JPH01236563A - Multi-purpose opening angle control device of electron microscope - Google Patents

Abstract

PURPOSE:To select the observation mode as min. probe dia. any as desired, by installing a lens for control of the angle of opening between two steps of condenser lenses and objective lens, and thereby varying the opening angle freely according to the purpose. CONSTITUTION:A lens 3 for control of the opening angle is installed between two steps of condenser lenses 1, 2 and objective lens 4 to vary the opening angle to any value according to the purpose. For an arbitrary probe current IF, the max. focusing depth is attained for a given observation magnification M without changing the objective stop dia. and the probe current IF. Thereby, button or key entry selects the observation mode to provide min. prove dia. and observation mode to provide max. focusing depth for any arbitrary probe current IF, wherein no variation in probe current IF is required.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention provides a multipurpose aperture angle in an electron microscope that can set an aperture angle according to an observation mode with a maximum depth of focus or an observation mode with a minimum probe diameter. Regarding a control device.

[Conventional technology]

In an electron microscope that observes and analyzes a sample by irradiating the sample with an electron probe, the opening angle α is set to minimize the probe diameter dp on the sample surface for any probe current used.

, t(IP), and the aperture angle is controlled to approach the aperture angle αt(IP) that maximizes the depth of focus l for a given observation magnification M. For example, in the past, the objective aperture diameter between the first or second stage focusing lens and the objective lens was changed, or the imaging position of the focusing lens was changed between the focusing lens and the objective aperture for a fixed objective aperture diameter. Is it a rough approximation of the opening monthly interest rate? I was doing 11.

[The invention tries to solve i! ! ! ) However,
The former method of changing the objective aperture diameter is cumbersome as it requires changing the aperture diameter each time depending on the purpose.
In the latter method, in which the objective aperture and diameter are kept constant in the two-stage focusing lens system, the entire probe current range (for example, 10-”
A to 10-'A), there is a problem in that it is difficult to set the opening angle to an ideal value.

FIG. 3 is a diagram for explaining the relationship between the aperture angle of the beam and the unevenness of the sample surface, where 4 indicates the objective lens and 5 indicates the sample surface.

Even if the beam is focused on the sample surface as shown at Pl in FIG. 3, if there are irregularities on the surface of the sample 5, the beam will spread and blur will occur as shown at P2.

At this time, in the case of the beam Bl having a small aperture angle, the width of the blur is smaller than that of the beam B2 having a large aperture angle. In this case, especially when the depth of focus is shallow, the irradiation beam changes greatly depending on the unevenness of the sample surface. There is a problem in that the characteristics of a strong (small change in detected X-ray intensity) spectrometer cannot be fully utilized.

The present invention solves the above-mentioned problems, and aims to provide a multipurpose aperture angle control device for an electron microscope that can arbitrarily change the aperture angle depending on the purpose.

[Means for Solving the Problems] To this end, the present invention provides an electron microscope having two stages of focusing lenses and an objective lens, in which an aperture angle control lens is disposed between the two stages of focusing lenses and the objective lens, It is characterized by a configuration in which the diameter of the objective aperture is not changed for any given probe current by controlling each lens (the aperture angle is set according to the observation mode).

[Effect]

In the multi-purpose aperture angle control device for an electron microscope of the present invention, an aperture angle control lens that can arbitrarily change the aperture angle depending on the purpose is arranged between the two-stage focusing lens and the objective lens. The tjQ observation mode that provides the minimum probe diameter and the observation mode that provides the maximum depth of focus can be freely selected without changing the objective aperture diameter and probe current with respect to the current.

〔Example〕

Examples will be described below with reference to the drawings.

FIG. 1 is a diagram showing one embodiment of a multipurpose aperture angle control device for an electron microscope according to the present invention, and FIG. 2 is a diagram for explaining the depth of focus. In the figure, 1 is the first focusing lens, 2 is the second focusing lens
A focusing lens, 3 an aperture angle control lens, 4 an objective lens,
5 is a sample, 6 is an objective aperture, 7 is an electron gun, 8 is a control unit, 9
indicates a scanning display section, and 11 to 14 indicate lens power supplies.

