JPS5911179B2 - Electron beam deflection device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in an electron beam device effective for observing selected area diffraction images using a scanning electron microscope.

In order to obtain a quasi-Kikuchi pattern from a microscopic region of a sample using a scanning electron microscope, it is necessary to angularly scan a well-paralleled electron beam centered on the microscopic region of the sample to be analyzed.

As an example of such an angular scanning method, as shown in FIG. There is a method that uses refractive power.

This method? When the angle of incidence on the sample is α, the electron beam is not fixed at one point on the sample due to the spherical aberration of the objective lens, but moves on the sample in proportion to α3.

This is a phenomenon in which the focal length of the objective lens for an electron beam that passes off-axis of the lens is apparently shortened, and the electron beam intersects with the optical axis closer to the lens than the sample surface.

Conventionally, in order to avoid movement of the electron beam due to this spherical aberration, an auxiliary lens was provided in the objective lens, and the auxiliary lens generated a lens magnetic field that was synchronized with the deflection angle by the deflection system and weakened the strength of the objective lens. A method was proposed in which the focal length of the objective lens was increased by applying an excitation current to the lens.

However, this method has manufacturing difficulties in that the optical axis of the objective lens and the optical axis of the auxiliary lens must be mechanically aligned, and the inductance of the auxiliary lens causes rapid changes in the input signal. On the other hand, it is difficult to respond faithfully to the lens strength, and it is difficult to perform fast angle scanning.

The present invention is aimed at eliminating the above-mentioned drawbacks, and as shown in an embodiment thereof in FIG. 2, two-stage deflection systems D1 and D2 are provided on the incident electron beam side of the objective lens 2. It is characterized by:

The deflection system D1 in FIG. 2 has the same function as the deflection system D in FIG. 1, and the deflection system D2 is for correcting the spherical aberration of the objective lens.

Now, when an electron beam is irradiated onto the sample 1 placed on the image plane of the objective lens 2 at an incident angle α with respect to the optical axis 3 of the objective lens, the spherical aberration coefficient of the objective lens is set as C8, and the electron beam is directed from the sample to the objective lens. To C8・α? It intersects the optical axis 3 at the approaching plane P1.

Therefore, the distance from the sample surface to the side opposite to the objective lens is △b=c
If an electron beam is incident on the objective lens from the object plane position 02, where the image plane position is plane P2, which is 8.α2 away, the electron beam intersects with the optical axis on the sample, and when performing angular scanning, it will be focused at one point on the sample. The irradiation position can be fixed.

In order to perform such deflection, the present invention provides two-stage deflection systems D and D2 on the electron beam incident side of the objective lens, and by arbitrarily varying the deflection angle β of these deflection systems, the apparent deflection position can be adjusted. 02 is moved so as to satisfy the above-mentioned conditions.

Next, conditions for performing the above-described deflection will be described.

In Fig. 2, the deflection angles by the deflection systems D, D2 are 01,
θ2, the image plane P1 of the objective lens, the object plane 01 and the deflection system D
1 and D2 are defined as shown in the figure.

In general, the focal length of the lens is f, and the object surface position a is a-△a.
When moving to , b becomes b + △b.

Therefore, the object surface distance 02 and the image plane P2 of the objective lens described above are
The moving amounts Δa and Δb are expressed by the following equations.

Also, from FIG. 2, the following equation holds true.

(a-△a-l2)・(θ1+θ2)==73 ,-θ
1 ””” (2) (a-△a)-(θ1+θ2
)=b'Ct"" (3)(1)? (
2) From equation (3)? can get.

This equation (4) t (5) is a scalar equation, and θ
1, indicating that θ2 faces the same direction as α.

When θ, θ2, and α in equations (4) and (5) are viewed as vectors, similarly, θ1, θ2, and α must be quantities in the same direction, so α2 must be a scalar product.

The deflection field in each deflection system consists of two deflection field components orthogonal to the optical axis 3. If these two directions are the X direction and the y direction, then the deflection angles θ, θ2 and the electron beam The incident angle α is expressed as follows, where i and j are unit vectors in each of the x and y directions.

