JP2003045370A - Scanning electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scanning microscope enabled to observe and analyze in a focalized state at all times even when a specimen is tilted and rotating against the direction of scanning. SOLUTION: The rotational angle θ of a specimen 7 is calculated by a control part 16 depending on arbitrary coordinates (x, y) of two points on one scanning line of an electron beam 4 in the observation/analysis region of the specimen 7, and a focus current variation amount ΔI at every pixels are calculated depending on the rotational angle θ, and a deviation of focus is prevented by superposing the variation amount on a focus current of an object lens 6. A focalized image is easily obtained for the entire observation/analysis region of the specimen 7 even when the specimen is tilted and rotating against the direction of scanning.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning electron microscope having a sample tilting function, and more particularly to a scanning electron microscope adapted to perform focus correction when a sample is tilted.

[0002]

2. Description of the Related Art A scanning electron microscope is also called an SEM, and a sample surface is scanned with an electron beam (electron beam) that is extremely narrowed to detect a signal having sample information, and an SEM image or EBSP is detected.
(Electron Backscatter Di
ffraction Pattern).

At this time, since the electron microscope has a deep depth of focus, it is convenient to obtain an image with a very good focus (focus) over the entire surface of the sample surface having irregularities, which is convenient and the sample is tilted. You can respond well to observation.

However, in the case of observation with high resolution, it is necessary to increase the opening angle of the electron beam and further reduce the probe diameter (electron beam diameter), but at this time the focal depth becomes shallow, so The focus shift becomes large when the sample is tilted. Therefore, this will be described below with reference to FIG.

First, as shown in FIG. 2A, when the sample observation surface is perpendicular to the scanning in the X and Y directions, z =
The focus of the electron beam 4 aligned with the i-plane (z coordinate on the observation plane, where z = 0 plane is the lower surface of the objective lens) does not shift even when scanning in the scanning (observation) area 24. . Here, e ⁻ represents an electron beam.

On the other hand, when the observation surface of the sample is tilted, for example, when the sample 7 is tilted at the angle α ° in the Y direction as shown in FIG. 2 (b), FIG. As shown, a focus shift occurs in the Y direction.

Therefore, in such a case, as shown in FIG. 2D, an objective lens 6 (described later) is provided for each X-direction scanning line.
The so-called tilted focus correction (dynamic focus) in which the focus current is changed is conventionally used.

This tilt focus correction method is applied not only to the observation but also to the EBSP method in which the crystal orientation analysis is carried out by using the "Kikuchi line" obtained by irradiating the sample with an electron beam after tilting the sample at a large angle. Is particularly effective.

[0009]

The above prior art does not consider the case where the sample is tilted and rotated with respect to the scanning direction of the electron beam, and has a problem in correcting the focus shift. . Conventional dynamic focus is
This was effective for the focus correction when the sample is tilted parallel to the X-direction scanning, but it is not possible to correct the focus shift when the sample is tilted and rotated with respect to the electron beam scanning direction. It is possible.

That is, as shown in FIG. 2 (c), when the sample is not only tilted but also further rotated with respect to the scanning direction of the electron beam, the focus of the scanning in the X direction is also increased. Since a shift occurs, the focus cannot be corrected even by using the dynamic focus described above.

As a result, according to the prior art, not only the focus shift at the time of SEM observation cannot be corrected, but also the EBSP
In the case of the method, there is a problem that it is difficult to detect the pattern of the fine crystal grains and to accurately calculate the crystal grain size due to the focus shift.

An object of the present invention is to provide a scanning electron microscope capable of performing observation and analysis always in a focused state even when a sample is tilted and rotated with respect to a scanning direction of an electron beam. To do.

[0013]

The above object is to detect the rotation angle of the sample with respect to the scanning direction of the electron beam in a scanning electron microscope of a system in which the incident angle of the electron beam with respect to the surface of the sample is tilted from the vertical direction for observation. Is provided, and the focus shift of the electron beam on the sample surface is corrected based on the detection result of the rotation angle.

