameter becomes the line segment QS, which is larger than the preset probe diameter r.

On the other hand, if the method of arranging the AP on the OL main surface is adopted, the above-mentioned problem does not occur.
In that case, since the number of apertures is limited, there is a probe current region that greatly deviates from the optimum opening angle. Therefore, an upper objective diaphragm system as shown in FIG. 1 is employed. According to this system, the probe can be irradiated onto the sample 2 at an optimum opening angle, but the above-described problem occurs. Come.

The case where the probe current is increased has been described above. When the probe current is decreased,
As long as the optimum opening angle decreases with the probe current, the probe diameter on the sample 2 becomes smaller than the preset probe diameter by the same argument.

The present invention has been made to solve the above-mentioned problem, and when analyzing by expanding the probe diameter on the sample, the probe diameter on the sample may be changed even when the probe current is changed. It is intended to provide an electron beam device.

[0018]

To achieve the above object, an electron beam apparatus according to the present invention comprises an electron gun, a focusing lens system, an objective aperture, an objective lens and a sample position arranged in this order. In an electron beam apparatus in which the probe diameter at the probe current and the sample position is adjustable,
If the probe current is changed while the probe diameter on the sample is set to a predetermined value, the changed probe current value, the set probe diameter on the sample, and the main It is characterized by comprising control means for changing the focal length of the objective lens so that the probe diameter on the sample does not change based on the distance between the surface and the sample.

[0019]

When the probe current is changed in a state where the probe diameter on the sample is set to a predetermined value, the control means determines the changed probe current value and the set probe diameter on the sample. The focal length of the objective lens is changed based on the distance between the main surface of the objective lens and the sample so that the probe diameter on the sample does not change.

Thus, even when the probe current is changed, the probe diameter on the sample does not change.
Sample analysis can be performed well.

[0021]

Embodiments will be described below with reference to the drawings.
Although the overall configuration of the electron beam apparatus according to the present invention is the same as that shown in FIG. 1, the operation of the control unit 6 is different. Hereinafter, the operation of the control unit 6 will be described.
The principle of how the above-described object can be achieved by performing the following operations will be described.

Now, the probe current is I _P0, and at this time, as shown in FIG. 2, the beam diameter on the OL main surface is R _0.
And the probe is focused on the point U, whereby the probe diameter on the sample 2 is assumed to be r. In this state, it is assumed that the probe current is increased to I _P ′ and the beam diameter on the OL main surface becomes R ′. If the focal length of the OL is not changed when the probe current is changed, the probe diameter on the sample 2 will deviate from the set value r, but as shown by the solid line 14 in FIG.
If the beam diameter on the main surface becomes R ', the probe diameter on the sample 2 can be kept at the set value r by making the probe focus on the point V.

When the probe diameter on the sample 2 is set to r, the probe diameter on the sample 2 is strictly a value obtained by combining the set probe diameter r and the minimum probe diameter r _P. Here, the minimum probe diameter r _P is the electron gun 1
Is the minimum value of the probe diameter which is limited by the luminance, OL aberration, diffraction effect and the like.

Therefore, the actual probe diameter on sample 2 is

[0025]

(Equation 1)

The minimum probe diameter r _P is usually 1 μm.
m, while r is usually 1 μm or more,
Moreover since there are more than twice the minimum probe diameter r _P is the normal, r is can be sufficiently large relative to the smallest probe diameter r _P, therefore (1) value is equal to r be able to.

Next, the arbitrarily given probe current I _P
In contrast, the beam diameter on the OL main surface is R (I _P ), and the beam diameter O when the probe is focused on the sample 2 at the optimum opening angle α
The focal length of L is expressed as f (0, R), and
When the probe is focused behind the sample 2 in FIG. 3, that is, when the beam diameter on the OL main surface is R, the probe diameter on the sample 2 is adjusted so as to have a set value r. The focal length of the OL is f (r, R)
It will be described as follows. Also, f (0, R), f (r,
Let the image distance in R) be w, w + △ w, respectively.

Now, referring to FIG.

[0029]

(Equation 2)

Holds, which is

[0031]

(Equation 3)

Is transformed.

Further, since the object distance is constant regardless of the excitation of the OL, for a given probe current I _P, the following equation (4) is generally established.

[0034]

(Equation 4)

Therefore, the following expression (5) is established from the expressions (3) and (4).

[0036]

(Equation 5)

Therefore,

[0038]

(Equation 6)

However, since this equation (6) generally holds, by substituting r = r and R = R 'into equation (6), the beam diameter on the OL main surface is R' and the sample It can be seen that the focal length at which the probe diameter on r is r can be obtained. That is, as described above, R 'is the probe current I
_Since the position and aperture of the AP are fixed in FIG. 1, f (0, R ′) is uniquely determined from the beam diameter R ′ on the OL main surface, and r Is a set value and w is a specified value, so that f (r, R ')
Can be obtained from equation (6).

