JPH09167587A - Scanning electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-magnification image with its small distortion by means of an object lens with its short focus distance. SOLUTION: This microscope is provided with deflectors 7a and 7b made of their upper and lower two stages spaced in a light-axis direction to scan a primary electron beam 2 two-dimensionally on a sample 8 and arranged, a secondary signal 14 generated from the sample is detected, and a sample image is obtained in a CRT12. When the two-dimensional deflection position of the primary electron beam on the sample is (X, Y), a deflection magnetic field BxU<pper> in an X direction and a deflection magnetic field BxU<pper> in a Y direction which are generated by means of a deflector 7a at the upper stage and a deflection magnetic field BxL<ower> in an X direction and a deflection magnetic field ByL<ower> in a Y direction which are generated by a deflector 7b at the lower stage are set as follows assuming the constants to be Ku, Kt, and θ: BxU<pper> =KuX, ByU<pper> =KuY, BxL<ower> =Kt (Xcos θ-Ysin θ), ByL<ower> =Kt (Ycos θ+Xsin θ).

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 for detecting a secondary electron generated from a sample by irradiating a primary electron beam to obtain a scanned image of the sample.

[0002]

2. Description of the Related Art In a scanning electron microscope, a primary electron beam can be two-dimensionally scanned on a sample or a position of the primary electron beam can be moved on the sample by a deflector arranged closer to an electron source than an objective lens. Done. At this time, if the trajectory of the primary electron beam deflected by the deflector deviates from the vicinity of the lens main surface of the objective lens, lens aberration occurs, the beam spreads, and the beam position on the sample deviates from the desired position. For this reason, the deflector is provided in two stages above and below the optical axis, and the deflection fulcrum (the point at which the trajectory of the electron swung back by the two-stage deflector intersects the optical axis again) is provided by the swing-back deflection method. It is arranged so that it is close to the main surface of. At this time, the X-direction and Y-direction deflection magnetic fields generated by the two-stage deflectors are aligned in the X-direction so that the deflection fulcrum is a fixed point regardless of the deflection amount of the primary electron beam. And Y-direction deflection coils are arranged.

[0003]

Generally, in order to obtain a high resolution image, it is necessary to shorten the focal length of the objective lens. Then, the distance between the deflection fulcrum and the sample becomes short, so when trying to deflect the beam over a wide area on the sample, the aberration of the lens becomes large and the image becomes blurred,
There was a drawback that distortion occurred. Therefore, there is a problem that it becomes more difficult to realize a low-magnification image without distortion and it is difficult to find an observation field of view as much as a scanning electron microscope that can obtain a high resolution.

The present invention has been made in view of the above problems of the prior art, and an object of the present invention is to provide a scanning electron microscope capable of obtaining a low-magnification image with less distortion even with a short focal length. And

[0005]

In the process of examining the cause of distortion of low-magnification images in a scanning electron microscope capable of obtaining high resolution, the present inventors set the deflection magnetic fields of the upper and lower deflectors appropriately to each other. It was found that by rotating the lens by an angle, a low-magnification image with small distortion can be obtained even if the focal length of the objective lens is short.

The present invention has been made based on such knowledge, and the orthogonal deflection coordinate axis of the upper deflector and the orthogonal deflection coordinate axis of the lower deflector are relatively rotated about the optical axis by a predetermined angle. It is characterized by Here, the orthogonal deflection coordinate axis of the deflector means the primary electron beam deflected on the deflector in correspondence with the scanning signals in the X and Y directions for two-dimensionally scanning the primary electron beam on the sample. It refers to the Cartesian coordinate axis that determines the deflection direction. The deflection direction of the primary electron beam is a direction orthogonal to the deflection magnetic field generated by the deflector and the optical axis.

The relative rotation of the orthogonal deflection coordinate axis of the upper deflector and the orthogonal deflection coordinate axis of the lower deflector is such that the positions of the coil winding slots of the upper deflector and the lower deflector are relative to each other around the optical axis. By rotating the coil of the upper deflector and the corresponding coil of the lower deflector by rotating relative to each other around the optical axis, or by arranging the coils without changing the coil arrangement, When the two-dimensional deflection position of the primary electron beam on the sample is set to (X, Y) by controlling the deflection current flowing in the coil of the lower deflector, or by combining the two, the upper deflector X-direction deflection magnetic field Bx ^Upper and Y-direction deflection magnetic field B
y ^Upper , the deflection magnetic field B in the X direction generated by the lower deflector
x ^Lower and Y direction deflection magnetic field By ^Lower are realized by making the following relationships with k _U , k _L and θ as constants.