Similarly, the above object is to provide a means for detecting the rotation angle of the sample with respect to the scanning direction of the electron beam in the scanning electron microscope of the type in which the incident angle of the electron beam with respect to the surface of the sample is observed while tilted from the vertical direction. The rotation angle of the sample is corrected based on the detection result of the rotation angle.

At this time, the means for detecting the rotation angle is constituted by means for detecting the rotation angle based on the coordinate positions of arbitrary two points on the same scanning line of the electron beam on the sample surface. You may do it.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION A scanning electron microscope according to the present invention will be described in detail below with reference to the embodiments shown in the drawings.
FIG. 1 is a schematic view of a scanning electron microscope according to an embodiment of the present invention. In this figure, an electron beam 4 generated by an electron gun 1, an extraction electrode 2, and an acceleration electrode 3 is shown.
Is focused into a fine beam by the electromagnetic objective lens 6 as well as the electromagnetic condenser lens 5, and is focused on the surface of the sample 7.

On the other hand, the magnification setting section 10 is set with an arbitrary magnification M from a predetermined input device (not shown). Then, this magnification M is input to the X-direction scanning control unit 11 and the Y-direction scanning control unit 12, and thereby the X-direction scanning control unit 1
1 and Y-direction scanning control unit 12 to X-direction scanning coil 8 and Y
A scanning current suitable for the set magnification M is supplied to each of the directional scanning coils 9, and as a result, the surface of the sample 7 is scanned by the electron beam 4.

When the surface of the sample 7 is irradiated with the electron beam 4 in this manner, secondary electrons 2 are emitted from the portion (point) where the electron beam 4 is converged.
1 is generated, and this secondary electron 21 becomes a secondary electron detector 22.
Detected by. Then, the signal generated by the secondary electrons 21 converted here is supplied to a display device (not shown) to obtain an SEM image display. Also, this S
Simultaneously with the EM image display, the signal detected by the secondary electron detector 22 is also stored in the image memory 23 as an image signal.

At this time, the value of the current supplied to the objective lens 6 is read as the focus current value by the focus current reading unit 15 and input to the control unit 16 composed of a microcomputer. At this time, the control unit 1
Further, the magnification M set in the magnification setting unit 10, the coordinate information and the tilt angle information set in the sample 7 by the stage control unit 14, and the image information stored in the image memory 23 are input to the unit 6. It is like this.

As a result, the control unit 16 controls the sample 7
Of the observation / analysis region designated on the surface of the sample 7 based on the calculated rotation angle θ °. The focus current change amount per pixel is sequentially calculated over the entire range, and the focus current change amount for each pixel is superimposed on the focus current by the objective lens power source 20 to control the focus on the entire observation / analysis region.

Next, in the case of the second embodiment, the control unit 16
Is recorded on the stage control unit 14 based on the calculated sample rotation angle θ °, then rotates the sample stage 13 by −θ °, and controls so that the rotation angle θ ° of the sample 7 becomes a zero value. This makes it possible to focus on the entire observation / analysis region by applying the same dynamic focus as in the prior art.

Further, in the embodiment shown in FIG. 1, the electrostatic auxiliary lens 25 is incorporated in the objective lens system, and the accuracy of the focus control can be further improved by the leverage. Next, the operation of this embodiment will be described with reference to the flowcharts of FIGS.

Here, the processing according to this flowchart is executed by the microcomputer of the control unit 16, and at this time, the control unit 16 instructs the operator of this scanning electron microscope to perform a predetermined operation each time. First, as shown in FIG. 4, the operator moves the sample stage 13 so that the portion to be observed on the surface of the sample 7 has a target visual field.
(501).

Although not shown, the operation at this time is executed by using a predetermined input device provided in the scanning electron microscope itself. Next, the observation magnification Mag (M) and the sample tilt angle Tilt (α °) are input from the input device 16 or the magnification setting unit 10 and the stage control unit 14 (502).