In FIG. 1, when an electron beam is emitted from an electron gun 7, this beam is focused by a first focusing lens 1 and a second focusing lens 2, and is irradiated onto a sample 5. This sample 5
The probe current IP applied to the objective aperture 6 with radius r is
is determined by the first focusing lens l and the second focusing lens 2.

The aperture angle control lens S changes the angle of the beam incident on the objective lens 4 without changing the probe current Ip, and consequently changes the aperture angle α of the beam incident on the sample surface. Here, each point between the electron gun 7, the first focusing lens 1, the second focusing lens 2, the aperture angle control lens 3, the objective lens 4, and the sample 5, and the distance between each principal surface are determined in order as shown in the figure. , Q, T, V, W. These values can be considered constant at all times, except for slight deviations in the positions of the lens principal surfaces that occur when the excitation of each lens is significantly changed.

Further, the objective diaphragm 6 is arranged near the main surface of the aperture angle control lens 3. The lens power supplies 11 to 14 are power supplies for driving the 11th bundle lens l, the second focusing lens 2, the aperture angle control lens 3, and the objective lens 4, respectively, and as described later, the control unit 8 controls the power supplies required for each lens. controlled to give a focal length of The operation display section 9 provides the control section 8 with information such as the acceleration voltage and probe current of the electron probe, observation magnification, and operation mode selection. In addition, the illustrated C, ZZ, Z
l is the distance from the main surfaces of the first focusing lens 1, second focusing lens 2, and aperture angle control lens 3 to the imaging point.

Next, the operating principle will be explained.

As shown in Fig. 2, when a beam with an aperture angle α is focused on the sample surface, the permissible range l in the vertical direction of the sample surface such that image blurring can be ignored with respect to the observation magnification M, that is, The depth of focus is given by 2α − (2, 1). Here, d is the probe diameter when focused, and r
is the normal resolution of the human eye or the amount of image blur that can be ignored.

Note that when emphasizing that the depth of focus P and probe diameter d are functions of the aperture angle α, they can be expressed as 1(α) and L(
α) etc. When 2<0, the depth of focus P is not defined and a limit is added to 1&magnification M.

Next, in order to find the probe diameter dp (α), first β: Brightness of the electron gun d9: Size of the TL electron gun virtual light source ■1: Acceleration voltage ΔV1; Voltage corresponding to the energy width of the electron beam or acceleration voltage Variation y % Relativistic correction value of acceleration voltage [~V(1+0.978 Xl0-6V) (Pol))
) λ: Wavelength of electron beam (~12.267r [input]) ΔI/I n Fluctuation of excitation current of objective lens 4 C3 + Spherical aberration coefficient Cc + Chromatic aberration coefficient α: Opening angle of electron probe incident on sample 5 (half apex corner). β-β(vm), d, = a* (v-). However, assuming that the acceleration voltage V and the distance W between the objective lens 4 and the sample 5 are constant, and specifying the type of electron gun, β, d9, λ, ΔI/L Cs, and C6 can be regarded as constant values. For a given probe current ■, the probe diameter d that receives most of it (for example, 70% or more) is d,! (α)±Aα−”+Bα:+Cαb
......(2,2) Represented by: Here it is.

Therefore, for such ap''(α), α, l(α
Find the conditions that α should satisfy so as to maximize ). For simplicity, if we set L=22 and h=r/M, we get (
Equation 2,1) becomes L-<h-dp)/α. Therefore, aα 3α aα dp Therefore, if we set 3L/aα−〇, then f (α)=2Cα6−2Aα−”10hdp =0α×α, so f(α)<0 α>α, and f(α)>0 However, , α is in the neighborhood of α... (2, 3) It can be seen that α-αL, which satisfies the following, gives a maximum value of 2. Note that f(α) is the opposite of α, sign.Realistic α required in actual equipment.

The value of can be easily determined by numerical calculation. Also, α, = αL (Ip 9M').