Here, subscripts such as l x and ly indicate the X and y directions, respectively.

Also, by raising α in equation (6) to the third power, 12=j2=1 and i j
When summarized using the relationship = O, the following equation (6)' holds true.

α3-i (α3X to ten・α2)+j(α3y+αy・α
2ωy...(6)' (6)? Substituting α and α3 in equation (6)' into equation (4), the equation for θ1 and 1 in the equation for θ1 in (6)
, J are equal, the following (7) t (s)
The equation is obtained, and in the same way, the equations for α and α3 in equations (6) and (6)' are substituted into equation (5), and the coefficients of i, j in the equation for θ2 and the equation for θ2 in (6) are obtained. Assuming that they are equal, the following equations (9) and (10) are obtained.

θIX-K1゜ctX-K2゜ax3-K2゜(OtX
゜ay2! ...(7) θ1y: K1may−K2*ay
s-K2●αy+αX2. ．． ．． (8) θ2X2K3・α
E1K4・Scream 3+K4・αe・αy2. ．． ．． (9) θ2
y=K3・α+he・αy30K4・αy・αX2...
(1o)y is obtained.

However, K1tK2 and KstK4 are expressed as follows.

K1, K2, K3, and K4 in equation (2) are constants determined by the positional relationship of the optical system and the spherical aberration coefficient.

Therefore, the deflection angle θI X tθl y tθ2 X 2θ2y in the X direction and y direction by the deflection systems DI and D2 is (
By changing the equations 7) to (10) to satisfy the equations, it is possible to angularly scan the electron beam with the electron beam fixed at one point on the sample.

Incidentally, information obtained by detecting signals generated from a sample by an angle-scanned electron beam is often displayed using a cathode ray tube.

In this case, the horizontal and vertical deflection angles of the electron beam in the cathode ray tube are determined by the incident angle component α of the electron beam irradiating the sample, respectively.
Well, it is normal to make it proportional to αa.

Therefore, since it is normal to make the sweep voltages vX and vy from the (sawtooth) sweep signal generation circuit applied to the deflection system of the cathode ray tube proportional to the sweep voltages vX and vy, assuming that the proportionality coefficient is 01, the following relationship holds true.

α=C −V jαy=C1−Vy−fl2)XIX In general, the current i flowing through the deflection coil and the deflection angle θ are expressed as follows, with the deflection sensitivity C2 of the deflection coil as a constant.

θ=C2 ・i ・・・・・・
...(L3) Now, the current and deflection sensitivity supplied to the deflection coils D1 and D2 for the sample irradiation electron beam are set to 11X, respectively.
, 11y l ''2Xt i2ytc3 ,C4, the above-mentioned equations (7) to (10) can be expressed as follows.

11X=K1・A1・iX-K2・A2・iX8-K2
・A2・ix-iy2 ・・・・・・・・・α
滲11y=K,・A,・ly K2・A2・t y3−K2・A2・iy−ix2 ・・・・・・・・・
・(L 12X=K3・B1・iX+K4・B2・ix
30K4・B2・I
・・・・・・・・・(16) 12y-K3・B1・iy+K
4・B2・iy3+K,・B2・i −iX2
......αηy However, AI, A2tB1, and B2 are expressed as follows.

All of these equations are equations derived when a magnetic field generating coil is used as a deflection means, but when a deflection means based on electrostatic deflection is used, 11 in equations (14) to αη above are obtained.
xtl2X, l1y, 12y are deflection voltages applied to each electrostatic deflection electrode V1 x t V 1 y t V2 z
,”2 Replace with y proportionality constants A1, A2, B1, B2
If A1', A2', B1' t B2' are replaced, the following relational expression is established.