Next, in the displayed target visual field, the (x, y) coordinates of the upper left end (P _max ) are recorded, and -L is set in the X direction so that the end point is located at the center of the screen. _X / 2M, Y
Only -L _Y / 2M direction, the image shift processing,
Alternatively, it is automatically moved by the control of the sample stage 13.

Here, M is the set magnification, L _X is the display distance in the X direction of the image, and L _Y is the display distance in the Y direction of the image, but the position of the end P _max is the x, y coordinate is
If it can be recorded together with the z-coordinate, the operator is not limited to the upper left end portion of the visual field, and the operator may specify it at any position. After the movement, the focus is adjusted by autofocus or manually, and the working distance (or focus current) at this position is calculated and recorded as the maximum working distance Z _max (503).

After the maximum working distance Z _max is recorded in this way, thereafter, by the same image shift processing or stage control, the original visual field is automatically changed by L _X / 2M in the X direction and L _Y / 2M in the Y direction. In this state, the observation / analysis area is determined (504), the image / analysis data size is input (505), the image acquisition time (s / pixel) and the Frame number are input (506). Thus, the observation / analysis time of the entire area total is calculated (507).

At this time, if the analysis time is inappropriate, the process returns to step 504, and the setting work is repeated until a proper time is reached. Then, after this processing 507, the operation is performed when the observation / analysis region is the entire target visual field.

First, the sample 7 in the scanning direction of the electron beam 4
Whether or not the rotation angle (θ °) is known, and if it is unknown, the (x, y) coordinate of the upper right end P _min on the target visual field is recorded, and this upper right end P _min is the center of the screen. As shown in FIG. 2, the image is automatically moved by L _X / 2M in the X direction and −L _Y / 2M in the Y direction by image shift processing or stage control, and the working distance (or The focus current) is calculated, and the calculated value is recorded as the minimum working distance Z _min (508). At this time, the position of the upper right end P _min is limited to the position on the scanning line for which this P _max is designated.

Then, after recording the minimum working distance Z _min , it is automatically moved by −L _X / 2M in the X direction, L _Y / 2M in the Y direction by image shift processing or stage control, and then returned to the original visual field. , Of the maximum working distance P _max already obtained
(x, y) coordinates, the minimum working distance Z _max just calculated, the (x, y) coordinates of the upper right end P _min , and the minimum working distance Z _m
The rotation angle (θ °) is calculated by in as described later (509). On the other hand, the rotation angle (θ °) may be known in some cases, so that it can be input at this time (51
0).

After the calculation or input of this rotation angle (θ °), the process shown in FIG. 5 is followed, first, from the magnification, the scanning distance in the X direction, and the number of pixels in the X direction, the focus per 1 pixel (1 pixel) in the X direction. The change amount Δf _X is calculated as described below.
Similarly, the focus change amount Δf _Y per 1 pixel in the Y direction is calculated as described later (511) (512).

The focus change amount Δ
The focus change amount Δf _Z per pixel in the Z direction during scanning in the X and Y directions is calculated from f _X and Δf _Y , as will be described later (513). Then, after recording (514) this focus change amount Δf _Z in a table, X,
The Y scan is started (515).

At this time, the focus current corresponding to the sequential scanning coordinates (x, y) is set from the focus change amount Δf _Z.
(516) When the scanning point becomes P _{(xm, yn)} , the scanning for one frame is finished (517). On the other hand, after calculating or inputting the rotation angle (θ °), the sample stage 13 is automatically rotated by −θ ° to set θ = 0 ° (518).
You may make it perform the process after (514).

In this case, the sample 7 is brought into a state in which it is not rotated, that is, the same state as in FIGS. 2 (b) and 2 (d).
In the process of 4), the focus change amount Δf _Z may be calculated with the focus change amount Δf _X = 0, or the dynamic focus correction may be performed as in the conventional technique. Observation and analysis can be performed in the state.

