JPH07201299A - Method and device for focusing in charged particle beam device - Google Patents

Abstract

PURPOSE:To realize a method and a device for focusing the charged particle beam which can be operated with small irradiation current, is not affected by the shape or position of the sample, and can achieve the focusing with high precision. CONSTITUTION:When the electron beam is focused, a control circuit 14 controls a driving source 16 of an object lens 3 and a deflection control circuit 17 of a deflecting coil 5. As a result, the driving source 16 supplies the exciting current which is changed stepwise to the object lens 3, and the deflection control circuit 17 reads the scanning signal from a scanning signal memory 18 and supplies it to the deflecting coil 5 every time when the step-shaped exciting current is changed. This scanning is provided by combining the linear scanning with the curved scanning. Most preferably, two crossing diagonal scanning is used in the case of the linear scanning, while the elliptical scanning and the circular scanning are sued in the case of the curved scanning.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focusing method and apparatus in a charged particle beam apparatus such as a scanning electron microscope capable of automatically focusing a charged particle beam.

[0002]

2. Description of the Related Art A scanning electron microscope has an automatic focusing function. In this focusing, the excitation of the focusing lens is changed stepwise, and a predetermined region of the sample is scanned with the electron beam in each excitation state, that is, in each focusing state of the electron beam. Electrons and backscattered electrons are detected, and detection signals are integrated for each focusing state. Then, the integrated values of the detection signals in each focusing state are compared, the focusing state when the maximum value is obtained is determined to be the focus position, and the excitation of the focusing lens is set in that state.

In such a focusing operation, there are the following five requirements for scanning the sample with an electron beam.
I can give you one. The first is that it can operate with a smaller irradiation current, and the second is that the operation time is short. And third, it must operate for all sample shapes. Furthermore, 4
The second is that it is not affected by the astigmatism of the electron beam.

By the way, in focusing, if unevenness exists over the entire surface of the scanning region of the sample, highly accurate focusing can be performed. However,
If the sample has a linear or circular characteristic part in a part of the scanning area of the electron beam and the other part is a smooth plane like the IC pattern, it is used for focusing. The integrated value of the detected signals thus obtained may have a small difference depending on each focusing state of the electron beam, and the accuracy of focusing may decrease. As a result, the fifth requirement is that there is no dependence on the sample position within the observation visual field.

In order to satisfy the above first and second requirements, while satisfying the third to fifth requirements,
It is necessary to scan the electron beam efficiently with a smaller number of pixels. Considering these points, a typical scanning shape currently used is considered.

The scanning shape of the electron beam at the time of focusing in the conventional scanning electron microscope is roughly divided into three methods. The first is an all-pixel type scanning in which a predetermined sample area is raster-scanned. As shown in FIG. 1, the second type is a concentric type in which the electron beam is scanned concentrically in a predetermined region of the sample. Third, as shown in FIG.
It is an 8-shaped scanning in which the electron beam is scanned in a 8-shaped pattern in a predetermined region of the sample.

[0007]

Each of the scanning shapes described above has its advantages and disadvantages. First, regarding all-pixel scanning, the observation target may be present in any part of the observation visual field, and the third requirement of the above requirements is best satisfied. On the other hand, when the number of pixels to be scanned is the same, the scanning time per pixel becomes shorter than that of other scanning shapes, and the SN ratio deteriorates. for that reason,
The first and second requirements cannot be satisfied.
Further, when the observed image includes astigmatism, peaks of the signal amount integral value appear at the focal lengths at both ends of the astigmatism, and the focal length with the circle of least confusion is not focused. Not satisfied.

Next, with respect to the concentric scanning, since the number of pixels to be scanned is small, the irradiation time of the electron beam per pixel can be set long and the SN ratio is good. Therefore, it is possible to operate with a smaller irradiation current as compared with the all-pixel type, and it is possible to satisfy the first and second requirements described above. Further, since it is basically circular, it becomes easy to focus on the focal length where the minimum circle of confusion is obtained even when the observed image contains astigmatism, and the fourth requirement is satisfied. However, in the case of a circular observation target such as a contact hole in a semiconductor sample, if the center of the contact hole is close to the center of the scanning shape, the overlap between the edge part of the hole and the scanning line of the electron beam will decrease, and the integrated signal amount Change becomes difficult to appear due to the change in excitation of each objective lens, and the third requirement is not satisfied.