V1X=K1・AjuvX−K2・A2′・VX3−K2
-A2' -VX-V 2-----・--(14)'
y V, y2K1・B1′・Vy−K.・B2'・Vy3-
K, -B2' -V -VX2 . ．． ．． ．． ．． ．．
．． ．． ．． (151/y V2X=K3・B1'・VX+K4・B2'・VX3+
K4-B2'-VX-Vy” −・−・・α
6)'v2y2K3・B1'・Vy1K4・B2'・v
y3+K2-B2'-V, "VX"""""aη' Next, an example of a deflection circuit that satisfies the relational expressions (14)' to (17)' will be explained based on FIG.

Figure 3 shows the first part of the optical system 4 that irradiates the sample with an electron beam.
This figure shows a circuit configuration for determining the deflection voltage V1X to be supplied to the electrostatic deflection electrode 5X of the third stage using the sweep voltage Vx tVy of the sweep signal generation circuit.

In the figure, 6 and 7 indicate sweep voltage generation circuits that generate sweep voltages VX and Vy, respectively, and a part of their output is directly applied to the deflection electrode 9xt9y of the cathode ray tube 8 via an amplifier (not shown). .

On the other hand, a part of the outputs of the circuits 6 and 7 are supplied to the squaring circuits 10 and 11, and the outputs of the squaring circuits 10 and 11 are supplied to the adding circuit 12.
is applied to obtain a signal of (V2+V2).

The signal is further multiplied by (-K2·A2') by the xy amplifier 13 and sent to the integrating circuit 14.
is applied to one input terminal of

Since the signal VX is applied to the other input terminal of the integrating circuit 14, the output of the integrating circuit 12 is -K2·A 2 '·
(vX3+vX-Vy').

The signal output is added to the signal output K1, A I ', VX of the amplifier 16 in the adder circuit 15, and v which satisfies the equation α→' is added.
1X=K,・A1'・VX-K2・A2'・vX3-K
An output signal of 2.A2'.vX-Vy2 is obtained.

The circuit shown in FIG.
The circuit for generating the deflection signals expressed by equations (19' to αD') is also constructed in the same manner as the circuit shown in FIG.

Furthermore, it goes without saying that when a deflection coil is used, a circuit having substantially the same configuration as the circuit shown in FIG. 3 is used.

Of the two-stage deflection system for the sample irradiation electron beam, the second
When the deflection system of the second stage coincides with the main surface of the objective lens, l2 in FIG. 2 becomes zero, and (7) t (s
), etc., K2 becomes zero.

Therefore, equation (7) t (8) becomes as follows, θ1X=K1. αE・・・・・・・・・
(7)'θ1y=Kpiαy...
...(8)' The deflection angle by the deflection system and the sample incident angle are proportional.

Since a single amplifier may be used instead of the circuit shown in FIG. 3 as a circuit satisfying these (force/, (S)/ expressions), the apparatus can be significantly simplified.

Next, when the second-stage deflection system D2 coincides with the object plane of the objective lens, a in Fig. 2 becomes a=l2, K1 in the αυ equation becomes K1=O, and (7) , (8) is as follows.

θ = 1K・αB−K2・αE・αy2 ・・・・・・
・(7) “IX2X θ1y=-K2・αy”−K2・αy・αe'...
...(8) "Also, if the first stage deflection system coincides with the object plane of the objective lens, a in FIG. 2 becomes a =
l! 1+l2, and K3 in the αη equation becomes K3=O (9), and the αO) equation becomes as follows.

θ2X=K4・αX3+K4・α・α2・・・・・・
・(9)′xy θ2 =K4・α3+K4・α ・α2 ・・・・・・
(10)'y y yx Also, if the second-stage deflection system coincides with the main surface of the objective lens, and at the same time the first-stage deflection system coincides with the object plane of the objective lens, the second stage deflection system coincides with the object plane of the objective lens, In the figure, l1=a t l 2 ”
'O, and K2tKs in equation (11) is K2=0,K
3=O, and equations (7) to (10) become as follows.

θ1X=K,・αE ・・・・・・
(7) ///θ1y=K1・αy...
...(8)//7θ2X=K4・αX3+K,・α
E・αy2・・・・・・(9) “′θ2y−K4・
αy'+K, ・αy・αX' ・・・・・・α ``Under the above-mentioned special conditions, equations (7) to (10) are simplified, so the configuration of the deflection circuit is also simplified accordingly. be converted into

FIG. 4 is a schematic diagram showing the main parts of a device according to still another embodiment of the present invention.