Bx ^Upper = k _U X By ^Upper = k _U Y Bx ^Lower = k _L (Xcos θ-Y sin θ) By ^Lower = k _L (Y cos θ + X sin θ) According to the present invention, even in a high resolution scanning electron microscope, a low magnification with a small distortion is obtained. Since it is possible to obtain an image, it is extremely effective for searching the field of view.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings. FIG. 1 is a schematic configuration diagram of a scanning electron microscope according to an embodiment of the present invention. The primary electron beam 2 emitted from the cathode 1 by the voltage V1 applied between the cathode 1 and the extraction electrode 3 is accelerated by the applied voltage Vacc applied between the cathode 1 and the acceleration electrode 4. Primary electron beam 2
Is focused on by the focusing lens 5 and the objective lens 6 controlled by the lens control power source 13 and irradiated onto the sample 8.
The sample is two-dimensionally scanned by upper and lower two-stage deflectors including an upper-stage deflector 7a and a lower-stage deflector 7b. 1
0 is an aperture.

The deflection signals of the deflectors 7a and 7b are controlled by the deflection controller 11 according to the observation magnification. The secondary signal 14 such as secondary electrons generated from the sample 8 by the irradiation of the primary electron beam 2 is detected by the secondary signal detector 9.
By using the output of the secondary signal detector 9 as a brightness modulation signal of the image display device 12 such as a CRT that uses the deflection signal from the deflection control unit 11 as a scanning signal, a magnified image of the sample is displayed on the image display device 12. To be done.

FIG. 2 shows an example of a two-stage deflector consisting of an upper deflector 7a and a lower deflector 7b. In this two-stage deflector, upper and lower winding frames 22 and 23 are fixed to a cylindrical member 21 having a hollow center so that a primary electron beam can pass, and a winding wire is wound around each winding frame 22 and 23 to deflect the deflection coil. Is formed. In the illustrated example, the upper winding frame 22 has eight winding slots 25. By winding a conductive wire around the winding slots 25, the coils 31a and 31b for generating the deflection magnetic field Bx ^Upper in the X direction are formed. , And Y-direction deflection magnetic field By ^Upper
The generating coils 32a and 32b are formed. Similarly, the lower winding frame 23 has winding slots 26 at eight locations in the illustrated example.
By winding a conducting wire around the winding slot 26, the coil 33 for generating the deflection magnetic field Bx ^Lower in the X direction is provided.
a, 33b and coils 34a, 34b for generating the Y direction deflection magnetic field By ^Lower are formed. The number of the winding slots 25 and 26 need only be such that a coil generating a deflection magnetic field in the X direction and a deflection magnetic field in the Y direction can be wound.
The number is not limited to eight.

Here, as shown in FIG. 3, the upper deflector 7
The winding frame 22 forming a and the winding frame 23 forming the lower deflector 7b are relatively rotated by θ with respect to the optical axis.
The deflector 7 is driven by the signal (X, Y) from the deflection controller 11.
a is a deflection magnetic field Bx ^Upper and Y in the X direction expressed by the following equation.
A deflection magnetic field By ^{Upper in the} direction is generated, and the deflector 7b generates a deflection magnetic field Bx ^{Lower in the} X direction and a deflection magnetic field By ^Lower in the Y direction, which are represented by the following equations. Here, k _U and k _L are constants determined by the shape and arrangement of the deflector. Further, the rotation angle θ is previously determined by experiments so that distortion and blurring of the low-magnification image are minimized.

Bx ^Upper = k _U X By ^Upper = k _U Y Bx ^Lower = k _L (Xcosθ-Ysinθ) By ^Lower = k _L (Ycosθ + Xsinθ) The X and Y directions of the magnetic field generated by the lower deflector 7b are used as a reference. Obviously, the above equation can be expressed as follows.

Bx ^Upper = k _U (Xcos θ-Ysin θ) By ^Upper = k _U (Ycos θ + Xsin θ) Bx ^Lower = k _L X By ^Lower = k _L Y FIG. It is a scanning electron microscope photograph of a copper mesh that is vertically and horizontally orthogonal to each other and is taken under an extremely short focal length of about 2 mm (display field is about 65 μm in length and about 75 μm in width). Distortion and blurring can be seen around the image. FIG.
According to the present invention, when the angle θ of FIG. 3 is set to 2 ° and the deflection magnetic fields of the upper deflector and the lower deflector are relatively rotated,
It is a scanning electron micrograph for the same copper mesh. Although the display field of view of FIG. 5 is the same as that of FIG.
Distortion and blurring around the image seen in are completely eliminated.
The rotation of the deflection magnetic fields of the upper and lower deflectors does not adversely affect observation at high magnification.

