JPH1167130A - Electron beam optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct oblique aberration, to improve the resolution of an electron beam optical device, and to prevent a sample from being damaged. SOLUTION: A scanning electron microscope to which the subject device can be applied is composed to have an electrostatic magnetic field composite object lens (a deceleration field). In this case, at least one deflection system 5 is installed, a corrected deflection magnetic field and a corrected deflection electric field are overlapped in the vicinity of an object lens 7, corresponding to a deflection orbit 3 of an electron beam deflected by the deflection system 5, and the corrected deflection magnetic field and the corrected deflection electric field correct oblique aberration to realize high resolution. Moreover, the deceleration field is formed, so that a high accelerating voltage is used and furthermore a sample damage is repressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron beam optical device used for an apparatus utilizing an electron beam, and more particularly to an electron beam optical device for a scanning electron microscope.

[0002]

2. Description of the Related Art The fields in which a scanning electron microscope is used are wide-ranging, such as the fields of advanced science such as semiconductor devices, liquid crystals and magneto-optical disks, materials such as metals and chemistry, medicine, biology, and medicine.

For example, in a semiconductor manufacturing process, measurement of a line width of a resist at the time of pattern formation is performed by a scanning electron microscope (SEM). When observing the line width or the like of the resist, it is preferable to perform inspection for residue, tailing, overhang, and foreign matter.

[0004] Inspection of residues, tailing, overhanging, and inspection of foreign matter require three-dimensional observation. When observing a sample three-dimensionally, it is necessary to incline and impinge the electron beam on the sample. However, in a conventional electron beam optical device, the electron beam is always incident perpendicularly on the sample. The scanning of the electron beam is performed by tilting the stage holding the sample so that the side surface of the object can be observed.

[0005] Lens aberrations include off-axis aberrations such as image coma, astigmatism, distortion, and curved off-axis chromatic aberration that occur when the object point is off the optical axis, and when the object point is on the optical axis. chromatic aberration,
There are axial aberrations such as spherical aberration. Either one affects resolution. In the conventional electron beam optical device described above,
The reason that the sample side is tilted and the electron beam is not tilted is that when the electron beam is tilted, the electron beam deviates from the optical axis of the objective lens, causing off-axis aberration and forming a small probe. This is because it was impossible to prevent the resolution from being lowered.

As an electron beam optical device for correcting off-axis aberrations, there is one shown in FIGS. 6 and 7 which is used in an electronic drawing apparatus. First, the electron beam optical device 1 shown in FIG. 6 will be described.

In the figure, reference numeral 2 denotes an optical axis of an electron beam optical device, 3 denotes a deflection trajectory of an electron beam, and 4 denotes a sample. A first scanning coil 5 (first deflection system), a second scanning coil 6 on the optical axis 2
(Second deflection system), an objective lens 7 (shown as an equivalent optical lens in the figure), which is a magnetic field type lens, and a correction deflection coil 8 arranged so as to be electrically superimposed on the lens magnetic field of the objective lens 7. Has been established.

An electron beam emitted from a filament (not shown) is deflected to the optical axis 2 by the first scanning coil 5 and deflected by the second scanning coil 6 in parallel with the optical axis 2. You. The deflection trajectory 3 is scanned with respect to the sample 4 by changing the deflection angle in a range of 0 to a predetermined angle. When the electron beam is scanned, the deflection trajectory 3 of the electron beam deviates from the optical axis 2 of the objective lens 7, but a correction deflection magnetic field is given by the correction deflection coil 8, and the deflection trajectory 3 and the objective lens 7 The magnetic field type objective lens 7 moves parallel without tilting so that the optical axes coincide.
Thus, off-axis aberrations when scanning are corrected.

Next, the electron beam optical device 10 shown in FIG. 7 will be described. The same components as those shown in FIG. 6 are denoted by the same reference numerals.

In FIG. 1, a first scanning coil 5, an objective lens 7 which is a magnetic lens (shown as an equivalent optical lens in the figure), and a lens of the objective lens 7 on the optical axis 2 of the electron beam optical device 10. Correction deflection coil 8 to superimpose on magnetic field
Are arranged.

