JPH05266850A - Scanning electron microscope - Google Patents

Abstract

(57) [Summary] [Object] The present invention relates to a scanning electron microscope, and an object thereof is to provide a scanning electron microscope in which alignment adjustment of an objective lens is easy. A scanning electron microscope having a focus wobbler function for adjusting alignment of an objective lens, wherein an excitation current setting means for setting a modulation amount of an objective lens alignment excitation current at the time of the focus wobble, and the objective lens alignment SEM image rotation angle setting means for setting a SEM image rotation angle proportional to the modulation amount of the excitation current, and proportional coefficient setting means for setting a proportional coefficient between the modulation amount of the objective lens alignment excitation current and the SEM image rotation angle. The proportional coefficient setting means is configured to compare the rotation angles of two SEM images acquired by two different objective lens alignment excitation currents and set a proportional coefficient that minimizes the difference in the rotation angles. To do.

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, and more particularly to a scanning electron microscope having an objective lens alignment adjusting function. In recent years, semiconductor integrated circuits,
In particular, many scanning electron microscopes using an electron beam are used for design verification and failure analysis of large-scale integrated circuits such as LSI (Large Scale Integrated circuit).

A scanning electron microscope irradiates a semiconductor integrated circuit to be inspected with an electron beam to detect secondary electrons reflected from the semiconductor integrated circuit, thereby performing design verification and failure analysis of the semiconductor integrated circuit. It is a thing.
However, in a scanning electron microscope using such an electron beam, an image obtained (hereinafter, SEM (Scanning Electron Electron
The accuracy of Microscope) image) deteriorates.

Therefore, it is necessary to accurately adjust the alignment of the objective lens.

[0004]

2. Description of the Related Art A conventional scanning electron microscope of this type is shown in FIG. 7, for example. This scanning electron microscope is roughly classified into a scanning electron microscope body 1 and
The scanning electron microscope main body 1 includes a lens barrel 3, a chamber 4, an electron gun 5, a polarizing coil 6, an objective lens 7, and a detector 8. The control system 2 includes a frame memory 9, Cursor overlay board 10, control circuit 11,
Alignment current control circuit 12, scan counter 1
3, SEM image rotation angle correction means 14, current setting circuit 15,
The wobbler control circuit 16 and the current amplifier 17 are included. Note that S is a sample to be inspected.

In the above configuration, when adjusting the alignment of the objective lens, a method called a focus wobbler is generally widely used. In the focus wobbler system, the alignment current control circuit 12 adjusts the X and Y of the objective lens alignment exciting current while observing the SEM image in which the exciting current of the objective lens 7 is modulated, and the SEM image of the objective lens exciting change is obtained. The center of expansion is SEM
This is a method of aligning the objective lens by aligning it with the center of the image.

[0006]

However, in such a conventional scanning electron microscope, the method called the focus wobbler is a very effective method for adjusting the alignment of the objective lens, but it is systematic. Since the adjustment method is not known, there is a problem that skill is required for the adjustment.

This is because the rotation of the SEM image due to a change in the objective lens alignment exciting current and the fact that the X and Y axes in the alignment current control circuit 12 do not correspond to the moving direction of the SEM image enlargement center. Is. FIG. 8 shows an SEM image of the conventional scanning electron microscope according to the change of the objective lens exciting current. It should be noted that FIG. 7A is an SEM image at the time of overfocus (excitation current in this case is I ₂ ), and FIG. 8B is an SEM image at just focus (excitation current in this case is I). c) is an SEM image at the time of underfocus (excitation current in this case is I ₁ ).

That is, two or more SEM images obtained by changing the objective lens alignment excitation current have differences in the degree of image blurring, magnification, image rotation amount, and the like. In particular, when the image rotates, the magnifying center position of the SEM image moves due to the exciting current, making it difficult to adjust the objective lens alignment exciting current. Therefore, in the scanning electron microscope, there is a problem that it is difficult to adjust the alignment of the objective lens.

[Object] Therefore, an object of the present invention is to provide a scanning electron microscope in which alignment adjustment of an objective lens is easy.