Further, with respect to the 8-shaped scanning, since the number of pixels to be scanned is small, there is a possibility that an accurate operation will not be performed if the observation target deviates from this. As a result, this scan cannot meet the fifth requirement.

The present invention has been made in view of the above points, and an object thereof is to operate with a small irradiation current and to perform focusing with high accuracy without being influenced by the shape and position of the sample. A method and apparatus for focusing on a charged particle beam are realized.

[0011]

A focusing method in a charged particle beam apparatus according to the present invention changes stepwise the intensity of a focusing lens for focusing a charged particle beam on a sample, and each time the change is made, A charged particle beam that scans the irradiation position of the charged particle beam on the sample, detects the optimum focusing lens strength based on the signal obtained with this scanning, and sets the focusing lens strength to that strength. The focusing method of the apparatus is characterized in that the scanning of the irradiation position of the charged particle beam on the sample is performed by combining linear scanning and curved scanning each time the focusing lens intensity changes stepwise.

Further, the focusing device in the charged particle beam apparatus according to the present invention comprises a focusing lens for focusing the charged particle beam on the sample, means for changing the intensity of the focusing lens in a stepwise manner, and the sample. Scanning means for scanning the irradiation position of the charged particle beam on the upper side, and scanning means for scanning the sample linearly and curvedly for each stepwise change of the focusing lens intensity. Signal generating means for supplying to the sample, a detector for detecting a signal obtained by irradiating the sample with the charged particle beam, and a signal from the detector is integrated each time the focusing lens intensity changes stepwise. It is characterized in that it is provided with an integrating means and means for setting the intensity of the focusing lens based on the integrated value for each step change.

[0013]

Focusing in the charged particle beam apparatus according to the present invention is performed by changing the focusing lens intensity stepwise,
The scanning of the irradiation position of the charged particle beam on the sample is performed by combining linear scanning and curved scanning. Further, particularly preferably, the linear scan uses two intersecting oblique scans, and the curved scan uses an elliptical scan and a circular scan.

[0014]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 3 shows a scanning electron microscope which is an embodiment of the present invention, and 1 is an electron gun. The electron beam EB generated from the electron gun 1 is focused on the focusing lens 2 and the objective lens 3.
Then, it is finely focused on the sample 4. Further, the electron beam EB is deflected by the deflection coil 5, and the irradiation position of the electron beam on the sample 4 is scanned. Secondary electrons generated by the irradiation of the sample 4 with the electron beam are detected by the secondary electron detector 6. The detection signal of the detector 6 is amplified by the amplifier 7 and then supplied to the filter 8 and the cathode ray tube 9.

The signal passed through the filter 8 is supplied to the integrator 11 via the absolute value circuit 10. The integration result of the integrator 11 is supplied to the peak detection circuit 13 via the AD converter 12. The peak value detected by the peak detection circuit 13 is supplied to the control circuit 14. 15 is an operation panel,
The operation panel 15 sends an instruction signal to the control circuit 14. The control circuit 14 includes a driving power source 16 for the objective lens 3 and the deflection coil 5.
The deflection control circuit 17 is controlled. The deflection control circuit 17 reads the scanning signal stored in the scanning signal memory 18 and supplies the signal to the deflection coil 5. The operation of such a configuration is as follows.

When observing a normal secondary electron image, the control circuit 14 controls the deflection control circuit 17 based on an instruction signal from the operation panel 15, and the deflection control circuit 17 sends a predetermined scanning signal to the deflection coil 5. It is supplied, and an arbitrary two-dimensional area on the sample 4 is raster-scanned by the electron beam EB. Secondary electrons generated by irradiating the sample 4 with the electron beam are detected by the detector 6. The detection signal is supplied to the cathode ray tube 9 synchronized with the scanning signal to the deflection coil 5 through the amplifier 7, and the secondary electron image of an arbitrary region of the sample is displayed on the cathode ray tube 9.