An electron beam emitted from a filament (not shown) is deflected with respect to the optical axis 2 by the first scanning coil 5, and the deflected angle is set within a range of 0 to a predetermined angle by the first scanning coil 5. By changing, the deflection trajectory 3 is scanned with respect to the sample 4. The scanning of the electron beam causes the deflection trajectory 3 of the electron beam to deviate from the optical axis 2 of the objective lens 7, but a correction deflection magnetic field is given by the correction deflection coil 8, and The objective lens 7 is tilted and moved at the same angle as the declination so that the axes coincide. Thus, off-axis aberrations when scanning are corrected.

[0012]

In order to observe a sample three-dimensionally as described above, it is necessary to tilt an electron beam with respect to the sample. When the stage is tilted, it is necessary to mount a laser interferometer in order to increase the position accuracy of the stage,
The laser interferometer is heavy and cannot be mounted on the tilt mechanism of the stage. For this reason, it is extremely difficult to perform tilt observation when the sample to be observed is extremely fine, such as a semiconductor element. In practice, measurement is always performed from the surface, and a three-dimensional image is not obtained. For this reason, it was not possible to obtain sufficient information because it was not possible to perform sufficient observation.

The electron beam optical devices 1 and 10 have been developed as electron drawing devices, and the acceleration voltage is as high as 50 to 100 KV. When observing a semiconductor material, the incident voltage needs to be 500 to 800 V, and in the electron beam optical devices 1 and 10, the semiconductor material is damaged. In this case, the angle of incidence on the sample is perpendicular or nearly perpendicular. For this reason, off-axis aberrations can be corrected but stereoscopic observation cannot be performed. Here, if the acceleration voltage is lowered so as not to damage the sample, the electron beam is confused by contamination or charge-up of the objective lens, and the resolution is deteriorated without being reduced as calculated. Further, when the acceleration voltage is low, the brightness at the electron gun is deteriorated due to the effect of space charge at the electron gun, and the theoretical brightness cannot be obtained. Further, axial aberrations such as chromatic aberration and spherical aberration are caused by an incident voltage (V _L : landing voltage) and an accelerating voltage (V) when an electron beam enters a sample.
₀ ), which is substantially proportional to V ₀ / V _L. Therefore, when the acceleration voltage is reduced, there is a problem that the aberration increases and the resolution decreases.

In view of such circumstances, the present invention corrects off-axis aberrations, improves the resolution of an electron beam optical device, and enables observation of a semiconductor element when the electron beam optical device is used in a scanning electron microscope. However, it is intended to enable both stereoscopic observation.

[0015]

According to the present invention, there is provided a scanning electron microscope having an electrostatic magnetic field compound objective lens (deceleration field), comprising at least one deflection system, and an electron beam deflected by the deflection system. The present invention relates to an electron beam optical device in which a correction deflection magnetic field and a correction deflection electric field are superimposed near an objective lens corresponding to a deflection trajectory of a line. The present invention relates to an optical device.

(1/2) rB '+ r'B + (1/2) r
Φ ″ + r′Φ ′ B: on-axis magnetic field distribution of the objective lens, Φ: on-axis electrostatic potential of the objective lens, r: distance from the optical axis, ': first derivative with respect to the optical axis coordinate (Z), ″: Second-order differentiation with respect to optical axis coordinate (Z) Off-axis aberration is corrected by the correction deflection magnetic field and correction deflection electric field, high resolution is realized, and a high acceleration voltage is used by forming a deceleration field, so that damage to the sample can be prevented. Suppress.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention. In the drawing, reference numeral 2 denotes an optical axis of an electron beam optical device, 3 denotes an electron beam deflection orbit, and 4 denotes a sample. The first on the optical axis 2
A scanning coil 5, an objective lens 7 (shown as an equivalent optical lens in the figure), and a correction deflection coil 8 are arranged so as to be superposed on the lens magnetic field of the objective lens 7. In order to correct the off-axis aberration of 7,
A correction deflection coil 8 is provided. A deceleration voltage is applied to the sample 4, a deceleration electrostatic lens 12 is formed by the deceleration field, and a correction deflection electrode 9 is provided for correcting off-axis aberration of the deceleration electrostatic lens 12. In this way, the magnetic field type objective lens 7 corrected by the correction deflection coil 8 and the deceleration electrostatic lens 12 corrected by the correction deflection electrode 9 overlap to form a corrected electrostatic magnetic field compound objective lens. .