[0010]

In order to achieve the above object, a scanning electron microscope according to the present invention is a scanning electron microscope having a focus wobbler function for adjusting an alignment of an objective lens. Excitation current setting means for setting the modulation amount of the lens alignment excitation current, SEM image rotation angle setting means for setting the SEM image rotation angle proportional to the modulation amount of the objective lens alignment excitation current, and the objective lens alignment excitation current A proportional coefficient setting means for setting a proportional coefficient between the modulation amount and the SEM image rotation angle is provided, and the proportional coefficient setting means determines the rotation angle of two SEM images acquired by two different objective lens alignment excitation currents. It is configured to compare and set a proportional coefficient that minimizes the difference in the rotation angles.

In this case, when the objective lens alignment exciting current is modulated on the SEM image, coordinate reading means is provided for designating the enlargement center of the SEM image and reading the coordinates of the enlargement center on the SEM image. By the coordinate reading means, three independent sets of objective lens alignment exciting currents (OAX, OAY), (OAX + δ, OA) are provided.
Y), the enlargement center (sx1, sy1), (sx2, sy2) of the SEM image obtained from (OAX, OAY + δ),
(Sx3, sy3) is read, and the objective lens alignment exciting current (oax0, oay0) for moving the enlargement center to the center (sx0, sy0) of the SEM image,

[0012]

[Equation 4]

It is desirable that the center of the SEM image at the time of focus wobbling and the center of the SEM image are overlapped with each other by repeating the above operation as required. Center of expansion (sx
1, sy1), (sx2, sy2), (sx3, sy
3) based on the linear transformation matrix elements m11, m12, m2
1, m22,

[0014]

[Equation 5]

The primary conversion matrix elements m11, m12, m21, and m22 obtained by the primary conversion processing consisting of
Based on the set of designated values (ix, iy), the objective lens alignment correction currents Ix, Iy are

[0016]

[Equation 6]

It is effective to provide correction value setting means for setting the objective lens alignment correction currents Ix and Iy as correction values of the objective lens alignment excitation current, which are obtained from the above.

[0018]

In the present invention, since the SEM image rotation angle is corrected in proportion to the modulation degree of the objective lens alignment exciting current,
Rotation of the SEM image is suppressed during focus wobbling.
That is, the objective lens alignment adjustment can be easily performed only by adjusting the objective lens alignment excitation current so that the center of enlargement of the SEM image coincides with the center of the SEM image.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing an embodiment of a scanning electron microscope according to the present invention, and is a schematic block diagram showing the overall configuration thereof. First, the configuration will be described.

In FIG. 1, the same numbers as the numbers given to the conventional example shown in FIG. 7 indicate the same parts. The scanning electron microscope of the present embodiment is roughly divided into a scanning electron microscope main body 1 and a control system 2, and the scanning electron microscope main body 1
Is composed of a lens barrel 3, a chamber 4, an electron gun 5, a polarizing coil 6, an objective lens 7 and a detector 8. The control system 2 includes a frame memory 9, a cursor overlay board 10 as a coordinate reading means, and a proportional coefficient setting. A control circuit 11 which is a means, an alignment current control circuit 12 which is an exciting current setting means,
It is composed of a scan counter 13, an SEM image rotation angle correction means 14 which is an SEM image rotation angle setting means, a current setting circuit 15, a wobbler control circuit 16, a current amplifier 17, and an alignment current primary conversion circuit 21 which is a correction value setting means. There is.

The objective lens alignment excitation current setting value by the alignment current control circuit 12 is written in a hardware register (not shown), the value of this register can be read from the control circuit 11, and the register setting value is the alignment current. It becomes an input value to the primary conversion circuit 21. FIG. 2 is a block diagram showing the configuration of the alignment current primary conversion circuit.