Next, a case of performing a normal electron beam focusing operation will be described. When the operation panel 15 is operated to instruct the focusing mode, the control circuit 14 controls the driving power supply 16 for the objective lens 3 and the deflection control circuit 17 for the deflection coil 5. As a result, the driving power source 16 supplies the objective lens 3 with a stepwise changing exciting current, and the deflection control circuit 17 outputs a scanning signal for operating a predetermined region of the sample each time the stepwise exciting current changes. Deflection coil 5
Supply to.

The secondary electron signal detected by the detector 6 in the focus state due to the stepwise excitation current is amplified by the amplifier 7 and then passed through the filter 8 to be converted into a positive signal by the absolute value circuit 10. To be made. The output of the absolute value circuit 10 is supplied to the integrator 11,
A signal based on one scanning of the electron beam for each excitation step of the objective lens 3 is integrated. The integrated value of the integrator 11 is converted into a digital signal by the AD converter 12 and then supplied to the peak detection circuit 13.

The peak detection circuit 13 stores the integrated value of the integrator 11 for each excitation step of the objective lens 3. FIG. 4 shows stored integral value changes at this time, where the vertical axis represents the integrated value and the horizontal axis represents the excitation intensity of the objective lens 3. The peak detection circuit 13 detects the excitation intensity of the objective lens 3 at the peak of the signal amount integral value change of the stored integral value, and supplies the value to the control circuit 14. Control circuit 1
Reference numeral 4 controls the drive power supply 16 to set the objective lens 3 to the excitation intensity at the peak, and the focusing operation is performed in this manner.

In the present invention, the scanning signal supplied from the deflection control circuit 17 to the deflection coil 5 in the above focusing operation is modified. That is,
Scanning of the electron beam for each excitation step of the objective lens 3 is performed by combining the five modes shown in FIG. First, the deflection control circuit 17 takes out the scanning signal stored in the scanning signal memory 18 for obliquely linear scanning shown in FIG. 5A, and supplies the signal based on this scanning signal to the deflection coil 5. . As a result, FIG.
The oblique scanning shown in (a) is performed.

Next, the deflection control circuit 17 includes a memory 18
Then, the scanning signal for performing the elliptical scanning shown in FIG. 5B is extracted and supplied to the deflection coil 5. Further, the deflection control circuit 17 causes the memory 18 to output a signal for performing the circular scanning shown in FIG. 5C, an elliptical scanning signal shown in FIG.
The oblique scanning signal shown in (e) is taken out, and the signal is sequentially supplied to the deflection coil. 5A and 5E are oblique linear scans, the respective straight lines are set to be orthogonal or close thereto. 5B and 5D are both elliptical scans, the major axis of each ellipse is set to be orthogonal or close thereto.

As described above, the scanning shown in FIG. 6 in which the scanning of the modes shown in FIGS. 5A to 5E is combined is performed for each excitation step of the objective lens 3. 2 detected by the detector 6 based on the scan shown in FIG.
The secondary electron signal is amplified by the amplifier 7, passed through the filter 8, converted into a positive signal by the absolute value circuit 10, and supplied to the integrator 11 to be integrated.

As a method of performing the scanning in the above-mentioned five modes, other than the method of storing the respective scanning signals and sequentially reading them and supplying them to the deflection coil 5, other methods can be used. . For example, a basic calculation formula may be prepared, the data of the scanning coordinate points may be created by calculation based on the calculation formula, and the scan signal of each mode may be created. In this case, the following is used as the basic calculation formula.

[0024]

[Equation 1]

Further, the deflection control circuit 17 described above has a sin
By including a circuit for drawing a circle in which COS and COS are combined and a circuit for rotating the scanning direction, the control circuit 14 is configured to generate a scanning signal in a desired mode in response to scanning start, stop, and conversion commands. May be.