The acceleration voltage of an electron beam emitted from a filament (not shown) is 3 KV (electron volt), the deceleration voltage is applied to the sample 4 at -2.2 KV, and the incident voltage V _L
Has a limit of 800V. Note that the value of the acceleration voltage and the value of the deceleration voltage to the sample 4 may be appropriately selected.

The electron beam is deflected with respect to the optical axis 2 by the first scanning coil 5, is deflected by a predetermined angle by the first scanning coil 5, and is deflected within a predetermined range around the deflection angle. The trajectory 3 is scanned with respect to the sample 4. The deflection trajectory 3 deviates from the optical axis 2 of the objective lens 7 when the electron beam is deflected, but a correction deflecting magnetic field is given by the correction deflecting coil 8 so that the deflection trajectory 3 and the optical axis of the objective lens 7 are shifted. The objective lens 7
Are tilted and moved at the same angle as the declination. Thus, off-axis aberrations when scanning are corrected. Further, the axial aberration is corrected by the deceleration electrostatic lens 12, and the off-axis aberration of the deceleration electrostatic lens 12 is corrected by the correction deflection electrode 9.

Next, the operation of the correction deflection magnetic field and the correction deflection electric field will be described.

The paraxial general formula including the objective lens when there is no correction deflection magnetic field and correction deflection electric field is expressed by the following equation (1).

[0023]

## EQU1 ## ω ″ + {′ ω ′ / 2} + {″ ω / 4} −i
[√ (η / 2ψ)] (Bω ′ + B′ω / 2) = 0

Next, the paraxial general formula including the correcting deflection magnetic field, the correcting deflection electric field, and the electrostatic field compound objective lens is expressed by the following equation (2).

[0025]

[Formula 2] ω ″ + {′ ω ′ / 2} + {″ ω / 4} −i
[{(Η / 2})] (Bω ′ + B′ω / 2) = − VF ₁
/ 2ψ + [√ (η / 2ψ)] ID _{1 In the} equation, η = e / m and ω is a complex representation of r.

Here, ψ is 軸 = constant = V ₀ (acceleration voltage) and ψ ′ = ψ ″ = 0 when only a magnetic lens having no deceleration field due to an on-axis electrostatic potential. Become.

[0027]

## EQU3 ## ω ″ −i [√ (η / 2V ₀ )] (Bω ′ +
B′ω / 2) = [√ (η / 2V ₀ )] ID ₁

The electron beam optical device 1 and the electron beam optical device 1
By adding a correction deflection field ID ₁ a (Bω '+ B'ω / 2) by 0 in the correction deflection coils 8, ω "=
0, the off-axis aberration is corrected by the correction deflection magnetic field, which is equivalent to the deflection trajectory 3 passing through the center of the objective lens 7.

Further, when there is a deceleration field, it becomes ψ ≠ constant, and a term of ψ′ω ′ + ψ ″ ω / 2 is added. This term represents off-axis aberration of the deceleration field. As the correction deflection electric field VF ₁ by the correction deflection electrode 9, (ψ′ω ′ +
By adding ψ ″ ω / 2), the off-axis aberration of the deceleration field is further corrected.

By applying a deceleration voltage to the sample 4, the acceleration voltage is maintained at a high voltage, and damage to the sample 4 is suppressed. As described above, the aberrations are the incident voltage V _L and the acceleration voltage V
_Since the ratio decreases substantially in proportion to the ratio of ₀ , V ₀ / V _L , on-axis aberration can be reduced. Thus, off-axis aberration and on-axis aberration can be corrected, and high resolution can be realized.

Next, a second embodiment will be described with reference to FIG.

In the second embodiment, a second scanning coil 6 is added to the first embodiment. With the addition of the second scanning coil 6, the sample 4 can be scanned around the optical axis while changing the tilt direction of the electron beam, and three-dimensional observation of the sample can be performed.

Next, a display device for displaying an image obtained by observing a sample from an oblique direction using the scanning electron microscope of the present invention shown in FIG. 1 will be described.

FIG. 3 is a schematic view of a sample viewed from the direction of the optical axis O of the scanning electron microscope. Reference numeral 13 denotes convex resist patterns arranged on a sample wafer. When the line is deflected in the positive direction of the X axis, the resist pattern is scanned in the area A, and in the reverse direction, the resist pattern is scanned in the area C. Similarly, the electron beam is deflected in the Y axis direction. In this case, the area B and the area D are scanned.