The alignment current primary conversion circuit 21 includes operational amplifiers OA1 and OA2 and four registers RG1 to RG.
G4 and the registers RG1 to RG4 store the primary conversion coefficients m11 to m22 supplied from the control circuit 11, respectively. As a result, in the alignment current primary conversion circuit 21, the alignment current control circuit 12
Input (ix, iy) from MDAC (integrating DAC)
The primary conversion is performed using a matrix, and the operational amplifiers OA1 and OA2 amplify it to obtain the actual objective lens alignment exciting current (I
x, Iy). It should be noted that the magnification in MDAC, that is, the primary conversion matrix elements k · m11 to k · m22 are set by the control circuit 11, and this primary conversion may be performed by digital calculation or input. You may perform it, after converting into an analog signal once.

FIG. 3 is a block diagram showing the structure of the SEM image rotation angle correction means. The SEM image rotation angle correction means 14
Operational amplifiers OA3 and OA4, and a register R for converting the modulation amount of the objective lens alignment excitation current into a rotation angle
G5 to RG8, and the control circuit 11 sets the values A and B (-B) of the registers RG5 to RG8. The set values A and B are based on the rotation angle designation value θ and the wobbler amplitude φ designated by the control circuit 11.

[0024]

[Equation 7]

It is calculated by As a result, in the SEM image rotation angle correction means 14, the scan coordinates (Sx, Sy)
The XY deflection current (defx, defy) is obtained from Figure 4
FIG. 3 is a block diagram showing a configuration of a wobbler control circuit. The wobbler control circuit 16 includes an operational amplifier OA5 and a wobbler signal generation circuit 22. Based on a mode setting signal designated by the control circuit 11, in the normal mode, the wobbler signal is used as the objective lens alignment exciting current from the current setting circuit 15. (Sine wave) is synthesized to generate an input signal to the current amplifier 17 which becomes an actual objective lens alignment exciting current.

In this embodiment, the scan counter 1
When receiving the frame scan end signal output after 3 to 1 frame, the wobbler signal is switched stepwise and held until the next frame scan end signal is received. Next, the operation will be described. First, the current value of the objective lens alignment exciting current is set to (OAX, OAY) (step 1), and the enlarged center coordinates (sx) of the SEM image at the objective lens alignment exciting current (OAX, OAY) are set.
1, sy1) is obtained (step 2).

Similarly, the current value of the objective lens alignment exciting current is set to (OAX + δ, OAY), and the objective lens alignment exciting current (OAX + δ, OAY).
The enlarged center coordinates (sx2, sy2) of the SEM image at step S3 are obtained (step 3), the current value of the objective lens alignment excitation current is set to (OAX, OAY + δ), and the objective lens alignment excitation current (OAX, OAY) is set.
Enlargement center coordinates (sx3, sy3) of SEM image at + δ)
Is required (step 4).

Next, the objective lens alignment exciting current is set so that the magnifying center coordinate coincides with the center coordinate of the SEM image (step 5), until the magnifying center coordinate and the center coordinate of the SEM image are substantially equal. The above steps 1 to 5 are repeated (step 6) to complete the rough objective lens alignment adjustment. After that, from the setting of the primary conversion matrix, the X and Y shifts at the center of expansion are set to be independently controllable (step 7), and the operator performs fine adjustment independently of X and Y (step 8). ..

FIG. 6 is a diagram showing an SEM image of the scanning electron microscope according to the present embodiment due to a change in the objective lens exciting current. Note that FIG. 7A shows SE at the time of overfocus.
M image, SEM image at the time of just focus,
FIG. 6C is an SEM image at the time of underfocus.
That is, if the enlargement center on the SEM is specified by a set of three independent objective lens alignment excitation currents, if the change in the objective lens alignment excitation current is proportional to the enlargement center position, the center of the SEM image is displayed. The set of objective lens alignment excitation currents for matching the centers of magnification should be known.

Therefore, the objective lens alignment adjustment can be systematically performed by repeating the above adjustment, and the movement of the enlargement center in the X and Y directions can be independently controlled. As described above, in this embodiment, the objective lens alignment adjustment can be roughly performed by a systematic method, and the fine adjustment is performed based on the information obtained there. Therefore, the systematic and efficient objective lens Alignment can be adjusted.