Next, a method of calculating the rotation angle θ in steps 508 to 510 described above will be described with reference to FIGS. 2 and 3. At this time, the scanning with the electron beam 4 is performed in the X direction parallel to the X axis and in the Y direction parallel to the Y axis (FIG. 2 (a)).
reference).

As shown in FIG. 2A, when the observation surface is parallel to the z = 0 surface (when the sample is not tilted), an arbitrary point in the scanning area 24 is set to point A ( x _a, y _a, When z _a), z-coordinate z _a of the point a is a z _a = i, therefore,
In this case it is not necessary to change the focus position (z _a).

On the other hand, the plane α when the observation surface of FIG. 2 (a) is tilted by α ° is y = y _d (y _d > 0) and forms an angle α ° with the z = i plane (FIG. 2 (b )-(D), FIG. 3 (a)).

Here, in the case of SEM, the scanning region 24 (x, y coordinates) of the electron beam 4 is irrelevant to the sample tilt angle (the same as when there is no sample tilt). the same x, terms that have a y-coordinate _{a '(x a, y a} , z a') knowing the z coordinate of the control the focus current in a scanning region above the plane alpha, can be focused.

[0039] Here, the 'z-coordinate z _a' of the point A, z _a '= i-a _{(y d -y a) · tanα} , the focus position (z _a') is changed as a function of the y coordinate You can do it. Further, as shown in FIGS. 2 (e) and 3 (b), the plane α rotates by θ ° with respect to the scanning direction of the electron beam (θ
The plane when (= unknown) is the plane θ.

[0040] At this time, the same x and the point A, a point on the plane θ with y-coordinate _{A "(x a, y a} , z a") a z-coordinate z _a "of,
If it can be expressed as a function of x, y, α, θ, point A "
_{(x a, y a, z} a ") by actual measurement, it is possible to determine the value of the rotation angle theta. Therefore, first, in FIG. 3 (a),
Since the z coordinate is expressed as a function of x, y, α, and θ, hypothetical points P, Q, and R are set. Then these points are x = x
and in collinear _a, it is set as follows.

First, letting point R be the intersection point with the y = 0 plane when a normal line of y = 0 passing through point A'is drawn, the coordinates of point R are (x _a , 0, z _a '). Is. Next, when the intersection point with the z = 0 surface when the normal line of the lower surface (z = 0 surface) of the objective lens 6 passing through the point R is drawn is the point Q, the coordinates of the point Q are (x _a , 0, 0, 0)
Is. When the intersection point of the line segment QR and the plane α is a point P, the coordinates of the point P are (x _a , 0, z _p ) (0 <z _p ≦ z
_a ').

[0042] Then, the line segment A'R _{first, A'R = y a ............ (1} ) and expressed, line segment QR is expressed as _{QR = z a '............ (2} ). Thus, these (1) and (2) 2, the line segment QP _{is, QP = z a '-y a} · tanα ............ (3) become.

Next, in FIG. 3B, the z coordinate is x,
Since it is expressed as a function of y, α, and θ, the assumption points P ′, Q ′,
Set R'as follows: Then, when a plane in which the y = 0 plane is rotated by θ ° with respect to the scanning direction of the electron beam is yθ and a normal line of the plane yθ passing through the point A ″ is drawn, the intersection point of this normal line and the plane yθ is defined as a point. If R ′ (x _R ′, y _R ′, z _R ′), the point R ′ is x _R ′ = (x _a −tan θ) · cos 2θ y _R ′ = − (x _a −tan θ) · sin θ · cos θ z _R '= z _a ".

[0044] In addition, the point R 'as a, z = 0 plane intersection point Q of the z = 0 plane at the time obtained by subtracting the normal line of _{the' (x Q ', y Q} ',
z _Q ′), the point Q ′ becomes x _Q ′ = (x _a −tan θ) · cos 2θ y _Q ′ = − (x _a −tan θ) · sin θ · cos θ z _Q ′ = 0.

