JPS6314811B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an astigmatism detection method for facilitating the astigmatism correction operation in a scanning electron microscope.

Since the resolution of a scanning image in a scanning electron microscope is primarily determined by the size of the electron probe, it is required to make the probe diameter as small as possible for high-resolution observation. There are various factors that determine the probe diameter, but since spherical aberration, chromatic aberration, etc. are unique to each device, in reality, in order to obtain the minimum probe diameter for a given device, it is necessary to select the optimal objective lens. Focus adjustment and astigmatism correction are required. Of these, focus alignment has been well researched,
Automatic focusing devices have already been put into practical use, and no special problems have arisen in their operation. However, astigmatism is difficult to detect and must be relied solely on the intuition of a skilled operator, which is a very troublesome operation for an amateur.

As an idea recently, for example, JP-A-55-
An astigmatism correction method as shown in the specification of the published publication No. 19798 has been proposed. This corrects astigmatism by applying a horizontal scanning signal and a vertical scanning signal to each quadrupole lens of an X- and Y-type astigmatism correction device consisting of two quadrupole lenses, and synchronizing with the scanning of the sample surface by an electron beam. The apparatus is modulated, and the information signal obtained from the sample is introduced as a brightness modulation signal into the cathode ray tube synchronized with the electron beam scanning to display a map-shaped X-Y image, and the information signal in the image is The bright line in the X and Y directions is aligned with the most focused X and Y coordinate position, and the direct current corresponding to the position of this bright line is supplied to both quadrupole lenses.

If such a method is used, the degree of astigmatism is expressed by the X and Y coordinate positions on the image, so even an amateur can easily perform the correction operation. However, in reality, it is necessary to select the coordinate positions that are in focus, that is, most of the points, so if the optimal position overlaps within a large pattern, the correction operation will not be possible. Therefore, there is a limit to the number of samples to be observed, and the observation magnification is only a few thousand times, making it completely unusable at tens of thousands of times, which is where high-resolution observation is performed.

SUMMARY OF THE INVENTION The present invention aims to solve the above-mentioned drawbacks, and provides a novel astigmatism detection method that makes the correction operation extremely easy.

FIG. 1 is a block diagram showing an example of an apparatus for carrying out the method of the present invention, and 1 indicates a scanning electron microscope column. An electron gun 2 is provided at the upper end of the column, and an electron beam (probe) emitted from the electron gun is focused by a focusing lens 3 and an objective lens 4 and projected onto a sample 5. One or two stages of X and Y deflection coils 6X and 6Y are placed between the focusing lens 3 and the objective lens 4, and a scanning sawtooth is connected to the horizontal scanning power source 7X and the vertical scanning power source 7Y via a magnification adjustment circuit 8. A wave signal is provided. As a result, the electron beam scans a certain area on the sample 5 two-dimensionally. Reflected electrons or secondary electrons scattered from various parts of the sample by this electron beam scanning are detected by a detector 9 and supplied to a cathode ray tube 12 as a brightness modulation signal via an amplifier 10 and an adder 11. The deflection coils 13X, 13Y of the cathode ray tube have the horizontal
Scanning signals are supplied from the vertical scanning power supplies 7X and 7Y, and raster scanning is performed in synchronization with the electron beam scanning of the sample, so that a luminance-modulated sample scanning image is displayed on the screen. 14X, 1
4Y is a quadrupole lens for astigmatism correction, which is actually incorporated into the objective lens 4, and arranged so that each magnetic pole (or electrode) is shifted by 45 degrees from each other. A current is supplied to 14X of the four-pole lens in the X direction from a power source consisting of clip circuits 15a and 15b, a variable DC power source 16, and an adder circuit 17. Moreover, clip circuits 18a and 18 are connected to 14Y.
b. Current is supplied from a power source consisting of a variable DC power source 19 and an adder circuit 20. The clip circuit 1
A horizontal scanning sawtooth signal as shown in FIG. 2a is supplied to switches 5a and 15b from a horizontal scanning power supply 7X via a switch 21. In addition, the Y direction clip circuit 18
A, 18b are supplied with a second voltage from the vertical scanning power source 7Y via a switch 22 interlocked with the switch 21.
Although not shown in the figure, a sawtooth signal having a much longer period than a is supplied. Each of the switches has three switching terminals, and in the switch 21, the a terminal is connected to the horizontal scanning power supply 7X, and the other b terminal is connected to the horizontal scanning power supply 7X.
and c terminals are not connected. Further, the switch 22 has a terminal b connected to the vertical scanning power supply 7Y, and the other terminals a and c are not connected.