[0031]

According to the present invention, the rotation of the SEM image can be suppressed during the focus wobble by correcting the SEM image rotation angle in proportion to the modulation degree of the objective lens alignment exciting current. Therefore, at the time of adjusting the objective lens alignment, the objective lens alignment adjustment can be easily performed only by adjusting the objective lens alignment excitation current so that the center of enlargement of the SEM image coincides with the center of the SEM image.

[Brief description of drawings]

FIG. 1 is a schematic block diagram showing the overall configuration of this embodiment.

FIG. 2 is a block diagram showing a configuration of an alignment current primary conversion circuit.

FIG. 3 is a block diagram showing a configuration of an SEM image rotation angle correction means.

FIG. 4 is a block diagram showing a configuration of a wobbler control circuit.

FIG. 5 is a flowchart for explaining an operation example of the present embodiment.

FIG. 6 is a diagram showing an SEM image according to a change in an objective lens exciting current in the scanning electron microscope of the present embodiment.

FIG. 7 is a schematic block diagram showing an overall configuration of a conventional example.

FIG. 8 is a diagram showing an SEM image according to a change in an objective lens exciting current in a conventional scanning electron microscope.

[Explanation of symbols]

1 Scanning Electron Microscope Main Body 2 Control System 3 Lens Barrel 4 Chamber 5 Electron Gun 6 Polarizing Coil 7 Objective Lens 8 Detector 9 Frame Memory 10 Cursor Overlay Board (Coordinate Reading Means) 11 Control Circuit (Proportional Coefficient Setting Means) 12 Alignment Current Control circuit (exciting current setting means) 13 Scan counter 14 SEM image rotation angle correction means (SEM image rotation angle setting means) 15 Current setting circuit 16 Wobbler control circuit 17 Current amplifier 21 Alignment current primary conversion circuit (correction value setting means) 22 Wobbler signal generation circuit S Sample

Claims (3)

[Claims] 1. A scanning electron microscope having a focus wobbler function for adjusting alignment of an objective lens, comprising: excitation current setting means for setting a modulation amount of an objective lens alignment excitation current during the focus wobble; and the objective. SEM image rotation angle setting means for setting a SEM image rotation angle proportional to the modulation amount of the lens alignment exciting current, the modulation amount of the objective lens alignment exciting current and the SEM
A proportional coefficient setting means for setting a proportional coefficient with respect to the image rotation angle, wherein the proportional coefficient setting means compares rotation angles of two SEM images acquired by two different objective lens alignment exciting currents, A scanning electron microscope characterized by setting a proportional coefficient that minimizes a difference in rotation angle. 2. When the objective lens alignment excitation current is modulated on the SEM image, coordinate reading means is provided for designating the enlargement center of the SEM image and reading the coordinates of the enlargement center on the SEM image. By the coordinate reading means, three sets of independent X, Y objective lens alignment exciting currents (OAX, OAY), (OA
X + δ, OAY), (OAX, OAY + δ) SEM image magnification centers (sx1, sy1), (sx2,
Objective lens alignment exciting current (oax0, oay) for reading sy2), (sx3, sy3) and moving the enlargement center to the center (sx0, sy0) of the SEM image.
0) to By repeating the operation as required,
The scanning electron microscope according to claim 1, wherein the center of enlargement of the SEM image and the center of the SEM image at the time of focus wobbling overlap. 3. The enlargement center (sx1, sy of the SEM image
1), (sx2, sy2), (sx3, sy3) based on the primary transformation matrix elements m11, m12, m21, m22
Is given by Is obtained by a primary conversion process consisting of the following, and based on the primary conversion matrix elements m11, m12, m21, m22 and a specified value (ix, iy) of a set of X and Y for objective lens alignment adjustment specified externally, The objective lens alignment correction currents Ix and Iy are given by From the objective lens alignment correction currents Ix, I
3. The scanning electron microscope according to claim 1, further comprising a correction value setting means for setting y as a correction value for the objective lens alignment excitation current.