Next, by setting aap/aα=0, it can be seen that α=αo1, which satisfies C α>0 ・old . . . (2, 4), gives the minimum value of the probe diameter dp, as is known.

As described above, for a given probe current Ip, the choice between maximizing the depth of focus 2 or minimizing the probe diameter d is αL or α. □It comes down to the choice.

Next, find the focal length of each lens to obtain these aperture angles α (■,).

First, let α be an arbitrary aperture angle of the beam incident on the sample. The radius R of the spread of beam l on the main surface of the objective lens 4
4 is R4(α)-w·α (2,5). The spread of the beam on the main surface of the aperture angle control lens 3 is limited to the radius r within the objective aperture for any probe current IP, so rs R.

Z, V-Z.

becomes. Therefore, the focal length Mr4 and the magnification M of the objective lens 4
4 is (V Zs (α)) + W (2, 7) MI (α) = □ VZs (α) (2, 8). - Also, the focal length Mr 3 and magnification M of the aperture angle control lens 3
, is the unknown Z determined by the probe current value ■ and the interlocking ratio k of the first focusing lens 1 and the second focusing lens 2 (L)
It can be expressed as (T Zz)+Zs (α) (2, 9). Here, from the law of conservation of brightness...
...(2,11) The magnification M3 of the aperture angle control lens 3 must satisfy the following condition with respect to the probe diameter d4 of the objective lens 4 obtained as follows.

M, MtM, M, =□ ・・・・・・(2,12) MI
, Mz is twice as large as that of the first focusing lens 1 and the 24i-th focusing lens 2. Here, from equations (2, 10) and (2, 12), MI MI = (TZz)・E However, since (2, 13), for a constant MI-MI, 2g E can be a value that does not depend on the opening angle α, but only on the probe Ti'll T p. In other words, α=α, and α=α. .

, the same E can be obtained for . Therefore, M, M, = (T-Z, (IP)) El (1p)...
...(2,14)

Next, by a known method, the first focusing lens 1 and the 24th!
When the focal length of the S-bundle lens 2 is f, , f, M,
M, , Z, (P fl) (Q fz) P r+...
...(2,15) ......(2,16) Therefore, by substituting it into equation (2,14), the interlocking ratio k of the first focusing lens 1 and the second focusing lens 2 is expressed as fl / rz = k (2, 1
7), then k (P+Q+T−-)f,” −(k (P0Q)T+P (Q+T)1 rz+PQ
T=0...(2,18)
Equation 0 (2, 16) arranged in the form of is the second focusing lens 2
Since this is a quadratic equation for the focal length f2 of
If we set P QT = (2, 19), a Ex ” +2 b fg +c=o+++...(
2,20) Therefore, from the condition of fz>0Sa(IP)>0, a(Ir)...(2,21) Also, by equation (2,15), fl (b)=k fl (b)・・・・・・
(2, 22). The value of this interlocking ratio is determined according to the convenience of the electron optical system, and may be a fixed value or may be changed depending on the probe voltage i1r or the purpose.

Substituting 2(2,21) and equation (2,22) into 2(2,16) will yield Z2(IP), and substituting this into 2(2,9) will yield fi(α, IP). It will be done.

Therefore, in the mode that deepens the depth of focus for any given probe current 1p, the control unit 8 uses the aperture angle αt(rp, M) obtained from equations (2, 3) to calculate f, (IP) see ( 2,
22) f2(lp) see (2
,21) f3 (a, (Ip ,M)lsee
(2,9) fa [αt (Tp, M) 1
See (2, 7)... (2, 23) The focal length of each lens can be changed as shown below.

Next, for any probe current I2, in the mode that minimizes the probe diameter, α is obtained from equations (2, 4).

Using pc(Ip), the control unit 8 fr(Ip)
see (2,22)fz (rp)
see (2,21)fr (
α. pt (IP) l see (2,9)
f< (cropt (IP)) see
(2, 7)... (2, 24) Just change the focal length of each lens.