Another method of rotating the deflecting magnetic field of the upper deflector and the deflecting magnetic field of the lower deflector will be described with reference to FIG. In this method, the angular positions around the optical axis of the corresponding deflection coils in the upper deflector 7a and the lower deflector 7b are the same, but the excitation current control method for deflecting the primary electron beam is different. There are features. When the two-dimensional deflection position of the primary electron beam on the sample is (X, Y), the deflection current Ix ^{Upper in} the X direction and the deflection current Iy ^{Upper in the} Y direction of the upper deflector 7a are the deflection amount signals X, The Y is amplified by the amplifiers 41 and 42 having the amplification factor d _U and set as follows.

Ix ^Upper = d _U X Iy ^Upper = d _U Y On the other hand, the deflection current Ix ^{Lower of the} lower deflector 7b in the X direction
And the deflection currents Iy ^{Lower in the} Y direction are the deflection amount signals X and Y.
Are multiplied by cos θ and sin θ in coefficient units 43, 44, 45 and 46, and are input to the addition amplifiers 47 and 48 to be added, whereby the following relational expressions are satisfied.

Ix ^Lower = d _L (Xcos θ−Y sin θ) Iy ^Lower = d _L (Y cos θ + X sin θ) where d _U and d _L are constants determined by the shape and arrangement of the deflector. The rotation angle θ is determined in advance by experiments so that distortion and blurring of the low-magnification image are minimized. The same effect can be obtained by controlling the deflection current as follows.

Ix ^Upper = d _U (Xcos θ-Y sin θ) Iy ^Upper = d _U (Y cos θ + X sin θ) Ix ^Lower = d _L X Iy ^Lower = d _L Y The orthogonal deflection coordinate axes of the deflector in the upper stage and the lower stage in FIG. 7 and FIG. Another method of rotating the orthogonal deflection coordinate axes of the deflector of FIG. In this method, as shown in FIG. 7, the angles around the optical axes of the deflection coils 51a, 51b and 53a, 53b corresponding to the upper deflector 7a and the lower deflector 7b, and 52a, 52b and 54a, 54b, respectively. The positions are the same, but the winding frame 23 of the lower deflector 7b
In the winding slot of the
3a, 53b, in addition to the Y-direction deflection magnetic field generating coil,
Auxiliary coils 57a, 57b and 58a, 58b for rotating the orthogonal deflection coordinate axis of the lower deflector by θ are wound in layers.

FIG. 8 shows the auxiliary connection relationship between the Y-direction deflection magnetic field generating coil of the lower deflector 7b. As shown in FIG. 8, Y-direction deflection magnetic field generating coils 54a and 54b
And the auxiliary coil coils 57a and 57b in the X direction are connected in series, and the impedance Z is connected in parallel to the auxiliary coils 57a and 57b. Therefore, the deflection current I for Y-direction deflection magnetic field generating coils 54a and 54b Y deflection
Although y flows, the deflection current Ixc flows through the auxiliary coils 57a and 57b in the X direction, and the remaining current is impedance Z.
Shunted.

When the magnetic field generated by the Y-direction deflection coils 54a and 54b is By and the magnetic field generated by the X-direction auxiliary coils 57a and 57b is Bxc, the Y-direction deflection current Iy is given.
Since the deflection magnetic field By ′ generated in step S1 is a combined magnetic field of By and Bxc, by the deflection magnetic field in the Y direction when the auxiliary coils 57a and 57b are not used, By ′ is the angle θ represented by the following equation. Rotate around an axis.

Θ = tan ⁻¹ (| Bxc | / | By |) = (Nxc · Ixc) / (Ny · Iy) = (Nxc / Ny) · {Z / (Zxc + Z)} where Zxc is a correction coil 57a and 57b, and Ny and Nxc represent the number of coil turns of the Y-direction deflection coils 54a and 54b and the auxiliary coils 57a and 57b, respectively. That is, the number of turns of the auxiliary coil Nxc
And the impedance Zxc are selected appropriately,
The deflection magnetic field in the Y direction can be rotated by the angle θ obtained in advance by an experiment.