Furthermore, the intersection point of the line segment Q'R 'and the plane θ is defined as a point P'.
Assuming that (x _P ', y _P ', z _P '), the point P'is x _P ' = (x _a −tan θ) · cos 2θ y _P '= − (x _a −tan θ) · sin θ · cos θ 0 < It can be expressed as z _P '≦ z _a ″.

[0046] Therefore, the line segment A "R _{'is, A"R' = x a} · sinθ + y a · cosθ ............ (4) , and the next line segment P'r 'is by (4) , P′R ′ = (x _a · sin θ + ya _a cos θ) · tan α (5) In this _{case, A "(x a, y} a, z a") and R 'z-coordinate of equal point P', Q ', R' are collinear and 0 <z _P '≦ z _a Therefore, z _a ″ = Q′P ′ + P′R ′.

The plane α and the plane θ are inclined at an angle (α °).
Are the same, and the surface when α = 0 ° (z = i surface)
Since the distance from to the lower surface (z = 0 surface) of the objective lens 5 is constant, QP = Q′P ′ = i−y _d · tan α. Therefore, (3) and (5) _{below, z a "= z a '} - a _{{y a (1-cosθ)} -x a · sinθ} · tanα ...... (6).

[0048] satisfied where (6), and to calculate θ, the two points z _a 'is in the same (x _a, y _a,
z _a ") coordinate is required, but the two points P _max and P _min are on the same scanning line, and therefore, it is possible to satisfy Eq. (6) from these coordinates, and therefore (6) ) Can be used to calculate the rotation angle θ, where Z _max <
Even in the case of Z _min , θ can be calculated in the same manner by the equation (6), so that it is not necessary to replace P _max and P _min .

Next, step 511 to step 5 in FIG.
13 and Δf _X , Δf _Y , Δ in step 516
A method of calculating f _Z and I _B will be described below. First, when the X coordinate of the scanning region is a ₁ , a ₂ , ... _Am , and the Y coordinate is b ₁ , b ₂ , ... B _n , the scanning distance X in the X and Y directions is X.
(m), Y (n) _{is, X (m) = a m} -a 1 = L X / M ............ (8) Y (n) = b n -b 1 = L Y / M ......... … (9).

[0050] Then, X-direction X (m) focus variation amount at the time of scanning Delta] f _X (m) is the (8) and _{(9), Δf X (m) = (} a m -a 1) · sinθ・ Tan α (10), which is the focus change amount Δf during Y (Y) direction Y (n) scanning.
_Y (n) is the (8) and _{(9), Δf Y (n) = {} b n -b 1 + (b 1 -b n) · (1-cosθ)} · tanα ...... (11 ).

Here, the scanning distance X (1), Y per pixel
_{(1), X (1) = a m} -a 1 / m-1 Y (1) = b n -b 1 / n-1 and it can be seen that constant.

Therefore, the focus change amount Δf _X per pixel in the X direction and the focus change amount Δf _Y per pixel in the Y direction can be expressed by the following equations, and these change at a constant rate. . Δf _X = Δf _{X (m)} / m-1 ...... (12) Δf _Y = Δf _{Y (n)} / n-1 ...... (13)

Here, if an arbitrary scanning position in each scanning region of X (m) and Y (n) is defined as a point B (a _s , b _t ), (0 <s ≦ m,
0 <t ≦ n, s and t are integers), point P at point B
The focus change amount Δf _Z from _max (a ₁ , b ₁ ) is (12)
From equation (13), Δf _Z = Δf _X (s−1) + Δf _Y (t−1) (14)

Therefore, the point P _max (a ₁ , b ₁ ) at the point B is
From the equation (14), the change amount ΔI of the focus current from is: I = k · Δf _Z (15) Here, k is a constant and is determined by the acceleration voltage, the number of coil turns of the lens, the shape of the lens, and the like.

Therefore, assuming that the focus current amount at the point P _max (a ₁ , b ₁ ) is I, the focus current amount I _{B at} this point B is I _B = I + kΔf _Z by the equation (15). ... (16).