The horizontal scanning signal from the switch 21 is also sent to comparison circuits 23a, 23b, and reference power supplies 24a,
When the two signals match, a narrow pulse signal as shown in FIG. 2e and f is generated. Further, the vertical scanning signal from the switch 22 is sent to comparison circuits 25a and 25b, and compared with the reference voltages of the reference power supplies 26a and 26b, and when the signal strengths match, a pulse similar to that described above is generated. Each comparison circuit 23a, 23b, 25a and 2
The output pulse signal from 5b is sent to the addition circuit 11 via the OR circuit 27, added to the video signal containing sample information, and supplied to the cathode ray tube 12.

In such a configuration, first, the DC power supplies 16 and 1
Adjust the output of switch 9 to zero, and also switch 21
and 22 are connected to the a terminal, the cathode ray tube 12
The sample image is displayed above. In this state, switch 2
2 is not connected, and only the horizontal scanning signal (FIG. 2a) is sent through the switch 21 to the clip circuits 15a and 15.
b and comparison circuits 23a and 23b. The clipping circuit 15a clips the horizontal scanning signal of _-V1 or higher and outputs a signal as shown in FIG. 2b. This signal is a scanning signal whose intensity changes over time only during one-third of the period on the negative side of one horizontal scanning signal, and has a constant intensity during the other periods.
On the other hand, the clip circuit 15b clips the horizontal scanning signal of +V ₁ or less, and outputs a scanning signal whose intensity changes over time only during the positive 1/3 period of one horizontal scanning signal, as shown in Fig. 2c. do. Clip circuit 15
The bribe signals a and 15b are sent to an adder circuit 17 and added to the output from the variable DC power supply 16.
In the initial state, the power supply is set to zero, so the output signal of the adder circuit is zero during the center period as shown in FIG. 2d. This signal is supplied to the X-direction quadrupole lens 14X for astigmatism correction, so
For each horizontal scan, the quadrupole lens
In the period of , it is excited to a continuously changing negative intensity, and in the next middle 1/3 period, the intensity becomes zero,
(Unexcited) Furthermore, during the last 1/3 period, it is excited to a continuously changing positive intensity. As a result, the cathode ray tube 12
As shown in Figure 3a, there are three
The equally divided central region B shows an image when the quadrupole lens 14X is not excited (constant intensity), and the left region A shows an image when it is excited to a continuously changing negative intensity. , In area C further to the right, an image is displayed when the magnet is excited to a continuously changing positive intensity. Therefore, in this image, the central area B is an image with the same focus state, but the areas A and C are images with consecutively different focuses in the horizontal direction. In the figure, to simplify the display, the focus state is expressed by the density of the vertical lines, and the denser the vertical lines, the better the focus is, that is, the state where there is no astigmatism.

By the way, since the horizontal scanning signal from the switch 21 is also sent to the comparison circuits 23a and 23b and compared with the reference voltage, if the reference voltage is set to -V ₁ and +V ₁ which are the same as the clip voltage, , Figure 2e,
A pulse signal such as f is obtained, and the signal g is transmitted through the OR circuit 27 to the cathode ray tube 1 as a brightness modulation signal.
2 will be introduced. As a result, bright lines (or dark lines) dividing each area are displayed between areas A and B and between B and C, as shown by X1 and X2. In FIG. 3a, the image in area B in the center is similarly blurred as a whole, and in area A, the blur is even more severe, and the blur becomes larger as it goes to the left. On the other hand, in area C, the vertical lines are much denser than in area A, and it can be seen that the focus is on the line along P. That is, it can be seen that astigmatism can be optimally corrected when a current (or voltage) with an intensity shown by P' in FIG. 2d is supplied to the quadrupole lens. Therefore, by operating the variable DC power supply 16 shown in Fig. 1 and raising the level of the constant value signal in the central area to P' in the positive direction, an image displayed in the central area B as shown in Fig. 3b is generated. becomes the optimal focus state, A,
In area C, an image is obtained in which blur increases as the distance from area B increases, and the blur in areas A and C becomes approximately symmetrical. After completing the correction in the X direction, turn switch 2 with the variable DC power supply fixed at that value.
Switch 1 and 22 to b terminals. As a result, the horizontal scanning signal is no longer supplied, and the vertical scanning signal is supplied to the clip circuits 18a and 18 via the switch 22.
b and comparison circuits 25a and 25b. As a result, the Y-direction quadrupole lens is supplied with a signal similar to that shown in Figure 2d, and the cathode ray tube 12 screen is supplied with a signal similar to that shown in Figure 3c.
An image as shown is displayed. In the same figure, D, E,
F is the area corresponding to A, B, and C in FIG.
These are bright lines that divide each area based on the outputs from a and 25b. In the Y direction as well, the magnitude of astigmatism can be detected by observing the focus state of the regions D, E, and F in the image, similar to the above-mentioned X direction. Then, by adjusting the variable DC power supply 19, an image with optimal focus on the central region E can be obtained. When this state is obtained, fix the power supply 19 to that value and switch the switches 21 and 22 to C. Since neither horizontal nor vertical scanning signals are supplied to the clip circuit and the comparator circuit, the cathode ray tube 12 A clear image with astigmatism corrected is displayed as shown in FIG. 3d.