Therefore, when the probe current is changed while the probe diameter r on the sample 2 is set, the control unit 6 obtains the focal length of the OL according to the equation (6), and determines the focal length of the OL. A process of supplying the achieved exciting current to the OL coil 5 is performed. Since the relationship between the OL exciting current and the focal length is known, the relationship may be stored in a ROM or the like, and the exciting current may be output using the value of the focal length as an input address.

By performing the above processing, for example, even if the probe current is increased and the beam diameter on the OL main surface is increased from R ₀ to R ′ in FIG.
Is changed, the probe is focused on the point V, so that the probe diameter on the sample 2 remains at r, and the probe diameter on the sample 2 does not change.

As described above, when the probe current is changed, the control unit 6 changes the focal length of the OL based on the equation (6).
Is a function of _P, f (0, R) is a function of R, that is, a function of probe current I _P, after all, the probe radius r and the probe current I on the sample 2 by the control unit 6 is set _The focal length is obtained based on _P and the distance w between the OL main surface and the sample 2.

As described above, the case where the probe current is increased has been described. Even when the probe current is decreased, the control unit 6 performs the operation of the equation (6) to control the focal length of the OL. The probe diameter above is the same as before the probe current was changed.

While one embodiment of the present invention has been described above, another embodiment will now be described. In the above embodiment, when the probe current is changed, the control unit 6 calculates equation (6) to obtain the focal length of the OL. In this embodiment, the control unit 6 uses the excitation parameter of the OL to make it easier. It is intended to obtain the focal length of OL by a simple calculation.

First, the principle of the operation will be described. Now, in FIG. 1, the number of coil turns of the OL coil 5 is N,
If the coil current is I and the relativistic acceleration voltage is V ^* ,
It is known that the OL excitation parameter J is represented by the following equation.

[0046]

(Equation 7)

It is also known that the excitation parameter J is approximately expressed using the focal length f. Therefore, the excitation parameter J is now calculated using the focal length f.

[0048]

(Equation 8)

If the probe irradiates the sample 2 at r = 0 in the equation (8) (hereinafter, the focal length of the OL when r = 0 is f
₀ ), that is, f = f (r, R) = f (0,
R) = f ₀ , the excitation parameter is J ₀ (= J
(F ₀ )), the change ΔJ of the excitation parameter required to change the focal length of the OL from f ₀ to f with the change of the probe current can be expressed by the following equation. it can.

[0050]

(Equation 9)

That is, when the probe current is changed, the probe diameter on the sample 2 does not change if the OL excitation parameter is corrected by the amount given by the equation (9).

Now, when substituting equation (6) into equation (9),

[0053]

(Equation 10)

Where

[0055]

[Equation 11]

Therefore,

[0057]

(Equation 12)

Therefore, equation (10) is

[0059]

(Equation 13)

Is as follows. Here, usually f ₀通常 w,
And probe diameter r on the sample 2 is sufficiently smaller than the beam diameter R on the OL principal surface, but now for example, f ₀ / w
= 0.9, r / R = 0.3

[0061]

[Equation 14]

From the equation (14), the equation (10) is changed to r / R
It can be seen that the error when developing and approximating to the first order is at most about 5%. The value of r / R = 0.3 is a value close to the maximum value of r / R.
Since R = about 0.01 to 0.1, the error is smaller.

Therefore, within this error range,

[0064]

(Equation 15)

Can be obtained.

Here, when the probe current is the reference probe current I _P0 , the beam radius on the OL main surface, that is, R (I _P0 ) is represented as R _0, and the probe diameter on the sample 2 is predetermined. It is assumed that the probe current I _P is changed when the reference probe diameter r ₀ is present. At this time

[0067]

(Equation 16)

Given the value of f ₀ , this value is given by f ₀ from equation (15).
_{(R (I P)) =} f 0 (I P), f 0 (R (I P0)) = f 0
(I _P0 ) and is represented by the following equation.

[0069]

[Equation 17]

By the way, f ₀ (I _P ) / f on the right side of the equation (16)
The value of ₀ (I _P0 ) is considered to be approximately 1 within the range of the first-order approximation (actually, when the probe current is in the range of 10 ⁻¹² to 10 ⁻⁵ A, the change of f ₀ is about 3% or less. So) From the formulas (15) and (16),

[0071]

(Equation 18)

Is obtained. Since the right side of equation (17) is the same as the left side of equation (9), if the probe current is changed, (1
The probe diameter on the sample 2 can be kept unchanged by correcting the OL excitation parameter by the amount obtained by the calculation on the left side of the equation (7).