Next, the results of observing the sample with the scanning electron microscope according to the above embodiment will be described with reference to FIG.
Here, the image of FIG. 6 is a result of SEM observation of a certain same sample under the same rotation angle θ.
Here, FIG. 6A on the upper side is an observation result when the processing according to the flowcharts of FIGS. 4 and 5 is not applied, and in this case, the sample is rotating in the scanning direction of the electron beam. It can be seen that it is difficult to grasp the surface condition of the entire field of view because defocus occurs in the upper right part and the lower left part of the figure.

On the other hand, the image of FIG. 6 (b) on the lower side is shown in FIG.
And the processing according to the flowchart of FIG. 5 is applied to change the focus current on a pixel-by-pixel basis, in this case, a focused SEM image is obtained over the entire field of view, and accordingly, the surface condition of the entire field of view is obtained. Can be easily grasped.

[0058]

According to the present invention, even when the sample is tilted and rotated with respect to the scanning direction of the electron beam, defocus correction can be obtained over the entire field of view, and therefore the surface of the entire field of view can be corrected. The state can be easily grasped. As a result, according to the present invention, it is possible to easily obtain a highly accurate analysis result by applying it to the crystal orientation analysis.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of a scanning electron microscope according to the present invention.

FIG. 2 is an explanatory diagram of an inclined state of a sample and an electron beam scanning region in a scanning electron microscope.

FIG. 3 is an explanatory diagram showing a relationship between an electron beam position and a sample in an embodiment of the scanning electron microscope according to the present invention.

FIG. 4 is a flowchart for explaining the operation of an embodiment of the scanning electron microscope according to the present invention.

FIG. 5 is a flow chart for explaining the operation of an embodiment of the scanning electron microscope according to the present invention.

FIG. 6 is an explanatory diagram of an observation result by a scanning electron microscope.

[Explanation of symbols]

1 electron gun 2 Extraction electrode 3 Accelerating electrode 4 electron beams 5 condenser lens 6 Objective lens 7 samples 8 X-direction scanning coil 9 Y direction scanning coil 10 Magnification setting section 11 X-direction scanning controller 12 Y-direction scanning controller 13 Sample stage 14 Stage control unit 15 Focus current reading unit 16 Control unit (microcomputer) 20 Objective lens power supply 21 Secondary electron 22 Secondary electron detector 23 Image memory section 24 scanning (observation) area 25 Electrostatic auxiliary lens

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Mion Nakagawa             882 Ichige, Ichima, Hitachinaka City, Ibaraki Prefecture             Inside the company Hitachi Science Systems (72) Inventor Noriyuki Kaneoka             882 Ichige, Ichima, Hitachinaka City, Ibaraki Prefecture             Ceremony company Hitachi measuring instruments group F-term (reference) 2G001 AA03 BA07 BA15 BA18 CA03                       FA10 GA01 GA06 GA13 HA13                       KA08 PA12                 5C001 AA05 AA06 CC04                 5C033 MM02

Claims (3)

[Claims] 1. An incident angle of an electron beam with respect to a surface of a sample is
In the scanning electron microscope of the method of observing while tilted from the vertical direction, a means for detecting the rotation angle of the sample with respect to the scanning direction of the electron beam is provided, and based on the detection result of the rotation angle, the electron beam on the sample surface A scanning electron microscope characterized by being configured to correct defocus. 2. The incident angle of the electron beam with respect to the surface of the sample is
In a scanning electron microscope of a type of tilting observation from a vertical direction, a means for detecting a rotation angle of a sample with respect to a scanning direction of an electron beam is provided, and the rotation angle of the sample is corrected based on the detection result of the rotation angle. A scanning electron microscope having the above structure. 3. The invention according to claim 1 or 2, wherein the means for detecting the rotation angle is based on coordinate positions of arbitrary two points on the same scanning line by the electron beam on the sample surface. A scanning electron microscope comprising a means for detecting the rotation angle.