Using the method described above, by observing the focus state of the image occurring in each area A, B, C or D, E, F, the state of astigmatism (size, direction, or correction) can be determined. degree),
Further, since the excitation of each quadrupole lens is adjusted so that the images in the central regions B and E are best focused, even an amateur can perform the stigmatization correction operation very easily. Furthermore, since the correction is made by optimizing the focus of the image within the central area, which has a large area, it is independent of the size of the pattern in the image, there is no restriction on the sample, and the magnification is very high. It can be fully utilized even at a magnification of 50,000 times or more.

Note that the above is an illustration of the present invention, and various changes are possible. For example, in the above example, the screen is divided into three sections, but the screen is not limited to this and may be divided into five or more odd numbers. Furthermore, although the display in the X direction and the display in the Y direction are temporally separated, it is also possible to display both the X and Y directions on one screen by switching the switches 21 and 22 during vertical scanning. good. Furthermore, using only the horizontal scanning signal, the horizontal scanning signal is clipped to clip circuits 15a and 15 during vertical scanning.
If the supply is switched from 18a and 18b to 18a and 18b, similar images can be obtained in the X and Y directions, as shown in FIG. 4, and the correction operation will be easier. Furthermore, clip circuits 15a and 15b
Horizontal and vertical scanning signals may be simultaneously supplied to 18a and 18b so that the quadrupole lenses 14X and 14Y work simultaneously.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of an apparatus for implementing the method of the present invention, FIGS. 2 and 3 are diagrams for explaining its operation, and FIG. 4 is a diagram showing another example of the present invention. 2... Electron gun, 3... Focusing lens, 4... Objective lens, 5... Sample, 6X and 6Y... Deflection coil, 7X
...Horizontal scanning power supply, 7Y...Vertical scanning power supply, 9...Detector, 11...Addition circuit, 12...Cathode ray tube, 14X and 14Y...Quadrupole lens, 15a, 15b and 1
8a, 18b... Clip circuit, 16 and 19... Variable DC power supply, 17 and 20... Addition circuit, 21 and 22... Changeover switch, 23a, 23b, and 25
a, 25b...comparison circuit, 24a, 24b and 26
a, 26b...Reference power supply, 27...OR circuit.

Claims (1)

[Scope of Claims] 1. Two sets of four that independently adjust the astigmatism of the electron beam in a device that scans the electron beam two-dimensionally.
When performing correction using a polar lens, one period of horizontal scanning is temporally divided into an odd number of three or more for the signal supplied to one set (X direction) of the quadrupole lens, and a signal having a predetermined width is divided into an odd number of three or more. The central region is used as a constant value signal, both peripheral regions are used as a signal that changes over time in relation to the horizontal scanning, and the signal supplied to the other set (Y direction) of the quadrupole lens is vertically One period of scanning is divided into an odd number of three or more, and a central region having a predetermined width is a constant value signal, and both peripheral regions are a signal that changes over time in relation to the vertical scanning. A method for detecting astigmatism characterized by dividing and displaying sample images corresponding to signals of respective regions supplied to each quadrupole lens on a cathode ray tube screen. 2. The non-contact device according to claim 1, which simultaneously supplies a signal to be supplied to the quadrupole lens of the one set (X direction) and a signal to be supplied to the quadrupole lens of the other set (Y direction). How to detect point aberration. 3. The supply of signals to the quadrupole lenses of one set (X direction) and the supply of signals to the quadrupole lenses of the other set (Y direction) are performed separately in time. The method for detecting astigmatism according to item 1.