Next, the actual operation will be explained.

When the control unit 8 is given the type of electron gun, acceleration voltage (2), operating path MW, etc., the control unit 8 refers to these parameters and sets the lens power supply 11 .
12 and calculate the focal lengths f, f2 of the first focusing lens 11 and the second focusing lens 2 using equations (2, 23) and (2, 2
Use the value calculated in 4).

Here, if a mode in which the depth of focus becomes deep is selected on the outline display section 9, the control section 8 also refers to the observation magnification M and sends data to the lens power source 13.14, and the aperture angle control lens 3 , the focal lengths fs and f4 of the objective lens 4 are set to values calculated by equations (2, 23). .

If the mode in which the probe diameter is minimized is selected on the display unit 9, the control unit 8 sends data to the lens power supply 13, 14, and the focal length of the aperture angle control lens 3 and objective lens 4 is Let f3 and f4 be values calculated by equations (2, 24).

As described above, it is possible to automatically set the aperture angle according to the purpose for any probe current without changing the objective aperture.

Furthermore, since the focusing lens has two stages, a sufficient reduction ratio can be obtained with a short lens barrel, and the liner tube can also be easily inserted into the pole piece. The aperture angle control lens 3 can now be operated to change the aperture angle according to the purpose.

Note that the present invention is not limited to the above embodiments, and various modifications are possible. For example, in the above embodiment, the case where the image forming point of the aperture angle control lens 3 is between the aperture angle control lens 3 and the objective lens 4 has been described, but when the image is formed behind the objective lens 4 or the sample 5 However, each lens can be linked in exactly the same way. In addition, the selection of the target mode (selection of maximum depth of focus or minimum probe diameter) explained in the present invention is α1,
α. , t, in an electron microscope or the like constructed with a three-stage focusing lens and an objective lens, the third-stage focusing lens is used as the aperture angle control lens, and the lens system is controlled in conjunction as described above. It may be configured as follows. Furthermore, in the embodiment, explanation was given for one objective aperture diameter, but
A plurality of apertures may be used if necessary.

(Effects of the Invention) As is clear from the above description, according to the present invention, an aperture angle control lens that can arbitrarily change the aperture angle according to the purpose is disposed between the two-stage focusing lens and the objective lens. Therefore, for any probe current I, the observation magnification M given without changing the objective aperture diameter and probe current I
The maximum depth of focus can be obtained. Further, for any probe current rr, the observation mode that provides the minimum probe diameter and the observation mode that provides the maximum depth of focus can be selected by button or key operation without changing the probe current rP. Furthermore, the two-stage focusing lens shortens the length of the lens barrel and allows the use of a liner tube, and the aperture angle control lens allows the aperture angle to be controlled without changing the probe current.

Furthermore, since each lens is controlled by an interlocking device, the focusing state does not change even if the aperture angle or the probe current (2) is changed.

Moreover, since the maximum depth of focus can be obtained as mentioned above, it is possible to use horizontal wavelength dispersive spectrometers and
It can exhibit the characteristics of a spectrometer that is resistant to sample irregularities, such as an EDS spectrometer.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of the multi-purpose aperture angle control device for an electron microscope according to the present invention, and FIG. FIG. 3 is a diagram for explaining the depth, and FIG. 3 is a diagram for explaining the relationship between the beam aperture angle and the unevenness of the sample surface. 1... First focusing lens, 2... Second focusing lens, 3
...Aperture angle control lens, 4...Objective lens, 5...
・Sample, 6...Objective aperture, 7...Electron gun, 8...
Control unit, 9...Scanning display unit, 11-14...Lens power supply. From and to Japan Electronics Co., Ltd.

Claims (1)

[Claims] (1) In an electron microscope that has two stages of focusing lenses and an objective lens, an aperture angle control lens is placed between the two stages of focusing lenses and the objective lens, and each lens is controlled to respond to any probe current. A multipurpose aperture angle control device for an electron microscope, characterized in that it is configured to set an aperture angle according to an observation mode without changing an objective aperture diameter.