Note that the difference between f ₀ (I _P ) and f ₀ (I _P0 ) is so different that the value of f ₀ (I _P ) / f ₀ (I _P0 ) is almost 1 within the range of the first-order approximation. Can be assumed, but even when the object distance greatly changes when the probe current is changed, the focal length of the OL is linked to the excitation of the focusing lens at the focus position (r = 0). Since means for keeping the in-focus state constant is known, here, f
The value of _{0 (I P) / f 0} (I P0) is to be considered as within the scope of first-order approximation is approximately 1.

Considering the physical meaning of each term on the left side of the equation (17), △ J (r, R ₀ ) indicates that the probe current _IP is the reference probe current I _P0 , and that Means the correction amount of the excitation parameter to be corrected when the probe diameter on the sample 2 is changed to r when the beam diameter is R ₀ . △ J (r ₀ , R) is the probe current I _P
Is changed to change the beam diameter R on the OL main surface, it means the correction amount of the excitation parameter to be corrected to keep the probe diameter on the sample 2 at the reference probe diameter r _0. In particular, △ J (r ₀ , R ₀ ) is a value when R = R ₀ in the correction amount of △ J (r ₀ , R).

Therefore, the probe current is set to the reference probe current I _P0, and the excitation parameter required to make the probe diameter on the sample 2 arbitrary r when the beam diameter on the OL main surface is R ₀ . When the correction amount △ J (r, R ₀ ) is theoretically or experimentally determined for each probe diameter r and tabulated (hereinafter, this table is referred to as a first table), and the probe current _IP is arbitrarily changed. In addition, the correction amount ΔJ (r ₀ , R) of the excitation parameter necessary for keeping the probe diameter on the sample 2 at the reference probe diameter r ₀ is obtained theoretically or experimentally for each probe current I _P and a table is obtained. (Hereinafter, this table is referred to as a second table), and the control unit 6 manages these two tables. The control unit 6 holds the value of △ J (r ₀ , R ₀ ).

When the probe current is changed while the probe diameter r on the sample 2 is being set, the control unit 6 determines from the first table based on the set probe diameter r. J (r, R ₀₎ with obtaining the correction amount of, calculated from the second table △ J (r _0, R) based on the modified probe current I _P, further △ J (r _0, R
_From the value of ( ₀ ), the right side of the equation (17) is calculated, and the correction amount of the OL excitation parameter is obtained.

Then, the control unit 6 determines how to change the focal length based on the correction amount of the excitation parameter (9).
The excitation current to be supplied to the OL coil 5 is determined from the relationship between the expressions (7) and (8) based on the expression, and the determined excitation current is supplied to the coil 5.

The relationship between the correction amount of the excitation parameter and the focal length is uniquely determined from the equation (9), and the relationship between the focal length and the excitation current is determined by the relativistic acceleration voltage since the number of coil turns N is constant. For example, since it is unambiguously determined from the relationship between Eqs. (7) and (8), instead of using Eq. (9) and Eqs. (7) and (8) as described above, the It is also possible to determine the relationship between the exciting currents in advance and make a table, so that the exciting current can be directly obtained from the obtained correction amount of the exciting parameter.

Although the embodiments of the present invention have been described above, it is apparent to those skilled in the art that the present invention is not limited to the above-described embodiments, and that various modifications are possible.

[0080]

As is apparent from the above description, according to the present invention, even if the probe current is changed, the probe diameter on the sample is automatically kept constant, so that the sample analysis can be performed well. And operability is improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a diagram for explaining a problem to be solved by the invention.

FIG. 3 is a diagram for explaining a general formula for a probe diameter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electron gun, CL1 ... 1st focusing lens, CL2 ... 2nd focusing lens, AP ... Object stop, OL ... Objective lens, 2 ... Sample, 3 ... Coil of 1st focusing lens CL1, 4 ... 2nd focusing lens CL2 Coil of the objective lens OL,
6: control unit, 7: operation unit, O: optical axis.

Claims (1)

(57) [Claims] An electron gun, a focusing lens system, an objective diaphragm,
In an electron beam apparatus in which the objective lens and the sample position are arranged in this order, and the probe current and the probe diameter at the sample position can be adjusted, when the probe diameter on the sample is set to a predetermined value. When the probe current is changed, the probe diameter on the sample is determined based on the changed probe current value, the set probe diameter on the sample, the distance between the main surface of the objective lens and the sample, and the like. An electron beam apparatus comprising: a control unit that changes a focal length of an objective lens so that the focal length does not change.