4A to 4D show observed images in the four regions A to D, respectively. In FIG. 4A, images including the left inclined portion of the convex resist pattern are observed. Similarly, FIG. 4B shows the resist pattern image including the image of the lower inclined portion, FIG. 4C shows the image of the right inclined portion, and FIG. 4D shows the resist pattern image including the image of the upper inclined portion.
As described above, the electron beam is deflected in at least four directions concentrically around the optical axis of the electron beam, and the resist pattern images viewed from the respective directions are stored, and the stored images are arranged side by side at the same time. By displaying the image on the top, it is possible to recognize a stereoscopic image of each of the convex resist patterns as viewed from substantially the entire circumferential direction.

FIG. 5 shows the four observation images in an overlapping manner. Incidentally, it is also possible to perform a calculation by a computer based on these four observation images to display a true stereoscopic observation image. When the scanning electron microscope of the present invention shown in FIG. 2 is used, it is possible to simultaneously observe the inclined surface around the resist pattern on the optical axis.

When the scanning electron microscope according to the present invention is used in the semiconductor manufacturing process, the throughput per unit time plays an important role in reducing the cost in the semiconductor industry. Since there is no need to tilt the sample in advance, the tilt mechanism of the stage can be omitted. Furthermore, when the sample is mechanically tilted, the field of view is shifted, so that an operation of returning the field of view is required, and much time is required. However, such a time loss can be eliminated and the throughput can be improved. Furthermore, since the mechanism such as the tilting mechanism can be omitted, the vibration resistance and the positional accuracy of the stage are greatly improved.

[0038]

As described above, according to the present invention, damage to a sample can be suppressed without lowering the accelerating voltage, and the invention can be applied to a scanning electron microscope, and high resolution can be realized. Since the electron beam is set at a high accelerating voltage, high resolution can be maintained for a long time without being affected by dirt on the aperture, theoretical brightness can be obtained, and a bright SEM image can be obtained. In addition, since a deceleration voltage is applied to the sample to form a deceleration electric field, a larger incident angle can be obtained with a small deflection angle, so that deflection aberration can be suppressed to a small value.

Further, since the electron beam can be incident on the sample while being inclined, three-dimensional observation can be performed without providing a mechanism such as a tilting mechanism. Further, since a mechanism such as a tilting mechanism can be omitted, the vibration resistance of the stage can be reduced. And the positioning accuracy is greatly improved.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing a first embodiment of the present invention.

FIG. 2 is a conceptual diagram showing a second embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a position of a sample with respect to an optical axis and a scanning range in the present invention.

FIG. 4 is an explanatory diagram of an image obtained corresponding to a position of a sample.

FIG. 5 is an explanatory diagram of an image obtained by combining FIGS. 4A, 4B, 4C, and 4D.

FIG. 6 is a conceptual view showing a conventional electron beam optical device.

FIG. 7 is a conceptual diagram showing another conventional electron beam optical device.

[Explanation of symbols]

 Reference Signs List 1 electron beam optical device 2 optical axis 3 deflection orbit 4 sample 5 first scan coil 6 second scan coil 7 objective lens 8 correction deflection coil 9 correction deflection electrode 10 electron beam optical device 12 deceleration electrostatic lens

Claims (4)

[Claims] A scanning electron microscope having an electrostatic field compound objective lens (deceleration field) has at least one deflection system, and corresponds to a deflection trajectory of an electron beam deflected by the deflection system. An electron beam optical device, wherein a correction deflection magnetic field and a correction deflection electric field are superimposed near an objective lens. 2. The electron beam optical device according to claim 1, wherein the following expression is substantially satisfied by at least one of the correction deflection magnetic field and the correction deflection electric field. (1/2) rB '+ r'B + (1/2) rΦ "+ r'
Φ 'B: axial magnetic field distribution of the objective lens, Φ: axial electrostatic potential of the objective lens, r: distance from the optical axis,': first-order derivative with respect to the optical axis coordinate (Z), ″: optical axis coordinate ( Second derivative with respect to Z) 3. The electron beam optical apparatus according to claim 1, wherein the electron beam is deflected on the surface of the sample into substantially concentric regions with respect to the optical axis to simultaneously display images in the respective regions. 4. The electron beam optical device according to claim 2, wherein four tilt images in two directions orthogonal to each other are superimposed and displayed simultaneously.