Although one embodiment of the present invention has been described in detail above, the present invention is not limited to this embodiment. For example, although secondary electrons are detected, reflected electrons may be detected. Further, in the embodiments, the scanning electron microscope has been described as an example, but the present invention can also be applied to an apparatus using an ion beam. Furthermore, although the excitation of the objective lens was changed during the focusing operation, an auxiliary lens of the objective lens may be provided and the excitation of the auxiliary lens may be changed. Furthermore, when focusing, the detection signals are integrated for each scanning and the integrated signals are compared to obtain the optimum focus position. However, the maximum amplitude value (peak-to-peak value) of the detection signal is detected. It may be done.

[0027]

As described above, the focusing in the charged particle beam apparatus according to the present invention is performed by linearly scanning the irradiation position of the charged particle beam on the sample each time the focusing lens intensity changes stepwise. And a curved scanning are combined. As a result, the shape of the sample,
The focusing operation can be performed with a low irradiation current regardless of the position and the presence or absence of astigmatism. In addition, focusing can be performed in a short time as compared with the all-pixel scanning method. That is, by using the five modes shown in FIG. 5, it is possible to scan a wide range on the observation screen. Further, although the object to be observed is generally considered to be in the center of the screen, the scanning method according to the present invention intensively scans the center of the screen, which is convenient for focusing the target portion to be observed. Becomes

Further, the scanning shape is constituted by, for example, the scanning modes shown in FIGS. 5 (a) and 5 (e) which are intersecting line segments, and the scanning modes shown in FIGS. 5 (c) to 5 (d) based on a circle. Therefore, even if there is astigmatism, it is possible to focus on the focal length where the circle of minimum confusion is formed.
Furthermore, when scanning concentrically, a circular sample such as a contact hole became a problem, but in the combined scanning mode of the present invention, the number of crossing with the edge portion of the circular sample is large and the signal amount is large. There is an advantage that changes in the integrated value can be easily obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a state of a conventional circular scan.

FIG. 2 is a diagram showing a conventional 8-shaped scanning state.

FIG. 3 is a diagram showing an embodiment of a scanning electron microscope according to the present invention.

FIG. 4 is a diagram showing a relationship between an excitation intensity of an objective lens and an integrated value.

FIG. 5 is a diagram showing each scanning mode according to the present invention.

FIG. 6 illustrates a combined scanning mode according to the present invention.

[Explanation of symbols]

 1 electron gun 2 focusing lens 3 objective lens 4 sample 5 deflection coil 6 detector 7 amplifier 9 cathode ray tube 12 AD converter 14 control circuit 15 operation panel 16 drive power supply 17 drive power supply 18 scanning signal memory

Claims (3)

[Claims] 1. The intensity of a focusing lens for focusing the charged particle beam on the sample is changed stepwise, and the irradiation position of the charged particle beam on the sample is scanned each time the intensity is changed.
Based on the signal obtained by this scanning, the intensity of the optimum focusing lens is detected, and the focusing lens intensity is set to that intensity. A focusing method in a charged particle beam apparatus, characterized in that a scanning of an irradiation position of a charged particle beam on a sample is performed by a combination of a linear scanning and a curved scanning each time a change occurs. 2. The focusing method in a charged particle beam apparatus according to claim 1, wherein the linear scan is two intersecting oblique scans, and the curved scan is an elliptical scan and a circular scan. . 3. A focusing lens for focusing the charged particle beam on the sample, a means for changing the intensity of the focusing lens in a stepwise manner, and a scan for scanning the irradiation position of the charged particle beam on the sample. Means, a scanning signal generating means for supplying to the scanning means a scanning signal for linearly scanning the sample and a scanning signal for curvilinear scanning at each stepwise change of the focusing lens intensity, and to the sample. Based on the detector that detects the signal obtained by irradiation of the charged particle beam, the integrating means that integrates the signal from the detector each time each step-like change in the focusing lens intensity, and the integrated value for each step-like change And a means for setting the intensity of the focusing lens.