Although the method of rotating the deflection magnetic field in the Y direction by θ is explained here, the deflection magnetic field in the X direction is also changed to Y.
Auxiliary coils 58a and 58b, which are wound on the direction-deflecting magnetic field generating coils 54a and 54b, can be rotated in exactly the same manner.

[0023]

According to the present invention, a low-magnification image with small distortion can be obtained even when the focal length of the objective lens is shortened in order to achieve high resolution.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a scanning electron microscope.

FIG. 2 is a schematic diagram showing an example of a deflector.

FIG. 3 is a diagram illustrating a relationship between an upper deflector and a lower deflector.

FIG. 4 is a low-magnification image obtained by a conventional scanning electron microscope.

FIG. 5 is a low-magnification image obtained by the scanning electron microscope of the present invention.

FIG. 6 is a control block diagram of a deflection current.

FIG. 7 is a diagram illustrating a relationship between an upper deflector and a lower deflector.

FIG. 8 is an explanatory view of coil wiring.

[Explanation of symbols]

1: cathode, 2: electron beam, 3: extraction electrode, 4: acceleration electrode, 5: focusing lens, 6: objective lens, 7a: deflector,
7b: deflector, 8: sample, 9: secondary electron detector, 10:
Aperture, 11: Deflection control circuit, 12: Image display device, 13:
Lens control power supply, 14: secondary signal, 21: cylindrical member: 2
2: upper winding frame, 23: lower winding frame, 25, 26: winding slots, 31a, 31b: X direction deflection magnetic field generating coil, 32a, 32b: Y direction deflection magnetic field generating coil, 33a, 33b: X-direction magnetic field generating coil, 34
a, 34b: Y direction magnetic field generating coils, 41, 42:
Amplifiers 43 to 46: Coefficient multipliers, 47 and 48: Summing amplifiers 51a and 51b: X direction deflection magnetic field generating coils,
52a and 52b: coils for generating a deflection magnetic field in the Y direction, 53
a, 53b: X-direction magnetic field generating coils, 54a, 54
b: Y-direction magnetic field generating coil, 57a, 57b: X-direction auxiliary coil, 58a, 58b: Y-direction auxiliary coil

Claims (4)

[Claims] 1. A scanning electron microscope that two-dimensionally scans a primary electron beam on a sample and detects a secondary signal generated from the sample to obtain an image of the sample. As a means for scanning, it is provided with upper and lower two-stage deflectors which are arranged apart from each other in the optical axis direction, and the orthogonal deflection coordinate axis of the upper deflector and the orthogonal deflection coordinate axis of the lower deflector are set at a predetermined angle around the optical axis. A scanning electron microscope characterized by being rotated relatively only. 2. A scanning electron microscope that two-dimensionally scans a primary electron beam on a sample and detects a secondary signal generated from the sample to obtain a sample image. As a means for scanning, a deflector having two upper and lower stages arranged apart from each other in the optical axis direction is provided, and when the two-dimensional deflection position of the primary electron beam on the sample is (X, Y), A deflection magnetic field Bx ^Upper in the X direction and a deflection magnetic field By ^Upper in the Y direction generated by the deflector;
A scanning electron microscope characterized in that a deflection magnetic field Bx ^Lower in the X direction and a deflection magnetic field By ^{Lower in the} Y direction generated in the lower deflector satisfy the following relationship with k _U , k _L and θ as constants. Bx ^Upper = k _U X By ^Upper = k _U Y Bx ^Lower = k _L (Xcosθ−Ysinθ) By ^Lower = k _L (Ycosθ + Xsinθ) 3. The scanning according to claim 2, wherein the coils of the upper deflector and the corresponding coils of the lower deflector are arranged so as to be relatively rotated about the optical axis by an angle θ. electronic microscope. 4. A scanning electron microscope that two-dimensionally scans a primary electron beam on a sample and detects a secondary signal generated from the sample to obtain a sample image. As a means for scanning, a deflector having two upper and lower stages arranged apart from each other in the optical axis direction is provided, and when the two-dimensional deflection position of the primary electron beam on the sample is (X, Y), deflection current Ix ^{Uppe r} and Y directions of deflection current Iy ^Upper in the X direction of the deflection coils, the deflection current Iy ^lower deflection current Ix ^lower and Y directions of the X direction of the lower deflection coil, h _U, h _L and θ A scanning electron microscope characterized by satisfying the following relationship as a constant. Ix ^Upper = h _U X Iy ^Upper = h _U Y Ix ^Lower = h _L (Xcosθ-Ysinθ) Iy ^Lower = h _L (Ycosθ + Xsinθ)