The device shown in the figure has two deflection coils D2 in the second stage shown in FIG.
The angle scanning is performed by a total of three stages of deflection systems D3 and D4.

The second and third stage deflection systems D3 and D4 are configured so that the relational expression θ1 holds true so that the electron beam passes through the center of the objective lens during normal sample surface scanning without angular scanning. There is.

On the other hand, the first stage deflection system D.

A (limited field) diaphragm 17 is provided on the objective lens side, and the focal length of the objective lens is determined so that a reduced image of the diaphragm is formed on the sample with this diaphragm as the object surface.

As shown in Fig. 4, the incident angle of the electron beam on the sample is α, and the deflection angles of the electron beam by the deflection coils D3 and D4 are θ3 and θ4, respectively.
and a deflection coil D.

The angle between the electron beam deflected by and the optical axis is θ.

Then, the geometrical relationship expressed by the following equation holds true.

(a-△a)・(θ4-03) a・θ to the snake.

...... (20) l3・θ3=l4・(θ4
-03) ...... (21) Meanwhile, the fourth
In the device shown in the figure as well as in the above-described device, the deflection angles θ3 and θ4 must be adjusted so that the relationship expressed by equation (1) holds true.

Therefore, by combining the equations (1), α9), and (20) t (2υ), the following equation is established.

@, Equation (c) is one-dimensional, but to think about it in two dimensions, α, θ3, and θ4 are regarded as vectors, and i and j are unit vectors in the X and y directions, respectively, and can be expressed as follows. .

(self) Raise α to the third power and get 12=j2=1 and i-j-0
When summarized using the relationship, the following equation (24)' holds true.

α3=i・(α3+α・α2)tenj・(ανtenαa・
α2x)xxy ......(24)' (auto) and (24)' α and α3 are (19), (2
Substitute into equations 2) and (23) and find θ.

, θ3, θ4, if it is assumed that the coefficients of i and j in equation (24) are equal, the following equation (29) is obtained.

However, Ko' and K' are expressed by the following equations.

Deflection system D is shown in Figure 4.

A configuration for performing normal sample surface scanning and angle scanning by switching the polarized currents supplied to t D3 and D4 is shown.

That is, the changeover switches 18a and 18b are interlocked with each other.
Deflection system D when switched to the a side terminal.

The excitation becomes zero, and a normal scanning signal and a sawtooth scanning signal are supplied from the scanning signal generating circuit 19 to the deflection systems D3 and D4.

Furthermore, when the changeover switch 18at18b is switched to the b side, a deflection signal that satisfies the above equation (25) is sent from the angle scanning signal generation circuit 20 to each deflection coil DO t D3 t
It is supplied to D4.

As explained in detail above, by performing angular scanning of the electron beam irradiating the sample using a two or more stage deflection system so as to satisfy a certain relationship, high-speed angular scanning can be performed without being affected by the spherical aberration of the objective lens. It is possible to do it without any trouble.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram for explaining the case of angular scanning of the electron beam irradiated to the sample, Fig. 2 is a schematic diagram for explaining the principle of angular scanning according to the present invention, and Figs. 3 and 4 are the diagrams according to the present invention. 1 is a schematic diagram showing the main parts of an apparatus for implementing the 1... Sample, 2... Objective lens, 8... Braun tube.

Claims (1)

[Claims] 1. In an apparatus in which an electron beam incident on an electron lens is made to be incident off-axis of the electron lens by a deflection system of two or more stages, and the electron beam always irradiates one point of the sample using the refractive power of the electron lens, When performing angular scanning of the incident angle α of the sample irradiation electron beam represented by angular components αE and αA in the X and Y directions orthogonal to the optical axis of the electron lens on a plane perpendicular to the optical axis of the electron lens, An electron beam deflection apparatus characterized in that means is provided for supplying a deflection signal given as a function of the angular components αe and αa to each of the deflection systems.